Phosphoric Acid Manufacturing: Using Raw Materials And Salfuric Acid 5l6d2y

“Manufacturing of Phosphoric Acid using Rock Phosphate and Sulfuric Acid”

Chapter No 1: Introduction A] History of Development Hennig Brandt (1630-1710) an Alchemist in Hamburg, discovered Phosphorous in 1969. Phosphorous burns in air and exist in nature as phosphate. Carl Wilbelm Scheel(1742-1786) and Johan Gottlieb Gahn (1745-1818) from Sweden studied the nature of bone , they dissolved bone ash [Ca3(PO4)2 in the form of hydroxypatite] in Sulfuric acid (H2SO4) so making phosphoric acid. In late 18th century M.M. Coignet of Lyan, improved the peltier process of making ash, acidifying it with sulfuric acid (H2SO4) to produce phosphoric acid. Ca3(PO4)2 + 3H2SO4 → 3CaSO4 + 2H3PO4 ∆

H3PO4 → HPO3 + H2O 4HPO3 + 12C → P4 + 2H2 + 12CO In the year 1870-1872 wet process phosphoric acid for fertilizer use was first produced commercially in , United States, Baltimore, Maryland for short period. Strength of phosphoric acid was increased as high grade of rock become available after 1900. The Israel mining industries (IMI) institute of research and development invented the hydrochloric acid route for making phosphoric acid in1950,which of incorporated solvent extraction. Hydrochloric acid (HCl) was chosen because of near by source of chloride from dead sea operation. Ca3(PO4)2 + 6HCl → 2H3PO4 + 3CaCl2 For summarizing (WPA) Wet Phosphoric Acid Process Technology took a big leaps forward with development of strong acid process in 1970. Between1927-1932 group such as Swiss with Dorr and other worked on the higher P2O5 concentration process. Their aim was to produce 40%-50% P2O5 acid directly at the filter. In 1932 Dorr built a tree - train plant at Trail, with the capacity of 40-50 tons per day, this process of producing 30-32% P2O5 acid at filter exist was known as strong acid process. After 1930, Nordendreen took out patent for the manufacturing of more concentrated (40% P2O5) Phosphoric Acid by means of formation of hemihydrates or anhydride but it was not until 1970. Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 1

“Manufacturing off Phosphoric A Acid using Rock Phosphatee and Sulfuricc Acid”

Inn 1966, Albrright and Wiilson investiggated some developmen d nts made by IMI I includinng thhe use of salt as souurce of soddium instead d of other sodium soources, (HC Cl) H Hydrochloric c acid was thhe byproducct of the pro ocess and thee cleaning of o WPA (W Wet Phosphoric acid) is donee by the solvvent Diisoprropyl ether(D DIPE) which h is identifieed byy IMI. The Methyl M Isobutyl Ketone (MIBK) waas used to seeparate nitricc acid (HNO O3 ) frrom Phospho oric acid. Affter that the Albright and d Wilson teaam was startted to develoop ouutlines of so olvent extraction processs. Inn 1979 Albrright and Wilson W had aadded a “Weet Process P Phosphoric Acid A (WPA))” pllant F5 to prroducing strong acid dirrectly from filter. f The prroduct acid was w separateed frrom calcium m sulfate hem mihydrate, w which was recrystallise r ed to gypsum m in order to t reecover the residual phosphoric vaalues. In 19 990, there w was the risee of purifieed Phosphoric acid (PWA) plants, p whicch were ablee to produce acid of suffficient qualitty too displace alll but the higghest purity acid demannds even at tthe early stagge. Over nexxt years, these teechnologies were develooped to a staage very closse to that stilll in existencce wenty first century. c tooday in the tw Structure of Phosphoric P acid: a

T bond lengths are: P=O: 1.52A⁰, The P― ―OH: 1.57 A⁰ A O― ―H: 1 A⁰ T (OH)-P-((OH) angle is The i 111⁰


B] Properties of Phosphoric Acid 1. Physical Properties: 1

Formula

H3PO4

2

Molecular weight

98.04 At normal temperature it is a

3

Appearance

colourless liquid or rhombic crystals

4

Physical state

Solid crystalline

5

Melting point / Freezing point

42.40C (1080F)

6

. Boiling point

2600C

7

Vapour pressure

0.0285 mm Hg at 200C

8

Density at 15.50C

1.583 gm/cc (75%), 1.694 gm/cc (85%) 15 centistokes (75%)

9

Viscosity at 200C

20 centistokes (80%) 28 centistokes (85%) 140 centistokes (100%) 1.710 at 600F

10

Specific gravity

11

Odour

12

Solubility

Soluble in water and ethanol

13

Refractive index

n20 /D 1.433

14

Non toxic in nature

15

Inodorous at ordinary temperature

Phosphoric acid is quite corrosive in nature. Its corrosive nature increases with increase temperature.

16

Vapour density

3.4(Air = 1)

17

Flash point

Not flammable

18

Auto ignition temperature

Not applicable

Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 3


2. Chemical Properties : 1. Molten Phosphoric acid slowly undergoes auto dehydration Auto dehydration of phosphoric acid 2H3PO4 → H4P2O7 + H2O 2. Acidic Properties: a) It is medium strong tribasic acid and this forms three series of salts i) Primary Phosphates (M1H2PO4) ii) Secondary Phosphates (M2HPO4) iii)Tertiary Phosphate (M3PO4) b) Dissociation of Phosphoric acid H3PO4 c)

H2PO4-

HPO4-2

PO4-3

Phosphoric acid (H3PO4) is moderately acidic, Primary phosphates are

weakly acidic, Secondary phosphates (HPO4-2) are weakly basic, Tertiary phosphates (PO4-3) are strongly basic. The last specie (PO4-3) largely hydrolyzed in water PO4-3 + HOH → HPO4-2 + OH-

Alpha plot for phosphoric acid and its conjugate (basehttp://ion.chem.usu.edu/sbialkow/Classes/3600/Overheads/H3A/H3A.html, 11/09/2014)



3. Basic Properties: a. Molten anhydrous phosphoric acid is a good conductor of electricity, because of self ionization and formation of phosphotacidium ion P(OH)4+ 2H3PO4 → H4PO4+ + H2PO4b. Due to slow auto dehydration of phosphoric acid in melts causes reaction such as 2H3PO4 ↔ H4P2O7 + H2O H4P2O7 + H2O ↔ H3O+ + H3P2O7-1 H3P2O7- + H3PO4 ↔ H2P2O7-2 + H4PO4Hence molten phosphoric acid has high content of ions.

4. Redox properties: a) Phosphoric acid in aqueous solution is very poor oxidizing agents b) Phosphoric acid is good reducing agent

5. Esterification of Phosphoric acid: a. Phosphoric acid forms mono ester with alkenes. H3PO4 + C3H6 ↔ H3PO4C3H8 b. Reaction of phosphoric acid with alcohols also gives phosphoric acid ester H3PO4 + R-CH2OH ↔ R-H2PO4CH2 c. Reaction of carbonate mineral with 100% phosphoric acid CaCO3 + H3PO4 → CaHPO4 + H2CO3 H2CO3 → H2O + CO2 (liq) CO2 (liq) → CO2 (gas)



C] Industrial Importance and Uses The following industries has the application of phosphoric acid as an intermediate or raw material in manufacturing of their products or sometimes refining of the products 1. Fertilizer industry : The important fertilizer products made from the phosphoric acid are: a. Triple super phosphate [CaH4(PO4)2.H2O] b. Ammonium phosphate [(NH4)2HPO4] c. Mono ammonium phosphate [(NH4)H2PO4] d. Merchant acid e. Super phosphoric acid f. Liquid fertilizers 2. Industrial phosphates : The principle industrial phosphates made from phosphoric acid are: a. Mono sodium phosphate [NaH2PO4] b.

Sodium acid phosphate [Na2H2P2O7]

c. Sodium meta phosphate [NaPO3] d. Disodium phosphate [Na2HPO4] e. Tetra pyrophosphate [Na4P2O7] f. Tri sodium phosphate [Na3PO4] g. Sodium tripolyphosphate[Na5P3O10] h. Dicalcium phosphate[CaHPO4] i. Tetra potassium pyrophosphate [K4P2O7]

3. Beverage Industry : Phosphoric acid is added to soft drinks as an acidifying agent which imparts desired tangy taste acid sourness to soft drinks. The pH value is maintained between 2-4. It is chiefly used in core type beverages but has also found application in preparation of phosphate beverages such as orange, lemon and cherry phosphates. The acidity furnished in the form of ortho phosphoric acid is beneficial to health.



4. Textile industry : Here the sults of phosphoric acid such as trisodium phosphates, disodium phosphates and sodium metaphosphates are generally used for performing the following purposes: a. Removal of grease and oil from cotton and wool b. Dyeing c. Degumming of silk [Removal of sericin & silk glue] d. Weighing of silk [Increasing the weight of silk fiber]

5. Sugar refining industry: A small amount of dil. Phosphoric acid solution is sprayed on the sugar in the centrifugal, which results in brightening and improvement of the colour of the product. The acid exists in several way in colour improvement a. Retains the pit of the syrup on the crystals b. Weak up slits of organic acids and release the volatile acids c. Precipitates dark colour organic salts as colourless phosphates

6. As a catalyst and oil refining agent : The three main processes where phosphoric acid used as catalyst to alter composition of hydrocarbons are: a. Dehydrogenation b. Polymerization c. Alkylation a. Dehydrogenation: Dehydrogenation is brought about mainly by decomposition or cracking of petroleum products of very high elevated temperature and pressure with the development of the catalyst. The desired results could be accomplished at lower temperatures. b. Polymerization : It is the reaction opposite to hydrogenation where phosphoric acid is again used as catalyst for reducing both temperature and pressure. Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 7


c. Alkylation : Alkylation is the reaction of dissimilar hydrocarbons. It is favoured by high pressure but can be carried out at reasonably low temperature with the help of the phosphoric acid and catalyst. 7. Photography: Free phosphoric acid is employed in the aniline process for: a. Reproduction of line subjects b. To regulate the acidity or alkalinity of the developer both. c.

Developing out papers by emulsion.

8. Rust removal (metal surface cleaning): Phosphoric acid is used to remove rust by direct application to rusted iron, steel tools, on other surfaces which changes the reddish brown iron oxide (rust) to ferric phosphate 2H3PO4 + Fe2O3 → 2FePO4 + 3H2O Liquid phosphoric acid is used for electroplating and often formulated as thick gel. The rust may also be removed via phosphate conversion coating. This coating provides the desired corrosion resistance also. 9. Water treatment : The main phosphate derivatives uses for this purpose are: a. Trisodium phosphate b. Tetra sodium pyrophosphate c. Mono sodium phosphate d. Di sodium phosphate All these phosphates leads to eutrophication of the water.

10. Fire retardants: Ammonium hydrogen phosphate decomposes on heating loses ammonia and produces phosphoric acid which slow down the combustion cellulose. The other phosphates used as fire retardants are urea phosphatestetrasis (hydroxyl methyi) phosphonium chloride and ammonium polyphosphate.



11. Detergents and soaps : Phosphoric acid salts such as trisodium phosphates and super phosphoric acid are the main constituents used in manufacturing of detergents and soaps

12. Dental cements: Phosphoric acid is widely used in dental cements giving good dental properties as follows: a. Hardness and high crushing strength b. Quick setting and strong adhesive properties c. Resistance to solvent effect of saliva d. Germicidal properties Two type of phosphate dental cements are: a. Zinc phosphate cements b. Silicate elements

13. Glasses: Phosphoric acid is used for making glasses with some modified properties as follows: a. Optical glasses having desired refractive index dispersion ratio. b. Glasses having high ultraviolet transmissions c. Fluorescent glasses d. Heat absorbing glasses e. Hydro fluoric acid resistance glasses

MISCELLANEOUS USES Phosphoric acid is used: 1. As a leavening agent 2. In the preparation of albumin derivatives 3. In the preparation of animal feed supplements 4. As a buffer agent for e.g buffer for high performance liquid chromatography 5. As the electrolyte in phosphoric acid fuel cell 6. As a pH adjusters in cosmetics and skin care product Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 9


7. As a catalyst in asphalt binding, liquefaction of wood, hydration of alkenes to produce alcohols. 8. In compound semiconductor processing, phosphoric acid is a common wet etching agent: for e.g. in combination with hydrogen peroxide and water it is used to etch in gas selective to lnP.



D] Economic Aspects 1. Demand-Supply position: Phosphoric acid has many applications as fertilizer & non-fertilizer products. Therefore the demand for phosphoric acid is a derived demand & the rate of growth in demand is largely dependent on the rate of growth in the sectors that use it as an input. The total global consumption of phosphoric acid increases from 37.1 Million Metric Ton in 2005/06 to 43.7 MMT in 2014/15. The table below shows the increase in phosphoric acid consumption [P2O5 basic] from 2005 to 2014 on calendar year basis. Table (1) World Fertilizer Consumption Year

Phosphoric Acid Consumption

% Growth in Consumption

(MMT) 2005

37.1

2006

39.0

+5.1%

2007

40.5

+3.8%

2008

41.8

+3.3%

2009

37.6

-5.4%

2010

40.6

+6.3%

2011

39.8

-3.3%

2012

40.3

-0.9%

2013

41.2

+2.2%

2014

43.7

+2.9% [P. Heffer, IFA, June, 2005/06/07/08/09/10/11/12/13/14]

The increase in the consumption of phosphoric acid leads to the increase in the demand of the same world demand of the phosphoric acid thus grow from 34.6 MMT in year 2006 to 43.6MMT in 2014 These increases in demand of phosphoric acid pressurize the industry to operate at higher capacity. World phosphoric acid capacity in 2007 decreased to 43.2 MMT, but rebound to 45 MMT of P2O5 in 2008 due to new projects in China & Morocco which



is further increased to 55.5 MMT of P2O5 in2014 because of new plants set-up during this year in countries like Jordan, Tunisia, Saudi-Arabia & Morocco. The increasing capacity impacts on the global potential supply of phosphoric acid which was observed to have a marginal surplus of more than 15% over the year of 2006-2004. Global phosphoric acid supply/demand projections show a stable potential balance which detailed in the table below. Table (2) World Phosphoric Acid Potential Supply/Demand Balance (Million Metric Tonnes, P2O5) Year

2010

2011

2012

2013

2014

Capacity*

47.8

51.0

52.3

53.8

55.5

Total Supply*

39.6

41.5

43.3

45.3

47.1

Fertilizer Demand*

31.3

32.8

34.2

35.5

36.6

Non-Fertilizer use*

5.5

5.6

5.6

6.0

6.2

Distribution Losses

0.7

0.8

0.8

0.8

0.9

Total Demand*

37.6

39.2

40.6

42.3

43.6

Balance*

2.0

2.3

2.7

3.0

3.4

% of Supply

5%

6%

6%

7%

7%

Increase [M. prud’homme, IFA, June 2010] *Definitions of various in context with the table: *Capacity: Here capacity is the effective/ Theoretical capacity, representing the maximum achievable production. *Supply: Supply is computed from the ‘effective capacity’, multiplied by highest operating rate achieved in the respective year. *Demand: There are two types of demand a. Fertilizer Demand b. Non-Fertilizer Demand a. Fertilizer Demand: It is the ability or willingness of farmers to buy fertilizer at a given probable consumption in one calendar year b. Non-Fertilizer Demand: Consumption as non-fertilizer use, referred to industrial use. Net non –fertilizer demand excludes the use of products that are recovered as a by-product from industrial process and then used as fertilizers. Total Demand = Fertilizer Demand + Non-fertilizer Demand + Distribution losses *Potential Balance: It is the difference between supply & total demand. Potential Balance = Supply – Total Demand



2. Import Export Data: Out of the total trade of approximately 5 million tonnes of phosphoric acid, India imports more than 2.5 million tonnes every year. However phosphoric acid is not freely traded & more than 50% of Indian transaction are by way of long term supply arrangements between producers & importers. About 1.2 million tonnes of phosphoric acid is imported by India from Morocco, which is about 50% of India’s total import. According to the Indian bureau of Mines, import of phosphoric acid decreased to 2 Metric Ton in 2010-2011 from 2.69MT in the previous year. Imports of acid considerably increased to 2.32 MT in 2011-2012. Imports are mainly from Morocco (47%), Senegal (17%), Tunisia (14%) & South Africa (18%). Table (1) Import of Phosphoric Acid during [2009-2012] Countries

2009-2010

2010-2011

2011-2012

Qty(Tonnes)

Qty(Tonnes)

Qty(Tonnes)

All countries

2692899

2008376

2324532

Morocco

1273174

860313

1084630

Senegal

332198

212676

392742

USA

222308

235231

323771

Tunisia

298101

227292

98371

South Africa

332770

353897

208469

Israel

142757

31322

66330

Saudi Arabia

5620

30381

-

UAE

-

13892

-

China

3998

9173

12074

Lebanon

-

7682

-

Indonesia

-

-

46929

Malaysia

-

549

34632

Ghana

-

-

14957

Other Countries

81973

77923

41627

[IMYB, 2009-10, 2010-11, 50th & 51th edition] India still depends on agricultural field which increases the demand of fertilizer. Hence 90-95% Phosphoric Acid produced in India is used for production of fertilizers. Therefore India never exports phosphoric acid on large scale.



The export of phosphoric acid decreases drastically to 11798 tonnes in 2010-11 from 143195tonnes in 2009-2010 which is further increased to 18674 tonnes in 2011-12.

Table (2) Export of Phosphoric Acid during [2009-2012] Countries

2009-2010 Qty

2010-2011 Qty

2011-2012 Qty

(tonnes)

(tonnes)

(tonnes)

All countries

143195

11798

18674

Bangladesh

46675

5229

13

Indonesia

56221

5405

18411

Saudi Arabia

8392

1001

-

UAE

23

24

23

Mozambique

18

36

20

Sri Lanka

3

19

10

Taiwan

21815

9

16

Oman

-

12

-

Kenya

-

52

-

Nepal

4

4

86

Japan

-

-

7

Nigeria

-

1

40

Sudan

-

-

1

Other Countries

10044

117

38 [IMYB, 2009-10, 2010-11, 2011-12]

If India wants to ensure phosphoric acid, availability through imports, companies need to participate in more production t ventures in countries rich in resource like Morocco, Senegal etc. and for long term supply arrangements. [Press Information bureau, 27 March 2008] 3. Manufacturing Facilities: (In India & in World) 1. Phosphoric acid is produced either by acidulation of rock phosphate by a mineral acid in wet process (i.e. by using H2SO4 or HCL) or by burning of phosphorous produced through electro-thermal process. [Dryden & M. G. Rao, 2010]



2. Process using H2SO4 is the most & most important and may be classified according to the hydrates form in which calcium sulfates crystallizes, viz, anhydrates (CaSO4), hemihydrates (CaSO4. ½ H2SO4) and dihydrates (CaSO4.2H2O). The hydrate form is controlled mainly by temperature and acid concentration. Table (1) gives the silent features of contemporary process technology (using H2SO4) in commercial use. Table (1) Silent Features of Contemporary Process Technologies for fertilizer grade Phosphoric Acid (H2SO4 route) Sr

Name of the No. of

No

Process

Data Furnished by Process Licensors

Separation Capitalized P2O5 recovery Energy

Gypsum

Steps

Quality

cost Rs.

in%

(Crores)

Requirement per ton P2O5 (KWH)

1

Dihydrate

1

29

95-96

125

Not good

2

Hemihydrate 2

31

98-98.5

110

Excellent

2

31

98-99

110

Excellent

Hemihydrate 1

25

93-94

100

Reported

-Dihydrates 3.

DihydrateHemihydrate

4

poor [Executive summary, 2003] 3. The features listed include P2O5 recovery efficiencies, specific energy consumption, temperature conditions required to be maintained in reactors & recrystallizer and quality of by-product gypsum from various process. 4. The conventional dihydrate process remains the most predominant because of its low capital cost, low operating temperatures and flexibility of operation. But the process suffers from relatively low P2O5 recovery & low strength of acid. Newer process which claims to overcome these limitations of the dihydrate process are hemihydrates-dihydrate (double filtration stage) and dihydrate-hemihydrate. 5. HCl acidulation process produces technical grade acid. After acidulation, the acid has to concentrated & purified before used in detergents or food industries. This process gives CaCl2 as a by-product which is difficult to dispose. Capital cost and



cost of production are higher than those sulfuric acidulation plants though the cost difference is reducing gradually with improving HCl process. 6. Electro-thermal reduction of phosphate rock produces very pure phosphoric acid, but the cost of production is extremely high because of high cost of power in India capital cost is highest for the plant based on this technology.

Table (2) Technology Status of Indian Industry (capacity in tones of P2O5/year) A. Plants Based on Conventional Dihydrate Process Sr.

Manufacturer

No 1

Installed

Process Licensor

Capacity FACT Limited,

33,000

Conductor Dorr Oliver, U.S.A.

Udyogmandal 2

EID parry, India GDFC Limited

Hindustan Dorr Oliver

10,696

Limited 3

Engineering

52,500

Societe-de-Prayon

Simon Carves

Belgium

Ltd. U. K.

Chemico, U.S.A. through

Hitachi Zosan

Hitachi zosen, Japan

Japan

Dorr Oliver, U.S.A. 4

Coromandel

91,000

Dorr Oliver, U.S.A.

Fertilizers Ltd 5

Albright Morarji &

Dorr Oliver Ltd, U.S.A.

17,385

Pandit Ltd.

Societe-de-Prayon

Charamsi

Belgium through

Morarji

Albright & Wilson U.K.

Chemical Co. Ltd.

6

Fact Limited,

1,18,800

Cochin

Societe-de-Prayon

FEDO

Belgium through FEDO

7

Hindustan Zinc

26,800

Not Available

Not Available

8

Hindustan Lever

41,850

Mac-him/Simchem

FEDO

2,25,000

Jacobs International Inc.

Hindustan

with Indian associate

Dorr oliver Ltd

Ltd. 9

Paradeep Phosphate Ltd.

Hindustan Dorr oliver Ltd.



B. Plants Based on Hemihydrate-Dihydrate (Nissan Process) Sr

Manufacturer

No

Installed

Process Licensor

Capacity

Engineering Conductor

1

RCF Ltd.

30,000

Nissan, Japan

PDIL

2

SPIC Ltd.

52,800

Nissan, Japan

Hitachi zosen Japan

3

HFC Ltd.

27,600

Nissan, Japan through,

PDIL

PDIL C. Plants Based on Dihydrate-hemihydrate (Central Glass Prayon Process) Sr No 1 2

Manufacturer Hindustan Copper Ltd FCI Ltd

Installed Capacity 68,000 1,19,000

Process Licensor Societe-de-Prayon Belgium Societe-de-Prayon Belgium

Engineering Conductor FEDO FEDO

D. Plants based on Hydrochloric Acid Process Sr

Manufacturer

No 1

Installed

Process Licensor

Capacity Ballarpur Industries

24,000


AEA (IMI

Krebs & Cie

Process)

Pvt. Ltd.

E. Plants Based on Thermal processes Sr.

Manufacturer

No 1

Installed

Process Licensor

Capacity Star Chemicals (Bombay)


6000

TVA, U.S.A.

-

Pvt. Ltd 2

Excel Industries

NA

TVA, U.S.A.

-

3

Transport Industry Ltd

NA

TVA, U.S.A.

-



Table (3) Technology Status of World in Manufacturing Phosphoric Acid (Capacity in tones of P2O5/year) A. Plants Based on Conventional Dihydrate process Sr.

Manufacturer

No 1

Installed

Process Licensor

Capacity Dorr Oliver, Ltd,


70,000

Self

-

34,000

Societe-de-Prayon

NA

U.S.A. 2

Simon Carves Ltd. , U.K.

3

Hitachi Zosen,

Belgium 65,000

Japan 4 5

Chemico, U.S.A. Jacobs

NA 14,000

Dorr Oliver Ltd,

Dorr Oliver Ltd,

U.S.A.

U.S.A.

Dorr Oliver Ltd,

Dorr Oliver Ltd,

U.S.A.

U.S.A.

Michim/Simchem

NA

International Inc. B. Plants Based on hemihydrates-Dihydrate Process Sr.

Manufacturer

No

Installed

Process Licensor

Capacity


1

Nissan, Japan

1,10,000

Self

-

2

PDIL, for two Plants

54,000

Nissan, Japan

Nissan, Japan

3

Hitachi zosen for one

32,000

Nissan, Japan

Nissan, Japan

Plants C. Plants Based on Hydrochloric Acid (IMI) process Sr.

Manufacturer

No 1

Installed

Process Licensor

Capacity AEA,

56,000


Self

Krebs & Cie Pvt. Ltd.

D. Plants Based on Thermal Process Sr.

Manufacturer

No 1

Installed

Process Licensor

Capacity TVA’s, U.S.A.

NA


Self

[Exectutive summary, 2009]



7. Research & development has been made during the last 30 years and modification in the process has been achieved. Some have modified effluent disposal system and have also developed methods for better utilization of gypsum. 8. FEDO, PDIL, GSFC Ltd, RCF and Ballapur Industries Ltd. Are the companies who have reported doing some R & D work pertaining to phosphoric acid. Unfortunately, there has been very little commercial application of in-house R & D work done in the country. 9. There are many Phosphoric acid producing Industries across the world. Here are some of those Industries with their capacities mentioned below in Table (3) 10. These Industries across the world had made lots of advancement in the manufacturing process, purification process of phosphoric acid. They had discovered the technology for the extraction of fluorine and uranium from the phosphor-gypsum. Some of these technologies are also being imported to India.



E] Brief Details about Raw Materials Used A. Rock Phosphate: 1. Introduction Finely ground rock phosphate is used as a source of P2O5 for Phosphoric acid production. The major part of world rock phosphate about 30 million tonnes per year is converted to phosphoric acid. The majority of the product phosphoric acid is further converted to fertilizer. A small proportion of rock phosphate is sold as cheap fertilizer. Rock phosphate is generally found in two forms: 1] Igneous Phosphate2] Sedimentary Phosphate. Rock phosphates in the apatite group are preferred such as fluorapatite (CaF), chlorapatite (CaCl), and Hydroxypatite (CaOH). Fluorapatite variants are mostly used i.e. Ca10(PO4)6F2. The phosphate rock is always complex. It contains several impurities. Total world reserves rock phosphate estimated to be over 65 billion tones, are sufficient to supply the world for almost 375 years at the current rate of consumption. In India Rajasthan is principle producing state, contributing 90% of total production followed by Madhya Pradesh with 10%. About 52% of the total production of rock phosphate is of grade 30-35% P2O5, 6% of 25-30% P2O5 grade, 1% of 20-25% P2O5 grade and 40% of 15-20% P2O5 grade. Only 25-30% requirement of rock phosphate is met through indigenous sources. The remaining requirement is met through import of rock phosphate. 2. Physical Properties: a. Formula

:Ca10(PO4)6F2

b. Physical state & appearance :Powder c. Color

:Grey & Yellow

d. Odor

:Odorless

e. Melting Point

:71400⁰C

f. Solubility

:Insoluble

g. Relative Density

:3.2 Kg/lit

h. Bulk Density(1% moisture) :1.75Kg/lit



Following are the general specification of rock phosphate recommended for use in manufacturing of phosphoric acid: Component

% in Rock phosphate

P2O5

32-34%

CaO

1.5% (max)

Fe2O3 + Al2O3

2.0% (max)

CO2

3.0% (max)

SO3

Should not be in sulfide form

SiO2

2.5-5% (max)

F

4% (max)

MgO

0.5% (max)

Cl

0.015% (max)

Organic Matter

1.5% (max)

3. Chemical Properties: i. Reaction of rock phosphate with sulfuric acid: Ca10(PO4)6F2 + 10H2SO4 + 20H2O → 6H3PO4 + 10(CaSO4.2H2O) ii. Reaction of rock phosphate with hydrochloric acid: Ca10(PO4)6F2 + 6HCL + 6H2O → 2H3PO4 + 3CaCl2 iii. Reaction of rock phosphate with phosphoric acid 2Ca10(PO4)6F2 + 14H3PO4 → 10Ca(H2PO4)2 + 2HF iv. Reaction of rock phosphate with silica gives elemental phosphorous 2Ca3(PO4)2 + 10C + 6SiO2 → P4 (Yellow Grade) + 6CaSiO3 + 10CO 4. Components of Rock phosphate: For production of phosphoric acid main criteria for rock phosphate are: a. P2O5 content: 33-38% P2O5 rock phosphate is considered as high grade. For production of phosphoric acid high grade rock phosphate is required. b. CaO content: It affects the sulfuric acid consumption. Each process of CaO needs an equivalent of 17.5 Kg sulfuric acid per ton of rock



phosphate. Relatively pure acid can be produced from rock phosphate containing very large amount of calcite. c. Fluorine: Usually occurs sedimentary rock as 10% of the P2O5 weight. Fluorine can be corrosive component if not enough silica is also not present. With high sodium content most of it will precipitate during phosphoric acid reaction. d. Sulfates: Existing sulfates in rock phosphate will save the corresponding amount of sulfuric acid during acidulation. e. SiO2: Reactive silica is needed to combine with fluorides to prevent corrosion. However highly reactive silica will depress the filtration rate. f. Al2O3, Fe2O3: Not a problem during manufacturing of phosphoric acid but afterward when using the phosphoric acid. Sludge formation with concentrated acid, builds water insoluble components in the phosphate fertilizer. g. MgO: Stays with acid phase. It increases viscosity strongly. h. Na2O: Precipitates as Na2SiF6 from acid. i. Organics: It forms during reaction. Dark cloudy solids suspended in product acid. Rock phosphate vary in composition around the world & even within a local mine,hence the need to analyze the rock on a regular basis as a part of phosphoric acid production plant control stratergy.

5. Manufacturing Processes: The phosphate rock having maximum P2O5 content & a minimum of impurities is suitable for the production of phosphoric acid. However rock phosphate contains different types of impurities & non-phosphate materials. It is important to remove or minimize all the contents of these impurities in rock phosphate. The separation of phosphate rock from impurities & non-phosphate materials for use in manufacturing of phosphoric acid consist of beneficiation, Drying and calcining at some operation and grinding. Rock phosphate from the mine is first sent to separate sand & clay and to remove impurities. The wet beneficiated rock phosphate may be dried or calcined depending on its organic content. Dried or calcined rock is ground in roll or ball mills to a fine powder. Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 22


In most of the cases to reach commercial grade of some 30% of P2O5, only screening or drying is required. For sedimentary rock suitable, suitable techniques for economic ore concentration re-crushing & screening or grinding followed by pneumatic particle size selection and washing and disliming by hydrocyclones or classifiers. 6. Uses and applications: a. It is used in production of phosphoric acid as a raw material. b. It is used in production of fertilizer such as single superphosphate, triple superphosphates and ammonium phosphate as a raw material. c. It is used in production of elemental phosphorous as raw material. d. In some cases it is directly used as a fertilizer. e. Rock phosphate is used in animal feed supplements, food preservatives, anticorrosion agent, cosmetics, fungicides, ceramics, water treatment and metallurgy.

B. Sulfuric Acid: 1. Introduction Beside rock phosphate sulfuric acid is second raw material needed for phosphoric acid production. It is preferred to utilize strong phosphoric acid in order to obtain high P2O5 content or washing efficiency. Sulfuric acid is the first choice for commercial processes, because it produces insoluble calcium sulphate (Gypsum) which can be easily filtered. Wet process phosphoric acid is the major world sulfuric acid consumer, with nearly 50% of the total production. Sulfuric acid is the highly corrosive strong mineral acid. For wet process phosphoric acid production generally 72-75% concentrated sulfuric acid is used. This concentration of sulfuric acid is obtained by mixing strong sulfuric acid(97% minimum conc.) and weak sulfuric acid(67% min conc.) in the mixing vessel.



2. Physical Properties: a. Molecular Formula

: H2SO4

b. Molecular Weight

: 98

c. Appearance

: Colourless viscous liquid

d. Odour

: Odourless

e. pH

: 0.3 (1 N solution)

f. Vapour Pressure

: <0.0012mmHg

g. Vapour Density

: 1.2 Kg/m3

h. Boiling Point

: 290⁰C

i. Melting Point

: 10.33⁰C

j. Decomposition Temperature : 340⁰C k. Solubility

: Soluble in water

l. Specific Gravity

:1.841

m. It dissolves most of the metals. n. It is conductor of heat & electricity Following general specification of sulfuric acid is recommended for use in manufacturing of phosphoric acid. Concentration

97% (min.)

Residue on ignition

0.054%

Iron (as Fe)

0.01%

Chloride (as Cl2)

0.0003%

Arsenic (as As2O3)

<1%

3. Chemical Properties: a. Reaction of sulfuric acid with water: It is highly exothermic reaction as of formation of hydronium ions. H2SO4 + H2O → H3O+ + HSO-4 b. Acid-Base reaction: Sulfuric acid reacts with most bases to give corresponding sulphate. Consider a reaction of sulfuric acid with copper oxide. CuO + H2SO4 → CuSO4 + H2O Sulfuric acid can also be used to dispose weaker acid from their salts. Consider reaction of sulfuric acid with sodium acetate. H2SO4 + CH3COONa → NaHSO4 + CH3COOH Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 24


c. Reaction with metals: Sulfuric acid reacts with metals producing hydrogen gas and salts. It attracts reactive metals such as iron, aluminum, zinc, manganese and nickel. Fe + H2SO4 → H2 + FeSO4 d. Reaction with sodium chloride: Sulfuric acid reacts with sodium chloride and gives hydrogen chloride gas and sodium bisulfate NaCl + H2SO4 → NaHSO4 + HCl e. Reaction with non-metals: Sulfuric acid oxidizes non-metals such as carbon & sulfur. C + 2H2SO4 → CO2 + 2SO2 + 2H2O S + 2H2SO4 → 3SO2 + 2H2O f. Electrophilic aromatic substitution: Benzene undergoes electrophilic aromatic substitution with sulfuric acid to give the corresponding sulfonic acid. O H2SO4, SO3

S

OH

‐H2O O

4. Manufacturing process a. process or DCDA process: Sulfur is burned to produce sulfur dioxides S + O2 → SO2 This is then oxidized to sulfur trioxides in the presence of vanadium oxide catalyst. 2SO2 + O2 ↔ 2SO3 The sulfur trioxide is absorbed into 97-98% H2SO4 to form oleum (H2S2O7) also known as fuming sulfuric acid. H2SO4 + SO3 → H2S2O7 H2S2O7 + H2O → 2H2SO4 b. Wet sulfuric acid process: Sulfuric acid is burned to produce sulfur dioxides S + O2 → SO2 This is oxidized to sulfur trioxides using oxygen and vanadium oxide as catalyst. 2SO2 + O2 ↔ 2SO3



Sulfur trioxide is hydrated into sulfuric acid SO3 + H2O → H2SO4 (g) Then it is condensed to get liquid 97-98% H2SO4 c. Sulfuric acid can be produced by burning sulfur in air and dissolving the gas produced in the hydrogen peroxide solution. SO2 + H2O2 → H2SO4

4. Uses & application a. It is sued as the main raw material in the phosphoric acid production. b. It is used in production of fertilizers such as ammonium sulfate, superphosphate and ammonium phosphate. c. It is used to produce various Chemicals such as zinc sulfate, alum etc. d. It is used in paper pulp & detergent Industries. e. It is used in the textile Industries to produce rayon and the artificial fiber. f. It is used in textile finishing. g. It is used in petroleum industries for petroleum refining. h. It is used in paint & pigment industries.



F] Justification 1. Syllabus requirement: The manufacturing of phosphoric acid by wet process using sulfuric acid and rock phosphate as raw material has been previously studied as part of syllabus in subject “Chemical Process-I” (6th Semester, Mumbai University). This study provided information about raw materials, process detail, flow sheet and engineering problems of manufacturing process. 2. Significance of phosphoric acid: Phosphoric acid serves as an intermediate product for most of the fertilizers used in India. India is agro based country. It requires large production of phosphoric acid based fertilizers which are ammonium phosphate, triple phosphate, and liquid mixed fertilizers. Phosphoric acid also used in dental science. 3. Uses and application: Phosphoric acid serves large application in fertilizers, pharmaceutical beverages, textile, oil refinery, sugar, soap and detergents and glass industry. Phosphoric acid used as catalyst to alter the composition of hydrocarbons in dehydration, polymerization and alkylation process. It also used in photography, rust removal, water treatment and fire retardant as main component. 4. Import and export requirement: Demand of phosphate fertilizers has grown rapidly in India. India has limited resource of rock phosphate therefore relied heavily on import of both phosphate rock and phosphoric acid for production of fertilizers. India is a great importer of phosphoric acid. Out of total trade of approximately 5 million tons. India imports more than 2.5 million tons per year because of great demand of fertilizers. India exports phosphoric acid to a very small extent 5. Storage, Handling, Transportation, Loading and unloading: Phosphoric acid is non-flammable, non-toxic and less corrosive in nature. Therefore it is easy to storage, handling and transportation. Storage facilities required for phosphoric acid are relatively more expensive than that of the solid fertilizer. Phosphoric acid is transported in rubber lined steel tankers or stainless steel tankers. The clarified acid is transported over long distance in special ocean going ships. In India phosphoric acid is imported in such ships.



6. Special significance in fertilizer industry: Phosphoric acid has find its ample use in fertilizer industry. It has a special significance in manufacturing of complex

fertilizers

like

nitrophosphate,

ammonium

nitrophosphate,

superphosphoric acid etc. also its serves a important raw material in the production of some industrial phosphate like sodium phosphate and calcium phosphate. 7. Importance of byproduct produced along with phosphoric acid: In manufacturing of phosphoric acid, important byproduct i.e gypsum and fluosilicic acid is obtained. This byproduct also has many industrial importances. Gypsum is soft sulfate mineral, it can be used as main constituents in many forms plasters, fluosilicic acid is used as fluoridation agents for drinking water. By considering all above points it is very much important to study the manufacturing of phosphoric acid using rock phosphate and sulfuric acid as a raw material in detailed.



Chapter No 2: Literature Survey

In this literature survey the review of various research articles published on international level and the information collected from patent papers are presented. Paul and Colin presented the method of production of phosphoric acid using two stage crystallization and filtration process employing a feed acid tank assembly and recovery solution tank assembly for production of high strength phosphoric acid with high recovery of P2O5. In this process Phosphoric acid and phosphate rock is dissolved in a reaction vessel to form the slurry. The slurry is then reacted in a first stage crystallization with sulfuric acid to produce calcium sulfate hemihydrates. The product acid is separated from the hemihydrates via filtration and the filter cake is then reacted with additional sulfuric acid to produce dihydrate calcium sulfate (Gypsum) and recovery solution. The gypsum is separated from the recovery solution via filtration and removed as a byproduct. Thus this process is producing high strength acid having concentrations of 39% P2O5 or higher and high P2O5 yields from the rock phosphate of 99% or greater commercial grade phosphoric acid has a P2O5 concentration or purity of about 50-54% where as food grade P2O5 has a concentration or purity of about 54-62%. [Kucera P. , Weyrauch C. G. ,2014] Macharro, Olveza and Larios studied the purification of industrial grade phosphoric acid (P2O5) by conventional electrodialysis. High concentrated phosphoric acid solution containing sulphates and chlorides as impurities is produced at anode. All other impurities are removed at cathode. Experiment was conducted using three compartment cell with anion and cation exchange membranes and industrial acid solution was introduced into the central compartment. The elemental analysis of diluted solution shows that the composition of magnesium, phosphate and sodium reduced in the central compartment. The ratios of the concentration of ions and the phosphate essentially unchanged by the process, consequently electrodialysis could not purify the acid in the central compartment. Migration of phosphate ion to the anolyte produced highly concentrated phosphoric acid solution containing sulphates and chloride impurities. Migration of phosphate ions across the membranes consumed



[Kucera P. , Weyrauch C. G. ,2014] large amount of energy. The Three compartment cell produced a highly concentrated phosphoric acid solution with ionic impurities in the anolyte. Such migration process consumes a large amount of energy and would therefore be extremely costly. Hence electrodialysis process is not commercially viable for purifying phosphoric acid. [Macharro J.J. , et. al, 2013]



[Macharro J.J. , et. al, 2013] Dr. Megy J.A. gives the outlines of the economics, environmental and quality advantages of the improved hard process over the wet acid process for the manufacture of phosphoric acid. The improved hard process will permit the use of leaner rock phosphate because it has greater tolerance of some common and magnesium. it will also reduce the environmental action of gypsum stacks because inert solid aggregate is the by product of the process not gypsum. This process produces super phosphoric grade acid at a significantly reduced cost compared with the wet acid process. In this process treated rock phosphate, silica and green petroleum coke are mixed in required proportions, dried and ground. Then aqueous slurry of wash waste clay or bentonite is added to the mixture to supply the moisture and clay to needed strong agglomarates. Product scrubber is used to produce phosphoric acid from stream processed from the kiln. The remaining gases from the product scrubber is fed to flue gas scrubber, where limestone is used to remove impurities from the flue gases. As long as the amount of water vapour in kiln gases is sufficiently low, the acid strength of 76% P2O5 is readily obtained. The hot kiln solids



are cooled with air, which is then used to dry the materials prior to grinding and agglomerates prior to entering the kiln. It utilises resources that are unsuitable or difficult to process by wet acid methods. Improved hard process also has several environmental advantageous. Regardless of the ore use, process produces solid waste in the form of amorphous agglomerates suitable for aggregate uses rather than gypsum piles. The improved Hard process is a dry process so threats to ground or surface water reduced. It also makes use of green sulphur petroleum coke. Improved hard process phosphoric acid is produced relatively pure, high concentration acid suitable for use of fertilizers. [Dr. Megy J. A., 2002]

[Dr. Megy J. A., 2002] Hotta and Kubota provides purification method of phosphoric acid, which includes bringing phosphoric acid containing arsenic with hydrogen halide, which is expected to be applicable to a broader range of use in the fields of food, medical and electronic material. The method given in the paper includes simple treatment step and required no special apparatus. The arsenic content of phosphoric acid drastically falls during an organic synthetic reaction accompanying production of hydrogen chloride in high concentration phosphoric acid. By bringing phosphoric acid into with hydrogen halide, the arsenic content of phosphoric acid can be reduced to 1ppm. When the is performed in the presence of a compound capable of generating hydrogen halide under acidic conditions, the arsenic removal effect is enhanced. Hydrogen halide causes arsenic in the phosphoric acid to convert to an arsenic



compound having a high volatility, such as arsenic halide and arsenic hydride and the compound evaporates at treatment temperature. The compound capable of generating hydrogen halide under the acidic condition is a halide of iron(II), copper(I) or tin(II), which is used to remove arsenic from the phosphoric acid. The arsenic content of 10 to 100 ppm of general industrial phosphoric acid can be reduced to about 1 ppm by treating phosphoric acid with hydrogen halide. The total cost for treatment is low and since sodium compound is not used, unlike the conventional method sodium does not remain in large amount in product phosphoric acid. [Hotta K. & Kubota F., 2005]

[Hotta K. & Kubota F., 2005] Smith & Jackson explains filter having a surface with plurality of perforations. At least one spray bar may be substantially fixed adjacent to the surface of the filter . A spray bar may include a plurality of nozzles for directing a pressure fluid toward the filter to move in a cylindrical while the spray bar directs pressurized fluid towards the filter . This method include drawing a vacuum on one side of the movable filter , opposite the mixture draw the phosphoric acid through the filter. Drawing vacuum on one side of the movable filter



result in calcium sulphate accumulation in the opening of the movable filter . Which include removing filter from the filter and causing the filter to move in cyclical manner and the filter is removed from the filter . During cyclical motion, a plurality of nozzles in the spray bar, may spray pressurized fluid towards the surface of the filter . This occurs through multiple cycle movement such that the spraying results in a sweeping motion across the surface of the filter . After a majority of openings in the filter are substantially free of calcium sulphate blockage spraying may be terminated. Fluid is sprayed at a pressure greater than 5000 psi to clean the filter . Wherein the fluid is sprayed at a flow rate between 450-900 gallons per minute at an angle perpendicular to the surface of the filter , which substantially cleans all the openings of filter and makes filter free of calcium sulphate. [Smith G. L. & Jackson J. D., 2010] Other literature shows how continuous melt suspension crystallization process is used for the purification of phosphoric acid, which consist of a. cascade of mixed product removal (MSMPR) crystallizer and a gravity wash column for subsequent solid liquid separation. The crystallization equipment consist of MSMPR crystallizer and a wash column (length 1200mm). Seed crystals were prepared from a solution of reagent grade acid (85% H3PO4 & specific gravity of 1.7) & cooled to -5oC to -10oC & size is maintain to 0.1 mm. After 1 hour of crystallization, magma is entrained to the top of wash column by screw propeller stirrer. Because of density difference between crystal and liquid, crystals in the magma settled down to from a loosely packed bed, which was melted to the bottom of column having temperature equal to melt temperature of phosphoric acid hemihydrate. Part of the melt is withdrawn while part of it returned as wash liquid & crystals is enabled. The melt containing impurities was extracted as residual at the top of column. Thus, feeding in the crystallizer and withdrawing of the product and the residual at the bottom & top of the column respectively allows the continuous operations of the crystallization equipment. Purity of the crude crystals descending in the wash column increases due to:I. II.

displacement of the mother liquor washing of the liquid layer adhering to the crystals



III.

Crystallization of pure melt on cold crystals.

IV.

Sweating of the crystals in with the hot melt or adiabatic recrystallization of the crystals.

Continuous suspension crystallizer to purify phosphoric acid ensures full suspension of crystals and without secondary nucleation. Also impurity concentration of the product is lowered. [Chen A., 2012]

1- mixed suspension mixed product removal crystallizer 2- scraped surface crystallizer to produce the crystals in the mother liquor 3- optional addxitional mixing vessel 4- wash column for efficient removal of crystal Process flow diagram for melt crystallization process [Chen A., 2012] According to Mollere P. and other collegues crystallization starts by adding relatively large amount if fine, relatively pure seed crystals to acid which has been cooled to supersatured conditions. Thereafter crystallization proceeds under conditions which substantially favor growth on the seeds and disfavor secondary nucleation. The process consists of a helical cooling coil device, agitated crystallization vessel, alternate acid cooler. Fig shows schematic outline with present invention of crystallization process. First the acid at a concentration of ( 62 - 65% P2O5) and a temperature between 30 0C and 60 0C is supply over the helical cooling coil inside



which es the coolant having temperature ranges from -20 to -15 0C. Thus the temperature of the acid between -9 and -13 0C is achieved. The overflow is then ed to the agitated crystallizer comprising of agitator shaft carrying impeller blades. The residence time within crystallizer should be less than 1 minute. For commercially crystallizer, residence time as short as 10 seconds is prefered. The acid is again ed through the acid cooler which is operated with respect to first cooler. Because agitation provides higher heat transfer rates due to which process becomes thermodynamically unstable and there may be the risk of increase in unwanted crystallization occurring in the device. It provides a continuous process, since the seed crystals always present in the crystallization vessel to cause continued crystallization of seed. Once the process setup it is self- sustaining. Using this method of seed preparation the process becomes continuous by only 1st seed supply (extraneous), also the acid of concentration (45 - 50% P2O5) is obtainable from the process. [Mollera P., et. al, apr 14, 2001]

1- helical cooling coil device 2- agitated crystallizer

3- alternate cooling coil Seed Crystallization Process [Mollera P., et. al, apr 14, 2001]



Carr J. M. proposed that filtration of wet process superphosphoric acid by vacuum filtration is greatly enhanced by the use of a filter aid having a statistically selected distribution of particle sizes. Super phosphoric acid is applied to the precoat by rotating drum through an acid container, with an supplement application being made by means of a distribution box. The applied acid thereafter es through the particles of filter aid with the solid impurities being filtered out or entrapped at or very near the surface of the precoat. A doctor knife rests against the outer surface of the precoat and serves to continuously remove a thin outer layer of the precoat and entrapped solid impuries. Since drum rotates in a counterclockwise manner, the knife continuously removes the material just before the precoat is re- exposed to the superphosphoric acid in acid container. In this matter, a fresh surface of precoat is continually presented to the super phosphoric acid. The filtered acid, meanwhile is drawn through the vacuum lines, the filter product line, the receiver and collected in the filter product tank. Filtration using filter aid in vacuum ensures 90% acid recovery from the slurry which is economically profitable with less input. [Cary J. M., Jr. et. al., 1995]

filtration using rotary filter [Cary J. M., Jr. et. al., 1995]



Pillai J. K explains how a digested slurry from a wet process phosphoric acid production process is dewatered by gravity assisted vacuum filtration using as a filtration aid an aqueous solution of a polymeric filtration and that has been subjected to high shear mixing to enhance the performance of the filtration aid and enhance the filtration. The yield of WPA process is encountered both an extreme need for filtration efficiency and extreme filtration conditions. Filtration aids, particularly polymeric filtration are often used to enhance the filtration process. Enhancement generally sought from filtration aids is:i. Increase in filtration rate ii. Minimization or elimination of solids in the filtrate iii. Minimization of liquid in the filter cake In WPA process the product obtained is desired to be in concentrated form, enhancing the filtration process and reducing the amount of acid associated with the filter cake are very important. The present invention provides a process for the dewatering of digested slurry from a wet process phosphoric production digestion process, in which an aqueous solution of polymeric filtration acid is added to the digested slurry. The dewatering is accomplished with gravity assisted vacuum filtration while the temperature of slurry remains in the range from 63 0C – 135 0C the liquid phase contains about 28 to 42 wt % soluble P2O5. [Pillai K. J., et. al., 2003]

Purification using gravity assisted vacuum filter [Pillai K. J., et. al., 2003] Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 38


Amin M. I. along with other collegues proposed the solvent Extraction of iron from Egyptian Wet process phosphoric acid is investigated. This process is important because of the presence of iron in phosphoric acid results in viscosity & consequently decreasing the filtration rate. Before iron extraction pretreatment is given to the phosphoric acid using diethyl-hexyl phosphoric acid. Crude acid is mixed with bentonite for approx 30mins & left for settle down. Flocculating agents such as Polycrylamide type is added to enhance the settlement of suspends. Extraction experiment is carried out in mechanically agitated thermostatic beaker containing iron in 50 ml phosphoric acid & 100 ml of D2EHPA. This mixture is agitated at 400 rpm speed. After 5 mins Organic & aqueous phase are separated. Iron conc. In aqueous phase is determined by adsorption spectrometer. Conc. of iron in organic phase is calculated by difference of iron in aqueous phase before & after extraction. Concentrated phosphoric acid (44% P2O5) was purified by solvent extraction using mixture of butanol & octanol in 3 steps. These trials are expensive because of applying scrubbing process. Removal of iron from wet process phosphoric acid is investigated by solvent extraction using solvent diethyl hexyl phosphoric acid. The % removal of Fe was 69.3- 99.9 % for Egyptian phosphoric acid & diluted phosphoric acid is a obtained without scrubbing process. These trails are expensive because of applying scrubbing process so the main goal of this work is extraction of iron from Egyptian wet process phosphoric acid without applying scrubbing during purification process.[Amin M. I., 2014] One of the journal gives us the basic overview of central prayon process for manufacturing/ production of phosphoric acid by Dihydrate-hemihydrate process. Overviews of major equipments are given along with process flow diagram. In his IFA symposium presentation, paul smith reviewed the processes presently available & examined the features of each process. He observed prayon enjoys considerable expertise in licensing Dihydrate plants, enjoying market leadership. Prayon pioneered the use of flash cooler to remove the heat of reaction. The flash cooler was originally designed in 1950’s & has progressively upgraded. Prayon currently offers the 4



Egyptian wet process phosphoric acid using organic solvent extraction [Amin M. I., 2014] Dihydrate process & 20 units have been installed capable of producing over 1000 Tonn/Day P2O5 acid. The traditional prayon reactor consist of a multicompartmented monolithic concrete reactor & a separate de-super saturation (Digestion section) which can be concrete or rubber & carbon brick lined, Carbon steel tanks. This process produces merchant grade self drying gypsum & a slightly stronger phosphoric



acid. Phosphoric acid production by dihydrate-hemihydrate 32-35% P2O5 process (Central prayon process) produces merchant grade gypsum & slightly stronger phosphoric acid. [Fertilizer International, 2011]

Central prayon process [Fertilizer International, 2011] Awwad N. S. proposed an article which gives successive process for purification of phosphoric acid produced by wet process are given & the extraction of organic such as iron, silica, fluorine, Uranium are also included. Materials & measurements: Commercial wet process phosphoric acid containing P2O5 57%, Fe 2.6%, F 0.7% & Uranium 50ppm. Determination of P2O5 & Fluorine: P2O5 was determined spectrochemically by a solution containing yellow molybdovanadate phosphoric acid complex. Extraction of silica from rice husk: Raw rice husk (RH) was stirred with the solution of 5% KOH at a weight ratio 1:12 (g/ml) of rice husk to solution respectively & heated to boiling for 30minutes mixture is kept overnight. Filtered & washed twice with distilled water until filtrate becomes neutral. To maintain the pH in range 5-7 10%HCl is added in filtrate. The precipitate formed was filtered dried & weighed.



Hydrogen peroxide was used as oxidizing agent to ensure that all Fe(2) & U(4) ions are oxidized to Fe(3) & U(6). The advantages of hydrogen peroxides are that it forms no precipitates with other reactants. The clays of iliminite were used to remove humic acid & suspended materials from crude phosphoric acid. Flocculating agents was added & the mixture was left for 10 minutes for settling. Extraction process was carried out by shaking 5ml of organic solvent with 5ml of crude acid in separating funnel for 15 min at 25⁰C. Maximum removal of organic matter such as Fe, Fluorine and Uranium is achieved by this process. Extraction of P2O5 can also be done by using various alcohols. [Awwad N. S., et. al., 2013] One of the arab journal gives short overview of the phosphoric acid production process (wet process) and the causes of corrosion in the various equipment of plant. The corrosion resistant materials to be used for all major equipments of plant are explained briefly. Corrosion prevention or resistance for a. Reactor: The first stage for which special material are required is the reactor for digesting the ore. The reactor vessel is made of cement or carbon steel with nonmetallic linings and sometimes augmented with acid resisting bricks. b. Mixers: The mixers are made up of stainless steels such as VDMR alloy 904 L(1.4539) or VDMR alloy 926 (1.4529). The application of this alloys are however limited by the increasing impurity in phosphate ores and higher temperature for digesting the ore. Material VDMR alloy 31 (1.4962) is particularly resistant to corrosion in phosphoric acid slurring. c. Filter: The filtration equipment also requires special materials. In particular good resistance to pitting and crevice corrosion is a criterion here. The material used for this plant section are for example 300 series stainless steels VDMR alloy 904 L(1.4539) or VDMR alloy 926 (1.4529). Even the higher resistance for pitting and crevice corrosion can be achieved with VDMR alloy 31(1.4560). d. Evaporator and heat exchanger: It is either equipped with metal tubes or is mode of graphite. It has been shown that metallic heat exchanger operates satisfactorily when hot water is used as heating medium. The solids settling on the tube walls, mainly gypsum are easily removed by routine cleaning, which prevents corrosion after



deposits. VDMR alloy 28 (1.4563) and VDMR alloy 31 (1.4562) are used for the heat exchangers.

Purification and extraction of phosphoric acid [Awwad N. S., et. al., 2013] e. Other equipments (plate HE , pumps and fans, equipments for the removal of gaseous fluorine compounds): Special stainless steel and special nickel alloys are used. VDMR

alloy 31(1.4562), which combines a high chromium content with



adequately high molybdenum content, demonstrates high resistance to uniform and localized corrosion in phosphoric acid under diverse conditions and is in fact superior to higher alloy materials in this respect. According to the causes of corrosion in the equipments of phosphoric acid production plant various corrosion preventable or resistance materials are suggested for all the major equipments. [Fertilizer Focus, 2014]

Wet phosphoric acid production process flow diagram [Fertilizer Focus, 2014]



Chapter no 3: Various Process Descriptions We have the following processes for the manufacturing of phosphoric acid along with their detailed description. 1] Acidulation of rock phosphate using H2SO4: There are five process for producing phosphoric acid using rock phosphate and H2SO4. They are: a. Di-hydrate process [26%-32% P2O5] b. Hemihydrate process [40%-52% P2O5] c. Di-hydrate – Hemihydrate process [32%-36% P2O5] d. Hemihydrate – Di-hydrate process [40%-52% P2O5] e. Hemihydrates – Recrystallization process [30%-32% P2O5] Reaction : 3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O 2CaSO4.1/2H2O + 3H2O

6H3PO4 +10CaSO4.1/2H2O + 2HF 2CaSO4.2H2O

A] Di-hydrate Process Process Description: The description process comprises four stages: grinding, reaction, filtration and concentration. For di-hydrate process particle size acceptable is (60%-70% less than 150 m), which are attained either by ball mill or rod mill. The tricalcium phosphate by reaction with conc. Sulphuric acid is converted into phosphoric acid and insoluble calcium sulphate. The operating conditions for di-hydrate precipitation are 26%-32% P2O5 and 70 0C- 80 0C. This temperature is controlled by ing the slurry through, a flash cooler or by air circulating cooler. Filtration separates the phosphoric acid from the calcium sulphate di-hydrate. On filtration, the gypsum is given at least two stages of washing to ensure a satisfactory recovery of soluble P2O5.



Process Block Diagram:

Di-hydrate Process Advantages: 1. Single stage filtration, simple in design. 2. Flexibility of rock phosphate. 3. Ease of operation. 4. Required lower grades of material of construction. 5. Low maintenance cost and high operating factors. 6. Easy transport of gypsum slurry. Disadvantages: 1. Produces acid at 26%-32% P2O5. 2. Normally requires steam for evaporation. 3. Acid has high level of Al and F. 4. P2O5 efficiency 94%-96%. 5. Requires weak acid storage and evaporation.



B] Hemihydrate Process Process description: The storage is similar to those of the di-hydrate process but grinding may be unnecessary. Reaction occurs and operating conditions are selected in this process so that the calcium sulphate is precipitated in the hemihydrates form. It is possible to produce 40%-52% P2O5 acid directly, with consequent valuable savings in energy requirements. Hemihydrate crystals tend to small and less well formed thus hemihydrates slurries tend to be more difficult to filter. Process block diagram:

Hemihydrate Process Advantages: 1. Single stage filtration. 2. Produces strong acid directly 40%-48% P2O5. 3. No intermediate storage if acid is produced at strength. 4. Uses coarse rock. 5. Ease of operation.



Disadvantages: 1. Limited number of rocks process industrially. 2. Large filter area required for 48% P2O5 acid. 3. High lattice loss, low P2O5 efficiency [90%-94%] 4. Produces impure hemihydrates. 5. Tight water balance. 6. Requires higher grade alloys. 7. Care required in deg and shut down. C] Hemihydrate – Di-hydrate Process Process description: It is possible to obtain 40%-52% P2O5 acid directly, by acidulating under hemihydrate conditions and separating hemihydrate before recrystallization, in this process. Thus the sequence is as follows: i.

Acidulate under hemihydrate conditions.

ii.

Separate product.

iii.

Recrystallize hemihydrates to di-hydrate.

iv.

Filter and return liquors to process.

The additional filter and the other equipment required, add to the typical cost of the plant enable savings to be made on evaporation equipment. Advantages: 1. It produces strong acid directly [40%-52% P2O5] 2. It produces pure acid (low SO4, Al, F) 3. Limited post precipitation. 4. Uses coarse rock. 5. Low sulphuric acid consumption. 6. High P2O5 efficiency [98.5%] 7. Produces a pure gypsum.



Disadvantages: 1. Two stage filtration, lower utilization. 2. Limited number of rock processed industrially. 3. Care required in design and shut down. 4. High recrystallization volume. 5. High capital cost investment. D] Di-hydrate – Hemihydrate Process Process description: In this process, although the reaction runs under di-hydrate conditions, it is not desirable to affect a very high degree of P2O5 recovery, during the separation of the acid from di-hydrate. The succeeding dehydration stage requires around 20%-30% P2O5 and 10%-20% sulphuric acid. The strength of product acid is 32%-35% P2O5. The sequence is as follows: i.

Acidulate under hemihydrate conditions.

ii.

Separate product.

iii.

Recrystallize hemihydrates to di-hydrate.

iv.

Filter and return liquors to process.

Advantages: 1. Flexible as to rock source. 2. Proven process. 3. Produces a pure hemihydrates. 4. High P2O5 efficiency (98%) 5. Higher acid strength [32%-36% P2O5] 6. Lower sulfuric acid consumption. 7. Gypsum may be used directly for plaster board or as a cement retarder, after

the additional of lime and rehydration in a storage pile. Disadvantages: 1. Two stage filtration, lower utilization. 2. High capital cost investment.



3. Requires steam for conversion. 4. Requires 35% acid storage and evaporator. 5. Rock slurry feed is unacceptable. 6. Requires final rehydration of hemihydrate to di-hydrate gypsum. 7. Normally, requires rock grinding. 8. Care required in design and shut down. 9. Requires sophisticated material of construction. F] Hemihydrate Recrystallization Process Process description: The acidulation occurs at hemihydrate conditions, the hemihydrates are then directly recrystallize to di-hydrate without intermediate hemihydrate separation as separate product. The flow diagram of this process resembles that of the multiple reactor dihydrate process with the exception that the attack reactor operates under hemihydrate conditions, while succeeding reactors operate under conditions favouring the rehydration of hemihydrate to di-hydrate gypsum. This is encouraged by seed dihydrate crystals recycled in the slurry form the filter feed. The product is no more concentrated than that obtained from di-hydrate gypsum but the gypsum is much purer. Advantages: 1. Single stage filtration. 2. Proven process with sedimentary rock. 3. Produces a pure gypsum. 4. Higher P2O5 efficiency (97%). 5. Slightly higher acid strength [30%-32% P2O5] 6. Lower sulfuric acid consumption. 7. Lower filter area. Disadvantages: 1. Requires a fine rock grind. 2. Requires sulfuric acid dilution. 3. Post precipitation before and after evaporation.



2] Acidulation of rock phosphate using HCl (IMI process): Reaction: Ca3(PO4)2CaF2 + 20HCl



6H3PO4 +10CaCl2 + 2HF

Process description: Phosphate rock is ground to 20 mesh screen and fed into a dissolver where an acid stream of conc. HCl plus make up wash water from the counter current decantation system is added. Fumes of CO2, HF, HCl are scrubbed for acid recovery. The mixture is fed to series of decantation units with overflow from the first settler moving to the counter current solvent extraction operations. The solid underflow goes to 2-3 washing thickners. Extraction of H3PO4 plus some free HCl is done in a battery of mixer settlers (a) with CaCl2 + CaF2 retained in aqueous phase. The extract is again ed through several more mixer settlers (b) for removal of trace impurities of Ca++ which are co-extracted. The extractant is aqueous reflux from the next unit transfer extractors (c) where water extracts H3PO4 and HCl from solvent phase. The washed solvent is recycled to a final series of mixer settlers (d) where trhe balance of HCl is extracted from raffinate phase of extractors (a). the acid free brine is send to steam stripping for solvent recovery. The aqueous acid raffinate from (c) is separated and concentrated in a triple effect evaporator to give three different overhead streams: i.

Alcohol-water overhead flash from the first effect which is condensed and returned to evaporators (a).

ii.

Dilute HCl from the second effect.

iii.

Conc. HCl from the third effect.

Advantages: 1. Advantage of this method is it produces less impurity level in phosphoric acid. 2. Phosphoric acid obtain from this process is >50 wt% P2O5. Disadvantages: 1. Due to the use fo HCl corrosion problem occurs.



3] Thermal Process: A] Using phosphate rock and blast furnace Reaction: Ca3(PO4)2 + 3SiO2 + 5C



2P + 5CO + 3CaSiO3

2P + 5CO + 5O2



P2O5 + 5CO2

P2O5 + 3H2O



2H3PO4

85-90% yield

Process description: Phosphate rock is pulverized and mixed with coke powder and binder is compressed to 5000 psi resulting into the briquettes. Briquettes are dried and charged along with sand and additional coke powder from top of the blast furnace. The preheated air (1000 – 11000C) is charged from bottom of the blast furnace via tuyere. A tuyere is cooled copper conical pipe numbering 12 in small furnace and up to 42 in large furnace through which hot air is blown in to the furnace. Preheated air leads to burning of briquettes giving temperature rise up to 13700C. The coke acts as reducing agent as well as fuels. About 760kg of coke is consumed in reduction of phosphate rock to phosphorous and remaining generates heat by combustion with air. Reaction is completed in the furnace itself producing P2O5 and calcium silicates as slag. The product gases also contain carbon monoxide and nitrogen along with dust particles. For purification, it is ed through cyclone separator and phosphorous condenser. Thus, P2O5 and elemental phosphorous are separated out. Hot P2O5 gases are cooled in the heat exchanger. Therefore, superheated steam is produced and a part of gas is taken into regenerative blast furnace. As a result the entire phosphorous and phosphorous pentoxide is cooled and purified before taken into hydrating towers. Purification of phosphoric acid includes removal of arsenic by hydrogen sulfide treatment followed by filtration.



Process flow diagram:

Manufacturing of phosphoric acid using Blast furnace Advantages: 1. Rock phosphate varying in impurity like sand is acceptable 2. Purity of phosphoric acid obtained by this process is over 85% 3. The yield of phosphate is about 85%-90% 4. There is no formation of gypsum as in acidulation process 5. Resulting phosphoric acid can be used in manufacturing of insecticides, pesticides, detergents Disadvantages: 1. Maintenance cost of blast furnace is much more than installation cost 2. It is difficult to maintain temperature over 1000 0C in blast furnace using preheated air 3. About 750 kg of coke is consumed per 1000 kg of phosphoric acid produced wich is much more than electric furnace process. 4. Briquette binder can affect the product purity and its rate of formation 5. Product phosphoric acid can not be used for fertilizer production



B] Using phosphate rock and electric furnace Reaction: Ca3(PO4)2 + 3SiO2 + 5C



2P + 5CO + 3CaSiO3

2P + 5CO + 5O2



P2O5 + 5CO2

P2O5 + 3H2O



2H3PO4

85-90% yield

ΔH= -364.8 kcals

ΔH = - 44.9 kcals

Process description: The phosphate rock is reduced to elemental phosphorous by the action of coke and heat in the presence of sand in electric arc furnace subsequent oxidation of phosphorous gives phosphorous pentoxide which on hydration gives the product phosphoric acid. Phosphate rock after proper grinding and primary purification is taken into sintering oven where it is nodulized and granulized so that fast oxidation of the separated phosphorous takes place. Temperature of 1095 0C is maintained in electric furnace so that maximum amount of elemental phosphorous extracted out and oxidation takes place. Since fluoride of phosphorous and calcium are the common impurity which reacts with sand giving flourosilicates as the slag. The gases from the furnace, phosphorous and carbon monoxide are removed by the suction process and the oxidation product P2O5 is taken into hydration column which gives P2O5 to H3PO4 at about 85 0C. Purification of phosphoric acid is carried out by H2S to remove Arsenic, H2SO4 to remove calcium salts and Silica to remove fluorides. All the byproducts are removed before concentrating the acid and filtering it as final product.



Process flow diagram:

Manufacturing of phosphoric acid using electric furnace Advantages: 1. Low grade phosphate rock can be used for this process 2. Iron and aluminium oxides are not objectionable as in wet process 3. The by product carbon monoxide is used as a fuel for calcinations 4. Purity of phosphoric acid obtained is over 85% 5. Yield of phosphate is about 87%-92% Disadvantages: 1. It is difficult to maintain 2400 0F temperature in electric furnace 2. Process requires large electricity to maintain high temperature in electric furnace 3. Electrode consumption for this process is also too high 4. Water is kept over phosphorous to avoid direct with the air because phosphorous oxidizes vigorously C] Oxidation and Hydration of phosphorous Reaction: 2P + 2½O2



P2O5 + 3H2O 

P 2 O5 2H3PO4

(94 – 97% yield)



Process description At the locations away from phosphate rocks mines from purified elemental phosphorous is oxidized and hydrated to give phosphoric acid. In the manufacturing process molten phosphorous is sprayed into combustion chamber along with preheated air and superheated steam. Combustion of phosphorous increases the temperature up to 1980 0C. Furnace design depends on the requirement with respect to quantity and quality. They are made of acid proof structural bricks, graphite, carbon and stainless steel. Process flow diagram:

Manufacturing of phosphoric acid by oxidation and hydration of phosphorous Advantages: 1. Phosphoric acid concentration is over 85% 2. Yield of the phosphate is about 94%-97%



3. Resulting of phosphoric acid can be used for manufacturing of insecticides, pesticides & detergents Disadvantages: 1. Process utilizes large amount of steam, which causes to increase the capital cost investment for the process 2. Glass wool filter is so costly and delicate equipment 3. Product phosphoric acid can not be used for fertilizer production 4] Acidulation of rock phosphate using nitric acid Reaction: Ca10(PO4)6F2 + 20 HNO3



6 H3PO4 + 10 Ca(NO3)2 + 2HF

Process description: When phosphate rock is treated with nitric acid, it forms phosphoric acid and soluble calcium nitrate. Rock phosphate is crushed in ball mill. Acidulation of crushed rock phosphate is processed in reaction vessel by using 40%-70% concentrated nitric acid. The reaction temperature is 120 0C- 130 0C and the reaction time ranges from 1-2.5 hrs crystal of calcium nitrate are form along with phosphoric acid. These crystals are separated from phosphoric acid by using filtration. Phosphoric acid is concentrated in evaporator. The product phosphoric acid contains 55%-72% P2O5. Advantages: 1. For manufacturing of phosphoric acid wet process are more popular due to increase demand for higher grade fertilizer 2. It is energy saving process as comparison with thermal process 3. Less expensive process as comparison with thermal process Disadvantages: Though thermal process is expensive but it gives extremely pure phosphoric acid, but this process produces phosphoric acid of 55%-72% P2O5.



COMPARATIVE STUDY OF ALL THE PROCESSES Sr

Name of the Capital

No process

Phosphoric P2O5

investment acid conc.

Energy

By-product

efficiency requirement information (/ton P2O5)

1

di-hydrate

29 crore

26%-32%

94%

125 KWH

60% gypsum

2

Hemihydrate

31 crore

40%-52%

98.5%

110 KWH

95% gypsum

Di-hydrate - 31 crore

32%-36%

98%

110 KWH

95% gypsum

- Di-hydrate 3

Hemihydrate 4

Hemihydrate

25 crore

40%-52%

90%-94% 100 KWH

40% gypsum

5

Hemihydrate

30 crore

30%-32%

97%

115 KWH

80% gypsum

rock 35 crore

70%-72%

95%

250 KWH

Insoluble

recrystallization 6

Using

phosphate

CaCl2

and HCl 7

Using

rock 45 crore

85%

87%-97% 4070 KWH

NA

Over 85%

85%-90% 3000 KWH

NA

85%-87%

94%-97% 150 KWH

NA

65%-72%

92%-96% 270 KWH

Ca(NO3)2 as

phosphate and

electric

furnce 8

Using

rock 42 crore

phosphate and

blast

furnce 9

By oxidation 45 crore and hydration of phosphorous

10

Using

rock 35 crore

phosphate

waste

HNO3



Chapter no 4: Justification for Selection of Process The choice among the processes available can be made only after intensive study, but influence mainly by the inter-related factors: A] Process Reliability: This is the most important factor influencing the selection. The process reliability depends upon the efficiency of plant, ease of operation, severity of operating conditions and maintenance down time after the reliability of a process. The hemihydrate – di-hydrate process for production of phosphoric acid using rock phosphate and sulfuric acid operates at less severe condition and has ease of operation and lower cost of maintenance, thus increasing the reliability. B] Adaptability for Various Rock Phosphates: The hemihydrate – di-hydrate process has been shown flexibility to use different types of rock phosphates without affecting the plant efficiency and the plant capacity. With the increasing price and decreasing quality of rock phosphate, the hemihydrate – dihydrate process give an easily filterable calcium sulphate with different types of rock phosphates. C] By-product Quality and Disposal: The hemihydrate – di-hydrate process produces excellent quality of gypsum as a byproduct which can further used in cement production and other uses. Other acidulation processes using H2SO4 produces relatively poor quality of gypsum which creates problem for filtration. Acidulation process using HCl (IMI process) produces calcium chloride as by-product which creates disposal problem. A poor quality of gypsum means cost of effluent treatment is high. D] P2O5 Recovery: P2O5 recovery and gypsum quality are inter-related . A higher recovery results in better quality of gypsum. A higher P2O5 results in better quality of gypsum produced. Di-hydrate process yields a lower recovery as compared with hemihydrate – dihydrate process. The hemihydrate – di-hydrate process has high recovery of P2O5 like electric furnace process.



E] Capital Investment and Maintenance Cost: The hemihydrate – di-hydrate process poses near about same capital investment and maintenance cost as other acidulation process, but this process has very low capital investment and maintenance cost as compared to thermal acid process. This process has ease of control system hence the maintenance cost is also lower than electric furnace process. F] Energy Requirement: The hemihydrate – di-hydrate process has same energy requirement as other acidulation process using sulfuric acid, but it has shown very less energy requirement as compared to thermal acid process. The hemihydrate – di-hydrate process shown that 110 KWH per ton of P2O5 production, where thermal acid shown that 4070 KWH required per ton of P2O5 production. G] Raw Material Cost: Rock phosphate is used as main raw material in the hemihydrate – di-hydrate process is very much cheaper than the elemental phosphorous used in manufacturing of phosphoric acid using oxidation and hydration process. Sulfuric acid is also compared to hydrochloric acid and nitric acid. By considering all the above points along with advantages and disadvantages of each and every process we have selected the hemihydrate – di-hydrate process for our project work



Chapter no 5: Details of Selected Process A] Process Description I] Phosphate Rock Grinding System: Unground rock from rectangular unground concrete silo in the rock phosphate silo area is fed to vibratory feeder. From the vibratory feeder the belt conveyor feds to bucket elevator which discharges unground rock phosphate into the main service unground rock phosphate bunker in the grinding unit building. The unground rock phosphate from unground rock phosphate bunker is fed by disc feeder to the ball mill. The capacity of this ball mill is 40 MT/hr. the crushed rock phosphate from the ball mill enters the classifier, via classifier feeder and the riser pipe. The coarser particles are fed back to the ball mill from classifier, while dust– laden air enters the cyclone, for the separation of proper size of ground rock phosphate. The fine dust from the cyclone is recycled back to the suction of main blower to atmosphere. A constant purge is maintained from the discharge of main blower through dust collector. The fine dust collected from the dust collector is mixed with the main stream of ground rock by screw conveyor and rotary air lock. For the purpose of drying rock hot air is introduced into the ball mill by air heater. Nominal capacity of ground rock phosphate silo is 350 tones of ground rock. II] Ground Phosphate Rock Handling System: Ground rock is transferred from the silo to the ground rock service bunker by pneumatic pump. Ground rock phosphate service bunker is fitted with the dust collector at the top with the high and low level alarm. The bunker has hold up capacity of 90 MT of ground rock phosphate. Process rock from the service bunker is drawn at constant rate by a continuous weight feeder and is fed to premixer through screw conveyor. The smooth flow of ground rock phosphate is kept by the aeration unit in the ground rock phosphate service bunker. It is desirable to check the accuracy of integrating mechanism of the weight feeder periodically.



Operating condition for ground rock feeding: a) Particle size of ground rock: minimum 90% thru 100 tyler mesh minimum 70% thru 200 tyler mesh b) Rock Analysis: prior to the feeding of rock phosphate analysis of following component should be made: moisture

:

1.5% (max)

CaO

:

52% (max)

P2O5

:

33% (min)

SO3

:

1.4% (max)

c) Rock feeding method: Feeding of the rock should be continuous and at constant rate consistent with the feeding of the acid. This is necessary to prevent imbalance of rock/acid ratio taking place in the reactor III] Sulfuric Acid Dilution and Return Acid System: a) Sulfuric acid system: The sulfuric acid dilution system can be operated at 98% sulfuric acid and dilution water or mixture of 98% sulfuric acid and 64% sulfuric acid available from nitric acid concentration plant. Case 1: when 98% sulfuric acid exclusively used, it is pumped from storage to the head tank. 98% sulfuric acid from the head tank and deminerlized water and dilution water are fed at predetermined rates through flow controller to the graphite mixing block to ensure that diluted acid is of 75% strength. The temperature of diluted acid is controlled by recirculation of process water. The diluted acid is mostly ed through mixing vessel.



Case2: when 64% sulfuric acid is available from HNO3 concentration plant, it is fed to head tank. Acid would be drawn at constant rate by the flow controller and fed to sulfuric acid mixing vessel. Where in it get mixed with the 98% sulfuric acid. Amount of dilution water required to obtain 75% sulfuric acid are mixed and cooled in dilution cooler, similar to case 1. b) Return acid system: The temperature of return acid in the return acid calibration tank is regulated by the flow ratio of steam by premixer temperature recording controller. The return phosphoric acid is adjusted to a designated level of P2O5 concentration i.e. 18%-21% P2O5 and metered within a accuracy of ±0.5% by flow controller before feeding to the premixer through mixing vessel. The return acid contains dissolved or supersaturated SiO2, CaSO4.2H2O and the temperature of acids is about 450C – 500C under the normal conditions when this return acid is mixed with the hot sulfuric acid (80 0C). the temperature of resultant mixed acid is between 60 0C – 75 0C. The solubility of gypsum in the consequently solid gypsum is separated from the mixed acid. Operating conditions for sulfuric acid & return acid: i) Concentration of sulfuric acid fed: 75% sulfuric acid in case 1 72.5% sulfuric acid in case 2 ii) Concentration of return acid: 18% -21% with permissible deviation in the concentration from the actual operating design 0.5% range. iii) Temperature control: In case 1, the temperature of diluted sulfuric acid is controlled by adjusting flow rate of cooling water to the sulfuric acid dilution cooler automatically and set of temperature control adjusted so that to maintain the temperature of premixer at designated level.



In case 2, the temperature of premixer controlled by adjusting flow rate of steam into return acid calibration tank automatically and the index of temperature controller should be set at the designated temperature. IV] Premixer and Digester Section: Dilute sulfuric acid and the return acid enter the mixing vessel with agitator which ensures adequate mixing of acids. The mixed acids enter the premixer along with rock phosphate through screw conveyor and whole mass is agitated by the agitator. The phosphoric acid and the hemihydrates gypsum is formed in premixer and digestor under carefully controlled conditions of reaction. This condition associated with the following factors, individually and in combination: a) Reaction temperature b) Combination of sulfuric acid and P2O5 concentration of acid mixture c) % Decomposition of phosphate rock attained under these stage d) SO3 / CaO ratio of the slurry e) The total retention time in this stage i.e. the reaction time Factors a and b individually or in combination have significant effect on the proper formation of hemihydrates calcium sulfates. The temperature condition of 90 0C 920C (±2 0C) is the optimum temperature in the premixer and digestor. Factor c under the decomposition stage, a certain minimum % of P2O5 in the rock phosphate must become water soluble. This % defined by the formula, % decomposition = [ 1- { [(A-B)/C] / [X/Y] } ] * 100 Where A = total P2O5 in Gypsum cake B = water soluble P2O5 in gypsum cake C = total CaO in gypsum cake X = total P2O5 in phosphate rock Y = total CaO in Phosphate rock



The objective of the factor of the process is for high P2O5recoveryand the production of quality gypsum. Factor d, the SO3/ CaO ratio of the slurry also influences the formation of large, well shaped crystal of gypsum. Factor e, the total retention time in the decomposition stage is associated with the stability of hemihydrate gypsum. The Nissan process requires hemihydrates gypsum should be formed in this stage. The hemihydrate stage should be transient stage which is

followed

by

formation

of

dehydrate

gypsum

through

hydration

and

recrystallization. Operating conditions: a) Temperature of slurry in premixer: 90 0C (accuracy of temperature control ± 2 0C) b) Temperature of slurry in digestor: 90 0C (accuracy of temperature control ± 2 0C) c) Decomposition % of Rock phosphate: 80%-90% d) Hydration ratio of CaSO4 : 0.5 – 0.7 e) Temperature controller: The temperature of the premixer controlled by the aid of the temperature recording controller by varying the temperature of the feed sulfuric acid in case 1. In case 2, the temperature of premixer is controlled by the aid of temperature recording controller by varying the temperature of the feed return acid. The temperature of the digestor should be kept at 90 0C- 98 0C by regulating the volume of draught air effecting surface cooling of the slurry in these tanks. V] Crystallization Section: The slurry through the last digestor processed through the three crystallizers in series. These vessels are agitated and connected by launders and here the hydration and crystallization of the gypsum is complete. The slurry is air cooled by an air blower, duct work and dispatches into each crystallizer. The slurry from the third crystallizer is collected in slurry pumping tank by slurry transfer pump to the filter feed splitter box, back to number 1 crystalleizer where it helps cooling and crystallization.



Slurry is also metered to filter and the excess over flows back to number 3 crystallizer. The total volume and the distribution of air to each crystallizers are controlled by dampers on air lines. Fumes are removed from the crystallizers by two numbers of exhausts fans and ed to atmosphere through the exhausts stack. The pressure in each crystallizer is kept slightly negative. The temperature in the crystallizers is controlled to maintain optimum declining temperature gradient for recrystallization and hydration. It is removed by temperature recorder. The solubility of calcium sulfate in its hemihydrate state rises rapidly with the drop in the temperature. The solubility of the di-hydrate calcium sulfate with maximum at about 50 0C is for less than hemihydrate in the crystallization stage, the slurry of relative high temperature containing hemihydrate calcium sulfate is cooled to a temperature below the hemihydrate – di-hydrate transition point. Solubility of hemihydrate increases considerably. In the actual operation of Nissan process, however, the solubility difference does not act directly as a motive force. The recrystallization of the hemihydrate to di-hydrate is dependent not only on the temperature but also on the ratio of the recycled feed slurry and the ratio of the recycled feed slurry and the ratio of new slurry feed respectively to slurry retained on the crystallizer. The actual motive force promoting the recrystallization is subject to the degree of super saturation of di-hydrate gypsum. The hydration and crystallization are frequently influenced by minor constituents of the rock phosphate, such as metallic sesquioxides, SiO2, fluorine and organic matter. As it is apparent considerable hydration heat is generated in this stage, necessiting a fairly large quantity of heat to be removed. This is accomplished in practice by air cooling, in the crystallization stage, the following factors are to be considered: 1. Retention time 2. Hydration temperature 3. Recycle ratio of feed slurry 4. Free sulfuric acid content in filtrate acid Most important factor in the Nissan process can be said to be the retention time. The term retention time is defined as the average retained time of the slurry in series of reaction vessels ignoring the effects of such recycle. This is expressed as:



Q=V/F Where,

Q = Retention time V = Effective volume of crystallizer F = feed rate of slurry from the decomposition

The necessary retention time in the crystallization is determined based on the reactivity of rock to be used. As previously mentioned, at low temperature di-hydrate gypsum would be formed under the some concentration is held constant. The temperature required for the formation of di-hydrate gypsum from the hemihydrate calcium sulfate can be found on the transition temperature curve. This temperature taken from reffering calcium sulfate phase diagram is somewhat higher than actual temperature applied in Nissan process. This difference is due to the fact that the rate of hydration of hemihydrate calcium sulfate is accelerated at the lower temperature than the conversion point. As it is well known, when the degree of super saturation is too high numerous crystals of very fine size are formed, a tendency of undesirable for manufacturing of phosphoric acid. The actual process conditions applied to crystallization stage has been established on consideration of the above two factors. That is crystallizer number 1 in order to minimize the hemihydrate supersaturation and to do prevent the formation of fine crystal nuclei, a higher temperature than the optimum is applied. In crystallizer number 3 a temperature necessary for quik and complete hydration is selected. With respect to factor 3, the ratio of the recycle slurry is between 1:1 and 2:1. The slurry recycled from the crystallizer number 3 accelerates crystallization by supplying seed crystals. When a proper recycle ratio is selected hydration can be easily promoted two times or quicker as compared with the conditions of no feed recycle or improper recycle. Operating conditions: a) Temperature of Crystallizers: 65 0C – 70 0C



Typical temperatures of crystallizers: i.

Crystallizer no 1: 63 0C

ii.


iii.


iv.


b) Decomposition %: Minimum: 98% (for Algeria and Tunisian rock) 98.5% (for Morocco, Jordan, Florida rock) c) Hydration ratio: 1.90 d) Slurry recycle ratio: 1:1 to 2:1 VI] Filtration Section: The phosphoric acid and the gypsum are separated by filtration using horizontal vacuum belt filter. The slurry from the crystallizer containing phosphoric acid along with the di-hydrate gypsum is ed to slurry pumping tank. The tank is cylindrical with the agitation, completely lined vessel of the size 4500 mm *4000 mm height having hold up capacity 56 m3. Slurry pumping tank pumps the slurry to filter feed splitter box. The filter feed splitter box is a vertical rectangular type of size 1500mm * 1200 mm * 1100 mm (l*b*h) having hold up capacity of 0.4 m3. The purpose of the splitter box is to maintain uniform flow rate of hydrated slurry to the filtration unit and excess supply of the slurry is fed back to the crystallizer. The filter used is horizontal vacuum belt filter, having effective filtration area is 29 m2. The slope of filtration unit is 0.6%, so adjusted in such way that the discharge end of the filter is slightly higher than the feed end. The vacuum is created by barometric condenser in the downstream which results into suction of phosphoric acid from gypsum is collected into three receivers. The phosphoric acid from the first receiver having 30% P2O5 acid is pumped to the clarifier and the phosphoric acid from the second receiver having 18% - 20% P2O5 is sent to the return acid tank. Phosphoric acid from third receiver is having 5% P2O5 is sent to 1st acid wash of the gypsum. The liquor ed from all the sections of the



filter under vacuum i.e. slurry feed acid wash, water wash, cake drying, cloth drying section all have entrained in them. The air from the filtrate separator on the slurry feed acid wash and the water wash sections of the filter es to primary spray condenser which removes any acidic vapours that may be in the air by scrubbing with process water. The decontaminated air then enters the primary vacuum pump. A similar arrangement applies for the cake drying section of the filter which has its own separator and secondary spray condenser and it is connected to secondary vacuum pump. The process water leaving the scrubber and es to the sealed tank respectively for disposal. Principal of washing of filter cake: 1) The washing effect is dependent mainly on two factors below under the conditions of cake thickness of the vacuum applied being considered to be constant. These factors are E, displacement efficiency and ‘n’ is washing ratio. Washing ratio is defined as : n = (volume of washing fluid) / (volume of liquor cake) Displacement efficiency E is defined as % of soluble matter washed of the filter cake to the total soluble matter before washing ratio n = 1, is assumed to be applied and this factor E is characteristic to the filter cake itself in gypsum cake washing, this depending on crystal size, uniformity of crystals size distribution etc. Taking the remaining ratio of the soluble matter in the cake as ‘R’ is given as follows: R= { 1 – [E/100] }^n Where R = remaining ratio of soluble matter in the cake E = displacement efficiency n = washing ratio



Thickness of filter cake: Cake thickness has significant meaning on washing effect. Cake washing time is required period for the time of disappearing of washed liquor from the surface of the cake. Cake drying time which is characteristic of filter cake and co-relating with the thickness of such cake is also necessary subsequent to the cake washing time and prior to the further washing of it. Minimizing remaining water soluble P2O5 is subject to the lowering the liquor content in the filter cake prior to its washing. Under the production requirement certain amount of dry solid must be separated within certain limited time. Operating conditions for filtration: a) Temperature of wash water: 50 0C – 65 0C b) Accuracy of flow meter slurry, wash water and wash acid: ±2% c) Vacuum required: 160 mmHg abs (maximum) d) Thickness of filter cake : 30 mm (minimum) Reaction: 3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O 2CaSO4.1/2H2O + 3H2O

6H3PO4 +10CaSO4.1/2H2O + 2HF 2CaSO4.2H2O



B] Process Block Diagram



CHAPTER NO 6: THERMODYNAMICS AND REACTION KINETICS

A] THERMODYNAMICS Introduction Thermodynamics is a branch of physics concerned with heat and temperature and their relation to energy and work. Thermodynamics checks the feasibility of the given reaction by using Gibb’s Free energy. Gibbs free energy can be calculated by using following formula: ∆G = ∆H - T∆S

……….[A]

Significance: 1. If ∆G is negative then, the proportion of products present will be large. 2. If ∆G is positive then, the proportion of products present will be small. 3. If ∆G is zero then, there will be equal proportion of products and reactants. 4. If ∆G is more negative than -40kJ/mol then, the reaction is regarded as having gone to completion. 5. If ∆G is more positive than about +40kJ/mol then, the reaction is regarded as not occurred at all. The formation reaction of phosphoric acid by hemihydrate- dihydrate process (Nissan Process) using sulfuric acid and rock phosphate is as follows:-

3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O  6H3PO4 + 10CaSO4.1/2H2O + HF ………(1) 2CaSO4.1/2H2O + 3H2O  2CaSO4.2H2O

………(2)

Where reaction (1) is carried at 920C in premixer and digester and reaction (2) is carried out at 570C in crystallizers.



Calculations 1. Standard values (at 250C) of heat of reaction(∆Ho) and entropy (∆So) for all component presents in the given reactions is given below: Components

∆Hfo(kJ/mol)

∆So(kJ/mol)

1.

Ca3(PO4)2CaF2

-1506.88

0.19056

2.

H2SO4

-814

0.1569

3.

H 2O

-258.83

0.06991

4.

H3PO4

-1254.36

0.15062

5.

CaSO4.1/2H2O

-1576.74

0.13054

6.

HF

-271.12

0.173.68

7.

CaSO4.2H2O

-2022.63

0.19414

Sr. No.

2. Data of specific heat capacity () for all the component present in the reaction is as follows: Components

.dT

. dT

Temperatures

920C

570C

920C

Ca3(PO4)2CaF2

-0.1873

0.1665

0.028

570C 0.0164 -3

H2SO4

0.1315

0.0628

2.55×10

H2 O

2.3092

1.1022

0.0107

1.617×10-3 0.0067

-3

H3PO4

0.00221

0.00105

0.043×10

0.027×10-3

CaSO4.1/2H2O

-0.0167×1010

-0.00371×1010

-614×103

-205×103

HF

2.967×103

-1.067×103

-12.4

-6.25

CaSO4.2H2O

13.063

6.239

0.254

0.16

3. Heat of reaction (∆Hf) can be calculated at any temperature using the formula: ∆Hf

= ∆Hf

o

+

∆. dT

………(3)

For reaction (1) . dT = Ʃ.dTproduct - Ʃ.dTreactants = -1.6699×109 – 12.2991 = -1.67×109 kJ/mol.K



∆Hfo = ƩHfoproduct- ƩHforeactant = -20612 +14089.79 = -6522.21 kJ/mol From equation (3), ∆Hf = -6522.21 + -1.67×109 ∆Hf= -1.67×109kJ/mol Similarly, for reaction (2) ∆Hf= 73.98×106 kJ/mol Also, ∆S = ∆S0 + ∆S0

∆

.dT

………(4)

= ƩS0product - ƩS0reactant = -8.62×10-3 kJ/mol

∆

.dT = Ʃ

.dTproduct -Ʃ

.dTreactant

= -6.14×106 kJ/mol From equation (4), ∆S = -8.62×10-3 + -6.14×106 ∆S = -6.14×106 kJ/mol Similarly, for reaction (2) ∆S = 0.409×106 kJ/mol



From equation [A], ∆G = ∆H - T∆S 1. For reaction carried out at temperature of 920C (365 K) ∆G1 = ∆H - T∆S = (-1.67×109) – 365(-6.14×106) ∆G1 = -3.91×109kJ/mol 2. For reaction carried out at temperature of 570C (3300C) ∆G2 = ∆H - T∆S = (73.98×106) – 330(0.409×106) ∆G2 = -60.99×106 kJ/mol

From the value of Gibb’s Free energy of both the reactions, we conclude that both reactions are feasible and goes to completion. B] REACTION KINETICS Introduction The formation reaction of phosphoric acid is completed in two steps. The first step is the first peak to dissolution of Fap and neutralization H2PO4- into H3PO4, and the second step to precipitation of CaSO4.1/2H2O according to the following reaction: For First Peak Reaction Ca10(PO4)6F2(s) +20{H+}sol → {10Ca+2 + 6H3PO4 + 2HF}sol

1st step

{Ca2+}sol + {SO42-}sol + 1/2 {H2O}sol → CaSO4·1/2H2O(s)

2nd step

With mean iteration enthalpy values as -15.0 and --12.0 kJ per mole of Ca-sulfate, respectively. For Second Peak CaSO4.0.5H2O(s) + 3/2 H2O → CaSO4.2H2O(s)



The dissolution rate of HH, rdis, is proportional to under saturation (Ce –[Ca]) where Ce is the Ca concentration corresponding to solubility of HH, whereas precipitation rate of DH, rpp, is of order rate 2 for Ca concentration, and so, rdis = k1(Ce − [Ca2+]) and

………..(1)

rpp = k2 [Ca2+]2

.……….(2)

Where k1 and k2 are constant rate. The global rate is controlled by the Ca2+concentration, so = rdis– rpp……….(3) d[Ca2+]/dt = k1(Ce - [Ca2+]) – k2 [Ca2+]2 Giving d[Ca2+]/dt + k2[Ca2+]2 + k1 [Ca2+] = k1Ce

………..(4)

On differentiating we get the following two solutions, [Ca2]=[1/2k2][−k1+C1th{C1t/2+lnC2}] and [Ca2+]=[1/2k2][−k1 + C1th{C1t/2 + ln(−C2)}]

………..(5) ………(6)

With C1 and C2 two constants depending both on k1, k2 and Ce. C1 = [4k1Cek2 + k12]1/2 and

………..(7)

C2 = [1/2k2Ce][4k1C1−2k12 - 8k1k2 Ce −4C1Cek2−4Ce2 k22]1/2

………..(8)

Let Q1 and Q2 denote the heat quantities released by hemihydrate dissolution and dehydrateprecipitation reactions respectively. Variation of each of these quantities is proportional tothe corresponding reaction rate as, dQ1/dt = VΔdisHk1(Ce − [Ca2+]) and

………..(9)

dQ2/dt = VΔppHk2[Ca2+]2

……...(10)

With ΔdisH and ΔppH the molar enthalpies of dissolution of HH and precipitation of DH, respectively.



The total energy is Q = Q1 + Q2 and is variation is expressed as, dQ/dt = VΔdisk1 (Ce − [Ca2+]) + VΔppΗk2[Ca2+]2

………(11)

The final heat flow expression is expressed as, dQ/dt = VΔdisHk1 [Ce −(1/2k2)(− k1 + C1th{C1t/2 + lnC2})] + VΔppHk2[(1/2k2)(−k1 + C1th{C1t/2 +lnC2})]2

…….....(12)

After performing the iterations on these expressions, we obtain the following values of six parameters, ΔHdis = 12.30 kJ/mol, Ce = 3.27×10-4mol/L ΔHpp= -24.90 kJ/mol, C2 = 59.33 We obtain, k1 = 5.00×10-5 s-1

………{reaction (1)}

k2= 1.99×10-3L/mol.s

………{reaction (2)}



Chapter No 7: Mass and Energy Balance A] Mass Balance Introduction A material balance is the ing of all materials enters, leaves, accumulated or is depleted in a process or in processes unit in a given time. All material balance calculation are based on the law of conservation of mass which states that matter can neither be created nor be destroyed during a process. The law of conservation of mass can also be stated as, 1. The total mass of all substances taking part in a process remains constant. 2. Within a given isolated system, the mass of a system remains constant regardless of the changes taking place within the system. 3. The total mass of various compartments remains constant during an unit operation or a chemical reaction. Hence, according to law of conservation of mass INPUT = OUTPUT + ACCUMULATION For steady state, accumulation=0 Therefore, INPUT = OUTPUT Material balance further classified as, 1. Material balance without chemical reaction. 2. Material balance with chemical reaction. General procedure for material balance: 1. Assume suitable basis for calculation. 2. Adopt weight such as gram or kgs in case of no chemical reaction. 3. Draw a block diagram for each equipment with inlet and outlet streams. 4. Search the unknown quantities or composition, flow rates. 5. In case of problem involving chemical reaction, search out a limiting component.



6. The quantity of reacting component appearing in the product stream is the quantity of that material remains unreacted.

PLANT CAPACITY: 150 MTPD 1MTPD = 1.1022 TPD = 1102.2kg/day = 45.925kg/hr Therefore, 150MTPD = 70.3 kmol/hr Basis: 70.3 kmol/hr of phosphoric acid leaving as product The reaction takes place in premixer and crystallizer respectively, therefore we carried the material balance across the following two units only. A] Across the premixer/ digester: Reaction: 3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O  6H3PO4 + 10CaSO4.1/2H2O + HF Diagram: F

M

Premixer/ digestor

Separation by filter

P

R

Where, F = Flowrate of total feed to the premixer R = Flowrate of recycle feed M = Flowrate of mixed feed P = Flowrate of desired product



Overall material balance: M=F+R To calculate the quantity of mixed feed, we use the stoichiometry principle to the above equation, i.

For rock phosphate [Ca3(PO4)2CaF2]: 6 kmoles of H3PO4 70.3 kmoles/hr of H3PO4

≡

3 kmoles of [Ca3(PO4)2CaF2]

≡

? kmoles of [Ca3(PO4)2CaF2] .

Therefore, kmoles of [Ca3(PO4)2CaF2] required =

ii.

For sulfuric acid [H2SO4]: 6 kmoles of H3PO4 70.3 kmoles/hr of H3PO4

≡

10 kmoles of H2SO4

≡

? kmoles of H2SO4 .

Therefore, kmoles of H2SO4 required =

iii.

= 117.167 kmoles/hr

For water [H2O]: 6 kmoles of H3PO4 70.3 kmoles/hr of H3PO4

≡

5 kmoles of H2O

≡

? kmoles of H2O .

Therefore, kmoles of H2O required =

iv.

= 58.583kmoles/hr

For hemihydrate gypsum [CaSO4.1/2H2O]: 6 moles of H3PO4

≡

10 moles of CaSO4.1/2H2O

70.3 kmoles/hr of H3PO4

≡

? kmoles of CaSO4.1/2H2O

Therefore, kmoles of CaSO4.1/2H2O required =

v.

= 35.15 kmoles/hr

.

= 117.167kmoles/hr

For hydrogen fluoride [HF]: 6 moles of H3PO4

≡

2 moles of HF

70.3 kmoles/hr of H3PO4

≡

? kmoles of HF

Therefore, kmoles of HF required =

.

= 23.433kmoles/hr



Therefore, Total Mixed Feed (M) = kmoles of [Ca3(PO4)2CaF2] + kmoles of H2SO4 + kmoles of H2O = 35.15 + 117.167 + 58.583 = 210.9 kmol/hr Now, recycle ratio ( ) = 1 : 1 Therefore, R= F Overall material balance, M = R+ F Therefore, M = 2F F=

= 210.9/2 = 105.45kmol/hr

Therefore, R = F = 105.45kmol/hr Actual amount of fresh feed required: Basis:- 105.45kmol/hr of fresh feed, The proportion of feed is, Ca3(PO4)2CaF2 : H2SO4 : H2O = 3: 10: 5 Amount of [Ca3(PO4)2CaF2] = Amount of H2SO4 =

.

.

= 31.635 kmol/hr

= 52.725 kmol/hr

Amount of H2O = 105.45 – (31.635 + 52.725) = 21.09 kmol/hr



UNIT

MATERIALS

PREMIXER

INPUT

OUTPUT

(kmol/hr)

(kmol/hr)

Ca3(PO4)2CaF2

31.635

--

H2SO4

52.725

--

H2 O

21.09

--

RECYCLE FEED

105.45

--

H3PO4

--

70.3

CaSO4.1/2H2O

--

117.167

HF

--

23.433

TOTAL

210.9

210.9

B] Across the crystallizers: Reaction: 2CaSO4.1/2H2O + 3H2O  2CaSO4.2H2O Diagram:

CRYSTALLIZER F

R

Where, F = Flowrate of feed in R = Flowrate of product out Basis: 117.167 kmol/hr of hemihydrate gypsum entering To calculate the quantity of other substance by applying stoichiometry to above equation. i.

For water [H2O]: 2 moles of CaSO4.1/2H2O

117.167 kmol of CaSO4.1/2H2O Therefore, kmols of H2O =

.

≡

3 moles of H2O

≡

? kmol of H2O

= 175.75kmol/hr



ii.

For dihydrate gypsum [2CaSO4.2H2O]: 2 moles of CaSO4.1/2H2O

117.167 kmol of CaSO4.1/2H2O Therefore, kmols of 2CaSO4.2H2O =

≡

2 moles of 2CaSO4.2H2O

≡

? kmol of 2CaSO4.2H2O .

= 117.167kmol/hr

Overall material balance: Flowrate of HH in + rate of accumulation of H2O = flowrate of DH out UNIT CRYSTALLIZER

MATERIALS

INPUT (kmol/hr)

OUTPUT (kmol/hr)

CaSO4.1/2H2O

117.167

--

H 2O

175.75

175.75

--

117.167

292.917

292.917

CaSO4.2H2O TOTAL For material balance in kg/hr Basis: 6890kg/hr of H3PO4 leaving UNIT

MATERIALS

INPUT (kmol/hr)

OUTPUT(kmol/hr)

Ca3(PO4)2CaF2

15564.42

---

H2SO4

5167.05

---

H2 O

379.62

---

PREMIXER/

RECYCLE FEED

2954.989

---

DIGESTOR

H3PO4

---

6889.4

CaSO4.1/2H2O

---

16989.215

HF

---

187.464

TOTAL

24066.079

24066.08

CaSO4.1/2H2O

16989.215

---

H2 O

3163.5

2108.997

CaSO4.2H2O

---

18043.718

TOTAL

20152.715

20152.715

CRYSTALLIZER



B] HEAT BALANCE Introduction The basis of energy balance of the system is the low of conservation of energy, which states that energy can neither be created nor be destroyed during a process, although it can be converted from one form to another and the total energy of an isolated system remains constant. This law is also called as first law of thermodynamics. For closed system, the energy balance equation is, [Final system energy] – [Initial system energy]= [Net energy transferred by system] General procedure for energy balance: 1. Assume a suitable basis of calculation. 2. Draw a block diagram of the process and lable the stream. 3. Determine the quantity of flowrates of all components with the help of material balance. 4. Determine the enthalpy of each stream components entering or leaving. 5. If a chemical reaction is involved where in heat is evolved or absorbed, it must be included in the energy balance equation. 6. If the heat capacity data is provided for component involved, choose a reference temperature on which they are based for the convenience of calculation. Energy Balance Calculation: Heat capacity (J/mol.K) is calculated at average temperature using these formulas. Sr. No.

COMPONENTS

value

1.

Ca3(PO4)2CaF2

15.72 + 0.1092T cal/mol.K

2.

H2SO4

0.4628 cal/g.K

3.

H2 O

267370-2.0901T + 8.125T2 – 0.014116T3 J/kmol.K

4.

H3PO4

0.7735 cal/g.0C

5.

CaSO4.1/2H2O

18.52 + 0.002197T – 156800/T2cal/mol.K

6.

HF

62.520 – 223.02T + 0.6297T2

7.

CaSO4.2H2O

46.8 cal/mol.K



Temperature of different equipments: 1. Premixer = 900C = 363K 2. Digester = 920C = 365K 3. Crystallizer(1st) = 530C = 326K 4. Crystallizer(2nd & 3rd) = 580C = 331K 5. Crystallizer(4th) = 630C = 336K Heat capacity data for different temperatures in (J/mol/K) Component

363K

365K

326K

331K

336K

Ca3(PO4)2CaF2

55.3536

55.578

51.3192

51.8652

52.4112

H2SO4

4.722×10-3

4.722×10-3

4.722×10-3

4.722×10-3

4.722×10-3

H2 O

671.035

671.689

650.118

653.948

657.484

H3PO4

7.89×10-3

7.89×10-3

7.89×10-3

7.89×10-3

7.89×10-3

CaSO4.1/2H2O

75.7728

75.7708

74.2708

74.4708

74.6957

HF

2.081

2.552

-5.720

-4.766

-3.781

CaSO4.2H2O

195.624

195.624

195.624

195.624

195.624

I.

Premixer:

The energy balance around a premixer is based on the following chemical reaction: 3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O  6H3PO4 + 10CaSO4.1/2H2O + HF To calculate Q: Q

=

[∑Hproduct - ∑Hreactant]

∑Hproduct

=

HH3PO4 + HCaSO4.1/2H2O + HHF + Hrecycle feed

∑Hreactant

=

HCa3(PO4)2CaF2 + HH2SO4 + HH2O

H (enthalpy) =

mdT

= m(365 – 298)

= m(67)



Rate In

Rate out

(kg/hr)

(kg/hr)

Ca3(PO4)2CaF2

15564.42

H2SO4

Component

(363 K)

H(enthalpy)

---

55.3536

57.72×106

5167.05

---

4.722×10-3

1634.720

H2 O

379.62

---

671.035

17.067×106

H3PO4

---

6889.4

7.89×10-3

3641.94

CaSO4.1/2H2O

---

16989.215

75.7728

86.247×106

HF

---

187.464

2.081

26137.54

Recycle feed

2954.989

---

1.23

0.243×106

∑Hproduct

=

86.276×106 J/kg

∑Hreactant

=

57.72×106 + 1634.720 + 17.067×106 +0.243×106

=

75.061×106 J/kg

Q

=

(86.276×106) – (75.061×106)

Qheat

=

11.215×103 kJ/kg

The amount of heat supplied to the premixer is 11.215×103 kJ/kg. ∑Hreactant =75.061×106 J/kg

II.

Digester:

PREMIXER

∑Hproduct= 86.276×106 J/kg

H (enthalpy) = mdT = m(365-363) = m(2) Rate In

Rate Out

(365K)

(kg/hr)

(kg/hr)

J/mol.K

Ca3(PO4)2CaF2

15564.42

---

55.578

1.73×106

H2SO4

5167.05

---

4.722×10-3

48.7976

H2 O

379.62

---

671.689

0.5099×106

H3PO4

---

6889.4

7.89×10-3

7151.073

CaSO4.1/2H2O

---

16989.215 75.7708

108.7147

HF

---

187.464

2.552

2.574×106

Recycle feed

2954.989

---

1.21

956.816

Component

H(enthalpy)



∑Hproduct

=

2.575×106 J/kg

∑Hreactant

=

1.73×106 + 48.7976 + 0.5099×106 + 7151.073

=

2.247×106 J/kg

Q

=

(2.575×106) – (2.247×106)

Qheat

=

0.328×103 kJ/kg

The amount of heat supplied to the digester is 0.328×103 kJ/kg. ∑Hreactant = 2.247×106 J/kg

III.


DIGESTER

1st Crystallizer: H (enthalpy) = mdT = m(326-365) = m(-39) Rate in

Rate out

(kg/hr)

(kg/hr)

CaSO4.1/2H2O

16989.215

H 2O CaSO4.2H2O

Component

(326K)

H(enthalpy)

---

74.2708

-49.21×106

3163.5

---

650.118

-80.209×106

---

18043.718

195.624

-137×106

∑Hproduct

=

-137×106 J/kg

∑Hreactant

=

-49.21×106 + -80.209×106

=

-129.419×106 J/kg

Q

=

(-137×106) – (-129.419×106)

Qcool

=

-7.581×103 kJ/kg

The amount of heat to be removed from 1st crystallizer is 7.581×103 kJ/kg. ∑Hreactant =-129.419×106 J/kg

st

1 crystallizer

∑Hproduct= -137×106 J/kg



VI.

2nd& 3rd Crystallizer: H (enthalpy) = mdT = m(331-326) = m(5) Rate in

Rate out

(kg/hr)

(kg/hr)

CaSO4.1/2H2O

16989.215

H 2O CaSO4.2H2O

Component

(326K)

H (enthalpy)

---

74.4708

6.326×106

3163.5

---

653.948

10.343×106

---

18043.718

195.624

15.64×106

∑Hproduct

=

15.64×106 J/kg

∑Hreactant

=

6.326×106 + 10.343×106

=

16.669×106 J/kg

Q

=

(15.64×106) – (16.669×106)

Qcool

=

-1.029×103 kJ/kg

The amount of heat to be removed from 2nd& 3rd crystallizer is 1.029×103 kJ/kg. ∑Hreactant= 16.669×106 J/kg

V.

2nd& 3rd crystallizer


4th Crystallizer: H (enthalpy) = mdT = m(336-331) = m(5) Rate in

Rate out

(kg/hr)

(kg/hr)

CaSO4.1/2H2O

16989.215

H 2O CaSO4.2H2O

Component

(326K)

H (enthalpy)

---

74.6957

6.345×106

3163.5

---

657.484

10.39×106

---

18043.718

195.624

15.648×106

∑Hproduct

=

15.648×106 J/kg

∑Hreactant

=

6.345×106 + 10.39×106

=

16.735×106 J/kg

Q

=

(15.64×106) – (16.735×106)

Qcool

=

-1.087×103 kJ/kg



The amount of heat to be removed from 4th crystallizer is 1.087×103 kJ/kg. ∑Hreactant= 16.735×106 J/kg

4th crystallizer

∑Hproduct=15.648×106 J/kg

OVERALL HEAT BALANCE Qtotal

=

Qpremixer + Qdigester – Qcrystallizers

Qtotal

=

11.215×103 + 0.328×103 – 7.581×103 -2(1.029×103) – 1.087×103

Therefore,

Qtotal

=

0.817×103 kJ/kg

Total heat supplied to the process is 0.817×103 kJ/kg



Chapter No 9: Design of Critical Equipment A] DESIGN OF BALL MILL: Function of ball mill is to ground the rock phosphate. Unground rock phosphate varies from 10cm to 0.05cm in size. For design of ball mill, we consider that unground rock phosphate has feed size of 0.08cm to 2cm. We have to crush this feed upto 0.007cm. Data: Feed size

: 20mm (20000 microns) to 2 mesh(841 micron)

Product size

: 0.007cm (200 mesh, 70%product es through it)

Sp. Gravity of rock phosphate: 1.1710 at 160C & 2.74 at 300C Work index of rock phosphate: 9092 Feed rate: 44tonnes/hr 1. Length & diameter of ball mill To determine length of ball mill and also diameter of ball mill we assume that 25 tonnes of unground rock phosphate is present in the mill at any time (56% of total feed rate) Now, D = 124.0 × C + 485.7 L = 85.71 × C + 1854 Where C = tonnes of feed D = diameter in mm L = Length in mm D = 124.2 × 25 + 485.7 = 3590.7 mm = 3.590m = 11.4829ft For deg purpose we consider diameter of ball mill is 12ft. D =12ft



L = 85.71 × 25 × 1854 = 3996.75mm = 3.99675m = 13.11ft ≈ 14ft For deg purpose we consider 10% excess length for suitable grinding condition L = 1.1 × 14 = 15.4ft Consider length of ball mill L = 16ft 2. Diameter of Ball Db = √

√

√

Where, Db = diameter Pf = size in µ of 80% ing in the fresh feed C = 200 for ball mill Wi = work index Vcr = percentage of critical speed Sgs = specific gravity of feed D = internal diameter of mill in m By substituting all the values, we get, Db = 42.72mm Ball mill run at 65% of critical speed For deg purpose we consider ball diameter is 50mm and 40mm Db = 50mm and 40 mm 3. Number of Balls: Volume of ball mill = × 122 × 16 = 1809.55ft Bulk volume of balls charge ratio to the volume of mills is known as filling ratio (F) and its range is 30- 45%. Assuming filling ratio to be 35%. Therefore, F =

Where, bulk volume of balls = F × volume of ball mill = 0.95 × 1809.55 = 633.34ft3



Volume of single ball for 50 mm dia= 4/3 × π × r3 =4/3 × π × 0.0823 = 230.9×10-3 ft3 Volume of single ball for 40 mm dia= 4/3 × π × r3 =4/3 × π × 0.0683 = 131.7×10-3 ft3

No. of balls for 50 mm dia=

.

=

.

No. of balls = 1985

No. of balls for 40 mm dia=

.

=

.

No. of balls = 2715 4. Critical speed and operating speed of ball mill and is given by,

ηc=

√

ηc=

√

. .

.

ηc= 0.3709 rps ηc= 22.25 rpm Operating speed is (65%) of critical speed

η = 0.65 × ηc η = 0.65 ×22.25 η = 14.46 rpm For balls we take alloy steel is IS-6079 as material of construction. Therefore total weight of balls is 61.7tonnes.



5. Power requirement of ball mill Using bond crushing law = 0.3162× Wi × (

√

√

)

Where, m = feed rate of rock phosphate Wi = work index P = power required to ball mill Dp = diameter of product particle Df = diameter of feed particle P = 44000 × 0.3162 × 9.92 × ( √ .

√

)

P = 478530.10kW P =641708.86 HP 6. Thickness of ball mill; t= Where, P = design pressure Di = internal diameter J = t efficiency f = design or permissible stress at design temperature Assuming internal pressure P = 0.3 N/mm2 Therefore, design pressure, p = 0.3 × 1.1 = 0.33N/mm2 Allowable stress at 300C = 30N/mm2 t=

.

. .

.

t = 23.82mm t ≈ 25 mm



B] DESIGN OF PREMIXER: Data: Type – cylindrical with agitation, completely lined vessel Agitator type – anchor agitation Size of vessel – 2275mm ID × 1465mm liquid height Hold up capacity – 5.95m3 Agitator rpm(N) – 120 Operating conditions: Temperature – 900C Pressure – hydrostatic M.O.C – carbon steel with butyl rubber lining + carbon brick lining Function – to mix up completely rock phosphate with sulfuric acid and phosphoric acid Diagram:

Typical configuration and dimensions of an agitated vessel Where, Da = diameter of agitator D = diameter of tank



HA = height of agitator from base of tank H = depth of liquid WB = width of baffles N = speed of agitator i. For anchor agitator = 0.9

Da = 0.9× D = 0.9 × 2275 Da = 2047.5mm ii. Power number (Np) = 215(Re)-0.955 , Re≤ 100 Where Re = Reynolds number ……..(1)

= To calculate,

ρf =

………(2)

where, m = mass of the material v = volume of the material Sr. No.

Component

m (kg/hr)

v (kg/hr)

1.

H2 O

379.62

0.3796

2.

H2SO4

5167.05

2.808

3.

Return acid

2954.989

8.2351

4.

Rock phosphate

15564.42

8.833

From equation (2), ρf = 1188.11 kg/m3 Assuming the viscosity (µ) to be 1 kg/m.s From eqn (1) we have, Re = 1188.11 × 120/60 × 2.0475 Re = 4865.31045 Re ≈ 4870



Thus the flow inside the premixer is turbulent iii. Pressure inside premixer: P = hydrostatic pressure ……..{given} = ρgh = 1188.11 × 9.81 × h Height of liquid is assumed to be 75% of the height of tank h = 0.75 × 1.465 = 1.09875m Therefore, P = 1188.11× 9.81× 1.09875 P = 12806.325 N/m2 iv. Power required (Np): Np = =

.

.

.

.

Np = power required = 2.722hP v. Shaft Design: Rated motor torque Tr, ×10 N-m

Tr =

Tr = 162.45 N-m Maximum torque for design, Tm Tm = 1.5 × Tr = 1.5 × 162.45 Tm = 243.686 N-m Design of shaft based on pure torsion, Tm =

× d3 × fs

Note: Shaft material: steel Permissible shear stress fs: 52N/mm2 Elastic limit in tension (fb): 240N/mm2 Modulus of elasticity (E): 1.95 × 105N/mm2



= 23866.973

d3 =

Therefore, d= 28.79×10-3m Design of shaft based on pure bending a) Fm = Tm/(0.75×Rb) Where Fm = bending force Rb = radius of impeller Therefore, Fm = 243.686/(0.75×1.02375) Fm = 317.376 N-m b) Mm = Fm× l Where, Mm= maximum bending moment l = length of shaft = 730mm ……{assumed} Mm = 317.376× 0.73 Mm = 231.684N-m c) Mm =

× fb × d3

d3 = Mm×32/(π× fb) d = 21.42×10-3m Design based on equivalent twisting moment, Te = √

2

2

Te = 336.244 N-m Te =

× 52 × d3

Therefore, d = 32×10-3m Design based on equivalent bending moment Me = ½(Mm + √

2

2

= ½(231.684 + 336.244)



× 240 × d3

Me =

d = 22.927× 10-3m selecting the maximum diameter of shaft as 32mm Design of shaft based on critical speed, Actual speed should not be between 66.6% to 130% of critical speed Nc Nc = N×100/66.6 Nc = 120×100/66.6or Nc = 180.180rpm Now, Nc = 946/δ1/2 Where, δ = appropriate deflection in mm δ = 27.565mm The maximum deflection due to each load acting independently is given by, δ= Weight(W) = mass × 9.81 = 0.236 × 106 N δ=

.

.

.

= 27.565×10-3

d = 21.44×10-3m Hence selected shaft size is 22mm



vi. Hub and Key Design. Hub diameter of agitator = 2d = 2 × 22 = 44mm Length of hub = 2.5d = 55mm Length of key = 1.5d = 33mm

= lb.fs = l.(t/2).fc

/ .

/

= 33 × b × 65 = 33×(t/2) × 130

b = t = 0.0103mm

vii. Blade Design: Carbon steel can be used as construction material for blade. Stress, f =

.

/

Therefore on substituting the values, f = 2362.86N/mm2 viii.

Uffing Design of stuffing box and gland

Internal diameter of stuffing box from eqn d1 = d + 2×( clearance between shaft and stuffing box) c = 0.2d + 5mm = 0.2(22) +5 = 9.4mm say 10mm d1 = 22 + (2×10) = 42mm Thickness of the gland flange = (d/8)× 12.5 = (22/8) × 12.5 = 16mm



Thickness of stuffing box body, t=

=

+ 6mm .

+6

Therefore t = 6.09mm Load on glands, F = ×P(d12 –d22) = db2 n ×ft Therefore, db = 1.6mm Use minimum stud diameter as 16mm and 4 no. of studs. Thickness of stuffing box flange = 1.75 db = 1.75× 1.6 Therefore,

ts= 28mm



C] Design of Horizontal Belt Filter

Thickness Of Filter Cake: For continuous filtration application the rate at which the cake is formed normally determines type of vacuum filter. Sr No

Process Type

Rate of cake built- up

Suitable equipment

1

Rapid filtering

0.1-10 cm/sec

Gravity pan, Horizontal table or top feed drum, continuous pusher type centrifuge

2

Medium Filtering

0.1-10 cm/min

Vacuum Drum or disc or pan or Vacuum belt filter, peeler type centrifuge

3

Slow filtering

0.1-10 cm/hr

Pressure filter, disc or tubular centrifuge

4

Clarification

Negligible cake

Precoat drum, filter aid system, sand deep bed filters

Mass of solids in filter cake = (1-Ɛ)*A*Ɩ*ρ Where, Ɛ=porosity of cake= 0.2 (for given cake) Ɩ =cake thickness ρ = solid density = 2350 Kg/M3 A = cross section area of filter cake= cloth width*cake thickness (Ɩ) Clothe width= 3.4 M Mass of solids in filter cake = 0.2986 Kg/sec 0.2986 = (1-0.2)*3.4*Ɩ*Ɩ*2350 0.2986 = 6392*Ɩ2 Ɩ = 6.8348*10-3 M/sec Ɩ = 4.1 mm/min OR 0.41 cm/min Hence for this thickness of cake horizontal belt vacuum filter is effective.



Area Of Filtration: C = It is solid concentration per unit volume of filtrate = wt. of solid/volume of liq C = 9754.33/(62500/1638) C = 255.64 Kg/m3 Cx = kg of solids per kg of slurry = 0.2986/202123 Cx = 0.135 Feed rate = 2.2123 Kg/sec Flow rate of filtrate V/tc = Feed rate*(Cx/C) = 2.2123*(0.135/255.817) V/tc = 101685*10-3 m3filtrate/sec V/Atc = (2*ΔP/Ctcµα)0.5 ΔP = pressure drop = 80,000 Pa µ = filtrate viscosity = 0.02 Ns/m2 α = (4.37*109)*(ΔP)0.3 = (4.37*109)*(80,000)0.3 α = 1.2924*1011m/kg sp. surface area of cake (So) =16077.31 m2 tc = filter cycle time = 90sec V/Atc = (2*ΔP/Ctcµα)0.5 101685*10-3/A = (2*80,000/90*0.02*1.2924*1011*255.5817)0.5 A= 25.6317 m2 Area for filtration should be 25-30 m2

Rate Of Filtration Q=A*ΔP/µ*[Rf+(C*α*V/A)] Rf = cake thickness/permeability for fine filter sheet Rf = 6.833*10-6/0.15*10-12 = 4.555*107 Q = 25.6317*80,000/0.02*[(4.555*107)+(255.5817*1.2924*1011*100.94/25.6317)] Q = 8.1099*10-7 m3/sec Q = 2.9195 lit/hr Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 102


Length Of Filter Cloth Head pulley dia (D) = 1400mm Tail pulley dia (d) = 1400mm Assuming Centre to Centre distance (C) = 15m Assuming quarter turn drive L = (π/2)*(D+d) + (C2+D2)0.5+(C2+d2)0.5 2

L = (π/2)* (1.4+1.4) + (1.4 +152)0.5 + (1.42+152)0.5 L = 32.329 M Peripheral Velocity of pulley V = velocity N = max. speed of motor = 12RPM V = π* Dia* N/60 V= π*1.4*12/60 V = 0.879 m/sec



Chapter No 9: Materials of Construction Material of Construction for Different Units: Rubber or brick lined digesters are used, with high alloy metallic components, (e.g. N08028, N06985, N06030, N0803). After filtration to remove precipitated saltsthe product acid concentration is roughly 35-44%. Stronger concentration of 5875% is obtained by vacuum evaporation. Evaporators are generally shell & tube exchangers with tubes of alloys 31(NJ08031), G-30(N06030) or G-3 (N06985). Materials: 1. Aluminum Alloys: Aluminum is resistant to phosphoric acid up to 20% conc. to 65ºC. Hot phosphoric acid is used to bright dip aluminum before anodizing. 2. Steel & cast iron: Steel forms a moderately protective phosphate film in the acid above 70% concentration & resistance may be further enhanced by arsenic salt. Unalloysed cast irons have the similar resistance in strong acids. Austentic nickel iron are resistant to slightly above room temperature but even the 14% silicon iron normally resistant to all concentrations at atm boiling point can be attacked by fluoride concentration. 3. Stainless steel: The straight chromium (ferritic or martensitic) grades of stainless steel find no of practical applications in phosphoric acid. Types 316L(S31603) & 317L(S31703) resist uncontaminated acid in all concentration to about 80ºC. However velocities above 1m/sec can cause erosion corrosion in valves & piping. It precisely because of contaminants in wet process acid that certain high alloy, high performance stainless steel were developed. Alloy 28(N08028), 31(N08031), & N08904 or N08700 have been successfully used in lieu of S31603 but N06985 & N06030 are usually preferred. 4. Copper & its alloy: Copper & its high strength alloys as well as C70600 heat exchangers tubes have been successfully used in unaerated acid in the absence of other



oxidizing agents (iron & copper). They have been the materials of choice in some high temperature organic synthesis under reducing conditions. 5. Nickel & its alloy: Chromium free alloys N02200 &N04400 are of small use in phosphoric acid, being less resistant than copper alloys under reducing condition &attacked under oxidizing conditions. The Molybdenum grade N10665 will resist pure acid of all concentrations to about 65ºC & up to 50% in atm boiling points. It is however rapidly corroded by oxidizing contaminants. Alloys of primary interest are the specialty alloy N06007 & 6% molybdenum stainless steel alloy 31(N08031). These were developed specially for wet process phosphoric acid 6. Reactive Metals: Titanium: because it is not a oxidizing acid phosphoric acid tends to attak titanium zirconium: zirconium resist pure acid to about 50% concentration, and above the atmospheric boiling temperature, but corrosion rates increases sharply above about 60% at 100ºC fluorides, zirconium equipment has been used to process a phosphoric acid-containing medium. Tantalu : It will resist all concentrations of phosphoric acid to about 190ºCif there is not more than a small fluoride concentration Niobium: Niobium resists phosphoric acid up to 95% & up to 60ºC 7. Noble Metals: Gold and platinum will resist all concentration of pure acid to the atmospheric boiling point silver corrosion rate increase from less than 1mpy in 15%acid at the atmospheric boiling point to 2mpy in 60% and 7mpy is 85% up to 250ºC (480 F) 8. Non-Metals: Organic: In the absence of solvent contamination conventional plastic, PVC, polyethylene, epoxyphenolic resist all concentration to the inherent temperature limitof the plastic. Troweled –epoxy coatings are used to protect concrete tank padsfromacid spillage. The fluorinated plastics are resistant to higher temperature. Inorganic: Carbon & graphite are inert in all concentrations of acidwith or without contaminants to at least 350ºC. A film of phosphate protects against oxidation. Some impervious graphite products are limited to about 170ºC by



the constituents of cemented connections. Glass is useful only at ambient temperature even without fluoride contamination. 9. Storage facilities: for product phosphoric acid Tanks FRP or N08700 or N08904 Piping’s FRP or S31603 Valves CF3M Pumps CF3M Gaskets Elastomeric, filled polytetra-fluoethylene (PTFE) or flexible graphite.



Chapter No 10: Cost Estimation Total operating cost (TOC) is the sum of: 1. Fixed Capital (FCC) 2. Working Capital (WC) Fixed capital cost is again comprised of: 1. Direct cost 2. Indirect cost 1. Direct Cost: Cost of material &labor involved in actual installation of complete facility complete under this category. This includes a) Purchased equipment’s cost (PEC) b) Installation including insulation & painting(IC) c) Instrumentation & controls installed(ICC) d) Piping installed (PI) e) Electrical Installed (EI) 2. Indirect Cost: Expenses which are not directly involved with material & actual installation of complete facility comes under this category. a) Engineering & supervision cost (E&SC) b) Construction expenses & contractors fee (CE & CC)

Working Capital: It is the sum of manufacturing cost & general expenses. 1. Manufacturing Cost: It is the sum of Direct production cost, Fixed charges & plant overhead cost. a) Direct production cost: comprise of i.

Raw Material Cost(RMC)

ii.

Operating labor Cost(OLC)

iii.

Utilities Cost(UC)

iv.

Maintenance & repair cost (M&RC)



b) Fixed charges: comprise of i.

Depreciation cost(DC)

ii.

Local Taxes (LT)

c) Plant overhead cost 2. General Expenses: General expense is the sum of istrative cost, Distribution & selling cost, Research & Development cost. GE=20% of (Total Capital Investments)

FORMULAE FOR CALCULATIONS: 1. For Total Capital Investments (TCI): WC = 20% of (TCI) And

TCI = FCI+WC TCI = FCI+ 0.2TCI

Therefore

FCI = (1-0.2) TCI TCI = FCI/0.8

2. TOC = FCC+WC 3. NET PROFIT = (Gross Income -Taxes) Gross Income = Total Income –Total Product Cost Where, Total Income = (Selling price × Quantity of product manufacturing) 4. RATE OF RETURN (ROR): ROR =

(NET PROFIT × 100)/Total Operating cost

5. PAY BACK PERIOD

=

TOC /(Profit+Depreciation)



CALCULATIONS: Direct cost: a) To calculate purchased equipment cost (PEC): COST (in `)

Sr No.

EQUIPMENT

1

Ball mill (Carbon steel, 10ft Dia×16ft length) 3

22,39,060

2

Mill Fan (44,000 M /hr)

3,09,506

3

Dust collector (150mm×3150mm long)

3,28,760

3

4

Vent Fan (15,000 M /hr)

6,12,600

5

Acid Mixing Tank

62,000

6

Premixer (2275mmID×1465mm ht)

64,680

7

Digester (2Nos,2820mm ID×3620mm ht)

3,61,136

8

Crystallizer (4 Nos, 6700mmID×7700mm ht)

29,70,680

9

Slurry Pumping Tank (4500mm ID×4000mm ht)

4,14,120

10

Filter feed splitter box (1500×1200×1000)mm

48,980

11

Horizontal Vacuum Belt Filter

36,95,870

12

Clarifier (7m Dia×4m ht)

1,01,522

13

Reaction fume scrubber

77,380

14

Storage Tank

42,340

PURCHASED EQUIPMENT COST (PEC)

` 11.28×106 /-

b) IC = 30% OF PEC IC = 0.3×11.28×106 IC = `30384×106 /c) ICC = 15% OF PEC ICC = 0.15×11.28×106 ICC = ` 1.692×106 /d) PC = 20% OF PEC PC = 0.2×11.28×106 PC = ` 2.252×106

/-



e) EC = 20% OF PEC EC = 0.2×11.28×106 EC = ` 2.252×106 /-

DIRECT COST

= PEC+IC+ICC+PC+EC =(11.28+3.384+1.692+2.256+2.256)×106

DIRECT COST

= ` 20.868×106/-

Indirect Cost: a) E&SC = 20% OF Direct cost E&SC = 0.2×20.868×106 E&SC = ` 4.1736×106/b) CE&CC = 30% of E&SC CE&CC = 0.3×4.1736×106 CE&CC = ` 1.252×106/INDIRECT COST = E&SC + CF&CC = (4.1736+1.252)×106 INDIRECT COST = ` 5.4256×106 /-

FIXED CAPITAL COST = DIRECT COST+INDIRECT COST = (20.868+5.4256)×106 FCC = ` 26.2936×106/Direct Production Cost: a) Raw Material Cost (RMC) =(8.3×215293.75)H2SO4+(648517.5×16)ROCK PHOSPHATE RMC = ` 12.163×106/b) Operating Labor Cost (OLC) = 10% of Total Product cost(TPC) TPC = Fixed charges/0.15 Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 110


Fixed Charges(FC) = Depreciation cost (DC) + Local Taxes (LT) DC=15% of FCC = 0.15×26.2936×106 DC = ` 3.944×106/LT = 20% of FCC = 0.2× 26.2936×106 LT = ` 5.2588×106/FC = DC+LT = (3.944+5.2588)×106 FC = ` 9.2028×106/Total Product Cost = FC/0.15 TPC = ` 61.325×106/OLC= 10% of TPC = 0.1×61.325×106 OLC= ` 6.1325×106/c) Utilities cost (UC):10% of TPC UC = ` 6.1325×106 /d) Maintenance & Repair Cost (M&RC):5% of FCC M&RC = 0.5×26.2936×106 M&RC = ` 1.3147×106/Direct Production cost= RMC+OLC+UC+M&RC DPC = (12.136+6.1352+6.1352+1.3147)×106 DPC = 25.7480×106/-

Plant Overhead Cost (POHC) = 10% of TPC POHC= 0.1×61.325×106 POHC= ` 6.1325×106 /-



Manufacturing cost (MC) = DPC+FC+POHC = (25.7481+9.2028+6.1352)× 106 MC = ` 41.0861×106 /-

GENERAL EXPENSES (GE) = 20% of TOC Therefore,

TOC = FCC+WC = FCC+ (0.2×TOC) FCC = TOC×(1-0.2) TOC = FCC/0.8 = (26.2936×106)/0.8 TOC = ` 36.617×106/GE = 0.2×36.617×106 GE = ` 7.3234×106/-

Working Capital (WC): WC = MC+GE = (41.0861+7.3234)× 106 Now, TOC= FCC+WC = (26.2936+48.4095) 106 TOC = ` 74.7031×106/Total Income: Total Income = Selling price×Quantity of product Manufactured = 12.6×688904×1000/24 Total Income = ` 83.835×106/Gross Income: Gross Income = Total Income – Total Product cost = (83.835 – 61.325)× 106 Gross Income = ` 22.483×106/-



NET PROFIT: NET PROFIT = Gross Income – Taxes = (22.483 -5.2588)× 106 NET PROFIT = ` 17.2242×106/-

Rate of Return (ROR) = (NET PROFIT/TOC) ×100 ROR = (17.2242/74.7031)×100 ROR = 43.0568%

Pay Back Period(PBP) = TOC/(Profit+ Depreciation) PBP = (74.7031×106)/(17.2242+3.944)×106 PBP = 2.529 Year



Chapter No 11: PLANT LOCATION/SITE The location of the plant can have a crucial effect on the profitability of a project, and the scope for future expansion. Many factors must be considered when selecting a suitable site. The principal factors to consider are: 1. Location, with respect to the marketing area. 2. Raw material supply. 3. Transport facilities. 4. Availability of labour. 5. Availability of utilities: water, fuel, power. 6. Availability of suitable land. 7. Environmental impact, and effluent disposal. 8. Local community considerations. 9. Climate. 10. Political and strategic considerations. A] Marketing area Phosphoric acid produced in bulk quantities; therefore the cost of the product per tonne is relatively low and the cost of transport a significant fraction of the sales price. The plant should be located close to the primary market. In an international market, there may be an advantage to be gained by locating the plant within an area with preferential tariff agreements. B] Raw materials The availability and price of suitable raw materials will often determine the site location. Plants producing bulk chemicals are best located close to the source of the major raw material; where this is also close to the marketing area. C] Transport The transport of materials and products to and from the plant will be an overriding consideration in site selection. If practicable, a site should be selected that is close to at least two major forms of transport: road, rail, waterway (canal or river), or a sea port. Road transport is being increasingly used, and is suitable for local distribution from a central warehouse. Rail transport will be cheaper for the long-distance Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 114


transport of bulk chemicals. Air transport is convenient and efficient for the movement of personnel and essential equipment and supplies, and the proximity of the site to a major airport should be considered. D] Availability of labour Labour will be needed for construction of the plant and its operation. Skilled construction workers will usually be brought in from outside the site area, but there should be an adequate pool of unskilled labour available locally; and labour suitable for training to operate the plant. Skilled tradesmen will be needed for plant maintenance. Local trade union customs and restrictive practices will have to be considered when assessing the availability and suitability of the local labour for recruitment and training. E] Utilities Chemical processes invariably require large quantities of water for cooling and general process use, and the plant must be located near a source of water of suitable quality. Process water may be drawn from a river, from wells, or purchased from a local authority. At some sites, the cooling water required can be taken from a river or lake, or from the sea; at other locations cooling towers will be needed. Electrical power will be needed at all sites. Electrochemical processes that require large quantities of power; for example, aluminium smelters, need to be located close to a cheap source of power. A competitively priced fuel must be available on site for steam and power generation. F] Environmental impact and effluent disposal All industrial processes produce waste products, and full consideration must be given to the difficulties and cost of their disposal. The disposal of toxic and harmful effluents will be covered by local regulations, and the appropriate authorities must be consulted during the initial site survey to determine the standards that must be met. An environmental impact assessment should be made for each new project, or major modification or addition to an existing process.



G] Local community considerations The proposed plant must fit in with and be acceptable to the local community. Full consideration must be given to the safe location of the plant so that it does not impose a significant additional risk to the community. On a new site, the local community must be able to provide adequate facilities for the plant personnel: schools, banks, housing, and recreational and cultural facilities. H] Land (site considerations) Sufficient suitable land must be available for the proposed plant and for future expansion. The land should ideally be flat, well drained and have suitable loadbearing characteristics. A full site evaluation should be made to determine the need for piling or other special foundations. I] Climate Adverse climatic conditions at a site will increase costs. Abnormally low temperatures will require the provision of additional insulation and special heating for equipment and pipe runs. Stronger structures will be needed at locations subject to high winds (cyclone/hurricane areas) or earthquakes. J] Political and strategic considerations Capital grants, tax concessions, and other inducements are often given by governments to direct new investment to preferred locations; such as areas of high unemployment. The availability of such grants can be the overriding consideration in site selection.



Chapter No 12: PLANT LAYOUT The economic construction and efficient operation of a process unit will depend on how well the plant and equipment specified on the process flow-sheet is laid out. The principal factors to be considered are: 1. Economic considerations: construction and operating costs. 2. The process requirements. 3. Convenience of operation. 4. Convenience of maintenance. 5. Safety. 6. Future expansion. 7. Modular construction. A] Costs The cost of construction can be minimised by adopting a layout that gives the shortest run of connecting pipe between equipment, and the least amount of structural steel work. However, this will not necessarily be the best arrangement for operation and maintenance. B] Process requirements An example of the need to take into process considerations is the need to elevate the base of columns to provide the necessary net positive suction head to a pump or the operating head for a thermosyphon reboiler. C] Operation Equipment that needs to have frequent operator attention should be located convenient to the control room. Valves, sample points, and instruments should be located at convenient positions and heights. Sufficient working space and headroom must be provided to allow easy access.



D] Maintenance Heat exchangers need to be sited so that the tube bundles can be easily withdrawn for cleaning and tube replacement. Vessels that require frequent replacement of catalyst or packing should be located on the outside of buildings. Equipment that requires dismantling for maintenance, such as compressors and large pumps, should be placed under cover. E] Safety Blast walls may be needed to isolate potentially hazardous equipment, and confine the effects of an explosion. At least two escape routes for operators must be provided from each level in process buildings. F] Plant expansion Equipment should be located so that it can be conveniently tied in with any future expansion of the process. Space should be left on pipe alleys for future needs, and service pipes over-sized to allow for future requirements. G] Modular construction In recent years there has been a move to assemble sections of plant at the plant manufacturer’s site. These modules will include the equipment, structural steel, piping and instrumentation. The modules are then transported to the plant site, by road or sea. The advantages of modular construction are: 1. Improved quality control. 2. Reduced construction cost. 3. Less need for skilled labour on site. 4. Less need for skilled personnel on overseas sites. Some of the disadvantages are: 1. Higher design costs. 2. More structural steel work. 3. More flanged connections. 4. Possible problems with assembly, on site.



H] General considerations Open, structural steelwork, buildings are normally used for process equipment; closed buildings are only used for process operations that require protection from the weather. The arrangement of the major items of equipment will usually follow the sequence given on the process flow-sheet: with the columns and vessels arranged in rows and the ancillary equipment, such as heat exchangers and pumps, positioned along the outside. SITE SELECTION The process units and ancillary buildings should be laid out to give the most economical flow of materials and personnel around the site. Hazardous processes must be located at a safe distance from other buildings. Consideration must also be given to the future expansion of the site. The ancillary buildings and services required on a site, in addition to the main processing units (buildings), will include: 1. Storages for raw materials and products: tank farms and warehouses. 2. Maintenance workshops. 3. Stores, for maintenance and operating supplies. 4. Laboratories for process control. 5. Fire stations and other emergency services. 6. Utilities: steam boilers, compressed air, power generation, refrigeration, transformer stations. 7. Effluent disposal plant. 8. Offices for general istration. 9. Canteens and other amenity buildings, such as medical centres. 10. Car parks. General Consideration: A] When roughing out the preliminary site layout, the process units will normally be sited first and arranged to give a smooth flow of materials through the various processing steps, from raw material to final product storage. Process units are normally spaced at least 30 m apart; greater spacing may be needed for hazardous processes.



B] The location of the principal ancillary buildings should then be decided. They should be arranged so as to minimise the time spent by personnel in travelling between buildings. istration offices and laboratories, in which a relatively large number of people will be working, should be located well away from potentially hazardous processes. Control rooms will normally be located adjacent to the processing units, but with potentially hazardous processes may have to be sited at a safer distance. C] The siting of the main process units will determine the layout of the plant roads, pipe alleys and drains. Access roads will be needed to each building for construction, and for operation and maintenance. D] Utility buildings should be sited to give the most economical run of pipes to and from the process units. E] Cooling towers should be sited so that under the prevailing wind the plume of condensate spray drifts away from the plant area and adjacent properties. F] The main storage areas should be placed between the loading and unloading facilities and the process units they serve. Storage tanks containing hazardous materials should be sited at least 70 m (200 ft) from the site boundary.



Chapter No 13: Health and Safety Aspects A] Rock Phosphate: [Ca10(PO4)6CaF2] : 1) General Information: Boiling Point

: N/A

Flash Point (Method Used)

: N/A

Fighting Procedure

:Rock Phosphate is an inorganic material. It is non flammable and non-hazardous.

2) Health Hazard Data Route(s) of Entry

:Inhalation, Skin, Ingestion

Health Hazards

:Acute and Chronic

Short term

:no effect other than nuisance dust.

Long term

:due to fluoride in the rock, large amounts over a long period of time may cause weight loss.

Emergency and First Aid Procedures: Eyes

: flush with water

Skin

: wash with water.

Call doctor if irritation persists. 3) Precautions for Safe Handling and Use 1. Steps to Be Taken in Case Material Is Released or Spilled: If uncontaminated, sweep up or collect and reuse as product. Use conventional housekeeping methods. 2. Waste Disposal Method: Use conventional housekeeping methods. Stay in accordance with State and Federal regulations. 3. Precautions to Be Taken in Handling and Storing



B] 98% SULFURIC ACID [H2SO4]: 1) General Information: a) Causes severe burns. b) Reacts violently with water. c) Contents may be under pressure of explosive, d) Flammable hydrogen gas. e) Highly reactive and capable of igniting combustible material on . f) Corrosive. Causes burns, tissue destruction, Can cause blindness. g) Harmful if inhaled. Causes upper respiratory tract irritation, lung irritation, chest pain, wheezing, shortness of breath, a burning h) sensation, tickling of the nose and throat, sneezing, Repeated exposure to high levels of sulfuric acid mist may cause etching i) tooth enamel in persons who breathe through their mouths. j) Harmful if ingested. Can cause irritation, abdominal pain, corrosion, burns to mouth and esophagus, death. k) When mists are released from this product they are considered to be probable or suspected human carcinogens 2) First aid measures for accidental: a) Hold eyelids open and flush with a steady, gentle stream of water for at least 15 minutes. Seek immediate medical attention. b) In case of , immediately wash with plenty of water for at least 15 minutes. Seek medical attention if irritation developes or persists. Remove contaminated clothing and shoes. Clean contaminated clothing and shoes before re-use. c) Remove victim from immediate source of exposure and assure that the victim is breathing. If breathing is difficult, ister oxygen, if available. If victim is not breathing, ister R (cardio-pulmonary resuscitation). Seek medical attention. d) Do not induce vomiting. If the person is conscious and has no trouble breathing a small (no more than one glass) amount of water may be given. Do not leave victim unattended. To prevent aspiration of the swallowed product,



lay victim on side with head lower than waist. If vomiting occurs do not reister water. Do not give anything by mouth to an unconscious person. e) Immediately obtain medical attention. 3) Fire hazard data: Flash Point: Not Applicable Not combustible. Strong oxidizers can react with reducing agents or combustibles producing heat and causing ignition. Reacts violently with water releasing heat and corrosive material. a) Not combustible. Use extinguishing method suitable for surrounding fire. Recommended (small fires): dry chemical. b) Firefighters should wear NIOSH/MSHA approved positive pressure breathing apparatus with full face-piece and full acidresistant protective clothing. Fight fire from maximum distance. 4) Evacuation Procedures and Safety: a) Personnel handling this material should be thoroughly trained to handle spills and releases. Do not direct hose streams into an unignited transportation spill (tank truck or tank car). b) Stop leak if it can be done without risk. Dike spill using absorbent or impervious materials such as earth, sand or clay. Dike or retain dilution water or water from firefighting for later disposal. c) Pump any free liquid into an appropriate closed container. Exercise caution during neutralization as considerable heat may be generated. Carefully neutralize spill with soda ash. Absorb neutralized spill with an inert absorbent. Scrape up and place in appropriate closed container. 5) Environmental and Regulatory Reporting: Do not flush to drain. Runoff from fire control or dilution water may cause pollution. Dispose of as a hazardous waste. Spills may be reportable to the National Response Center (800-424-8802) and to state and/or local agencies. Large spills should be handled according to a predetermined plan.



6) Handling and storage: a) Do not breathe vapors and mists. Do not get on skin or in eyes. This product reacts violently with bases liberating heat and causing spattering. b) When diluting an acid, ALWAYS add the acid slowly to water and stir well to avoid spattering. NEVER ADD WATER TO ACID. c) Store in tightly closed containers. Store in an area that is dry, well-ventilated, diked with impermeable material, Freezing point varies with concentration. Maximum recommended storage temperature = 104F (40C). Corrosion rates increase at elevated temperatures. C] PHOSPHORIC ACID (H3PO4): 1) General Information: a) Clear, colorless solution with caustic odor. b) Routes of Entry: Skin, eyes, inhalation and ingestion. 2) First aid information: a) Inhalation of mists can cause corrosive action on mucous membranes. Symptoms include burning, choking, coughing, wheezing, laryngitis, shortness of breath, headache or nausea. Move casualty to fresh air and keep at rest. Get medical attention if symptoms persist. b) Symptoms include eye burns, watering eyes. Rinse with plenty of water for a minimum of 15 minutes and seek medical attention immediately. c) Symptoms include burning, itching, redness, inflammation and/or swelling of exposed tissues. Immediately flush with plenty of water for at least 15 minutes while removing contaminated clothing and wash using soap. Get medical attention if necessary. d) Do Not Induce Vomiting. Causes corrosive burns of the mouth, gullet and gastrointestinal tract if swallowed. Symptoms include burning, choking, nausea, vomiting and severe pain. Wash out mouth with water and give a glass of water or milk. Get medical attention immediately.



3) Fire-fighting measures Product is not flammable. a) Use appropriate media for adjacent fire. Cool containers with water, keep away from common metals. b) Wear self-contained, approved breathing apparatus and full protective clothing, including eye protection and boots. c) Emits toxic fumes under fire conditions. d) Material can react with metals to produce flammable hydrogen gas. Forms flammable gases with aldehydes, cyanides, mercaptins, and sulfides. 4) Accidental release measures a) use of personal protective equipment. b) Cleanup personnel need personal protection from inhalation and skin/eye . Evacuate and ventilate the area. Prevent spillage from entering drains. Cautiously add water to spill, taking care to avoid splashing and spattering. Neutralize diluted spill with soda ash or lime. Absorb neutralized spill with vermiculite or other inert absorbent material, then place in a suitable c) Created onrelease to the environment may be subject to federal/national or local reporting requirements. Dispose of all waste or cleanup materials in accordance with local regulations. Containers, even when empty, will retain residue and vapors. 5) Handling and storage a) Use with adequate ventilation. Wash thoroughly after using. Keep container closed when not in use. b) Store in cool, dry well ventilated area. Keep away from incompatible materials. Drains for storage or use areas for this material should have retention basins for pH adjustment and dilution of spills.



c) Provide local exhaust, preferably mechanical. d) If necessary use an approved respirator with acid vapor cartridges. e) Wear chemical safety glasses with a face shield for splash protection. f) Wear neoprene or rubber gloves, apron and other protective clothing appropriate to the risk of exposure. g) Provide eyewash stations, quick-drench showers and washing facilities accessible to areas of use and handling. Have supplies and equipment for neutralization and running water available. D] Gypsum (CaSO4.2H2O): 1) General information: a) Exposure to airborne dust may cause immediate or delayed irritation or inflammation. Eye by large amounts of dry powder or splashes of wet gypsum dust may cause eye irritation. b) Direct may cause irritation by mechanical abrasion. c) Gypsum may contain trace amounts of free crystalline silica. Prolonged exposure to respirable free silica can aggravate other lung conditions and cause silicosis, a disabling and potentially fatal lung disease. d) Exposure to gypsum dust may cause irritation to the moist mucous membranes of the nose, throat, and upper respiratory system. 2) First aid measures: a) Immediately flush eyes thoroughly with water. Continue flushing eye for at least 15 minutes, including under lids, to remove all particles. b) Wash skin with cool water and pH-neutral soap or a mild detergent. Seek medical treatment if irritation persists or later develops. c) Remove to fresh air. Seek medical help if coughing and other symptoms do not subside. d) Do not induce vomiting. If conscious, have the victim drink plenty of water and call a physician immediately.



3) Accidental release measures a) Collect dry material using a scoop. Avoid actions that cause dust to become airborne. Avoid inhalation of dust and with skin. b) Wetting of spilled materials may be beneficial to minimize generation of airborne dusts. 4) Handling and Storage: Respirable crystalline silica-containing dust may be generated during processing, handling and storage. Use well equipped ventilation, masks and goggles.



Chapter No 14: Pollution Control Measures 1) Control measures for dust emission from grinding section: Maharashtra Pollution Control Board limit: 150 Mg/Nm3 Measures: a) Vent fan damper should be fully open b) Pulse air pressure should be OK (about 7 Kg/cm2) c) If pulse sir pressure is below 6 Kg/cm2 then instrument and mechanical maintenance department should be informed for corrosive action. d) Differential pressure across dust collector should be less than 150 mmwc e) Checking dust emission from central laboratory f) If dust emission is more than 150 Mg/Nm3 then dust collector bags should be inspected and replaced damaged bags. Control emission of fluorine fumes at work place: Maharashtra Pollution Control Board limit: 25 ppm at stack and below 3 ppm at work place Measures: a) Premixer, digester duct inspection cover should be in closed position b) Closing ducting and launder covers, if covers are open c) Ensure cleaning of exhaust duct daily to avoid choking in exhaust duct d) If ducting is in choked condition, then mechanical maintenance department should be informed to drop the ducting for cleaning. e) Venturi water valve to venturi scrubber should be open



Control measures for minimizing generation of large quantity and bad quality effluent: Measures: To improve effluent quality: a) Isolate equipment to be cleaned b) Recycle slurry to process c) Dilute the balance slurry with water before draining d) Checking that foaming is not taking place in filtrate compartments etc. to prevent acid spillage e) Ensure antifoam dosing in filtrate compartments etc. in case of foaming f) Check no leakage is taking place from phosphoric acid, slurry warman pumps, etc. g) Isolate pump in case of leakage and inform mechanical maintenance department for attending leakage h) Check filter wash system is working in auto mode to prevent acid overflow from filtrate compartments, return acid tank i) If filtrate wash system is not working in auto mode then instrumentation department should be informed j) Analyzing effluent pH, total phosphate and maintain records k) Plant effluent is recycled back in process To reduce in effluent generation quality: a) Ensure dry cleaning of equipment, floors to minimize effluent generation. Water is not to be used for cleaning b) Use only minimum water for final cleaning of equipments if required c) Transferring the sludge collected from equipment after dry cleaning to crystallizer to recover P2O5 content from gypsum Analyze effluent quantity trend and maintain records and take appropriate actions in case of deviations in quality and quantity of effluents.



Chapter No 15: Any Other Related Topic 1. HYDROFLUOSILISIC ACID (BYPRODUCT) SEPARATION: i.

This is the ultimate product of hydrogen fluoride (HF) which is obtained from first reaction as follows 3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O → 6H3PO4 + 10CaSO4 + 1/2H2O + 2HF

ii.

For fluorine recovery the hydrogen fluorine evolved in fluorine in reaction as above may react with excess silica present in rock to form silicon tetra fluoride which when hydrolyzed to form fluosilisic acid. These reactions are 4HF + SiO2 → SiF4 + 2H2O 3SiO4 + 2H2O → 2H2SiF6 + SiO2

iii.

These hydrofluosilisic acid was found to have the following specifications. a. H2SiF6 : 14% min b. P2O5 : 1-1.5% max c. Suspended Solids : 2-3% max

iv.

H2SiF6 has found various applications in numerous field as follows a. In water fluoridation b. In ceramics to increase hardness c. As Disinfectant in copper & brass vessel in breweries d. In building works for hardening cement, plaster of paris, concreate flooring, preserving masonry. e. As general Disinfectant f. In technical plants g. As wood preservative & impregnating compounds h. In electroplating i. In the manufacture of sodium, Ammonium, Magnesium, Zinc, Copper, Barium, Lead & other fluosilicates It serves the following industries: Sodium, silica, glass, cement, fluoride industries.



v.

SEPARATION TECHNIQUE: a. EVAPORATION UNIT: i.

The product acid from storage tank is pumped to vacuum evaporator which is associated with catch all separator on top

ii.

Here the actual separation of phosphoric acid & hydrogen fluorine takes place

iii.

The phosphoric acid is separated from bottom ,HF gasses are obtained from top which reacts with SiO2 to produce silicon tetrafluoride SiF4

iv.

These fumes then ed to next unit

b. FUMES SCRUBBING UNIT: i.

The fumes of SiO4 so obtained is scrubbed here with the help of make-up water which is carries from the condensate tank

ii.

In the scrubbing process SiO4 reacts with water to form H2SiF6, Hydrofluosilisic acid

c. BAROMETRIC CONDENSOR UNIT: i.

Fumes of hydrofluosilisic acid are condensed & pumped to storage tank.

vi.

Material of Construction: The H2SiF6 acid storage tank is made up of carbon steel with inner of the tank is rubber lined

vii.

From here it taken up for transportation to desired destination

2. URANIUM EXTRACTION FROM WET PROCESS PHOSPHORIC ACID: (LIQUID MEMBRANE APPROCH) i.

Uranium occurrence is usually less therefore extraction of uranium is done from WPPA(Wet Process Phosphoric Acid)

ii.

Earlier it was done by solvent extraction process. In 1970’s research on liquid membrane approach for separation of uranium from wet process phosphoric acid was initiated at exxon research &Engg. co.



iii.

Liquid membrane technology utilizes water-in-oil emulsion as vehicle to simultaneous removal of uranium from WPPA feed & concentrate it

iv.

The simultaneous extraction & stripping operation of LM process results in kinetically controlled uranium separation , removing some of limitations of the thermodynamically dominated solvent extraction process

v.

In contrast to solvent extraction LM extraction exhibited an inverse temperature response, insensitivity to phosphoric acid strength of 5-8 m & insensitivity to concentration of complexing agent in the range of 0.070.14 m

WORKING: i.

The process sequence involved emulsifying the aqueous strip phase into an organic phase containing a surfactant &complexing agents

ii.

The emulsion was then dispersed in aqueous feed phase, which contained the species to be removed. During this complexing step, the desired species was transferred & concentrate in emulsified aqueous internal phase

iii.

Upon completion of contracting, the emulsion &raffinate phases are disengaged in gravity settler.

iv.

The last step in LM process is the isolation of the concentrated aqueous internal phase

ADVANTAGES: The unique aspects of this technique was that extraction & stripping were affected simultaneously than sequentially in conventional solvent extraction 3. ENVIRONMENTAL IMPACT OF PHOSPHOGYPSUM: i.

Usually the phosphogypsum obtained from WPPA is disposed on agricultural land

ii.

This phosphogypsum is laden with heavy metals compounds etc. which are capable of detoriating the land quality & capacity

iii.

Heavy metals includes Arsenic, cium, chromium, etc. which are very poisonous if intake will done by plants

iv.

In order to reduce the toxic contents from the phosphogypsum, they are treated with various solvent extraction process or many different technologies



Chapter no. 16: Conclusion Our project at “Rashtriya Chemical and Fertilizers Ltd.” Is “Manufacturing of phosphoric acid using rock phosphate and sulfuric acid as a raw materials” of capacity 150 MTPD. The process is called wet process which gives phosphoric acid of fertilizer grade i.e. of 27 – 30 %. This acid acts as an intermediate raw material in manufacturing of other fertilizers like suphala, ANP, DAP etc. This project has provided us with the opportunity of implementing the knowledge of chemical engineering aspects whichever we had studied through our academic courses into practical considerations. Thus helps us in improving our concepts related to the same. And throughout we cover the topics such as mass balance, energy balance, design of critical equipment, thermodynamics etc. and realize that whatever covered theoretically varied a lot with what happens practically. As this project was guided to us by two of our guides (internal and external) which were bounded with abundance of knowledge had helped us in every way in improving our skills and thought us new things like habit of reading journals, magazines, books, art of making presentation, writing bibliography etc, which is a great present to us and will be helpful throughout our life. We show our gratitude towards them for guiding us and shaping and moulding us to the best. Here we conclude our project by experiencing lots of industrial knowledge and enthusiasm.



Chapter No 17: BIBLIOGRAPHY 1. A. V. Slack, M. Dekker, “ Phosphoric acid”, Arnold publication, vol 1, part 1, page no 1-67, 1968 2. Aimei Chen, Jaiwen Zhu, Bin Wu, Kui Chen, Lijun Ji, “Continuous Melt Suspension Crystallization Of Phosphoric Acid”, Journal of crystallization process & technology 2012, 2, 111- 116 3. Carr J. M., Jr. et al., “Filtration Of Wet Process Superphosphoric Acid”, Patent No US4547295, Oct 15,1995 4. Dr. Helena, “Corrosion resistant alloys for the production of phosphoric acid”, Fertilizer focus May 2014 5. Dr. Joseph A. Megy, “A credible alternative to the WFA process”, Fertilizer international , May – June 2002 page no. 81-89 6. Egon Wiberg & Nils Wiberg, “Inorganic Chemistry”, Wiely publication, 2000, page no 427-440 7. Gary Lee Smith, Jimmy Dwayne Jackson, “Method of filtering phosphate utilizing a rotary table filter or horizontal table filter”, Patent No US7736526B2, Jun 15, 2010 8. J. J. Macharro, J.C.Olveza, A. Larios, H.M. Hemandez – Hernandez, M.E. Alkantara – Garduno, G.Orozco, “Electro dialysis of phosphate in Industrial Grade

Phosphoric

Acid”,

Hindwai

Publishing

corporation,

ISRN

electrochemistry, vol 2013, ID 865727, Oct 31, 2013 9. Kiyoshi Hotta, Fuyuhiko Kubota, “Method for purification of phosphoric acid and high purity of phosphoric acid”, Patent No US6891039B1, Mar 1, 2005 10. krishnan J. Pillai, Lawrence J. Connelly, Oak Lawn, William K. Gustafson, “Method Of Dewatering A Wet Process Phosphoric Acid Slurry”, Patent No US5185135, Feb 9, 2003 11. M. I. Amin, H.M. Kamal & M. M. Gouda. Nuclear Materials Authority, Cairo Egypt, “Decreasing iron content from Egyptian wet process phosphoric acid using organic solvent extraction”, International Journal of advanced research 2014, volume 2, Issue 1, 1019-1026 12. Mollere P., Vivian C. Astley, Jody J. Taravella, Harvey, “Process For Seed Crystal Preparation For Phosphoric Acid Crystallization”, Patent No US4657559, Apr 14, 2001



13. N. S. Awwad, Y. A. El-Nadi & Mostafa M. Hamed, “Process for purification & Extraction of phosphoric acid”, Arab journal of Nuclear Science & Application, 46(1), (66-77) 2013 14. “Phosphoric acid production by Dihydrate-hemihydrate 32-35% P2O5 process (Central Prayon Process)”, Fertilizer International 441, March-April 2011 15. T. Austin, “Shreves Chemical Process Industries”, Tata Mcgraw Hill Publication, 5th edition, 2012, page no 1-21 (phosphorous industry) 16. HRD department, RCF Ltd., “Trombay Process and Services”, 2009 edition, Page no. 178-188, 224-238 17. http://ion.chem.usu.edu/~sbialkow/classes/3600//overheads/H3A/H3A.html, 11/9/2014 18. Indian Minerals Yearbook, “Apatite and Rock Phasphate”, 51st edition, 2012 19. Paul Kucera & C. G. Weyrauch, “Production of phosphoric acid by two stage crystallization process”, publication no. US2014/0255287 A1, Sep 11, 2014 20. Pierre Bicker, M. Dekker, “Phosphoric Acid And Phosphate: Raw Materials, Technology & Economics of Wet Process”, CRC press, 2nd edition, 1988, Page no 1-8, 12-31 21. Rao M. Gopala, M. Sitting, “Dryden’s Outline for Chemical Technology”, 2nd edition, page no 141-149 22. Robert Noyes, “Phosphoric Acid by Wet Process”, Noyes Development corporation, 1962, page no. 1-11 23. Rodney Gilmour, “Phosphoric Acid Purification, Uses, Technology & Economics”, CRC press, 2014, page no. 1-25, 37-46 24. S. J. Kauwenbergh, “World Phosphate Rock Reserves and Resources”, IFDC publication, page no 3-13 25. The Fertilizer Association Of India, “Handbook of Fertilizer and Technology”, 6th edition, 2001, page no. 77-87, 123-131 26. Vasant Gowarikar, V. N. Krishnamurti, Kalyani Paranjpe, Manik Paranjpe, “Fertilizer Encyclopedia”, Wiley-Blackwell, 1st edition, 2009 27. William H. Waggman, “Phosphoric Acid and Phosphatic Fertilizers”, Reinhold Publishing Corporation, 2nd edition, 1962 28. R. K. Sinnot, “Chemical Engineering Design”, Volume 6, 4th edition, 2005, Page no. 34-130, 194-240, 892-906 29. www.livestrong.com, 20/9/2014 Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015]                  Page | 135


30. http://www.wikipedia.org/wiki/phosphoricacid.html, 20/9/2012 31. http://www.aerofiltri.it/phosphate.html, 20/9/2014


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