Foaming Index 4731j

Characteristics of Foaming Slag in Smelting Reduction Processes Dr S K Dutta, Member R Sah, Non-member Smelting reduction processes, without using coke, are alternative ironmaking technologies for production of hot metal. Gases, which are generated due to reaction, cannot be removed from the reactor without foaming of the slag. Characteristics of foaming slag are important in the smelting reduction processes. Main foaming parameters are foaming index and foam life.In this paper measurement of foaming index and influence of additives on foam are discussed. Keywords : Alternate ironmaking ; Non-coking coal ; Slag foam ; Foaming index ; Foam life

INTRODUCTION Blast furnace (BF) ironmaking technology has dominated the world scenario as most economic and widespread resource of iron used in steelmaking. Till today, BFs have played a major role in achieving high degree of gas utilisation. However, this dominancy of BF technology has been facing problems due to shortage of metallurgical coke and higher investment cost1. As shown in Table 1, the availability of coking coal in India is limited (15.4% only of the total reserve). while it has a huge reserve of non-coking coal2. The need of an alternative ironmaking technology arises to complement BF process in order to produce hot metal using noncoking coal. Such processes are known as Smelting Reduction (SR) Processes. The term smelting reduction is used to designate processes for the production of hot metal without using metallurgical coke3. Recently, the smelting reduction process, for production of liquid iron, has received considerable attention due to its many advantages, such as lower capital cost (due to absence of auxiliary units), high production rate, and the diversity of charging materials4. In most of the smelting reduction processes, coal and iron ore are injected into an iron bath, the main reactions are the cracking of the coal and the reduction of iron oxide in the slag phase by solid carbon and carbon dissolved in metal. Therefore, a large amount of CO and H2 gases are evolved when a high production rate is maintained. The gases at the slag-metal or slag-carbon interface, as a result, form bubbles and the volume of the slag increases extensively due to foaming5, ie, the gases cannot come out from the reactor through the slag phase without foaming. Slag foams are formed, when gas bubbles entrapped in the slag can not readily coalesce and foam

Reserves, Mt 28 031 148 284 5 978 182 293

FOAMING PARAMETERS Foaming Index Considerable research activities were concentrated toward understanding the foaming behaviour of slag in the past decade, the major contribution coming from Fruehan and co-workers4,5,7,8. They measured the foaming behavior of different slags. To quantify the foaming behaviour, Ito and Fruehan5 defined the foaming index (Σ, s) of the slag as s

Percentage of Total 15.4 81.3 3.3 100.0

Dr S K Dutta is with the Metallurgical Engineering Department, Faculty of Technology and Engineering, M S University of Baroda, Vadodara 390 001; and R Sah is with the Mechanical Engineering Department, Institute of Technology, Nirma University, Ahmedabad. This paper was received on September 12, 2005. Written discussion on the paper will be received until January 31, 2006.

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Foaming slag provides a large surface area and the chemical reactions proceed more favourably. Slag foaming becomes the production rate limiting step in the process. At a high production rate, the slag can foam out of the reactor, which is somewhat similar to the slopping phenomenon in oxygen steelmaking4. The foaming slag is also important, because it is the medium for post combustion and heat transfer, which is the key to an energy efficient process. Hence, foamed slag plays an important role in heat transfer from the post combustion flame to the bulk slag in the reactor. Therefore, slag foaming is important in the smelting reduction process, and it is critical to understand the fundamental features of slag foaming in the process.

∑ = h / Vg

Table 1 Coal reserves in India2 Type of Coal Coking coal Non-coking coal Lignite Total

comprises a system of tightly packed bubbles separated from one another by thin films of liquid slag. In true foam, the liquid is eliminated from the films separating the bubbles by drainage and thus a high viscosity, by retarding the rate of drainage, tends to stabilise the foam6. On the other hand, the energy requirement for formation of foam increases with increasing surface tension, and hence, low surface tensions are favourable for both formation and durability of foam.

(1)

where h is the height (cm) of the foam at steady state when gas with superficial velocity ( V gs , cm /s) is ed through it. s The superficial gas velocity ( V g ) is defined as

V gs = Q g / A

(2)

where Q g is a volumetric gas flow rate (cm3 /s) and A is crosssectional area (cm2) of the reactor. The superficial gas velocity is also correlated to the void function (α), IE(I) Journal-MM

volumetric fraction of gas, and the actual gas velocity ( V g ,cm /s) V gs = α V g (3) The foam height (h) is expressed as a function of void function and foam layer thickness (L , cm) h =αL (4)

Therefore, Rate of volume change due to bubble rupture = k NVb

(8)

where k, N and Vb are the rate constant for bubble rupture (s-1), total number of bubbles, and average volume of a gas bubble (cm3), respectively. The total volume of foam and the bubbles volume are can be related by:

Finally, foaming index is expressed in of the foam layer and actual gas velocity as ∑ = L / Vg (5)

α = NVb / V (9) where α is the average void fraction and V is the volume of foam (cm3).

From equation (5) it is clear that foaming index means the average gas travelling time through the foamed layer. This equation is valid when void function (α) is independent of foam height (h), ie, void function can be assumed as constant. The foaming index was found to be independent of reactor size for reactor diameter greater than 3 cm and depends only on the physical properties of the slag4. Knowing the foaming index of the slag, the gas evolution rate, and the reactor size, the foam height in any process can be calculated.

Using equations (8) and (9), equation (7) can be written as:

The foaming index means the foaming ability of the slag in the foam caused by blowing gas. So, the foaming index has been correlated as a function of the physical properties such as the density, viscosity, and surface tension of the liquid slag. Zhang and Fruehan7 have demonstrated that the foaming index is also inversely proportional to the gas bubble size. For dimensional analysis, Jiang and Fruehan4 have assumed that the foaming index is a function of all the variables and dimensional constants that may affect the foaming index (Σ). Therefore, ∑ = f ( µ, σ, ρ, d b )

(6)

where µ, σ, and ρ are the viscosity (g /cm-s), surface tension (g /s2), and density (g /cm3) of slag respectively, and db is the gas bubble diameter (cm).

d V / d t = Q − kαV

(10)

where Q is the rate of gas generation or injection (cm3/s). If foam is produced in a reactor of uniform cross sectional area, the equation (10) can be written as:

d h / d t = V gs − kαh

(11)

Foaming index is defined by equation (1), so equation (11) becomes:

d h / d t = ( h / ∑ ) − kαh

(12)

Therefore, at steady state, equation (12) can be written as:

∑ = 1/( αk )

(13)

The bubble rupture takes place due to drainage of the liquid. The average foam life (τ, s) is defined as10

τ = (1/V0 )∫ t d V

(14)

where V0 is an initial liquid volume (cm3) in foam. Again, V = V0 (1 − e −kt )

(15)

Therefore, Foam Life The foam volume is determined by the balance equation9 Rate of change of foam volume = {(rate of gas generation or injection)-(rate of volume change due to bubble rupture)} (7) The gas bubble rupture on the top layer of foam causes a decrease in foam volume because of gas escape. Bubble rupture inside the foam leads to bubble coalescence and, consequently, a change in the liquid film thickness between the bubbles and their packing. Coalescence of bubbles also leads to a decrease in foam volume. Besides, nonuniform bubbles, which are produced by coalescence, make the foam unstable. Hence, the bubble rupture rate can be assumed to be proportional to the number of bubbles. Assume that the kinetics of bubble ruptures follow first order rate equation. Vol 86, October 2005

d V / d t = kV0 e −kt

(16)

Combining equations (14) and (16) can be written as: τ = k ∫ t e −kt d t = 1/ k

(17)

Foam life (τ) is the time (s) required to drain the liquid entrapped between two consecutive layers of bubbles, the rate constant (k) for bubble rupture is inversely proportional to foam life (τ). Again by combining equations (13) and (17), can be written as:

τ = α∑

(18)

Equation (18) shows the relationship between foaming index and foam life. For an ideal slag (ie, a slag of constant void fraction) the foaming index is equal to the average foam life. 55

Fruehan and co-workers5,8 used a molybdenum disilicide resistance furnace for experiment. Slag was taken in an alumina crucible (41 mm intenal diameter and 300 mm height). Argon gas was introduced into the molten slag through an alumina tube (1.57 mm intenal diameter). When foam height reached a steady state level, the foam-gas interface was detected by two molybdenum wire probes (0.76 mm φ). They observed that the foam height increases linearly with the increasing superficial gas velocity (Figure 1). The foaming index is obtained from the slope of the line shown in Figure 1. Similar experiments were also carried out at different temperatures. As shown in Figure 2, the foaming index decreases with increasing temperature because of a decrease in viscosity and an increase in surface tension. Similar observations were also made by Wu, et al 11. Ito and Fruehan5 found that the foaming index was independent of reactor diameter (>3.2 cm) and wall effects were small. Foaming index decreased with increasing basicity (B = CaO / SiO2) upto a maximum (B=1.2 to 1.22) and then increased (Figure 3) at 1673K due to presence of second phase particles (CaO or 2CaO. SiO2). The

surface tension increased and viscosity decreased with increasing CaO in slag. Therefore, low surface tension and high viscosity stabilised the slag foam. On the other hand, foaming index increased with increasing basicity, when basicity was greater than the liquidus composition. This was because solid particles such as 2CaO. SiO2 and CaO precipitated at higher CaO content and the particles significantly increased the foam stability. Therefore, the precipitation of second phase particles had a larger effect than the increase in surface tension and decrease in viscosity on foam stability. Table 2 shows the laboratory experimental data of foaming index for smelting reduction slag at 1773 K. 20 CaO-SiO2-FeO-Al2O3 FeO=30%, Al2O3=3%-5%

10

Σ,s

MEASUREMENT OF FOAMING INDEX

5 1573 K 1673 K

2 1

2.0

1.5

1.0

0.5

0

1.50

Foaming height, cm

CaO/(SiO2+Al2O3) 1.25

Figure 3 Relation between foaming index Σ and the basicity ratio of the slag at 1573K and 1673 K

1.00

Table 2 Foaming Index for Smelting Reduction Slag at 1773 K

0.75

Basicity (CaO/SiO 2 )

Slag Composition

0.50 1.00

30% CaO 60% SiO2 -

1.25

-

0.50 0.25 0.00 0.00

0.50

1.00 2.00 2.50 1.50 Superficial gas velocity, cm/s Figure 1 The relation between foam height and gas flow rate for a 48 % CaO - 32 % SiO2 -10 % Al 2O3 - 10 % FeO slag at 1873 K 1.50

1.50

45% CaO 30% SiO2 10% MgO 15% Al2O3

1.10

37.2% CaO 33.8% SiO2 18% MgO 11% Al2O3 41.2% CaO 25.8% SiO2 8% MgO 15% Al2O3 48.7% CaO 24.3% SiO2 7% MgO 47.7% CaO 18.3% SiO2 9% MgO 10% Al2O3

Forming index

1.25

1.00 1.60 0.75 2.00 0.50 1700

1850 1800 1900 Temperature, K Figure 2 Effect of temperature on foaming index for a slag containing 48% CaO, 32% SiO2, 10% Al 2O3, and 10% FeO

56

1750

2.60

Addition to the Slag, % 10% CaF2 5% 7.5% 10% 12.5% 15% 0% 3% 5% 7.5% 10% 15% 0% 1% 3% 6% 9%

FeO FeO FeO FeO FeO FeO FeO FeO FeO FeO FeO FeO FeO FeO FeO FeO -

Foaming Σ, Index (Σ, Σ,s)

Reference

2.000 1.400 1.200 0.900 0.800 0.750 0.600 1.300 0.900 0.800 0.800 0.700 2.900 2.000 1.600 1.300 1.200

Jiang and Fruehan 4

0.387

1.073 Wu, et al 20% CaF2

0.681

20% CaF2

1.170

11

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Jiang and Fruehan 4 have conducted slag forming measurements in of the foaming index on reduction smelting slags (CaO - SiO2 - FeO, CaO - SiO2 - MgO - Al2O3 - FeO) at 1773 K and found that the slag foam stability decreases with increasing FeO (>2 %) content and basicity. For the slag system (CaO - SiO2 - FeO), no stable foam was observed at very low FeO content (< 2%). As percent of FeO increases, the slag foaming index goes through a maximum and then decreases. Foams formed from gases, resulting from chemical reactions on metal surfaces, have significantly smaller bubbles and more stability.

Foam

Pellet

INFLUENCE OF ADDITIVES ON FOAM

Ito and Fruehan 5 studied the effect of P2O5, sulphur, MgO, and CaF2 on foaming of slag. Potassium (K), phosphorous (P), and sulphur (S) are surface active components, which lower the surface tension of the slag. P2O5 slightly increases foaming index (Σ) whereas marginally decreases foaming index indicating surface tension alone does not determine slag foamability. CaF2 decreased foaming index by lowering the viscosity of the slag. Large addition of CaF2 significantly decreases the foam stability by increasing CaO solubility and consequently dissolving some of the second phase particles. MgO increases foaming index probably because it increases the amount of solid particles in the slag. Zhang and Fruehan 7 found that the anti-foam effect of coke or coal char particles was primarily contributed by the non-wetting nature of the carbonaceous materials with the liquid slag. Wu, et al 11 also investigated foaming behaviour of slag with addition of additives such as coal, coke, graphite and CaO. The effect of different coke size on the foam behaviour of slag at 1773 K is shown in Figure 5. The foam height increases with fine powder coke (76 µm and 105 µm) and decreases with grain coke (1 mm and 3 mm). Effect of number of particle and size of coke on foaming index for laboratory experimental data is shown in Table 3 7. In the smelting reduction process of the thick slag layer, it is very important to keep slag height stable without abnormal slag foaming. Adding carbonaceous material can control the slag foaming. Vol 86, October 2005

Dense-slag

Figure 4 Iron ore pellet in foaming slag Table 3 Effect of Number of Particle and Size of Coke on Foaming Index at 0.5 Slag Basicity7 Diameter of Coke Particle, mm 3

Number of Particles 0 2 0 1 4 0 1 5 0 2 4

6

8

10

6

Foaming Index (Σ, s) 1.65 1.10 1.80 0.90 0.62 3.60 1.30 0.51 2.00 0.75 0.47

Added 76 µm coke Added 110 µm coke No additive Added 1 µm coke Added 3 µm coke

5 Foam height, ∆h, cm

Coke can reduce slag foaming in steelmaking processes. It was reported that top injection of coke was very effective in controlling excess foaming during smelting reduction of iron chrome ore. The use of carbonaceous particles in controlling foaming had been experimented on 1 t smelting reduction furnace at Sakai Works, Nippon Steel Corporation, Osaka-fu, Japan as pilot scale bath-smelting experiments12. Zhang and Fruehan 7 observed that the foam height was found to decrease significantly with the increase of the ratio of the carbonaceous particles to that of the slag. X-ray examinations showed that small gas bubbles ruptured and spread over the surface of a coke particle present in the slag. Then several spread bubbles coalesced and evolved from the top of the coke particle as a single larger bubble. Wettable particles showed a completely different behaviour when interacting with the foaming slag. The X-ray images of an iron ore pellet in the foamed slag is shown in Figure 4 7. The iron ore pellet is totally immersed in the foaming slag. The foam grew and ed the pellet without any gas bubbles being ruptured or coalesced, even when some of the mechanical movement was applied to the pellet. The wettability of the solid particle with the liquid slag plays a key role in slag foaming.

4 3 2 1 0

2 4 3 Flow rate,V, cm-s-1 Figure 5 Effect of various coke sizes on form behavior of sample slag at 1773 K Hara and Ogino 13 also studied the effect of surface active components 0

1

on foaming of slag. They found that vigorous foaming appears when the slag contains components, which stabilize the bubble, especially surface active components such as SiO2, P2O5, and CaF2. 57

The mechanism of the stabilization of the foam is considered to be a surface tension driven flow, namely, the Marangoni effect. This effect also plays an important role in the suppression of foaming by coke addition. SUMMARY The slag foaming is an important factor for the smelting reduction process. To quantify the foaming behavior, the foaming index (S) of the slag is measured. The foaming index means the foaming ability of the slag in the foam caused by injecting of gas. For an ideal slag (ie, a slag of constant void fraction) the foaming index is equal to the average foam life. The control of the foaming index is required for steady state operation in the smelting reduction process.

REFERENCES 1. S K Dutta and R Sah. Proce of Asia Steel Inter Conf, vol I, April 9 - 12, 2003, Jamshedpur, p 1.d.4.1. 2. P Bhattacharya, S S Chatterjee, B N Singh and S Prasad. Proce of Inter Conf on Alternative Routes of Iron and Steelmaking, September 15 - 17, 1999, Perth, Australia, p 151. 3. H A Fine, R J Fruehan, D Janke and R Steffen. Steel Research, vol 60, nos 3 and 4, March-April 1989, p 188. 4. R Jiang and R J Fruehan. Metall Trans B, vol 22B, August 1991, p 481. 5. K Ito and R J Fruehan. Steel Research, vol 60, nos 3 and 4, March-April 1989, p 151. 6. D R Gaskell. Steel Research, vol 60, nos 3 and 4, March-April 1989, p 182.

Foaming index decreases with increasing basicity up to a maximum and then increases due to presence of second phase particles (CaO or 2CaO.SiO2). The slag foam stability decreases with increasing FeO content and basicity. The foaming index decreases with increasing temperature because of a decrease in viscosity and an increase in surface tension. The anti-foam effect of coke or coal char particles was primarily contributed by the non-wetting nature of the carbonaceous materials with the liquid slag. Adding carbonaceous material can control the slag foaming. Wettability between the particle and the slag is the key factor in determining the ability of the particle to control foaming of slag.

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7. Y Zhang and R J Fruehan. Metall Trans B, vol 26B, August 1995, p 813. 8. B Ozturk and R J Fruehan. Metall Trans B, vol 26B, October 1995, p 1086 . 9. A K Lahiri and S Seetharaman. Metall Trans B, vol 33B, June 2002, p 499. 10. J J Bikerman. ‘Foams’. Springer-Verlag, New York, USA, 1973, p 168. 11. K Wu, W Qian, S Chu, Q Niu and H Luo. Iron Steel Inst Jpn Int, vol 40, no 10, October 2000, p 954. 12. Y Ogawa, H Katayama, H Hirata, N Tokumitsu and M Yamauchi. Iron Steel Inst Jpn Int, vol 32, no 1, January 1992, p 87. 13. S Hara and K Ogino. Iron Steel Inst Jpn Int, vol 32, no 1, January 1992, p 81.

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