Cryogenic Recycling 1p2y6w

NEED FOR RECYCLING  The problem of processing scrap is a major

concern with the ever increasing production and population.  Automobile industry is a major contributor of scrap, especially scrap steel and rubber (tires).  Rubber (tires) recycling not only has economic value, but also helps reduce the disposal problem of a worldwide production of more than 300 million tires per year, leading to land filling.  From coast-to-coast and also in the heartland, landfill costs and tipping fees have risen dramatically in the last few years and are expected to continue to rise.

NEED FOR RECYCLING (contd…)  A number of landfills have closed their doors to tires and scrap steel, already making re-utilization of scrap by manufacturers a viable economic alternative.  Concern over nitrosoamines, heavy metal contamination, and concern about effects on the water table of deteriorating rubber products further drives the incentive toward recycling of these materials.

 From an environmental standpoint, cryogenic recycling makes solid economic sense and is in the best interest of both manufacturer and consumer.

WASTE TO A USEFUL RAW MATERIAL  In rubber processing there will always

be some scrap, which needs to be minimized, otherwise it will lead to land filling.  The cryogenic recycling system can economically turn the scrap into a raw material again - even if we produce as little rubber scrap as 200 kg per day.  It is widely believed that the finer the rubber powder reintroduced into compound, the less visible the defects and change of properties.

WASTE TO A USEFUL RAW MATERIAL (contd…)  Cryogenic treatment processes can reduce rubber scrap to less than 100 micron powder.

 The rubber granules are put through a highly efficient cooling conveyor, sized for particular purpose.  The liquid nitrogen embrittles the rubber crushing it, before it is fed into the mill for grinding.  Steel scraps in auto-industries can also be treated

by cooling it to cryogenic temperatures where impact resistance offered is minimal and the scraps can easily be shredded.

TEMPERATURE EFFECT ON FAILURE RATE  Straining rate is the rate

of change of shape of the object without it breaking.  It can be observed from the graph that the strain rate decreases with decrease in temperature.  The advantage of this fact could be utilised in the cryogenic recycling process

SCRAP SEPARATION PRINCIPLE  Many scrap materials are mixtures of materials having

vastly different ductility at common low temperatures which permit separation.  At 210K, rubber is very brittle & steel is not as brittle.  Thus, an automobile tire can be cryogenically crushed

to give rubber powder and separated steel.  Copper, aluminium and stainless steel retain their ductility at 145K, but steel does not.  Thus articles in a cryogenic crusher would provide steel particles and copper uncrushed which can be separated by various screening and magnetic processes.

SCRAP RECYCLING PROCESS

RECYCLING PROCESS  The cryogen, liquid nitrogen at 77K is in the cryo tank

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and is thermally insulated. A conveyor is used to charge the scrap into the tank through a tunnel. It is charged to the cryogen bath and cooled to 145K. Cooling of the scrap simultaneously leads to vaporisation of liquid nitrogen. Then,the scrap is removed from the bath and is broken by a breaker. A curtain is used to seal the cryogen on the withdrawal of scrap steel from the tank.

RECYCLING PROCESS (contd…)  The scrap is cooled counter currently with the vaporised effluent nitrogen which rises from the

cryo tank and is exhausted through the tunnel.  Additional liquid nitrogen may be introduced to make up for the losses in the system.  A gas recycle duct is employed with a blower to recycle refrigerant gas from the tunnel back to the refrigeration recovery tunnel.  If, we consider steel for instance, the required refrigeration is approximately1gm of liquid nitrogen for 45.5gm of steel.

UTILITIES OF RECYCLING  The recycled scrap can be used as a raw material in

various applications.  About 12 million scrap tires a year are made into rubberised asphalt which is used to resurface federal interstates and highways in many states in the USA.  In northern Virginia, road builders have combined shredded tire rubber with cement to form ‘whisper walls’ that reflect sound waves from traffic and reduces noise level, sparing local residents’ ears.  They are also used for tracks, RR crossings and tennis courts. Also, some are burned in coal fires for power and cement plants reduces mercury pollution.

FUTURE SCOPE  As opportunities for disposal of scrap materials diminish, similar opportunities arise by necessity

for alternative uses of recycled rubber materials.  Advances in state-of-the-art partitioning and separation technology will allow particle size reduction costs.  Printed circuit boards (PCBs) in tvs, computers, & other electronic items produce lot of scrap. The rich content of precious metals like gold, silver, palladium in PCB scraps provides a strong economic justification for their recycling.

FUTURE SCOPE (contd…)  The liquefied gas industry is less susceptible to price

increases associated with petroleum dependent materials and technologies.  Thus increases in processing costs in the cryogenic recycling industry are forecast to increase at a rate lower than the general increase in the majority of petroleum-derived materials used in rubber.  This encourages utilization of the technology for continued cost reduction and performance improvement.

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