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CRYOGENIC RECYCLING. 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).
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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.
RECYCLING PROCESS • The cryogen, liquid nitrogen at 77K is in the cryo tank 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.
