Scrap Shredding

 January 10, 2016 

There are many scrap yards which use gigantic shredders to process scrap metal from flattened automobiles, household appliances, and a wide range of other steel industrial and consumer equipment. These shredders are driven by motors ranging from a few thousand up to ten thousand horsepower.

Years ago these items were shredded with snag-tooth type low rpm shredders. These were pre-shredders. They went out in the 1960's with the invention, by someone named Newell, of the giant swing hammer shredders now in service everywhere.

Along with the scrap, a large flow of water is sprayed into the shredders. The water adds important lubricity, and it is also used to cool the heating which comes with the shredding operation (all those horsepower go into heat enegy). The water is also added partly to suppress fire which occurs from time to time when they hit a gasoline tank or propane bottle.

California had regulated VOC emissions from the shredders, so gasses from the shredder must go through a combustion furnace. This is not required in other states. California has equally exotic regulations governing disposal of sludge from the wastewater; it must combined with cement as part of chemical treatment.

The flow from the shredder, with all its water, is run past magnets to separate ferrous metal. That leaves a flow of fluff and non-ferrous. This flow is screened into coarse, medium and fines. These three flows are run past eddy current separators. The non-ferrous scrap is repelled, and it is thrown the furthest from the end of the belt conveyor. The fluff falls a shorter distance, thus separating the flows. Magnets hold any residual ferrous metal to the belt, and it is scraped off into a third flow.

The fluff is made up of the plastic, rubber hoses and moldings, wood scraps, foam rubber from car seats, fabric, headliners, insulation, and other non-metallic trash. This is from the mopeds, busses, stoves, washing machines, bed springs and everything else which is fed into the shredder.

It is interesting that flattened cars in England have 2% wood by weight. People must be disposing of pallets and scrap lumber along with the cars.

Vincent's interest was in separating the water from the fluff. This was an extremely challenging application for a screw press. Some tramp metal is always entrained with the fluff. And some of the fluff can be thick plastic boards, conveyor belting, hydraulic hoses, and other things which tended to jam a press unless it had high torque capacity. These larger pieces required the screw press to have a large diameter screw.

The material is also very abrasive, which requires restoring hardsurfacing to the screw of the press every month or so.

The material has high freeness – the water tends to fall easily from it. This allowed a screw press with a short L/D (length of the screen divided by the diameter of the screw) to be used.

Unfortunately for Vincent, a solution more practical than a screw press turned out to be a large industrial compactor.

It was fun while it lasted.

Still, there are other viable applications in the scrap yard. These include dewatering WWTP sludge and recovering oil from automotive oil filters. Some yards add magnetite to the fluff in order to separate certain forms of plastic. It was found that the magnetite, which is costly, could be separated into the press liquor.