Each year, tens of thousands of old car bodies, washing machines and refrigerators disappear into huge shredding machines at metal-recycling plants in Australia’s capital cities.

A sludgy mess

Electromagnets extract the large steel fragments for melting in electric arc furnaces and further separation processes remove larger fragments of non-magnetic metals, like brass and aluminium.

The separation process leaves large volumes of watery sludge containing small particles of steel, copper, brass, lead aluminium and other non-metallic compounds. Recyclers typically send the sludge for disposal in landfill or toxic-waste dumps.

A new approach

CSIRO’s Mr Warren Bruckard has been investigating whether metals recyclers could apply mineral processing techniques to recover potentially valuable products from the sludge, while at the same time reducing its volume and toxicity.

Mr Bruckard’s team conducted a detailed analysis of sludge from a recycling plant operated by Smorgon Steel subsidiary Metalcorp Recyclers at Laverton, in Victoria. The team determined that it contained approximately 3.1 kilogram of recoverable copper and 33 kilograms of clean steel per tonne of dry feed.

The proposed process for extracting the metal involves the following steps:

• hydraulic classifying to remove material such as plastic, foam and rubber
• magnetic separation to concentrate clean steel
• gravity concentration in a jig to separate low-density materials like calcite and glass from the high-density metals like copper, lead and brass.

Paying for itself

Mr Bruckard said that, at some metal recycling plants, the value of the recovered products could more than offset the cost of installing and operating the add-on process in the future.

He believes metal recyclers will use this or similar technology in the future, as landfill costs continue to rise and legislation to regulate toxic wastes becomes more stringent.