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There's gold in that there dust
By David Derbyshire
TURN again, Dick Whittington. The streets of London may not be paved with gold, but they are dusted with platinum. Among the discarded cigarette butts, chewing gum and burger wrappers lie minute particles of some of the world's most precious metals.
Now Veolia, the waste management firm, is to extract valuable palladium from the tens of thousands of tons of dust recovered from the roads and pavements of Britain's cities each year.
Worth about £15 a gram, the metal is used in mobile phones, computers, dental fillings and white gold. It is also used in catalytic converters in vehicles, where it coats a ceramic structure on the exhaust pipe and converts harmful hydrocarbons, carbon monoxide and oxides of nitrogen into harmless compounds.
Platinum and rhodium, another valuable metal, can reduce the level of the harmful emissions as well. Minuscule quantities of each metal enter the atmosphere along with the clean emissions from the exhaust before settling on the streets.
If the scheme to extract palladium proves successful next year, the company plans to mine other metals from the debris.
Soaring landfill taxes have now made stronger the economic case for recycling material from street waste.
Richard Kirkman, head of technology and waste at Veolia, said: "We have brought together two technologies — conventional soil washing, which removes plastics and metals and glass from street sweepings, and a technique that we use to remove palladium from the fine dust that is left after we have washed the soil.
"The precious metals are in trace amounts but they are quite valuable."
A new processing plant near Rugby, in the West Midlands, will sift through 30,000 tons of road sweepings each year, allowing 90% of the debris to be recycled.
Large stones, glass, plastic and cans will be removed before magnets pick out steel and iron. Aluminium cans are then recovered, while vibrating conveyor belts partition smaller pieces of plastic, twigs and grit.
The remnants are mixed with water, filtered, shaken, spun and sieved while "smart sponges", bags filled with absorbent polymers, remove oils.
The resulting fine grey dust is treated with chemicals and passed through membranes to extract the palladium. Veolia estimates it will extract 5kg of the metal annually, worth £80,000 at current prices.
About 300 tons of palladium are mined each year, most from sites in South Africa, Russia and Zimbabwe. Almost two-thirds is used in catalytic converters.
Although recoverable amounts are minuscule, they are similar to the concentrations extracted from mined ore.
Angela Murray, of Birmingham University, has a spin-off company called Roads to Riches which will launch another scheme to recover metals from waste next year. "In South Africa, ore contains palladium at two to 10 parts per million,' she said. "We can produce material from road dust that contains seven or eight parts per million."
The volume of metal recycled in Britain fell from 15m tons in 2007 to 13m tons last year.
Ian Hetherington, director general of the British Metals Recycling Association, said: "There is much more metal out there that can be captured technically. We are very interested in seeking out new ways of doing it."
|Copyright David Derbyshire 2011|