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24/09/11

The streets are paved with... platinum

One ambitious recycling company believes that the gutters of London contain neglected treasures, reports David Derbyshire

The streets of London may not be paved with gold, but they are dusted with platinum. For among the discarded cigarette butts, burger wrappers and chewing gum littering urban streets lie some of the world's most precious metals.

The minuscule particles of platinum — and related "platinum group metals" such as rhodium — are spewed from car exhausts fitted with catalytic converters.

Until recently, this invisible dusting of riches was judged too difficult and costly to retrieve. Now one of the country's biggest waste firms has unveiled plans to extract precious metals from some of the debris and dust. Next year, the UK's first dedicated plant will begin processing thousands of tonnes of street sweepings. Prospecting in the gutter may sound far-fetched. But at a time of soaring landfill taxes, engineers say recovering such metals has finally become economic.

Richard Kirkman, head of technology and waste at the recycling firm Veolia, said: "We have brought together two technologies — conventional soil washing, which removes plastics and metals and glass from street sweepings, and a technique which 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."

Palladium is a rare lustrous silverywhite metal discovered in 1803 by London chemist William Hyde Wollaston, who named it after the asteroid Pallas which had been found the previous year.

About 300 tonnes is produced each year, mostly in South Africa, Russia and Canada. An estimated 24,000 tonnes remains in the earth's crust. Some 60% ends up in catalytic converters — the devices that remove some of the most toxic pollutants from engine exhausts.

Roughly a fifth of the world's supply of palladium goes into mobile phones, computers and flatscreen televisions, while the rest goes into dental fillings and jewellery. It is used to make white gold.

Demand and prices have soared in the past few decades as catalytic converters have become compulsory round the world. Today it sells for about 500 an ounce.

About 70% of the precious metals in catalytic converters end up on the roadside as dust. Veolia is building its processing plant at Ling Hall near Rugby, Warwickshire, to sift through road sweepings from main cities and roads.

The unit will first sieve out the larger pieces of stone, glass and plastic and rubbish from the sweepings. Magnets will pick out steel and iron, while aluminium cans will be extracted using a device called an eddy current separator. Vibrating conveyor belts will then joggle the rest of the waste into plastics, twigs and grit.

Once the large recyclables have been taken away, the smaller grit and dust will be mixed with water and filtered, shaken, spun and sieved again. Oils will be removed using "smart sponges" — bags filled with absorbent polymers. Finally, the fine grey dust that is left can be processed chemically and put through membranes to extract the palladium.

By the end of the process, at least 90% of the road sweepings will have been turned into useful products.

Aside from the metals and recyclables, the plant will produce compost from the leaves and twigs, aggregates for the construction industry from glass fragments and stones, and soil conditioner. The remaining mixed waste made of useless fragments of plastics and laminates will go to landfill.

The company estimates that its new plant will be able to sort 30,000 tonnes of road dust a year and yield 5kg of palladium. At current prices, that will be worth approximately 80,000.

The Veolia trials will collect only palladium. However, if they are a success, the company will look at mining street dust for other valuable metals, including platinum.

Veolia is currently testing the technology.

Over the August bank holiday, its sweepers collected waste from the streets of Notting Hill, west London, after the annual carnival. That dust is now being processed at its existing palladium recovery plant in Ellesmere Port, Cheshire.

Other engineers have their sights set on the precious metals lying in the gutter. Angela Murray of Birmingham University will start work next year on a trial to remove platinum and rhodium, as well as palladium, from road dust collected in the West Midlands.

Her spin-off company, called Roads to Riches, has patented its own processing technique. "We have shown that it can be economic to do this," she said.

The amount of precious metal collected in road dust varies from street to street and from month to month. But a single, compact sweeper can collect about two tonnes of dust and debris. That can contain metals worth 5 to 10.

"They all come back to the central depots at the moment so it won't cost any more money to collect the dust. And not sending it to landfill will save money," Murray said.

The amounts of metal may be tiny but they are similar to the concentrations dug up in ore from mines.

"In South Africa, ore contains palladium at two to ten parts per million," said Murray. "We can produce material from road dust that contains seven or eight parts per million."

There isn't just an economic case for recovering palladium and other precious metals — there's also a political one. Approximately 75% of all platinum group metals are produced in South Africa. Russia and Zimbabwe are also exporters.

"One country has a near monopoly," said Murray. "But the South African mines also suffer from problems with power outages. Europe is extremely vulnerable to price fluctuation."

This month the British Geological Survey produced an index to highlight the chemical elements most at risk. The BGS looked at the abundance of elements in the earth's crust, their economic importance, where they are mined and the political stability of those countries. Platinum group metals, including palladium, were ranked among the most vulnerable.

The search for new sources of metals comes as the volume of metal recycling in this country fell from 15m tonnes in 2007 to 13m tonnes last year, according to the British Metals Recycling Association.

Ian Hetherington, the director-general, blamed the economic downturn. "If people aren't buying new fridges they are not scrapping old ones. It's the same for cars — the motor industry activity is a key influence on recycling. There is much more metal out there in our society that can be captured technically. We are very interested in seeking out new ways of doing it." The chemicals industry is watching the experiments with interest — but also a dose of scepticism. Jonathan Butler at Johnson Matthey, a leading manufacturer of speciality chemicals, said: "Recycling provides a useful secondary source of platinum group metals and, as such, is important in meeting future demand in a sustainable way.

"Primary reserves of platinum group metals extend to many decades using current mining techniques. Having adequate supplies is crucial to ensuring that these metals continue to improve the quality of our everyday life in future, whether that is in cleaning up air pollution, generating energy cleanly and efficiently, or in developing new medical treatments for an ageing population."

Butler welcomed attempts to recover platinum group metals from waste and road dust, but added: "It is important to remember that the amount of platinum group metals potentially recovered in this way is negligible in terms of the overall size of platinum group metal supplies."


Copyright David Derbyshire 2011