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High commodity prices have encouraged the use of mineral-munching bugs to extract metals from waste or low-grade ore.Even the sleekest gadget depends on the mucky business of digging stuff out of the ground.

Mobile phones and computers use copper for their wiring and rely on cobalt ,germanium,lithium,nickel,platinum and tantalum for other components.Electric motors need magnets made of “rare earth” elements such as neodymium.

Rising prices and Chinas tightening grip on supplies of rare earth elements (it accounts for about 97% of production) have heightened the appeal of finding other sources of supply.The result is a growing interest in the use of rock eating bacteria to extract metals from low grade ores, mining waste or industrial effluent.

Rock eating bacteria such as Acidithio-bacillus and Leptospirillum are naturally occurring organisms that thrive in nasty, acidic environments.They obtain energy from chemical reaction with sulphides, and can thus accelerate the breakdown of minerals.Base metals such as iron, zinc,copper and cobalt occur widely as sulphides, and more valuable metals such as gold and uranium are also present in the same bodies of ore.With a little help from the metal munchers these metals can be released in a process called bioleaching.

This approach has its pros and cons, to recover large quantities of metals quickly from ores with a high metal content, smelting remains the most profitable route.Bioleaching is slower but it is also cheaper, making it well suited for treating ores and mining wastes with low metal concentrations.It is also generally cleaner. Material containing poisonous elements such as arsenic is unsuitable for smelting because of the risk of pollution.

For many years Bioleaching has been used to recover gold from ores that are hard to break down using heat treatment(known as “roasting”).The bacteria are set to work in huge stirred tanks, called bio reactors, containing ground up rocks and dilute sulphuric acid.The bacteria change one form of iron found within the ore (ferrous iron) to another (ferric iron) and tap the energy released. In acidic solutions ferric iron is a powerful oxidising agent.

It breaks down sulphide minerals and releases any associated metals.In the past it has been hard to recover metals other than gold profitably in this way. High commodity prices mean that bioleaching of a variety of metals has become an attractive prospect in recent years.In 2008, for example, a new venture started operating in Talvivaara, Finland.

It was set up as the result of a European research project called Bio Shale, which showed that bacteria could recover nickel,copper,lead,silver,zinc,cobalt, rhenium,selenium,tin,gold,platinium,palladium and uranium from Europe’s extensive but underexploited “black shale” deposits.

Last year the Talvivaara Min=ing Company produced over 10,000 tonnes of nickeland 20,000 tonnes of zinc from local shales, and it plans to deliver 90,000 tonnes of zinc, 1,800 tonnes of cobalt and 15,000 tonnes of copper a year by 2012.In 2010 the firm started to extract uranium and expects to produce 350 tonnes a year, other metals will follow.

In Chile, which has 30% of the worlds copper deposits, BHP Billiton, a mining giant, has set up two bioleaching operations in the past three years, each aiming for around 200,000 tonnes of copper production a year.The Finnish and Chilean ventures both use bioheaps, which are vast, carefully engineered piles of ground up ore (in Chile, 2,000 metres long, over 100 metres wide and nearly 20 metres high) irrigated from above with dilute sulphuric acid laced with bacterial cultures, and aerated from below.

Using many different organisms within the heaps gives the best results.Meanwhile a Canadian firm, BacTech Mining Corporation, which sells a bioleaching process for gold extraction has set up a new division to apply the technology to mining waste.

At the town of Cobalt in Canada it plans to remove toxic elements such as arsenic from old silver mine tailings and extract cobalt, nickel and silver.Metals can also be extracted from polluted water, where sources near old disused mines are also contaminated.In Germany a firm called GEOS has set up a pilot plant on a coal mining site to clean the iron laced groundwater using bacteria.

Organisms with slightly different dietary preferences, known as sulphur-reducing bacteria show promise for removing dissolved metals from liquid industrial waste.Paques, a firm based in the Nederlands , has a commercial process for recovering zinc using such bacteria.Interest in bioleaching shows no sign of abating, as part of a European project called ProMine . Geologists are mapping Europe’s mineral resources to a depth of 5km in and effort to stimulate the mining industry and reduce dependence on imports.Metal munchers look set to be very busy.

 
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