New technologies offer metals environmental boost
Date: 29-Oct-01
Country: UK
Author: Karen Norton
Faced with the depletion of many easy-to-process ores, miners are delving deeper for seams that are harder to exploit and they are looking for ways to limit the expense.
Bacteria and acids may replace the traditional heating of ore and melting of metal at energy-hungry, fume-belching smelters. In other cases, new technologies might ameliorate environmental side-effects from smelting.
Experimental bio-leaching or pressure-acid leaching (PAL), where bacteria and acid draw metal from ore or from the intermediate processing form in concentrates, could improve metal yields while cutting costs and the greenhouse gas emissions which are blamed for global warming.
"The number one goal is to lower world production costs, but there are some other benefits and in the long-term environmental implications could be exceptionally beneficial," said Dan Brebner, global mining analyst at UBS Warburg.
Most interest is centred on exploiting the bacteria that have lived for many centuries in acid mine waters to eat through concentrates from primary sulphide ores, as shallower and more easily mined oxide and secondary sulphide ores are depleted.
Bacteria accelerate the leaching of concentrates from primary ores which often have low metal grades but high precious metals content and high levels of impurities such as arsenic, making established leaching techniques uneconomic.
FOCUS ON COPPER AND NICKEL
Research in bio-leaching is focused on copper and to a lesser extent on nickel. Zinc, cobalt, manganese and vanadium may be treated in future.
Bio-leaching and the leaching of concentrates from sulphide ores bypass the traditional metal smelting process and avoid the sulphur dioxide emissions thought to cause respiratory problems.
As in conventional mining, the waste from bio-leaching must still be collected and stored in a reservoir. It is easier to clean up in liquid form, if properly contained, than as a gas.
Water consumption and power needs were also lower in bio-leaching, some analysts said, but others were more cautious.
In new aluminium technology, carbon dioxide emissions are reduced when more durable nickel/iron inert anodes are used in aluminium smelting rather than normal carbon ones.
South African miner Gencor began using bugs to break down difficult-to-exploit ores encased in sulphides and carbons to recover gold in the mid-1980s.
The onus is now on base metals because of the impending need to treat harder-to-break down primary sulphides. Melting of such concentrates normally incur penalties because smelters say it is harder to remove impurities and precious metals.
Bacteria have lived in mines for thousands of years but their role in accelerating the breakdown of metal sulphides was discovered only in the 1950s/60s. Once broken down, contained metals pass into a solution, which is treated to recover metal.
In 2003 Chile's Codelco and BHP Billiton Plc plan to bring onstream a $50 million copper plant in Chile using bioleaching.
A number of other firms are involved in similar projects and nickel and zinc bio-leach projects are also being evaluated.
But for nickel, many analysts see more promise in pressure acid leaching than experimental bio-leaching, to deal with laterite ore which is deeper down and harder to treat than nickel sulphides, which are running low.
"PAL technology has the potential to be revolutionary, but it has yet to be proven," Brebner said.
The process, where nickel is extracted from laterite ores at high pressure and temperature with sulphuric acid, has been plagued by technical problems since the first projects started up in Australia in the late 1990s.
To catch on more widely, production costs must fall to below $1 a lb from $2 a lb, and industry leaders Inco Ltd and Falconbridge would need to opt for them, Brebner added.
While the leaching of ore to extract intermediate concentrates is at the forefront of research, some others are looking at leaching t