With shortages of essential metals predicted, E&T discovers that the mining industry is turning to bacteria in its quest to make use of lower-grade ores, and even to extract metals from century-old mining waste.
Many of the technologies expected to form the basis of a European low-carbon, high-tech economy - solar panels, electric cars, fuel cells, wind turbines, and energy efficient LED lights among them - are under threat, thanks to looming shortages and soaring prices of metals essential to their production.
Yet Europe has ores and minerals of its own, many of which have been economically exploitable in the past. So with impending scarcity - whether for political reasons or depletion of natural resources - in mind, European scientists are examining the potential to extract metals from low grade unexploited ores, mining wastes and process-wastes using mineral-munching bacteria.
Bio-mining or bio-leaching, as the technology is called, relies on certain microorganisms' ability, in the presence of water and air, to act as catalysts to the break down of minerals that contain ferrous iron (Fe2+) and/or sulphides.
The bugs produce ferric iron (Fe3+), a powerful oxidising agent, and sulphuric acid as by-products (see boxout on opposite page). Conveniently, ores featuring metals such as copper, zinc, uranium, cobalt or gold often contain iron or are sulphides, or they co-exist in deposits with such substances. 'Iron and sulphur have no commercial value in these ores but by liberating them, the bacteria also liberate the more valuable metals,' says Professor Barrie Johnson, who runs the acidophile research team at the University of Bangor.
Metals are freed in the form of dissolved salts, such as copper sulphate, which can be recovered easily in subsequent electrolytic or chemical processes. Gold and silver remain inert as pure metals but are exposed by the oxidisation of the surrounding material and recovered by mixing the oxidised metal-bearing solids with lime to raise the pH, and then applying standard methods for precious metal recovery.
Bacterial metal extraction techniques have been used since the mid-1980s to extract gold from refractory ores not readily processed by any competing technology. In this case, the cultures are set to work in huge stirred tanks, called bioreactors, containing a slurry of ore concentrates and dilute sulphuric acid. Cobalt has also been recovered in bioreactors in Uganda recently, but generally this technology has struggled to compete with more established methods for processing mineral concentrates such as roasting, where the infrastructure - smelters, for example - already exists.
Six years ago, researchers from across the EU, including Johnson and another British microbe expert Dr Paul Norris from the University of Warwick, started experimenting with rock-eating bacteria in the EU-funded BioShale project. The idea was to see whether such bugs could recover metals from the continent's extensive black shale deposits - the Kupferschiefer formation, for instance, extends over 600,000km from England to Poland - which have reserves of copper, nickel, lead, silver, zinc, cobalt, molybdenum, rhenium, vanadium, selenium, tin, bismuth, gold, platinum, palladium and uranium. Many of the scientists also worked on a project called BioMine (2005 to 2008), looking at the bacterial recovery of metals from ores and concentrates, mining wastes, and metal-bearing scrap.
One of the results of this research is Talvivaara, an area in Finland noted for its sulphidic black shale ores, where bacteria are now helping to produce towards 50,000t of nickel per year (3 per cent of annual global nickel production) from deposits that had not previously been considered economically viable. The ores have a nickel content of 0.2 per cent and recovery rate is around 80 per cent. The same deposits also contain 0.1 per cent copper, 0.5 per cent zinc, and smaller percentages of cobalt and uranium, which the company hopes will deliver 60,000 to 65,000t of zinc, 1,200t of cobalt, 10,000t of copper and about 350t of uranium per annum. A zinc recovery line was added in June 2010 and a permit to extract uranium was under consideration.
Talvivaara is an example of bioheap leaching, a cheaper alternative to using bioreactors that has gained interest within the last five to 10 years. In Chile, which has 30 per cent of the world's copper deposits, the global mining company BHP Billiton has set up two bioheap leaching operations in the last three years, each targeting about 200,000t of copper production a year.
Bioheaps are vast, carefully engineered piles of ground ore. In Escondida in Chile, they are each 2km long, 125m wide and18m high. Side by side, they occupy an area of 2km by 5km, and several heaps ('lifts') are being stacked upon each other up to 126m high as the operation develops.
As well as being irrigated from above with dilute sulphuric acid, heaps are aerated from beneath to provide the microorganisms with oxygen and carbon dioxide. The mineral-degrading microbes are brewed in nearby inoculation ponds, so they can be introduced in the irrigation water. The leached metals in solution are collected at the base. 'For a heap-leaching operation to be effective, you need to have a good diversity of organisms to inoculate the heaps because they are so vast and heterogeneous. You will get small pockets where conditions vary and different environments will favour certain organisms,' says Johnson, who is working with Rio Tinto on such projects as part of a Royal Society industrial fellowship. 'We find one ore type will suit one consortia of organisms and another might be better with a different mixture.'
For hard to break down deposits such as the copper-rich chalcopyrites, there are ongoing discussions about whether heaps could be run at high temperatures, according to Paul Norris, whose group works closely with BHP Billiton. 'We found many years ago that the extent of copper extraction from chalcopyrite was roughly proportional to the temperature.
'At higher temperatures, secondary mineral deposition and precipitates coating the mineral surfaces are either reduced or seem to present a less impermeable barrier to ferric iron, which is the oxidising agent in these systems,' he explains.
In Chile, it is hard to retain heat in the heaps because of their large surface area and high altitude, but Talvivaara may be a working example of high temperature heap-leaching, points out Norris. Temperatures of nearly 90C were reached in a trial heap-leaching test, and according to a paper by Maria Riekkola-Vanhanen of the Talvivaara Mining Company, extreme thermophile archaea that thrive at 60C and above have been found in the heap community, in addition to the standard 20C to 40C mesophiles. However, the presence of reactive pyrrhotite (FeS) in the ore makes it unclear how much of the metal extraction might result from bacterial activity and how much from purely chemical oxidation processes.
Bioreactors have generally been confined to gold recovery but Canadian firm BacTech, which has a patented process called BACOX used in commercial gold plants in China and Australia to process between 20,000 to 75,000t of concentrate (50,000 ounces to 150,000 ounces of gold) per year, has recently set up a green division to apply the technology to remediation. BacTech is exploring projects in unnamed former-communist countries in Europe, and Ontario in Canada. At the town of Cobalt in Ontario, the plan is to remove arsenic from old silver mind tailings and as part of the process capture cobalt, nickel and silver.
'First and foremost, we are trying to remediate the environment. That means oxidizing the sulphides in the tailings to eliminate the production of sulphuric acid from natural oxidation, and removing - or making benign - toxic elements such as arsenic, bismuth, antimony, mercury and so forth. But by doing that, we hope to also liberate and recover valuable metals,' explains Paul Miller, VP of technology and engineering at the BacTech Mining Group.
Not going to waste
Bacterial recovery of metals held in process waste streams, acid mine drainage and polluted water is also gaining traction. In Germany, as part of the ProMine project (see boxout on opposite page), a pilot biotreatment plant has been set up on a lignite-mining site by a German company called GEOS. Its aim is to clean the iron-polluted ground water by precipitating a mineral called schwertmannite (Fe3+16O16(OH)12(SO4)2).
Barrie Johnson's Bangor group is looking at the microbiology of the system: 'Schwertmannite has some useful properties, such as being able to absorb toxins like arsenic. It is a nanoparticle product, so even though iron itself has very little value, this mineral does, - says Johnson, who is also working on projects to harness another kind of bacteria, known as sulphur or sulphate-reducing bacteria (SRB) in waste processing.
Sulphate-reducing bacteria have been known for some time but more recently acid- and metal-tolerant species new to science have been isolated at mine sites in Spain and Anglesey, North Wales. They use sulphate or sulphur as their terminal electron acceptors, producing hydrogen sulphide as a by-product. As a low cost mechanism for precipitating dissolved metals as sulphides, they have a great deal of potential. A team of scientists from the Universities of Birmingham and Dundee, for instance, has recently managed to recover palladium and platinum from liquid wastes using SRBs.
Two commercial-scale systems are also operating in Europe. The THIOTEQ process developed by the Dutch company Paques, uses SRB to produce hydrogen sulphide which when it contacts the metal-rich waste stream or process water, forms insoluble metal sulphides. Paques has also developed a related wastewater treatment technology (BIOMETEQ) that uses SRB housed in self-cleaning sand filters to facilitate the selective recovery of metals.
The main engineering challenge in using SRBs, according to Johnson, is in adjusting the acidity of the environment and species to separately precipitate multiple different metal sulphides. 'For example, if we have a stream containing both copper and zinc, to take copper out and leave zinc behind you need to run the system at very low pH, which means using very different types of bacteria to those used in the BacTech and Paques processes,' he says.
A bright, shiny future?
Employing bacteria to recover metals from a range of sources remains a niche commercial activity in Europe but interest is clearly growing. Bioheap leaching requires space, whereas bioreactor-based techniques are somewhat sensitive to the ups and downs of metal prices. However, the larger the range of metals contained in a low-grade ore or process waste, the more viable are both approaches.
The existence of 'penalty' elements such as arsenic, bismuth and antimony can improve commercial viability. For instance, at Lubin, an active copper mining site in Poland that was examined within BioShale and BioMine, conventional smelting operations nearby have limited the attractiveness of biomining, but says Norris: 'This could change if any ores become available that contain enough toxic metals to make smelting less attractive.'
In the case of remediation projects, BacTech's Paul Miller suggests that the intelligent way forward, particularly if the metals recovered are strategically important, would be for local and national governments to agree to step in to support a clean-up operation if metal prices dropped below a certain value.