Microbe miners - bioleaching for metal recovery
As richer mineral pickings continue to dwindle, interest is growing in applying bioleaching to a growing range of projects. Guy Richards highlights some of the R&D taking place in this niche arena.
As far as the mining industry is concerned, bioleaching – the extraction of metals from their parent ores using microbes – is a niche technology, with relatively few applications compared with the mainstream alternatives of smelting and other established hydrometallurgical processes. But this could change as high-grade ore bodies become increasingly rare and are found in more remote areas, and the importance of reclaiming metals from waste grows.
Bioleaching comes into its own for treating lowgrade ores that cannot be processed economically by traditional means. Using naturally occurring microbes makes it more environmentally friendly than established processes, it is flexible and easily controlled, and delivers high metal recovery rates – 80–90% in many cases. In addition, as bioleaching requires no sophisticated equipment, it can be operated by relatively unskilled labour and is therefore suitable for use in remote locations.
However, bioleaching is slow, often taking hundreds of days and so delaying cash flow, and since the process occurs in strongly acidic conditions, care has to be taken to stop any toxic chemicals leaking into the environment. Another drawback is that once the process is started, it is difficult to stop.
The technology is currently used to treat sulphide ore bodies and concentrates to extract cobalt, copper, gold, nickel, uranium and zinc, among others. The exact method and bacteria vary according to the mineralisation of the ore, but in general, iron- and sulphur-oxidising microbes (those such as Leptospirillum ferriphilum and Acidithiobacillus ferrooxidans) are used to catalyse the oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) and sulphur to sulphate, in acidic conditions. With a mineral such as chalcopyrite (CuFeS2) for instance, Cu2+ ions are removed from the acid solution – typically sulphuric acid (H2SO4) – and passed through an electrowinning process where the metal is collected on cathodes.
The process is engineered in three main ways:
- tanks or bioreactors – used principally for sulphidic-refractory gold
- concentrates – heaps using crushed ore
- stockpile or leach dumps using run-of-mine ore – typically for secondary copper ores
Chalcopyrite accounts for about 80% of the global copper resource, but, according to bioleaching consultant Dr Corale Brierley, heap/stockpile bioleaching is the only known process that can extract copper from the mineral cost-effectively. In fact, she says, more than 15% (at least 3.14 million tonnes) of copper produced worldwide each year uses the technology. Niche though the technology may be, its contribution is already significant.
According to Rick Gilbert, Vice-President of technology at USA copper major Freeport-McMoRan, that contribution only looks set to grow. ‘As oxide ores are depleted and sulphide ores become predominant, bioleaching will become more widely practised,’ he says.
That said, there is still much that isn’t fully understood about the process in terms of how to optimise it for some applications and how to apply it to others. Dr Brierley explains that the global bioleach community can be split into four groups:
- molecular biologists, who do fundamental research
- applied researchers at universities and organisations such as the BRGM in France, the CSIRO in Australia and Mintek in South Africa
- consultants such as herself who transfer R&D into companies
- corporate R&D departments at mining companies such as Freeport-McMoRan and Newmont
Working at the fundamental level are microbiologists such as Dr Chris Bryan, of the University of Exeter’s Environment and Sustainability Institute, UK. Bryan explains, ‘While mineral bioleaching systems have been used commercially in varying forms since the 1960s, the fundamental microbiology that drives the process has been black-boxed to some extent. Take the analogy of driving a car. While we understand some basic inputs and outputs – for example, accelerator pedal equals go faster – we know much less about the mechanics of the motor, so if things go wrong we don’t know why or how to fix it.
‘My focus is on the microbial interactions in heap leaching operations. These systems are incredibly complex, with huge variations in many elements, such as temperature, pH, partial pressure [pp] of CO2, and ppO2. What fascinates me is how the microbial communities respond to these changes and how that might affect heap performance. Elucidating these relationships can help with heap design and operational decisions,’ he says.
The work of the French Geological Survey, the BRGM, is broadly similar in some respects, in that its teams are seeking to understand the role of microbes, particularly in terms of population dynamics. Dr Patrick d’Hugues, Head of the Waste and Raw Material Unit at the Bureau, explains, ‘Population dynamics means following the changes in a bioleaching population depending on the operating conditions. We then try to optimise the population’s composition to increase the performance of the process.’ This work, he says, is currently being rolled out with an undisclosed Polish partner to treat a polymetallic ore.
Another strand of development is a new type of bioleaching reactor that is partway between the two established technologies of heap and tank leaching. ‘The work focuses on a new type of agitation-aeration system that should reduce investment and operating costs of bioleaching compared with tank leaching,’ says d’Hugues. ‘It could also be applied to mineral processing and mining wastes, and it would be more efficient than heap leaching.’
Currently, R&D work is underway in the copper industry for reprocessing mineral waste.
The CSIRO, in Australia, is also developing and optimising various processes, including one that focuses on the adaptation of salt-tolerant microbes for bioleaching low-grade ores with saline waters. The aim here, says research team leader Dr Anna Kaksonen, is to address the issue that some leach liquors may contain elevated concentrations of salts as a result of evaporation of water, dissolution of gangue minerals or the use of seawater in arid regions where fresh water is not easily available.
‘Another area of research has been the development of biotechnical unit processes for iron oxidation and precipitation,’ she says. ‘These can be used for regenerating the ferric iron oxidant for the oxidative leaching of sulphide ores, as well as for removing excess iron from leach liquors originating from leaching of sulphide or oxide ores.’
At a more fundamental level, her colleague Dr Miao Chen is studying the mineral-solution interface in bioleaching systems. ‘The physical chemistry of the interface is not well understood,’ she says, ‘and much of our research has been conducted at the Australian Synchrotron facility, which enables us to visualise the bioleaching process at very high resolution and see what the bacteria are doing to the mineral substrate as they break it down.’
Over in South Africa at national mineral research organisation Mintek, Petrus van Staden, manager of the biotechnology division, says its R&D work can be broadly divided into two strands – small/low-grade ore bodies of base metals and uranium, and minerals-related effluents and waste.
‘In the first strand, one important research topic has been providing technologies for unlocking previously uneconomic mineral resources due to low grade or small size, based on innovations in heap leaching (bio- or chemical) and minerals sorting,’ he explains. ‘As a result, we have seen a number of feasibility studies re-opened for reassessing previously abandoned projects, based on the latest technologies. ‘There are biological tools that can be brought to bear on the treatment of mine waters and extracting value from waste,’ he adds, ‘but this requires combination with other technologies, so the need for collaborative research is inescapable.’
Bioleaching in action
For mining companies, it’s more of a mixed picture. Freeport-McMoRan employs bioleaching at several of its copper operations – at Morenci and Bagdad in the USA, Cerro Verde in Peru, and El Abra in Chile. Gilbert explains, ‘The decision whether or not to use bioleaching depends on the mineralisation characteristics and economics, which are specific to an individual orebody. But it has been effective at all leaching operations where it has been applied, with increased and accelerated recovery of copper.’
Newmont, meanwhile, is focusing on pre-oxidative treatment of refractory gold ores, work that resulted in a large-scale demonstration test at its Carlin mine in Nevada, USA, and subsequent commercialisation of the technology in the mid-1990s. Currently, the gold producer is demonstrating the bioleaching of refractory, enargite-dominant ore at the Minera Yanacocha gold mine in Peru. Senior Director Mike Skurski explains that the process is being tested on-site due to its relatively lower construction and operating costs compared with milling, and says that preliminary results of current testing appear promising. Although he admits, ‘Economic conditions and the mineralogy of orebodies we currently mine and process have resulted in us moving away from bio-oxidative pre-treatment of gold ores, however it remains a potential viable alternative on smaller ore bodies or operating mines that lack conventional treatment options.’ Currently, he adds, Newmont has no production of copper via bioleaching.
This disparity between the pragmatism on the part of some miners and the optimism in the academic community led bioleaching consultant Dr David Dew to call for a forum for objective discussion of the technology. He explains, ‘Often bioleaching conferences paint an optimistic view of the growth and future of bioleaching applications, but they do not highlight commercial failures or challenges faced in the industry, and lack awareness of alternative technologies. A forum where both positive and negative experiences of industry are presented, to highlight areas of research that would have the greatest impact on commercial operations, would be beneficial to the development of the technology.’
Dew continues, ‘However, any such forum would depend on industry taking the lead and showing a willingness to participate, collaborate and share experiences – a potential problem for technology providers. And, initially, it may need to focus on one aspect of bioleaching, such as heap leaching of lowgrade ores or tank leaching of concentrates.’
This could lead eventually to what Brierley calls the ultimate goal with the technology – in situ leaching, in which she says bioleaching would play a central role. As the world becomes increasingly urbanised and populations encroach on existing and future mine sites, it is inevitable, she says, that deposits will have to be exploited by pumping a leach solution into an ore body via a borehole, circulating the solution through the rock, dissolving the target metal and extracting the metal through a second borehole.
Only when the mining industry reaches this stage will bioleaching be able to shake off its niche status.