|
A fascinating article about Bio-mining written by Chris that featured in the Camborne School of Mines Journal recently.
Kennecott Bingham Canyon copper mine (photo by Ravell Call / The Deseret News via AP)
Biohydrometallurgy: a tale of tiny miners
Dr Chris Bryan, Lecturer: Sustainable Mining and Minerals Resourcing
Introduction
Mining is perhaps the second oldest profession in the world (well, certainly the third oldest) but as with most things it seems, nature has been doing it for far longer. As life coalesced from the primordial soup, there was no oxygen in the atmosphere and photosynthesis was not even a glimmer in evolution’s eye. For millions of years, all life on Earth was underpinned by energy from mineral sources rather than sunlight – early life was lithotrophic, literally ‘rock-eating’.
Exposed sulfide minerals such as pyrite were abundant and microorganisms got their energy through the oxidation of the iron and sulphur they contained. These activities produce a corrosive mix of ferric iron and sulphuric acid which attacks nearby minerals resulting in the dissolution and deportment of many metals contained therein. Today we do not tend to find naturally occurring outcrops of sulphide minerals, but where such minerals are exposed to moisture and air through extractive activities they are rapidly colonised by the decedents of these ancient tiny miners. In the absence of any preventative measures this microbial weathering can produce metal-rich acid mine drainage (AMD) causing severe environmental damage.
Nevertheless, humans have been exploiting the process since antiquity, albeit unknowingly: in 166 AD a Greek naturalist and physician described in situ leaching of permeable copper ores in Cyprus while the observation by Diego Delgardo in the 1500’s of scrap iron being transformed into copper (simple displacement chemistry) in the Rio Tinto surely fuelled alchemists’ beliefs that base metals could be converted to gold[1, 2].
Now known as biomining or biohydrometallurgy, it has been used commercially since the early 1960’s with the construction and irrigation of heaps for the recovery of copper at the Kennecott Bingham Canyon Copper mine. Between 1980 and 1998, the amount of the world’s copper produced from biomining operations increased from 10% to 25%[3, 4]. The biooxidation of gold-bearing arsenopyrite ores is the basis of the BIOX® process developed by Gencor in the 80’s. In these processes microbial activity renders occluded gold accessible to cyanide, leading to recovery rates of 95-98% and up to 3% of the world’s gold is produced this way.
Realism
Since the 1992 Earth Summit in Rio, the concept of sustainable technologies and development has become very popular. Biohydrometallurgical extraction procedures find favour in this respect as they are “almost without exception more environmentally friendly” than physicochemical processes[1]. At the first International Biohydrometallurgy Symposium (IBS) in the late 1970’s, there was great optimism about the application of biohydrometallurgy to the mining industry. It was claimed at the time that this was a technology that could offer a greener, more efficient alternative to high temperature pyrometallurgical processing of ores and concentrates, and would eventually replace conventional roasting operations.
More than 30 years later and clearly that has not happened, nor does it look likely to do so. The early evangelicalism has given way to a more pragmatic assessment: biohydrometallurgy is a tool like many others in the mineral processing tool box. While biomining can fulfil certain roles extremely competently, it remains a niche technology and often cannot compete on a purely economic basis with pyrometallurgy and pressure leaching.
Challenges
The earliest commercial biomining systems were not designed to specifically promote microbial activity. Although this has since changed, there is still a tendency to ‘black-box’ the microbiological processes that are fundamental to successful biohydrometallurgical application. Without understanding these highly complex interactions we cannot target key operational parameters and biohydrometallurgy will never realise its full potential, whatever that may turn out to be.
Many supergene copper deposits overlay large deposits of low-grade chalcopyrite. The grades typically preclude conventional flotation and smelting, yet with increasing copper prices and decreasing grades globally, they are increasingly targeted for exploitation. With relatively low CAPEX and OPEX, biomining should be quite suitable for marginal and low-grade materials. However, the dissolution mechanisms of chalcopyrite are poorly understood and while bioleaching of low grade chalcopyrite ores has been demonstrated, there is much work to be done to understand and optimise this process.
Notwithstanding the technical challenges – and perhaps as a result of promising so (too?) much early on – there is a feeling at the moment that biohydrometallurgy has been consigned to an option of last resort. It is tested with complex deposits that have proven unsuitable for other options and when it too fails it is seen as a failure of biohydrometallurgy generally rather than the impossible nature of the target material. Thus, one of the foremost challenges for biohydrometallurgy may be one of information and education; making sure that it is at least tabled as a processing option at the outset rather than something to consider when all else has failed.
Current activities at CSM
Much of my previous work has focussed on the more fundamental aspects of biomining in tanks and heaps, especially microbial colonisation and succession [5-8]. However, I’ve always been interested in the application of biomining to mine wastes, such as the bioleaching of tailings at the former Kasese Copper mine in Uganda for the recovery of cobalt. There is nothing unique about the Kasese mine history and such examples must exist elsewhere. Thus we are looking at the possibility of reprocessing mine wastes and low grade or marginal deposits in Cornwall and elsewhere in Europe. We have an extensive collection of mineral-oxidising microorganisms which operate over a range of temperatures and salt concentrations. Many of these have commercial potential.
Further to this, Europe has identified 14 elements upon which it is critically reliant on importation. This list includes many rare earth elements essential for high tech products and components. At the same time, Europe produces vast quantities of electronic waste which contain concentrations of many critical metals comparable to those of their natural ores. We are currently developing co-processing methods which could see biohydrometallurgy used as part of a broader urban mining process for the recovery of these and other metals.
Cornwall hosts a wealth of mine wastes and as a result produces huge amounts of AMD. In Cornwall as elsewhere this must be remediated at significant cost, in perpetuity. While some legal departments may argue over the definition of ‘perpetuity’, this is somewhat Clintonesque – we will be remediating mine drainage for as long as water drains from them. We are currently looking into the growth of algae in AMD. To some extent, this has been previously investigated – and largely rejected – as a stand-alone remediation option. However, it does show some promise as way of valorising AMD as part of a wider remediation strategy. Any value derived from the production of, for example, biofuels, or through the reduction of contaminant loading may help reduce the overall remediation costs while not necessarily being economically viable as a stand-alone process.
As with all work in developing novel biomining processes or improving the existing state-of-the-art, the critical thing to consider is its place in the wider picture – how might this novel or improved process integrate into existing flow-sheets? How significant are the improvements?
Finally, through the development of a Biohydrometallurgy module offered as part of our Minerals Engineering Masters programme we hope that future generations of human miners will have an appreciation of the potential of the world’s smallest.
Further Reading
1. Rawlings, D.E., Heavy metal mining using microbes. Annual Review of Microbiology, 2002. 56: p. 65-91.
2. Brombacher, C., R. Bachofen, and H. Brandl, Biohydrometallurgical processing of solids: a patent review. Applied Microbiology and Biotechnology, 1997. 48(5): p. 577-587.
3. Brierley, C.L., How will biomining be applied in future? Transactions of Nonferrous Metals Society of China, 2008. 18(6): p. 1302-1310.
4. DaSilva, E., Biotechnology: developing countries and globalization. World Journal of Microbiology and Biotechnology, 1998. 14(4): p. 463-486.
5. Bryan, C.G., et al., The effect of CO2 availability on the growth, iron oxidation and CO2-fixation rates of pure cultures of Leptospirillum ferriphilum and Acidithiobacillus ferrooxidans. Biotechnology and Bioengineering, 2012. 109(7): p. 1693-1703.
6. Bryan, C.G., et al., The efficiency of indigenous and designed consortia in bioleaching stirred tank reactors. Minerals Engineering, 2011. 24(11): p. 1149-1156.
7. Govender, E., C.G. Bryan, and S.T. Harrison, Quantification of growth and colonisation of low grade sulphidic ores by acidophilic chemoautotrophs using a novel experimental system. Minerals Engineering, 2012.
8. Watling, H., et al., Effects of pH, temperature and solids loading on microbial community structure during batch culture of a polymetallic ore. Minerals Engineering, 2012.
|