Recent biomining highlights

Biomining '20 was scheduled to begin today in Falmouth, but not to be of course. So as a poor substitute, I have put together a short, but by no means comprehensive, review of some of the highlights in the development of Biomining reported at MEI's Biohydrometallurgy conferences during the last decade. I hope that you biotech specialists might fill in the gaps.

Although there is evidence that bioleaching was used in the Rio Tinto area in Spain prior to Roman occupation, for the recovery of copper, as well as in China some 2000 years ago, it is only over the past 40 years that biohydrometallurgy has been recognised increasingly as an emerging technology for extraction of metal values from recalcitrant minerals, low grade ores or mineral resources carrying penalty metals.  This has led to the development of commercial tank and heap leaching processes, processing concentrates and crushed ores to liberate metals of interest through bioleaching of base metals and biooxidation to enable subsequent recovery of gold and PGMs.  

At Biohydromet '18, Sue Harrison, of University of Cape Town, said in her keynote lecture that biotechnology is set to have an increasingly important role,  not only in the treatment of primary ores and concentrates, but in the quest for the circular economy, and is likely to have a major role in remediation, treatment of tailings, electronic and other wastes, and as a potential aid to processes such as flotation. Processes based on biohydrometallurgy have potential to deliver environmental benefits over competing extraction approaches and to enhance the degree of extraction from the overall resource.

Currently, said Prof. Harrison, the recognition of the relevance of biohydrometallurgy in a broader context is growing.  Key aspects include the need to account for unintentional bioleaching reactions on the disposal of waste rock and tailings and the need for the long term prevention of such reactions to enable appropriate handling of waste rock and restoration and rehabilitation of prior mine sites with associated protection of water resources.  Further, limited global resources of key metals highlight both the need to process mineral resources of decreasing grade, smaller size of deposit and increasing complexity and the ability to extract metals from secondary sources for re-use.  In the former, biohydrometallurgy has potential to expand technological approaches.  In the latter, with an increasing focus on the circular economy, the sources of metals or modern-day ‘ores’ are changing to include secondary resources such as waste electrical and electronic equipment (WEEE) and municipal solid waste (MSW). These present new challenges for biohydrometallurgists.

The metal constituents of Printed Circuit Boards (PCBs) primarily include copper, lead, aluminium, tin and iron alongside other heavy metals such as nickel, zinc and cadmium.  The ongoing generation of electronic waste (eWaste), driven by rapid electronic and technological innovation, has provided a metal-rich waste stream with potential to form a key resource from which metals may be recovered and recycled in line with the desire for an increasingly circular economy.  Bioleaching has demonstrated promise as a processing option for the recovery of valuable base metals from eWaste and pretreatment of the PCB for further recovery of precious metals. The microbes generate the leach agent (ferric iron) that facilitates solubilisation of the metals embedded within PCBs. 

At Biohydromet '14 Corale Brierley, of Brierley Consultancy, USA,  highlighted some interesting facts, such as that 50% of all copper mined in the world at that time was in the last 25 years, and that 18-20% of global copper production was by bioleaching. She stressed that the growth in renewable energy was set to dramatically increase copper use (see also Is Zero Carbon by 2050 attainable?)

Heap bioleaching of primary copper ores has great potential for low grade ores not amenable to processing by other means, but there are also limited applications for heap bioleaching of refractory gold ores, low grade nickel sulphide deposits and low grade sulphidic uranium ores. The time frame required for recovery of the metal inventory in heap bioleaching of sulphidic minerals has significant impact on the economic analysis of the process. One factor influencing this time frame is the time required from construction of the heap to achievement of an active bioleaching system. This is affected to a large extent by the microbial colonisation of the low grade sulphide ores.

Factors impacting this colonisation were considered by Sue Harrison in her keynote lecture at Biohydromet ’12. These factors include initial microbial attachment to the ore, the development of firmly attached biofilms, the location of the microbial community with respect to the ore, the kinetics of microbial growth on the ore surface and its subsequent impact on microbial ecology.

At the Biohydromet '14 panel discussion (MEI Blog 28 July 2014) Pieter van Aswegen, of PMet Consulting, South Africa, highlighted some of the challenges for biooxidation and some of the aspects which could make it a very serious competitor to pressure oxidation, which is the preferred technique in North America for the treatment of refractory gold ores.

Solids content has always been one of the major limitations for biooxidation, and in the past if more than 2000 tpd of concentrate were treated, pressure oxidation was the most economic method. Initially 10% solids was used, but in 1989 Fairview gold mine in South Africa increased to 20% solids, making biooxidation a viable alternative. Pieter said that the largest plant was in Uzbekistan with 1000m3 tanks treating 2000 tpd of concentrates, and developments in new agitators and impellers have allowed the start up of recent plants in Tanzania using 40% solids for high gas dispersion applied in cyanide destruction operations.

A major constraint on solids content is the bacteria, and how robust they are at 30% solids and above. Pieter felt that very little work has been done on this, but it is important to aim for 20-30% solids, as this reduces capital costs due to smaller tanks and lower retention times. Lower retention times can be achieved by using thermophiles, but these are not as robust as mesophiles, which limits solids content to around 15% in most cases.

Pieter also highlighted another challenge for biooxidation of gold ores, the reduction of cyanide consumption, which is much higher than with pressure oxidation, due to the generation of elemental sulphur which consumes cyanide. Typically 10-40 kg/t of cyanide is consumed in biooxidation, compared with around 2 kg/t in pressure oxidation. Some work is being done using thermophiles in the final tanks to oxidise the elemental sulphur in order to reduce overall cyanide consumption. Barrie Johnson of Bangor University pointed out that the high cyanide consumption in biooxidation could restrict the implementation of this technology in some countries, as cyanide usage is banned, and a major target for bio and hydrometallurgists should be to continue to look for alternatives to cyanide, as if this could be removed from the circuit biooxidation would have more widespread use.

Biomining '21 sponsor Newmont Mining Corporation pioneered the investigation, development, and commercial-scale implementation of refractory gold whole-ore heap biooxidation, during a period spanning 1988-2009 at Carlin, Nevada. Basic and applied research and development from 1988-1999 included laboratory test work and increasingly larger pilot test heaps culminating in the full-scale implementation of a process that was estimated to contribute 120,000-180,000 oz/year to Carlin’s production between 2000-2005. Key parameters that influenced performance of the on-off heap biooxidation process, and factors that led to the discontinuation of the operation were described by Frank Roberto, in his keynote lecture at Biohydromet '16.

Outotec's BIOX® process was developed for the pre-treatment of refractory concentrates ahead of conventional cyanide leaching for gold recovery. As the gold is encapsulated in sulfide minerals such as pyrite and arsenopyrite, the gold is prevented from being leached by cyanide. The BIOX process destroys sulfide minerals and exposes the gold for subsequent cyanidation, thereby increasing the achievable gold recovery. Traditionally, cyanide consumption represents a significant operating cost in most gold leach circuits with BIOX® plants included. 

At Biohydromet '18 Craig van Buuren, of Outotec Biomin, South Africa, discussed how, with an ever increasing environmental and commercial emphasis on cyanide utilisation, expanding its BIOX® technology to meet this challenge saw Outotec continue to develop its MesoTherm technology.  This is a hybrid two-stage process using mesophiles to realise the initial primary stage oxidation and thereafter, using a thermophilic culture to complete the sulphide oxidation.

Mesophile (40-45°C) bioleaching is exploited in the BIOX® process, and for the recovery of nickel from pyrrhotite/pentlandite. While such plants perform well, the process is unable to leach copper from recalcitrant copper sulfides. Thermophile (70-80°C) bioleaching of copper from chalcopyrite concentrates has been developed to commercial pilot scale. While a technical success, the economics of thermophile bioleaching for copper recovery are borderline at best, driven largely by the requirement of pure oxygen and the restricted operating pulp density (~15 %). At Biohydromet '18 Chris Bryan, of BRGM, France, detailed the development of a two-stage continuous system for the recovery of nickel and copper from a polymetallic sulfide concentrate. The concentrate is partially leached by a mesophile consortium, solubilising the majority of the nickel, before a thermophile consortium solubilises the copper and remaining nickel.

The potential of in-situ leaching of mineral reserves has been under consideration with biohydrometallurgy of key interest owing to the potential for ongoing regeneration of leach agents. At Biohydromet '14 Jim Brierley, of Brierley Consultancy, USA felt that future mines might utilise some form of a process similar to the hydrofracturing technology ('fracking') to release shale gas, thus opening up a buried resource. Benefits could include reducing the footprint of mining and development of new technologies for extraction of critical earth resources. Biohydrometallurgists would play an important role in advancing new technologies, but obviously not working alone in developing such in situ technology- it would need the involvement of metallurgists, geologists, rock engineers and others.

To prepare for this Jim said that we should be researching how microorganisms behave under high hydrostatic pressures, anaerobic and other conditions yet to be defined. This would be a complex technology only applicable for use with highly specific amenable ore bodies and would need to meet all economic, environmental, safety and societal concerns.

Biohydrometallurgy has had a poor reputation in certain circles, as a technology that is too unpredictable, but at Biohydromet '12, Chris Bryan highlighted that it is often used as an avenue of last resort on ores that are untreatable with other methods. It is a complex technology and there is a need for an all-encompassing approach, with fundamental understanding of microbiology, engineering, mineralogy, hydrometallurgy etc.

Now we look forward to Biomining '21. Updates are at #Biomining21, and there is a current call for abstracts.

Current Biomining '21 sponsors

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