Biooxidation research challenges

Last month's Biohydromet '14 ended with an excellent panel discussion on the future of biohydrometallurgy, chaired by MEI Consultants Patrick D'Hugues of BRGM France and Sue Harrison of University of Cape Town, with panellists Pieter Van Aswegen of PMet Consulting, South Africa, Paul Norris of University of Exeter, UK, and Jim Brierley of Brierley Consultancy, USA.

Pieter, Patrick, Paul, Jim, Chris Bryan and Sue

This one hour session covered many important aspects of the future role of biohydrometallurgy, some of which are highlighted below.

Pieter van Aswegen began by highlighting 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. Today the largest plant is 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. He 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's message to researchers was therefore not to be conservative, and to make real efforts to aim for 20-30% solids. Rob Hille remarked that the University of Cape Town team has been working on this for the past 2 years with Biomin, using mesophilic and thermophilic bacteria at 31% solids.

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.

Paul Norris spoke on behalf of the academics, and made the important point, discussed previously on the blog, that researchers should be aware of the wealth of past work which is out there. Biomet conferences have been held in various parts of the world for about 38 years now, and there have been hundreds of papers published, so there is always a danger of reinventing and recycling material which has been effectively hidden in the early literature.

He stressed that academics should continue to search for more useful microorganisms, as there are areas in which those currently available are inadequate for industrial use, solids tolerance in stirred tanks, discussed by Pieter, being a notable area, particularly at high temperature. It is important that workers in the wide range of countries represented at Biohydromet '14 look at relevant natural and mining sites in their countries. It is not difficult to find new microorganisms and then to screen them to assess their effectiveness.

Looking further ahead, Jim Brierley felt that future mines might utilise some form of a process similar to the hydrofracturing technology developed by petroleum engineers 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.

Could this be the future for the minerals industry? It would need a new mind-set, and how would we manage it to make it work? 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 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 is a rapidly evolving field, and we look forward to seeing how things have progressed, both technologically and socially, at Biohydrometallurgy '16. But in the meantime I invite comment- what do you feel about the role of biotechnology in future mining operations?