Rejecting arsenic from copper ores
A process combining flotation, roasting and waste stabilisation may provide an economically viable and more environmentally friendly technique for exploiting untapped high-arsenic copper ore deposits.
Scientists at Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) say the results of the first techno-economic evaluation of the arsenic-reduction technology at laboratory-scale are encouraging and warrant further development.
Copper concentrators and smelters will feel the benefits, says Warren Bruckard, Research Program Leader for Mineral Processing and Agglomeration at CSIRO. ‘Under normal circumstances, when high-arsenic copper ores are processed, the arsenic is concentrated along with the copper. When these concentrates get to the smelter, the arsenic gets dispersed into the biosphere in various streams, causing concerns for smelter operators and the people who live around them.’
Smelter operators are therefore imposing stricter penalties on the amount of arsenic they will allow in the concentrates that feed them. Bruckard says generally anything above 2,000ppm is fined, while arsenic levels above 5,000ppm are not accepted.
‘Those limits will be coming down further, meaning concentrators who mine and beneficiate the copper ores have to either reduce the arsenic levels by blending high and low arsenic ores, or avoid mining some sections of the orebody,’ he says. ‘This is not sustainable in the long term.’
The CSIRO technique introduces another flotation step in the flowsheet after the primary stage in which copper (and arsenic) are separated from the gangue minerals. This second stage exploits the differing pulp potentials at which copper and arsenic minerals float to find a ‘window’ of potential where separation can be achieved.
‘Each ore will have a different window depending on which copper and arsenic minerals are present,’ explains Bruckard.
The process produces separate low-arsenic and high-arsenic concentrates.
The low-arsenic substance can be sold off to smelters at no penalty, while the high-arsenic product is roasted at about 700ºC in a fluid bed reactor. The volatility of the arsenic ensures it is fumed off into a gas feed where, once cooled and oxidised, it precipitates into an arsenic oxide solid.
This material is then immobilised in a low temperature ceramic and is safe for disposal into the ground, claims Bruckard. ‘We can concentrate the arsenic into a small volume stream, so it is not just dispersed into the biosphere, but isolated, treated and stabilised.’
The calcine from the roast, meanwhile, is high in copper and can be smelted directly.
The preliminary techno-economic evaluation has revealed that, despite additional operational and capital costs, the process should lead to an AUS$0.6M/y increase in net revenue over the conventional processing route, due to a higher yield of copper concentrate.
The assessment is based on the assumption that the conventional method could produce a concentrate with a low enough arsenic level. For a high arsenic orebody that cannot be mined and processed in this way at present, the economic and environmental benefits of the new flowsheet increase further.
The team at CSIRO is now leading a project on this that is managed by AMIRA, an independent association of mineral companies, headquartered in Melbourne, Australia. Anglo American Chile Ltd and Rio Tinto are sponsoring the programme.
The aim is to move the work to pilot scale, as well as improve fundamental understanding of the mechanisms at work and adapt the technology to other copper penalty elements such as bismuth and antimony. The researchers have also explored its use on nickel sulphide ores that are high in arsenic.
Further information: Csiro