R&D firm unveils new gold bioleaching process

A biochemical process developed by a firm based in Bozeman, Mont., is showing potential as an inexpensive, clean alternative to cyanidation in gold extraction.

Yellowstone Environmental Science is developing the process for use in gold and silver recovery. The biohydrometallurgical process uses two types of naturally occurring bacteria — one aerobic, the other anaerobic.

Biotechnology is currently used by companies such as Newmont Gold as a pretreatment step in otherwise conventional precious metal processing.

However, the new process is different in that it does not employ cyanide or any other toxic reagents. It is the result of ongoing research aimed at developing a heap-leach process that is both profitable and relatively non-polluting.

The process consists of two steps, the first of which involves the bio-oxidation of low-grade refractory ore with an aerobic bacterium (Thiobacillus ferrooxidans) on the heap. In the second step, heap drainage is mixed with an anaerobic, sulphate-reducing bacterium and reapplied to the heap. The sulphate-reducing bacterium converts the dissolved sulphate in the heap drainage into a dissolved, bisulphide, leaching agent, which combines with the gold in the heap. The gold is then collected on charcoal or zinc dust, with any excess sulphur recovered as a byproduct. Hydrogen is also consumed in the second step of the process, neutralizing acid waste waters.

The capital cost of cyanidation is roughly the same as that of the new process. However, operating costs may prove to be substantially lower. And since sulphur compounds are less persistent than cyanide, the environmental risks, and attendant costs, could also be lower.

The work is attracting the attention of several mining companies, including Pegasus Gold, Barrick Gold and Placer Dome.

The research was funded by the U.S. National Science Foundation, and Yellowstone is seeking funding elsewhere for further development and commercialization of the process.

Meanwhile, it has filed for patents in the U.S. and other countries.