Gold and silver mines are often located in pristine environments that are highly susceptible to pollution by mining and metals recovery activities. While biotechnologies have been developed for freeing precious metals from refractory ores, cyanidation is still the process of choice for extracting gold and silver values. This Small Business Innovation Research Phase II project will continue the development of a novel biohydrometallurgical process for extracting gold and silver from bio-oxidized ore. The project investigates the use of a bisulfide leaching solution as an effective, but more environmentally friendly and less costly, alternative to the conventional cyanidation process. However, in recent years, the addition of the environmental costs to the total cost of mining gold and silver has stimulated a search for options that are more environmentally acceptable. The proposed bisulfide leaching approach takes advantage of the fact that sulfate-reducing bacteria are capable of both (1) producing dissolved sulfides which act as a complexing agent for precious metals and (2) consuming hydrogen gas generated by the complexation (anaerobic corrosion) reaction. During Phase II, a laboratory prototype will be designed, constructed, and operated; its performance will then be compared to that of the conventional cyanidation process. The prototype will include subsystems for bio-oxidation, gold extraction, and gold recovery. A change in the basic technology of gold and silver extraction is proposed that will reduce mining-related pollution at its source. The proposed process can be applied in leaching precious metals (gold, silver and platinum group metals) from their ores and from metal scrap, which is a multibillion market worldwide. If Phase I results are borne out, bisulfide leaching could reduce chemical reagent costs. The proposed biohydrometallurgical process could provide a means of gold and silver recovery that is both profitable and relatively nonpolluting..
