Biocatalyzed leaching of precious metal values

Abstract

A biohydrometallurgical apparatus and method for extracting precious metals, including gold and silver, from oxidized ore by means of leaching with a neutral bisulfide solution instead of cyanide, thiourea, thiosulfate, or other conventional lixiviants is disclosed. In a preferred embodiment, a first process step of conventional bio-oxidation of ore particles is accomplished to free precious metals dispersed or occluded within the ore. A portion of the acidic, base-metal sulfate leach solution produced by bio-oxidation is introduced to an anaerobic reactor. In a heap leach embodiment of the process, the anaerobic reactor is a side-stream reactor. In an alternative slurry (e.g.,vat) leaching embodiment, the anaerobic process may occur on-line. A preferably non-toxic electron donor, such as acetate or methanol (which does not bind effectively to activated carbon), is added to the anaerobic reactor to enrich within it a mixed culture of sulfate-reducing bacteria. The anaerobic reactor may be operated in a pH-stat mode by adding sufficient acidic sulfate solution to maintain a neutral pH in the reactor. In a second process step, the oxidized ore (possibly covered and submerged to exclude oxygen) is leached by recirculating the neutral bisulfide lixiviant saturated with H.sub.2 S through it in a leaching reactor. Precious metal values are recovered from the pregnant bisulfide solution by contact with activated carbon or other conventional techniques.