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Comment Europe

Significantly reduce cyanide consumption in refractory gold-ore treatment

By Gerald Ondrey |

Traditionally, cyanide consumption with conventional bio-oxidation residues is higher than with residues produced through other oxidative technologies. The Outotec MesoTherm BIOX process, based on its proven mesophile BIOX process, offers an easy, cost-effective upgrade path that can cut cyanide consumption by as much as 50% compared to conventional bio-oxidation, says Jan van Niekerk, director, Gold Process Solutions, Metso Outotec Corp. (Espoo, Finland; www.mogroup.com). The formation of thiocyanate — a common and stable cyanide species traditionally formed as a further byproduct — is also reduced by more than 50%, he says.

The MesoTherm BIOX process enhances the established mesophile BIOX process by combining mesophile bio-oxidation technology with a thermophile oxidation stage. The thermophile stage operates at a higher temperature (about 65°C) than the mesophile stage (40–42°C), thereby achieving an even more-effective overall sulfide-oxidation step, says Van Niekerk.

In the complete process (flowsheet), fresh concentrate slurry — which contains gold, sulfide minerals (such as pyrite and arsenopyrite) and gangue minerals (such as silicates or dolomite) — process water and some nutrients are fed to the mesophile stage. There, bacteria oxidize some of the sulfide minerals, releasing iron, arsenic and sulfate into solution (the gold remains inert). This partially oxidized slurry is fed to an interstage thickener to remove some dissolved species prior to the thermophile stage. The partially oxidized solids are pumped to the thermophile reactors, where the sulfide is further oxidized to achieve near 100% oxidation, explains Van Niekerk. The BIOX product then undergoes a three-stage, counter-current decantation (CCD). The washed oxidized solids from the final CCD thickener are then pumped to the leaching section (carbon in leach; CIL), where cyanide is added to dissolve the liberated gold.

The new process is currently being implemented at the Fairview BIOX plant in South Africa, where the mine has converted one of its secondary mesophile reactors into a thermophile reactor. This reactor was operated as a production reactor, as part of the process. The plan is to convert more of the secondary reactors to thermophile reactors over time, says Van Niekerk.

gold-ore treatment

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