Companies weigh pros, cons of autoclaving, bioleaching

Both processes eliminate toxic gas emissions, convert sulphur as well as arsenic into more stable compounds and discharge them into tailings ponds on site. While autoclaving is generally more expensive, capital and operating costs of either process depend on the location and size of the orebody, energy input and other factors. For small and remote producers, bioleaching seems a more viable choice. “The transportation of lime to a remote location in the Northwest Territories versus Central Nevada, the difference in material costs delivered to the site can be as much as $300 per tonne,” says P.T. O’Kane, Wright Engineers’ vice- president of marketing. (Lime is used to adjust the pH in the pressure leaching process.)

So far, Canadian miners have opted for autoclaving over bioleaching. American Barrick Resources (TSE), for one, has been using the technology to pretreat sulphur gold ores at the Mercur mine, 56 km southwest of Salt Lake City, Utah, for the past two years.

Wright Engineers was the company that completed the studies, process and detailed design and the construction management for the autoclave circuit at the Mercur mine. O’Kane, however, does not dismiss the viability of bioleaching. He shares his views on the two new technologies with The Northern Miner. Excerpts:

Bioleaching has tremendous potential for use in treating refractory gold ores and concentrates. Once it has shown that bioleaching can be operated at a commercial scale, we believe a number of bioleach plants will be built. We also believe that both autoclaving and bioleaching will remain as process possibilities because of the present differences in process capability. For instance, if only partial oxidation of sulphides is necessary for significantly higher gold recoveries, then bioleaching may be the preferred process. But if complete destruction of the sulphides is required, with the present state of process development of bioleaching, pressure oxidation is the preferred process. Bioleaching Advantages Disadvantages Low temperature process Long residence time required, Runs at atmospheric pressure even for partial oxidation of Low degree of process control the sulphides required Only been successfully run at Possible to control the extent of demonstration plant scale sulphide sulphur oxidation Low degree of arsenic precipitation

within the oxidation step Autoclaving Advantages Disadvantages Proven technology High temperature process High degree of sulphide Requires a high degree of sulphur oxidation control High degree of arsenic An oxygen plant is required to precipitation within the oxida- supply the necessary oxygen tion step