SUMMARY GEOCOAT® PROCESS DESCRIPTION

The GEOCOAT® process uses iron and sulfur oxidizing microorganisms to facilitate the oxidation and leaching of sulfide minerals in an engineered heap environment. These include the mesophiles (moderate temperature) bacteria Thiobacillus ferrooxidans, Thiobacillus thiooxidans, and Leptospirillum ferrooxidans, and thermophilic (high temperature) microorganisms such as the Archaea Sulfolobus and Acidianus.

After concentration by conventional processes, typically flotation or gravity concentration, the sulfide minerals are coated onto a support which may be barren (waste) rock or a low-grade sulfide or oxide material. The support is closely sized, typically -25 +6mm, and the concentrate forms a coating less than a millimeter thick on the surfaces. The mass ratio of concentrate to carrier rock is typically in the range of 1:5 to 1:10. The coating is applied by contacting the thickened concentrate slurry with the support rock as it discharges from the stacking conveyor onto the heap. This results in the formation of a thin, relatively uniform coating on the support rock surface. The adherent coating is not washed out of the heap by solution application or even during heavy rain.

The relatively uniform size of the support rock particles results in large interstitial spaces within the heap, offering very low resistance to air and solution flows. Low pressure fans supply air through a system of perforated pipes placed under the heap. The air flow rate is adjusted to remove heat to control the temperature within the heap to the optimum range for bacterial activity. The air also provides the oxygen necessary for the oxidation reactions.

The large interstitial spaces, combined with the thin concentrate layer, create ideal conditions for biooxidation. The sulfide mineral grains and the attached bacteria are constantly exposed to the downward flowing solution, and the countercurrent flow of air. This results in the efficient transfer of oxygen and rapid oxidation rates. Typically, oxidation is complete within 60 to 120 days, whereas in a whole-ore heap, times of several hundred days do not necessarily result in complete oxidation, even when the ore has been agglomerated. The larger void spaces and rigid support provided by the sized support rock particles in the GEOCOAT® heap prevent the compaction typical of whole-ore heaps and ensure uniform distribution of air and solution to all parts of the heap.

In the basic GEOCOAT® process for refractory gold concentrates, the concentrate is coated onto a barren, essentially inert, support rock. After biooxidation, the heap is reclaimed and the concentrate is separated from the support rock by wet screening. The washed support rock is recycled for re-coating with fresh concentrate.

A potentially attractive option is to coat the sulfide concentrate onto a low-grade sulfide material which would otherwise have to be stockpiled or dumped as waste. The bacterial action in the concentrate coating is also effective in oxidizing the sulfide minerals in the support rock, making additional metal values available for recovery. This may allow sub-cutoff grade material to be brought into the economic reserve. An alternative is to use a screened and sized portion of the ore as support medium. The rest of the ore is ground and floated, producing the concentrate which is coated onto the support rock fraction for biooxidation.

Downstream processing operations depend on the purpose of the biooxidation or bioleaching process. In the treatment of refractory gold ores, the gold remains in the solid residue, which is removed from the pad for additional treatment, typically cyanidation. In the processing of copper and other base metal sulfides, the valuable metal is solubilized and is recovered from the leach solution, while the residue remains on the pad. An “on-off” type pad is used for refractory gold ores with the oxidized material being off-loaded for further processing, and the pad reused. However, for copper and other base metal ores, a permanent pad may be used, with the pad area being expanded as required. Alternatively, additional lifts of coated support rock may be stacked on top of the first.

The GEOCOAT® process is also applicable to the treatment of concentrates containing both gold and copper values. The heap is bioleached to produce a solution from which copper is recovered by solvent extraction and electrowinning. The residual, leached material is unloaded from the pad and screened to remove the oxidized concentrate containing the gold values. Gold is recovered from the oxidized concentrate by cyanidation in agitated tanks.

Figure 1 shows flowsheet options for the application of the GEOCOAT® process to the treatment of refractory gold ores. Figure 2 is a schematic flowsheet for the processing of a base metal concentrate, in this case copper.