HEAP LEACHING

The preferred alternatives in heap design and their motivations are identified as: normal heap leaching, to be generally applied wherever possible; flooded vat leaching, to be applied where wetted ore lacks adequate cohesion or where limited space necessitates it; and thin layer leaching, to be applied where deleterious vat chemistry necessitates it. A number of features are likely to be incorporated in the optimization of typical local heap leach operations, including pre-cyanidation, pre-agglomeration, retaining walls, staged (counter-current) heap treatment, control of calcium levels, and reduced pH with trickle irrigation. The following technologies associated with heap leaching are expected to become preferred methods: adsorption using high pKa weak-base resin columns or, alternatively, staged heap leaching with direct electrowinning (where scaling is controllable and chloride levels are relatively low); and direct smelting of bullion in gas-fired kilns for larger installations, or Micron-type production of bullion flake by electrodeposition onto aluminium with subsequent NaOH treatment for smaller installations. To overcome many problems resulting from the chemistry inherent in Western Australian gold ores and groundwaters, there is considerable motivation to move away from alkaline cyanidation towards acid leach technologies.