Leaching processes

Leaching is a hydrometallurgical method that permits the beneficiation of gold ores from deposits which would else be uneconomic because of their fine gold distribution or their relatively low gold grades. Leaching processes, however, are not limited to such ores.

The chlorination process, which is obsolete by now, used chlorine gas that was conducted through wet, precrushed gold ore forming soluble gold chloride that could be washed out with water. The dissolved gold was eventually precipitated from the aqueous solution by ferrous sulphate. The rather common cyanide leach process is based on the selective solubility of gold in a weak cyanide solution. The chemical principles of gold leaching by sodium cyanide are the following:

4Au +8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH

The essential requirements for a good gold recovery are adequate cyanide and oxygen levels as well as a pH – value between 10 and 12. The normal cyanide consumption per tonne of ore is in the order of 100 – 500g. The presence of cyanide soluble minerals, so called cyanicides or cyanide killers (e.g. secondary copper sulphides) increases cyanide consumption; under certain conditions cyanide consumption can reach levels that render a leaching operation uneconomic. The approximate tolerable level for cyanide soluble copper is definitely below 0.5%.

Presuming ideal conditions the solubility rate of pure metallic gold is 3,25mg/cm² /hour. At such a rate a gold grain of 30 microns size would dissolve within 9 hours, a 150 microns diameter gold particle would need almost 45 hours. Therefore leaching processes are often combined with gravity circuits that serve for the recovery of coarse gold particles.

A cyanide leaching process can be conducted under different technical conditions. An effective technique for well leachable high grade gold ores is agitation leaching. Thereby an ore pulp with grain sizes of < 100 microns is permanently agitated during its leaching in special tanks (e.g. Pachuca tanks). The leaching period is generally less than 24 hours. The gold recovery lies usually above 90%.

A second technique is vat leaching, which also requires well leachable ores. The required grain sizes of < 1 cm are much coarser as compared to agitation leaching because in vat leaching the ore is not agitated in the leaching tanks and only coarse material warrants a good permeability. Grain sizes of < 2 mm are disadvantageous as they tend to block the free circulation of the leaching agent. The leaching period is 2 – 4 days and gold recovery lies in the order of 70 – 80%.

The cheapest but slowest technique is heap leaching. It is a process usually applied to low grade gold ore. Basically the ore is piled to a given height on an inclined impermeable surface, a so called leach pad. A sprinkler system provides a continuous spray of alkaline cyanide solution that percolates through the ore dissolving the gold. The gold bearing or pregnant solution is collected and pumped to a gold recovery plant. The amount of material contained in a heap can reach more than 100.000 t. The leaching time ranges from several weeks to a few months. The gold recovery hardly exceeds 70%.