Extractive Metallurgy of Gold

Gold has been important as precious metal since early times. The extractive metallurgy and recovery of gold ores has experimented a special interest each time that the international price takes a high value. Then, techniques of extraction and treatment trend to improve gold recovery each time more and more. Process selection is an iterative procedure which usually starts as soon as exploration has established the presence of gold mineralization in sufficient grade and tonnage for the orebody to be considered a potentially economic reserve. The effort devoted to process selection is related to the degree of certainly of the grade and reserve estimations and their absolute values, such as attractiveness of the deposit. The risk associated with the development of gold projects can be minimized planning a good work metallurgical program, and considering aspects that have been considered in the project: capital, profitability, risk and environmental factors.

The mineralogical properties of an ore determine its response to the various process options and indicate the potential environmental impact of its treatment. The mineralogical characteristics are determined from the ore composition and textural properties. Such date is used in conjunction with metallurgical testwork results and information from other similar orebodies for process selection and flowsheet development.

The metallurgical response of an ore to a proposed treatment scheme directly determines the economics of the process, or combination of processes used. The factors to be considered in the evaluation are: Recovery of gold and other valuable minerals; Quality of product, and the need for further processing; Treatment rate; capital cost; Operating cost; Environmental impact; and Technical risk.

The first extraction process usually involves gravimetric equipment. This is the most common gold recovery system used in many plants, however, some equipment such as the sluice box, has not changed significantly for many years. Other gravimetric devices are the shaking table, centrifugal concentrators, and jigs. They are used when there is free gold and the recovery is not problematic.

Other alternative very interesting to concentrate gold is froth flotation. The flotation of the free gold and gold bearing sulphides are done to produce a gold rich concentrate, which can be treated by cyanidation, regrinding and cyanidation, intensive cyanidation, oxidative pretreatment and cyanidation or direct smelting. Also, flotation of free gold sulphides can produce a sulphide-free tail for cyanidation. Most the time when there is flotation of gold ores the next process is cyanidation. All hydrometallurgical gold extraction routes use a leaching step to produce a gold solution as an intermediate product. Currently diluted alkaline cyanide solutions are use exclusively for gold dissolution, although chlorine media has been use in the past. Others lixiviants such as thiourea, thiosulphate, bromide and iodide solutions are also potential alternatives to cyanide leaching, but none has yet been used commercially.

Cyanide leaching can be applied in several forms: agitated leaching, heap or dump leaching, vat leaching, and intensive leaching. Agitated leaching systems are use for the treatment of ground slurries or reclaimed tailings. The rich solution can be treated by activated carbon or powder zinc (Merryl Crowe Process). In the first case, the gold content in the solution is upgraded and finally is sent to an electrolytical treatment to obtain cathodes that are smelted. The second option let to obtain a metallic cement formed by gold, silver and some particles of zinc. In both cases the final product is sent to a smelting process that gives the called gold bullion.