Cyanide process

Gold cyanidation (also known as the cyanide process or the MacArthur-Forrest process) is a hydrometallurgical technique for extracting gold from low-grade ore by converting the gold to a water-soluble coordination complex. It is the most commonly used leaching process for gold extraction.

Production of reagents for mineral processing to recover gold, copper, zinc and silver represents approximately 13% of cyanide consumption globally, with the remaining 87% of cyanide used in other industrial processes such as plastics, adhesives, and pesticides. Due to the highly poisonous nature of cyanide, the process may be considered controversial and its usage is banned in a small number of countries and territories.

In 1783, Carl Wilhelm Scheele discovered that gold dissolved in aqueous solutions of cyanide. Through the work of Bagration (1844), Elsner (1846), and Faraday (1847), it was determined that each atom of gold required two cyanide ions, i.e. the stoichiometry of the soluble compound.

The expansion of gold mining in the Rand of South Africa began to slow down in the 1880s, as the new deposits being found tended to pyritic ore. The gold could not be extracted from this compound with any of the then available chemical processes or technologies.

In 1887, John Stewart MacArthur, working in collaboration with brothers Dr. Robert and Dr. William Forrest for the Tennant Company in Glasgow, Scotland, developed the MacArthur-Forrest process for the extraction of gold ores. By suspending the crushed ore in a cyanide solution, a separation of up to 96 percent pure gold was achieved.

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