Bagration (1844), L Elsner (1846) and Faraday (1847) worked out the stoichiometry, but it wasn't applied to gold ores until 1887, when the MacArthur-Forrest Process was developed in Glasgow, Scotland by John Stewart MacArthur, funded by the brothers Dr Robert and Dr William Forrest.
The chemical reaction is called the Elsner Equation as follows :
It is an electrochemical process in which oxygen picks up electrons from the gold at a cathodic area, whilst gold ions are rapidly complexed by the cyanide around the anodic area to form the soluble aurocyanide complex.
In 1896 Bodländer confirmed that oxygen was necessary, something that was doubted by MacArthur, and discovered that hydrogen peroxide was formed as an intermediate.
The ore is comminuted (using grinding machinery), and may be further concentrated by froth flotation or by centrifugal (gravity) concentration, depending on the mineralogy of the ore. The alkaline ore slurry can be combined with a solution of sodium cyanide or potassium cyanide, however many operations utilize calcium cyanide as this is often the most cost effective form for industrial use.
It is critical to avoid the release (volatilization) of cyanide as hydrogen cyanide because this gas is highly toxic; hydrogen cyanide boils at 26 °C, barely above room temperature. Cyanide ions may become hydrogen cyanide gas when they acquire free protons.
In order of economic efficiency, the common processes for recovery of the solubilized gold from solution are (certain processes may be precluded from use by technical factors):
Obviously most mines handle large quantities of cyanide without hitting the headlines, but famous cyanide spills include :
|1985-91||Summitville||USA||Leakage from leach pad|
|1995||Omai||Guyana||Collapse of tailings dam|
|1998||Kumtor||Kyrgyzstan||Truck drove over bridge|
|2000||Baia Mare||Romania||Collapse of tailings dam|
|2000||Tolukuma||Papua New Guinea||Helicopter dropped crate into rainforest|
|2001||Tarkwa||Ghana||Overflow from tailings pond|
In the EU, industrial use of hazardous chemicals is controlled by the so-called Seveso II Directive (96/82/EC as amended by 2003/105/EC), which replaced the original Seveso Directive (82/501/EEC as amended by 87/216/EEC and 8/610/EEC) brought in after the 1976 dioxin disaster. "Free cyanide and any compound capable of releasing free cyanide in solution" are further controlled by being on List I of the Groundwater Directive (80/68/EEC) which bans any discharge of a size which might cause deterioration in the quality of the groundwater at the time or in the future. The Groundwater Directive was largely replaced in 2000 by the Water Framework Directive (2000/60/EC).
In response to the Baia Mare spill, Brussels introduced Directive 2006/21/EC on the management of waste from extractive industries. Article 13(6) requires "the concentration of weak acid dissociable cyanide in the pond is reduced to the lowest possible level using best available techniques", and at most all mines started after 1 May 2008 may not discharge waste containing over 10ppm WAD cyanide, mines built or permitted before that date are allowed no more than 50ppm initially, dropping to 25ppm in 2013 and 10ppm by 2018.
Under Article 14, companies must also put in place financial guarantees to ensure cleanup after the mine has finished. This in particular may affect smaller companies wanting to build gold mines in the EU, as they are less likely to have the financial strength to give these kinds of guarantees.
The industry has come up with a voluntary Cyanide Code that aims to reduce environmental impacts with third party audits of a company's cyanide management.
Sulfuric acid leaching of Igarape Bahia gold-copper ore for copper extraction--an ore pretreatment for gold recovery by cyanidation
Aug 01, 2002; Abstract The high copper grade in the Igarape Bahia (Brazil) gold-copper ore restricts the direct application of the classical...
Leaching kinetics and mechanisms of surface reactions during cyanidation of gold in the presence of pyrite or stibnite
May 01, 2005; Abstract This paper quantifies the detrimental effects of pyrite and stibnite on gold leaching kinetics. Means to reduce the...