Gold cyanidation
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Gold cyanidation (also known as the cyanide process or the MacArthur-Forrest Process) is a metallurgical technique for extracting gold from low-grade ore by converting the gold to water soluble aurocyanide metallic complex ions. As of 2005 it is the most commonly used process for gold extraction. Due to the highly poisonous nature of cyanide, the process is controversial.
History
The original discovery of gold cyanidation was made in 1783 by the chemist Carl Wilhelm Scheele, the discoverer of cyanide.The modern cyanide process, the MacArthur-Forrest Process, was developed in Glasgow, Scotland in 1887 by John Seward MacArthur, funded by the brothers Dr Robert and Dr William Forrest.
The reaction
The chemical reaction is called the Elsner Reaction; its stoichiometry is:
The process and its uses
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 is combined with a solution of sodium cyanide or potassium cyanide.
The negatively charged cyanide ions (anions) release the positively charged gold ions (cations) from the ore as a metal complex. The gold oxidizes to form the soluble aurocyanide metallic complex, NaAu(CN)2.
Process improvement
The Effect of pH
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.
- CN- + H+ = HCN(g)
Effect of Lead Nitrate
Lead nitrate can improve gold leaching speed and quantity recovered, particularly in processing partially oxidized ores.Effect of Dissolved Oxygen
Oxygen is one of the reagents consumed during cyanidation, and a deficiency in dissolved oxygen in solution can slow leaching speed. Air or pure oxygen gas can be bubbled through the pulp to increase the dissolved oxygen concentration.Intimate oxygen-pulp contactors are used to increase the partial pressure of the oxygen in contact with the solution, thus raising the dissolved oxygen concentration much higher than the saturation level at atmospheric pressure. Oxygen can also be added by dosing the pulp with hydrogen peroxide solution.Pre-aeration and Ore Washing
In some ores, particularly partially sulfidized ores, aeration (prior to the introduction of cyanide) of the ore in water at high pH can render elements such as iron and sulfur less reactive to cyanide, and therefore the gold cyanidation process more efficient.The oxidation of iron to iron (III) oxide and subsequent precipitation as iron hydroxide avoids cyanide losses due to the formation of ferrous cyanide complexes.The oxidation of sulfur compounds to sulfate ions avoids the consumption of cyanide to thiocyanate (CNS-) byproduct.
Gold Recovery from Solution
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):
- Carbon in pulp
- Merrill-Crowe process
- Electrowinning
- Resin in pulp
Controversy
The process is controversial, due to the highly toxic nature of cyanide. The worst environmental mining disaster in United States history occurred at Summitville mine, rendering 27 km (17 miles) of a Colorado river devoid of life. There have also been notable disasters in Kyrgyzstan, French Guiana and Romania (where cyanide spills in Baia Mare contaminated the waters of Tisza).
The American state of Montana and several countries have banned cyanide mining. A recent controversy was in Romania about the project of Roşia Montană, where Gabriel Resources, a Canadian company intends to use this process.
While cyanide is toxic, it breaks down rapidly when exposed to sunlight. Careful handling (due to the inherent dangers) has meant most of the many mine sites using this technique have not produced mining disasters.
External links and references
- [Efforts at a cleaner process]
- [A different commercial method that does not use toxic cyanide]
- [Historical notes - essay]
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