Gold cyanidation
Gold cyanidation or the cyanide process is a metallurgical technique for extracting gold from low-grade ore via the use of cyanide compounds. It is at present the most important and most-often used process for this purpose. Due to the highly toxic nature of cyanide, the process is controversial.
History
The process traces to 1887, though the original discovery was made in 1783 by the chemist Carl Wilhelm Scheele, the discoverer of cyanide.
The modern cyanide process was developed in Glasgow, Scotland in 1887 by J. S. MacArthur and others.
The reaction
The chemical reaction is called the Elsner Reaction; its stoichiometry is:
The process and its uses
The ore is comminuted (ground to a fine powder in a revolving cylinder with steel balls), 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 anions release the gold cations from the ore as a metal complex. The gold oxidizes to form the soluble aurocyanide metallic complex, NaAu(CN)2.
The Effect of pH
Lime (calcium hydroxide) or soda (sodium hydroxide) is added to ensure that the pH during cyanidation is maintained over pH 10.5. Although the cyanidation process is slowed down at higher alkalinities, it is critical to avoid the volatilization of cyanide as toxic hydrogen cyanide gas:
CN- + H+ = HCN(g)
Effect of Lead Nitrate
Lead nitrate can improve gold leaching kinetics and recoveries, particularly in 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 kinetics. Air or oxygen gas can be bubbled through the pulp to increase the dissolved oxygen concentration. Intimate oxygen-pulp contactors can be 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 hydrogen peroxide solution.
Preaeration and Ore Washing
In some ores, particularly partially sulfidized ores, preaeration of the ore in water at high pH can passivation cyanicide elements such as iron and sulfur. The oxidation of iron to Fe(III) and subsequent precipitation as iron hydroxide avoids cyanide losses to ferrous cyanide leaching. The oxidation of sulfur compounds to sulfate ions avoids the consumption of cyanide to thiocyanate (CNS-) byproduct.
Gold Recovery from Solution
The solubilized gold can be recovered from solution by zinc cementation (using the Merrill-Crowe Process), but since its introduction in 1985, the adsorption on activated carbon using the carbon in pulp (CIP) is used in most industrial applications where the presence of competing silver or copper does not prohibit its use.
Controversy
The process is controversial, due to the highly toxic nature of cyanide; see also Summitville mine, the worst environmental mining disaster in United States history, which rendered 17 miles of a Colorado river devoid of life. The American state of Montana and several countries have banned cyanide mining. There have also been notable disasters in Kyrgzstaan, French Guyana and Romania.