Gold cyanidation (also known as the cyanide process or the MacArthur-Forrest 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, 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:

4Au + 8NaCN + O₂ + 2H₂O → 4NaAu(CN)₂ + 4NaOH

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.

• Carbon in Pulp
• The Merrill-Crowe Process
• Electrowinning
• Resin in Pulp

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.

External links and references

• Canadian Rockhound on the cyanide process
• Efforts at a cleaner process
• A different commercial method that does not use toxic cyanide
• Historical notes -essay