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Cyanidation

Definition:

Cyanidation is a metallurgical technique for extracting gold from low-grade ore by converting the gold to a water soluble coordination complex. It is the most commonly used 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.

Application

The ore is comminuted using grinding machinery and is sometimes further concentrated by froth flotation or by centrifugal (gravity) concentration, depending on the ore. The alkaline ore slurry can be combined with a solution of sodium cyanide or potassium cyanide, however many operations utilize calcium cyanide, which is more cost effective. The extracting solution is maintained at an alkaline pH to prevent release of toxic hydrogen cyanide. 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 slows leaching rate. Air or pure oxygen gas can be purged through the pulp to maximize 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.

Perspective

Despite being used in 90% of gold production, due to the highly poisonous nature of cyanide, the process is controversial and its usage is banned in a number of countries and territories. Although aqueous solutions of cyanide degrade rapidly in sunlight, the less-toxic products, such as cyanates and thiocyanates, may persist for some years. The famous disasters have killed few people — humans can be warned not to drink or go near polluted water — but cyanide spills can have a devastating effect on rivers, sometimes killing everything for several miles downstream.

Other Beneficiation Methods

Electrostatic Beneficiation
Flotation Beneficiation
Gravity Separation Beneficiation
Magnetic Separation
Roasting
Wet Separation