cyanide process

cyanide process. Also known as cyanidation, it is a hydrometallurgical technique for extracting gold and silver from low-grade ores. The method, pioneered in the late 19th century, involves dissolving the precious metals using a dilute solution of sodium cyanide or potassium cyanide. It revolutionized the mining industry by making the exploitation of low-concentration deposits economically viable, though it is associated with significant environmental risk.

Overview

The fundamental principle relies on the high affinity of cyanide ions for gold and silver, forming stable, water-soluble complexes. The process typically involves finely grinding ore and agitating it with the cyanide solution in large tanks or vats. After the metals are dissolved, they are recovered from the "pregnant solution" through techniques like carbon adsorption or zinc precipitation. This method largely supplanted older, less efficient techniques such as amalgamation and is a cornerstone of modern extractive metallurgy. Its efficiency allows for the economic processing of ores with grades as low as one gram of gold per ton of material.

History

The discovery that gold could dissolve in cyanide solutions is credited to Carl Wilhelm Scheele in 1783. However, the practical industrial application was developed independently by John Stewart MacArthur and the brothers Robert W. Forrest and William Forrest in Glasgow, Scotland, in 1887. Their patent led to the formation of the Cassel Gold Extracting Company. The process was first successfully implemented on a large scale at the Crown Mine in New Zealand in 1889 and later at the Robinson Deep Mine in the Witwatersrand basin of South Africa. Its adoption transformed Witwatersrand into the world's premier gold-producing region and was a key factor in the Second Boer War.

Chemistry of the process

The dissolution of gold in an aerated cyanide solution follows the Elsner's Equation: 4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH. Oxygen is a critical reactant, acting as the oxidant. The resulting dicyanoaurate(I) ion, [Au(CN)₂]⁻, is highly stable. For silver, a similar reaction occurs, forming [Ag(CN)₂]⁻. The process is typically conducted in a slightly alkaline environment (pH 10-11) maintained with calcium hydroxide to prevent the formation of lethal hydrogen cyanide gas. The kinetics and efficiency are influenced by factors like cyanide concentration, particle size, temperature, and the presence of other minerals like copper sulfides.

Application in gold extraction

Modern application involves several key stages. Crushed and ground ore is subjected to either tank leaching in agitated vessels or heap leaching on large impermeable pads. In carbon-in-pulp and carbon-in-leach circuits, activated carbon granules adsorb the gold complex from the slurry. The loaded carbon is then treated in a elution column with a hot caustic cyanide solution to strip the gold, which is finally recovered by electrowinning onto steel wool cathodes or by zinc precipitation (the Merrill-Crowe process). Major operations utilizing this technology are found globally, from the Super Pit in Australia to sites in Nevada, Ghana, and Peru.

Environmental and safety concerns

The use of toxic cyanide compounds presents major hazards. Catastrophic failures of tailings dams, such as those at the Baia Mare facility in Romania (2000) and the Mount Polley mine (though primarily a water breach), have led to widespread ecosystem contamination and fish kills. The threat to wildlife, particularly birds, from exposed tailings ponds is significant. Strict international codes, like the International Cyanide Management Code, have been developed to improve practices. Furthermore, the potential for acid mine drainage from waste rock can create long-term water pollution issues, complicating site closure and remediation efforts at places like the Berkeley Pit.

Alternatives and developments

Research into less toxic lixiviants is ongoing due to environmental pressures and community opposition. Thiosulfate leaching has been studied as an alternative, notably for ores containing copper or carbonaceous material that interfere with cyanidation. Halogen-based methods and the use of thiourea have also been investigated, though often with higher costs or technical challenges. Biological extraction using microorganisms like Acidithiobacillus ferrooxidans (bioleaching) is established for copper but less so for gold. Current developments focus on improving cyanide destruction in tailings, using technologies like the INCO process (sulfur dioxide/air), and enhancing recycling within the circuit to minimize consumption and discharge.