What is gold cyanidation?

Gold cyanidation extracts gold from the ground.

Gold cyanidation, also called cyanide leaching, is a process used to extract gold from raw ore taken from the ground. It uses cyanide to dissolve the gold within the rock, which itself is not cyanide-soluble. The gold is then stripped into a liquid form that can be treated to remove the cyanide. Almost 90% of all gold mined commercially is made by cyanidation. The process has been controversial since its inception due to the toxic nature of cyanide and the threat it poses to the environment and to the people working in the extraction facilities.

The cyanidation process begins after gold has been discovered and the raw ore is separated from the ground, usually by explosive means. The ore is crushed to better facilitate the leaching process. Fragmenting ore into finer pieces is called pile leaching. Processing the ore immediately without crushing it is known as dump leaching. Each process, however, uses cyanide to remove the gold from the ore.

Depending on what other metals are present in the ore, preliminary processes may be required to ensure productive and effective extraction. One such process is washing the ore, or submerging the ore in high pH water, known as an alkaline solution. An alkaline solution of calcium oxide is often used to neutralize potential acids, after which the solution is either flooded with air or aerated. These methods limit the extent to which iron and sulfide, commonly found in ore, interact with cyanide. The use of calcium oxide pre-cyanidation helps ensure that no hydrogen cyanide, a highly toxic form of cyanide, is released during the process.

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The gold cyanidation process is generally conducted in an outdoor environment, although an indoor facility that meets safety standards is sometimes employed. A cyanide salt such as potassium cyanide, sodium cyanide or calcium cyanide, the most popular choice, is mixed with water and then applied to the ore. This part of the process is complete when most of the gold obtained has been liquefied and removed.

The time required for almost complete cyanidation of gold varies from 10 hours to 44 hours and depends on the size of the gold particles present in the ore. The more oxygen present at the time of cyanidation, the less time the process takes. When the gold is sufficiently dissolved, it is recovered by one of two methods. It can be adsorbed onto large particles of carbon that are filtered from the ore. In the Merrill-Crowe precipitation process, oxygen is removed from the solution, which is then infused with a zinc powder and passed through a filter.

The environmental risks of using gold cyanidation are numerous, especially as the process often takes place outdoors. If proper safety precautions are not taken, there can be serious consequences for workers and surrounding ecosystems. While steps are taken to ensure that no hydrogen cyanide develops, other forms of cyanide still pose a danger to exposed organisms. Harmful chemicals, including nitrates and thiocyanates, are created during cyanidation, although their impact is much less extensive than a cyanide spill. Gold mining companies must abide by strict security measures to prevent such an event from occurring.

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