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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 soluble in cyanide. The gold is then drawn out in a liquid form that can be treated to remove the cyanide. Almost 90% of all gold extracted commercially is done so by cyanidation. The process has been controversial since its inception due to the poisonous nature of cyanide and the threat it poses to the environment and the people working in the extraction facilities.
The cyanidation process begins after the gold has been discovered and the raw ore separated from the ground, often by explosive means. The ore is ground up to better facilitate the leaching process. Breaking up the ore into finer pieces is called heap 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 necessary to ensure a productive and effective extraction. One such process is ore washing, or submersion of the ore in water with a high pH, known as an alkaline solution. A calcium oxide alkaline solution is often used to neutralize potential acids, after which, the solution is flooded with air, or aerated. These methods limit the extent to which iron and sulfide, commonly found in ore, interact with the 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.
The process of gold cyanidation is usually conducted in an outdoor setting, though an indoor facility, that meets safety regulations, 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 obtainable gold has liquified and been removed.
The amount of time required for near-complete gold cyanidation ranges from as little as 10 hours to as long as 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 will take. When the gold has sufficiently dissolved, it is recovered by one of two methods. It can be adsorbed onto large carbon particles 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 hazards of using gold cyanidation are numerous, especially since the process often takes place outdoors. If proper safety precautions are not taken, there can be serious consequences for the workers and the surrounding ecosystems. Although measures are undertaken 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, though their impact is far less extensive than a cyanide leak. Gold extraction companies must abide by strict safety measures to prevent the occurrence of such an event.