Gold cyanidation is a process that uses cyanide to dissolve gold from ore. It is controversial due to its poisonous nature and environmental risks. The process involves grinding the ore and using cyanide to extract the gold, which is then removed in liquid form. Pre-processing may be required to ensure effective extraction. The process can take up to 44 hours and involves risks to workers and ecosystems. Gold mining companies must comply with safety measures to prevent cyanide leaks.
Gold cyanidation, also called cyanide leaching, is a process used to extract gold from ore taken from the ground. It uses cyanide to dissolve the gold within the rock, which, by itself, is not soluble in cyanide. The gold is then extracted into a liquid form which can be treated to remove the cyanide. Nearly 90% of all commercially mined gold is via 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 mining facilities.
The cyanidation process begins after the gold has been discovered and the ore separated from the ground, often by explosive means. The ore is ground to better facilitate the leaching process. The breaking of ore into finer pieces is called heap leaching. Immediate processing of the ore without crushing is known as landfill leaching. Each process, however, uses cyanide to remove the gold from the ore.
Depending on what other metals are present in the ore, pre-processing may be required to ensure productive and effective extraction. One such process is washing the ore, or soaking the ore in water with a high pH, known as an alkali solution. An alkaline solution of calcium oxide 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 ores, interact with cyanide. The use of precyaniding of calcium oxide helps ensure that no hydrogen cyanide, a highly toxic form of cyanide, is released in the process.
The gold cyanidation process is usually conducted in an external environment, although an internal facility that meets safety standards is sometimes employed. A cyanide salt, such as potassium cyanide, sodium cyanide, or calcium cyanide, the most common 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 liquefied and been removed.
The amount of time required for almost complete gold cyanidation varies from as little as 10 hours up 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 will take. When the gold has sufficiently melted, it is recovered by one of two methods. It can be adsorbed on large carbon particles which are filtered out of 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 since 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 is evolved, other forms of cyanide still pose a danger to exposed organisms. Harmful chemicals are created during cyanidation, including nitrates and thiocyanates, although their impact is much less extensive than a cyanide leak. Gold mining companies must comply with strict safety measures to prevent such an event from occurring.
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