Gold extraction refers to the methods used to remove gold from its raw state in gold ore. A number of processes are employed to achieve extraction, including separating the gold from its surroundings by physical force, exposure to heat, or chemical means. The ore is typically mined first, though this is not always necessary, and then subjected to an extraction method appropriate for its quality and location.
One of a number of precious metals, gold is found in the ground, either in alluvial deposits, non-cohesive sediment, or other rocks. It is found in formations of veins and flaky grains or as whole nuggets and is recovered from the earth by methods such as placer mining, which includes panning, sluicing, and dredging.
For sluicing, a large box with ridges is set up at the base of loose rock. It collects the gold as water pushes the less dense components of the ore over the ridges. Dredging is the method by which a team of extractors funnel loose soil from the bottom of a lake or stream into a sluice box floating in a body of water. Less dense metals are pushed away as the gold sinks to the box's base. The majority of the world's gold supply, however, is the product of hard rock mining, conducted in either open pits or in underground mines, up to 12,800 feet (3,900 meters) deep.
Prior to the middle of the 1900s, gold excavation from hard rocks was less of a concern, as most people searching for gold did so by utilizing placer mining methods. These miners sought out alluvial deposits thought to be rich in gold, usually flakes or grains, and often used water as a medium for breaking apart the soil and recovering the gold. This method of recovery is tedious, time consuming, and requires a large number of workers to be successful, especially on a commercial level.
After the middle of the 20th century, new technologies were developed that allowed ore found in hard rock to be successfully retrieved without severely compromising its quality or quantity. Various mineral processing methods are used, often in conjunction with one another, to extract the ore from its rock encasement. Comminution, hydrometallurgy, and pyrometallurgy are used to extract the gold.
Comminution is often one of the first steps in gold extraction. Extractors break apart the hard rock containing the ore. This is not done in every extraction, but it is often used to remove high quality or difficult to recover ore.
Hydrometallurgical processes are usually accomplished by leaching. For heap leaching, the rock is crushed into smaller pieces and placed over a liner. A cyanide solution is introduced, which liquifies the gold, releases it from the rock, and allows it to drain away for processing. This method of gold extraction, also known as gold cyanidation, takes several months. Dump leaching is used to extract mostly lower quality gold and metals. It does not require the rock to be broken into small pieces and is cheaper and less effective.
In some cases, gold cannot be removed by cyanidation alone. Pyrometallurgy can also be used for gold extraction as well. This can include smelting, in which, the ore is reduced to a molten state as well as drying and roasting, in which other materials in the rock are oxidized to prevent them from hindering the extraction process.