Gold Mining

Gold mining consists of the processes and techniques employed in the removal of gold from the ground. There are several techniques by which gold may be extracted from the earth.

US Land and Minerals

Placer mining

Panning

Gold panning is mostly a manual technique of sorting gold. Wide, shallow pans are filled with sand and gravel that may contain gold. The pan is submerged in water and shaken, sorting the gold from the gravel and other material. As gold is much denser than rock, it quickly settles to the bottom of the pan. The panning material is usually removed from stream beds, often at the inside turn in the stream, or resting on the bedrock bed of the stream, where the density of gold allows it to concentrate. This type of gold found in streams or dry streams are called placer deposits.

Gold panning is the easiest technique for searching for gold, but is not commercially viable for extracting gold from large deposits, except where labor costs are very low and/or gold traces are very substantial. It is often marketed as a tourist attraction on former goldfields. Before production methods can be used, a new source must be identified. Panning is a good way to identify placer gold deposits so that they may be evaluated for commercial viability.

Metal detecting

With a metal detector, a person may walk around an area and systematically scan below the surface. The sensor can give a positive reading for a quantity of gold to a depth of as much as a meter below the surface. As the device is easy to operate and highly mobile, this method of prospecting is very popular among gold miners.

Sluicing

Using a sluice box to extract gold from placer deposits has been a common practice in prospecting and small-scale mining throughout history to the modern day. A sluice box is essentially a man-made channel with riffles set in the bottom. The riffles are designed to create dead zones in the current to allow gold to drop out of suspension. The box is placed in the stream to channel water flow. Gold-bearing material is placed at the top of the box. The material is carried by the current through the box where gold and other dense material settles out behind the riffles. Less dense material flows out of the box as tailings.

Larger commercial placer mining operations employ screening plants, or trommels, to remove the larger alluvial materials, such as boulders and gravel, before concentrating the remainder in a sluice box or jig plant. These operations typically include diesel-powered, earth-moving equipment, including excavators, bulldozers, wheel loaders and rock trucks.

Dredging

Although this method has largely been replaced by modern methods, some dredging is done by small-scale miners using suction dredges. These are small machines that float on the water and are usually operated by one or two people. A suction dredge consists of a sluice box supported by pontoons, attached to a suction hose which is controlled by the miner working beneath the water.

State dredging permits in many of the United States gold-dredging areas specify a seasonal time period and area closures to avoid conflicts between dredgers and the spawning time of fish populations. Some states, such as Montana, require an extensive permitting procedure, including permits from the U. S. Corps of Engineers, the Montana Department of Environmental Quality, and the local county water-quality boards.

Some large suction dredges (100 hp+ 10 inch) are used in commercial production throughout the world. Small suction dredges are much more efficient at extracting smaller gold than the old "bucket line" was. This has improved the chances of finding gold. Smaller dredges with 2 to 4-inch (100 mm) suction tubes are used to sample areas behind boulders and along potential pay streaks, until "color" (gold) appears.

Other larger scale dredging operations take place on exposed river gravel bars at seasonal low water. These operations typically use a land-based excavator to feed a gravel-screening plant and sluicebox floating in a temporary pond. The pond is excavated in the gravel bar and filled from the natural water table. "Pay" gravel is excavated from the front face of the pond and processed through the floating plant, with the gold trapped in the onboard sluicebox and tailings stacked behind the plant, steadily filling in the back of the pond as the operation moves forward. This type of gold mining is characterized by its low cost, as each rock is moved only once. It also has low environmental impact, as no stripping of vegetation or overburden is necessary, and all process water is fully recycled. Such operations are typical on New Zealand's South Island and in the Klondike region of Canada.

Hard rock mining

Hard rock gold mining is done when the gold is encased in rock, rather than found as particles in loose sediment. Hard rock mining produces most of the world's gold. Sometimes open-pit mining is used, such as at the Ft. Knox Mine in central Alaska. Barrick Gold Corporation has one of the largest open-pit gold mines in North America, located on its Goldstrike mine property in northeastern Nevada. Other gold mines use underground mining, where the ore is extracted through tunnels or shafts. South Africa has the world's deepest hard-rock mine, which mines gold from as deep as 3900 meters under the ground.

Byproduct gold mining

Gold is also produced by mining in which it is not the principal product. Large copper mines, such as the Bingham Canyon mine in Utah, often recover considerable amounts of gold and other metals along with the copper. Some sand and gravel pits, such as those around Denver, Colorado, may recover small amounts of gold in their washing operations. The largest-producing gold mine in the world, the Grasberg mine in Papua, Indonesia, is primarily a copper mine.

Gold ore processing

In placer mines, the gold is recovered by gravity separation. For hardrock mining, other methods are usually used.

Cyanide process

Cyanide extraction of gold may be used in areas where fine-gold bearing rocks are found. Sodium cyanide solution is mixed with finely-ground rock that is proven to contain gold and/or silver, and is then separated from the ground rock as gold cyanide and/or silver cyanide solution. Zinc is added to the solution, precipitating out residual zinc, as well as the desirable silver and gold metals. The zinc is removed with sulfuric acid, leaving a silver and/or gold sludge that is generally smelted into an ingot then shipped to a metals refinery for final processing into 99.9999% pure metals.

Advancements in the 1970s have seen activated carbon used in extracting gold from the leach solution. The gold is absorbed into the porous matrix of the carbon. Activated carbon has so much internal surface area, that fifteen grams (half an ounce) has the equivalent surface area of the Melbourne Cricket Ground (18,100 square meters). The gold can be removed from the carbon by using a strong solution of caustic soda and cyanide. This is known as elution. Gold is then plated out onto steel wool through electrowinning. Gold specific resins can also be used in place of activated carbon, or where selective separation of gold from copper or other dissolved metals is required.

The cyanide technique is very simple and straightforward to apply and a popular method for low-grade gold and silver ore processing. Like most industrial chemical processes, there are potential environmental hazards presented with this extraction method in addition to the high toxicity presented by the cyanide itself. This was seen in the environmental disaster in Central-Eastern Europe in year 2000, when during the night of 30 January, a waste pond dam breakage at the Sasar gold mine reprocessing facility near Baia Mare in northern Romania released approximately 100,000 m³ of wastewater contaminated with heavy metal sludge and up to 120 tons of cyanide into the Tisza River.

Cradle

A cradle was rocked back and forth while water was poured over it. The sand and gravel was washed through the screen of the cradle, leaving the gold behind.