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An Overview of Precious Metal Mining and Refining Techniques

Gold Prospecting, Mining, and Refining | Gold Processes and Uses

gold prospecting and mining
Gold as it’s found in igneous rock.

Tiny flakes of gold can be found scattered throughout igneous rocks. On occasion however, gold is found concentrated in lodes or veins, which form when molten material is forced into cracks and fissures in the surrounding rock. Rocks exposed on the Earth’s surface are relentlessly scoured by wind and water.

 

The process of erosion eventually deposits fragments of rock, and the gold it contains, into streams where they are churned into ever-smaller pieces. When gold is released from the rock, it settles and concentrates into streams and riverbeds. These gold deposits are referred to as alluvial, or placer deposits .

 

gold mining in Australia
Australia’s largest open cut gold mine, known as the Super Pit.

To strike it rich, prospectors search for exposed igneous rock and traces of eluvial  or alluvial  gold. Most alluvial gold consists of tiny grains, but occasionally a large nugget is found. The largest gold nugget ever found was discovered in Moliagul, Victoria, Australia in 1869. It weighed 156 pounds (70.92 kg) and was christened the “Welcome Stranger.”  

 

gold nuggets from mining
Gold nuggets in a pan. Image by Nate Cull.

Prospecting for gold in alluvial deposits requires few tools, but it is slow, backbreaking work. Early prospectors “panning” for gold, placed soil or river gravel in large pans with flat bottoms.

 

Water was added to the pan and it was gently rocked and swirled. This washed away lighter material, leaving heavier gold particles behind. Some believe that the legend of the Golden Fleece comes from an early variation of this method of prospecting. If small quantities of gold were found in panned samples, the prospectors moved their search upstream, in an attempt to locate the source of the gold.

 

gold mining during the Klondike
A mining operation during the Klondike gold rush.

 

gold mining rocker box
A typical 19th century rocker box or cradle.

To speed production, prospectors often turned to rocker boxes and sluices. Rocker boxes operated on the same principle as panning, but processed greater quantities of rock at once. Loose rock was placed in a large wooden box that was then filled with water and rocked like a cradle. Sluicing requires a stream of water to be directed over crushed rock in a shallow wooden trough. As the water flows through the sluice, gold is trapped in notches at right angles to the flow.

 

prospector and metal sluice
A prospector leaning over a sluice looking for gold.

Modern prospectors are armed with geological maps and satellite information. Gravel is extracted from deep water with dredgers or power shovels. Heavy machinery is employed to extract ores in large-scale open pit mines. Although hydraulic mining was invented by the Romans, who developed the technique to break up rock with powerful jets of water, it has evolved over the centuries.

 

hydraulic mining in California
A hydraulic mining operation in California (c. 1857–1870).

Today, explosives loosen rock, and loose sediment is channeled through sluices with the aid of water jets. Hydraulic mining is not an environmentally sensitive means of retrieving gold. It upsets the flow of rivers and damages adjacent land. Alluvial, or other surface deposits, account for about 20 percent of the world’s gold production.

 

open pit gold mining
Open pit mining at the Carlin Trend Mine in the U.S.

In some countries, notably Russia and South Africa, gold is found in reefs , gold-bearing lodes, or veins, which extend up to three miles below the surface. Shafts are sunken into the Earth, and horizontal passages, called crosscuts, are driven along the reef. Miners work in temperatures of about 130º F (~55º C). Huge amounts of rock are removed and up to three tons of rock may be required to produce an ounce of gold. This method of mining can be extremely expensive.

 

underground gold mining
Underground mining at the Australian Kalgoorlie mines in 1951.

 

underground gold mining
Heavy equipment is used to extract gold in underground mines such as the Campbell mine in Ontario, Canada.

After the rock containing gold is removed from the mine, it is ground into dust in a crusher. Several methods have been developed to remove the gold from this dust. In the past, mercury trapped the gold by forming an amalgam . However, mercury is extremely poisonous so its use has been curtailed. Instead, more efficient processes using cyanide have been developed.

 

processing molten gold
The processing of molten gold, bullion, before being cast into molds.

When diluted sodium cyanide is added to the crushed rock, it dissolves the gold. The gold is separated from the cyanide solution and smelted to remove some of its impurities. The molten gold, called crude bullion, is then cast into molds. The gold is further refined into pure material by electrolytic refining , chlorination, or acid leaching processes.  

 

Each country has enacted its own legislation to govern mining and other related issues such as environmental quality, resettlement, and biodiversity conservation. Some countries have better records than others, but in general, a growing awareness of environmental issues has increased governmental oversight. “Dirty Gold” is a term employed by advocates who wish to see changes in the social and environmental aspects of gold mining.

map of cyanide spill in Romania
The disastrous cyanide spill at a gold mine reprocessing facility in Baia Mare, Romania in 2000.

 

The use of cyanide is an important aspect of gold mining that needs to be carefully managed. The most significant environmental risk is its potential to leach into soil and groundwater. The International Cyanide Management Code is the accepted authority for use of cyanide in gold mining, although it is a voluntary industry program. Companies that become signatories to the code demonstrate compliance by having their operations audited by an independent third party.

 

A series of international reports over the last decade led to the development of the Initiative for Responsible Mining Assurance (IRMA), a global effort to develop a system of independent third party assurance for the mining sector. IRMA was established in 2007 by a group of mining companies, retailers, trade associations, and advocacy groups that aim to identify mining standards and establish a governance model.

 

Although most gold is produced from alluvial and other surface mining, or from underground reef mining, some gold is produced as a by-product when copper, lead, and uranium ores are refined.  An estimated 30 percent of the gold in the U.S. is produced by these means.

refining gold process
The exacting process of refining gold.

 

Experts estimate that the oceans contain about a hundred times more gold than humans have ever extracted from the surface of the Earth. In the 1920s, faced with staggering war debt, German scientists attempted to extract gold from seawater. Unfortunately, they found that the cost of extraction far outstripped the value of the gold recovered.

gold production in Utah
Gold is produced as a by-product at the Bingham Canyon Copper Mine in Utah.

 

An estimated 130,000 to 140,000 tons of gold have been mined since antiquity. About four-fifths of this gold was mined in the last 100 years, and over half of this gold was extracted in the last 50 years.  If all the gold that has ever been mined were fashioned into a cube, it would measure 63 feet in each direction. Over half of all mined gold has come from a single country, South Africa, which also contains the world’s richest goldfield, the Witwatersrand region of the Transvaal.

 

Now that we know all about the process of extracting and refining gold, we will next learn about Platinum Mining and Refining | Platinum Processes and Uses.

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