Gold Mining Basics
For thousands of years, gold has been highly valued because of its beauty and relative scarcity. Not only has it been used in jewelry and other valuable works of art, but it has also served as the basis for exchange and barter, and as the standard for many modern currencies. Until recently, most of the gold bullion produced each year ended up in the vaults of government treasuries or central banks. However, because it has many desirable physical and chemical properties, such as resistance to oxidative corrosion and superior electrical and heat conductivity, many other uses of gold have become popular in modern times. Gold is now used in such diverse businesses as dentistry, electronics, and the aircraft-aerospace industry.
Gold ranks 80th in order of abundance of the elements of the earth’s crust. It occurs in many different geological environments and in many different kinds of rock. There are two principal types of deposits where gold is concentrated enough to make commercial mining viable: primary (lode and disseminated) deposits, and secondary (placer) deposits. Primary deposits are found in hard-rock. Lode deposits (which provide most of the gold produced today) are where gold has been concentrated in cracks in rocks, in the form of high grade veins. Gold that is widely distributed throughout a large volume of rock is in a disseminated deposit. Placer deposits are formed when gold erodes from primary deposits, and is carried downstream as metallic particles in the form of dust, flakes, grains or nuggets. Placer deposits are usually found in the bottom of a valley. In some cases, rain water washed the eroded gold particles into a stream, where they sank to the bottom. Over millions of years, placer deposits can become compressed into rocks. Deposits of native gold (pure gold), which are rare, are usually found in placer deposits.
Modern technology and advances in the earth sciences (such as geology and chemistry) have greatly improved mineral exploration techniques. Many deposits on the surface (the easiest to locate) have already been found, but new, sophisticated techniques are now helping with the discovery of underground ore deposits. Geological maps are valuable tools for finding promising areas for exploration. Knowledge of geological processes and mineral distribution patterns enable geologists to compare known deposits to the physical and chemical characteristics of other rocks. After the geology of an area appears to be favorable, technology such as airborne and ground geophysics, remote sensing and geochemistry are used to examine a property. The information gathered is used to designate areas where greater investigation is warranted.
Exploration geochemistry can also help locate deposits of ore. Small amounts of minerals that occur in surface rocks, soil, plants and ground water are sampled and analyzed. The presence of unusual amounts of certain elements can indicate that large deposits of valuable minerals, such as gold, are underground.
Once an area has been targeted for further investigation, trenching is used to obtain samples for testing if the minerals are fairly close to the surface. Otherwise, samples are obtained by drilling. Various tests are run to determine if mineralization is present. For example, nitric acid can be used to confirm the presence of gold in other materials.
When significant mineralization is found, additional drilling is used to determine the size and parameters of the deposit. The additional samples obtained are also analyzed for their mineral composition. If the deposit contains enough gold to make production economically feasible, metallurgical tests are then run to determine the quality of the metal and the appropriate processing technique for removing it.
Lode deposits are removed by hard-rock mining, where miners drill and blast the rock to remove it from the ground. The topography of the area and physical characteristics of a deposit will determine what kind of mine is built. Open-pit mining may be used if a deposit is near the surface, at least in the initial stages. Holes are drilled and filled with explosives, which are detonated to break up the ground. The ore is then loaded into trucks and hauled to a mill for processing. If a deposit is underground or if open-pit mining is not economically feasible, underground mining is done. A horizontal tunnel called an adit, or a vertical tunnel called a shaft, is dug. Stopes (short tunnels leading from the adit or shaft) are then dug to access the ore. Holes in a face (a surface containing ore) are drilled and filled with explosives. After detonation, the broken ore is loaded into trucks and hauled to a mill.
Once at the mill, the ore is crushed into smaller loose rock. From the crusher, high-grade ore goes into a series of grinding mills where it is ground into a fine powder. Low-grade ore instead goes directly to the separation process, where mineral particles are removed from the host rock.
Placer deposits are usually removed by heavy equipment scooping up the rock, sand, and gravel from the valley bottom where the mineralization occurs. The gold is recovered by gravity separation. The ore is sifted at the site by washing it with water. The gold can be separated out from the other materials because it’s heavier, and it sinks to the bottom. It’s essentially the method used by amateur miners who pan for gold by hand. Hydraulic mining can also be used to remove placer deposits, although it can be damaging to the environment because it may cause erosion at the site, and mud in streams below the site. Where large amounts of loose gravel and sand or soil are poorly packed, water cannons can be used to strip away entire hills of the material, which is then run through a sluice (a wooden trough with riffles). The gold sinks to the bottom of the sluice and is recovered.
There are several methods of extracting gold from ore. Low-grade ore goes directly to a leaching process, where the gold is leached from the host rock using a sodium cyanide solution. The cyanide dissolves the gold, which is then collected. For high-grade ore, processes differ depending on the type of ore involved, in order to achieve maximum recovery of the gold. Oxide ore, like low-grade ore, goes directly to the cyanide leaching process. Refractory ore contains carbon and so has to be heated to 1000 degrees Fahrenheit before it can go to the leaching process. The high heat burns off sulphide and carbon, so that the gold can then be leached from the ore. Sulphide refractory ore doesn’t contain carbon, and it is oxidized in an autoclave in order to safely separate the sulphide before it goes to the leaching process.
After the leaching process, the gold is extracted from the cyanide solution and deposited onto activated carbon, from which it is chemically stripped. Next it is melted in a furnace called a smelter, along with a chemical mixture called “flux.” The flux combines with all impurities except silver, and floats on top of the gold. The flux is poured off and the molten gold is poured into molds to create dore bars, which are about 90% pure gold. The dore bars, or ingots, are then sent to a refinery where any remaining impurities are removed through further processing to create 99.99% pure gold.
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