Chemical Mining Methods for Gold
The myth of glittering solid gold veins being found lining mine walls are not at all representative of the modern methods of gold mining. Most gold is contained in minute amounts in other rock, called ore. To get the gold, chemical processes have been invented that will separate the mineral from the other materials.
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Mercury Amalgamation
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The process of bonding mercury to gold locked within ore is one chemical process that can extract very clean gold from rock. Clean means gold that is not oxidized with rust or coated with other minerals. Mercury bonds readily to clean gold and absorbs it, which results in the amalgam, or mix of the two minerals. Ore is crushed to expose the gold, then treated with mercury and a little water to help wash away ore. The mercury bonds to gold particles it contacts. A pulp is created from this combination, from which the mercury then must be removed, or retorted. A sealed process in which the material is heated very slowly to 1,500 degrees to vaporize and collect the mercury is used to separate the two, leaving behind the gold.
Cyanidation
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The most common method of chemical extraction of gold is by applying a weak solution of cyanide to finely ground ore to remove gold particles. The ore is treated with cyanide solution, added with lime in large vats called agitators where it is stirred and aerated for 24 to 48 hours. The cyanide actually dissolves the gold, and when the pulp is separated from the liquid, the solution is filled with gold particles. Zinc dust is added to the solution, the zinc acts to separate the gold from the liquid and form another amalgam containing gold particles. Acid is used to wash away the zinc, leaving the gold behind.
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Heap Leaching
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Treating ore with cyanide is done in another more simplified method known as heap leaching. This method is a low-cost method but has environmental concerns because it is done directly at the site without a refinery. A polyethylene liner is laid down and set in such a way that liquid can collect in a pool at the bottom and be channeled to a man-made pond. Ore is heaped on the liner, and a cyanide solution is sprinkled over it, dissolving the gold and allowing it to collect in a pool on the liner. This is channeled off into the pond to be further processed to remove the gold from the solution. This method is faster and less expensive but has a lower gold recovery rate.
Vat Leaching
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Vat leaching also employs the use of a cyanide solution, but this is used in finely ground ore with gold that will be the most difficult to extract. Because it is an expensive method, this is not frequently used. A vat is filled with the ore, then solution is added. The vat is a sealed impermeable container. Once the gold is dissolved into the solution, either another metal or a cathode with an electrical charge is added. Either will attract the gold, which can then be extracted from the solution.
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References
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