Mercury Usage in Gold Mining and Why It Is a Problem
In alluvial, colluvial or eluvial placer deposits, mercury is used for the extraction of secondary gold by gravity methods. Thanks to Mercury's intrinsic properties, it allows for the separating of gold from other materials easily. "Mercury readily forms alloys with other metals called amalgams (...) Mercury is amalgamated with gold to facilitate the recovery of gold from its ores." wrote Anne Marie Helmenstine, our About Guide for chemistry.
It is heated to evaporate the mercury, leaving the gold behind to recover it from the 50% mercury and 50% gold amalgam.
Subsistence artisanal small-scale gold mining is a way to survive for an estimated 10-15 million miners in 70 countries, including approximately 3 million women and children. Surprisingly and on top of being the world’s largest employer in gold mining and representing 90% of the gold mining workforce worldwide, small-scale gold mining produces 15% of the annual gold production.
The Guyana Shield region (Surinam, Guiana and French Guiana), Indonesia, The Philippines and part of Western Africa’s coast (e.g., Ghana) are particularly impacted by the phenomenon. Under the socio-economic and political conditions found in the small-scale gold mining operation, the use of mercury is often considered as the easiest and most cost-effective solution for gold separation.
Mercury Watch, an organization "dedicated to collecting, analyzing, and publically serving information about mercury released to the environment," claim that "artisanal scale gold mining is the single largest demand for mercury in the world. An estimated 1400 tons of mercury were used by Artisanal scale gold mining miners globally in 2011".
Why Is the Use of Mercury a Problem?
Any of the four stages of the small-scale gold production process, namely amalgamation, separation of amalgamation, removal of excess mercury, and burning of the remaining amalgam, release mercury into the environment. As a result and along Wikipedia, "11% of the human-generated sources of mercury (50% of the total, the other half comes from natural sources, such as the volcanic activity) comes from gold production. The three largest point sources of mercury emissions in the U.S. are the three largest gold mines. The hydrogeochemical release of mercury from gold-mine tailings has been accounted as a significant source of atmospheric mercury in eastern Canada".
It is acknowledged to be second only (though quite far behind…) to coal combustion as a source of human-generated mercury emission in the atmosphere.
It is well known that mercury is highly toxic, causing damage to the nervous system at even relatively low levels of exposure. According to the World Health Organization: "Elemental and methylmercury are toxic to the central and peripheral nervous system. The inhalation of mercury vapor can produce harmful effects on the nervous, digestive and immune systems, lungs and kidneys, and may be fatal. (…) Neurological and behavioral disorders may be observed after inhalation, ingestion or dermal application of different mercury compounds. Symptoms include tremors, insomnia, memory loss, neuromuscular effects, headaches and cognitive and motor dysfunction. (…) Children are especially vulnerable and may be exposed directly by eating contaminated fish. Methylmercury bioaccumulated in fish and consumed by pregnant women may lead to neu¬rodevelopmental problems in the developing fetus. Transplacental exposure is the most dangerous, as the fetal brain is very sensitive. Neurological symptoms include mental retardation, seizures, vision and hearing loss, delayed development, language disorders and memory loss. In children, a syndrome characterized by red and painful extremities called acrodynia has been reported to result from chronic mercury exposure."
Mercury can contaminate the atmosphere and water at a very long distance, demanding, therefore, a global response to reduce at the lowest possible its uncontrolled use by the small-scale gold mining industry.