The long history of gold extraction is closely tied to the unique properties of liquid mercury. Amalgamation is the process where mercury is used to dissolve or wet fine particles of metals, most notably gold and silver, to form an alloy known as an amalgam. This technique provided a relatively simple way to separate precious metals from crushed ore, establishing itself early in the history of mining. Understanding the quantitative limit of gold that mercury can hold has significant implications for both historical mining efficiency and modern environmental concerns.
The Chemistry of Amalgamation
Mercury is the only common metal that exists as a liquid at room temperature, giving it a unique advantage in forming alloys. When fine gold particles contact elemental mercury, the gold atoms are readily attracted to the mercury matrix through metallic bonding. The resulting amalgam is a true alloy, not a simple physical mixture, and can be liquid, paste-like, or solid depending on the metal ratio. Initially, gold dissolves into the liquid mercury, but as more gold is incorporated, the substance transitions into various solid intermetallic compounds, such as AuHg2 and Au3Hg.
Limits of Gold Solubility in Mercury
The holding capacity of mercury for gold is substantial but not limitless. At standard room temperature, liquid mercury can dissolve a small amount of gold, approximately \(0.1\%\) to \(1\%\) by weight, forming a true solution. The amalgamation process continues beyond this initial solubility limit through the formation of solid gold-mercury compounds. The overall gold content in a working amalgam can reach a much higher concentration, often between \(40\%\) and \(50\%\) by weight, where these solid compounds are suspended within the excess liquid mercury.
Historical Use in Gold Extraction
The high affinity between gold and mercury has been exploited for millennia, with evidence of its use dating back over 3,500 years. This process was particularly effective in separating fine gold particles, sometimes called “flour gold,” from large volumes of ore. In placer mining, mercury was added to sluice boxes to catch fine gold from river sediments. In hard rock mining, crushed ore was washed over mercury-coated plates where the gold adhered and formed an amalgam.
Retorting
Miners collected the soft amalgam and separated the gold by heating it in a process called retorting. This heating vaporized the mercury, leaving behind concentrated, relatively pure gold.
Health and Environmental Hazards of Mercury Amalgamation
The method’s effectiveness was overshadowed by the toxicity of mercury, particularly during the recovery stage. Heating the amalgam in the open air, often without a closed retort, releases highly toxic elemental mercury vapor into the atmosphere. Inhaling this vapor causes severe, long-term neurological effects, including tremors and memory loss, known historically as “mad hatter disease.”
Environmental Contamination
Mercury lost during mining contaminates waterways and soil, where microbes transform it into methylmercury. This potent neurotoxin bioaccumulates in the aquatic food chain, building up in fish and shellfish. The consumption of contaminated seafood poses a significant health risk to humans and wildlife. International efforts, such as the Minamata Convention on Mercury, now work to reduce or eliminate mercury use in artisanal and small-scale gold mining.