Amalgamation Process
Also: Amalgamation, Mercury Process, Mercury Amalgamation
Historical gold extraction process in which liquid mercury dissolves gold and silver from ore and binds them as an amalgam, which is then separated by heating.
The amalgamation process is one of the oldest chemical methods for gold extraction and was for centuries the most important technique in mining. It uses mercury (Hg), which has the property of selectively dissolving gold and silver from crushed ore and forming a liquid alloy mixture – the so-called amalgam. The recovered amalgam is then heated: mercury evaporates at 356.7 °C, and the remaining crude metal (a so-called Doré bar) is subsequently further refined at the refinery.
Working Principle
The process runs in three steps:
- Crushing and grinding of the gold-bearing ore to fine rock (powder).
- Mercury addition – the ore is brought into contact with liquid mercury (in drums, shaking tables, or by direct stirring). Gold and silver preferentially bond to the mercury; barren rock remains behind.
- Retort heating – the separated amalgam is heated in a closed retort. Mercury vapour condenses and is (ideally) collected and reused; the remaining metal has a fineness of typically 60–80 %.
Historical Significance
The process was already used by the Romans and reached its peak between the 16th and 19th centuries – particularly in the silver mines of Latin America (Potosí, Zacatecas) and during the California Gold Rush (1848) and in Australia. During this era, a large proportion of the world's gold and silver supply came from amalgamation operations.
Environmental Problems and Current Use
Mercury is highly toxic and persists in ecosystems. Uncontrolled mercury discharge poisons soils, water bodies, and food chains. The Minamata Convention (UN, 2013, in force 2017) obliges signatory states to progressively reduce and ban mercury in industrial mining.
In modern large-scale mines, the amalgamation process has been entirely superseded by cyanide leaching and other hydrometallurgical processes. However, it continues to be widely used in artisanal and small-scale gold mining (ASGM) – particularly in sub-Saharan Africa, South America, and South-East Asia – because the equipment is inexpensive and the process is easy to handle. ASGM sectors are estimated to account for around 35–40 % of global anthropogenic mercury emissions.
Comparison with Modern Processes
| Process | Metal Recovery | Environmental Risk | Capital Expenditure |
|---|---|---|---|
| Amalgamation | 60–90 % | Very high (Hg) | Very low |
| Cyanide leaching | 90–98 % | High (CN⁻) | Medium |
| Flotation + smelting | 85–95 % | Low to medium | High |
The current gold price influences which processes can be operated economically – higher prices make more complex, cleaner technologies viable. The material value of already recovered metal can be calculated with the Melt Value Calculator.
In Brief
The amalgamation process is historically significant but ecologically problematic: mercury persists in the ecosystem and endangers people and nature. Modern large-scale mines have long since replaced it with safer processes; in informal small-scale mining it remains widespread despite international bans.