Amalgamation Process
Also: Amalgamation, Mercury process, Mercury amalgamation
A historical gold recovery 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 of gold recovery and was for centuries the most important technique in mining. It uses mercury (Hg), which has the property of selectively dissolving gold and silver out of ground ore and forming a liquid alloy mixture – the so-called amalgam. The amalgam obtained is then heated: mercury evaporates at 356.7 °C, and the remaining raw metal (a so-called dore bar) is subsequently refined further in the affinage stage.
Operating Principle
The process runs in three steps:
- Crushing and grinding of the gold-bearing ore into fine rock (flour).
- Mercury addition – the ore is brought into contact with liquid mercury (in barrels, on shaking tables or by direct stirring). Gold and silver bind preferentially to the mercury; barren rock remains behind.
- Retort heating – the separated amalgam is heated in a closed retort. Mercury vapour condenses and is (ideally) captured 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 from the 16th to the 19th century – above all in the silver mines of Latin America (Potosí, Zacatecas) as well as during the gold rush in California (1848) and Australia. In that era, a large part of the world's gold and silver supply came from amalgamation operations.
Environmental Problem and Present-Day Use
Mercury is highly toxic and persists in ecosystems. Uncontrolled mercury discharge poisons soils, waters 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In modern large-scale mines, the amalgamation process has been completely superseded by cyanide leaching and other hydrometallurgical processes. However, in artisanal and small-scale gold mining (ASGM) – above all in sub-Saharan Africa, South America and South-East Asia – it continues to be widely used, because the equipment is cheap and the process simple 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 cost |
|---|---|---|---|
| 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 even more elaborate, cleaner technologies profitable. You can determine the material value of metal already recovered with the melt value calculator.
In Brief
The amalgamation process is historically significant but environmentally problematic: mercury remains in the ecosystem and endangers people and nature. Modern large-scale mines replaced it long ago with safer processes; in the informal small-scale sector it is still widespread despite international bans.