Amalgamation Process
Also: Amalgamation, Mercury process, Mercury amalgamation
A historic gold-recovery method 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 recovering gold and was for centuries the leading technique in mining. It uses mercury (Hg), which has the property of selectively dissolving gold and silver out of ground ore to form a liquid alloy mixture — the so-called amalgam. The amalgam obtained is then heated: mercury vaporises at 356.7 degrees C, and the raw metal left behind (a so-called doré bar) is afterwards further purified in refining.
How it works
The process runs in three steps:
- Crushing and grinding of the gold-bearing ore into a fine powder.
- 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 is left behind.
- Retort heating — the separated amalgam is heated in a closed retort. Mercury vapour condenses and is (ideally) captured and reused; the metal left behind has a fineness of typically 60-80%.
Historical significance
The method 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 (Potosi, Zacatecas) and during the gold rushes in California (1848) and Australia. In that era, a large share of the world's gold and silver supply came from amalgamation operations.
Environmental problems and use today
Mercury is highly toxic and persists in ecosystems. Uncontrolled mercury release poisons soils, waters, and food chains. The Minamata Convention (UN, 2013, in force 2017) obliges signatory states to phase down and ban mercury in industrial mining.
In modern large-scale mines, amalgamation has been completely displaced by cyanide leaching and other hydrometallurgical processes. However, it remains widespread in artisanal and small-scale gold mining (ASGM) — above all in sub-Saharan Africa, South America, and South-East Asia — because the equipment is cheap and the process simple to operate. ASGM sectors are estimated to contribute around 35-40% of global anthropogenic mercury emissions.
Comparison with modern methods
| 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 viable. You can work out the material value of metal already recovered with the melt-value calculator.
In brief
The amalgamation process is historically important but environmentally problematic: mercury remains in the ecosystem and endangers people and nature. Modern large-scale mines long ago replaced it with safer processes; in informal small-scale mining it remains widespread despite international bans.