What Is Gold Amalgamation?
Amalgamation is a gold extraction technique that exploits mercury's unique ability to form a liquid alloy — called an amalgam — with gold and silver. The process has been used for centuries, dating back to Roman mining operations, and remains relevant in certain artisanal and small-scale gold mining (ASGM) contexts today, though its use is increasingly regulated due to environmental and health concerns.
The Science Behind Amalgamation
Mercury (Hg) is a liquid metal at room temperature that readily wets and bonds to native gold particles when they come into contact. This property — called wettability — allows mercury to selectively capture gold from a slurry of crushed ore while leaving most other minerals behind.
The key chemical process is straightforward:
- Mercury physically coats and surrounds free gold particles.
- Gold dissolves into the mercury to form a semi-solid amalgam (typically 30–50% gold by weight).
- The amalgam is physically separated from the remaining slurry by its higher density.
- Heat is then applied to volatilize the mercury, leaving behind sponge gold.
Step-by-Step Amalgamation Process
- Ore Preparation: The ore is crushed and milled — often in a ball mill or stamp mill — to liberate fine gold particles from host rock.
- Mercury Addition: Mercury is introduced to the ore slurry in a mixing vessel, rotating drum, or directly onto a copper amalgamation plate coated with mercury.
- Contact Time: The mixture is agitated to maximize contact between mercury and gold particles. This typically takes 10–30 minutes depending on ore grade and gold particle size.
- Amalgam Separation: The amalgam settles due to its high density. It is separated by screening or gravity, then squeezed through chamois leather or canvas to remove excess mercury.
- Retorting: The amalgam is placed in a retort — a sealed vessel — and heated to approximately 350–400°C. Mercury vaporizes, is captured by a condenser, and is recovered for reuse. What remains is sponge gold.
- Smelting: The sponge gold is melted in a furnace to produce a gold bullion bar or doré.
Types of Amalgamation
| Type | Description | Common Use |
|---|---|---|
| Plate Amalgamation | Ore slurry flows over copper plates coated with mercury | Small-scale alluvial operations |
| Barrel Amalgamation | Ore and mercury tumbled in a rotating drum | Hard rock artisanal mining |
| Pan Amalgamation | Mercury added to ore in a grinding pan | Historical; rare today |
Limitations and Considerations
While amalgamation is simple and low-cost, it has significant drawbacks:
- Mercury toxicity: Elemental mercury and its vapors are highly toxic. Improper handling causes neurological damage and environmental contamination.
- Gold recovery rate: Amalgamation works well only on coarse, free-milling gold. Fine, refractory, or sulfide-bound gold is largely unrecoverable by this method.
- Regulatory restrictions: Many countries have restricted or banned mercury use in mining under the Minamata Convention on Mercury.
For operations where amalgamation is still practiced, proper retorting equipment and mercury containment protocols are essential to minimize exposure and environmental release.
Alternatives to Amalgamation
Modern operations often replace amalgamation with gravity concentration (using shaking tables or centrifugal concentrators) followed by cyanidation or flotation. These methods offer higher gold recovery rates and eliminate mercury-related hazards entirely.