The amalgamation is a traditional and highly efficient gold extraction technique that utilizes the affinity between mercury (Hg) and native gold to form an amalgam (a gold-mercury alloy), thereby separating and concentrating gold particles. Based on its production method, it can be categorized into internal amalgamation and external amalgamation.
Internal Amalgamation
Internal amalgamation is a gold recovery method where mercury (Hg) is directly added to grinding or mixing equipment, enabling thorough contact between mercury and gold particles in the ore to form amalgam (gold-mercury alloy). This technique selectively recovers gold and is particularly effective for coarse gold particles (0.1–2 mm), especially in ores with high free gold content.
Common Equipment
Common equipment includes amalgamation barrels, ore crushers, amalgamation pans, or specialized small ball mills, rod mills, etc. Internal amalgamation is conducted in sealed containers, enabling effective control of mercury pollution.
Amalgamation Barrel
- A drum-type device lined internally with wear-resistant materials (such as rubber or steel plates).
- During operation, ore, water, and mercury are added. Rolling or agitation promotes contact between gold particles and mercury.
Amalgamation Pan
Traditional manual or mechanically stirred equipment is used for small-scale production.
Ball Mill + Amalgamation
In some gold processing plants, mercury is added directly during ball milling, enabling simultaneous grinding and gold recovery.
Process Description
- Ore Crushing: Raw ore is crushed to an optimal particle size (typically 1–3 mm) to liberate gold grains.
- Amalgamation:
- Ore, water, and mercury (Hg: Au weight ratio ~3:1–5:1) are mixed in barrels or pans.
- Stirring or tumbling continues for 4–12 hours to ensure thorough gold-mercury contact, forming liquid/paste amalgam.
- Amalgam Separation: Slurry is settled or filtered; denser amalgam (Au-Hg alloy)collects at the bottom.
- Amalgam Distillation: Amalgam is heated in retorts at 350–450°C to vaporize and recover mercury, leaving porous gold (purity 60–90%).
- Tailings Treatment: Residual slurry containing trace mercury undergoes activated carbon adsorption or sulfide precipitation.
Applicability
This method is commonly employed when gold-bearing ores contain negligible amounts of copper, lead, and zinc minerals, lack sulfides that readily cause mercury to pulverize, and feature relatively coarse gold grain sizes. It is also frequently used in placer gold mines to separate gold from other heavy minerals.
Primary Drawback (Mercury Pulverization)
During ore crushing, mercury readily fractures into minute particles. These particles become encapsulated by films of base metal oxides, lubricating oils, and fine mineral silt, losing their cohesive ability and resulting in mercury pulverization. Pulverized mercury is difficult to recover, leading not only to mercury loss but also to the entrainment of some gold.
External Amalgamation
The external amalgamation is a type of mercury amalgamation gold extraction process. Its characteristic lies in the mercury not being directly mixed with the ore slurry. Instead, it is spread over the surface of mineral processing equipment (such as mercury plates or sluice boxes). As the slurry flows over it, gold particles come into contact with the mercury to form amalgams, separating them from other gangue minerals. In gold processing plants, amalgamation plates are typically installed at the discharge outlet of ball mills, primarily to capture coarse-grained free gold within the milled ore product.
Primary Equipment
1. Amalgamation Plate
Construction: Consists of three parts: support frame, bed surface, and amalgamation plate.
- Fixed Type: Currently, China predominantly uses a fixed flat amalgamation plate, though stepped types and those with central collection channels are also employed.
- Amalgamation Plate Material: Commonly used materials include red copper plates and silver-plated copper plates, with silver-plated copper plates yielding the best results.
Installation: For ease of silver plating and replacement, electrolytic copper plates are typically cut into small sections measuring 400–600 mm wide and 800–1200 mm long. After silver plating, these sections are laid in overlapping fish-scale patterns on the bed surface according to the frame's inclination. The bed surface must be completely sealed to prevent slurry leakage.
2. Riffled Sluice Box
A layer of mercury is spread across the bottom to enhance gold particle collection.
Process Description
- Slurry Preparation: Crush raw ore to 0.1–3 mm to fully expose gold particles.
- Mercury Plate Application:
- Apply liquid mercury evenly (approx. 0.05–0.2 mm thick) onto the plate to form a wetting layer.
- A small amount of alkali (Na₂CO₃) may be added to enhance mercury fluidity and reduce oxidation.
- Slurry Flow Over Mercury Plate:
- The slurry slowly flows over the mercury plate surface (flow velocity 0.2–0.5 m/s). Gold particles, being denser, sink and form amalgams upon contact with mercury.
- Light gangue minerals are washed away by the water flow.
- Scraping and processing of amalgam: Amalgam is periodically scraped off, then subjected to pressure filtration to remove residual mercury before distillation for gold recovery.
- Mercury recovery: Mercury is recovered through distillation and condensation and recycled.
Key Technical Parameters
The determination of mercury plate area relates to the ore processing volume, ore properties, and the role of amalgamation in the process flow:
Slurry flow conditions: Flow layer depth 5–8 mm, flow velocity 0.5–0.7 m/s.
Mercury plate quota:
- Conventional processing: 05–0.5 m²/ton·day.
- For capturing large free gold particles only (with subsequent flotation/cyanidation for tailings): 1–0.2 m²/ton·day.
Feed conditions: Feed concentration 10–25%, feed particle size 3–0.4 mm.
Mercury consumption: Typically 3–8 g/ton.
Mercury Exposure Control & Environmental Protection in Amalgamation
Mercury can enter the human body through the skin, mucous membranes, and respiratory tract in liquid, salt, or vapor form. It accumulates in organs such as the kidneys, liver, brain, and bones, causing acute or chronic poisoning. Mercury vapor poses the greatest hazard.
Emission Standards
- Airborne mercury levels must not exceed 0.01–0.02 milligrams per cubic meter.
- The maximum allowable concentration of mercury and its compounds in industrial wastewater is 0.05 milligrams per liter.
Protective Measures
To safeguard the environment and worker health, facilities handling mercury must strictly implement the following protective measures:
- Operating Procedures: Containers holding mercury must be sealed to prevent vapor leakage. Operators must wear protective gear to avoid direct skin contact with mercury. Eating, smoking, or storing food in mercury-containing rooms is prohibited.
- Ventilation and Exhaust: Enhance ventilation in mercury mixing workshops and gold refining rooms. High-risk operations like mercury paste washing must be conducted in enclosed cabinets equipped with ventilation systems.
- Facility Construction: Floors in mercury-handling facilities should be constructed with non-mercury-absorbing materials and sloped at 1–3%. Walls and floors must remain smooth.
- Cleaning and Purification
Floors must be regularly scrubbed with soapy water or potassium permanganate solution (1:1000).
2. Mercury collection devices should be installed beneath work cabinets and in outdoor sewage pits to prevent mercury runoff.
3. Workshops handling mercury should periodically purify air using manganese dioxide absorption (achieving up to 99% mercury vapor absorption efficiency).
Summary of Gold Amalgamation Process
The amalgamation method, as a traditional gold extraction process with a long history, was once widely used due to its simple equipment, low investment, and easy operation. It demonstrates exceptionally high recovery efficiency for coarse liberated gold (especially clean-surfaced native gold), enabling "early and maximal recovery" to minimize gold loss in subsequent flotation or cyanidation circuits.
However, this process presents significant limitations: it struggles to recover fine gold particles efficiently and is sensitive to ore composition (high sulfur or clay contents can cause mercury powdering). The most critical issue is mercury contamination, posing severe threats to both environmental safety and operator health.
Due to tightening environmental regulations, mercury amalgamation has seen drastically reduced applicability. Some countries have completely banned its use, while in China, it is now only permitted in specific gold mines and small-scale operations, strictly paired with rigorous protective measures. In placer gold mining, amalgamation remains employed to separate gold from heavy minerals, whereas in lode gold processing, it primarily functions as an auxiliary step within integrated circuits, complementing flotation, gravity separation, or cyanidation for capturing coarse free gold.
Looking ahead, with increasing environmental restrictions and advancements in gravity separation technology (e.g., centrifugal concentrators, shaking tables), mercury amalgamation will progressively be replaced by non-toxic alternatives, eventually fading from mainstream gold extraction. Its current role should be defined as a transitional supporting process under strict control—never a standalone solution.