Formation, nature and structure of amalgam

In the production process, gold and other minerals in pellet form in contact with the mercury. At this time, other minerals are not trapped by mercury and flow with the slurry, and the gold particles are wetted by mercury to capture (Fig. 1) and diffuse into the interior of the gold particles. The process of diffusion of mercury into the gold particles is to form AuHg _{2 on the} surface of the gold particles, and then gradually diffuse into the deep part of the gold particles to form Au ₂ Hg until a solid solution of Au ₃ Hg is formed ( FIG. 2 ). At the center of the amalgamated gold, there is usually no gold that forms an amalgam with mercury. The gold particles must be in contact with mercury for about 1.5 to 2 hours before they can be completely amalgamated, so only fine gold is fully amalgamated during the amalgamation time.

Figure 1 State of mercury when it comes into contact with gold and other minerals

Figure 2 Amalgamation process of gold particles

Hansen (1958) plotted the accepted mercury-gold balance diagram as shown in Figure 3. Mercury can dissolve 0.06% of gold at 20 ° C. As the temperature increases, the mobility of mercury increases and the solubility of gold increases. Gold can form a solid solution with 15% mercury at 20 °C. This is the maximum ratio of gold mercury compounds that can form solid solutions.

Figure 3 Mercury-gold two-phase equilibrium diagram (Hansen, 1958)

However, in the practice of amalgamation, it is impossible to achieve a balance between gold and mercury. Because amalgam is often composed of gold particles with a surface covering mercury, mercury gold compounds, and liquid mercury containing a small amount of gold (including excess). The resulting amalgam is a silvery white paste mixture consisting of a compound and a solid solution having the same properties as a general alloy. Amalgam contains less than 10% gold in liquid form, while gold containing up to 12.5% ​​is dense. When the amalgam is heated to 400 ° C, the mercury sublimation is separated from the amalgam in an elemental state. And the amalgam is easily decomposed at a temperature lower than the melting point to precipitate excess mercury.

Mercury paste scraped from industrial production, washed with water and squeezed out excess mercury to obtain a dense solid mercury paste. The amount of gold in the solid amalgam is related to the size of the pressing force and the compactness of the filter cloth, which is usually very close to the content of AuHg ₂ (32.95% Au). And the thickness of the amalgamated gold particles will directly affect the gold content of the mercury paste. When the coarse-grained gold is mixed with mercury, the amalgamation of the gold grain center is incomplete, and the mercury paste contains gold up to 40% to 50%. When fine-grained gold is mixed with mercury, the amalgam of the gold particles is complete and the surface area is large. There is a lot of mercury attached, and the mercury paste contains only 20% to 25% of gold.

Mercury paste contains other metal minerals, quartz or gangue debris in addition to gold and mercury. These substances are mostly mechanical mixtures and are not compounds of mercury. However, a small amount of metals such as silver and copper contained in the mercury paste are the result of amalgamation of these metal parts.

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