First, the theoretical basis of amalgamation

Although the amalgamation of gold has a long history, the systematic research work on the theory of mercury amalgamation has only made great progress in recent decades. Mercury amalgamation is based on the selective wetting (capture) of gold particles in the slurry by liquid metal mercury, which separates it from other metal minerals and gangues, and then the mercury diffuses into the trapped gold particles. Amalgam (alloy). The amalgam is then distilled in a mercury vaporizer to separate the mercury from the amalgam to obtain gold.

In the process of amalgamation, the contact between the surface of the mercury and the surface of the gold particles in the slurry is carried out in an aqueous medium. When gold is in contact with mercury, the new contact surface formed between them replaces the original gold and water and the interface between mercury and water, thereby reducing the surface energy of the phase and destroying the contact between the surface of the mercury and the surface of the gold particles. Hydration layer. At this point, mercury rapidly diffuses along the surface of the gold particles and promotes a reduction in surface energy at the interface. As mercury diffuses into the interior of the gold particles, it forms a mercury gold compound (amalgam) and simultaneously releases heat. This exothermic reaction is the result of the interaction between atoms.

The reason why mercury selectively wets gold and diffuses into the interior of the gold particles is that the surface of the gold particles has the thinnest oxide film. It is well known that when a metal surface is in contact with air, it is oxidized to form an oxide film. However, compared with other base metals, gold has the slowest oxidation rate and the thinnest oxide film. This is the root cause of gold wetting and amalgamation of mercury. In addition to gold, silver , copper , zinc , tin and cadmium can also be combined with mercury to form amalgam, and even platinum can form platinum amalgam with the participation of zinc or sodium. However, the surface of silver and platinum can form a dense, hard oxide film, and amalgamation is difficult. Other base metals are difficult to remove due to the oxide film on the surface, and cannot directly form an amalgam.

Second, the formation, nature and structure of amalgam

During the production process, gold and other minerals are in granular contact with 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.

Third, factors affecting amalgamation

In the process of mercury amalgamation, the wetting effect of mercury on gold is affected by the particle size of gold and the degree of monomer dissociation, the composition of gold and mercury, the pH of the slurry medium, the concentration and temperature of the slurry, the mineral composition, and the process, equipment and operation of the amalgamation process. Factors such as conditions. The main influencing factors are:

(1) The particle size of gold and the degree of monomer dissociation. The size, shape, structure of the gold particles, and the effect of the continuum on the mercury amalgamation effect are mainly determined by the degree to which the gold particles dissociate from the minerals encapsulating it, that is, the grinding (for the sand mine, which is the scrubbing and crushing operation) . One of the distinguishing features of the amalgamation process is the use of higher pulp concentrations and larger grinding ore size. In general, gold particles suitable for amalgamation are between -1 and +0.1 mm. The mercury-mixed test results of the former Irkutsk Institute of the Soviet Union on sand remineralized concentrates are shown in Fig. 4. In the grinding cycle of a mine in China, the maximum particle size of the amalgamation of the mercury-mixed plate is 0.15mm. The gold particles dissociated in this particle size range can be immediately mixed with mercury, and the recovery rate can reach 77% to 78.8%. It can be seen that under the conditions of perfect amalgamation, the effect of mercury amalgamation is mainly determined by the degree of monomer dissociation of natural gold. When the gold particles are small and covered by the slime or the film, the mercury amalgamation effect is not good. When the ore is concentrated under the condition of large concentration of pulp, the fine gold particles below 0.03 mm are easily lost with the slurry, and it is not easy to form an amalgam with the mercury on the mercury plate, so that the recovery rate is lowered.

Figure 4 Mercury mixing efficiency of gold with different particle sizes in sulfuric acid medium

1--1 to +0.2 mm; 2--0.2 to +0.074 mm; 3--0.074 mm

(2) The composition of the gold particles. In all gold deposits, the color of gold is higher than the color of vein gold. In the vein gold, the color of gold in the oxidized ore is higher than the color of gold in the original ore. In natural gold, the main component except gold is silver. The amount of silver in natural gold determines the color, specific gravity and its ability to reflect visible light. In addition, native gold still contain copper, nickel, iron, zinc, lead and other impurities. In the practice of amalgamation, “pure” gold is most likely to be mixed with mercury, and excessive content of heavy metal impurities will affect the quality of mercury and reduce the effect of mercury. When the gold particles are in contact with pyrite, gangue, etc., it takes a long time to collect at the end of the mercury plate.

The surface of the gold particles may be covered by different films, and the thickness of the film is from less than 1 μm to 100 μm. The material forming the film includes a substance brought into and mechanically adhered to the gold particles during the grinding process, iron oxide formed after the iron is worn, components of the gold mineral and substances formed by the reaction of the impurities, and oil for the working machine. Deterioration caused by quality. These films should normally be removed during the grinding or before the amalgamation, which approach is to add lime, cyanide, chloride, heavy chromate, permanganate, lead oxide or an alkali agent.

(3) The composition of mercury. The use of mercury containing gold, silver and a small amount of heavy metals (both copper, lead and zinc are less than 0.1%) is better than using pure mercury. When zinc amalgam is used in a dilute sulfuric acid medium, not only can gold be captured, but platinum can also be collected. However, when the heavy metal impurities are excessively present in the mercury, the surface of the mercury is concentrated to greatly reduce the surface tension of the mercury, and the mercury wetting ability to gold is lowered. For example, when mercury contains 1% copper, the diffusion process of mercury on gold is 30-60 min; when copper contains 5%, the diffusion process takes 2~3 h. Mercury contains 0.1% to 5.0% zinc, so it will not wet the gold, and will not spread into the gold particles. Mercury containing less than 0.05% zinc has good wettability to gold. When a large amount of copper or iron is mixed into the mercury, the amalgam becomes hard and brittle and then pulverized. Therefore, a large amount of iron filings are mixed during grinding, or the ore contains oxidized minerals that are easily oxidized, or the strong mechanical action of mercury and ore surfaces, or heavy metal ions generated in the slurry, can cause mercury pulverization during the amalgamation process. Mercury contains gold and silver, which accelerates the wetting process of mercury on gold. When the gold and silver content is 0.17%, the wetting ability of mercury to gold can be increased by 70%; when the content of gold and silver is 5%, it can be doubled.

Mercury surface will be contaminated oil, clay, talc, graphite, arsenic compounds, a number of sulfide and antimony, copper, tin and other metals, and decomposition of organic matter, soluble iron, copper sulfate and other substances.

Mercury is contaminated because harmful impurities or compounds formed during operation form a very thin film on the surface of the mercury beads. It is spread over the same layer of barrier on the mercury beads, so that mercury and gold cannot be in direct contact. Moreover, the pollution film has a certain adsorption force on mercury, and it constantly swims on the surface of the mercury bead, but is not easily separated. As a result, the mercury beads are continuously divided and surrounded by the movement and gradually become smaller, and finally powdered. In severe cases, the slurry will flow completely and lose the ability to capture gold. However, the pollution of mercury is generally light in most mines, the pulverization is not very obvious, and the recovery of gold is still effective. However, there are also a few mines. The ore contains some non-stoichiometric quantities of pyrite, pyrrhotite, pyrite and other minerals. A small part of these minerals are unstable in the crystal lattice and will be freed at any time. Oxidation to elemental sulfur. In amalgamation, it reacts with mercury to form mercury sulfide on the surface of mercury. This type of contamination is generally difficult to overcome unless pre-treatment to remove elemental sulfur prior to amalgamation. Sulfide minerals of antimony and arsenic can also form mercury, arsenic and mercury compounds with mercury, and the surface of the contaminated mercury causes chalking. The entry of iron into mercury creates a gray-black film on the surface of the mercury that separates the mercury into a large number of tiny balls. The early sodium amalgam or distillation purification method can effectively prevent the hard brittleness and pulverization of mercury.

Another reason for mercury pulverization is caused by the over-grinding of mercury, which is a microsphere surrounded by a water film.

(4) Slurry temperature. Mercury is liquid at room temperature and has a melting point of -38.89 ° C and a boiling point of 357.25 ° C. When the slurry temperature is too low, the viscosity of mercury is large and the wettability to gold is poor. As the temperature rises, the activity of mercury increases, which is advantageous for the amalgamation operation. However, when the temperature is too high, the mobility of mercury will increase and some of the mercury will be lost with the slurry. A factory found that the recovery rate of mercury in mercury-mixed plates was the highest in 2, 3, 4, and November, up to 23%; in winter (12-January), it was reduced by 1%; in summer (from June to September), it was reduced by 4%. about. When the slurry is at 10 ° C, the evaporation rate of mercury is 1.43 mg m (m ² ·min); when the temperature of the slurry is changed within the range of 10 to 40 ° C, the evaporation of mercury increases by about 1.2 to 1.5 times for every 10 ° C increase in temperature. .

(5) Concentration of pulp. In the overflow external mercury, the concentration of solid materials in the slurry should generally not exceed 10% to 25%, and the concentration of mercury in the grinding cycle is about 50%, so as to ensure sufficient sedimentation speed of the gold particles. Because the external amalgamation relies on the effect of the slurry flowing in the tank, the gold particles are stratified by the difference in density, and the mercury on the surface of the natural gold particles and the mercury amalgamation is fully contacted to achieve the purpose of collecting gold. In the grinding cycle of a mine, the amalgamation of the plate has been proved by many years, they have obtained the best recovery rate with 50% pulp concentration and 0.15mm grinding maximum particle size. The internal amalgam is captured by mercury at the moment when the surface of the gold particles is exposed when the natural gold particles are dissociated in the crushing equipment. For example, in the grinding machine, the main reason is to consider the grinding efficiency. Mercury is mixed under this condition because the concentration of the slurry is between 60% and 80%. When the internal mixing of mercury is carried out in the mining machine, the boring machine and the mercury mixing cylinder, the concentration of the slurry is preferably 30% to 50%. However, after the end of the internal amalgamation operation, in order to collect the amalgam and mercury dispersed in the slurry for recovery, the slurry should be diluted to a lower concentration.

(6) The pH of the pulp. In the acidic medium or cyanide solution (NaCN concentration 0.05%), the operation of amalgamation is better, especially for the treatment of ore with complex nature and more harmful impurities. Because in the process of amalgamation, the oxide film formed on the surface of gold particles and mercury can be dissolved by acid or cyanide. Figure 5 shows the mercury-mixing efficiency of gold in different media in a former gold deposit in the former Soviet Union.

Figure 5 Mercury mixing efficiency of gold in different media

1-neutral medium; 2-acid medium (3% to 5% sulfuric acid); 3-alkaline medium (lime solution)

The presence of a base can precipitate soluble salts and eliminate the effects of oil quality. For example, in the case of grinding, the soluble salts are produced due to the oxidation and electrochemical reaction of the sulfides (in the case of internal amalgamation), so that the mercury is covered by the base metal salts; in addition, the oily substances (mechanical lubricants) are mixed during the grinding. , will affect the normal process of the amalgamation process. These adverse effects can be eliminated by mixing mercury in an alkaline medium. The use of lime as a modifier can neutralize mineral acid, reduce soluble salts, prevent the influence of sulfides, reduce the harm of oil, and can agglomerate extremely fine-grained slime and reduce the viscosity of the medium. However, excessive lime can inhibit the inclusion of golden iron ore and reduce the rate of mercuryation. The usual amount of lime added is 4% to 5% of the amount of the amalgam concentrate.

(7) The nature of the mercury plate and the slope of the operation. Ammonia plates are usually made of silver-plated copper plates and also with pure silver plates. The latter has higher investment costs, the board surface is too smooth, and the gold-scraping effect is not ideal. The former has a small investment cost and the board surface is rough, which can enhance the mercury-absorbing capacity and is beneficial to the gold selection.

The slope of the mercury plate is too large, the slurry flow rate is fast, and the gold particles have less chance of contact with the mercury plate and are easily lost. If the slope is too small, the ore is easy to accumulate on the mercury plate, and the gold particles are not easy to contact with the mercury plate, which will also weaken or even lose the role of gold. The slope of the suitable mercury plate selected by a mine is 8 degrees.

(8) The amount and frequency of mercury added. The amount of mercury added and the number of additions are one of the factors affecting the gold selection effect. Add mercury to the mercury plate should be uniform and appropriate. Mercury is highly mobile in the summer and should be added less frequently. In winter, it is usually added every 2 to 4 hours, and the well should be added as appropriate. The amount of mercury added per ton of ore depends on how much gold is contained in the ore. The number of times of scraping mercury also depends on how much or so gold is contained in the ore. When the ore contains less gold, since the mercury on the mercury plate is thin at the same time, the gold can be scraped once every 24 hours, which is beneficial to the formation of mercury gold film, improve the re-acquisition of gold, and prolong the pure operation time of the mercury plate. . When the ore contains a lot of gold, it can be scraped once per shift.

(9) Water. The water used should be free of acids, heavy metal sulfate ions and organic matter. In order to purify the water, in some cases, lime or other chemicals may be added to the water for pretreatment, so that the above harmful substances are precipitated together with the additive to obtain a better effect.

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