Cyanide slurry was filtered, washed with output solution containing gold (commonly known as expensive solution or mother liquor), which still contains a small amount of precipitated sludge is difficult and suspended particles, typically zinc substitution should be clarified and the recovery of gold and then degassed. Figure 13 shows a concise flow of clarification, degassing and zinc addition of a gold-containing solution. The widely used clarification equipment is a frame clarifier, followed by a filter press. Some small mines use sand filter boxes and sedimentation tanks. The sand filter box is laid on the false bottom of the box. The filter cloth is respectively equipped with a gravel layer with a thickness of 120-150 mm and a fine sand layer with a thickness of 60 mm. Although the sand filter box has a simple structure, it is the same as the sedimentation tank. Low, the clarification effect is poor. For this reason, it is often used in conjunction with a frame clarifier. [next]
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The mother liquor produced by slurry filtration and washing contains a small amount of slime and suspended particles which are difficult to precipitate. Their presence contaminates the surface of zinc, reduces the precipitation rate of gold and consumes cyanide in the mother liquor. A frame clarifier, a filter press, a sand filter tank or a sedimentation tank can be used for clarification to remove slime and suspended solids from the mother liquor. 
The biggest impact on production during clarification operations is that the filter cloth is clogged with carbonate, sulfide or slime deposits. In order to eliminate these harmful effects, the intermediate sump between filtration and clarification is usually eliminated, and the time during which the gold-containing solution is in contact with the air is shortened to reduce the dissolution of carbon dioxide in the air into the solution. And clean the washing and clarifying equipment regularly and wash the filter cloth with dilute hydrochloric acid with a mass fraction of 1% to 1.5% to remove the calcium carbonate precipitate.
1) Zinc replacement gold principle The replacement of immersion gold is to add metal zinc to the noble liquid after purification and deoxidation. After the displacement reaction, the gold in the solution is replaced by a metal state and precipitated, and the zinc is dissolved in the alkaline cyanide solution. .
The process of zinc replacement gold is an electrochemical reaction process, and the precipitation of gold is the result of the formation of galvanic couples of zinc and gold. In the galvanic couple, zinc is the anode and gold is the cathode. The gold cyanide ion [Au(CN)] 2 - acts on the anode and moves with zinc under the action of the galvanic current, converting zinc into zinc cyanide ion [Zn(CN) 4 ] 2- and entering the solution, gold It is reduced and precipitated, and its reaction formula is as follows:
2Au(CN) 2 - +Zn=====2Au↓+Zn(CN) 4 2-
The reaction is rapid and the replacement is complete.
When the cyanide concentration and the alkali concentration in the solution are small, the oxygen dissolved in the solution redissolves the precipitated gold and oxidizes the zinc to form a hydroxide precipitate. The Na 2 Zn(CN) 4 formed in the above reaction also decomposes to form a zinc cyanide precipitate.
In solutions with higher concentrations of cyanide and alkali, in addition to zinc cyanide anion, zinc dissolves and releases hydrogen as follows:
Zn+4CN - +2H 2 O=====Zn(CN) 4 2- +2OH - +H 2 ↑
Zn+2OH-=====ZnO 2 2- +H 2 ↑
The hydrogen generated in the above reaction, if concentrated on the surface of the zinc, generates polarization and forces the displacement reaction to stop. If zinc contains a small amount of lead or if an appropriate amount of lead salt is added during the replacement process, the polarization will be reduced or disappeared. At the same time, the hydrogen reacts with the dissolved oxygen in the solution to form water, which can reduce or even prevent the gold which has formed a precipitate from being desolved, and the metal zinc can no longer be oxidized. [next]
Under normal zinc powder replacement conditions, the concentration of cyanide should be controlled to about 0.02% and about 0.01% of calcium oxide in the gold-containing solution entering the displacement sedimentation tank. In the case of zinc wire replacement, some cyanide plants do not remove the dissolved oxygen in the solution to completely eliminate the harmful effects on the gold precipitated.
The cyanide solution usually contains less lead. Since the combination of lead and zinc can improve the precipitation of gold, an appropriate amount of lead nitrate or lead acetate is often added to the mother liquor. However, excessive lead may cause many edge reactions leading to increased zinc consumption and slow and incomplete precipitation of gold, or contamination of the precipitate by precipitation of Pb(OH) 2 . Therefore, generally only 5 to 10 g of lead nitrate is added to each ton of mother liquor.
The concentration of sodium cyanide and the concentration of oxygen in the zinc precipitation method are closely related to the recovery rate of gold. It has been shown by experiments that when the concentration of gold in the cyanide solution is 15 mg/L, NaCN is 0.015%-0.07%, and NaOH is 0.015%. When the oxygen content is 0 to 3.1 mg/L, the amount of zinc added is 1 g/L. When the NaCN concentration is increased, the consumption of zinc is increased due to the tendency to form a precipitate. When the oxygen content of the solution is 1 mg/L, the recovery rate of gold can reach 97%-100%, and when the oxygen content is increased to 30 mg/L, the recovery rate of gold is only 78%-80%.
2) Zinc wire precipitation method Zinc wire replacement method The process of replacing gold from cyanide solution began in 1889. This method puts the zinc wire in the sedimentation tank. When the gold-containing solution flows through the sedimentation tank, the solution and The zinc wire is sufficiently contacted to cause a displacement reaction, and as a result, the gold powder is displaced and precipitated at the bottom of the tank.
The construction of the zinc wire replacement sinking gold box is shown in Figure 14. The sedimentation tank can generally be made of wood, steel or cement concrete, usually divided into 5~10 grids, with a total length of 3.5~7m, a width of 0.45~1m and a depth of 0.75-0.9m. The screen is placed on the iron frame with a hole diameter of 3.36~1.68mm (the mesh is 6-12 mesh).
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The gold-containing leachate flows in from the first grid. There is no zinc wire installed here. Only the concentration of the solution is clarified and adjusted (cyanide is added here), and then from bottom to top, successively flow into the cells containing the zinc wire, and the gold content in the solution The downstream is getting lower and lower. To sink gold from the poor liquid containing low gold, it is advisable to use fresh zinc wire to improve the efficiency of sinking gold. Generally, in the last grid through which the solution flows, fresh zinc wire is loaded. After a period of use, it moves up countercurrently and moves to the first lattice of zinc-rich wire. The zinc wire gradually becomes smaller, finer, broken, and the ability to sink gold. Reduce, remove the panning, thicker, longer zinc wire, and return to reuse. The golden mud that sinks to the bottom of the box contains a lot of broken zinc wire and other impurities, which are taken out and further processed. For every 1kg of gold produced, it takes 4~20kg of zinc wire.
When the gold-containing solution flows through the tank, it is in contact with the zinc wire for about 17-20 minutes, during which time about 99% of the gold is replaced. In the production practice, the handles fixed at the center of the screen are gently lifted up and down, and the zinc wire is loosened and hydrogen bubbles are released, and the gold mud is separated from the zinc wire to sink the bottom of the groove. After a period of time, the old zinc wire that can continue to be used in the box is moved to the first few cells of the box, and the new zinc wire is added to the next few cells, so that the solution containing low gold and the new zinc wire with strong replacement force can be obtained. Contact, increase the precipitation rate of gold. When loading the zinc wire, it must be loosened and spread evenly. Pay special attention to the four corners in each grid to prevent the solution from flowing through the cavity and reduce the replacement effect.
The sedimentation tank usually discharges gold mud 1-2 times a month. The extracted zinc wire is sieved out of the gold mud by a cylinder and used for replacement of the next batch. The gold mud is discharged from the discharge port and filtered and recovered in a filter box or a filter press.
Although the zinc wire replacement method has the advantages of simple equipment, easy operation, and no power consumption, the zinc wire consumption is large and the NaCN consumption is also large (the noble liquid used for the zinc wire replacement method is generally not degassed, and the zinc is high. Oxygen solution will oxidize to form a white precipitate), gold mud contains high zinc and the equipment occupies a large area. Therefore, the zinc wire replacement method has been replaced by the zinc powder replacement method in large and medium-sized mines.
3) Zinc Powder Precipitation Method The zinc powder replacement precipitation method began in 1894 when gold was recovered from gold-containing solutions. It is currently the most widely used method. The equipment for zinc powder replacement used filter presses and displacement tanks early. Merrill Claufa, which was later developed, is a typical method in the zinc powder displacement precipitation method. Its equipment and methods have not only withstood the many years of production practice of the Merrill Crouch plant, but have also been selected by some of the world's major cyanide plants.
The so-called zinc powder method is to mix the zinc powder with the gold-containing solution, and then send it to the filter. The replaced gold powder and the excess zinc powder enter the filter cake, and are separated after gold removal. [next]
The zinc powder used in the zinc powder displacement precipitation method is obtained by distilling zinc. The zinc content of zinc powder should be 95%~97%, lead is about 1%, and the particle size is less than 0.01mm (97% is ~0.04mm in the US). Both coarse zinc and ZnO reduce the displacement precipitation effect. The use of blue powder produced by the zinc smelting plant, containing ZnO mass fraction of about 10% to 15%, is not good for sinking gold. Because these ZnOs do not act as precipitates, they completely enter the gold mud. Zinc powder is easily oxidized and should be stored and transported in a sealed container.
The surface area per unit mass of zinc powder is much larger than that of zinc wire, which makes the zinc powder sink gold much more efficiently than zinc wire.
Gold-containing solutions often contain high dissolved oxygen due to aeration during cyanidation and contact with air during operation. The presence of a large amount of oxygen causes the precipitation of gold in the solution to be slow and incomplete when zinc is added to the solution, and the precipitated gold is desolventized and the consumption of zinc is increased.
In order to reduce the side effects of oxygen in the leachate on zinc powder immersion (zinc consumption, gold dissolution), degas (deoxidation) before adding zinc powder. Degassing is carried out in a vacuum degassing tower, the construction of which is shown in Fig. 15. The degassing tower is a cylinder with a volume of 0.5 to 1 m 3 and the exhaust pipe is connected to a vacuum pump. The leachate enters the tower from the top of the tower, is sprayed on the wooden strip by the inlet pipe, collides with the wooden strip and is splashed to form fine water droplets, so that the surface area of ​​the solution is increased. At this time, under the suction of the vacuum pump, the dissolved oxygen in the solution is extracted by the vacuum pump to achieve degassing, and the gas is discharged from the exhaust port. In order to maintain a certain level of degassing liquid in the tower, the tower is equipped with a buoy, which automatically adjusts the liquid level by connecting the counterweight to the butterfly valve on the inlet pipe. In this way, the opening and closing of the butterfly valve is controlled by the level of the liquid surface. The degassed droplets are collected in the lower part of the tower body, discharged from the liquid discharge port, and sent to the zinc powder by the centrifugal pump.
Zinc powder replacement has the advantages over zinc wire replacement: zinc powder is cheaper than zinc wire; gold replacement precipitation is more complete; gold mud contains low zinc content; low processing cost; low zinc powder consumption; zinc powder replacement is easy to automate. Therefore, it has been widely adopted at home and abroad. The disadvantages of the zinc powder method are: large equipment, large investment, and high energy consumption.
There are four methods for the specific application of zinc powder replacement method: