Cyanide gold extraction process (6)

â‘  filter replacement zinc powder precipitation. This method consists of a tape or other form of feeder that continuously feeds the zinc powder into the conical mixing tank and replaces it in the filter (Figure 16). In addition to partially deoxygenated solution gas tank to discharge conical mixing tank mixed with zinc to zinc slurry discharged from the groove bottom, the rest except for using submersible centrifugal pump (centrifugal pump is immersed in a solution containing gold bath, in order to prevent the intake air) pumped The gas and liquid are combined and sent to a filter press or a frame filter, and the filter is filtered to produce gold mud and separate the lean liquid.

2 displacement tank zinc powder replacement precipitation method. This is a method of gold displacement and precipitation in a displacement precipitator (Fig. 17) which is a conical bottom circular groove. Four frames with a bag filter are mounted on the four walls corresponding to the groove, and are radially fixed on the center tube. The frame is in a "U" shape, the filter is placed at one end, and the other end is separated from the degreased liquid manifold. The branch pipes are connected. The gold stripping liquid manifold surrounds the outside of the tank body, communicates with the filter frame through the branch pipe, and the main pipe is connected to the vacuum pump and the centrifugal pump. [next]

The zinc powder replacement tank is composed of a main part such as a mixing tank and a zinc powder replacement precipitator. After the gold-containing solution after degassing, centrifugal pump into a mixing tank, while the powder feeder to the vessel to give a suitable amount of added zinc dust and lead salts. The gold-containing liquid and the zinc powder are mixed into a slurry in the tank, and then automatically flow into the cone space in the lower part of the zinc powder precipitator, and are stirred by the propeller. On the bracket in the middle of the precipitator, four filter frames are arranged, and a U-shaped tube with a hole is used as a skeleton. One end of the bag filter is blocked, and one end is connected to a vacuum pump. After the zinc slurry is stirred, the filtrate is withdrawn by the suction of the vacuum pump, and the precipitated gold powder and excess zinc powder are deposited on the filter cloth. The gold-containing solution contacts the zinc powder deposited on the surface of the filter cloth to replace the gold, and when the surface layer of the filter cloth reaches a certain thickness, it is discharged. In order to prevent the operation from being interrupted by the discharge of the gold mud, the precipitators should be connected in parallel 2 to 3 and used alternately. [next]
After the degassing solution and the zinc powder are mixed into the mixing tank, they are fed into the displacement precipitator from the bottom of the tank, and the zinc slurry rises along the central tube under the action of the propeller and the small impeller. The gold mud is deposited on the filter cloth by the suction of the vacuum pump, and the lean liquid is filtered through the branch pipe and discharged from the manifold. According to the production practice, the replacement of gold is mainly not caused when it is mixed with zinc powder, but when the gold-containing solution is filtered through the zinc powder layer on the surface of the filter cloth, the surface of the filter cloth can be quickly applied after the displacement of the displacement tank is started. A zinc powder precipitation layer is formed on the surface, so that at the beginning of the filtration, zinc powder which forms more than half of the total amount of the zinc powder precipitation layer is directly added to the open displacement sedimentation tank to facilitate the precipitation of the gold sludge. Although the displacement sedimentation tank is open and the air is in direct contact with the surface of the zinc paste, the zinc slurry is inhaled due to the rapid filtration speed and the low stirring force of the slow-rotating propeller and the small impeller (for stirring the upper zinc paste). Not much oxygen. Since the gold mud is intermittently discharged, when the continuous displacement precipitation is performed, 2 to 3 replacement sedimentation tanks should be provided for alternate use.
Lead nitrate or lead acetate is dropped from the mixing tank onto the surface of the zinc powder to form a lead film on the surface of the zinc powder to enhance the replacement capacity of the zinc powder. The lead salt is added in an amount of 10% by mass of the zinc powder. When the NaCN and CaO of the gold-containing solution are as low as 0.014% and 0.018%, respectively, the precipitation effect of gold is also very good. The gold-depleted liquid is measured once every hour by colorimetry, and if the gold content exceeds 0.15g/m ^3, it returns to the same. deal with. The consumption of zinc powder is from 15 g/m ³ to 50 g/m ³ (depending on the gold content of the gold-containing solution).
3 Continuous addition of zinc powder replacement precipitation method. The replacement operation of the continuous addition of zinc powder replacement precipitation method (Fig. 18) of Merrill Crow Cyanide Plant is to directly pump the mother liquor after degassing to the emulsifier, and continuously add the zinc powder to the emulsifier and emulsify the solution through the zinc powder feeder. . The amount of zinc powder added is 15~70g per ton of liquid. The precipitation of gold occurs essentially immediately after the addition of zinc. The emulsified solution was replaced in a vacuum precipitation chamber and gold was precipitated. Over a period of time, more than 99% of the gold in the solution is reduced and precipitated, and the poor liquid contains about 0.02 g/t of gold. Filtration of the precipitate from the solution typically uses a Sock or frame filter or filter press, and more widely used is a Stellar filter. During continuous production, the period of cleaning the precipitate from the filter is 3~28d. The cleaned sediment is sent to the smelting plant for smelting the combined gold ingot.

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Using computer-controlled continuous Merrill Crowe gold and silver plus zinc replacement MC2000 system, has been developed to complete the hydrometallurgical industrial companies, and applied to large American and Mongolia that states Grubbs Tucker gold. The system automatically samples every 15 minutes, automatically adjusts the amount of zinc powder added according to the measurement results, and automatically controls various operations.
4 zinc powder cake filtration replacement method. The use of a filter press zinc powder cake to replace the gold-containing cyanide solution can reduce the consumption of zinc and increase the gold content of the gold mud. The poor liquid contained in the zinc powder cake displacement can be reduced to trace amounts.
4) Factors affecting zinc replacement immersion gold 1 cyanide concentration and alkalinity. Gold requires a certain concentration of cyanide and alkalinity during the replacement process in order to dissolve the zinc and expose the fresh surface. Too low cyanide concentration and alkalinity will not only reduce the rate of gold substitution, but more importantly, the zinc cyanide complex will decompose, and the resulting insoluble zinc cyanide precipitate covers the surface of zinc, preventing gold from contacting with zinc. Excessive cyanide concentration and alkalinity, although beneficial to the purification and displacement filtration of the noble liquid, the dissolution rate of zinc is increased, the consumption is increased, and unnecessary waste is caused. Therefore, in the production practice, the cyanide concentration is generally controlled to be 0.04% to 0.06%, and the alkalinity is 0.01% to 0.02%.
2 oxygen concentration. The oxygen contained in the solution is not conducive to gold replacement, because the gold that has been replaced may be redissolved in the presence of oxygen in the cyanide solution. At the same time, the oxygen in the solution will accelerate the dissolution of zinc, and form a zinc hydroxide precipitate to reduce the replacement effect. Therefore, it is necessary to deoxidize before the replacement of the noble liquid, and the oxygen content in the solution is required to be less than 0.05 mg/L.
3 The amount and quality of zinc. When zinc is used to replace gold, its amount and quality play a decisive role in the replacement effect.
The amount of zinc is related to the cyanide concentration and alkalinity in the solution, oxygen and impurities (including impurity ions, suspended matter, etc.), temperature and other factors, and more importantly, the surface area of ​​zinc. Since gold substitution is performed on the surface of zinc, the larger the surface area, the faster the replacement rate, the better the effect, and the less the zinc consumption. Sometimes due to the above factors, the consumption of zinc is very small according to the reaction formula, but in actual production, the amount is tens or even hundreds of times the theoretical calculation. Generally, the consumption of zinc wire per cubic meter is 200~400g, and the consumption of zinc powder is 15~50g.
The quality of zinc is also an important factor affecting the efficiency of displacement. Whether it is zinc wire or zinc powder, the zinc content must be greater than 98%. It is required to be strictly protected from moisture, agglomeration, high temperature baking or oxidation, and can not be soaked by acid water or alkaline water. At the same time, the zinc powder particle size is -325 mesh greater than 95%, and the zinc wire is thin and thin (ie, 1-3 mm wide and 0.2 thick). ~0.4mm), and can not be placed for a long time after cutting.
4 temperature. The reaction rate of zinc replacement gold is accelerated with the increase of temperature. When the temperature is lower than the temperature, the replacement efficiency is affected to some extent. Below 10 ° C, the reaction rate will be very slow. However, if it is higher than 30 °C, it will not only increase the consumption of zinc, but also accelerate the replacement and precipitation of other impurity ions. Therefore, it is generally preferred to control 15~30 °C.
5 cleanliness of precious liquid and the content of impurities. The precious liquid must be clear and transparent before it enters the replacement. If it contains turbidity or oil, it will contaminate the zinc surface or form a film to cover the surface of the zinc. Therefore, it is a powerful measure to improve the replacement efficiency by strengthening the clarification and purification filtration of the noble liquid and making the suspended matter less than 5 mg/L. [next]
If the solution contains sulfate, thiosulfate and divalent iron cyanide ions, it will inhibit the gold precipitation. In particular, compounds containing sulfides, copper cyanides, arsenic and antimony , even at very low contents, significantly reduce the replacement efficiency of gold, and the degree of reduction increases with increasing content, and even stops the reaction. The hazards of these materials are that they are easily deposited on the zinc surface to passivate or form a film to cover the zinc surface. The lowest concentrations they affect are:
Sulfide 4.5 × 10 ^-4 mol; 锑 1.65 × 10 ^-4 mol;
Copper cyanide 6 × 10 ^-3 mol; arsenic 2.3 × 10 ^-4 mol.
6 lead salt. A small amount of lead in zinc or an appropriate amount of lead salt before replacement can accelerate the replacement of gold. In addition to forming a galvanic couple with zinc, the action of lead can also react with divalent sulfur ions in solution to form lead sulfide precipitates. However, excessive lead will cover the surface of zinc, which will not only affect the replacement efficiency, but also bring harm to the next step. Therefore, it is recommended to add 10~100g of lead acetate or lead nitrate per cubic meter of precious liquid in production.
5) Rapid control analysis of gold content in lean liquid For the control analysis of the gold content of lean liquid after zinc replacement and activated carbon adsorption, the fast colorimetric method of Canada Kemingke Kang Mine can be used. The mine continuously adds zinc powder to replace the lean liquid of precipitated gold, and samples are taken once every hour for control analysis, and the measurement time is about 15 minutes. The steps are:
Use a 1500mL conical flask to take 1000mL of lean liquid, add 25mL of saturated NaCN solution, 7 drops of saturated lead acetate solution and 2g of zinc powder, shake it by hand for about 1min, transfer the precipitated sponge lead into porcelain cup, add 10mL of aqua regia and heat to near dry. Add HCl 5 mL and continue to evaporate to about 2 mL and transfer to a small tube. After cooling, carefully add saturated SnCl ₂ 4 drops for colorimetry. The gold content of the different color samples is:
Pink: 0.02 g/t; blue: 0.04 g/t; light purple: 0.06 g/t; dark purple: 0.08 g/t (if precipitation occurs 0.1 g/t); black: >0.10 g/t.
4. Gold mud treatment
The gold mud produced by adding zinc precipitation from the gold-containing cyanide solution generally contains no more than 20% gold. Table 2 shows the mass fraction of the gold mud component of a factory.

As can be seen from Table 2 above, the treatment of gold mud is mainly to remove zinc, lead and sulfur. The processing method of gold mud mainly adopts the fire process, and the wet process has been adopted in recent years. [next]
The fire process is generally three steps of acid dissolution, calcination and smelting.
1) Acid solution The so-called acid solution is to use a solution of dilute sulfuric acid (H ₂ SO ₄ mass fraction of 10% to 15%) as a solvent gold mud, so that the components soluble in dilute sulfuric acid in the gold mud are dissolved and separated from the gold mud. Zinc is easily soluble in dilute sulfuric acid:
Zn+H ₂ SO ₄ =====ZnSO ₄ +H ₂ ↑
Soluble substances such as copper are also dissolved; silver in gold mud may also be dissolved in small amounts.
Since a large amount of hydrogen is generated during acid dissolution, the acid-dissolving operation must be carried out in a tank having a mechanical stirring device, and a hood should be provided on the tank to allow the hydrogen to be discharged in time.
In order to reduce the amount of zinc in the gold mud, the coarser zinc particles and zinc wire are sieved by a sieve before pickling. In order to prevent the reaction from causing splashing, adding dilute acid should not be too fast, and should be cooled by cold water.
If the gold mud contains arsenic, it will generate hydrogen arsenic gas; it may also produce hydrogen cyanide gas. These are all harmful gases, so the acid bath must be sealed and provided with a hood.
The consumption of sulfuric acid is generally 1.5 times the mass of zinc. The acid dissolution time is generally 3 h and clarified for 3 h.
The composition of the gold mud after pickling has undergone significant changes. Table 3 shows the gold mud components (mass fraction) of a factory after acid dissolution.

It can be seen from Table 3 above that after acid dissolution, the gold content in the gold mud is significantly increased, the zinc content is significantly reduced, and the lead content is also increased, because lead is left in the gold mud in the form of lead sulfate.
After acid dissolution, liquid-solid separation is carried out, and the gold mud is washed with water and filtered, and the gold mud becomes a filter cake.
2) Calcination The filter cake also contains water and water of crystallization as well as certain compounds. The purpose of the calcination is to remove these moisture and turn the sulfides and sulfates into oxides, which creates favorable conditions for the next smelting.
When roasting, put the filter cake in the iron pan, put it into the heating furnace and slowly heat it. The temperature should be controlled within the range where carbonate, sulfate and cyanide can be dissociated, but the solid material should be prevented from melting, generally the highest temperature control. At around 600 °C.
The roasting of some plants in South Africa is carried out in an electric furnace. Each furnace can hold 6~12 iron plates. Each iron plate can hold 60kg of gold mud. The roasting time is 16h, and the roasting temperature is kept at 600~700°C.
In order to prevent the gold mud from being attached to the iron pan by heat, first apply a coating such as lime to the inner wall of the iron pan. In order to oxidize the impurities during firing, an appropriate amount of saltpeter may be added to the filter cake as an oxidizing agent. In order to avoid the scattering of hot gold mud, the roasting process should not be stirred. If the temperature is too high during calcination, the cold material can be slowly added to cool down.
Some small gold clay fired plant, it can be carried out in the pan, with the heating coal or coke, called "sand speculation." The calcined gold mud is called calcine, and the calcine is sent to smelting.

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