Precipitated gold from the pregnant solution containing the gold with zinc and aluminum Replacement replacement method. In the production practice, it is further divided into zinc (aluminum) wire replacement method and zinc (aluminum) powder replacement method. However, in the calcium-containing cyanide solution, aluminum forms calcium aluminate and precipitates together with gold. Therefore, aluminum can only be precipitated from NaOH as a pH adjuster and cyanide solution. The zinc substitution method is widely used today in the continuous vacuum precipitation method of zinc powder used in the Merrill Crowe factory. The cyanide noble liquid is replaced by the zinc gold precipitation and smelting commonly known as "cyanide gold mixed". .
1. The principle of zinc replacement of precipitated gold and the factors affecting gold precipitation
The gold in the zinc replacement solution is carried out according to the following reaction formula:
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:
Zn+ 
O 2 +H 2 O=Zn(OH) 2 ↓
Na 2 Zn(CN) 4 +Zn(OH) 2 =2Zn(CN) 2 ↓+2NaOH
The resulting zinc hydroxide and zinc cyanide form a white film precipitate on the surface of the metal zinc, which prevents the gold and silver from completely precipitating from the cyanide solution.
In solutions with higher cyanide and base, in addition to the formation of Zn(CN) 4 2 - complex anions, zinc also releases hydrogen from the well as follows:
4NaCN+Zn+2H 2 O=Na 2 Zn(CN) 4 +2NaOH+H 2 ↑
2NaOH+Zn=Na 2 ZnO 2 +H 2 ↑
This reaction increases the consumption of zinc and releases a large amount of oxygen. However, hydrogen reacts with dissolved oxygen in the solution to form water, which can reduce or even prevent the dissolution of the precipitated gold. Metal zinc can also be oxidized.
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. When zinc wire is replaced, cyanide and alkali concentrations are correspondingly higher because some plants do not degas the solution. Of course, it is preferable to remove the dissolved oxygen in the solution through the degassing tower before the zinc-containing solution is replaced by the gold-containing solution to completely eliminate the harmful influence on the gold of the replacement precipitate.
Typically the cyanide solution containing less amount of lead, since lead can be improved in conjunction with zinc precipitation of gold, it is often added to an appropriate amount of lead acetate or lead nitrate to the mother liquor. However, excessive lead will cause zinc to be consumed due to many edge reactions, and the precipitation of gold will be slow and incomplete, or the precipitate will be contaminated by the formation of Pb(OH) 2 precipitate, so it is only for each ton of mother liquor. Add 5 to 10 g of lead nitrate.
The presence of copper produces a metallic copper precipitate that consumes zinc. Mercury will form an alloy with zinc.
The presence of sulfur ions produces ZnS and PbS precipitates that contaminate the metal zinc.
Since the cyanide solution contains calcium and hydroxide ions, the presence of nickel can seriously affect the precipitate. Therefore, the nickel in the Kurr Addison plant's lean liquid is nearly 90 × 10 -6 .
The relationship between zinc precipitation method and sodium cyanide concentration, oxygen concentration and gold recovery rate shows that when the cyanide solution is gold 15mg/L, NaCN 0.015%-0.07%, NaOH 0.015%, oxygen 0-3.1mg ∕L, the amount of zinc added is 1g ∕L. When the NaCN concentration is increased, the consumption of zinc is increased due to the tendency to form a precipitate. When the solution contains 1 mg ∕L of oxygen, the recovery rate of gold can reach 97%-100%, and when the oxygen content increases to 30 mg ∕L, the recovery rate of gold is only 78%-80%.
Second, zinc wire replacement precipitation method
The zinc wire replacement process for the recovery of gold from cyanide solution began in 1889. The zinc wire displacement sedimentation tank (Fig. 1) is generally wood, steel or concrete. It is usually divided into 5 to 10 grids, with a total length of 3.5 to 7 m, a width of 0.45 to 1 m, and a depth of 0.75 to 0.9 m. The screen is placed on the iron frame with a hole diameter of 3.36 to 1.68 mm (the mesh is 6 to 12 mesh). The zinc wire is turned into a scrap of 0.02-0.04 mm thick and 1 to 3 mm wide by metal zinc on a lathe, or the molten metal zinc is continuously and uniformly poured on a water-cooled high-speed rotating pig iron cylinder to form pellets.
Figure 1 Zinc wire replacement sedimentation tank
1-box; 2-box edge; 3-lower baffle; 4-upper baffle;
5-screen frame; 7-zinc wire; 8-gold mud; 9-vent; 10-handle
When the gold-containing solution flows through the tank, the contact time with the zinc wire is about 17 to 20 minutes, and during this 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 regularly to loosen the zinc wire and release hydrogen bubbles, 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 releases gold mud once or twice a month. After the extracted zinc wire is separated into gold mud by a cylindrical sieve, the sieved zinc wire is used for the next batch replacement. 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 (4 to 20 kg of zinc is required for the production of 1 kg of gold), and the consumption of NaCN is also large (due to the expensive use of the zinc wire replacement method). The liquid is generally not degassed, zinc will oxidize in a high oxygen solution to form a white precipitate), the 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.
Third, zinc powder replacement precipitation method
The zinc powder displacement 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 at the Merrill Crouch plant, but have also been selected by some of the world's major cyanide plants.
The zinc powder used in the zinc powder displacement precipitation method is obtained by distilling zinc. Zinc powder should contain 95% to 97% zinc, about 1% lead, and particle size less than 0.01mm (US regulations 97% -0.04mm). Both coarse zinc and ZnO reduce the displacement precipitation effect. The blue powder produced by the zinc smelting plant contains about 10% to 15% of ZnO, which is unfavorable 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.
(1) The filter press zinc powder replacement precipitation method. This method consists of a tape or other type of feeder that continuously feeds the zinc powder into the conical mixing tank and replaces it in the filter (Figure 2). The deaerator solution of the degassing tank is placed in a conical mixing tank and mixed with zinc powder to form a zinc slurry discharged from the bottom of the tank, and the rest is pumped with a submersible centrifugal pump (centrifugal pump immersed in a pool containing gold solution to prevent inhalation of air). The degassing liquid is combined with a filter press or a frame filter, and the filter is filtered to produce gold mud and separate the lean liquid.
Figure 2 Filter press zinc powder replacement equipment system
1-degassing tower; 2-vacuum pump; 3-conical mixing tank; 4-feeder; 5-centrifugal pump;
6-submersible centrifugal pump; 7-filter press; 8-gold mud tank; 9-lean tank; 10-centrifugal pump
(2) Replacement tank zinc powder replacement precipitation method. This is a method of gold displacement and precipitation in a displacement precipitator, the equipment used is shown in Figure 3. The displacement precipitator is a circular groove with a conical bottom. A frame of four spreading bag filters is mounted on the four walls corresponding to the slots, and is radially fixed to the center tube. The frame has a "U" shape with a filter at one end and a branch at the other end of the degreased liquid manifold. 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.
Figure 3 Displacement tank zinc powder replacement equipment system
1-degassing tower; 2-straight air pump; 3-submersible centrifugal pump; 4-mixing tank;
5-feeder; 6-displacement sedimentation tank; 7-bag filter;
8-center tube; 9-spiral pulp; 10-center shaft; 11-small impeller;
12-transmission mechanism; 13-branch; 14-main pipe and vacuum pump; 15-centrifugal pump
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 suction gold mud is deposited on the filter cloth by means of the vacuum pump, and the lean liquid permeation filter cloth is discharged from the manifold through the branch pipe. According to the production practice, the replacement of gold precipitates mainly does not occur when mixing with zinc powder, but when the gold-containing solution passes through the zinc powder layer on the surface of the filter cloth. In order to rapidly form a zinc powder precipitation layer on the surface of the filter cloth after the displacement of the displacement sedimentation tank, it is necessary to directly add zinc powder which forms more than half of the total amount of the zinc powder precipitation layer to the open displacement sedimentation tank at the start of filtration. Conducive to the precipitation of gold mud. Although the displacement sedimentation tank is open and the air is in direct contact with the surface of the zinc paste, the zinc paste is not as fast as the filtration speed is fast, and the stirring force of the slow-rotating propeller and the small impeller (for stirring the upper zinc paste) is weak. How much oxygen is inhaled. Since the gold sludge is intermittently discharged, when the continuous displacement precipitation is performed, 2 to 3 replacement precipitation tanks should be provided for alternate use.
Lead nitrate or lead acetate is dropped into the zinc powder surface from the mixing tank by a drip tube 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 weight 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 deal with. The consumption of zinc powder depends on the gold content of the gold-containing solution from 1 to 5 g ∕m 3 to 50 g/m 3 .
(3) The Merrill Crow factory continuously adds zinc powder to replace the precipitation method. The replacement operation of Merrill Claufa (Fig. 4) is to directly pump the degassed mother liquor to the emulsifier, and the zinc powder is continuously added to the emulsifier through a zinc powder feeder and emulsified with the solution. The amount of zinc powder added is 15 to 70 g 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. After a suitable time, more than 99% of the gold in the solution is reduced and precipitated, and the poor liquid contains about 0.02 g 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 to 28 days. The cleaned sediment is sent to the smelted gold ingot.
Figure 4. Equipment system of the Merrill Crowe method (Woodcock, 1976)
The computer-controlled Merrill Crow's MC2000 system, which continuously replaces gold and silver with zinc powder, has been developed by the Hydrometallurgical Industry Company and applied to the Grubb Star Gold Mine in Montana, USA. 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) Using a filter press zinc powder cake to filter and replace the gold-containing cyanide solution, which 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.
Fourth, the rapid control analysis of gold content in poor liquid
The control analysis of the gold content of the lean liquid after zinc substitution and activated carbon adsorption can be carried out by the fast colorimetric method of the Canadian Kemingke Kang Mine. The mine continuously adds zinc powder to replace the lean liquid of precipitated gold, and samples are taken once every hour for control analysis. The measurement time is about 15 min. The steps are:
1000 mL of the lean liquid was taken with a 1500 mL conical flask, 25 mL of saturated NaCN solution, 7 drops of saturated lead acetate solution and 2 g of zinc powder were shaken by hand for about 1 min. The precipitated sponge lead was transferred to a porcelain cup, and 10 mL of aqua regia was heated and evaporated to near dryness, and then 5 mL of HCl was added to continue evaporation to about 2 mL and transferred into a small test tube. After cooling, a saturated color of SnCl 2 4 was carefully added dropwise for colorimetry. The gold content of different color samples is:
Pink: 0.02g∕t;
Blue: 0.04g∕t;
Light purple: 0.06g∕t;
Dark purple: 0.08g ∕t (if the precipitation is 1.0g ∕t);
Black: >0.10g/t.
Locking Pliers,Grip Locking Pliers,Basic Locking Pliers,Alloy Steel Grip Locking Pliers
Ningbo Anyun Tai Metal Products Co., Ltd. , https://www.eurokingtools.com