Study on Extraction of Gold and Silver from Silver and Gold Concentrate Calcine by Water Chlorination

1986 Tongbai silver mine into production since, there are more than 20 independent silver mine gradually completed and commissioned, which has become an important source of silver concentrate our silver production. However, because of the low grade ore silver, complex mosaic mineral composition, the produced silver (fine) small amounts of other mineral can be recovered by conventional cyanidation silver and gold, the silver concentrate most difficult process. Pretreatment methods such as chlorination roasting, pressurized oxidation, nitric acid oxidation, and chloride-pressure oxidation can greatly improve the recovery of silver, but for the lowest-priced and chemically active silver in precious metals. These processes still lack economic competitiveness. Therefore, most of the silver concentrates are processed in the corresponding lead or copper smelting furnace, while gold and silver are comprehensively recovered from the anode mud. However, due to the shortcomings of the smelting process, such as long recovery process and serious capital pressure, how to deal with silver-bearing ore economically is still an important problem that metallurgical workers at home and abroad fail to solve.
The hydrazine chlorination method was once the main method for extracting gold from ore. However, as the leaching gold ore resources are depleted, the proportion of refractory gold ore is gradually increasing, especially the requirements for environmental protection are becoming stricter. It faces the dual challenges of economic and environmental protection. Therefore, the water chlorination method has been re-emphasized. At present, a number of factories have been put into production. For example, the gold leaching rate of gold concentrate leaching in South Africa by water chlorination method is 99%, and the immersion gold rate of arsenic- free blasting of 110g/t in the former Soviet Union is 98. %, while the United States treated with chlorination of arsenic and carbon ore, the leaching rate was 94% when leached for 18h, and the chlorine consumption was 17.5kg/t. This paper intends to conduct a preliminary study on the roasting process of a carbonaceous silver-gold concentrate and the gold extraction process by water chlorination.
1 Experimental part
1.1 Raw materials and reagents  The sample is a flotation silver-gold concentrate of a silver mine, and the material with a particle size of -0.074mm accounts for about 95%. Phase analysis showed that the major sulphide minerals FeS _2, and with a small amount of galena, sphalerite, tetrahedrite; major gangue silicates such as sericite; graphitic carbon and has a higher content of The chemical composition and the chemical phase analysis results of gold and silver are shown in Table 1 and Table 2, respectively, where CT is the total carbon content; Corg is the organic carbon content other than carbonate.

1.2 Process Since the treated silver concentrate is a carbonaceous high-sulfur ore, if it is directly treated by water chlorination, the main sulfide will react with chlorine as follows (taking pyrite as an example):

FeS ₂ +7Cl ₂ +8H ₂ O=FeCl ₂ +12Cl ^- +2SO ₄ ^2- +16H ⁺

It is not difficult to see from the above formula that each dissolved 1kg of pyrite (0.533kg of sulfur) will require at least 4.13 kg of chlorine. Assuming that the sulfur in the concentrate is all present in pyrite, the sulfur of 1t of this concentrate is dissolved. Kg chlorine gas, calculated according to 1200 yuan / t chlorine gas, the cost of only chlorine gas is as high as 2352 yuan, obviously the cost of the drug is unacceptable. Therefore, this study used a roasting process to convert most of the sulfides into SO ₂ (industrial production can be considered for acid production) to reduce the consumption of chlorine in the chlorination of water. The experimental process flow is shown in Figure 1. [next]

Fig.1 Schematic diagram of the process flow of water-chlorination treatment of silver-gold concentrate

In the experiment, the calcined sand was first prepared and re-milled by XZP-100 vibrating mill for 1 min, and then used for water chlorination to extract gold, and then silver was recovered from the leaching slag. The chlorine immersion gold is carried out in a conical flask on a temperature-controlled magnetic stirrer. The chlorine gas is passed through the buffer bottle, the potential and pH are measured by a pHS-2C precision acidity meter; the immersion silver test is heated by an electric furnace. The 6402 type electronic relay and the mercury contact type thermometer are controlled by temperature, and the JB-50 electric mixer is stirred in a beaker with a lid.
1.3 Analytical methods The content of gold and silver in silver concentrate, calcine and solid slag samples is analyzed by fire assay, while the silver in liquid phase is determined by atomic absorption spectroscopy, except in the case of water chlorination immersion gold, unless otherwise specified. Analysis of gold.
2 Results and discussion
2.1 Preparation of silver concentrate calcination Due to the limitation of the nature of carbonaceous silver concentrate, roasting is often difficult to achieve the purpose of improving the leaching rate of gold and silver. Usually, it is necessary to add sulfate or chloride for sulfation or chlorination roasting. The preparation conditions of the experimental calcination are as follows: when more than 5% of the additive is present, the segmentation roasting is adopted, that is, the first-stage calcination temperature is 400 ° C, the calcination time is 1 h; the second-stage calcination temperature and the calcination time are respectively It is 400 to 670 ° C and 4 h. The obtained calcined water is subjected to fine grinding and slurry adjustment, and the water is used for chlorination immersion gold.
2.2 Water chlorination method for gold extraction In the process of water chlorination and gold extraction, the residual sulfide in the calcine is also oxidized, and the silver will remain in the water chlorination slag as silver chloride; then the traditional cyanidation method is adopted. The thiosulfate method or the thiourea method can recover the silver in the water chlorination leaching gold slag, and the recovery rate of silver will be greatly improved.
2.2.1 Effect of temperature on gold leaching rate In order to maintain a sufficient amount of free chloride ions to ensure the stability of gold chloride ions, 10-20 g/L NH ₄ Cl should be added during the water chlorination immersion process. Under the fixed leaching conditions (potential greater than 1.0V, time 6h), the effect of water chlorination leaching temperature on gold leaching rate was investigated. The experimental results are shown in Fig. 2. The curve in Figure 2 shows that when the temperature is raised from room temperature to 45 ° C, the leaching rate of gold is increased by about 16%, but the temperature continues to increase, and the leaching rate of gold decreases slightly because of the increase in temperature. The solubility of chlorine gas is reduced, and the side reaction of chlorine consumption is intensified. The color of the leachate is gradually changed from yellowish green to yellowish brown, and the leaching slag rate is significantly reduced (such as leaching for 12 h, the slag rate at room temperature and 45 ° C is 93% and 74%), that is to say, a large amount of iron oxide is also dissolved, which is unfavorable for subsequent gold and silver recovery. In addition, since the gold content of the calcine is low, a room temperature leaching process is preferred. [next]

Figure 2 Effect of temperature on gold leaching rate
NH ₄ Cl: 20 g/L HCl: 0.8 mol/L L/S: 4:1 t: 6 h

2.2.2 Effect of leaching time on gold leaching rate The effect of leaching time on immersion gold rate was investigated at normal temperature and suitable chlorine flow rate (potential adjustment of aqueous solution). The experimental results are shown in Fig. 3. The curve of Fig. 3 shows that the leaching rate of gold increases remarkably with the leaching time, but the leaching time is preferably 10 h. At this time, the gold leaching rate is 96.1%, and the slag contains gold of about 0.7 g/t.

Figure 3 Effect of leaching time on gold leaching rate
NH ₄ Cl: 20 g/L HCl: 0.8 mol/L L/S: 4:1

2.2.3 Reagent consumption of water chlorination process Due to the small scale of experiment and the difficulty of clogging of chlorine gas cylinder valve during the experiment, the parameters such as chlorine gas consumption have not been examined in detail, but according to the literature, water chlorination treatment When the sulfur content is 0.46% of the calcine, the chlorine consumption is 35 kg/t. Although the sulfur content of the calcined sand in this experiment is more than 1%, the quality of the prepared calcined sand is judged by whether the dilute sulfuric acid solution can leach a large amount of silver, so the sulfide content in the calcine will be very low, so the chlorine gas The consumption does not seem to exceed the level reported in the literature. In addition, according to the literature, the reagent costs of chlorine, sodium cyanide and thiourea are 1.0, 1.5 and 2.5 US dollars per ton, respectively. It can be seen that the reagent cost of the water chlorination method is not inferior to the cyanidation method, which may be In addition to environmental factors, water chlorination and gold extraction is one of the reasons for the active research in the field of carbonaceous gold mining.
2.3 Silver recovery in water chlorinated gold slag The bismuth thiosulfate method is one of the most studied methods of non-cyanide gold extraction in recent decades. Except for the insoluble silver compound formed with iron oxide, the silver in the water chlorinated gold slag is converted into easily soluble silver chloride, etc. Therefore, the attempt to recover silver is carried out by the thiosulfate method. The leaching conditions are: room temperature, pH value 9, liquid-solid ratio of 25:1, Na ₂ S ₂ O ₃ concentration of 20-25 g/L and leaching time of 3 h. At this time, the optimum leaching rate of silver is about 94.1%. ~95.6%.
3 Conclusion
3.1 The composition of the carbonaceous silver concentrate is complicated and difficult to be treated by the traditional cyanidation method. However, the leaching rate of gold and silver by calcination-water chlorination immersion gold-thiosulfate immersion can reach 96.1% and 95.6%, respectively. . The method adopts non-cyanide reagent leaching at room temperature, and the leaching rate of gold and silver is high, and is suitable for use in small gold and silver mines.
3.2 Due to the limitations of laboratory experiments, many parameters such as chlorine consumption, gold and silver recovery in the precious liquid have yet to be further studied.

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