About 80% of the world's zinc is currently produced by wet processes. The silver (0-300 g/t) in the zinc concentrate is mostly enriched in the zinc leaching residue (100-600 g/tAg) after the calcination and leaching steps in the wet zinc smelting process. In today's increasingly scarce resources and increasing silver consumption, the recovery of silver from zinc leaching slag has important practical significance for rational use of the slag and increased economic benefits. At present, research methods for recovering silver from zinc leaching slag include thiourea method, chloride salt method, immersion smelting method, and flotation method. The flotation method is attractive due to its simple process, low production cost and good enrichment effect. 1. Types and chemical composition of zinc leaching residue The types and chemical compositions of common wet zinc leaching slags are shown in the table below. Type of leaching residue and its chemical composition As can be seen from Table 1, zinc hydrometallurgy leaching residue contains many worthy recovered valuable metal, has a very important economic value. Second, the physical properties of silver in zinc leaching residue The zinc leaching slag has a fine particle size (-0.074 μm is about 90%), and more than 90% of the silver is distributed in fine particles of -200 mesh. The existence of silver is complicated. Most of the silver is present in silver sulfide and natural silver (about 80%). A small amount of silver exists in the form of compounds such as silver oxide, silver chloride, silver sulfate, silver silicate and silver iron vanadium . The general physical properties of silver in the above zinc leaching slag make the premise basis for the recovery of silver from the flotation process. 3. Progress in flotation of silver in zinc leaching residue The method for recovering silver by flotation in zinc leaching slag can be directly floated according to the nature of the slag or indirect flotation after a certain pretreatment and then flotation. Zhuzhou smelting plant [1] uses butylammonium black drug as collector , No. 2 oil as foaming agent, natural pH 4～5, pulp concentration 40%～50%, adopts a coarse, three fine, three-sweep process The process of flotation leaching slag, the technical and economic indicators obtained are: concentrate 2% ~ 3%, tailings 97% ~ 98%, silver recovery rate 55% ~ 75%. The leaching residue contains 200-400 g/t of silver, the concentrate contains 6000-15000 g/t of silver, and the tailings contains 500-120 g/t of silver. The disadvantage is that when the concentration of zinc ions is high, the flotation index is easily deteriorated. Yi Liang lead zinc [2] Acid leaching residue containing Pb10.4%, Zn2.75%, Ag175.2g / t, Fe17.37%, neutral leaching, acid leaching and heating three sections of the leaching acid leaching, and then float The selected method recovers lead, zinc and silver, and obtains a concentrate containing Pb 55.47% and Ag 654.3 g/t. The silver recovery rate is 67.9%, and the flotation reagent cost is 11.5 yuan/t slag. Belgian Barron Electric Zinc Plant [3] uses flotation method to produce silver concentrate containing 24kg/t of silver from high temperature and high acid zinc leaching residue. The total recovery rate of silver is over 92%. The reason for the success of the flotation method is that the plant has a step of effective hot acid leaching, which can decompose the possible iron bismuth compound [(K, Na, Pb, Ag) Fe 3 (SO 4 ) (OH) 6 ] to release silver. . The South African patent and the Japanese patent [4 , 5] report the method of recovering silver by zinc leaching residue flotation. The residue is treated with H 2 SO 4 solution (150-200 g/L) at about 95 ° C for 6-8 h, then the solid is filtered, washed, and the slurry is tempered with water. At pH 1-5, 80.2% silver can be selectively recovered by flotation of silver with a sulfide collector. Japan’s Akita zinc smelting plant [6] conducted a flotation process to recover silver from zinc leaching slag as early as 1963. And in 1973 industrial production. After 1978, it was changed into sulphated roasting leaching slag flotation silver. The flotation was carried out in a slurry with a pH of 3.5 to 4. The collector was sulfhydryl benzothiazole (350 g/t). When the leaching residue contains 215 g/t of silver, the flotation silver concentrate contains 4150 g/t of silver, 8.98 g/t of gold, the recovery of silver is about 75%-80%, and the recovery of gold is 20%-30%. Japanese patent [7] proposed a flotation process of sulphation roasting, leaching, and H 2 S treatment to recover gold, silver, and copper in zinc leaching slag. 5kg zinc leaching residue (Cu 3.13%, Pb 6.58%, Zn 17.33%, Fe 29.33%, Au 8.00g / t, Ag 57.0g / t) mixed with 0.98L concentrated sulfuric acid, sulfated at 650 ° C The mixture was calcined for 1 h, then leached with water at 80 ° C, filtered through H2S, and the solid was ground and floated with MIDC foaming agent and AP404 collector. The concentrate contained Cu 25.21%, Pb 5.39%, Zn1. 64%, Fe 28.44%, Au 28.41 g/t, Ag 2450 g/t. Liang Jingdong [6] studied the flotation of wet zinc smelting super acid leaching residue. The butyl ammonium black drug is used as the collector, the sodium sulfide is the activator, the No. 2 oil is the foaming agent, and the process of pH=2 uses a coarse, one sweep, and one blank selection process from the silver-containing 428.6g/ t, gold 0.86g / t, lead 18.22% of the slag obtained silver 998.5g / t, gold 1.87g / t, lead 34.16% mixed concentrate, the recovery rate was 93.14%, 84.59% and 75.59%. According to reports [8] , the use of butyl ammonium black drug 450g / t, SN-9150g / t, activated carbon 3000g / t, pine oil 200g / t prescription and a crude two fine three sweep process to treat neutral leaching residue, A concentrate containing 5608.66 g/t of silver was obtained with a recovery of 76.5%. The prescription is characterized by the use of activated carbon as a carrier to facilitate the recovery of fine silver. Huang Kaiguo [9] used Na 2 S as a regulator, butyl xanthate and auxiliary collector XY as a collector, RB as a foaming agent, and floated to recover silver in the zinc leaching residue under pH=6. The results show that the closed-circuit test of one rough selection, one selection and one sweep can obtain silver concentrate with silver content of 4369.73g/t and recovery rate of 79.44% from zinc leaching residue containing 498.10g/t. Liang Jingdong [6] studied the high-acid leaching residue of low-pollution iron smelting zinc smelting plant for ore recovery and recovery of silver. According to the characteristics of the leaching slag, sodium sulfide is used as the activator, butylammonium black and butyl sulphate are the collectors, and the No. 2 oil is the foaming agent. The crude silver flotation process is used to obtain the silver concentrate grade. 31kg / t, a good indicator of silver recovery rate of 91.57%. A Canadian company [10] developed an effective process for the recovery of lead, silver and gold from neutral leaching slag and acid-decomposed jarosite residue and hematite slag by vulcanization-flotation. The vulcanization of silver and lead is the addition of a certain amount of sulfide, which is carried out at a low pH and at a controlled rate. In order to increase the particle size of the sulfide, artificial lead and silver sulfide are added as seed crystals. In dithio phosphate and phosphite as dithiophosphate collector, flotation conditions at pH2 ~ 4. The recovery rates of lead, silver and gold are 85% to 90%, 90% to 95% and 80%, respectively. A representative concentrate contains 50% to 60% lead, 3 to 5 kg/t silver, and 10 to 15 g/t gold. British Patent [11] reported a method for flotation of iron-containing zinc leaching slag after selective vulcanization. The impure yttrium slag slag is leached with sulfuric acid, and the residue is vulcanized. The flotation of the dithiophosphate type collector 242 can obtain the recovery rate of 94.8% of Au, 88.5% of Ag and 93.9% of Pb. Shen Xiangyu [12] carried out research on the recovery of silver in the iron-smelting process of zinc smelting. Using super acid leaching, the silver iron bismuth compound in the decomposition slag releases lead and silver. Then sodium sulfide sulfide, tetrabutylammonium aerofloat, No. 2 oil and coal oil auxiliary flotation collector silver. As a result, the grade of silver ore was 961.1 g/t, the yield of silver concentrate was 16.5%, the grade was 4456.0 g/t, and the recovery rate was 76.54%. A new process for recovering silver from zinc leaching slag by the leaching flotation combined method developed in Russia [13] . In the zinc slag studied, silver is present in the form of silver oxide, which accounts for 65% to 70%, and the remaining silver is bound to the surface of the sulfide. The soluble silver oxide in the slag was leached at a temperature of 323 K with a 1:1 solution of 5 g/L sodium thiosulfate or ammonium thiosulfate; most of the insoluble sulfide was recovered by flotation. The experimental results show that the total recovery of silver is about 93.8%, and the recovery rate of the original flotation method is only 70%. This new method has been adopted by a new factory. The Noranda Research Center [14] targets silver minerals and compounds present in zinc leaching slag, natural silver minerals (silverite, horn silver and natural silver), synthetic silver ore (chemically precipitated silver chloride, silver sulfide) Laboratory flotation studies were carried out with silver-yellow iron sputum and silver-bearing lead pyrite, as well as pyrite and sphalerite deposited on the surface. The results show that all silver minerals and silver-containing compounds can be floated in a medium with a high recovery rate using conventional sulfide collectors in addition to various ferromanganes. Experiments with actual zinc leaching slag prove that flotation is a feasible treatment. It is known from the above literature that the common collectors for the silver in the leaching slag of the flotation wet zinc smelting process are xanthate, black drug, hydrocarbyl dithioformate, sulfhydryl benzothiazole, etc., and the foaming agent is No. 2 For oil or the like, the activator is Na 2 S or the like. In summary, the method of flotation of zinc leaching residue to recover silver is feasible. However, different zinc leaching properties, as well as mineral zinc concentration and pH, lead to a large difference in flotation between different plants. Since the presence of silver in the leaching residue is complicated and a part of the silver is wrapped, pretreatment with sulfuric acid, sodium sulfide or the like can improve the flotation index. The research report of the combination drug system shows that the selection of the combination drug system contributes to the improvement of the silver recovery rate. The leaching flotation combined method combining mineral processing and metallurgical disciplines is directed to the composition characteristics of the slag, leaching silver oxides, and then flotation recovering the unleached silver-containing sulfides to obtain the highest silver recovery. Fourth, the development direction of zinc leaching slag flotation research First, the study of the occurrence of silver in the leaching slag should be strengthened. The choice of silver recovery method depends to a large extent on the composition of the dross. As can be seen from the above review, the different properties of the leaching slag have a great influence on the index of silver recovery by flotation. Finding the state of silver in the leach residue helps to select a reasonable process and medication system to improve the flotation index. Secondly, research should be carried out on the combination of zinc leaching slag flotation reagents and the development of new high-efficiency collectors. Practice has shown that the use of combination agents can increase the silver recovery rate and reduce the dosage of the drug. Therefore, finding a more effective combination of collectors is an important way to improve the silver flotation index. In addition, the combination of the combination of agents and the mechanism of action need to be explored. Aiming at the characteristics of fine grain size and complex shape of zinc leaching slag, combined with the modern flotation agent structure theory and molecular design principle, a new high-efficiency silver collector with good selectivity and no influence on the presence of a large amount of zinc ions in the pulp was developed to improve silver. The sorting efficiency reduces the grade of silver in the tailings. It is very beneficial to study the potential-controlled flotation technology of leaching slag by using modern sulfide ore potential-controlled flotation theory. The theory is a new theoretical system formed by the combination of quantum chemistry and electrode process kinetics in the sulphide ore flotation process. The potential-controlled flotation technology developed by this theory is an innovation of sulfide ore flotation technology, but the research in zinc leach residue is still blank. In addition, the basic theoretical research on the recovery of silver by wet zinc leaching slag flotation is basically blank. Therefore, it is urgent to strengthen the basic theoretical research, such as the influence of a large amount of zinc ions in the slag on the flotation recovery of silver, and the flotation solution chemistry. A corresponding theoretical system should be established to guide industrial production. Finally, the research on the treatment process combining physical beneficiation and chemical beneficiation is strengthened. It is often difficult to increase the silver recovery rate by using only a single single physical beneficiation method after obtaining a certain silver recovery rate. With the development of chemical beneficiation, the selection of a suitable selective metallurgy combined process can strengthen the comprehensive recovery of silver to obtain better treatment indicators. This method is a new research direction for the treatment of zinc leaching slag, and is in line with the development trend of mineral processing technology . references: [1] Mineral Processing Manual Editorial Board. Mineral Processing Handbook [M], Volume 4. Beijing: Metallurgical Industry Press, 1990, 487-491. [2] Meng Fanyu. Treatment and Comprehensive Utilization of Lead and Zinc Smelting Waste Slag [C]. Proceedings of the Second National Conference on Comprehensive Utilization of Mineral Resources, 1988-185. [3] You Lihui. The Silver Flotation Production of Laoshan Company [J]. Zhuye Technology, 1990, 18(1): 64~65. [4] South African Patent [P]: ZA7602986, 1977. [5] Japanese Patent [P]: JP-A-52-35197, 1977. [6] Liang Jingdong. Flotation Theory and Practice of Metallurgy[M]. Beijing: Metallurgical Industry Press, 1995, 186~191. [7] Tian Guangrong, like gold, silver and platinum group metals recovery method [M] Chongqing: Science Press, Chongqing Branch, 2001 - 1965,2000. [8] Deng Hua. Improvement of Silver Flotation [J]. Zhuye Technology, 1994, 22 (4): 50. [9] Huang Kaiguo. Flotation of silver from zinc leaching residue [J]. Journal of Central South University of Technology, 1997, 28 (6): 530 ~ 532. [10] Xu Wenxian. Recovery of lead, silver and gold from the residue of zinc smelting process by vulcanization-flotation method [J]. Non-ferrous metal abstracts, 1991 Supplement: 89-97. [11] British Patent [P]: GB2084491, 1975. [12] SHEN Xiang黔.Study on Recovering Silver in the Process of Zinc Smelting by Iron Tanning Process[J].Mining and Metallurgical Engineering,1992,12(2):51～55. [13] Li Hua. Leaching flotation combined method for recovering silver from zinc slag [J]. Foreign metal ore dressing, 1996, (8): 22~24. [14] Flotation of silver minerals and their precipitates in acidic media [J]. Foreign metal dressing, 1990, (10): 53. Author unit Department of Mineral Engineering, Central South University of Technology (Zhou Guohua, Xue Yulan, Jiang Yuren) China Nonferrous Metals Industry Bureau (He Boquan)
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