Gold extraction and environmental protection

I. Introduction

Our country is rich in gold resources of a country. Since the reform and opening up, especially after the 1980s, the annual production of gold in China has increased at an average rate of 10%. In 1999, China's annual gold output reached 169.1 tons, ranking second in the world after South Africa, the United States, Australia and Canada.

As early as 4000 BC, China began its gold production activities. After the founding of New China, China's gold production technology has made great progress. In the 1950s, the re-election method was adopted. In the 1960s, the mercury- mixing and flotation methods were mainly used. In the 1970s, the whole mud cyanidation method was adopted. The cyanide carbon slurry (CIP) method and the carbon leaching (CIL) method are mainly adopted and promoted. According to incomplete statistics, the current production of gold by cyanidation accounts for more than 60% of the country's total gold production; there are still a large number of small and medium-sized enterprises that still use the amalgamation method to recover coarse-grained monomer gold for environmental belts. Came to pollution.

Like many countries in the world, China faces two major challenges in resource and environmental protection in the process of developing gold production. First, with the gradual reduction of gold resources in the province, it is necessary to develop new technologies to deal with poor and difficult gold resources. Second, with the increasingly strict requirements of environmental protection, especially after the promulgation of “China Environmental Protection**” in 1989, it forced enterprises to accelerate the study of clean production processes while controlling the environmental pollution of gold production, so as to promote the gold production technology. Development in the direction of no pollution or less pollution.

Second, the recovery of coarse-grained monomer gold and environmental protection

(1) Re-election method

Gravity sorting using the difference between gold and gangue density is the oldest method for people to recover gold from gold mines. Due to the low production cost of the re-election method and the low environmental pollution, it has been used up to now.

The re-election method plays an important role in the gold ore dressing in China. For the riverbed gold suitable for gold mining, depending on the nature of the ore, a single fixed chute process, a chute-jigging-shaker process or a three-stage jig process is used. The residual gold, slope product and alluvial sand gold are washed, agitated and choked, washed, centrifuged, and shaken-washed. The gold concentrate obtained from the gold mining or ship mining needs to be further selected by re-election, magnetic separation, electric selection or a combination of these methods, and the quality of the smelting by fire method is 85% to 92%. Finished gold. Table 1 lists the technical and economic indicators for mineral processing of some gold mining and ship mining in China.

Table 1 Technical indicators for gold ore dressing production

The re-election method is mainly used in the grinding process to recover coarse-grained monomer gold in the beneficiation or extraction process of the vein gold mine. In some of the selection and smelting plants in Shandong Province of China, the jig is generally used for rough selection, and the coarse concentrate is selected by the shaker to obtain gold concentrate, which is sent to smelting and refining to produce gold. The recovery rate of re-election work gold in the grinding circuit depends on the crude gold content of the ore and the monomer dissociation degree of the gold in the grinding circuit. The general recovery rate fluctuates in the range of 15% to 45%.

(2) Mercury amalgamation

The amalgamation method is an effective method for recovering coarse-grained monomer gold. Due to the use of fully enclosed ventilation in the operation room and scraping of mercury paste under water coverage, the mercury concentration in the air of the amalgamation operation reached 0.0044 mg/m ³ , and the mercury concentration in the air when the mercury paste was applied reached 0.0088 mg/m ^{3 .} The tailings wastewater discharged from the gold-smelting plant has a mercury content of 0.0034 mg/L, and the tailings pond overflow water contains mercury of 0.0096 mg/m ³ , which can be lower than the national required emission standards (see Table 2).

Table 2 National environmental standards for mercury and its compounds

In order to solve the environmental pollution caused by the amalgamation operation, various scientific research units in China cooperated with the gold mines, carried out a large amount of scientific research work, and adopted re-election (jigging) instead of amalgamation in the grinding circuit of many gold mining plants in Shandong Province. Gained results and promoted applications. In order to replace the amalgamation operation, Canada developed the Knelson concentrator as early as the 1970s, and gradually improved and was promoted and applied in the 1980s. The equipment is a high-efficiency centrifugal beneficiation equipment. In the East Malartic gold mine in northwestern Quebec, Canada, the 76cm Nielsen concentrator replaces the jig to recover monomer gold, which enables the recovery of re-election work gold. The rate has increased from 10.8% to 40%. At present, the equipment has been applied in hundreds of grinding circuits in gold selection plants around the world. With the continuous development of equipment for environmental protection in the world and the new and efficient recovery of coarse gold, the amalgamation operation in the gold smelting plant will eventually be completely replaced.

Third, cyanide gold extraction and tailings purification treatment

(1) Cyanide gold extraction process

Since the 1887 release of the first Englishman John Macarthur and so a patent processing gold and silver ore with cyanide, New Zealand (1889), South Africa (1890), United States (1891) were established cyanide leaching - Replacement of zinc dust The gold extraction plant [7]; carbon slurry (CIP) method is an important development of the cyanide gold extraction method. In 1973, the United States established the first carbon slurry gold extraction plant in Homestake, followed by South Africa. Australia and even the world's gold producing countries have been promoted; in turn, activated carbon or resin was added during the cyanide leaching process, and developed into carbon leaching (CIL) and resin pulp (RIP) methods.

The application of cyanide leaching-zinc chip replacement gold extraction process in China began in the 1970s. In 1985, it designed and built two small-scale carbon pulp plants in Linghu and Chiweigou. In 1986, the Zhangjiakou gold mine in Hebei introduced carbon extraction technology (CIL) from abroad to solve the problem of low recovery rate of the gold-containing oxidized ore in the mine using amalgamation-flotation process. The recovery rate increased from 73% to 93%. A gold mine in Liaoning replaced the original cyanide leaching-zinc powder replacement process with carbon leaching (CIL) method, which increased the recovery rate of gold from 82.93% to 91.4%. The carbon leaching technology and equipment have been digested, absorbed and developed, and have been rapidly promoted and applied throughout the country. Up to now, there are more than 50 carbon leaching plants of various production scales in China, and it has become the main process of gold production in China.

In order to meet the development of small and rich gold resources, in 1988, the Beijing Research Institute of Mining and Metallurgy developed a portable carbon leaching gold plant with a daily processing capacity of 30 t. In 1990, after a gold mine in Henan was put into production, it was obtained. Good results, under the conditions of 5.82 g / t of ore containing gold, the cyanidation leaching rate reached 92.27%, and the recovery rate of finished gold reached 89.82%. The resin pulp gold extraction method is a new technology developed after the carbon leaching method. Three different cyanide resin pulp (RIP) gold extraction plants have been built in Anhui, Hebei and Xinjiang, and have achieved good production. index. For example, a gold mine in Xinjiang adopts a re-election-cyanide-resin gold extraction process. The total recovery rate of gold is 92%. Due to its high resin adsorption capacity, low production cost and strong adaptability, it has already shown its application prospects. In addition, the use of cyanide heap leaching to treat low-grade gold ore has also made good progress in China.

For example, the heap leaching scale of a gold mine in Xinjiang has reached 50,000 tons, and the largest heap leaching mine has processed more than 160,000 tons of ore per year. The gold recovery rate of heap leaching can reach 70%, which is to develop low-grade gold in China. Mineral resources provide an economical and reasonable treatment.

(2) Purification of cyanide tailings

Cyanide is an effective immersion agent recognized by the world. The cyanide leaching process plays an extremely important role in gold production in countries all over the world. According to incomplete statistics: the world's 60 largest gold producing areas and 23 famous gold enterprises, the production of gold by cyanidation accounted for 81.30% of its total output, of which carbon slurry method and carbon leaching method accounted for cyanide method. 53.87%. This shows that the amount of cyanide used in gold production in the world is large.

As is known to all, cyanide is a highly toxic substance, and countries around the world have strict restrictions and requirements on the discharge of gold production waste. In order to prevent environmental pollution caused by cyanide waste residue, China National Environmental Protection Agency and Gold Administration have formulated national standards and water quality standards for cyanide emissions, which stipulate that the maximum allowable concentration of industrial wastewater is 0.5 mg/L, and the drinking water contains CN- More than 0.05 mg/L. At present, there are roughly the following methods for purifying tailings used in cyanide gold extraction plants in China:

1. Chlorination method

Chlorination is one of the main methods used in China to purify cyanide tailings. According to the local conditions of the cyanide plant, chlorine gas, liquid chlorine, bleaching powder, etc. can be selected and used. In fact, they enter the tailings slurry to produce hypochlorous acid, which in turn decomposes cyanide into non-toxic substances.

For small cyanide gold extraction plants in remote mountainous areas, the bleaching powder method is often used. For example, a gold mine in Henan belongs to a cyanide gold-plating plant with a daily processing capacity of 30 t. The cyanide tailings are added with bleaching powder of 8.5 kg/t. After two-stage treatment for 2 h, the tailings can contain CN-from 205. After the mg/L is reduced to 0.48 mg/L, it is discharged into the tailings pond.

For large and medium-sized cyanide gold extraction plants, the tailings can be alkaline chlorination or acid chloride. For example, cyanide tailings from a gold mine in Hebei Province, using lime added, chlorine gas under alkaline conditions (pH 9 ~ 11), tail slag after 2 h purification treatment, can reduce its CN-content from 200 mg / L 0.5 mg/L or less; while the cyanidation tailings of another gold mine in Hebei Province adopts the acid chloride method, the closed-end operation is carried out in the acid stage, so that the tailings and chlorine gas are fully mixed and oxidized under acidic conditions to make the tail The slag containing CN- decreased from 500 mg/L to 8.5 mg/L in the acidic phase (pH=2), and then decreased to 0.3-0.4 mg/L in the alkaline phase (pH=12). Compared with the alkaline chlorination method, due to its fast oxidation rate, processing time is shortened by 1/3, and production cost is 42% lower, it is being promoted and applied in domestic gold mines.

2. Acidification

The acidification method directly treats the cyanide tailings with H ₂ SO ₄ to form HCN, and generates NaCN by lye absorption to achieve the dual purpose of purifying cyanide tailings, which is suitable for high cyanide-containing concentration. Tail residue purification treatment. For example, the gold concentrate of cyanide tailings in a gold mine in Shandong Province, China, contains CN-up to 1242 mg/L. The mine uses H2SO4 to acidify the tailings slurry. By blowing air into the slurry, HCN enters the NaOH. The washing device absorbs and generates NaCN for recycling. The production practice shows that the acidification method can reduce the CN- in the tailings to 3.2 mg/L, and then use the bleaching powder treatment or the pipeline aeration method to discharge the CN- in the wastewater to 0.5 mg/L. The method can make the recovery rate of NaCN in the tailings more than 80%, and the benefit can compensate the cost of treating the tailings wastewater, and is an economical and reasonable method for treating cyanide tailings.

3, other methods

Activated carbon adsorption catalytic oxidation is a new method for developing and treating cyanide-containing wastewater in recent years. The method is to pass air in the cyanide-containing wastewater to ensure sufficient oxygen, and to decompose and achieve the purpose of removing cyanide under the action of a catalyst. The industrial test of a gold mine in Hebei Province proves that the treatment with this method can reduce the CN-containing wastewater from 250 mg/L to less than 0.5 mg/L.

Adopting the zero-discharge process of natural purification and cyanide wastewater recycling is another important way to solve the environmental pollution caused by cyanide-containing wastewater. The research and production practice of a gold mine in Liaoning Province proves that the tailings wastewater containing CN-70～100 mg/L of the cyanide plant of the mine is sent to the tailings pond without treatment, and contains CN-22~ after natural clarification. The return water of 59 mg/L was returned to the gold extraction plant for recycling. This treatment method not only reduced the cyanide leaching index, but also reduced the cost of wastewater treatment by 82%. The premise of adopting the zero-emission process: First, the water consumption of the gold-plating plant must be greater than the wastewater discharge; second, the tailings pond has no leakage and will not cause groundwater pollution. Where conditions are met, it is economically justified to use a zero-emission process.

Fourth, research and development of non-cyanide gold extraction process

As the world increasingly stringent environmental protection requirements, greatly promoting research and development of non-cyanide gold extraction process agents, including: chloride, thiourea, thiosulfate, bromide method, lime sulfur Law and so on.

(a) Chlorination

Chlorination is one of the earliest methods of chemical gold extraction. The advantage is that the gold leaching speed is fast, and the carbon-containing material adsorbing gold is inactivated. The disadvantage is that the acidic chlorine-containing solution is highly corrosive and requires high anti-corrosion materials for the equipment.

As early as the 16th century, brine was used to extract silver from ore. In 1880, Australia and Canada used gold chlorination to extract gold from roasted gold. In 1966, Finkelstein and others used the perchlorination method to extract gold from the Merril type mud mine in South Africa. The United States used hypochlorite pretreatment on the Carlin-type carbonaceous gold ore to make the gold leaching rate from 33% increased to over 90%. In 1988, Newmont Corporation of the United States further developed flash chlorination technology, which greatly shortened the chlorination time, reduced chlorine consumption by 25%, increased gold leaching rate by 6%, and introduced FeCl3 heap leaching. Gold law. Peru and France have developed a chlorination leaching process for the combination of salt, sulfuric acid and manganese dioxide. But so far, only a few countries such as South Africa have been used for industrial production using the water chlorination process.

Since 1988, the Beijing Research Institute of Mining and Metallurgy has carried out research on the gold extraction process of chlorination, and has made good progress through research on various ore types. For the gold-bearing arsenic bismuth- sulfur gold ore in Guizhou Miaolong, the leaching rate of gold is up to 91.44% by roasting-water chlorination method; for a gold mine in Hebei, it contains Au 12.21 g/t and As 0.6%. The material is treated by chlorination and gold extraction. Compared with the cyanidation method, the leaching time is shortened by 19 h, the gold leaching rate is increased by 1.69%, and the silver leaching rate is increased by 74.45%. For a gold-containing material in Gansu Province, 1 t of chlorination is carried out daily. The expansion test of gold shows that when the fineness of grinding is -0.074 mm, 50%, under the condition of 4 g/t of gold, the leaching rate of gold can reach 85%-90% after chlorination for 6 h. The chlorine consumption is 37-40 kg/t ore. In addition, the Hunan Institute of Nonferrous Metals used FeCl ₃ gold-electrowinning process for Longshan dipping slag, the leaching rate of gold was 98%, and the electric volume rate was over 98%. The calcination-water chlorination method was used to raise gold for Pingjiang gold mine. Process, gold leaching rate of 97%. Yantai University used a hypochlorite leaching process for the Rushan Sanjia gold concentrate, and the gold leaching rate reached 95%. Kunming University of Science and Technology used MnO ₂ , NaCl and H ₂ SO ₄ to chlorinate low-temperature (280 ° C) chlorination of gold ore containing As and Sb in Beibei, and achieved good results and progress.

The chlorination process is an ancient and promising method. Although many research work has been done in the field of chlorination and gold extraction, it is still in the stage of engineering research. In order to enable the process to be used for production as soon as possible, it is necessary to vigorously develop cheap and renewable chlorinating agents, reduce their consumption, and solve technical problems such as equipment corrosion.

(2) Thiourea method

The thiourea method is considered to be a promising method for non-cyanide gold extraction. Before 1941, the Soviet scholar Plasin (Плaсии) and others first studied the thiourea gold-soluble technology, and after more than ten years of research, laid the foundation for the thiourea method to extract gold from ore. In 1968, Romanian scientists and technicians further studied the thiourea process gold extraction process. After 1976, South Africa, the United States, Australia, Canada and other countries have carried out research on thiourea gold extraction, the main direction of which is to study the adaptability to ore and the feasibility of refractory gold. As early as 1972, the Soviet Union established the first industrial test plant for gold extraction by thiourea in the Lebegine gold mine. The test found that the method has two major problems: excessive consumption of thiourea and serious gold surface passivation. Subsequently, through continuous research by scientists from various countries, it was found that the introduction of SO _{2 in the} process of gold extraction by thiourea can greatly reduce the consumption of thiourea and significantly reduce the surface of elemental sulfur passivated gold particles, thus accelerating the thiourea method. The industrialization process of gold.

The domestic thiourea process gold extraction process began in 1972, and Changchun Gold Research Institute and other units have carried out a lot of research work. The thiourea leaching is generally carried out in an acidic medium (pH < 1.5), usually using Fe3+ as the oxidizing agent. For the three arsenic-containing gold-sulfur flotation concentrates of a gold mine in China, the comparative test results of thiourea leaching and cyanide leaching (see Table 3) show that the leaching rate of gold is higher than that of cyanide by thiourea method. Law. An expanded test with a daily throughput of 600 kg verified the results of small tests with an average leaching rate of more than 92%. A gold mine in Guangxi has used the thiourea method to raise gold. After several industrial tests, it has been found that the thiourea method has the advantages of high leaching speed and low toxicity, but the thiourea is expensive, the production cost is high, and the production process Poor stability limits its promotion and application. 0 kg/t ore. In addition, the Hunan Institute of Nonferrous Metals used FeCl ₃ gold-electrowinning process for Longshan dipping slag, the leaching rate of gold was 98%, and the electric volume rate was over 98%. The calcination-water chlorination method was used to raise gold for Pingjiang gold mine. Process, gold leaching rate of 97%. Yantai University used a hypochlorite leaching process for the Rushan Sanjia gold concentrate, and the gold leaching rate reached 95%. Kunming University of Science and Technology used MnO ₂ , NaCl and H ₂ SO ₄ to chlorinate low-temperature (280 ° C) chlorination of gold ore containing As and Sb in Beibei, and achieved good results and progress.

(III) Thiosulfate method

In the non-cyanide gold extraction study, the thiosulfate method is superior to the chlorination method and the thiourea method because it is leached in an alkaline medium. As early as 1957, the Soviet Union studied the dissolution thermodynamics of pure gold in ammonia thiosulfate solution. 1972 Japanese study of Cu ^{2 +} S ₂ O ₂ ^{- 3} gold dissolution catalysis and its influencing factors. ³ Leaching Research ^- 1981 US _{2 2} O S containing copper, manganese were Refractory Gold. In 1983, the Soviet Union obtained a patent for the treatment of complex gold deposits containing copper and manganese with thiosulfate. The United States buttonmont company established a bacterial pretreatment (carbon and arsenic) thiosulfate heap leaching plant in the Carlin Gold Mine.

Domestic research on thiosulfate leaching gold started late. After 1984, Shenyang Gold Institute, Shenyang Mining and Metallurgy Institute, Northeastern University, Institute of Chemical Metallurgy, Chinese Academy of Sciences, Central South University of Technology and other units carried out thiosulfate gold extraction Basic theoretical research and technical development work of the method. According to the report, although thiosulfate has the advantage of high leaching index, the leaching conditions are demanding and require leaching at a temperature of 50-60 ° C. The leaching system requires SO ₂ or sulfite as a stabilizer and Cu ^{2 +} as a catalyst. Wait. The test results of a copper-bearing gold concentrate in Shandong Province by thiosulfate method show that for gold concentrate containing Au 153.62 g/t, Ag 443.5 g/t and Cu 4.71%, under normal temperature and pressure conditions, Compared with the cyanidation method, the amount of leaching agent was equivalent by the thiosulfate method, but the leaching time was shortened by 18 h, and the leaching rates of gold and silver were increased by 8.22% and 25.73%, respectively. At present, the domestic thiosulfate method gold extraction process is still in the engineering research stage, and needs to be further improved and promoted.

(4) Bromination

Similar to chlorine, bromine is a strong immersion agent. As early as 1881, Shaff obtained a US patent for bromination by bromination. Later, the research was interrupted due to the widespread application of the cyanidation gold extraction process. Until the 1990s, with the emphasis on environmental protection, the bromination method re-emerged attention and research. In 1990, the United States introduced the organic bromide Geobrom series of agents: 3113, 3400, 5500, of which the Geobrom 3400 containing Br 34% had the best effect. In addition, Australia has achieved good results with inorganic bromide as a gold leaching agent.

In recent years, the relevant domestic units have carried out research on gold extraction of gold flotation gold concentrate containing 32.5 g/t by bromination method. The test results show that the flotation gold concentrate is desulfurized by calcination at 600-650 °C in HCl medium. The addition of NaCl and bromine water for 12 h can make the gold leaching rate reach 99%, but the research work is still in the laboratory stage, and the research in the United States and Australia has entered the practical stage of engineering.

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