Gold extraction and environmental protection (2)

Gravity separation or extraction process in the beneficiation of gold ore vein, mainly for grinding coarse free gold recovery circuit. 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%.
2.2 amalgamation [4]
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 mercury content in the tailings wastewater discharged from the gold-removing plant is 0.0034 mg/L, and the overflow water in the tailings pond contains 0.0096 mg/m3, which can be lower than the national emission standards (see Table 2).
Table 2 National environmental standards for mercury and its compounds
Table 2 National environmental and discharge standards of mercury and its compounds
Type Type Hazardous Substances Maximum allowable concentration Industrial wastewater Mercury and inorganic mercury compounds 0.05 mg/L (in Hg)
Liquid domestic water mercury <0.001 mg/L
Farmland Irrigation Water Mercury <0.001 mg/L
Gas residential area atmospheric mercury daily average 0.0003 mg/m ³
Workshop Air Metallic Mercury 0.01 mg/m ³

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 76 cm Nielsen concentrator replaces the jig to recover monomer gold, which enables re-election work. The recovery rate has increased from 10.8% to 40% [5]. 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.
3 Cyanide gold extraction and tailings purification treatment
3.1 cyanide gold extraction process since 1887 Englishman John Macarthur and other published after the first one with the cyanide process gold and silver ore patent [6], New Zealand (1889), South Africa (1890), United States (1891), respectively A cyanide leaching- zinc chip replacement gold extraction plant was established [7]; the carbon slurry (CIP) method is an important development of the cyanide gold extraction method. In 1973, the United States established the first charcoal in Homestake. The pulp gold extraction plant was subsequently promoted in South Africa, Australia and even the world's gold producing countries; and then activated carbon or resin was added during the cyanide leaching process to develop 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% [8]. 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, Beijing Research Institute of Mining and Metallurgy developed a portable carbon leaching gold extraction plant with a daily processing capacity of 30t [9]. In 1990, it was obtained after a gold mine in Henan was put into production. The better result is that under the condition of 5.82 g/t of ore containing gold, the cyanidation leaching rate is 92.27%, and the recovery rate of finished gold is 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 re-election-cyanide-resin gold extraction process, and the total recovery rate of gold is 92% [10]. Due to its high resin adsorption capacity, low production cost and strong adaptability, it has already demonstrated its application. prospect. 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.
3.2 Purification of cyanide tailings Cyanide is an effective leaching agent recognized to date. The cyanide leaching process plays an extremely important role in gold production in countries around 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% [11]. 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:
3.2.1 Chlorination method [12]
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 30t cyanide gold extraction plant, and the cyanide tailings are added with bleaching powder of 8.5kg/t. After two treatments for 2h, the tailings can contain CN-from 205mg/L. After being reduced to 0.48 mg/L, it was discharged into the tailings pond.

HDPE Resin

High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. HDPE is commonly recycled, and has the number "2" as its resin identification code.It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, HDPE is used in the production of plastic bottles, corrosion-resistant piping, geomembranes and plastic lumber.