The requirements of comprehensive utilization of mineral resources and the influence of energy, environmental protection, and other factors have promoted the development of hydrometallurgy, thus forming many different leaching methods. Refractory gold needs used pretreatment to help to concentrate gold. Wet chemical treatment is one branch of ore pretreatment and has the advantages roasting process doesn’t have. Those are no complex secondary leaching issues caused by under-burning and over-burning; well-adapted and select different reagents according to different ores. According to the different media, the pretreatment can be divided into alkaline and acid leaching pretreatments. Depending on the reaction conditions, it can be divided into atmospheric pressure and hot pressure pretreatments. Generally speaking, there are five chemical pretreatments of refractory gold.
Refractory Gold Chemical Pretreatments
1. Atmospheric Pressure AlkalineLeaching Pretreatment
Atmospheric pressure alkaline leaching pretreatment is to oxidize and treat the relevant components of the ore by adding chemical reagents under normal pressure, and the medium is alkaline.
An experimental study on enhanced alkaline leaching pretreatment of an arsenic-bearing gold concentrate was carried out at a normal temperature and normal pressure. The polymetallic minerals in the gold concentrate are mainly metal sulfides, mainly pyrite, arsenopyrite, and orthorhombic Arsenite. The test adopts the comprehensive separation method of physics and chemistry and uses the edge grinding and leaching process. The main equipment uses a tower mill and leaching machine to perform ultra-fine grinding on the arsenic-containing gold concentrate, and then uses the enhanced pretreatment stirring tank at normal temperature and normal pressure. Strengthen the alkaline leaching pretreatment, so as to remove arsenic, desulfurization, or fully dissociate gold and sulfide, and then carry out cyanide leaching to achieve the purpose of efficient gold extraction. The method has the advantages of environmental protection, simple process, short process, and small investment.
2. Atmospheric Acid Treatment
Atmospheric acid treatment is usually the oxidation treatment of refractory ore with Permonosulfuric acid. Permonosulfuric acid is a more potent oxidizing agent than H2O2 and is stable at lower pH values. Peroxymonosulfuric acid is obtained by adding H2O2 to concentrated sulfuric acid:
H2O2＋H2SO4（concentrated）＝H2SO5＋H2O
Peroxymonosulfuric acid can oxidize sulfide ore, and it has a better effect on the oxidation of arsenopyrite. Lakshmanan (according to G. V. Weert, 1988) has reported that Permonosulfuric acid is used to achieve an effect similar to that of water-phase oxidation. Compared with traditional roasting and pressure oxidation methods, the processing cost is lower, but this method has not been used industrially.
3. Wet Chlorination Method
The water chlorination method is used for the direct gold extraction of carbonaceous gold ores. Unlike gold extraction, the reagent used in pretreatment is not chlorine gas, but hypochlorite, high-valent ferric chloride, and copper salts and chlorination. sodium etc. High-valent iron and copper salts are ideal oxidants for pre-leaching a range of sulfide minerals. The test shows that the order of leaching sulfides from high-priced iron salts is from difficult to easy: molybdenite, pyrite, pyrite, diamonte, sphalerite, galena, chalcocite, pyrrhotite; The order of leaching sulfides from high-priced copper salts from difficult to easy is pyrite, chalcopyrite, galena, sphalerite, chalcocite. Through the decomposition of sulfide, the encapsulated gold particles can be released, which facilitates cyanide leaching.
4. HNO3Decomposition Method
The use of HNO3 to oxidize arsenopyrite can fully decompose the sulfide in the raw material, thereby enriching Au exponentially, which is beneficial to the recovery of Au. The former Soviet Union used HNO3 to treat arsenic gold ore, and As and S were oxidized into arsenous acid and sulfuric acid, so as to achieve the purpose of entirely dissociating and encapsulating gold, and the recovery rate of gold extraction by cyanidation of leaching residue was more than 95%. The acid consumption of this method is 0.18-0.6t, but the introduction of oxygen or air can accelerate the oxidation process and reduce the consumption of HNO3 by 1/2-1/3. The former Soviet Union’s Baykov Institute of Metallurgy used 5% to 10% HNO3 to treat Au-containing pyrite and arsenopyrite for 1 hour at 75 to 85 °C and a solid-liquid ratio of 1:7.5 at atmospheric pressure. When oxygen or air is introduced, Fe98%, As97%, and S92% are dissolved in the solution, Au is left in the slag, the recovery rate of gold extraction by oxidation can reach 94%~96%, and the KCN unit consumption is 3~4kg/t concentrate. The toxic nitrogen oxide gas generated in the process is returned to the operation by absorbing and regenerating HNO3. Guangdong Nonferrous Metals Research Institute used this method to deal with the acid consumption of Xinjiang Karamay Gold Mine of 939kg/t. It can be seen from some indicators of the oxidative decomposition of HNO3 that the leaching rate of Au after treatment by this method is as high as 95%. But the Achilles heel of this method is that the acid consumption is too large. Although the HNO3 consumption can be reduced by means of pressurized oxygen leaching, it still needs 0.1-0.3t per ton of ore. HNO3 needs to be regenerated by distillation at 350 °C, which is difficult to achieve in the industry, and As not only cannot be used but also needs to be cured. It can be considered that the possibility of industrial application of this method is minimal unless the grade of Au is very high, Otherwise, it is not economical.
5. Hot Press Chemical Pretreatment
It is well known that autoclaved oxygen leaching in a closed vessel can increase the reaction speed and reach the reaction endpoint in a shorter time. The use of hot-pressed oxygen leaching for the pretreatment of refractory ore can improve the cyanidation rate of gold and reduce the consumption of cyanide. The hot-pressed oxidation method is mainly used for the pretreatment of sulfide ores and arsenopyrite. According to the different medium environments, it is divided into hot-pressed oxygen acid leaching and alkali leaching. Hot pressure oxygen acid leaching can be used for the pretreatment of sphalerite, chalcopyrite, galena, copper-zinc sulfide ore, nickel-cobalt sulfide ore, gold-bearing pyrite, and arsenopyrite; hot pressure oxygen alkali leaching mainly refers to The principle of hot pressure oxygen ammonia leaching is that some metal cations can form soluble complexes with ammonia, and at the same time, under the condition of hot oxygen pressure, a large amount of S is oxidized by SO42－.
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