Up to now, the main method widely used in industrial production to degrade the gold in the sulfide by submicron is still the oxidative roasting method. However, when dealing with high arsenic-containing sulphidic concentrate, this approach seriously pollute the environment when decomposed gold. In addition, a portion of the gold is inevitably lost in the volatile product during the firing process. Therefore, hydrometallurgy is the most promising method for treating gold-arsenic-containing sulfide concentrates.
Soviet researchers used nitric acid to decompose gold-arsenic-containing sulfide concentrates. Nitric acid is a potent oxidizing agent pyrite, arsenopyrite and non-ferrous metal sulfide. When the sulfide is decomposed with nitric acid, copper , zinc , iron, cadmium and arsenic can be transferred into the solution, while lead , gold and antimony are enriched in the insoluble residue. The advantage of using nitric acid decomposition is that it is possible to completely regenerate.
The chemical reaction formula for the oxidation of metal sulfides with nitric acid in the presence of oxygen is as follows:
MeS 2 +2HNO 3 +3O 2 =MeSO 4 +H 2 SO 4 +2NO 2 , (1)
2NO 2 +H 2 O=HNO 3 +HNO 2 substance (2)
MeS 2 +HNQ+3O 2 +H 2 O=MeSO 4 +H 2 SO 4 +HNO 2 (3)
The process of the reaction formulae (1) to (3) can be carried out under the following conditions; this process must be carried out in a closed loop, and the mixture of nitrogen dioxide and oxygen is repeatedly passed through the slurry. After the necessary conditions are met, the suboxide of nitrogen is oxidized to nitrogen dioxide during the recycling of nitrous acid gas. In the subsequent contact with the slurry, they are converted to nitric acid or directly involved in the oxidation of the sulfide.
The authors have studied the decomposition of nitric acid in gold-containing flotation concentrates. More than 90% of the gold in this concentrate is wrapped in pyrite and arsenopyrite. When the sulfide concentrate is leached by nitric acid under normal pressure in a reaction tank with a gas agitator, the process parameters are as follows: the concentration of nitric acid is 150-200 g/L, and the ratio of solid to liquid is 1:5 (7.5). The temperature is 75~85 ° C and the leaching time is 2 hours. The nitrogen dioxide precipitated during the reaction is mixed with oxygen (air) and continuously pumped into the regeneration system of nitric acid. The gas withdrawn from the reaction tank is absorbed by water in the injection device of the closed circuit. The spray device includes a container for water (acid), a cooler, a pump, and an ejector. After the nitrogen dioxide is absorbed by the water through the ejector, a certain concentration of nitric acid is obtained. These nitric acids can be recycled.
When tested under the optimum sulphate strip, 96% of sulfide sulphur, 98% of arsenic and 97% of iron were transferred to a nitric acid solution. Arsenic and sulfur are oxidized to arsenious acid and sulfuric acid. The iron in the solution is present in the form of iron sulfate. The non-ferrous metals are all transferred to the solution. Gold, silver (in the presence of chloride ions), lead and antimony are concentrated in the insoluble residue (leaching residue). The yield of the leaching residue varies depending on the mineral composition of the concentrate, and is generally between 25% and 34%. After the cyanide treatment, the recovery rate of gold is 92~94%, and when it is smelted together with copper concentrate, the recovery rate of gold is 96~98%.
Acidic mother liquors containing residual nitric acid, dissolved non-ferrous metals, iron and arsenic should be denitrified at 350 ° C to regenerate nitric acid. The solid residue after denitration should be calcined at 650--700 °C. Under these conditions, iron combines with arsenic to form iron arsenate which is insoluble in water. It is similar to natural minerals - scorodite. In the heat-treated product, in addition to iron arsenate, there are also non-ferrous metals in the form of their sulfates. These non-ferrous metals can be leached with water. The residue after leaching is sent to the dumping site for storage. See the illustration for the decomposition process of the gold-arsenic-containing sulfide concentrate.
Depending on the mineral composition, the amount of nitric acid required to decompose a ton of gold-containing sulfide concentrate is 0.1 to 0.3 tons of concentrated nitric acid or 0.18 to 0.6 (56% concentration of nitric acid). When it also contains a metallic metal or a rare metal, it is economically advantageous to use such an acid consumption. According to the above theory of decomposing sulfides with nitric acid (reaction formulas (1) to (3)), if a mixture of nitrogen dioxide and oxygen can be recycled, the consumption of nitric acid can be minimized. At the same time, nitric acid can also act as a catalyst during the leaching process. This makes nitric acid not only effective in the treatment of rich concentrates (for example, molybdenum ore, scheelite , etc.), but also in the treatment of other gold-containing products.
The test for decomposing the sulfide concentrate with nitric acid was carried out under a gas phase circulation condition using a styling test apparatus. Continuous conversion of nitrogen oxides to nitrogen dioxide is a necessary condition for this process. This is because nitrogen dioxide can participate in the oxidation of metal sulfides. At this time, the consumption of nitric acid will be reduced by 1/2~2/3 compared with the theoretical consumption. This is due to the maximum use of nitrogen dioxide in the oxidation of sulfides. In many cases, it is also possible to use nitrogen gas from the nitrogen plant to replace the nitric acid to decompose the sulfide concentrate.
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