Cyanide bath leaching gold is currently the most widely used method for private mining of gold. It has the advantages of high extraction rate, strong adaptability to ore, relatively low cost and relatively simple process flow.
The basic principle is that the leaching of gold ore, zinc powder or zinc wire and then to extract gold from the leaching solution with an oxygen-containing cyanide solutions. Considering the factors such as the solubility, chemical stability, consumption cost and environmental pollution of cyanide on gold, the most commonly used cyanide is NaCN.
The basic chemical reaction formula is:
Although the pool leaching method is widely used, due to the lack of technical strength in many small mines, the production technology of leaching gold in cyanide tank is not enough, resulting in high production cost, high pollution, low leaching rate and recovery rate of gold. The phenomenon, so that the economic benefits are not high, and even appear to be damaged.
In actual production, the minimum entry grade (not less than 2g·t - 1 ) of the mine should be comprehensively determined based on production cost, leaching rate, recovery rate and expected price of sales price. When mining mineral deposits with large gold grade changes, the analysis of Au should be carried out frequently, and the results of the analysis should be used to guide the mining of the ore body. The ore that does not reach the pool grade should be removed.
In addition, in order to determine the optimal NaCN concentration and calculate the leaching rate, Au analysis of the ore and tailings entering the pond must be performed. Using Au's rapid analysis method (which I have recommended for an improved rapid analysis method), data can be acquired quickly to guide production.
The author has conducted inspections on some mines in Baise and Yulin, and has done some research on the production technology of pool leaching and gold extraction.
First, the choice of cyanide
Cyanides which can be used for gold dissolution are: KCN, NaCN, NH4CN and Ca(CN) 2 . When selecting cyanide, considering the factors such as the solubility of cyanide in gold (see Table 1), chemical stability, cost of consumption, environmental pollution, etc., most of the mines use NaCN.
Table 1 Relative solubility of cyanide
Second, the choice and control of cyanide solution concentration
The concentration of cyanide solution and the concentration of oxygen in the solution are the two main factors determining the dissolution rate of gold. They have a great influence on Au leaching rate, leaching period and control of production cost, so master the leachate.
The cyanide concentration plays an important role in improving economic efficiency.
The NaCN concentration of the leachate is mainly related to the minerals entering the pool ore and the grade containing Au, and the concentration increases with the increase of the Au grade. At the same time, different amounts of cyanide-depleting impurities are present in different ores. Conventional cyanide concentrations are generally between 0.03% and 0.1%. Table 2 shows the conventional selection of controlling the concentration of the cyanide solution.
Table 2 Selection of cyanide solution concentration
Too high a concentration of NaCN will result in waste of the drug, an increase in cost, and a large environmental pollution; if the cyanide concentration is too low, the leaching rate becomes slow and the leaching rate becomes low.
The cyanide concentration can be determined by AgNO3. AgNO3 reacts with NaCN to form AgCN, and the excess silver ion and dimethylaminobenzyl tannin are pink as the titration end point, which is calculated by the following formula.
Using this method, the NaCN concentration of the leachate at different times was monitored at any time, and the NaCN concentration of the leachate was adjusted according to the standard of Table 1.
Third, we must strictly control the acid and alkali solution of the leachate
(1) NaCN in aqueous solution should be kept at a pH of 9.4 to 11 to maintain stability. When the pH is low, NaCN will be hydrolyzed:
A part of the generated HCN will volatilize, causing loss and harm.
(2) Carbon dioxide and sulfuric acid (H 2 SO 3 , H 2 SO 4 ) in the solution also react with cyanide to form HCN.
Effect (iii) associated minerals: pyrite oxidation, such as when, in addition to generating a sulfuric acid (H 2 SO 4), but also generate some ferrous sulfate (FeSO 4), and generated with a cyanide Fe (CN) 2. FeSO 4 can be oxidized to Fe 2 (SO 4 ) 3 when there is alkali and oxygen in the solution, and then reacts with alkali to form Fe(OH) 3 , which does not react with cyanide. Thus, the addition of a base serves to protect cyanide, and the added base is referred to as a protective base.
Lime is now generally used to adjust the pH of the solution. It is important to note that the pH of the water must be adjusted to 11 before NaCN is placed. At the same time, the pH value of the leachate should be measured frequently during the leaching process. When the pH value is lowered, lime should be added for adjustment.
Fourth, the impact of ambient temperature
The pool leaching cyanidation method has certain requirements on the ambient temperature. Figure 1 shows the relationship between the dissolution rate of gold in sodium cyanide solution and temperature. From Fig. 1, we can see that the temperature requirement of leaching can be satisfied by generally maintaining 15 to 25 °C.
Fig.1 Relationship between dissolution rate of gold in sodium cyanide solution and temperature
Table 3 shows the results measured under different temperature conditions. It can be seen from the table that when the temperature is close to 5 °C, production should be stopped.
Table 3 Measurement results under different temperature conditions
V. Other influencing factors
1. For some non-precious metal minerals, pretreatment is required to reduce cyanide consumption and increase leaching rate. Common methods include baking, pickling, and the like. The appropriate method can be selected depending on the type of mineral. Gold does not change during the roasting process.
2. The size and shape of the gold grain exposure of the ore also affect the gold leaching speed. The larger the particle size, the slower the dissolution rate. Therefore, the gold mine is pre-ground prior to cyanidation, so that the gold particles are fully exposed, and the leaching rate can be improved in the leaching cycle. In the ore, the gold grains have a spherical shape, a sheet shape, a dendritic shape, an inner cavity, and other irregular shapes. The spherical gold is smaller than the surface, and the leaching speed is slow. The other shapes of gold have a larger specific surface than the spherical gold, and the leaching speed is faster.
3. The concentration of the slurry and the content of the slime directly affect the diffusion rate of the solvent and the contact of the solvent with the gold particles. Therefore, the ore concentration must be reduced for the ore containing more ore.
4. Appropriate stirring can accelerate the dissolution of gold, shorten the leaching period and increase the leaching rate.
references
[1] Li Dingxin, Wang Yonglu, et al. Extraction and refining of precious metals [M]. Changsha: Central South University of Technology Press, 2000.
[2] Huang Zhenqing, et al. Concise gold practical manual [M]. Shenyang: Northeast Normal University Press, 1991.
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