To determine the best conditions for acidic thiourea leaching of gold, iron, respectively Jiabu La sulfuric acid concentration, the concentration of thiourea, sulfuric acid concentration, the temperature, the rate of washing, the solid acid ratio, type and amount of oxidizing agent such factors and leaching rate of gold extraction The impact of the rate was studied in detail:
1) Effect of iron sulfate concentration The components of the test-treated golden iron ore concentrate are shown in Table 1. When the solid acid ratio is 30%, the relationship between the leaching speed and the concentration of ferric sulfate is measured by using different concentrations of Fe 2 (SO 4 ) 3 as shown in Fig. 1.
Table 1 Â Chemical analysis of gold-bearing iron ore concentrate | |||||||||||||
element | Au | Ag | As | TFe | Si | Ca | Al | S | Mg | Na | Mn | Cu | C |
content | 50 | 8.5 | 1 | 22.6 | 3.66 | 3.15 | 3.66 | 15.7 | 1.69 | 1.51 | 0.28 | 0.04 | 1.05 |
It can be seen from the figure that when the concentration of the substance of Fe 2 (SO 4 ) 3 is increased from 0.0037 mol/L (1.5 g/L) to 0.0153 mol/L (6 g/L), the gold leaching speed increases and the concentration exceeds After 0.153 mol/L, the leaching speed did not change.
2) Influence of sulfur concentration on sulfur leaching rate As shown in Fig. 2, the leaching speed increases with the increase of sulfur pulse concentration, and reaches the optimum value at 0.197mol/L (15g/L). That is, the leaching time is 60 min, and the extraction rate of gold is about 95%.
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The optimal concentration of thiourea varies greatly depending on the raw materials and experimental conditions. The data reported by the researchers are quite different. The best concentration of thiourea determined by another foreign research report is 4g/L, while China has a large scale in recent years. The optimum mass fraction of thiourea used in industrial tests is 0.2%-0.3%.
Figure 2 above also shows that the passivation occurs when the concentration is too high. That is, when the amount of dissolved gold reaches a maximum value, the concentration of thiourea is continuously increased, and the amount of dissolved gold is decreased. This is because the decomposition of thiourea causes elemental sulfur to precipitate on the gold surface and form a passivation film, thereby hindering the dissolution of gold.
3) Effect of sulfuric acid concentration Gold dissolution can be carried out in a sulfuric acid solution of sulfuric acid, hydrochloric acid and nitric acid, but it is most preferable in a sulfuric acid sulfuric acid solution. When the acidity is not large (mass fraction 0.1%-0.5%), the dissolution can be carried out, and when the acidity is 0.1% to 2%, the gold has a relatively stable dissolution rate. Sulfuric acid not only plays a role in coordination in the process of leaching gold from thiourea, but also protects against the decomposition of thiourea, so it is a regulator and a protective acid. Many studies have reported that as the concentration of sulfuric acid increases, the leaching rate of gold increases significantly, that is, the lower the pH control, the higher the leaching rate of gold.
Figure 3 is a study by the Gabra study. It is indicated that the pH of the leachate varies with time at different initial concentrations of sulfuric acid. With a concentration of 0.0175 mol/L sulfuric acid, the pH was increased from 3.25 (250 mV) to 5.7 (110 mV) in 1 h, and the pH was changed from 1 (260 mV) to 2.7 (250 mV) with 0.175 mol/L sulfuric acid. The optimum sulfuric acid concentration should be 0.175 mol/L.
Effect 5) an oxidizing agent in an acidic thiourea system, the choice of oxidant ferric sulfate, ferric chloride, hydrogen peroxide, sodium peroxide, ozone, oxygen, air, potassium bromate, weight chromium and potassium, but Excessive oxidizing agents will cause thiourea oxidation loss and should not be used. Gabra compared the effects of three oxidants of ferric sulphate, oxygen and air on the leaching speed, and obtained a set of curves as shown in Figure 4, which showed that the highest extraction rate of gold was obtained with iron sulphate, followed by oxygen, and only air. A little lower than oxygen. It should also be pointed out that the choice and amount of oxidant directly determines the potential of the solution. For example, the acid thiourea immersion gold test in the Carlin-type gold mine in the United States determines that the solution potential should be controlled at +150 mV. He also believes that increasing the potential does not necessarily lead to a higher gold extraction rate. The potential range offered by Gabra is +110~+260mV, and Charlie is specified as +140mV.
When Fe 3+ is used as the oxidant, the mass concentration of Fe 2 (SO 4 ) 3 is controlled at 3g/L, and Gabriel should be limited to 6g/L, and then the Fe 3+ concentration is increased, and the gold leaching speed is no longer improve.
It should also be noted that the Fe 3+ oxidant has a special convenient condition that it is not necessary to add Fe 3+ oxidant when treating the pyrite type mineral.
6) Influence of temperature Gabriel's research indicates that in the range of 2-35 °C, the extraction rate of gold increases slightly with temperature, and the temperature coefficient of gold extraction rate is 1.09-1.10, but the temperature is as high as 60 °C. The urea begins to decompose, which in turn causes the extraction rate to decrease (see Figure 5).
7) Effect of solid acid ratio Under the test conditions of Gabra, a 50% solid acid ratio can obtain a 94% gold recovery rate, and a solid acid ratio lower than a 50% gold extraction rate. The effect of solid acid on the immersion speed is shown in Figure 6. [next]
A more consistent view of the kinetics of thiourea dissolution gold is that the rate at which gold is dissolved in the acidic thiourea solution in the presence of an oxidant is controlled by the diffusion process. Xie Furui (Shivrin) et al reported that the diffusion coefficient of silver and gold thiourea complex ions (298K).
D[Au(TU) 2 + ]=3.3×10 -6 cm 2 /s
D[Ag(TU) 3 + ]=4.7×10 -6 cm 2 /S
D(TU)=1.1×10 -6 cm 2 /S
In summary, the kinetics of thiourea dissolution gold is analyzed from the viewpoint of galvanic corrosion and diffusion, mainly related to thiourea concentration and diffusion coefficient, oxidant concentration and diffusion coefficient, contact area of ​​gold particles and solution and thickness of diffusion layer. When oxygen is used as the oxidant, if the concentration of thiourea is high and the oxygen concentration is low, the dissolution rate of gold increases as the oxygen concentration increases.
It should be emphasized here that when processing gold-bearing ores, increasing the fineness of the grinding is beneficial to increase the leaching rate of gold, but also causes other impurity elements associated with gold to enter the solution more, so the grinding The fineness must be appropriate.
Increasing the temperature of the solution can increase the diffusion rate of thiourea and increase the diffusion coefficient; the effect of solution temperature on the diffusion coefficient is far less than the effect on the reaction rate. As the temperature of the solution increases, the decomposition of thiourea is accelerated, and the reaction rate of the impurity element dissolved together with gold with thiourea and sulfuric acid is also accelerated. In general, this will result in a large consumption of reagents, and the leaching rate of gold will decrease. In practice, thiourea leaching of gold is usually carried out at room temperature, by adding acid followed by thiourea, adding sulfuric acid and thiourea in a tank, or adding acid in batches to prevent local overheating of the slurry. The leaching temperature increases.
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