## 3 Results
### Chemical Treatment Process  
#### 3.3.1 The Effect of the Initial pH of Acidification Conditions  
In order to study the influence of the initial pH value of acidification conditions on the effect of copper precipitation, the initial pH value of elution was adjusted by sulfuric acid, and the addition of sodium hydrosulfide was controlled to be 0 mg/L. The elemental content analysis of the solution after full reaction with different initial pH values is shown in Figure 7. Figure 7 shows that the acidification treatment caused the volatilization of HCN to a certain extent, resulting in a decrease in the total cyanide concentration of the solution [18]. The acidification process in elution water at room temperature could be described as [19]:  
\[2\text{NaCl}+\text{H}_{2}\text{SO}_{4}\rightarrow\text{Na}_{2}\text{SO}_{4}+2 \text{HCN} \tag{1}\]  
\[4\text{NaCu}(\text{CN})_{2}+2\text{H}_{2}\text{SO}_{4}\rightarrow 2\text{Cu}_{2}( \text{CN})_{2}+2\text{Na}_{2}\text{SO}_{4}+4\text{HCN}\dagger \tag{2}\]  
\[\text{Cu}_{2}(\text{CN})_{2}+2\text{NaSCN}\rightarrow 2\text{CuSCN}\downarrow+2\text{NaCN} \tag{3}\]  
\[\text{Ca}^{2+}+\text{H}_{2}\text{SO}_{4}\rightarrow\text{CaSO}_{4}\downarrow +2\text{H}^{+} \tag{4}\]  
The effect of acidizing copper precipitation increased with the decrease in the initial pH value. When the initial pH of acidification reaction decreased from 8.64 to 1.5, the concentration of copper ions in the filtrate after the reaction decreased from 704.58 mg/L to 139.86 mg/L. When the pH value was reduced to 1.8, the copper ion concentration did not decrease as the pH value decreased, so the initial pH value of the optimal acidification reaction was 1.8.  
Figure 6: Effect of secondary air-drying time on the leaching toxicity of backwashed cyanide tailings.  
3.3.2. The Effect of Sodium Hydroosulfide Dosage  
After the acidification reaction, 140 mg/L copper ions were in the solution, which may adversely affect the effect of subsequent membrane treatment and the service life of the membrane. Therefore, in this study, sodium hydrosulfide was added for deep copper deposition to improve the copper recovery rate [20]. In order to study the influence of the amount of sodium hydrosulfide on the effect of copper deposition, the conditional experiment of the amount of sodium hydrosulfide was carried out under the condition that the initial pH value of the acidification reaction was 1.8. The vulcanization process in elution water at room temperature could be described as [21,22]:  
\[S^{2-}+Cu^{2+}\rightarrow CuS\downarrow \tag{5}\]  
\[2 Cu(CN)_{3}{}^{2-}+3H_{2}SO_{4}+S^{2-}\rightarrow Cu_{2}S\downarrow+6 HCN+3 SO_{4}{}^{2-} \tag{6}\]  
The elemental content analysis of the solution after treatment with different sodium hydrosulfide dosages is shown in Figure 8. It can be seen from Figure 8 that when the dosage of sodium hydrosulfide was 50 mg/L, the concentration of copper ions in the solution was reduced from 140.3 mg/L to 70 mg/L, and the copper sinking effect was obvious. With the increase in the amount of sodium hydrosulfide, the concentration of copper ions after treatment was maintained at about 10 mg/L, so the appropriate amount of sodium hydrosulfide was 100 mg/L.