2.1 Cyanide waste slag washing test
Clean water was used for washing the cyanide slag during the washing process. In this study, we examined the impact of washing water pH and volume on the levels of [CN]T (The total cyanide content includes free cyanide and complex cyanide) and [Cu2+]T (Total concentrations of Cu2+ and Cu (CN)n(n−1)−) in cyanide slag, aiming to decrease their concentration and comply with the storage standards for tailings dam.
2.1.1 Effect of washing water pH
Cyanide waste slag is usually alkaline. In this part, the cyanide slag was rinsed with acidic water of varying pH levels, and the content of the cyanide ([CN]T) and copper ions ([Cu2+]T) after rinsing was evaluated. The test results are shown in Fig. 2. The washing water (washing solution) obtained was analyzed for its elemental composition, as shown in Fig. 3.
It can be observed from Fig. 2 and Fig. 3 that when the pH of the washing water was low, the concentration of [CN]T and [Cu2+]T in the washing solution was also low. As the pH value of the washing water increases, the content of the cyanide slag gradually decreases and then stabilizes, while the concentration of [CN]T and [Cu2+]T in the washing solution initially increases and then stabilizes. When the pH of the washing water was 3.0, the concentrations of [CN]T and [Cu2+]T in the cyanide slag were 10.36 mg/L and 15.39 mg/L, respectively. The concentrations of [CN]T and [Cu2+]T in the washing solution were 1103 mg/L and 803.75 mg/L, respectively. When the acidity of the washing water was too high (pH < 3.0), the free cyanide in the cyanide slag reacted directly with [Cu2+]T during washing, forming precipitates such as CuCN. These precipitates were mixed in the cyanide slag and could not be recovered. After comprehensive consideration, the pH value of the washing water was chosen to be 3.0.
2.1.2 Effect of washing water volume
To investigate the impact of washing water volume on the washing and pressure filtration of cyanide slag, a washing test of cyanide slag was conducted at a pH value of 3.0 for the washing water. The content of cyanide slag under various washing water conditions and the concentration of the cyanide slag washing solution are shown in Fig. 4 and Fig. 5, respectively.
According to Fig. 4 and Fig. 5, the content of [CN]T and [Cu2+] T in cyanide slag decreases as the amount of washing water increases. When the volume of washing water was 0.8 m3/t, the concentration of [CN]T in the content of cyanide slag was 4.56 mg/L, which was below the 5 mg/L threshold required by the national standard. This meets the criteria for storage in the tailings pond. With the increase in washing water, the concentration of [CN]T and [Cu2+]T in the cyanide slag washing solution decreased to 579.36 mg/L and 446.22 mg/L, respectively, and stabilized. Therefore, the volume of washing water used in the experiment was 0.8 m3/t.
2.1.3 Toxicity analysis of cyanide slag
The cyanide slag was transferred to a Zero Headspace Extractor (ZHE). Install the ZHE, slowly pressurize to remove the headspace, and close all valves. The ZHE was fixed on the flip-flop oscillation device, the speed was adjusted to 30 ± 2 r/min, with oscillation performed at 23 ± 2°C for 18 ± 2 hours. After the oscillation stopped, the ZHE was taken down to check for any device leaks. If the ZHE device leaks, it should be resampled for leaching. The leaching solution was collected, refrigerated, and stored for analysis. The concentration of each element, as well as cyanide and thiocyanate in the leaching solution, was tested.
From Section 2.1.1 to 2.1.2, it can be seen that the pH of the washing water is 3.0, and the volume of the washing water is 0.8m3/L. The leaching toxicity test results of the cyanide slag before and after washing are presented in Table 2, and the elemental content in the washing solution of the cyanide slag after washing is displayed in Table 3.
Table 2
Leaching toxicity test results of cyanide slag (mg/L)
Sample
|
[CN]T
|
Cu
|
Fe
|
Pb
|
Zn
|
Cr
|
As
|
Cd
|
Origin cyanide waste slag
|
40.61
|
31.69
|
0.31
|
0.03
|
<0.01
|
0.023
|
0.016
|
0.045
|
Washed cyanide slag
|
3.09
|
4.36
|
0.11
|
0.01
|
<0.01
|
0.01
|
<0.01
|
0.01
|
Limited standards*
|
5
|
120
|
120
|
1.2
|
120
|
15
|
1.2
|
0.6
|
Note: * indicates China's gold industry cyanide slag pollution control technical specifications (HJ943-2018) |
Table 3
Element analysis in the washing solution (mg/L)
Element
|
[CN]T
|
Cu2+
|
Cu (CN)n(n−1)−
|
Fe3+
|
Pb 2+
|
Zn2+
|
As5+
|
Cd2+
|
pH
|
concentration
|
589.28
|
124.08
|
312.14
|
0.16
|
0.02
|
0.02
|
0.01
|
0.01
|
10.50
|
The data in Table 2 shows that the leaching toxicity of [CN]T in the origin cyanide waste slag is 40.61 mg/L, and that of the washed slag is 3.09 mg/L, meeting the storage requirements for the tailings dam. It can be observed from Table 3 that the pH of the washing solution is 10.5, the [CN]T content is 589.28 mg/L, the Cu2+ content is 312.04 mg/L, the Cu(CN)n(n−1)− content is 124.18 mg/L, and small amounts of Pb2+, Zn2+, Cd2+, Cr3+, and other elements. The recovery of Cu2+ /Cu(CN)n(n−1)− and CN− from the washing solution will be discussed in Section 2.2, and Section 2.3, respectively.
2.2 Cu2+/Cu(CN)n(n−1)− precipitation test in washing solution
2.2.1 Principle of Cu2+/Cu(CN)n(n−1)− precipitation in washing solution
Sulfuric acid and NaHS were added to the washing solution to facilitate the Cu2+/Cu(CN)n(n−1)− precipitation reaction. The variation of Gibbs free energy with temperature during the Cu(CN)n(n−1)− precipitation reaction is shown in Fig. 6. When the reaction temperature range was 0°C ~ 100°C, the Gibbs free energy of the main chemical reactions was negative, indicating that the reaction could proceed spontaneously. As the temperature increased, there was a slight decrease in the change of Gibbs free energy of the reaction, indicating an increased spontaneous tendency of the reaction with rising temperature. The Eh-pH diagram of the Cu-C-N-S-H2O system is shown in Fig. 7. As the pH value decreases, the Cu(CN)n(n−1)− in the washing solution initially transitions from Cu(CN)43− to Cu(CN)32−, and then from Cu(CN)32− to CuCN. CuS was produced by the reaction of NaHS with Cu2+. Specifically, when the pH was less than 4.2, Cu(CN)n(n−1)− was eliminated from the washing solution in the form of CuCN precipitate. CuCN precipitation can be decomposed by introducing a specific oxidation potential. This allows CuCN to produce Cu(OH)2 under alkaline conditions and Cu(CN)n(n−1)− under acidic conditions.
2.2.2 Effect of pH value
It can be seen from Table 3 that the washing solution pH is 10.5. In this part, the washing solution was acidified with sulfuric acid solution, which lowered the pH from 10.5 to 2.5, while stirring at a speed was 200r / min. The impact of various pH values on the [CN]T and [Cu2+]T contents of the washing solution is shown in Fig. 8.
Figure 8 shows a significant change in the pH value of the washing solution with varying concentrations of [Cu2+]T and [CN]T. As the pH value decreases, the concentration of [Cu2+]T in the washing solution decreases from 539.65 mg/L to 150.69 mg/L, and the [CN]T concentration decreases from 640.91 mg/L to 340.26 mg/L, eventually stabilizing. The [CN]T content decreased as the pH value decreased, possibly due to the volatilization of HCN caused by an acidification reaction. The lower the pH of the washing solution, the more effective the [Cu2+] T precipitation. However, when the pH is between 2.5 and 4.5, the concentration of [Cu2+]T remains relatively stable. Therefore, the optimal pH for acidification is 3.5.
2.2.3 Effect of NaHS dosage
The washing solution was acidified by adjusting the pH in order to precipitate [Cu2+]T. After acidification, the concentration of [Cu2+]T in the washing solution remained at 150 mg/L. The primary product of acidified [Cu2+]T precipitation was CuCN, while the main product of sulfide [Cu2+]T precipitation was CuS (See Fig. 7). The solubility product of CuCN at room temperature is 1.602×10− 9, and the concentration product of CuS is 2.4×10− 17. The solubility product of acidified [Cu2+]T precipitation product was high, and the [Cu2+]T precipitation was incomplete. Therefore, a small amount of NaHS needs to be added to improve the precipitation rate of [Cu2+]T for deep [Cu2+]T precipitation. To investigate the impact of NaHS dosage on [Cu2+]T precipitation, the pH of the washing solution was adjusted to 3.5, the NaHS dosage ranged from 50-250mg/L, and the stirring speed was set at 200r/min. The test results for NaHS dosage are shown in Fig. 9.
It can be observed from Fig. 9 that the concentration of [Cu2+] T and [CN]T in the washing solution gradually decreased and then stabilized with the increase in NaHS dosage. When the NaHS dosage was 200mg/L, the concentrations of [Cu2+]T and [CN]T decreased to 5.42mg/L and 258.36mg/L, respectively, and the precipitation rate of [Cu2+]T was 98.99%. When the dosage of NaHS exceeded 200mg/L, the concentrations of [Cu2+]T and [CN]T did not change significantly. The dosage of NaHS had a significant effect on [Cu2+]T precipitation, so the NaHS dosage used in the experiment was 200mg/L.
2.3 Recovery test of CN− in washing solution
2.3.1 The principle of gaseous membrane decyanation
Under acidic conditions (pH < 4), the temperature was 30 ~ 40°C, CN− will combine with H+ in water to form HCN. Because of its volatility and the gas membrane's characteristics of being 'breathable and impermeable', the solutions on both sides were insoluble, while HCN gas could pass freely. The washing solution flows through the exterior of the gas membrane module's membrane wire, and HCN is transferred to the membrane surface through the boundary layer of the feed solution. The washing solution on the membrane surface reaches a phase equilibrium with the gas phase (air) retained in the membrane pore and then passes through the membrane in the form of gas to the other side. The phase equilibrium between the gas in the pores on the membrane surface and the NaOH solution outside the membrane wire (Yan 2021) was neutralized and absorbed by the NaOH absorption solution inside the membrane wire of the membrane module. This process forms a non-volatile NaCN which removes and recovers the cyanide in the washing solution. If the amount of NaOH in the tube was sufficient, the HCN in the washing solution could be reduced to a very low level. The gas membrane had a structure with a braided central tube for water distribution and a baffle, which could reduce the cyanide concentration in the solution to less than 1 ppm. The decyanation principle of gas membrane is shown in Fig. 10.
2.3.2 Effects of flow rate and membrane stage
When the flow rate of the NaOH solution in the gaseous membrane was controlled at 0.8 m3/h and the pH value of the washing solution after [Cu2+]T deposition was 3.5, the study examined the impact of the washing solution flow rate and the number of gaseous membrane stages on the treatment effectiveness of [CN]T. The test results are shown in Fig. 11.
It can be seen from Fig. 11 that with the decrease in flow rate, the concentration of [CN]T in the solution after primary and secondary membrane treatment decreases. However, as membrane treatment continued to increase, the concentration of [CN]T in the solution did not change significantly. When the flow rate is low, HCN has enough time to pass through the membrane hole in gas form and reach the other side of the membrane. HCN was neutralized and absorbed by the NaOH absorption solution on the surface of the membrane components. At a flow rate of 0.4 m3/h, a two-stage membrane was employed and the [CN]T concentration in solution was reduced to 2.21 mg/L. The removal rate of [CN]T compound was 99.02%. To ensure the effective decyanation of the solution, the flow rate of the washing solution is 0.4m3/L, and the film stage is 2 stages.
2.4 Process cycle test
The feasibility of the process involving "cyanide slag washing, washing solution Cu2+/Cu (CN)n(n−1)− precipitation, and gaseous membrane recovery of CN−" was verified through cyclic testing. The process cycle flow is shown in Fig. 12. The pH of the washing water was adjusted to 3.0, and the volume of washing water used was 0.8 m3/t. The copper and cyanide in the cyanide slag were effectively removed by this cycle process. The washing solution was pumped into the copper ion precipitation tank through the metering barrel, and the initial pH of the solution was adjusted to 3.5 by adding sulfuric acid. After a reaction period, the copper ion precipitator, 200 mg/L of NaHS was added. After the NaHS solution was pumped into the plate and frame filter press using the pneumatic diaphragm pump, the filtrate underwent additional filtration through the precision filter. The filtrate from the precision filter was pumped into the gaseous membrane module through an acid-resistant pump. In the process of gas membrane treatment, the flow rate of the washing solution was 0.4 m3/h, and it is suitable to use 2-stage gas membranes. The cyclic test results of cyanide slag washing and comprehensive utilization of copper and cyanide are shown in Table 4.
Table 4
Cycle results of comprehensive utilization of copper ion and cyanide in washing solution
Decyanation solution cycle times
|
Leaching toxicity of cyanide slag (mg/L)
|
process*
|
Concentration (mg/L)
|
[CN]T
|
Cu2+/Cu(CN)n(n−1)−
|
[CN]T
|
Cu2+/Cu(CN)n(n−1)−
|
First cycle
|
2.89
|
6.35
|
①
|
652.45
|
529.36
|
②
|
332.74
|
5.42
|
③
|
2.21
|
5.21
|
Second cycle
|
3.11
|
6.46
|
①
|
680.36
|
564.98
|
②
|
225.38
|
3.26
|
③
|
3.64
|
5.11
|
Third cycle
|
2.76
|
5.95
|
①
|
656.32
|
545.56
|
②
|
296.34
|
4.37
|
③
|
2.64
|
3.26
|
Note: ①②③ See Fig. 13 |
Table 4 showed that the leaching toxicity of cyanide slag after three cycles of washing and pressure filtration was less than 5mg/L, which meets the storage standard of a tailings dam. After the cyanide slag washing solution was treated by copper ion precipitation and gas membrane method, the copper ion precipitation rate and cyanide adsorption rate in the washing solution were high, and the comprehensive utilization effect was remarkable. The residual concentrations of [CN]T and Cu2+/Cu (CN)n(n−1)− in the solution were greatly reduced, and the two concentrations were only 3 mg/L and 4 mg/L respectively. The results of cycle test show that the cyanide slag can be treated harmlessly and utilized comprehensively by this process. At the same time, the process has good stability and feasibility.