Monograph
Comparative experimental study on the harmless treatment of cyanide tailings through slurry electrolysis

https://doi.org/10.1016/j.seppur.2020.117314Get rights and content

Highlights

  • Three kinds conditions of slurry electrolytic for treating cyanide tailings.

  • Treatments for cyanide and minerals in tailings are concentrated in the same reactor.

  • Cyanide tailings are directly treated by slurry electrolysis.

  • Cyanide content in cyanide tailings decreased obviously during electrolysis.

  • Mineral in cyanide tailings oxidation dissolution was intensified during electrolysis.

Abstract

In this work, the cyanide tailings were treated with slurry electrolysis technology. The removal of cyanide and the regularity of the oxidative dissolution of the main minerals in tailings during direct electrolysis (ET), air electrolysis (ETA), and NaCl electrolysis (ETC) were compared and analyzed. X ray diffraction, backscattered electron analysis, mineral liberation analysis, and other analytical methods were performed to analyze and characterize the mineral composition and interlocking relationship of cyanide tailings. Results indicated that cyanides and typical minerals, such as pyrite and pyrrhotite, in cyanide tailings would undergo a certain degree of oxidation and dissolution in the ET, ETA, and ETC systems, and the processing effect of the ETC system was more obvious than that of other systems. After the treatment, the mass loss of cyanide tailings could reach 8.62%, which was 5.10% and 1.36% higher than the mass loss in ET and ETA, respectively. The removal rates of CNT, CN, Cu, Zn, and Fe were 92.07%, 97.17%, 86.31%, 98.24%, and 93.03%, respectively. The relative contents of pyrite and pyrrhotite decreased by 8.82% and 4.65%, respectively. The particle size occupancy rates of pyrite greater than 50 μm and pyrrhotite greater than 15 μm were 0.24% and 3.36%, respectively. The mineral liberation degrees of typical minerals increased significantly, and the mineral liberation degrees of pyrite and pyrrhotite increased by 16.46% and 13.20%, respectively. The oxidation of cyanides and minerals in cyanide tailings under the ETC system mainly involved electrochemical indirect oxidation. Cl migrated to the anode through electrogeneration to generate Cl2/ClO in situ. CN and metal cyanide ions that migrated near the anode were oxidized by Cl2/ClO to produce CO2, N2, and metal cations. Metal cations returned to the cathode where they were reduced and deposited. Pyrite, pyrrhotite, and other minerals in cyanide tailings could also be oxidized and dissolved by ClO. Therefore, the particle size of minerals decreased, and the degree of mineral liberation increased. As electrolysis proceeded, the interlocking relationship between the minerals was destroyed, and more minerals were exposed in the form of monomers. These phenomena provided favorable conditions for the subsequent comprehensive recovery of valuable metals from cyanide tailings.

Introduction

In China’s gold industry, the cyanidation of gold extracted from ore is a simple, low cost, and effective gold extraction method with the widest application range and the longest use time. In gold cyanide extraction, a large amount of cyanide tailings produced. The annual discharge of cyanide tailings by China's gold industry exceeds 24.5 million tons. Cyanide tailings contain a high amount of undissolved cyanide and valuable metals, performing storage or landfill treatment alone not only causes damage to the environment but also leads to a great waste of resources [12], [7]. Therefore, research on the harmless and resource-based treatment of cyanide tailings is important for the environmental protection and sustainable development of China’s gold industry.

Research on the comprehensive utilization of cyanide tailings mainly involves pretreatment–secondary gold and silver extraction, comprehensive copper, iron, and zinc recovery, and harmless treatment. The first two methods focus on the comprehensive recovery of valuable resources, whereas the latter focuses on cyanide destruction. Comprehensive recovery has a long and complicated process and mainly utilizes the flotation method. Harmless treatment is simple, and it is the most direct and effective method for solving the problem of pollution caused by cyanidation and gold extraction. Harmless treatment mainly includes chemical oxidation processes, electrolytic processes, biological oxidation, and natural attenuation [18]. Cyanide tailings are processed with SO2/air (INCO) after ore extraction [6]. When SO2, lime, and Cu2+ dosages are 7.7, 0.04, and 0.01 g/L, the concentration of free cyanides decreases from 364 mg/L to 0.4 mg/L. Kitis [9]treated cyanide tailings after gold extraction from Ovacik gold ore through hydrogen peroxide oxidation and found that the free cyanide concentration decreases from 144 mg/L to less than 1 mg/ L. Cakrillo – Pedroza [4]treated cyanide tailings through ozonation, and indicated that the concentration of free cyanides from 375 mg/L to 7 mg/L when the pH of the solution is 11.2 and the blowing amount of ozone is 0.1 g/min. Adjei [1] conducted experiments and found that Pseudomonas, Bacillus, Alcaligenes, Acinetobacter, and Burkholderia have a certain ability to degrade cyanide. For example, Pseudomonas decreases the free cyanide concentration of cyanide tailings from 45 mg/L to 0.4 mg/L. At the Marlin gold mine in Guatemala, Saarela [16]found that cyanide ions are destroyed at an anode when a cathode collects metals during electrochemical oxidation. Valiuniene [19]treated cyanides and reduced the free cyanide concentration from 260 mg/L to 13 mg/L. In summary, harmless treatment significantly affects the treatment of cyanide tailings, but some problems have yet to be solved during treatment process to achieve industrialization. Harmless treatment processes mainly include two types: the direct oxidation of slurry and the oxidation of cyanide-containing wastewater after slag washing. The former consumes a large amount of reagents and requires strict process conditions. The latter requires washing the tailing slag and treating the cyanide-containing solution after separation. The processes are complicated and consume high amounts of water. Therefore, a simple process, low cost, and effective treatment method should be investigated for the future development of the gold industry.

In this study, cyanide tailings were harmlessly treated through slurry electrolysis technology, which involves the washing of cyanide tailings, the electrolytic oxidation of cyanide, the electrolytic deposition of metal ions, and the oxidative dissolution of minerals concentrated in the same reactor. During electrolysis, the electric field strengthens mass transfer and promotes the diffusion of cyanide ions adsorbed on the surface of tailings into a solution. In indirect electrochemical oxidation, cyanides are removed, and minerals are oxidized through the oxidation of Cl2/ClO produced via chloride ion anodization. The interlocking relationship between the minerals in cyanide tailings was destroyed, the mineral liberation degree of the minerals increased, and favorable conditions were created for the subsequent recovery of valuable metals. The whole process is shown in Fig. 1. This process had the advantages of a simple process, a low operation cost, and good treatment results. The intensive analysis and optimization of the process were important for Chinese gold smelting enterprises to promote potentiality, increase efficiency, save energy, and reduce emission.

Section snippets

Materials

Cyanide tailings were obtained from the gold concentrate of Taibai gold ore in Shaanxi Province. Analysis showed that the contents of the total cyanide (CNT) in tailings and free cyanide (CN) were 1574.21 and 275.82 mg/L, respectively. The results of the chemical composition analysis and mineral composition analysis of cyanide tailings are shown in Table 1. The chemical composition analysis of the cyanide tailings (Table 1) indicated that the main valuable metals were sulfur, iron, and

Changes in the mass of cyanide tailings

Slurry electrolysis experiments were performed in the ET, ETA, and ETC systems. Table 2 shows the changes in the mass of the cyanide tailings before and after electrolysis.

Table 2 indicates that the masses of cyanide tailings treated with the three systems underwent certain losses. The lowest mass loss rate was observed in ET system, followed by ETA system. The largest mass loss rate was found in the ETC system. The mass loss of cyanide tailings during electrolysis was due to the removal of a

Conclusion

  • (1)

    Slurry electrolysis technology can be feasibly used to harmlessly treat cyanide tailings. Comparing the experimental results revealed that the effect of the ETC system on the oxidative dissolution of cyanide and major minerals was more obvious than that of the other systems. After the treatment, the mass loss rate of cyanide tailings was 8.62%, and the removal rates of CNT, CN, Cu, Zn, and Fe were 92.07%, 97.17%, 86.31%, 98.24%, and 93.03%, respectively. The relative contents of pyrite and

Formatting of funding sources

Funding: This work was supported by the National Natural Science Foundation of China (51774227), Key Fund of Shaanxi Natural Science Foundation (2018JZ5011), and Innovation Capability Support Program of Shaanxi (2020TD-028).

Author contribution

Yi Chen: Experiments, data curation and writing - original draft, formal analysis. Yonghui Song: Supervision. Yao Chen: Project administration. Xinwei Zhang: Validation. Xinzhe Lan: Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (21)

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