Elsevier

Minerals Engineering

Volume 188, October 2022, 107833
Minerals Engineering

A kinetic-mechanistic study of cyanide degradation which can be contained in mining tailings dams using a divided electrolytic cell

https://doi.org/10.1016/j.mineng.2022.107833Get rights and content

Highlights

  • CN can be electro-degraded using a divided electrolytic cell with graphite.

  • CN is electro-oxidized by two pathways depending on the processing time.

  • 800 ppm of CN can be electro-degraded during 120 min.

  • A current efficiency of 100% can be achieved.

  • The electrolytic system can degrade 100% of CN contained in mining tailings dams.

Abstract

Cyanidation is considered the most employed process to recover precious metals such as silver and gold. However, cyanide containing wastes can be hazardous for the environment. Therefore, the soil electroremediation technique was systematically studied to eliminate/degrade the cyanide contained in mining tailing dams using an electrolytic cell with cationic separator and cheap-stable carbon electrodes. The extent and kinetics of cyanide degradation was firstly analyzed in synthetic solutions using the electrolytic cell at different current density, ionic conductivity, and cyanide concentration. The electrolytic system was also evaluated for the treatment of a sample obtained from a mining tailing dam containing cyanide. The results revealed that 100 % of cyanide can be degraded from synthetic solutions containing 200 ppm CN at 60 A/m2, 0.25 M NaOH and room temperature. It was also found that 100 % of cyanide can be degraded from the real samples obtained from a Mexican mining tailing dam. The kinetic analysis revealed that cyanide degradation can occur through two pathways: a) in the case of tests containing 500 and 800 ppm NaCN, the rate determining step of the cyanide degradation to cyanate species follows a first order reaction with respect to the cyanide ions, and b) when the tests are carried out with 200 ppm NaCN, the same first order reaction controls the process kinetics from 0 to 30 min, while at t > 60 min, the rate determining step for cyanide degradation is modified and limited by the occurrence of parasite reactions. The proposed mechanism for cyanide degradation is also consistent with the thermodynamic calculations. The results also revealed that complete cyanide electro-degradation can be achieved with a current efficiency of 100 %.

Introduction

Free cyanide is the term typically employed to describe the cyanide ion (CN), and hydrogen cyanide (HCN) (Kuyucak and Akcil, 2013). Simple cyanide species are referred to the cyanide salts (KCN, NaCN, etc.) which are easily dissolved in water (Huertas et al., 2010). Around 90 % of the gold production is carried out by cyanidation (Mudder and Botz, 2004). The metal-cyanide complexes can be classified according to the their stability, such as: strong complexes containing iron (Larsen et al., 2004), gold, cobalt (McDougall, 1980), etc. and weak complexes composed of copper and zinc (Dash et al., 2009; F. Nava et al., 2007). It is well known that cyanide is stable and can be toxic for human beings at certain conditions, e.g., 2.7 ppm of cyanide in blood can cause the death in any human being (Baud, 2007). Despite of this, cyanide is massively employed for the recovery of silver and gold in many hydrometallurgical plants (Snmpe, 2001); furthermore, it is also used in other mining processes, such as the flotation of certain minerals, where cyanide is employed to separate some minerals which contain sulfur (Janetski et al., 1977), it also inhibits the flotation of pyrite and pyrrothite (Zhao et al., 2016). Due to this type of operations, it is possible to find mining tailings dams which contain 0–25 ppm of cyanide species (Dash et al., 2009), as can be seen this concentration range is lower than that found in some cigarettes (0.5 mg/cigarette), however, from an environmental viewpoint, the accumulation of cyanide species in the mining tailings dams can represent a problem in the future (Kuyucak and Akcil, 2013).

Nowadays, there are different alternatives to treat the industrial wastes which contain cyanide, these alternatives can be classified depending on its final use, e.g., destructive, recovery and adsorption processes (Dai et al., 2012). The destructive processes can be carried out through the occurrence of oxidation or biological reactions, the main objective of these processes is to perform the complete decomposition or degradation of cyanide to less toxic species (Parga et al., 2003). As regards the recovery processes, the cyanide is recovered and then transformed into sodium or potassium cyanide salts, which can be reutilized in cyanidation operations (Dai et al., 2012, Tsolele et al., 2019). Finally, the adsorption processes are focused in the recovery of soluble cyanide through its adsorption on different materials, however, the adsorbed cyanide must be completely desorbed and decomposed (Akcil, 2003). It is worth mentioning that the environmental impact of these processes can be decreased through the utilization of greener technologies which can permit the decomposition of CN to less hazardous species. Among the different treatments it is of interest the electro-oxidation method, which has been studied with electrochemical techniques using different anode materials e.g. Al, Cu, Ni, Ti (Iordache et al., 2004) where different mechanisms for cyanide electro-oxidation have been proposed. In this sense Virbickas et al. (2021) have mentioned that cyanide electro-oxidation on platinum can occur according to next reaction (Eq.1):CN-+2OH-CNO-+H2O+2e-

Cañizares et al., 2005, Iordache et al., 2004, Lanza and Bertazzoli, 2002 have also proposed the possibility to produce N2 from the cyanate electrolytic decomposition, as shown in next reaction (Eq. (2)):2CNO-+4OH-2CO2+N2+2H2O+6e-

However, Arikado et al. (1976) studied the oxidation mechanism of cyanide using the polarization technique and graphite electrodes. The authors suggested two types of mechanisms which can take place at low or high OH concentration. In the case of the low OH– concentration the proposed mechanism is shown in next Eqs. (3–6):CN-CN+e-CN+CN-(CN)2+e-(CN)2+4H2O(COO)2-2+2NH4+

As regards the scenario of high OH concentration, the reaction mechanism considers the same discharge step of cyanide ions shown in Eq. (3) followed by the next reactions (Eqs. (6), 7):CN+OH-HOCN+e-HOCN+OH-CNO-+H2O

As can be seen the reaction mechanism is still in discussion. Despite of this controversy, the cyanide electro-oxidation using undivided electrolytic cells has been proposed as an alternative to eliminate the cyanide (Cañizares et al., 2005, Lanza and Bertazzoli, 2002) contained in some mining wastes generated during the cyanidation of precious metals. Cañizares et al. (2005) analyzed the cyanide degradation using an undivided electrochemical cell i.e., without an ionic membrane, and used an anode composed of boron doped with PbO2 and diamond, the authors showed a 100 % degradation of cyanide from an aqueous solution containing 200 ppm CN– at 10 mA/cm2 during 5 h. From a mechanistic viewpoint (Cañizares et al., 2005) suggested the oxidation of cyanide ions to cyanate followed by the cyanate oxidation to CO2 and N2 using a sulfate medium as shown in Eqs (1 and 2); however, if the electrolytic system is operated in the presence of chloride ions (which was added to increase the solution conductivity), this species is also oxidized on the anode producing hypochlorite, which is able to oxidize the cyanide species as shown in Eqs (8–10).Cl-+2OH-ClO-+H2O+2e-2CN-+5ClO-+H2O2HCO3-+N2+5Cl-2CN-+5ClO-+2OH-2CO3-2+N2+H2O+5Cl-

Abdel-Aziz et al. (2016) studied the oxidation of cyanide using an undivided electrolytic cell with graphite and stainless steel as anode and cathode, respectively, in the presence of chloride ions. The authors also suggested the in-situ formation of HClO which co-oxidizes the cyanide ions, as shown in Eq. (11). It is worth mentioning that HClO is formed from the oxidation of chloride ions on the anode. However, it is also possible to co-produce toxic chlorine gas, which is also considered an oxidizing agent. Therefore, it is of interest to study and develop alternative electrochemical processes which inhibit the production of chlorine gas.5HClO+2NaCN2NaCl+2CO2+N2+H2O+3HCl

On the other hand, Lanza and Bertazzoli (2002), also studied the cyanide degradation in an undivided electrochemical cell using DSA anodes in the presence of NaOH, Na2SO4 and Na2CO3 containing solutions. This electrolytic system permits the complete decomposition of 50 or 700 ppm cyanide during 2 h at 100 A/m2. It appears that the energetic consumption is considerably high; however, it can be expected considering that DSA anodes have a narrow electro-activity window, i.e., the oxygen evolution reaction is favored in this anodic material ((Fuentes-Aceituno et al., 2008).

As can be noticed, the electrolytic decomposition of cyanide is an interesting alternative; however, different issues must be improved, e.g., the high energetic consumption, the low current efficiency, and in some cases the generation of toxic chlorine gas and other chloride species. In order to improve the electrolytic decomposition of cyanide, it can be interesting to use a selective ionic membrane which can avoid parasite reactions, and thus increasing the current efficiency. Actually, this approach has been reported in different types of electrochemical applications, e.g., electrodyalisis, where anionic and cationic membranes are used to permit the selective migration of anions or cations (Chen and Hu, 2020, Foulkes, 2000, Min et al., 2012). Therefore, it can be interesting to include a cationic membrane into the electrolytic cell to avoid the migration of cyanide ions from the anode to the cathode compartments, and thus increase the current efficiency and the kinetics of the cyanide electro-oxidation.

This paper aims to study systematically the cyanide degradation in an electrolytic cell which has the anode and cathode compartments separated by a cationic membrane to maximize the current efficiency. It is also intended to use less expensive anode and cathode materials e.g., graphite, which can increase the kinetics of cyanide electro-degradation without the addition of NaCl in the anodic compartment to decrease the occurrence of parasite reactions, e.g., chloride ions electro-oxidation. Finally, the results of this research can be used to determine the most favorable electric and aqueous solution conditions to accelerate the cyanide degradation which can be contained in mining tailing dams. Furthermore, this information is also useful to comprehend the reaction mechanism involved.

Section snippets

Cyanide oxidation tests using a divided electrolytic cell

The electro-oxidation tests were carried out using an acrylic electrochemical cell of 1L capacity. The cell contained 2 graphite plates as electrodes (10x10 cm and 1.0 cm width), the exposed surface of each electrode was 50 cm2. Both electrodes were connected to a power supply (Novark Technologies DCE 25/12-1DSP), as shown in Fig. 1. The anode and the cathode of the cell were separated by a cationic membrane (cmi-7001), with the aim to study systematically the electro-oxidation of cyanide

CN electro-degradation kinetics using synthetic solutions

This section presents a kinetic analysis of the cyanide electro-degradation varying the NaOH concentration from 0.1 to 0.5 M at different NaCN concentration (200–800 ppm).

Fig. 2 illustrates the degradation of cyanide at room temperature using 0.1 M NaOH varying the current density with 800 (a), 500 (b) and 200 (c) ppm NaCN. Fig. 2 (a), (b) and (c) show that cyanide degradation at 20 A/m2 corresponds to 20, 40 and 75 % at 120 min for 800, 500 and 200 ppm NaCN, respectively; this behavior can be

Conclusions

The current and electrolyte concentration have a substantial influence on the oxidation rate of CN due to the fact, that an increase in the electrical current favors the electrons flow in the cell, while the cyanide and NaOH concentration increases the ionic strength and conductivity of the system, facilitating the electrochemical reaction, i.e., cyanide oxidation.

The best conditions for cyanide degradation using the electrolytic cell corresponds to 200 ppm CN–, 0.25 M NaOH, 0.2 A at room

CRediT authorship contribution statement

E.G. Sierra-Alvarado: Validation, Investigation. S. Valle-Cervantes: Investigation, Writing – review & editing. R. Lucho-Chigo: Writing – review & editing. M.D.J. Rodríguez-Rosales: Writing – review & editing, Resources. J.C. Rojas-Montes: Formal analysis, Visualization. J.C. Fuentes-Aceituno: Writing – review & editing, Visualization, Methodology. V.J. Martínez-Gómez: Writing – original draft, Conceptualization, Methodology, Visualization.

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.

Acknowledgements

The authors are grateful to CONACyT for the scholarship awarded to E.G. Sierra-Alvarado

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