Recycling gold and copper from waste printed circuit boards using chlorination process

Abstract

Waste electric and electronic equipment (WEEE) usually contains many recoverable and valuable elements, including gold and copper. Primitive technologies, such as aqua regia leaching and calcination, are still widely used in China, which has caused serious pollution in local environments. Chlorination is a feasible alternative for the recovery of metallic elements due to the higher dissolution rate, non-polluting character and selective leaching of different metals by controlling the redox potential. In the present study, an efficient and less-polluted chlorination process to recycle gold and copper from waste PCBs was investigated. This work is based on a physical processing with a crush–pneumatic separation–corona electrostatic separation dealing with waste PCBs, which provided the raw material for chlorination process. The influence of different pretreatments and experimental parameters was studied. The result showed that more than 90% of copper could be selective recovered by controlling the Eh of leaching solution from 400 to 800 mV. The supercritical process had a better effect with a leaching yield of metallic elements over 99% percent, for both gold and copper. The ball milling process had different influence on gold and copper based on milling time. The leaching yield of copper increased with increase in ball-milling time. For gold, there was an optimal time around 10 min at 20 s⁻¹ frequency of milling. The leaching yield of gold could reach more than 99% after a pretreatment with ball milling for 10 min when the leaching time was 90 min, the leaching temperature was 40 °C and the initial concentration of sulfuric acid was 100 g L⁻¹. This study could contribute significantly to metallic element recycling of high value-added WEEE and provide technological parameters for industrial application in the future.

---

Introduction

With the rapid development of electronic industries, a large amount of waste electric and electronic equipment (WEEE) is constantly generated worldwide.¹ WEEE usually contains many recoverable and valuable elements, including gold and copper, which have the highest value and recoverable amount. The treatment of WEEE is not only an issue about environmental protection, but also an important way to recycle resources. Generally, the electronic components are fixed onto a printed circuit board (PCB) in virtually every piece of electronic equipment. It has been reported that PCBs in a personal computer contain gold amounting to 80 g t⁻¹,² which is higher than that in some gold ores. Due to the shortage of high-grade gold resources with large-scale gold ore mining,^3,4 the recoverable gold in WEEE will become one of the important sources of gold. Gold is generally plated on printed circuit board contacts to increase the hardness of the contact and to take advantage of its good conductivity to minimize contact resistance of the circuit. The main part of the contact is copper foil. There is only a thin layer of gold plated on the copper foil. Hence, the separation of gold and copper needs to be taken into account in the leaching process. Presently, a physical processing with a crush–pneumatic separation–corona electrostatic separation has been proved to be a feasible method for industrialization of waste PCB recycling.⁵ A metal concentrate could be generated after the process. However, this process cannot achieve the separation of gold or copper from other metal materials.

Hydrometallurgy is widely used in gold extraction.⁶ Cyanidation process is the most frequently used method in conventional hydrometallurgy.⁷ However, this process could not achieve the selective leaching of copper and gold⁸ and has environmental-related disadvantages such as a highly toxic effluent.⁹ As shown in Fig. 1, in China, primitive technologies are still widely used in family-run workshops to recover gold from PCBs. The electrolytic process has a high cost that prevents the application to small-scale treatments. The leaching process with aqua regia could produce large amount of strong acid waste liquid, which results in serious pollution of the local environment.^10–12

Fig. 1 Widely-used process for recycling gold from waste PCBs in China.

Due to the higher dissolution rate,¹³ non-polluting character^14,15 and selective leaching of different metals by controlling the redox potential, chlorination is a feasible alternative method for gold recovery.^16,17 Thus far, some research efforts have used chlorination as an alternative method. Donmez B. et al.^18,19 have studied the chlorination leaching of gold from decopperized anode slime. The extraction yield can reach more than 90%. Baghalha¹³ used chloride/hypochlorite solutions in the leaching of oxide gold ore, which reached 67% gold extraction in 4 h. Nam K. S.²⁰ used chloride/hypochlorite to recover gold from tailings and reached 80% gold extraction. The studies indicate that raw material played a big role in the chlorination leaching. However, few studies focused on the chlorination leaching of gold from PCBs.

Moreover, pretreatment is an important method to improve the leaching yield and purity of metal. However, traditional pretreatment methods, such as roasting, acid leaching, alkaline leaching and bio-oxidation,³ will generate large amounts of waste water or highly polluting gas.^20,21 Different from refractory gold ores, there is no sulfide or carbonate in PCB. Thus, oxidation is not the key process in the pretreatment of PCBs. Hence, ball milling as a physical method with low pollution and cost should be taken into consideration in the pretreatment process. In recent years, supercritical water (T ≥ 374 °C, P ≥ 22.1 MPa) has become known as an environment-friendly method in many academic sectors. As the dissociation constant (K_w) for water is about three orders of magnitude higher at the critical point,²² supercritical water is both a strong acid and alkali. It is an ideal pretreatment method due to the utilization of a non-toxic reactant.^23,24 There are many influencing factors in chlorination processes such as temperature, reaction time, acidity, chloride concentration, and agitation rate. Some of the influencing factors should be investigated for the optimization of the gold leaching process.

In this work, we try to establish an efficient and less-polluted process to selectively recycle gold and copper from metal concentrate, which comes from waste PCBs after a crushing–electrostatic separation process. The objectives of this research are as follows: (1) to evaluate the effectiveness of crushing and supercritical water pretreatment on gold and copper recovery; (2) to evaluate the effectiveness and feasibility of the selective leaching of gold and copper by a chlorination process and optimize the experimental parameters to increase the leaching yield.

Experimental

Materials

In this study, waste memory sticks collected from WEEE recycling factory in Shanghai were used as the representative of waste PCBs. The gold-plated parts were sheared from the waste memory sticks for the experiment. The metal concentrate was collected after the crushing–electrostatic separation process and regarded as raw materials in the following experiments. The mass fraction of mainly metallic elements was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) after acid digestion and this is presented in Table 1. The non-metal part of raw material was mainly consisted of glass fiber and reinforced resin.

Table 1 The mass fraction of the main metals of raw material (wt%)

Au	Cu	Al	Fe	Ni	Pb
0.2339	20.78	0.8529	0.1424	0.1478	0.0868

---

In this experiment, sodium chlorate was taken as the chlorinating agent. The reagents used in this study (NaClO₃, NaCl and H₂SO₄) were of analytical grade, and the solutions were pre-made separately.

Apparatus

The leaching experiments were carried out in a glass Erlenmeyer flask immersed in a thermostatically controlled water bath with a constant temperature circulator, which was equipped with an adjustable speed magnetic stirrer. The Eh of the solution was monitored and controlled using a Mettler Toledo experimental pH meter FE20 with an ORP probe in all experiments. The particles needing pretreatment were ball milled with a Retsch mixer mill MM400. The size of the particles was measured by a Laser Particle Size Analyzer MAE-3000. The surface morphology of the particles was observed with a Nova NanoSEM 230 scanning electron microscope. A semi-batch reactor was used for the supercritical pretreatment process in the present study. The schematic diagram of the reactor is shown in Fig. 2.

Fig. 2 Schematic diagram of the semi-batch reactor.

Methods

In a typical experiment run for leaching copper, the known weight of sample particles and 150 g L⁻¹ sulfuric acid solution were added into the reactor. When the temperature achieved 80 °C, sodium chlorate was added continuously into the reactor to maintain the Eh of the solution within a range of 400 to 600 mV. After 8 hours leaching, the samples were filtered. The filtrate and chlorination residue were collected separately. The filtrate L₁ was diluted with distilled water to a certain volume. The chlorination residue and 150 g L⁻¹ sulfuric acid solution were added back into the reactor for leaching gold. 25 g L⁻¹ sodium chlorate and 75 g L⁻¹ sodium chloride were added into the reactor after reaching the desired temperature. The initial Eh of the solution was between 1000 to 1200 mV. The remaining samples were filtered after leaching for 2 hours. The filtrate L₂ was also diluted with distilled water to a certain volume. Finally, the concentration of metallic elements of copper and gold leachate was determined by ICP-OES. In the process, the agitation was set at a certain speed, and the solid–liquid ratio was 1 : 8.

The parameters in a gold-leaching experiment were optimized, including initial concentration of sulfuric acid, reaction time and temperature. The parameter settings in the optimization experiment are presented in Table 2.

Table 2 Parameter settings of the optimization experiment^a

Time	Temperature	Initial acidity
a t (min): leaching time; T (°C): leaching temperature; C (g L^−1): initial concentration of sulfuric acid.
t = 30, 60, 90, 120, 180, 240, 300; T = 40; C = 150	T = 30, 40, 50, 60, 70; t = 120; C = 150	C = 50, 100, 150, 200, 250; T = 40; t = 120

---

The flowchart of leaching process is shown in Fig. 3.

Fig. 3 The flowchart for leaching of waste PCBs.

The metallic elements' leaching yield (R) is calculated by the following equation:

where R is the leaching yield of metallic elements, M₀ is the initial amount of metallic elements in sample, V is the volume of the leaching solution after distilling, and C is the concentration of the metallic elements in the leaching solution.

Results and discussion

Pretreatment experiment

In this experiment, the leaching yield of copper was calculated by the copper concentration in L₁, and the leaching yield of gold is calculated by the gold concentration in L₂.

Effect of ball milling

The raw materials were ball-milled by the MM400 mixer mill for 10, 30 and 60 minutes. The frequency of mixer mill was set at 20 s⁻¹. The size distribution of crushed particles is presented in Table 3.

Table 3 The size distribution of crushed particles^a

Fine crushing time (min)	Dx(10) (μm)	Dx(50) (μm)	Dx(90) (μm)
a Where Dx(n) is the particle size at the cumulative frequency of n%.
0	14.7	325	1490
10	8.56	177	639
30	5.69	86.5	338
60	5.14	60.6	242

---

The leaching experiments were conducted with 10 g of metal concentrate with different ball milling times. The results are presented in Fig. 4.

Fig. 4 Leaching yield of particles with different fine crushing times: (a) gold and (b) copper.

As shown in Fig. 4, with increasing ball milling time, the leaching yield of copper increased from 77.06% to 97.54%. The leaching yield of gold increased from 48.25% to 89.98% after a 10 minute ball milling. However, the leaching yield of gold decreased from 89.98% to 43.75% when the ball milling time increased from 10 minutes to 60 minutes.

To improve the separation rate of gold and copper, an experiment using two-stage leaching with controlled redox potential was conducted for the particles with 10 min ball milling. After the typical run, the residue and sulfuric acid were added back into the reactor for another 8 h leaching at a range from 400 to 600 mV. The result showed an additional 6.99% of copper was leached out from residue.

Effect of supercritical process

The supercritical pretreatment process was conducted with 10 g of metal concentrate without ball milling. Supercritical conditions were set at 400 °C and 23 MPa with a holding time of 60 min. After supercritical process, the particles remaining in the reaction tube were collected for the leaching process. The leaching yields of metallic elements are shown in Fig. 5 with the comparison of that after 10- and 60-min ball milling and the raw material.

Fig. 5 Comparison of leaching yield of metallic elements.

As shown in Fig. 5, the supercritical process had the advantage of higher leaching yield of metallic elements. Both the leaching yield of gold and copper could reach more than 99%.

SEM photograph

The scanning electron microscope photographs for particles with different pretreatment are shown in Fig. 6.

Fig. 6 Surface morphology of different particles (800× magnification): (a) raw material; (b) particles after 10 minute ball milling; (c) particles after supercritical process.

Analysis for pretreatment

Fig. 6 shows that the ball milling process resulted only in changing the particle size. On the one hand, the ball milling could achieve the separation of metallic and metalloid parts and enlarge the specific surface area of metallic particles. On the other hand, with decreasing particle size, the surface energy of particles increase, which results in the agglomeration of particles. The metallic particles were partly wrapped by reinforced resin, which is the main component of the substrate of PCBs. In the PCBs, most of the copper existed as copper foil, whereas gold mostly existed as a plated metal on the contact. Due to the small amount and the thinness of the gold-clad layer, it was considerably easier for gold to be crushed into smaller particle sizes. Hence, the wrapping of reinforced resin had a greater influence on gold, which led to the decrease of leaching yield of gold with longer ball milling time, whereas the leaching yield of copper was still increasing.

However, Fig. 6 also shows that the particle surface was totally different after the supercritical process. Due to the change of dissociation constant at the critical point, supercritical water became a strong oxidizing agent. Supercritical water is a solvent used to recycle organic polymers due to its low viscosity, high mass transportation coefficient, diffusivity and solubility. On the one hand, the supercritical process changed the surface morphology of particles. On the other hand, the non-metal parts of PCBs were resolved into low molecular weight organic matter after the supercritical process, which meant a more complete separation of metal and non-metal parts. Hence, the leaching yield of metallic elements increased after the supercritical process. However, the supercritical process demands high energy consumption due to the reaction conditions of high temperature and pressure, which might hamper industrial application.

The separation rate of copper could reach over 90% after the result of two-stage leaching. However, the results also indicated that the reaction rate in the second stage leaching had a significant decline due to the decrease of leachable metal. Hence, leaching multiple times was not an efficient way to increase the separation rate and leaching yield of metallic elements without changing other experimental conditions.

Optimization experiment

The experiments were conducted with 100 g of particles after 20 s⁻¹ ball milling for 10 minutes. After the typical procedure for leaching copper, 5 g of the residue (R₁) was used in each experiment for the gold leaching. The parameter settings are presented in Table 2. The leaching yields were calculated by the concentration of metallic elements in L₂. The concentration of metallic elements is presented in Table S1.†

Effect of leaching time

As shown in Fig. 7, with the increase of leaching time from 30 min to 90 min, the leaching yield of gold increased from 69.97% to 99.60%. However, when the leaching time was continuously increased from 90 min to 300 min, the leaching yield of gold decreased from 99.60% to 51.52%. For copper, when the time increased from 30 min to 180 min, the leaching yield is about 0.3% of the total raw material, and the leaching time had little influence on the leaching of copper. The copper remaining in the residue was hard to leach without changing the experimental parameters. Moreover, the leaching yield of copper showed an increase from 0.30% to 0.49% when the leaching time increased to 300 min.

Fig. 7 Leaching yield at leaching time: (a) gold and (b) copper.

Effect of leaching temperature

As shown in Fig. 8, when the experiment temperature increased from 30 °C to 40 °C, the leaching yield of gold increased from 80.57% to 90.12%. When the temperature continuously increased to 60 °C, there was a significant decline in leaching yield. However, there is a rising trend of leaching yield of gold as the leaching temperature increased to 70 °C. In addition, the leaching yield of copper increased with the increase of temperature. It was found that the trend of rise is more remarkable at high temperature.

Fig. 8 Leaching yield at leaching temperature: (a) gold and (b) copper.

Effect of initial concentration of sulfuric acid

As shown in Fig. 9, the leaching yield of gold peaked from 100 g L⁻¹ to 150 g L⁻¹ concentration of sulfuric acid. When concentration of acid decreased to 50 g L⁻¹ or increased to 250 g L⁻¹, in both cases a significant decline of leaching yield of gold became evident. For copper leaching, the leaching yield increased gradually with increasing concentration of acid. However, the influence was not evident, particularly when the concentration of acid ranged from 50 g L⁻¹ to 150 g L⁻¹.

Fig. 9 Leaching yield at different initial concentration of acid: (a) gold and (b) copper.

Interaction of other metals

As waste PCB has a complicated composition, including many other metals, the interaction of metals should be studied. According to results shown in Table 1, Al, Fe, Pb and Ni were chosen for further study. The leaching yields were calculated by the concentration of metallic elements in L₂.

As shown in Fig. 11, with increase in leaching time, neither Al nor Fe showed an evident change in leaching yield where the fluctuation was less than 3%. However, both Al and Fe showed a significant increase of leaching yield with increase in leaching temperature. Compared with Fig. 8, it was the same trend as that for copper. Fig. 11(c) indicates that Fe was more sensitive to the change of acidity of solution.

Fig. 10 Eh–pH diagram for: (a) Au–Cl–H₂O system (25 g L⁻¹ NaClO₃, 75 g L⁻¹ NaCl and 0.29 g L⁻¹ Au); (b) Cu–Cl–H₂O system (25 g L⁻¹ NaClO₃, 75 g L⁻¹ NaCl and 25.96 g L⁻¹ Cu).

Fig. 11 Leaching yield of Al, Fe, Ni and Fe under different experimental conditions: (a) time; (b) temperature; (c) initial concentration of sulfuric acid.

The leaching yield of Pb was significantly higher than that for other metals. Due to the low solubility of PbSO₄, Pb was hardly leached in the first-stage leaching process. Fig. 11(b) and (c) showed that the temperature and acidity had an influence on the leaching yield of Pb with an optimized leaching condition. However, the influence was limited. The leaching yield of Ni did not have an evident change in different experimental conditions. Furthermore, the concentration of Ni was low in the leaching liquor L₂. Hence, the interaction between Pb, Ni and gold was not the main factor influencing the leaching efficiency of gold.

The results proved that the interaction of gold and other metals leaching was evident in high temperature. The competitive relationship of gold and other base metals would result in a decrease of gold leaching yield with increase in temperature. However, as the amount of these metals was considerably less than the amount of copper, the interaction was very limited in the leaching system, particularly at low temperatures.

Principle and thermodynamic analysis

Main chemical reactions that occur in gold extraction process can be described by the following equations:^25,26

Au⁺ + e⁻ → Au, E⁰ = 1.691 V (2)

Au³⁺ + 3e⁻ → Au, E⁰ = 1.498 V (3)

AuCl₂⁻ + e⁻ → Au + 2Cl⁻, E⁰ = 1.113 V (4)

AuCl₄⁻ + 3e⁻ → Au + 4Cl⁻, E⁰ = 0.994 V (5)

2Au + 3Cl₂ + 2Cl⁻ → 2[AuCl₄]⁻ (6)

As sodium chlorate was the chlorinating agent in this experiment, the reactions in the leaching solution can be described as following equations:

2ClO₃⁻ + 12H⁺ + 10e⁻ → Cl₂ + 6H₂O, E⁰ = 1.468 V (7)

ClO₃⁻ + 6H⁺ + 6e⁻ → Cl⁻ + 3H₂O, E⁰ = 1.45 V (8)

The chemical equation indicates that the gold ion could only exist in solution after the formation of complex ions with chloride ion, as the complex ion decreased the electrode potential of gold from 1.498 V to 0.994 V. The leaching solution should be in an acidic condition, and the hydrogen ions would be consumed in the leaching process.

The typical Eh–pH diagram for the Au–Cl–H₂O system (25 g L⁻¹ NaClO₃, 75 g L⁻¹ NaCl and 0.29 g L⁻¹ Au) and the Cu–Cl–H₂O system (25 g L⁻¹ NaClO₃, 75 g L⁻¹ NaCl and 25.96 g L⁻¹ Cu) used in this study are shown in Fig. 10 (calculated by HSC Chemistry 7.0).

The information from Au–Cl–H₂O system model indicated that, gold in the leaching solution is mostly stable as an AuCl₄⁻ complex and that the stability zone for AuCl₃²⁻ is narrow. Generally speaking, when the Eh of solution is between 900 mV and 1500 mV, and the pH of solution is below 8, the gold can be leached into solution as the AuCl₄⁻ complex. However, if the Eh potential is too high (e.g., above 1200 mV) and the pH of solution is above 5, the gold chloride will convert to a more stable Au(OH)₃.²⁷ Hence, to maintain gold in the solution, the leaching condition should be controlled with the 800–1200 mV Eh range and the pH below 4. The Cu–Cl–H₂O system model showed that, when the Eh of solution is above 0 mV and the pH of solution is below 4, copper can be leached into solution. The lower pH and higher Eh were better for the leaching of copper. The selective leaching of cooper could be achieved if the Eh of leaching solution were controlled between 0 to 800 mV, and the pH of solution is below 4.

Fig. 10 shows that the gold and copper had a competitive relationship in the leaching system. Due to the complicated composition of PCBs, the leaching yield of gold will be influenced by copper and the other metallic elements remained in the residue. As presented in chemical reaction formula eqn (7), with increase in time, the H⁺ in the leaching solution would be expended and the emission of Cl₂ might occur due to the open system. As a result, the concentration of chlorinating agent will decrease and the pH of solution would increase, which had a greater influence on the leaching of gold due to the stability of AuCl₄⁻. The Au³⁺ in leaching solution would tend to convert to Au(OH)₃ under this condition, which resulted in a significant decreasing of leaching yield of gold and an increasing of leaching yield of copper after 180 min leaching.

Generally speaking, the increase in temperature would accelerate the reaction rate and present as an increase in leaching yield of metallic elements. However, as the leaching system was not closed, if the leaching temperature was too high, the gas emission of Cl₂ would increase, which would lead to the decrease in concentration of chlorate and in the leaching yield of gold. When the temperature increased from 50 °C to 70 °C, the temperature had a more significant influence on the leaching of metal, which resulted in a recovery in the leaching yield of gold at 70 °C, as shown in Fig. 8. Due to the competitive relationship of metallic elements in the leaching system, the leaching yield of gold was influenced by the leaching of other metallic elements such as copper at the same time. Hence, the leaching yield of gold decreased within a certain range of temperature.

Chlorate exists in many forms in aqueous solution. The acidity of the solution played a big role in the amount of different forms present. Due to the high standard electrode potential of gold, the gold leaching process was more sensitive to changes in the form of chlorate. As shown in Fig. 10, if the pH of the leaching solution is too high, the gold chloride would tend to be converted to Au(OH)₃. However, if the pH of the leaching solution is too low, the solubility of Cl₂ would decrease due to the movement of solubility equilibrium in the direction of producing gas, which resulted in the decrease in chlorinating agent and led to the decreased leaching yield of gold. Moreover, a lower pH had a positive influence on the leaching of copper. As the concentration of chlorinating agent was not the main influencing factor, the leaching yield of copper would increase continuously with the increase in acidity, which would have a negative influence on the leaching of gold resulting from the competitive relationship. Hence, there is an optimal condition for gold leaching at around 100 g L⁻¹ of sulfuric acid.

Conclusions

Chlorination leaching could achieve the selective leaching of gold and copper in the waste PCBs. The pretreatment could contribute in a major way to increase the leaching yield of metallic elements. Both the supercritical process and the ball milling with appropriate time had a positive influence on the leaching of metallic elements from waste PCBs. The supercritical process had a better effect but with higher cost. Compared to conventional gold leaching method, this method can recovery both copper and gold with high purity. Low amounts of hazardous waste gases or liquids were produced during the entire process. This process provides a less-polluted, economic and efficient way for recycling gold and copper from waste PCBs (Fig. 12).

Fig. 12 The flowchart of chlorination leaching process for the selective recovery of gold and copper.

Acknowledgements

This work was supported by the Development Program of China (863 program 2012AA063206).

Notes and references

1. H. G. Ni and E. Y. Zeng, Environ. Sci. Technol., 2009, 43, 3991–3994.
2. L. Theo and IEEE, in Proceedings of the 1998 IEEE International Symposium on Electronics and the Environment, IEEE, New York, 1998, pp. 42–47.
3. Z. Ping, Z. Xin-jiong, L. Kun-fang, Q. Guang-ren and Z. Ming, Int. J. Miner., Metall. Mater., 2012, 19, 473–477.
4. M. W. Ojeda, E. Perino and M. D. Ruiz, Miner. Eng., 2009, 22, 409–411.
5. J. Li, H. Lu, J. Guo, Z. M. Xu and Y. H. Zhou, Environ. Sci. Technol., 2007, 41, 1995–2000.
6. J. Ficeriova, P. Balaz and E. Gock, Acta Montan. Slovaca, 2011, 16, 128–131.
7. J. Vinals, C. Nunez and O. Herreros, Hydrometallurgy, 1995, 38, 125–147.
8. W. F. Liu, T. Z. Yang and X. Xia, Trans. Nonferrous Met. Soc. China, 2010, 20, 322–329.
9. A. Filcenco-Olteanu, T. Dobre, E. Panturu, R. Radulescu and N. Groza, Rev. Chim., 2010, 61, 223–226.
10. B. H. Robinson, Sci. Total Environ., 2009, 408, 183–191.
11. C. F. Shen, S. B. Huang, Z. J. Wang, M. Qiao, X. J. Tang, C. N. Yu, D. Z. Shi, Y. F. Zhu, J. Y. Shi, X. C. Chen, K. Setty and Y. X. Chen, Environ. Sci. Technol., 2008, 42, 49–55.
12. P. P. Sheng and T. H. Etsell, Waste Manage. Res., 2007, 25, 380–383.
13. M. Baghalha, Int. J. Miner. Process., 2007, 82, 178–186.
14. L. S. Strizhko, R. I. Normurotov and D. B. Kholikulov, Russian Journal of Non-Ferrous Metals, 2009, 50, 348–352.
15. L. S. Strizhko, R. I. Normurotov and D. B. Kholikulov, Russian Journal of Non-Ferrous Metals, 2009, 50, 449–451.
16. C. J. Ferron, C. A. Fleming, D. Dreisinger and T. O'Kane, Chloride as an alternative to cyanide for the extraction of gold – Going full circle?, Mineralogical Soc Amer, Chantilly, 2003.
17. E. Y. Kim, M. S. Kim, J. C. Lee and B. D. Pandey, J. Hazard. Mater., 2011, 198, 206–215.
18. B. Donmez, F. Sevim and S. Colak, Chem. Eng. Technol., 2001, 24, 91–95.
19. B. Donmez, Z. Ekinci, C. Celik and S. Colak, Hydrometallurgy, 1999, 52, 81–90.
20. K. S. Nam, B. H. Jung, J. W. An, T. J. Ha, T. Tran and M. J. Kim, Int. J. Miner. Process., 2008, 86, 131–140.
21. Z. Liu and T. Yang, Precious Met., 2014, 35, 79.
22. P. E. Savage, Chem. Rev., 1999, 99, 603–621.
23. J. L. Song, H. L. Fan, J. Ma and B. X. Han, Green Chem., 2013, 15, 2619–2635.
24. P. T. Anastas and M. M. Kirchhoff, Acc. Chem. Res., 2002, 35, 686–694.
25. K. Changsok, T. A. Zhang, Y. Zeng, H. Jongsu, G. Z. Lu and X. L. Jiang, Rare Met. Mater. Eng., 2012, 41, 569–572.
26. C. Kim, T. Zhang, W. Mu, T. Jong, Y. Zeng and X. Jiang, Chin. J. Rare Met., 2012, 36, 129–134.
27. Z. Wang, D. Chen and L. Chen, Chin. J. Rare Met., 2006, 30, 703–706.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra16231e

---