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Electro-oxidation of sphalerite in weak alkaline sodium chloride solution
Hydrometallurgy 157 (2015) 127–132
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Electro-oxidation of sphalerite in weak alkaline sodium chloride solution Zhan-fang Cao ⁎, Ming-ming Wang, Hong Zhong ⁎, Zhao-hui Qiu, Pei Qiu, Ya-jun Yue, Guang-yi Liu, Shuai Wang College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, PR China
a r t i c l e
i n f o
Article history: Received 5 May 2015 Received in revised form 10 July 2015 Accepted 10 August 2015 Available online 12 August 2015 Keywords: Sphalerite Electro-oxidation Weak alkaline Mechanism
a b s t r a c t The technology of Zn extraction from sphalerite concentrate by using electro-oxidation in weak alkaline sodium chloride solution has been investigated in this paper. The effects of several parameters on Zn recovery during the electro-oxidation have been investigated, and the optimized conditions are listed as follows : pH in the range of 8.5–8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, solid–liquid ratio ≥ 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm. Under these experimental conditions, sphalerite concentrate can be oxidized efficiently. Furthermore, the electro-oxidation mechanism of sphalerite in weak alkaline NaCl solution has been studied. It was found that the elemental sulfur and the SO2− 4 were the major existing forms of S from sphalerite and Zn would precipitate in the form of Na2Zn3(CO3)4 in the case where Na2CO3 served as electrolyte pH regulator. To prevent SO2− 4 from accumulating in the electrolyte and precipitate Zn as Zn(OH)2 instead of Na2Zn3(CO3)4, Na2CO3 was replaced by Ca(OH)2 to stabilize the pH of electrolyte, and the effect of the addition of Ca(OH)2 on Zn recovery was investigated. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The commonly used hydrometallurgical technologies to treat sphalerite are roasting-acid leaching–electrowinning and oxygen pressure acid leaching processes (Mei et al., 2001; Mulaba-Bafubiandi and Waanders, 2005). However, SO2 generated and discharged at the roasting stage in the former process will inevitably pollute the air (Peng et al., 2005; Santos et al., 2010). As to the latter process, the elevated temperature and high pressure required in the leaching step make the process not cost-effective (Zhang et al., 2008) and difficult to be performed. Considerable attention has been paid to some other methods of dealing with sphalerite in recent decades. The utilization of ferric sulfate and ferric chloride to leach sphalerite has been reported by some researchers, and the relevant investigations on the kinetics of dissolution of spalerite have been carried out (Bobeck and Su, 1985; Lochmann and Pedlik, 1995; Palencia and Dutrizac, 1991; Santos et al., 2010). The leaching of sphalerite with ammonia as lixiviant under high pressure oxygen condition has been studied deeply (Sarveswara Rao and Ray, 1998; Sarveswara Rao et al., 1993; Wang et al., 2011). Microbial leaching of sphalerite has been practiced and investigated for a long time (Ake and Stig, 1997; Wang et al., 2006). Furthermore, some researchers combined bioleaching and chemical leaching as an integrated process to treat zinc sulfide (De Souza et al., 2007). The studies on nonoxidative dissolution and oxidative leaching of sphalerite by hydrochloric acid and hypochlorous acid respectively have been reported in
⁎ Corresponding authors.
http://dx.doi.org/10.1016/j.hydromet.2015.08.005 0304-386X/© 2015 Elsevier B.V. All rights reserved.
some papers (Cho, 1987; Majima et al., 1981). Some investigators succeeded to achieve a high leaching efficiency of Zn from sphalerite in alkaline solution with the help of chemical conversion of lead carbonate (Zhang et al., 2008). Besides, the electro-oxidation leaching has also been employed to treat sphalerite, which has been reported by a few researchers (Qiu, 1999). In the conventional electro-oxidation process in which acid chloride solution is usually adopted as its electrolyte (Arslan and Duby, 2003), the sulfide minerals are leached at the anode and the metal ions are reduced at the cathode simultaneously. The leaching of the sulfide minerals at the anode is often attributed to the oxidation of mineral particles by current, Fe3+/Cu2+ in the electrolyte and the oxidized chloride species generated at the anode and to the nonoxidative dissolution of mineral particles in acid electrolyte (Gordy, 1973; Kruesi, 1972; Qiu, 1999; Zhang et al., 1998). As the traditional electro-oxidation leaching is implemented in the strong acid chloride solution, the emission of Cl2 which is widely recognized as the air pollutant will be inevitable in the process and severely restrict the development of electro-oxidation technology. Therefore, the electro-oxidation of sphalerite in weak alkaline sodium chloride solution, which has not been reported yet, was investigated in this paper. It has been confirmed in our previous work that the electrooxidation leaching of molybdenite in weak alkaline sodium chloride solution is feasible (Cao et al., 2010). Additionally, as pointed out by some other researchers, it is theoretically much easier for sphalerite than for molybdenite to be oxidized in the electro-oxidation process (Qiu, 1999). Hence, the possibility of the electro-oxidation leaching of sphalerite in weak alkaline sodium chloride solution has been investigated in this paper. The experimental tests on how the process parameters-
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Fig. 1. X-ray diffraction result of the sphalerite concentrate. Fig. 3. Effect of NaCl concentration on Zn recovery. Experimental conditions: pH = 8.5– 8.8, reaction time = 1 h, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
electrolyte pH (regulated by Na2CO3), NaCl concentration, reaction time, liquid–solid ratio, anodic current density and stirring speedaffected the electro-oxidation of sphalerite were carried out. Moreover, the reaction mechanism in the process has been investigated and discussed. The addition of Ca(OH)2 rather than Na2CO3 to the electrolyte to stabilize weak alkaline environment has been studied.
2. Experimental 2.1. Concentrate sample and reagents The XRF (EDX1800B X-ray Fluorescence meter produced by Skyray Instrument Co., Ltd., Kunshan, China) result of sphalerite concentrate used in all electro-oxidation tests is listed as follows (%): Zn 60.3, S 28.09, Fe2O3 4.42, SiO2 2.88, Al2O3 1.39, Cu 0.09, Cd 0.14, Pb 0.38. XRD (Rigaku 2500 X-ray Diffraction meter produced by Rigaku Corporation, Japan, scanning range of 5°–80°, scanning speed at 8°/min, JADE 5.0 analysis software, these conditions are applied to all XRD analyses in this paper) result of the concentrate sample (Fig. 1) shows ZnS was the major phase, while the particle size was in the range of 10 μm to 100 μm with 83% less than 40 μm (MS-2000 Laser particle size analyzer, Britain). In all electrooxidation tests, except for water which was tap water, all reagents involved in the tests were chemical grade. The reagents employed to analyze Zn2 + content in solution and precipitate SO24 − were analytical grade.
Fig. 2. Effect of pH on Zn recovery. Experimental conditions: NaCl concentration = 2 mol/ L, reaction time = 1 h, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
2.2. Electro-oxidation experiments 2.2.1. Instruments and determination of Zn2+ A self-made septum-free electrolytic bath made of plexiglas (7.5 cm × 5.5 cm × 5.5 cm) was used in the electro-oxidation tests. DSA electrode (dimensionally stable electrode, RuO2, IrO2, SnO2 coated mesh electrode) and graphite rod were adopted as anode and cathode respectively. The pH meter (pHS-3C pH meter produced by INESA Scientific Instrument Co., Ltd., Shanghai, China) was used to monitor electrolyte pH, the magnetic stirrer (DF-101S magnetic stirrer produced by Greatwall Scientific Industrial And Trade Co., Ltd, Zhengzhou, China) to sustain the suspension of mineral particles and the S22 Regulated Power Supply System (Chenhua Instruments Co., Ltd., Shanghai, China) to provide applied potential between anode and cathode in electro-oxidation tests. In all electro-oxidation experiments, Zn from sphalerite precipitated in a certain form which depended on the specific alkaline environment. To analyze the mass of Zn recovered, the residue containing Zn precipitate was firstly dissolved in HCl solution. Then, the EDTA titration method (0.1 mol/L EDTA, xylenol orange used as indicator, sodium thiosulfate used as masking agent, HAc–NaAc buffer responsible for stabilizing solution pH at 5.4–5.9) was adopted to analyze Zn2+ content.
2.2.2. Procedure All electro-oxidation experiments were carried out at room temperature with current of 1.5 A and electrolyte volume of 100 ml. The other specific conditions are given in Section 3.
Fig. 4. Effect of reaction time on Zn recovery. Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
Z. Cao et al. / Hydrometallurgy 157 (2015) 127–132
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Fig. 7. Effect of stirring speed on Zn recovery. Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2.
ray diffraction and gravimetric method. Additionally, to prove the existence of elemental sulfur, another experiment was carried out to oxidize most of ZnS. Subsequently the residue was acidized to dissolve and remove Zn precipitate, dried and then analyzed by X-ray diffraction. Under conditions optimized by Na2CO3 system, the replacement of Na2CO3 by Ca(OH)2 to stabilize weak alkaline environment in the process was investigated. The residue was dried and then analyzed by Xray diffraction.
3. Results and discussions Fig. 5. Effect of S/L ratio on Zn recovery. Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
With the adoption of Na2CO3 to stabilize alkaline environment in sodium chloride solution, the effects of these process parameters, namely pH, NaCl concentration (mol/L), reaction time (h), S/L ratio (solid/liquid ratio, g/ml), anodic current density (A/m2) and stirring speed (rpm) on Zn recovery were investigated. Furthermore, to study the mechanism of the electro-oxidation of sphalerite in weak alkaline sodium chloride solution in which Na2CO3 served as pH regulator, three parallel experiments were performed under conditions optimized by above process conditions tests to investigate the existing forms of Zn and S from sphalerite concentrate. After electro-oxidation, the residue was dried and then analyzed by X-ray diffraction. Excess BaCl2 was added to the leachate acidized by HCl solution to precipitate SO2− 4 , and the generated precipitate was analyzed by X-
Fig. 6. Effect of anodic current density on Zn recovery. Experimental conditions: pH = 8.5– 8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, S/L ratio = 0.03 g/ml, stirring speed = 800 rpm.
3.1. Electrode reactions In the electro-oxidation of sphalerite, the oxygen evolution reaction (Eq. (1)) and the chlorine evolution reaction (Eq. (2)) are probably the main anodic reactions, and the former should make little contribution to the oxidation of the concentrate. The generated Cl2 is able to react with water, as shown in Eq. (3), leading to the formation of HClO. Afterwards, the dissociation of HClO, which depends on the solution pH, gives rise to the generation of ClO−(Eq. (4)). The relevant literature reveals that ClO− will predominate the solution with solution pH exceeding 7.5(Cho, 1987). Furthermore it has been pointed out that with pH in the range of 8.14–8.58 (17–70 °C), the mole ratio of ClO− to HClO is 1: 0.07 in the electrolysis of NaCl solution (Chen, 1993). Therefore, it can be reasonably assumed that the generated Cl2 is converted into ClO− and Cl−(Eq. (5)) according to the experimental conditions in this paper, and the overall conversion of Cl− to ClO− can be described as Eq. (6). Additionally, partial ClO− can be further oxidized into ClO− 3 electrochemically at the anode (Eq. (7)) (Arslan and Duby, 1997)
Fig. 8. X-ray diffraction of Ba precipitate.
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2H2 O þ 2e ¼ H2 þ 2OH−
ð8Þ
ClO− þ H2 O þ 2e ¼ 2OH− þ Cl−
ð9Þ
3.2. Effect of pH on Zn recovery With pH ranging from 8.5 to 10, the major chloride oxidant generated at the anode should be ClO− which is responsible for the electrooxidation of sphalerite in weak alkaline environment. As indicated in Fig. 2, the percentage of Zn recovered decreased along with the increase of pH in the range of 8.5–10, which suggests that the oxidation of sphalerite concentrate by ClO− was suppressed by the increasing pH. Hence, in the following experiments, the pH value was controlled at between 8.5 and 8.8.
Fig. 9. X-ray diffraction of the residue obtained in Na2CO3 system.
despite the fact that it is no more than 2.3% when pH ranges from 8.14 to 8.58 (17–70 °C) (Chen, 1993). At the cathode, besides the hydrogen evolution reaction (Eq. (8)), the reduction and conversion of ClO− to Cl− can also take place (Eq. (9)). It is noted that the mole of electrons passing throng the electrodes is equal to that of OH− consumed at the anode or that of OH− generated at the cathode no matter which anodic reaction (Eqs. (1), (6) and (7)) or cathodic reaction (Eqs. (8) and (9)) takes place. Consequently, the solution pH can be stable theoretically with the absence of other reactions consuming or supplying OH−. 4OH− −4e ¼ O2 þ 2H2 O
ð1Þ
3.3. Effect of NaCl concentration on Zn recovery Seen from Fig. 3, the rising NaCl concentration facilitated the oxidation of sphalerite concentrate when the NaCl concentration was ≤3 mol/ L. The possible reason is that the amount of Cl− which could be converted into ClO− to oxidize sphalerite in the electro-oxidation process was inadequate with relatively low NaCl concentration, thus the rising NaCl concentration led to the increase of ClO− followed by the enhanced Zn recovery when NaCl concentration was ≤3 mol/L. However, when the NaCl concentration exceeded 3 mol/L, the Zn recovery was not affected much by NaCl concentration. Therefore, 3 mol/L was selected as the concentration of NaCl in the following experiments. 3.4. Effect of reaction time on Zn recovery
−
2Cl −2e ¼ Cl2
ð2Þ
Cl2 þ H2 O ¼ HClO þ Hþ þ Cl−
ð3Þ
HClO ¼ Hþ þ ClO−
ð4Þ
The effect of reaction time on Zn recovery (Fig. 4) shows that when the reaction time was ≤2.5 h, the average oxidation rate of sphalerite remained high, afterwards it distinctly slowed down and the percentage of Zn recovered reached the maximum with reaction time = 5 h. The possible reason for the trend is that with the increase of the reaction time, more ZnS was oxidized to the solution and then converted into a Zn precipitate, which meant that the actual S/L ratio had dropped down and this might suppress the oxidation rate of sphalerite. To verify the explanation, the following experiments were carried out by adopting different initial S/L ratio with reaction time = 3 h. 3.5. Effect of S/L ratio on Zn recovery
Cl2 þ 2OH− ¼ ClO− þ Cl− þ H2 O
ð5Þ
Cl− þ 2OH− −2e ¼ ClO− þ H2 O
ð6Þ
− 6ClO− þ 6OH− −6e ¼ 2ClO− 3 þ 4Cl þ 3=2O2 þ 3H2 O
ð7Þ
It is indicated in Fig. 5(a) that with the same reaction time = 3 h, the initial S/L ratio had tremendous effect on the mass of Zn recovered (it is noted that with any S/L ratio listed, there was still considerable amount of Zn in the concentrate after electro-oxidation). When the initial S/L ratio was ≤0.06 g/ml, the increase of the initial S/L ratio remarkably facilitated the oxidation of sphalerite, suggesting that the reason for the falling average oxidation rate of sphalerite with reaction time ≥ 2.5 h in Fig. 4 was in fact the shortage of sphalerite in electrolyte. It can be seen from Fig. 5(b) that with the reaction time = 1 h, the rise of the initial S/L ratio could hardly affect the oxidation of sphalerite
Table 1 Three parallel experiments aimed to study the existing forms of S. Test number electric quantity (Ah) #1 #2 #3
Zn recovered (g) Theoretical actual 1.5 1.5 1.5
0.455 0.455 0.455
SO2− proportion (%) 4
Mass of BaSO4 (g) Theoretical actual 0.519 0.500 0.542
1.86 1.79 1.94
1.487 1.281 1.332
79.9 71.5 68.5
Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
Z. Cao et al. / Hydrometallurgy 157 (2015) 127–132
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Fig. 10. X-ray diffraction result of the acidized residue (reaction time = 5 h).
Fig. 12. X-ray diffraction of the residue obtained in Ca(OH)2 system.
when it was bigger than 0.03 g/ml. Therefore, in the following experiments, the reaction time and the initial S/L ratio were set at 1 h and 0.03 g/ml respectively.
3.8. Electro-oxidation mechanism of sphalerite in weak alkaline sodium chloride solution with Na2CO3 as pH regulator
3.6. Effect of anodic current density on Zn recovery As shown in Fig. 6, the oxidation of sphalerite was affected by the anodic current density to some extent. The proportion of Zn recovered rose as the anodic current density grew when it did not surpass 1342 A/m2, and that was probably because the generation of ClO− rather than O2 became the dominant anodic reaction. Afterwards it dropped down with increasing the anodic current density, and that might be attributed to the enhanced formation of ClO− 3 which possessed the weaker oxidative ability compared with ClO−. Hence, the anodic current density was fixed at 1342 A/m2.
3.7. Effect of stirring speed on Zn recovery Fig. 7 reveals that when the stirring speed was in the range from 200 to 600 rpm, quicker agitation was able to enhance the oxidation of sphalerite concentrate, while once it reached 600 rpm, the continuing increase of stirring speed would exert little influence on Zn recovery. The possible explanation for this trend is that the diffusion of ClO−, which contributed to the oxidation of sphalerite, was much more sensitive to the change of the stirring speed when it was low (200 to 600 rpm). Hence, the stirring speed was set at 800 rpm in the subsequent experiments.
Fig. 11. Comparison of Zn recoveries in Ca(OH)2 system and Na2CO3 system. Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
According to the XRD results of the precipitates generated by the addition of BaCl2 to the acidized leachate (Fig. 8, almost all peaks in line with the standard peaks of BaSO4) and the residue (Fig. 9), it is confirmed that SO24 − was an existing form of S from sphalerite, and Zn would precipitate in the form of Na2Zn3(CO3)4. Thus, the possible oxidation route of sphalerite is given as Eq. (10). þ − 2− 12ClO− þ 3ZnS þ 4CO2− 3 þ 2Na ¼ 12Cl þ 3SO4 þ Na2 Zn3 ðCO3 Þ4 ↓
ð10Þ The purpose of the three parallel experiments in Table 1 is to investigate the possibilities of other existing forms of S through analyzing the relationships among the electric quantity, the mass of Zn recovered and that of BaSO4. In Table 1, the ‘Zn recovered-theoretical’ column refers to the theoretical mass of Zn recovered based on electric quantity, the assumption that Eqs. (6) and (8) are the exclusive anodic reaction and cathodic reaction respectively and Eq. (10). The ‘Zinc recovered-actual’ column refers to the actual mass of Zn recovered in the experiments. As shown in Table 1, the theoretical values are still much smaller than the actual ones was although with 100% current efficiency, which suggests that SO2− 4 not the only existing form and there should be other forms of S in lower valence state. The ‘mass of BaSO4-theoretical’ stands for the theoretical mass of BaSO4 calculated according to the actual mass of Zn recovered the mole of which should be equal to that of BaSO4 obtained was the only existing form, while ‘mass of BaSO4-actual’ repreif SO2− 4 sents the actual mass of BaSO4 obtained. The relevant data indicate that the former were distinctly bigger than the latter, suggesting again that proportion’ column rethere were other existing forms of S. The ‘SO2− 4 fers to the ratio of S based on the actual mass of BaSO4 to the total S the mole of which is equal to that of Zn recovered in experiments. The rewas the major but not exclusive existing form sults indicate that SO2− 4 of S from sphalerite. Fig. 9 demonstrates no elemental sulfur was found in the residue, and the possible reason was that its quantity was too small to be found compared to other components. Hence, to convert and remove as much ZnS as possible in the residue to make sulfur stand out in the XRD result, a single experiment was performed with the reaction time = 5 h. XRD result of the acidized residue in Fig. 10 confirms the existence of sulfur, suggesting that sulfur and sulfate were the main existing forms of S after the electro-oxidation of sphalerite in weak alkaline sodium chloride solution. The continuing generation of Na2Zn3(CO3)4 meant the demand for CO2− 3 , therefore it was necessary to add Na2CO3 to the electrolyte conin the electrolyte, Zn stantly in the process. Otherwise with little CO2− 3
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would precipitate as Zn(OH)2 and then the extra consumption of OH− would make the electro-oxidation system move toward acidification. Eq. (10) indicates that SO24 − would accumulate in the electrolyte, which may be not beneficial to the recycling of electrolyte. In addition, Na2Zn3(CO3)4 precipitate is not the suitable source for Zn electrowinning process. Hence, Ca(OH)2 was adopted to replace Na2CO3 in the and Zn2+ (Eq. (11)). following experiments to precipitate SO2− 4 4ClO− þ ZnS þ CaðOHÞ2 ¼ 4Cl− þ CaSO4 ↓ þ ZnðOHÞ2 ↓
ð11Þ
3.9. Electro-oxidation of sphalerite concentrate in Ca(OH)2 system Fig. 11 shows that the electro-oxidation rate of sphalerite concentrate in Ca(OH)2 system was slightly higher than that in Na2CO3 system despite the fact that the maximum percentages of Zn recovered in both systems were similar. As shown in Fig. 12, the addition of Ca(OH)2 rather than Na2CO3 to the electrolyte indeed made SO24 − precipitate as CaSO4. However, the main existing form of Zn was ZnO instead of Zn(OH)2 besides ZnS according to the XRD result. The possible reason is that Zn(OH)2 could be decomposed to ZnO as the residue was dried at 80 °C before XRD analysis. Hence, with using Ca(OH)2 to maintain the weak alkaline reaction environment, the electrolyte can be recycled accumulating and Zn(OH)2 obtained is a suitable Zn with little SO2− 4 source for the electrowinning process. 4. Conclusions It has been confirmed that the process parameters, namely pH, NaCl concentration, reaction time, solid/liquid ratio, anodic current density and stirring speed influence Zn recovery to a certain extent during the electro-oxidation of sphalerite concentrate implemented in the weak alkaline sodium chloride solution. The optimized conditions are listed as follows : pH in the range of 8.5–8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, solid–liquid ratio ≥ 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm. In this process, S from the concentrate will be mainly converted into sulfur and sulfate. In the case where weak alkaline environment is regulated by Na2CO3, Zn from sphalerite will precipitate in the form of Na2Zn3(CO3)4, while Zn will precipitate as Zn(OH)2 with Na2CO3 replaced by Ca(OH)2 and will precipitate as CaSO4 simultaneously. With using Ca(OH)2 inSO2− 4 can be prevented stead of Na2CO3 to regulate the electrolyte pH, SO2− 4 from accumulating in the electrolyte which will be recycled and Zn(OH)2 obtained is suitable for the electrowinning process. Acknowledgments The authors are thankful to the Open-End Fund for the Valuable and Precision Instruments of Central South University (CSUZC201504), the
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Electro-oxidation of sphalerite in weak alkaline sodium chloride solution Zhan-fang Cao ⁎, Ming-ming Wang, Hong Zhong ⁎, Zhao-hui Qiu, Pei Qiu, Ya-jun Yue, Guang-yi Liu, Shuai Wang College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, PR China
a r t i c l e
i n f o
Article history: Received 5 May 2015 Received in revised form 10 July 2015 Accepted 10 August 2015 Available online 12 August 2015 Keywords: Sphalerite Electro-oxidation Weak alkaline Mechanism
a b s t r a c t The technology of Zn extraction from sphalerite concentrate by using electro-oxidation in weak alkaline sodium chloride solution has been investigated in this paper. The effects of several parameters on Zn recovery during the electro-oxidation have been investigated, and the optimized conditions are listed as follows : pH in the range of 8.5–8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, solid–liquid ratio ≥ 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm. Under these experimental conditions, sphalerite concentrate can be oxidized efficiently. Furthermore, the electro-oxidation mechanism of sphalerite in weak alkaline NaCl solution has been studied. It was found that the elemental sulfur and the SO2− 4 were the major existing forms of S from sphalerite and Zn would precipitate in the form of Na2Zn3(CO3)4 in the case where Na2CO3 served as electrolyte pH regulator. To prevent SO2− 4 from accumulating in the electrolyte and precipitate Zn as Zn(OH)2 instead of Na2Zn3(CO3)4, Na2CO3 was replaced by Ca(OH)2 to stabilize the pH of electrolyte, and the effect of the addition of Ca(OH)2 on Zn recovery was investigated. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The commonly used hydrometallurgical technologies to treat sphalerite are roasting-acid leaching–electrowinning and oxygen pressure acid leaching processes (Mei et al., 2001; Mulaba-Bafubiandi and Waanders, 2005). However, SO2 generated and discharged at the roasting stage in the former process will inevitably pollute the air (Peng et al., 2005; Santos et al., 2010). As to the latter process, the elevated temperature and high pressure required in the leaching step make the process not cost-effective (Zhang et al., 2008) and difficult to be performed. Considerable attention has been paid to some other methods of dealing with sphalerite in recent decades. The utilization of ferric sulfate and ferric chloride to leach sphalerite has been reported by some researchers, and the relevant investigations on the kinetics of dissolution of spalerite have been carried out (Bobeck and Su, 1985; Lochmann and Pedlik, 1995; Palencia and Dutrizac, 1991; Santos et al., 2010). The leaching of sphalerite with ammonia as lixiviant under high pressure oxygen condition has been studied deeply (Sarveswara Rao and Ray, 1998; Sarveswara Rao et al., 1993; Wang et al., 2011). Microbial leaching of sphalerite has been practiced and investigated for a long time (Ake and Stig, 1997; Wang et al., 2006). Furthermore, some researchers combined bioleaching and chemical leaching as an integrated process to treat zinc sulfide (De Souza et al., 2007). The studies on nonoxidative dissolution and oxidative leaching of sphalerite by hydrochloric acid and hypochlorous acid respectively have been reported in
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http://dx.doi.org/10.1016/j.hydromet.2015.08.005 0304-386X/© 2015 Elsevier B.V. All rights reserved.
some papers (Cho, 1987; Majima et al., 1981). Some investigators succeeded to achieve a high leaching efficiency of Zn from sphalerite in alkaline solution with the help of chemical conversion of lead carbonate (Zhang et al., 2008). Besides, the electro-oxidation leaching has also been employed to treat sphalerite, which has been reported by a few researchers (Qiu, 1999). In the conventional electro-oxidation process in which acid chloride solution is usually adopted as its electrolyte (Arslan and Duby, 2003), the sulfide minerals are leached at the anode and the metal ions are reduced at the cathode simultaneously. The leaching of the sulfide minerals at the anode is often attributed to the oxidation of mineral particles by current, Fe3+/Cu2+ in the electrolyte and the oxidized chloride species generated at the anode and to the nonoxidative dissolution of mineral particles in acid electrolyte (Gordy, 1973; Kruesi, 1972; Qiu, 1999; Zhang et al., 1998). As the traditional electro-oxidation leaching is implemented in the strong acid chloride solution, the emission of Cl2 which is widely recognized as the air pollutant will be inevitable in the process and severely restrict the development of electro-oxidation technology. Therefore, the electro-oxidation of sphalerite in weak alkaline sodium chloride solution, which has not been reported yet, was investigated in this paper. It has been confirmed in our previous work that the electrooxidation leaching of molybdenite in weak alkaline sodium chloride solution is feasible (Cao et al., 2010). Additionally, as pointed out by some other researchers, it is theoretically much easier for sphalerite than for molybdenite to be oxidized in the electro-oxidation process (Qiu, 1999). Hence, the possibility of the electro-oxidation leaching of sphalerite in weak alkaline sodium chloride solution has been investigated in this paper. The experimental tests on how the process parameters-
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Fig. 1. X-ray diffraction result of the sphalerite concentrate. Fig. 3. Effect of NaCl concentration on Zn recovery. Experimental conditions: pH = 8.5– 8.8, reaction time = 1 h, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
electrolyte pH (regulated by Na2CO3), NaCl concentration, reaction time, liquid–solid ratio, anodic current density and stirring speedaffected the electro-oxidation of sphalerite were carried out. Moreover, the reaction mechanism in the process has been investigated and discussed. The addition of Ca(OH)2 rather than Na2CO3 to the electrolyte to stabilize weak alkaline environment has been studied.
2. Experimental 2.1. Concentrate sample and reagents The XRF (EDX1800B X-ray Fluorescence meter produced by Skyray Instrument Co., Ltd., Kunshan, China) result of sphalerite concentrate used in all electro-oxidation tests is listed as follows (%): Zn 60.3, S 28.09, Fe2O3 4.42, SiO2 2.88, Al2O3 1.39, Cu 0.09, Cd 0.14, Pb 0.38. XRD (Rigaku 2500 X-ray Diffraction meter produced by Rigaku Corporation, Japan, scanning range of 5°–80°, scanning speed at 8°/min, JADE 5.0 analysis software, these conditions are applied to all XRD analyses in this paper) result of the concentrate sample (Fig. 1) shows ZnS was the major phase, while the particle size was in the range of 10 μm to 100 μm with 83% less than 40 μm (MS-2000 Laser particle size analyzer, Britain). In all electrooxidation tests, except for water which was tap water, all reagents involved in the tests were chemical grade. The reagents employed to analyze Zn2 + content in solution and precipitate SO24 − were analytical grade.
Fig. 2. Effect of pH on Zn recovery. Experimental conditions: NaCl concentration = 2 mol/ L, reaction time = 1 h, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
2.2. Electro-oxidation experiments 2.2.1. Instruments and determination of Zn2+ A self-made septum-free electrolytic bath made of plexiglas (7.5 cm × 5.5 cm × 5.5 cm) was used in the electro-oxidation tests. DSA electrode (dimensionally stable electrode, RuO2, IrO2, SnO2 coated mesh electrode) and graphite rod were adopted as anode and cathode respectively. The pH meter (pHS-3C pH meter produced by INESA Scientific Instrument Co., Ltd., Shanghai, China) was used to monitor electrolyte pH, the magnetic stirrer (DF-101S magnetic stirrer produced by Greatwall Scientific Industrial And Trade Co., Ltd, Zhengzhou, China) to sustain the suspension of mineral particles and the S22 Regulated Power Supply System (Chenhua Instruments Co., Ltd., Shanghai, China) to provide applied potential between anode and cathode in electro-oxidation tests. In all electro-oxidation experiments, Zn from sphalerite precipitated in a certain form which depended on the specific alkaline environment. To analyze the mass of Zn recovered, the residue containing Zn precipitate was firstly dissolved in HCl solution. Then, the EDTA titration method (0.1 mol/L EDTA, xylenol orange used as indicator, sodium thiosulfate used as masking agent, HAc–NaAc buffer responsible for stabilizing solution pH at 5.4–5.9) was adopted to analyze Zn2+ content.
2.2.2. Procedure All electro-oxidation experiments were carried out at room temperature with current of 1.5 A and electrolyte volume of 100 ml. The other specific conditions are given in Section 3.
Fig. 4. Effect of reaction time on Zn recovery. Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
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Fig. 7. Effect of stirring speed on Zn recovery. Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2.
ray diffraction and gravimetric method. Additionally, to prove the existence of elemental sulfur, another experiment was carried out to oxidize most of ZnS. Subsequently the residue was acidized to dissolve and remove Zn precipitate, dried and then analyzed by X-ray diffraction. Under conditions optimized by Na2CO3 system, the replacement of Na2CO3 by Ca(OH)2 to stabilize weak alkaline environment in the process was investigated. The residue was dried and then analyzed by Xray diffraction.
3. Results and discussions Fig. 5. Effect of S/L ratio on Zn recovery. Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
With the adoption of Na2CO3 to stabilize alkaline environment in sodium chloride solution, the effects of these process parameters, namely pH, NaCl concentration (mol/L), reaction time (h), S/L ratio (solid/liquid ratio, g/ml), anodic current density (A/m2) and stirring speed (rpm) on Zn recovery were investigated. Furthermore, to study the mechanism of the electro-oxidation of sphalerite in weak alkaline sodium chloride solution in which Na2CO3 served as pH regulator, three parallel experiments were performed under conditions optimized by above process conditions tests to investigate the existing forms of Zn and S from sphalerite concentrate. After electro-oxidation, the residue was dried and then analyzed by X-ray diffraction. Excess BaCl2 was added to the leachate acidized by HCl solution to precipitate SO2− 4 , and the generated precipitate was analyzed by X-
Fig. 6. Effect of anodic current density on Zn recovery. Experimental conditions: pH = 8.5– 8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, S/L ratio = 0.03 g/ml, stirring speed = 800 rpm.
3.1. Electrode reactions In the electro-oxidation of sphalerite, the oxygen evolution reaction (Eq. (1)) and the chlorine evolution reaction (Eq. (2)) are probably the main anodic reactions, and the former should make little contribution to the oxidation of the concentrate. The generated Cl2 is able to react with water, as shown in Eq. (3), leading to the formation of HClO. Afterwards, the dissociation of HClO, which depends on the solution pH, gives rise to the generation of ClO−(Eq. (4)). The relevant literature reveals that ClO− will predominate the solution with solution pH exceeding 7.5(Cho, 1987). Furthermore it has been pointed out that with pH in the range of 8.14–8.58 (17–70 °C), the mole ratio of ClO− to HClO is 1: 0.07 in the electrolysis of NaCl solution (Chen, 1993). Therefore, it can be reasonably assumed that the generated Cl2 is converted into ClO− and Cl−(Eq. (5)) according to the experimental conditions in this paper, and the overall conversion of Cl− to ClO− can be described as Eq. (6). Additionally, partial ClO− can be further oxidized into ClO− 3 electrochemically at the anode (Eq. (7)) (Arslan and Duby, 1997)
Fig. 8. X-ray diffraction of Ba precipitate.
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2H2 O þ 2e ¼ H2 þ 2OH−
ð8Þ
ClO− þ H2 O þ 2e ¼ 2OH− þ Cl−
ð9Þ
3.2. Effect of pH on Zn recovery With pH ranging from 8.5 to 10, the major chloride oxidant generated at the anode should be ClO− which is responsible for the electrooxidation of sphalerite in weak alkaline environment. As indicated in Fig. 2, the percentage of Zn recovered decreased along with the increase of pH in the range of 8.5–10, which suggests that the oxidation of sphalerite concentrate by ClO− was suppressed by the increasing pH. Hence, in the following experiments, the pH value was controlled at between 8.5 and 8.8.
Fig. 9. X-ray diffraction of the residue obtained in Na2CO3 system.
despite the fact that it is no more than 2.3% when pH ranges from 8.14 to 8.58 (17–70 °C) (Chen, 1993). At the cathode, besides the hydrogen evolution reaction (Eq. (8)), the reduction and conversion of ClO− to Cl− can also take place (Eq. (9)). It is noted that the mole of electrons passing throng the electrodes is equal to that of OH− consumed at the anode or that of OH− generated at the cathode no matter which anodic reaction (Eqs. (1), (6) and (7)) or cathodic reaction (Eqs. (8) and (9)) takes place. Consequently, the solution pH can be stable theoretically with the absence of other reactions consuming or supplying OH−. 4OH− −4e ¼ O2 þ 2H2 O
ð1Þ
3.3. Effect of NaCl concentration on Zn recovery Seen from Fig. 3, the rising NaCl concentration facilitated the oxidation of sphalerite concentrate when the NaCl concentration was ≤3 mol/ L. The possible reason is that the amount of Cl− which could be converted into ClO− to oxidize sphalerite in the electro-oxidation process was inadequate with relatively low NaCl concentration, thus the rising NaCl concentration led to the increase of ClO− followed by the enhanced Zn recovery when NaCl concentration was ≤3 mol/L. However, when the NaCl concentration exceeded 3 mol/L, the Zn recovery was not affected much by NaCl concentration. Therefore, 3 mol/L was selected as the concentration of NaCl in the following experiments. 3.4. Effect of reaction time on Zn recovery
−
2Cl −2e ¼ Cl2
ð2Þ
Cl2 þ H2 O ¼ HClO þ Hþ þ Cl−
ð3Þ
HClO ¼ Hþ þ ClO−
ð4Þ
The effect of reaction time on Zn recovery (Fig. 4) shows that when the reaction time was ≤2.5 h, the average oxidation rate of sphalerite remained high, afterwards it distinctly slowed down and the percentage of Zn recovered reached the maximum with reaction time = 5 h. The possible reason for the trend is that with the increase of the reaction time, more ZnS was oxidized to the solution and then converted into a Zn precipitate, which meant that the actual S/L ratio had dropped down and this might suppress the oxidation rate of sphalerite. To verify the explanation, the following experiments were carried out by adopting different initial S/L ratio with reaction time = 3 h. 3.5. Effect of S/L ratio on Zn recovery
Cl2 þ 2OH− ¼ ClO− þ Cl− þ H2 O
ð5Þ
Cl− þ 2OH− −2e ¼ ClO− þ H2 O
ð6Þ
− 6ClO− þ 6OH− −6e ¼ 2ClO− 3 þ 4Cl þ 3=2O2 þ 3H2 O
ð7Þ
It is indicated in Fig. 5(a) that with the same reaction time = 3 h, the initial S/L ratio had tremendous effect on the mass of Zn recovered (it is noted that with any S/L ratio listed, there was still considerable amount of Zn in the concentrate after electro-oxidation). When the initial S/L ratio was ≤0.06 g/ml, the increase of the initial S/L ratio remarkably facilitated the oxidation of sphalerite, suggesting that the reason for the falling average oxidation rate of sphalerite with reaction time ≥ 2.5 h in Fig. 4 was in fact the shortage of sphalerite in electrolyte. It can be seen from Fig. 5(b) that with the reaction time = 1 h, the rise of the initial S/L ratio could hardly affect the oxidation of sphalerite
Table 1 Three parallel experiments aimed to study the existing forms of S. Test number electric quantity (Ah) #1 #2 #3
Zn recovered (g) Theoretical actual 1.5 1.5 1.5
0.455 0.455 0.455
SO2− proportion (%) 4
Mass of BaSO4 (g) Theoretical actual 0.519 0.500 0.542
1.86 1.79 1.94
1.487 1.281 1.332
79.9 71.5 68.5
Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
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Fig. 10. X-ray diffraction result of the acidized residue (reaction time = 5 h).
Fig. 12. X-ray diffraction of the residue obtained in Ca(OH)2 system.
when it was bigger than 0.03 g/ml. Therefore, in the following experiments, the reaction time and the initial S/L ratio were set at 1 h and 0.03 g/ml respectively.
3.8. Electro-oxidation mechanism of sphalerite in weak alkaline sodium chloride solution with Na2CO3 as pH regulator
3.6. Effect of anodic current density on Zn recovery As shown in Fig. 6, the oxidation of sphalerite was affected by the anodic current density to some extent. The proportion of Zn recovered rose as the anodic current density grew when it did not surpass 1342 A/m2, and that was probably because the generation of ClO− rather than O2 became the dominant anodic reaction. Afterwards it dropped down with increasing the anodic current density, and that might be attributed to the enhanced formation of ClO− 3 which possessed the weaker oxidative ability compared with ClO−. Hence, the anodic current density was fixed at 1342 A/m2.
3.7. Effect of stirring speed on Zn recovery Fig. 7 reveals that when the stirring speed was in the range from 200 to 600 rpm, quicker agitation was able to enhance the oxidation of sphalerite concentrate, while once it reached 600 rpm, the continuing increase of stirring speed would exert little influence on Zn recovery. The possible explanation for this trend is that the diffusion of ClO−, which contributed to the oxidation of sphalerite, was much more sensitive to the change of the stirring speed when it was low (200 to 600 rpm). Hence, the stirring speed was set at 800 rpm in the subsequent experiments.
Fig. 11. Comparison of Zn recoveries in Ca(OH)2 system and Na2CO3 system. Experimental conditions: pH = 8.5–8.8, NaCl concentration = 3 mol/L, S/L ratio = 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm.
According to the XRD results of the precipitates generated by the addition of BaCl2 to the acidized leachate (Fig. 8, almost all peaks in line with the standard peaks of BaSO4) and the residue (Fig. 9), it is confirmed that SO24 − was an existing form of S from sphalerite, and Zn would precipitate in the form of Na2Zn3(CO3)4. Thus, the possible oxidation route of sphalerite is given as Eq. (10). þ − 2− 12ClO− þ 3ZnS þ 4CO2− 3 þ 2Na ¼ 12Cl þ 3SO4 þ Na2 Zn3 ðCO3 Þ4 ↓
ð10Þ The purpose of the three parallel experiments in Table 1 is to investigate the possibilities of other existing forms of S through analyzing the relationships among the electric quantity, the mass of Zn recovered and that of BaSO4. In Table 1, the ‘Zn recovered-theoretical’ column refers to the theoretical mass of Zn recovered based on electric quantity, the assumption that Eqs. (6) and (8) are the exclusive anodic reaction and cathodic reaction respectively and Eq. (10). The ‘Zinc recovered-actual’ column refers to the actual mass of Zn recovered in the experiments. As shown in Table 1, the theoretical values are still much smaller than the actual ones was although with 100% current efficiency, which suggests that SO2− 4 not the only existing form and there should be other forms of S in lower valence state. The ‘mass of BaSO4-theoretical’ stands for the theoretical mass of BaSO4 calculated according to the actual mass of Zn recovered the mole of which should be equal to that of BaSO4 obtained was the only existing form, while ‘mass of BaSO4-actual’ repreif SO2− 4 sents the actual mass of BaSO4 obtained. The relevant data indicate that the former were distinctly bigger than the latter, suggesting again that proportion’ column rethere were other existing forms of S. The ‘SO2− 4 fers to the ratio of S based on the actual mass of BaSO4 to the total S the mole of which is equal to that of Zn recovered in experiments. The rewas the major but not exclusive existing form sults indicate that SO2− 4 of S from sphalerite. Fig. 9 demonstrates no elemental sulfur was found in the residue, and the possible reason was that its quantity was too small to be found compared to other components. Hence, to convert and remove as much ZnS as possible in the residue to make sulfur stand out in the XRD result, a single experiment was performed with the reaction time = 5 h. XRD result of the acidized residue in Fig. 10 confirms the existence of sulfur, suggesting that sulfur and sulfate were the main existing forms of S after the electro-oxidation of sphalerite in weak alkaline sodium chloride solution. The continuing generation of Na2Zn3(CO3)4 meant the demand for CO2− 3 , therefore it was necessary to add Na2CO3 to the electrolyte conin the electrolyte, Zn stantly in the process. Otherwise with little CO2− 3
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would precipitate as Zn(OH)2 and then the extra consumption of OH− would make the electro-oxidation system move toward acidification. Eq. (10) indicates that SO24 − would accumulate in the electrolyte, which may be not beneficial to the recycling of electrolyte. In addition, Na2Zn3(CO3)4 precipitate is not the suitable source for Zn electrowinning process. Hence, Ca(OH)2 was adopted to replace Na2CO3 in the and Zn2+ (Eq. (11)). following experiments to precipitate SO2− 4 4ClO− þ ZnS þ CaðOHÞ2 ¼ 4Cl− þ CaSO4 ↓ þ ZnðOHÞ2 ↓
ð11Þ
3.9. Electro-oxidation of sphalerite concentrate in Ca(OH)2 system Fig. 11 shows that the electro-oxidation rate of sphalerite concentrate in Ca(OH)2 system was slightly higher than that in Na2CO3 system despite the fact that the maximum percentages of Zn recovered in both systems were similar. As shown in Fig. 12, the addition of Ca(OH)2 rather than Na2CO3 to the electrolyte indeed made SO24 − precipitate as CaSO4. However, the main existing form of Zn was ZnO instead of Zn(OH)2 besides ZnS according to the XRD result. The possible reason is that Zn(OH)2 could be decomposed to ZnO as the residue was dried at 80 °C before XRD analysis. Hence, with using Ca(OH)2 to maintain the weak alkaline reaction environment, the electrolyte can be recycled accumulating and Zn(OH)2 obtained is a suitable Zn with little SO2− 4 source for the electrowinning process. 4. Conclusions It has been confirmed that the process parameters, namely pH, NaCl concentration, reaction time, solid/liquid ratio, anodic current density and stirring speed influence Zn recovery to a certain extent during the electro-oxidation of sphalerite concentrate implemented in the weak alkaline sodium chloride solution. The optimized conditions are listed as follows : pH in the range of 8.5–8.8, NaCl concentration = 3 mol/L, reaction time = 1 h, solid–liquid ratio ≥ 0.03 g/ml, anodic current density = 1342 A/m2, stirring speed = 800 rpm. In this process, S from the concentrate will be mainly converted into sulfur and sulfate. In the case where weak alkaline environment is regulated by Na2CO3, Zn from sphalerite will precipitate in the form of Na2Zn3(CO3)4, while Zn will precipitate as Zn(OH)2 with Na2CO3 replaced by Ca(OH)2 and will precipitate as CaSO4 simultaneously. With using Ca(OH)2 inSO2− 4 can be prevented stead of Na2CO3 to regulate the electrolyte pH, SO2− 4 from accumulating in the electrolyte which will be recycled and Zn(OH)2 obtained is suitable for the electrowinning process. Acknowledgments The authors are thankful to the Open-End Fund for the Valuable and Precision Instruments of Central South University (CSUZC201504), the
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