Effect of impurities in recycling water on Pb-Ag anode passivation in zinc electrowinning process
LIU Hui1, WANG Yun-yan1, CHAI Li-yuan1, XIAO Hai-juan1, 2, PEI Fei1, SHU Yu-de1
1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083 China;
2. Changtian International Engineering Corporation, MCC, Changsha 410008, China
Received 16 July 2010; accepted 12 December 2010
Abstract:
Effect of impurities in recycling water on Pb-Ag anode passivation in zinc electrowinning process was investigated by linear scan voltammetry. Results show that passivation process would be affected in the presence of Cl- and F- in recycling water. It was highly advantageous to take H2SO4 concentration as 180 g/L, Mn2+ concentration as 3-5 g/L and F- less than 42 mg/L. However, passivation process would not be affected when Cl- concentration was less than 13 mg/L without any other ions, or when mass ratio of Mn2+ to Cl- existing in electrolyte was 8, where Cl- concentration could reach up to 625 mg/L.
Key words:
zinc electrowinning process; Pb-Ag anode; passivation; impurities; recycling water;
1 Introduction
Nonferrous metals industry shares huge water consumption in China [1-2]. A large amount of acidic sulfate wastewater containing heavy metals from non-ferrous metal industry was discharged, which would lead to the lavishness of water resource and seriously affect safety of residential drinking water due to persistent pollution of toxic heavy metals [3-5].
Although water consumption per ton of product has presently been declined in non-ferrous metal smelter in China, water recycling ratio is still less than 85%. The only way to solve this problem is to improve the recycling ratio of purified water and to achieve zero discharge of wastewater [6]. Various methods have been developed to treat heavy metal-containing wastewater. For example, limestone neutralization process is conventionally applied for its advantages such as low investment, mature technique, simple and convenient operation. However, the purified water produced by this method may result in some problems for high concentrations of Ca2+, SO42-, F-, Cl- and so on, which would influence and limit the reutilization of industrial wastewater [7-8].
There are many parameters which influence Pb-Ag anode passivation in zinc electrowinning process, such as substrate metal, contents of Ag, Ca and Sr in anode, component of electrolyte, potential, current density, temperature and pretreatment of electrode [9-11]. Components of electrolyte, such as concentrations of H2SO4, Mn2+ and Cl-, are decisive factors because they affect process of Pb-Ag anode passivation and performance of passivation layer directly [12-14]. Impurities in reused water may influence zinc electrowinning process, and then lead to decreasing quality of zinc. Therefore, it is of great significance to establish main indexes for recycling process of purified water. For example, after heavy metal-containing wastewater from Zhuzhou Smelter Group, Hunan province, China, was treated by biologics, which was exploited by our research group, concentrations of Zn, Cd, Pb, Cu and As of purified water reached Water Quality Standard for Drinking Water Sources (CJ 3020—93). However, removal of main anions including SO42-, Cl- and F- was not so remarkable, which may result in another problem: purified water with chloride ions and fluoride ions may affect Pb-Ag anode passivation process when reused to zinc electrowinning process. Therefore, it is of considerable interest to investigate and establish main indexes of Pb-Ag anode passivation in zinc electrowinning process.
Objective of this work is to investigate effect of impurities, such as concentrations of H2SO4, Mn2+, Cl- and F-, on passivation process of Pb-Ag anode in zinc electrowinning process.
2 Experimental
2.1 Electrolytes
Reagents used in this study included H2SO4, ZnSO4·7H2O, KCl, NaOH, MnSO4·H2O, NaCl and NaF, and electrolytes were prepared from analytical grade chemicals and double-distilled water.
2.2 Experiment of linear scan voltammetry
Linear scan voltammetry was carried out in a three-electrode system consisting of a lead-silver electrode as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum sheet (1 cm×1 cm) as counter electrode. Open circuit potential—time curves were recorded before experiments of linear scan voltammetry.
Lead-silver electrode was prepared in laboratory. Lead-silver wire was collected from Zhuzhou Smelter Group, Zhuzhou City, Hunan Province, China. In brief, a lead-silver wire with diameter of 6 mm was inserted into a glass tube with 15 cm in length and 2 cm in diameter. Epoxy resin adhesive was filled into the glass tube to fix lead-silver wire. Circular cross-section of one end of lead-silver wire exposed outside and the other end extended. After 24 h, the exposed surface of the lead-silver wire was polished with 12 μm-abrasive paper, dipped in acetone for 30 min and washed with double-distilled water, and the final product was regarded as working electrode (Pb-Ag anode).
Experiments were thermostated at 313 K in thermostat water bath. Scan rate was set as 1 mV/s over the potential range from stable potential to 2.0 V, while stable potential was determined by open circuit potential—time curves. Potentials described in this study were all recorded vs SCE.
2.3 Effect of different indexes on Pb-Ag anode passivation process
Effect of H2SO4 concentration on Pb-Ag anode passivation process was investigated using ZnSO4 solution with a zinc concentration of 50 g/L as base electrolyte. H2SO4 concentrations were adjusted to 0, 50, 100, 150, 180 and 210 g/L, respectively.
Effect of Mn2+ concentration on Pb-Ag anode passivation process was studied on the basis of H2SO4-ZnSO4 solution, in which concentration of Zn2+ was 50 g/L, concentrations of H2SO4 were 50 g/L and 180 g/L, respectively. Mn2+concentrations were 0, 0.5, 1.5, 3, 4 and 5 g/L, respectively.
Effects of Cl- concentration, Mn2+/Cl- mass ratio and F- concentration on Pb-Ag anode passivation process were investigated on the basis of H2SO4-ZnSO4 solution, in which concentration of Zn2+ was 50 g/L, H2SO4 was 180 g/L. Conditions of experiments were controlled as follows: Cl- concentrations were controlled as 0, 13, 105, 203, 253, 302 and 407 mg/L; Mn2+ concentrations were 3.0 g/L; Mn2+/Cl- mass ratios were 1, 3, 5, 8 and 10; F- concentrations were 0, 24, 42, 52 and 64 mg/L.
3 Results and discussion
3.1 Effect of H2SO4 concentration on Pb-Ag anode passivation process
3.1.1 Effect of H2SO4 concentration on open circuit potential-time curves
Effect of H2SO4 concentration on open circuit potential—time curves of Pb-Ag anode can be sketched in Fig. 1. Changes of potential could be classified into three stages: stable stage, unstable stage and stable stage. Corrosion potential of Pb-Ag anode shifted negatively with the gradual increase of H2SO4 concentration, while the trend of negative shift became smoother at higher H2SO4 concentrations. The reason was that the whole system could be regarded as a corrosion cell when the applied current was zero. In anode of corrosion cell, following reactions occurred at higher H2SO4 concentrations [15]:
H2SO4→H++HSO4- (1)
Pb+HSO4- 
PbSO4+H++2e (2)
Anode surface was separated from the solution with PbSO4, which covered Pb-Ag anode surface owing to its low solubility in H2SO4 solution. Coverage of PbSO4 on anode surface increased with reaction time and when Pb-Ag anode surface was completely covered by PbSO4, corrosion process reached stable state, that is to say, the corrosion potential was stable.
Fig. 1 Open circuit potential—time curves of Pb-Ag anode at different H2SO4concentrations
3.1.2 Effect of H2SO4 concentration on anodic polarization curves
Anodic polarization curves of Pb-Ag anode at different H2SO4 concentrations are depicted in Fig. 2, and the fitting parameters are listed in Table 1. It appears clearly that anode polarization curves can be divided into four regions. From corrosion potential (φcorr) to passivating potential (φp) is the first region named as active dissolution region, in which dissolution current of Pb-Ag anode increased with potential. In the second region, from φp to activation potential (φF), which is named as transition region, condition on Pb-Ag anode surface was rapidly changed and anodic current decreased sharply from φP. The third region began from φF. In this region Pb-Ag anode surface was in the state of passivation and dissolution current was very small. Thus, it is named as passivation region. And then when potential was over passivation potential (φtr), it came into the fourth region (over passivation region). This region has characteristics similar to the first region, that is, current increased with the enhancement of potential.
Decrease of current in Fig. 2 indicates that PbSO4 crystal formed and grew rapidly on anode surface, and then metal was oxidized gradually in the sulfate layer. Owning to high over potential generated by oxygen evolution on surface of PbO2 layer, when potential was less than 
, the main reaction was the formation of intermediary, such as PbSO4 crystal and PbO2. When potential was more than, metals oxidation and oxygen evolution took place synchronously. With the decrease of unoxidized anode surface area, oxidation rate of the anode decreased, while oxygen evolution rate and the anodic polarization current increased with the reaction time, when anodic potential was high enough, current density could be used to express the oxygen evolution rate.
Fig. 2 Anodic polarization curves of Pb-Ag anode at different H2SO4concentrations
Figure 2 and Table 1 also show that when H2SO4 concentration changed from 0 to 100 g/L, φp and φF shifted negatively, while φtr shifted positively, so the passivation region extended. Additionally, Imax and Ip increased gradually. However, when the concentration of H2SO4 was more than 100 g/L, φP and φF turned to shift positively, which in turn shortened the passivation region, yet Imax and IP increased continuously. When the concentration of H2SO4 was 180 g/L, φP, φF and IP reached their minimums, while φtr and Δφ reached their maximums; values of φP, Imax, φF, φtr and Δφ were -0.605 8 V, 0.097 2 mA, -0.493 9 V, 1.129 8 V and 1.623 7 V, respectively. The region of maintaining passivating current was in the domain 10.3-58.8 μA.
3.2 Effect of Mn2+ concentration on Pb-Ag anode passivation process
3.2.1 Effect of Mn2+ concentration on open circuit potential—time curves
Open circuit potential—time curves of Pb-Ag anode at different Mn2+ concentrations are depicted in Fig. 3 and Fig. 4. Corrosion potential exhibited a negative shift with the increase of Mn2+ concentration. When H2SO4 concentration increased from 50 g/L to 180 g/L, the negative shift value of corrosion potential increased. The corrosion principles of Pb-Ag anode in electrolyte with or without Mn2+ were similar. When SO42- concentration increased, the following equation showed left shift:
(3)
So, HSO4- concentration in electrolyte increased, which is beneficial to Pb-Ag corrosion.
3.2.2 Effect of Mn2+ concentration on anodic polarization curves Figures 5 and 6 show anodic polarization curves of Pb-Ag anode at different Mn2+ concentrations, and corresponding fitting parameters are listed in Table 2 and Table 3.
Table 1 Fitting parameters of anodic polarization curves of Pb-Ag anode at different H2SO4 concentrations
Fig. 3 Open circuit potential—time curves of Pb-Ag anode at different Mn2+ concentrations (ρ(H2SO4)=50 g/L)
Fig. 4 Open circuit potential—time curves of Pb-Ag anode at different Mn2+ concentrations (ρ(H2SO4)=180 g/L)
When Mn2+ existed in electrolyte, φF showed little change, φtr shifted negatively and IP increased slightly (Fig. 5 and Fig. 6). Comparison between Fig. 5 and Fig. 6 reveals that when Mn2+ concentration was in the range of 1.5-3.0 g/L and H2SO4 concentration increased from 50 g/L to 180 g/L, fitting parameters showed following change under the same concentration of Mn2+: φP shifted negatively, φF and φtr shifted positively, Imax increased, passivation region Δφ became wide and IP decreased. Thus, H2SO4 concentration in this work was taken as 180 g/L, that is, H2SO4/Zn2+ mass ratio was 3.6.
When H2SO4 concentration was set as 180 g/L and Mn2+ concentration changed from 0.5 g/L to 3.0 g/L, φp, Imax, φF, φtr, Δφ and Ip had little change. However, φP and φF shifted negatively while φtr shifted positively, Imax and IP decreased, and passivation region Δφ widened when Mn2+ concentration was higher than 3.0 g/L. Consequently, the concentration of Mn2+ was chosen as 3-5 g/L in electrolyte.
Fig. 5 Anodic polarization curves of Pb-Ag anode at different Mn2+concentrations (ρ(H2SO4)=50 g/L)
Fig. 6 Anodic polarization curves of Pb-Ag anode at different Mn2+concentrations (ρ(H2SO4)=180 g/L)
Table 2 Fitting parameters of anodic polarization curves of Pb-Ag anode at different Mn2+ concentrations (ρ(H2SO4)=50 g/L)
Table 3 Fitting parameters of anodic polarization curves of Pb-Ag anode at different Mn2+concentrations (ρ(H2SO4)=180 g/L)
3.3 Effect of Cl- concentration on Pb-Ag anode passivation process
3.3.1 Effect of Cl- concentration on open circuit potential—time curves
Open circuit potential—time curves of Pb-Ag anode at different Cl- concentrations are shown in Fig. 7. The negative shift of corrosion potential with increase of Cl- concentration made passivation process more complex. When Cl- concentration was higher than 302 mg/L, the time for reaching steady state was shortened. There was an obvious competition on activate adsorption points between Cl- and HSO4-; moreover, Cl- accelerated the corrosion process after contacting Pb-Ag anode surface via the surface void of PbSO4, and the formed PbCl2 precipitate was absorbed on Pb-Ag anode surface and made system stable.
Fig. 7 Open circuit potential—time curves of Pb-Ag anode at different Cl- concentrations
3.3.2 Effect of Cl- concentration on anodic polarization curves
Anodic polarization curves of Pb-Ag anode at different Cl- concentrations are shown in Fig. 8, and fitting parameters are listed in Table 4. When Cl- concentration changed from 0 mg/L to 13 mg/L, φP and φF shifted positively, φtr shifted negatively, Δφ decreased. Imax increased by 42.2 μA, while IP changed little. When Cl- was more than 13 mg/L, φp decreased by 0.295 5 V, φF declined by 0.280 6 V. Moreover, φtr changed by 0.038 2 V, which made Δφ wide. However, Imax increased by 0.295 5 mA, and Ip also increased evidently. Consequently, passivation process would not be affected only under the condition that Cl- concentration was less than 13 mg/L when there was only Cl- existing in electrolyte.
Fig. 8 Anodic polarization curves of Pb-Ag anode at different Cl- concentrations
At present, there are two theories for interpretation of metal passivation process destructed by Cl- ions. One is phase film formation theory: Cl- pushes O2 out of the film or passes through the film void. The other is adsorption theory: adsorption capacity of Cl- is so strong that it takes the place of O2 and is absorbed on electrode surface. Therefore, protection layer of metal oxides would be replaced by non-protective, soluble metal chloride layer. The following reaction occurs:
Pb+2Cl-→PbCl2+2e (4)
CAO [16] considered when Cl- enriched in solution near the partial surface of metal, it would increase absorption of anion on metal surface, decrease ion resistance of passivation layer and destroy protection ability. On the other hand, it would accelerate the dissolution of passivation layer for complex compounds formed between Cl- and metal ions.
3.4 Effect of Mn2+/Cl- mass ratio on Pb-Ag anode passivation process
3.4.1 Effect of Mn2+/Cl- mass ratio on open circuit potential—time curves
Open circuit potential—time curves of Pb-Ag anode at different Mn2+/Cl- mass ratios are shown in Fig. 9. When Mn2+/Cl- mass ratio was less than 3, corrosion potential showed negative shift with the increase of the mass ratio. Corrosion potential and time needed for reaching steady state changed little when Mn2+/Cl- mass ratio was more than 3.
3.4.2 Effect of Mn2+/Cl- mass ratio on anodic polarization curves
Anodic polarization curves of Pb-Ag anode at different Mn2+/Cl- mass ratios are shown in Fig. 10, and fitting parameters are listed in Table 5. It can be seen that when Cl- existed in electrolyte, φP and φF shifted negatively while φtr shifted positively, Δφ widened, Imax and IP were less than those without Cl-.
Fig. 9 Open circuit potential—time curves of Pb-Ag anode at different Mn2+/Cl- mass ratios
Fig. 10 Anodic polarization curves of Pb-Ag anode at different Mn2+/Cl- mass ratios
When Mn2+/Cl- mass ratio increased from 1 to 3, φp, φF and φtr changed positively, Δφ widened. Imax decreased by 0.088 3 mA, while Ip changed little.
When Mn2+/Cl- mass ratio changed in the range of 3-8, φp and φF shifted positively, φtr shifted negatively, Δφ diminished, and there was a minor increase for Imax and Ip.
When Mn2+/Cl- mass ratio was 8, Imax reached the minimum of 0.030 5 mA, while IP and Δφ did not change too much. φP, φF and φtr showed negative shift.
When Mn2+/Cl- mass ratio was more than 8, Imax and IP increased again, φP, φF and φtr increased up to -0.944 5 V, -0.918 9 V and 1.224 5 V, respectively. Δφ increased by 4.7 mV.
Table 4 Fitting parameters of anodic polarization curves of Pb-Ag anode at different Cl- concentrations
Table 5 Fitting parameters of anodic polarization curves of Pb-Ag anode at different Mn2+/Cl- mass ratios
Therefore, the optimal Mn2+/Cl- mass ratio in electrolyte was 8, while Cl- concentration could be 375 mg/L and Mn2+ 3 g/L.
3.5 Effect of F- concentration on Pb-Ag anode passivation process
3.5.1 Effect of F- concentration on open circuit potential—time curves
Open circuit potential—time curves of Pb-Ag anode at different F- concentrations are shown in Fig. 11. The corrosion potential showed negative shift with increase of F- concentration, while the time needed for reaching steady state increased. As F is the most active halogen, corrosion ability of hydrofluoric acid is great in aqueous solution. Metal fluoride formation would accelerate the dissolution of metal. And PbF2 precipitate resulted in formation of a protection layer through being adsorbed on surface of Pb-Ag anode, which made the system stable.
Fig. 11 Open circuit potential—time curves of Pb-Ag anode at different F- concentrations
3.5.2 Effect of F- concentration on anodic polarization curves
Anodic polarization curves of Pb-Ag anode at different F- concentrations are shown in Fig. 12, and fitting parameters are listed in Table 6. φp and φF decreased by 0.211 7 V and 0.195 4 V, respectively, Imax enhanced little, φtr fluctuated gently, and Δφ increased by 0.189 7 V with increasing F- concentration from 0 mg/L to 42 mg/L. However, when F- concentration was 42 mg/L, Imax enhanced to 0.14 mA, Ip decreased by the range of 0.003 0-0.036 6 mA. Further increasing F- concentration, φP and φF showed positive shift, Δφ diminished while Imax and IP increased again. Consequently, F- concentration in electrolyte should be less than 42 mg/L.
Fig. 12 Anodic polarization curves of Pb-Ag anode at different F- concentrations
Corrosion mechanism of passivation layer with F- is in accordant with other halogen ions. Furthermore, due to halogen ion with the smallest radius, F- could achieve stronger activation and permeability, which made the destruction of passivation layer worse. And the damage to the passivation layer was serious when Pb-Ag anode was in passivation region.
4 Conclusions
1) It was highly recommended for passivation process of Pb-Ag anode when the concentration of H2SO4 in electrolyte was 180 g/L, Mn2+ was in 3-5 g/L and F- was less than 42 mg/L.
2) Pb-Ag anode passivation process would not be affected when Cl- concentration was less than 13 mg/L without any other ions or when Mn2+/Cl- mass ratio was 8, while Cl- concentration could reach 625 mg/L.
3) Existence of Cl- and F- in purified water would affect Pb-Ag anode passivation process.
Table 6 Fitting parameters of anodic polarization curves of Pb-Ag anode at different F- concentrations
4) From the viewpoint of Pb-Ag anode passivation process, the purified water with SO42-, F-, Cl- produced by limestone neutralization process could be used to zinc electrowinning.
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回用水中杂质离子对锌电解过程中Pb-Ag阳极钝化的影响
刘 恢1，王云燕1，柴立元1，肖海娟1, 2，裴 斐1，舒余德1
1. 中南大学 冶金科学与工程学院，长沙 410083；
2. 中冶集团 中冶长天国际工程有限责任公司，长沙 410008
摘 要：采用线性扫描伏安法研究了回用水中杂质离子对锌电解过程中Pb-Ag阳极钝化的影响。结果表明：回用水中Cl-和F-的存在将会影响电极的钝化过程。当溶液中H2SO4浓度为180 g/L，即酸锌比为3.6，Mn2+浓度为3~5 g/L，F-浓度小于42 mg/L时有利于Pb-Ag阳极钝化；而当溶液中只存在Cl-时，Cl-浓度必须低于13 mg/L才不会影响电极的钝化过程；当溶液中同时存在Mn2+和Cl-且锰氯质量比为8时，电极的钝化过程不会受到Cl-的影响，此时溶液中Cl-的含量最高，可以达到625 mg/L。
关键词：锌电解；Pb-Ag阳极；钝化；杂质；回用水
(Edited by YANG Hua)
Foundation item: Project (50925417) supported by China National Funds for Distinguished Young Scientists; Project (50830301) supported by the National Natural Science Foundation of China; Project (08JJ3020) supported by the Natural Science Foundation of Hunan Province, China; Project (2008SK2007) supported Key Program of Science and Technology of Hunan Province, China; Porject (2009ZX07212-001-01) supported by Key Project for Water Pollution Control and Management Technology of China
Corresponding author: WANG Yun-yan; Tel: +86-731-88830875; Fax: +86-731-88710171; E-mail: wyy@csu.edu.cn
DOI: 10.1016/S1003-6326(11)60912-6
