Trans. Nonferrous Met. Soc. China 24(2014) 893-899

Anodic behavior and microstructure of Al/Pb-Ag anode during zinc electrowinning

Yong-chun ZHANG¹, Bu-ming CHEN¹, Hai-tao YANG¹, Zhong-cheng GUO^1,2, Rui-dong XU¹

1. Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China;

2. Kunming Hendera of Science and Technology Co. Ltd, Kunming 650106, China

Received 6 March 2013; accepted 5 November 2013

Abstract: Anodic behaviors and oxygen evolution kinetics of Pb-0.8%Ag and Al/Pb-0.8%Ag anodes during the initial 24 h zinc electrowinning were investigated with cyclic voltammetry (CV) curves and electrochemical impedance spectroscopy (EIS). The results reveal that the anodic behaviors and reaction kinetics of the two anodes vary a lot during the anodic polarization which indicate the formation and stabilization of anodic layer. Compared with conventional Pb-0.8%Ag anode, Al/Pb-0.8%Ag anode has longer time of anodic polarization. At the very beginning of anodic polarization, the two anodes all exhibit higher potential of oxygen evolution reaction (OER) since the reaction is controlled by the transformation step of intermediates. Then, its OER potential is largely diminished and OER rate is deduced from the formation and adsorption of the first intermediate (S-OH_ads). In the prolonged anodic polarization, the anodic potential of Al/Pb-0.8%Ag gradually decreases and the final value is more stable than that of conventional Pb-0.8%Ag anode. On the anodic layer after 24 h of anodic polarization, the OER potential is controlled by the formation and adsorption of intermediate. The microstructures of Al/Pb-0.8%Ag and Pb-0.8%Ag anodes after 24 h of anodic polarization were analyzed by scanning electron microscope (SEM).

Key words: zinc electrowinning; oxygen evolution kinetics; electrochemical impedance spectroscopy; oxygen evolution reaction potential

1 Introduction

The demand for energy-saving and higher standard of cathode zinc depends on the lower OER potential and corrosion rate of anodes used in zinc electrowinning. Lead based anodes have been used in the zinc electrowinning for more than a century due to their low cost and availability. However, the conventional lead based anodes operate at relatively high oxygen over-potential, which results in significant energy consumption, and suffer from corrosion which takes place at a slow rate during zinc electrowinning. Then researches [1-5] point that amount of silver (0.2%-1.0%) alloyed with lead decreases the oxygen overvoltage and increases the corrosion resistance of anodes used in zinc electrowinning. The polished surface of the Pb-0.69% Ag alloy anode has the lowest over-potential followed closely by that of Pb-0.6%Ag and Pb-0.58%Ag alloys anodes, showing that properties of conventional Pb-Ag anodes depend on their Ag content. The shape of anodes also can influence their properties. For instance, compared with conventional Pb-Ag anodes, porous Pb-Ag anodes save energy during initially zinc electrowinning [6,7], but their performances will be decreased when the non-conducting phases of MnO₂ and PbSO₄ are filled in their holes during zinc electrowinning. In addition, Ca, Bi, Co²⁺ and Mn²⁺ also can change the performances and anodic behaviors of conventional Pb-Ag anodes during zinc electrowinning [8-10].

Pb/Pb-Co₃O₄ [11-16] and Pb/Pb-MnO₂ [17-19] composite anodes have higher electro-catalytic activity and corrosion resistance for copper and zinc electrowinning. But the Co₃O₄ and MnO₂ particles are segregative during the preparation process of Pb-Co₃O₄ and Pb-MnO₂ composite coatings. So, the anodic processes are heterogeneous on the anodes during metal electrowinning, leading to shorter service time. WC, PANI, PbO₂ and CeO₂ particles which have been electrodeposited with Pb on aluminum-matrix are used as anodes in zinc electrowinning. Despite their higher corrosion resistance and electro-catalytic activity, Al/Pb-PANI-WC [20] and Al/α-PbO₂-CeO₂-TiO₂ anodes [21] are still the new anode materials in the laboratory. PANI and WC particles are non-uniform in composite anodes for segregation during the preparation of Al/Pb-PANI-WC and Al/α-PbO₂-CeO₂-TiO₂ composite anodes. PANI/WC anode prepared by pressing is easy to spit and peel off during the anodic polarization, leading to shorter service time. α-PbO₂-CeO₂-TiO₂ composite coating is also easy to peel off from the aluminum matrix due to the characteristics of α-PbO₂.

Aluminum has a lower cost, density and electrical resistivity than lead, which can possibly reduce the bath voltage and mass of aluminum-based anodes for zinc electrowinning.

In this work, Al/Pb-0.8%Ag anode was prepared by electrodeposition from a synthetic lead methanesulfonate electrolyte. These researches present the results of a study which aims to extend the new work and utilize some of the benefits of Al-matrix and Pb-0.8%Ag composite coating. Therefore, the goal is to develop a new method for preparation of Pb-0.8%Ag composite anode which can offer improved performance in zinc electrowinning.

2 Experimental

2.1 Process method

The alloy materials were obtained by electro- deposition on 30 mm×40 mm aluminum plates from a synthetic lead methanesulfonate electrolyte consisting of 0.5-2 g/L organic additives, 100-200 g/L Pb(CH₃SO₃)₂ and 0.5-1 g/L AgCH₃SO₃. The electro deposition was carried out at pH values in the range of 1.5-2 at a cathode current density J_c=1 A/dm² with mechanical bubbling for 24 h at 40 °C.

The aluminum-matrix was pretreated by buffing (mechanical buffing with fiber grinding wheel), degreasing (soak in fatty alcohol-polyoxyethylene ether degreaser at 40 °C for 4-12 h), caustic wash (soak in 12 g/L NaOH solution at 20-40 °C for 120-180 s), acid pickling (38 g/LH₂SO₄ at 20-40 °C for 10-30 s), zinc immersion (soak in 12 g/L NaOH, 15 g/L ZnO, 10 g/L KNaC₄H₄O₆·4H₂O, 2 g/L FeCl₃×6H₂O solution at 20-40 °C for 60 s), acid pickling (3-10 s), zinc immersion (at 20-40 °C for 50 s) and nickel electroplating for 1 min.

2.2 Measurement and analysis

The electrochemical workstation (CHI760C) with three-electrode systems was used for measuring cyclic voltammetry curves and EIS characterization for Al/Pb-0.8%Ag and conventional Pb-0.8%Ag anodes in the synthetic zinc electrowinning electrolyte consisting of Zn²⁺(50 g/L), H₂SO₄ (150 g/L) and Mn²⁺ (5 g/L). The counter electrode was platinum plate and the reference electrode was Hg/Hg₂SO₄ electrode (SSE) with a potential φ_SSE=+0.64 V. The two anodes were used as the working electrode. In addition, SEM and XRD were used for determining surface microstructure and phase of the two anodes after 24 h anodic polarization.

The anodic polarization was performed at 500 A/cm² in a glass at a constant temperature of 35 °C. The CV scanning rate was 30 mV/s and the range of CV scanning was from -1.2 to 2.0 V. The frequency range of EIS measuring was 0.01-10 kHz and the applied anodic potential for EIS measuring was 1.85 V.

3 Results and discussion

3.1 Cyclic voltammetry curves

As shown in Figs. 1 and 2, cyclic voltammetry curves of conventional Pb-0.8%Ag (Fig. 1) and Al/Pb-0.8%Ag (Fig. 2) anodes after 0, 12 and 24 h anodic polarization are nonreversible, so Eq. (1) obtained by BARD and FAULKNER [22] can be used in this case. Classic Butler-Volmer kinetics model which is necessary before equations can be derived for most electrochemical approaches apply in this part, then a plot of lnk_f(φ) versus φ should be line with a slope (-αF/RT). Based on researches of Refs. [23,24] and Frank-Condon principle, Eq. (2) can be obtained and α depends on potential in a particular way.

                 (1)

                          (2)

where k_f(φ) is the potential-dependent rate constant as a function of potential φ; φ′ is an electrode versus a reference at different scanning speed v; k_f is the heterogeneous rate constant for reduction; D_o is the diffusion coefficient of O; I(t) is the current semi-integral; I₁ is the limit value of I(t); i(t) is the experimental current; F is the faraday constant; E′ is the electron energy,  E^Θ is the standard potential of an electrode or couple; α is the transfer coefficient.

Fig. 1 Cyclic voltammetry curves of conventional Pb-0.8%Ag anode after 0, 12 and 24 h of anodic polarization

Fig. 2 Cyclic voltammetry curves of Al/Pb-0.8%Ag anode after 0, 12 and 24 h of anodic polarization

Reorganization energy for electron transfer λ can be obtained as follows:

                                  (3)

where λ_i is the contribution from reorganization of species O; and λ₀ is the contribution from reorganization of the solvent.

Considering electrode reaction and ignoring self-exchange reaction and λ_i, equation can be obtained as follows [22]:

                                  (4)

where  is the standard Gibbs free energy of forward action.

In this case, i(t) is probably related to the pretreatment of the aluminum matrix and the type of additives in the electrolyte used for the preparation of composite coatings, and D_o is probably related to the microstructure of anodic layer of the two anodes during anodic polarization. The CV scanning rates for Pb-0.8%Ag and Al/Pb-0.8%Ag anodes are all 30 mV/s. The higher electrocatalytic activity can promote the anode reaction, and the OER potential and  of anode reaction decrease. So, α increases and -αF/(RT) decreases.

As shown in Fig. 1, the curves of Pb-0.8%Ag anode after 0, 12 and 24 h of anodic polarization show two anodic peaks and two cathodic peaks [25,26]. The first peak A is produced by the reaction as follows:

                      (5)

A well shape passive region ensues, followed by PbO₂ formation and oxygen evolution peak B. In this peak only β-PbO₂ is formed, as shown by absence of a divided reaction peak C [27]. The division of the peak can be attributed to the presence of two phases, α-PbO₂ and β-PbO₂. After 12 h of anodic polarization (curve 2), the lead sulphate area of peak A is the shifting of this peak to positive potential values, but that of peak B is the shifting to negative potential values. So, the area from lead sulphate to β-PbO₂ is decreased. After 24 h of anodic polarization (curve 3), the area from lead sulphate to β-PbO₂ is narrower than curve 2. At the reverse scanning, the potential values of peaks C and D are the shifting of this peak to more positive with an increase of anodic polarization time, the reduction peaks C and D are also increased by a higher peak and increase the width of the passive zone, leading to the longer time of anodic polarization.

As shown in Fig. 2, anodic behaviors of Al/Pb-0.8%Ag anode are similar to Pb-0.8%Ag anode after 0, 12 and 24 h of anodic polarization. With the increase of anodic polarization time, the peak A is the shifting of this peak to positive potential values and peak B is the shifting to negative potential values, leading to narrower area from lead sulphate to β-PbO₂. At the reverse scanning, the potential values of peaks C and D are the shifting of this peak to more positive, the reduction peaks C and D are also increased by a higher peak and increase the width of the passive zone, leading to the longer time of anodic polarization. However, the peak B area of Al/Pb-0.8%Ag anode is narrower than Pb-0.8%Ag anode, which is due to its lower oxygen evolution reaction potential.

As shown in Fig. 3, Al/Pb-0.8%Ag anode has longer time of anodic polarization than Pb-0.8%Ag anode after 24 h of anodic polarization in lower potential zone, which is probably due to the different surface resistances. The surface resistance is probably related to the β-PbO₂ phase and microstructure of the two anodes.

Fig. 3 Cyclic voltammetry curves of Pb-0.8Ag and Al/Pb- 0.8%Ag anodes after 24 h of anodic polarization

3.2 EIS characterization

Figure 4 shows the typical potential-time curves of Pb-0.8%Ag (curve 1) and Al/Pb-0.8%Ag (curve 2) anodes during the beginning 24 h of anodic polarization. Al/Pb-0.8%Ag anode exhibits excellent electro-catalytic activity towards oxygen evolution reaction during the beginning 24 h of anodic polarization. The activity diminishes largely due to the change of the surface state for new microstructure and phase. In the prolonged anodic polarization, the anodic potential gradually decreases and the potential is lower than Pb-0.8%Ag anode after 24 h of anodic polarization. The whole anodic polarization process can be expressed by the following formation of PbSO₄ and PbO₂ phases. The two anodes have different anodic behaviors during 24 h of anodic polarization which can be studied with EIS. As shown in Fig. 5, the new phase β-PbO₂ is found on the surface of the two anodes after 24 h of anodic polarization. So, the conclusion can be deduced that β-PbO₂ phase has been produced during the initial 24 h of anodic polarization.

Fig. 4 Time-potential curves of Pb-0.8%Ag and Al/Pb-0.8%Ag anodes during 24 h anodic polarization

Fig. 5 XRD patterns of Pb-0.8%Ag and Al/Pb-0.8%Ag anodes after 24 h of anodic polarization

The oxygen evolution reaction steps are generally proposed as the mechanism of OER on active metal oxides electrode [28,29]. The oxygen evolution reaction by decomposition of water in acid medium can be described as follows:

                      (6)

                 (7)

                 (8)

               (9)

                          (10)

where S stands for active site on oxide surface, S-OH_ads and S-O_ads are adsorption intermediates. Reaction (8) is normally considered to be obeyed by compact morphology and reaction (9) is obeyed by cracked morphology. In addition, reaction (10) generates O₂. According to the mechanism discussed above, the oxygen evolution reaction is kinetically controlled by Eq. (1) through reactions (7)-(10) during 24 h anodic polarization. Based on the faradic impedance Z_f of such irreversible electrode reactions with one state variable besides electrode potential can be written as follows:

                        (11)

where R_t is the charge transfer resistance of the electrode reaction and always has a positive value; R_a and C_a are the equivalent resistance and pseudo-capacitance associated with adsorption of intermediate (S-OH_ads), respectively. As shown in Fig. 6, C_dl is the double-layer capacitance and R_s is the uncompensated solution resistance. By theoretical calculation, R_fC_f represents the capacitive response of oxide layer, R_f and C_f are the resistance and capacitance of the oxide layer, respectively. In the simulation, constant phase elements (CPEs) are used instead of capacitors, C_dl and C_a, to fit the experimental data. The CPE can obtain a good approach to the surface roughness, physical non-uniformity or the non-uniform distribution of surface reaction site. The impedance CPE can be written as

                             (12)

where Q is the capacity parameter and n is accounting for the deviation from the ideal behaviors. Moreover, the double layer capacitance C_dl is coupled with the uncompensated solution resistance R_s and the charge transfer resistance R_t according to the equation as follows [30]:

               (13)

The simulated EIS patterns and parameters of Pb-0.8%Ag and Al/Pb-0.8%Ag anodes are shown in Figs. 6 and 7. In the simulation of those spectra with one capacity arc, it shows that the simulated and experimental data can reach a good agreement by the proper circuit in Fig. 8. Such phenomenon may be explicable below. It has been wildly proposed that the formation of anodic layer on lead-based anode is mainly according to the following two processes: 1) Pb→PbSO₄→β-PbO₂; 2) Pb→PbSO₄→α-PbO₂→β-PbO₂. Therefore, during the formation process of anodic layer, considerable layer resistance exists probably due to non-conducting PbSO₄ and α-PbO₂. As listed in Table 1, the charge transfer resistance R_t only takes little part of the whole resistance in this case, while the film resistance R_f and adsorption resistance R_a dominantly determine the final resistance values. In general, the sum of R_f and R_a values of the two anodes largely decrease during anodic polarization, indicating that the adsorption of intermediate becomes easier with the PbO₂ layer. The film resistance of Pb-0.8%Ag anode is larger than Al/Pb-0.8%Ag anode. Moreover, the change of Pb-0.8%Ag anode adsorption resistance is different from that of Al/Pb-0.8%Ag anode adsorption resistance during the 24 h of anodic polarization. It also has the lowest adsorption resistance on its fruit surface which is due to the different microstructures.

Fig. 6 EIS of Pb-0.8%Ag anode after fitting

Fig. 7 EIS of Al/Pb-0.8%Ag anode after fitting

Fig. 8 Electrical equivalent circuit used to simulate impedance

In addition, the double layer capacitance C_dl is calculated by Eq. (13). As listed in Table 1, the generally increasing C_dl values of the two anodes reveal that the anodic surface becomes more prone to the adsorption of anions by prolonging the time of anodic polarization. The behavior of adsorption pseudo-capacitance during the anodic polarization is evaluated by the Q_dl values obtained in the CNLS fit. Since the pseudo-capacitance represents the coverage of intermediate on the anodic surface, its generally increasing trend during the anodic polarization indicates that the intermediate S-OH_ads can be more easily adsorbed on the stable PbO₂ layer as well. Besides，between the two anodes, Al/Pb-0.8%Ag anode presents larger Q_dl values in latter anodic polarization which is probably due to the thick, micro-porous(as shown in Fig. 9) and β-PbO₂-rich anodic layer, showing higher corrosion resistance and electro-catalytic activity.

Table 1 Equivalent circuit parameters for Pb-0.8%Ag and Al/Pb-0.8%Ag anodes

Fig. 9 SEM images of Pb-0.8%Ag anode after 24 h of anodic polarization (a) and Al/Pb-0.8%Ag anode before (b) and after (c) 24 h of anodic polarization

As shown in Fig. 9, the SEM images of Pb-0.8%Ag anode after 24 h of anodic polarization and Al/Pb- 0.8%Ag anode before and after 24 h of anodic polarization are shown clearly. The microstructure of Al/Pb-0.8%Ag anode after 24 h of anodic polarization (Fig. 9(c)) is characterized by rectangular crystallites of PbSO₄, possessing an order orientation and concentrating. The microstructure of Pb-0.8%Ag anode after 24 h of anodic polarization (Fig. 9(a)) is characterized by irregular crystallites of PbSO₄, possessing a uniform tetragonal symmetry and chaotic orientation [24] which is probably conducive to the diffusion of O₂. The results above are probably due to the microstructure of Al/Pb- 0.8%Ag anode (see Fig. 9(b)) and its aluminum-matrix.

4 Conclusions

1) Compared with conventional Pb-0.8%Ag anode, Al/Pb-0.8%Ag anode has longer time of anodic polarization, and the microstructure on anodic layer is characterized by rectangular crystallites of PbSO₄, processing an order orientation after 24 h anodic polarization which is probably due to the preparation method for Al/Pb-0.8%Ag anode and its aluminum- matrix.

2) At the beginning of anodic polarization, Al/Pb-0.8%Ag anode firstly presents fairly high OER potential controlled by the transformation of intermediate due to the existence of non-conducting phases of MnO₂ and α-PbO₂. After 24 h anodic polarization, the film impedance nearly disappears and the reaction resistance is dominantly composed by the adsorption resistance. The adsorption resistance and pseudo-capacitance of the Al/Pb-0.8%Ag anode are much smaller and larger than those of Pb-0.8%Ag anode, respectively. Such phenomenon can be well interpreted by XRD analysis on these stable anodic layers, which show that Al/Pb-0.8%Ag anode presents obviously more conducting and active β-PbO₂ phase on the anodic layer.

3) Compared with conventional Pb-0.8%Ag anode, Al/Pb-0.8%Ag anode has lower surface resistance and larger Q_dl due to the more β-PbO₂ phase, showing higher corrosion resistance.

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Al/Pb-Ag阳极在锌电积过程中的阳极行为和微观结构

张永春¹, 陈步明¹, 杨海涛¹, 郭忠诚^1,2, 徐瑞东¹

1. 昆明理工大学 冶金与能源工程学院，昆明 650093；2. 昆明理工恒达科技有限公司，昆明 650106

摘  要：采用循环伏安曲线(CV)和电化学交流阻抗谱(EIS)研究Pb-0.8%Ag 和Al/Pb-0.8%Ag阳极在最初24 h锌电积过程中的阳极行为和析氧动力学。结果表明：2种阳极材料在阳极极化过程中的阳极行为和动力学的多种变化表明阳极表面膜的形成和稳定。与传统的Pb-0.8%Ag阳极相比，Al/Pb-0.8%Ag阳极具有较长的极化时间。 在最初的电积过程中，阳极表面中间产物的转变使2种阳极都表现出较高的析氧电位。随着析氧电位降低，析氧反应速率可以由阳极表面最初的生成物和中间产物S-OH_ads推断。随着电积的持续进行，Al/Pb-0.8%Ag阳极的析氧电位逐渐降低，Al/Pb-0.8%Ag阳极比Pb-0.8%Ag 阳极先达到稳定状态。电积24 h后，阳极电位由阳极表面的生成物和中间产物控制。采用扫描电子显微镜观察Al/Pb-0.8%Ag 和Pb-0.8%Ag阳极锌电积24 h后的微观形貌。

关键词：锌电积；析氧动力学；电化学交流阻抗；析氧电位

(Edited by Chao WANG)

Foundation item: Project (51004056) supported by the National Natural Science Foundation of China

Corresponding author: Zhong-cheng GUO; Tel: +86-871-8352598; Fax: +86-871-8352599；E-mail: guozc012@163.com

DOI: 10.1016/S1003-6326(14)63140-X