Pb-Ag-Co Composite Anode Material for Zinc Electrowinning

Rare Metal Materials and Engineering Volume 43, Issue 12, December 2014 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2014, 43(12): 2889-2892.

ARTICLE

Preparation and Properties of Al/Pb-Ag-Co Composite Anode Material for Zinc Electrowinning Yang Haitao1,2,

Chen Buming1,

Liu Jianhua3,

Guo Zhongcheng1,4,

Zhang Yongchun1,

Xu ruidong1 1

Kunming University of Science and Technology, Kunming 650093, China;

2

State Key Laboratory of Multiphase Complex Systems, Institute

of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; 4

3

Central South University, Changsha 410083, China;

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

Abstract: The Al/Pb-0.3wt%Ag-0.01wt%Co and Al/Pb-0.3wt%Ag composite anode materials were prepared by electrodeposition. The electrochemical properties of the composite anode in 50 g/L Zn2+ and 150 g/L H2SO4 solutions were investigated by anode polarization curves, cyclic voltammetry (CV) curves and Tafel curves. The surface structure was characterized by SEM and EDS. The results show that the cobalt addition can reduce the oxygen evolution potential and improve the corrosion resistance. Compared with cast Pb-0.3wt%Ag, the anodized layer of the Al/Pb-0.3wt%Ag and Al/Pb-0.3wt%Ag-0.01wt%Co are more dense, the grains are finer. Key words: zinc electrowinning; Al/Pb-Ag-Co; anode; electrochemical behavior; microstructure

Over the past few decades, electrowinning had become the preferred method for the final recovery of metals such as copper, zinc, nickel, manganese and cobalt. Nearly 20% of the world production of copper and more than 85% of the world zinc are carried out via hydro-electrometallurgical routes[1]. In the zinc electro-winning industry, the Pb-Ag(0.5~1.0wt%) anode is widely used[2,3]. While, there are still some shortcomings of Pb-(0.5~1.0wt%)Ag anode[4], such as the pollution of the cathode product, noble metal consumption and the poor mechanical properties. Due to these disadvantages of the leadsilver anode, the good characteristics of the anodes have been continued to search for alternative anodes. Cobalt has attracted the most investigations on the performance of lead-based anode. It reveals that Ag could be successfully replaced by cobalt, the anodic layer containing Ag or Co shows a high strength and protects the surface relief after prolonged electrolysis[5]. M. Petrova[6] studied that the

electrochemical and corrosion properties of the ternary and quaternary lead alloys Pb-0.18%Ag-0.012%Co, Pb-0.2%Ag -0.03%Co and Pb-0.2%Ag-0.12%Sn-0.06%Co. It was showed that the cobalt addition could reduce the polarization and improve the corrosion resistance of the alloy, and the maximum concentration of cobalt in cast lead anodes was usually in the range 0.003~0.02 wt%. However, the manufacturing was difficult to control and the process was complicated, which restricted their commercial use[7] . In this paper, the electrodeposition method was employed, aiming to reduce the consumption of silver and cobalt, which are distributed more evenly. The beneficial effect could be achieved by a more finely grained and more homogeneous microstructure. Aluminum was used for matrix because of its good electrical conductivity and mechanical strength. Methanesulfonate system was used as the electroplating liquid which met the environmental requirements.

Received date: May 15, 2014 Foundation item: National Natural Science Foundation of China (51004056); Opening Foundation of Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences (KKZ6201152009); Specialized Research Fund for the Doctoral Program of Higher Education (20125314110011); Applied Basic Research Foundation of Yunnan Province (2010ZC052); Applied Basic Research Key Plans Program of Yunnan Province (2014FA024) Corresponding author: Chen Buming, Ph. D., Associate Professor, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, P. R. China, Tel: 0086-871-68352598, E-mail: [email protected] Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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2

Results and Discussion

2.1 Anodic polarization curves The anodic polarization curves of the Al/Pb-0.3wt%Ag, Al/Pb-0.3wt%Ag-0.01wt%Co and cast Pb-0.3wt%Ag are presented in Fig.1. The relationship curve of potential, E (MSE), and current density (i) was employed in this research. Anodic polarization experiments were carried out at a constant scan rate of 10 mV·s-1 from an initial potential of 1.3 V (MSE) to a final potential of 2.3 V (MSE). The changes in the oxygen evolution potential of the different anodes can be clearly observed in the synthetic zinc electrowinning electrolyte. Over the studied polarization range, the polarization curve of Pb-0.3wt%Ag cast alloy anode sample presents the highest potential of oxygen evolution whereas that of the Al/Pb-0.3wt%Ag-0.01wt%Co anode presents the lowest at a current density of 500 A·m-2(0.05 A·cm-2). As shown in the inset diagram of Fig. 1, compared with the Pb-0.3wt%Ag cast alloy anode, the potentials of oxygen evolution of the Al/Pb-0.3wt%Ag-0.01wt%Co and Al/Pb-0.3wt%Ag decrease by about 40 mV and 20 mV, respectively, at a current density of 500 A·m-2 (0.05 A·cm-2).

0.14 -2

The composite anode materials were obtained by electrodepositing them onto aluminum plates (20 mm×20 mm×2 mm). The pretreatment of aluminum substrates included degreasing and chemical etching. The plating bath composition and the process conditions of the composite anode material were as follows: 100 g·L-1 (CH3SO3)2Pb, 100 g·L-1 CH3SO3H, 80 g·L-1 C6H5Na3O7, 1 g·L-1 AgNO3, 0.5 g·L-1 CoSO4, 80 g·L-1 NaOH, 0.5 g·L-1 C102H151O39N31, pH=1~2, the cathodic current density Jk=1 A·dm-2, T=25 ℃ and with mechanical agitation. The average thicknesses of Pb-Ag and Pb-Ag-Co composite coatings were all 30 µm. The contents of Ag and Co in the Al matrix were tested by atomic absorption spectrometry (AA-6300C) and visible spectrophotometer (722G). An electrochemical workstation (CS350, Corrtest, China) with three electrode systems was used during measurement of the anodic polarization curves and cyclic voltammetry curves in a synthetic electrolyte of 50 g·L-1 Zn2+ and 150 g·L-1 H2SO4 at 35 °C. The working electrodes were the prepared anode experimental samples (1#: Pb-0.3wt%Ag cast alloy, 2#: Al/Pb-0.3wt%Ag, 3#: Al/Pb-0.3wt%Ag-0.01wt%Co.) with 1 cm2 left by epoxy resin, the reference electrode was a mercurous sulfate electrode (MSE) of Hg and Hg2SO4/sat.·K2SO4, and the counter electrode was a platinum plate with an area of 6 cm2. The working electrode and counter electrode were connected by a saturated K2SO4 agar salt bridge. All potentials shown in the figures were against the MSE. An FEI–Quanta 200 scanning electron microscope was used to observe the microscopic surface morphology.

2.2 Cyclic voltammetry curves The cyclic voltammetry (CV) curves of the Al/Pb- 0.3wt%Ag, Al/Pb-0.3wt%Ag-0.01wt%Co, and cast Pb-0.3wt% Ag are presented in Fig.2. The scanning of the potential was carried out from -0.8 to 2.5 V (MSE) at a sweep rate of 10 mV·s-1. Curves register two characteristic anodic(a, b) and two characteristic cathodic peaks(c, d) which were described by Rashkov8]. Some studies[8,9] suggested that the first anodic peak a was due to the reaction Pb→PbSO4, the second anodic peak b was the reaction of the outer layer of PbSO4→PbO2 (α-PbO2 and β-PbO2). The two cathodic peaks c and d are the reduction reaction of PbO2(α-PbO2 and β-PbO2)→PbSO4 and PbSO4→Pb, respectively. The current value of the anodic peak a occurs almost under the same potential. The cathodic peaks c of Al/Pb-0.3wt%Ag and Al/Pb-0.3wt%Ag-0.01wt%Co electrodes have a higher current value than that of the cast Pb-0.3wt%Ag alloy electrode, moreover the value of Al/Pb-0.3wt%Ag-0.01wt%Co is the highest. It can be concluded that the generated PbO2 amount of Al/Pb-0.3wt%Ag- 0.01wt%Co is the most in the anode. In other words, the Al/Pb-0.3wt%Ag-0.01wt%Co possesses the best catalytic activity for oxygen evolution.

Current Density/A·cm

Experiment

1# Pb-0.3% Ag 2# Al/Pb-0.3% Ag 3# Al/Pb-0.3% Ag-0.01% Co

0.12 0.10 0.08 0.06 0.04

0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 0.05 2.10

1#Pb-0.3%Ag 2#Al/Pb-0.3%Ag 3#Al/Pb-0.3%Ag-0.01%Co

2.14

2.18

2.22

2.26

2.30

0.02 0.00 1.2

1.4

1.6 1.8 2.0 Potential/ V(vs. MSE)

2.2

Fig.1 Anodic polarization curves of three electrodes in 50 g·L-1 Zn2+ +150 g·L-1 H2SO4 solution

0.6 Current Density/A·cm-2

1

Pb-0.3wt% Ag Al/Pb-0.3wt% Ag Al/Pb-0.3wt% Ag-0.01wt% Co

0.4 0.2

b

a

0.0 -0.2

d

-0.4

c

-0.6 -0.8

-0.6

0.0 0.6 1.2 Potential/V(vs. MSE)

1.8

Fig.2 Cyclic voltammetry curves obtained after 24 h polarization

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2.3 Tafel curves The Tafel curves of the three electrodes were recorded from –1.4 V to –0.2 V with a scanning speed of 10 mV·s-1 in 50 g/L Zn2+, 150 g/L H2SO4 solution. The experiments were performed at 35 ℃. The Tafel curves are presented in Fig.3. The Tafel curves were fitted and the results are shown in Table 1. Generally speaking, the corrosion resistance of alloys is improved with the corrosion potential ascending[10]. It can be seen from the Table 1 that the corrosion potential of the Al/Pb-0.3wt%Ag composite anode is higher than that of cast Pb-0.3wt%Ag, and the corrosion potential of Al/Pb-0.3wt%Ag0.01wt%Co is the highest. But the order of corrosion current is opposite. It can be concluded that the corrosion resistance of Al/Pb-0.3wt%Ag-0.01wt%Co and Al/Pb-0.3wt%Ag are better than that of cast Pb-0.3wt%Ag. 2.4 Microstructure analysis The SEM images of Fig.4a~4c present the microstructures of Pb-0.3wt% Ag cast alloy, the Al/Pb-0.3wt% Ag and Al/Pb-0.3wt%Ag-0.01wt%Co after 24 h zinc electrowinning. It can be seen that the microstructures change obviously with the different electrodes. The structure of Pb-0.3%Ag is characterized by small size grains and loose. The Al/Pb-%0.3Ag and Al/Pb-0.3%Ag-0.01%Co anode, compared with the cast Pb-Ag alloy electrode, possess more uniform grain size, a clear outline of grain and denser microstructure. When cobalt and Pb-0.3wt%Ag are co-deposited on the surface of aluminum matrix, the microstructure is improved. Perhaps the addition of Co would make Ag elements in the basal body more even and the grain more refined. So the lower oxygen evolution potential is obtained. It can be seen from Fig.4d that the side A is aluminum matrix and side B is the composite coating of Pb-Ag. At the side of the aluminum matrix, the content of aluminum is 100%. At the side of the composite coating, the content is mainly Pb and Ag, and which are evenly distribution.

1#Pb-0.3% Ag

-2

Electrode

Ecorr/V

icorr/A·cm-2

1# Pb-0.3wt%Ag

-0.625

0.477

2# Al/Pb-0.3wt%Ag

-0.522

0.234

3# Al/Pb-0.3wt%Ag-0.01wt%Co

-0.419

0.213

1#

a

2#

b

3#

c

d

B

-1

10 log (i/A·cm )

Table 1 Corrosion current density and corrosion potential

A

2#Al/Pb-0.3% Ag

-2

10

3#Al/Pb-0.3% Ag-0.01% Co -3

10

-4

10

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Potential/V(vs. MSE)

Fig.4 Microstructure of the three electrodes after 24 h zinc electrow-

0.2

0.4

Fig.3 Tafel plots of three different electrodes in 50 g·L-1 Zn2+ + 150 g·L-1 H2SO4 solution

inning (a~c); the elemental analysis of Al/Pb-0.3wt%Ag (d)

Fig.5 is the EDS results for the composite Al/Pb-Ag-Co. The content of cobalt in the electrode is almost 0.01wt%. The content of silver in the electrode is higher than 0.3wt%.

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a

3.8

b

Pb

Element OK PbM AgL CoK

Intensity/kcps

3.0 2.3 1.5 0.8

O Co

0.0 0.00

Ag

4.00

Co

8.00

wt% 10.01 89.63 00.35 00.01

and 20 mV, respectively, at a current density of 500 A·m-2. The Al/Pb-0.3wt%Ag-0.01wt%Co possesses the highest corrosion potential, the lowest corrosion current. In other words, The Al/Pb-0.3wt%Ag-0.01wt%Co possesses the best corrosion resistance. 3) Compared with the cast Pb-0.3%Ag alloy electrode, Al/Pb-0.3wt%Ag and Al/Pb-0.3wt%Ag-0.01wt%Co anode, possess more uniform grain sizes, clear outlines of grains and denser microstructures. The addition of Co would make Ag elements in the basal body more even and the grain more refined. So the lower oxygen evolution potential is obtained.

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Fig.5 Microstructure (a) and EDS analysis (b) of the Al/Pb-Ag-Co

2000, 57: 109

anode

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3 Conclusions

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Journal[J], 1999, 34: 198

1) The Al/Pb-0.3wt%Ag-0.01wt%Co and Al/Pb-0.3wt%Ag composite anode material can be prepared by electrodeposition technology. 2) Compared with the Pb-0.3wt%Ag cast alloy anode sample, the oxygen evolution potentials of Al/Pb-0.3wt%Ag -0.01wt%Co and Al/Pb-0.3wt%Ag decrease by about 40 mV

8 Rashkov S T, Stefanov Y, Noncheva Z et al. Hydrometallurgy[J], 1996, 40: 319 9 Dobrev T, Valchanova I, Stefanov Y et al. T I Met Finish[J], 2009, 87: 136 10 Kim J H, Lee H, Lee S P. Journal of Alloys and Compounds[J], 2003, 348: 293

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