Elsevier

Hydrometallurgy

Volume 173, November 2017, Pages 125-133
Hydrometallurgy

The effects of halides in the electrowinning of zinc. I. Oxidation of chloride on lead-silver anodes

https://doi.org/10.1016/j.hydromet.2017.08.015Get rights and content

Highlights

  • •

    In the absence of manganese, oxidation of chloride does not occur on PbSO4 but readily on PbO2.

  • •

    Oxidation of chloride occurs under mass transport control in the electrowinning of zinc in the absence of manganese.

  • •

    The rate of oxidation of chloride is significantly lower in the presence of manganese in the range 1 to 5 g/L.

  • •

    The use of periodic current reversal does not increase the rate of oxidation of chloride in the presence of manganese.

  • •

    The rate of oxidation of chloride in the presence of manganese is too low to be of value in controlling chloride levels.

Abstract

A study of the kinetics of the anodic oxidation of chloride ions on the surface of lead-silver anodes used for the electrowinning of zinc has shown some interesting and practically important results. Oxidation of chloride does not occur on a PbSO4 surface but occurs readily on a PbO2 surface. Oxidation of chloride occurs at the mass-transport controlled rate at the operating potentials of anodes during the electrowinning of zinc in the absence of manganese in the electrolyte. However, the rate of oxidation of chloride ions is significantly lower in the presence of manganese ions in the concentration range 1 to 5 g/L. These results have been compared with plant observations that confirm the lower rate of chlorine generation in the presence of manganese. The use of periodic current reversal does not increase the rate of oxidation of chloride ions in the presence of manganese ions. The rate of anodic oxidation of chloride in the presence of manganese ions is too low to be of practical value in controlling the chloride levels during the electrowinning of zinc.

Introduction

Halide ions such as chloride and fluoride are often present as impurities (1 to 500 mg/L) in electrolytes used in the electrowinning of zinc. In the case of chloride, oxidation to chlorine is possible at lead-silver anodes and this can create health and safety problems in zinc tankhouses. Although oxidation of fluoride ions to fluorine is not possible at the operating anodic potentials, the detrimental effect of fluoride ions on the adhesion of zinc to the aluminium blanks is well known (Andrianne et al., 1980, Han and O'Keefe, 1982). Several reported studies (Kelsall et al., 2000, Yoshida et al., 1997, Lashgari and Hosseini, 2013) on the behaviour of chloride have suggested that the extent of oxidation to chlorine is significant. Others (Bennet, 1980, Pakhomova and Marenkova, 1966, Hierzyk et al., 1969) have found that increased concentrations of manganese(II) ions in the electrolyte reduce the release of chlorine and a mechanism was suggested in which chlorine is reduced by manganese(II) which is then oxidised to manganese(III). No convincing evidence for such a mechanism was provided. There is also little data in the literature on the effect of chloride ions on the rate of corrosion of lead-silver anodes during the electrowinning of zinc.

It was considered useful to study the anodic oxidation of chloride ions in the absence and presence of manganese(II) ions which are present in zinc electrolytes in concentrations ranging between 2 to 10 g/L. Finally, it is of interest to establish which anodic reactions are possible on these anode materials in the potential region between the formation of lead sulfate and its oxidation to one of several possible lead dioxide phases. The effect of manganese oxide layers on the anodic reactions is also of importance. A subsequent paper will deal with the effects of chloride ions on anode corrosion.

Section snippets

Experimental procedure

The electrochemical studies were performed using a conventional three-electrode system. A modified thermostatted Metrohm glass cell covered with a detachable lid was used as a reaction vessel. The working electrode was a rotating disc (5 mm diameter) of Pb-0.42% Ag rolled alloy. The disc was core-drilled from 9 mm commercial alloy sheets and the resulting cylinder attached to a stainless steel stub with conducting silver epoxy and subsequently coated with an insulating epoxy resin.

The reference

Anodic reactions in the absence of chloride and manganese ions

In order to compare the anodic reactions in the presence of chloride ions, a limited study was made of the well-known anodic behaviour in the absence of chloride and Fig. 1 shows the cyclic voltammogram of the rotating alloy electrode in a solution of 165 g/L sulfuric acid at 40 °C. The sweeps were initiated in a positive direction from the rest potential that was close to − 0.35 V.

Oxidation of lead to a passivating surface of PbSO4 occurs in peak (a).PbSO4+H++2e=Pb+HSO4−E°=−0.304Vat40°C

Further

Conclusions

This study of the oxidation of chloride ions on the surface of anodes used for the electrowinning of zinc has shown some interesting and practically important results.

  • 1.

    In the absence of manganese, oxidation of chloride ions does not occur on a PbSO4 surface but occurs readily on a PbO2 surface that forms at potentials greater than about 1.9 V.

  • 2.

    Oxidation of chloride occurs at the mass-transport controlled rate at the operating potentials of anodes during the electrowinning of zinc in the absence of

Acknowledgements

The authors would like to acknowledge the financial and technical support of the industry sponsors of the AMIRA Projects P705B and P705C under which auspices this research work was conducted. We also would like to thank Teck Metals Ltd. for granting approval to publish this research.

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