Roles of tannic acid and gelatin in Zn electrowinning and their inhibition mechanisms investigated via electrochemical methods

doi:10.1016/j.hydromet.2020.105390

Hydrometallurgy

Volume 195, August 2020, 105390

https://doi.org/10.1016/j.hydromet.2020.105390 Get rights and content

Highlights

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Effects of tannic acid and gelatin in Zn electrowinning were studied.
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CE decreased from 85.73% to 72.09% under tannic acid concentration increased from 10 to 400 mg/L.
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Inhibition mechanisms of organic for Zn deposition was investigated via electrochemical methods.

Abstract

In this study, the roles of tannic acid and gelatin in Zn electrowinning were investigated. The results indicated that the addition of 10 mg/L of gelatin promoted Zn electrowinning and increased its current efficiency (CE) from 89.55% to 91.8%. However, the CE was only 77.47% when the electrolyte contained 50 mg/L of gelatin. As the concentration of tannic acid in the electrolyte increased from 10 mg/L to 400 mg/L, the CE decreased from 85.73% to 72.09%, which represented declines of 4.27% and 19.5%, respectively, compared with that of normal Zn electrowinning conditions in the absence of tannic acid. With increase in the concentrations of tannic acid and gelatin, the cell voltage increased and CE decreased sharply, which eventually resulted in a significant increase in the unit consumption of direct current (DC). The mechanisms by which tannic acid and gelatin inhibited the kinetics of Zn plating were additionally researched using electrochemical methods. The results showed that tannic acid and/or gelatin in high concentrations in the electrolyte significantly inhibited the deposition of Zn on the cathode by increasing the overpotential, reducing the deposition rate, and covering the electrode surface, which led to the appearance of agglomerates and needle-like structures on the surfaces of the Zn sheets.

Introduction

The process of Zn electrowinning is sensitive to the presence of organic compounds, which can seriously affect normal Zn hydrometallurgical operations (Guillaume et al., 2007; Mureşan et al., 1996b; Wang et al., 2019). The literature indicates that the presence of organic impurities in excessively high concentrations in the electrolyte can significantly affect the current efficiency (CE), unit consumption of direct current (DC), and quality of Zn deposited on the cathode in terms of mechanical properties and purity (Ivanov, 2004; Mackinnon and Brannen, 1982). Various organics, including extractants, flotation reagents, flocculants, tannic acid, acid mist suppressants, and gelatin, are introduced into Zn hydrometallurgical processes to increase the leaching rate of metal ions or improve working conditions and product quality (Cheng et al., 2004; Hosny, 1993; Mureşan et al., 1996a; Recéndiz et al., 2007; Tripathy et al., 2004). However, the CE, unit consumption of DC, and quality of deposited Zn are observably affected by the excessive presence of organic impurities in the electrolyte.

MacKinnon (1994) determined that the CE of Zn deposition increased and the deposition morphology changed when the electrolyte contained 10 mg/L of Dowfroth (a foaming agent). Majuste et al. (2015) investigated the relationships between the quality of Zn sheets and concentrations of flotation collectors, flocculants, and lubricating oils. They determined that the CE was merely 21.6% when the electrolyte contained 50 mg/L of flotation collectors. Moreover, pores were formed on the surface of the Zn sheets, and the nucleation overpotential for Zn deposition increased. Alfantazi and Dreisinger (2003) reported the effects of two surfactants, i.e., orthophenylene diamine and sodium lignin sulfonate, on Zn electrowinning and noted that the CE decreased from 90% to approximately 60% when the concentration of orthophenylene diamine was only 3 mg/L. Previous works have mainly focused on the effects of flotation reagents, acid mist suppressants, extractants, and surfactants on Zn electrowinning. However, there are other compounds of significance in Zn hydrometallurgy, such as tannic acid, which is widely used to recover Ge, and gelatin, which is usually employed to improve the quality of Zn sheets. In the recovery of Ge, the amount of tannic acid added is usually 35–45 times the content of Ge. Generally, the electrolyte is continuously recycled, and hence, tannic acid and gelatin in the electrolyte accumulate and their concentrations increase after a certain duration, thereby obstructing the process of Zn electrowinning. Hence, it is of scientific significance and practical value to research systematically the influences of tannic acid and gelatin on Zn electrowinning.

In the present work, the influences of tannic acid and gelatin on the cell voltage, energy consumption, CE, and surface morphology of deposited Zn during Zn electrowinning were investigated. The cyclic voltammetry (CV) curve, cathodic polarization curve, overpotential for Zn deposition, and kinetics data at different concentrations of tannic acid and gelatin were researched to explore their action mechanisms. The study was conducted to understand better the behaviors of tannic acid and gelatin in Zn hydrometallurgy. It aids in determining the amount of organic matter to be added during production and implementing optimization procedures to minimize any negative impact on industrial operations.

Section snippets

Reagents and equipment

The electrolyte was prepared using analytically pure ZnSO₄·7H₂O, MnSO₄·H₂O, concentrated H₂SO₄ (98%), and distilled water. The concentrations of Zn²⁺ ion, H₂SO₄ and Mn²⁺ ion were 50, 160, and 5 g/L, respectively. The addition amounts of SrCO₃ and gelatin were 2 and 0.1 kg/t, respectively. Tannic acid and gelatin were obtained from a Zn hydrometallurgical enterprise in Yunnan province. The concentrations of other ions impurities are shown in Table 1. The surface morphology of the sample was

Effects of tannic acid and gelatin on cell voltage, CE, and unit consumption of DC

The effects of tannic acid, gelatin, and their co-addition on the cell voltage, CE, and DC unit consumption of Zn electrowinning were investigated. These results are illustrated in Table 2. As the concentration of organic matter in the electrolyte gradually increases, the cell voltage and DC unit consumption of Zn electrowinning increase while CE significantly decreases. Under normal electrowinning conditions in the absence of tannic acid, the cell voltage is 3.112 V. The cell voltage increases

Conclusion

The effects of organic matter such as tannic acid and gelatin, individually and in combination, on Zn electrowinning were investigated. The addition of 10 mg/L of gelatin improved the CE from 89.55% to 91.8%. However, as the amounts of tannic acid and gelatin in the electrolyte increased, the cell voltage increased and CE decreased sharply, which eventually resulted in a significant increase in the unit consumption of DC. In particular, the CE was only 72.09% and 49.29% with the individual

Declaration of Competing Interest

No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51464024, 51864042, and 51804220).

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