Modelling the Effect of Solution Composition and Temperature on the Conductivity of Zinc Electrowinning Electrolytes

Zinc electrowinning is an energy-intensive step of hydrometallurgical zinc production in which ohmic drop contributes the second highest overpotential in the process. As the ohmic drop is a result of electrolyte conductivity, three conductivity models (Aalto-I, Aalto-II and Aalto-III) were formulated in this study based on the synthetic industrial electrolyte conditions of Zn (50–70 g/dm3), H2SO4 (150–200 g/dm3), Mn (0–8 g/dm3), Mg (0–4 g/dm3), and temperature, T (30–40 °C). These studies indicate that electrolyte conductivity increases with temperature and H2SO4 concentration, whereas metal ions have negative effects on conductivity. In addition, the interaction effects of temperature and the concentrations of metal ions on solution conductivity were tested by comparing the performance of the linear model (Aalto-I) and interrelated models (Aalto-II and Aalto-III) to determine their significance in the electrowinning process. Statistical analysis shows that Aalto-I has the highest accuracy of all the models developed and investigated in this study. From the industrial validation, Aalto-I also demonstrates a high level of correlation in comparison to the other models presented in this study. Further comparison of model Aalto-I with the existing published models from previous studies shows that model Aalto-I substantially improves the accuracy of the zinc conductivity empirical model.

Understanding the zinc discharge from sodium sulphate solution in the presence of surfactants

This paper aims to understand the effect of surfactants on zinc discharge in sulphate solutions on a solid electrode and to collect new experimental data that would give a deeper insight into the processes involved in industrial electrolysis. Anionic and cationic coagulants (flocculants) and an anionic frother were used as surfactants. Electrolysis was conducted in the potential region of –1050 to –1250 mV (Ag/АgCl) in stationary and dynamic conditions in sodium sulphate solution under intensive stirring. The authors obtained comparative data on the zinc discharge current in electrolyte with and without frother at the scan velocities of 2 to 100 mV/sec. It is noted that at higher scan velocities (>10–20 mV/sec) and in the initial electrolysis phase the zinc discharge process develops in a mixed mode. In this case, as the study showed, the positive effect of the frother on zinc discharge is most distinguished. A reaction order was designed based on zinc ion with four potentials to prove that the zinc electrowinning process develops in a mixed mode. It is shown that the addition of frother raises the reaction order from 1.2 to 1.5, which is attributed to a larger effective cathode surface area. The data obtained in a galvanostatic mode under intensive stirring conditions indicate that, at the current density of 1.7 mA/cm2, the electrode polarization is 1.6 times lower in the presence of cationic coagulant and almost 3 times lower in the presence of anionic coagulant. The data given in this paper are also indicative of a changing electrolysis mode. Under stirring, a transition is observed from a diffusion mode of zinc ion reduction to a mixed one. The experimental data obtained under intensive stirring conditions in sodium sulphate solution with frother, as well as anionic and cationic coagulants are in line with the theory of electrochemical processes.