Novel electrowinning of zinc from zinc oxide compound in low temperature ionic liquid

Abstract

A novel method for zinc electrodeposition was investigated by using a urea and choline chloride in a molar ratio of 2:1 low temperature deep eutectic solvent as electrolyte. The dissolution of zinc oxide was investigated using FTIR and corresponding solubility measurement was carried out in ICP-AES. Through direct comparison of IR Peak identified for the [ZnO∙Cl∙urea] complex, optimized solubility was found at a temperature of 100°C. The dissolution limit is approximated in FTIR with 1.23 mol/L ZnO in the Urea/ChCl eutectic mixture and is verified in ICP-AES with a measurement of 90584 ppm at 100°C. The reduction potential of Zn2+ to Zn was found at -1.1V, using Ag as reference electrode. The reduction of Zn and its corresponding stripping process was characterized as irreversible and controlled by diffusion. The diffusion coefficient was confirmed with an average value of 1.89 ×10-8 cm2/s at 100°C. The nucleation process was characterized as instantaneous by comparing with 3-D nucleation model. Process variables such as temperature and surface lubricant concentration were investigated in order to improve deposit layer surface quality. Temperature was proven to have large impact of the formation of deposit layer as nodular cauliflower like structure was formed at the top of each particle at 100°C. The addition of [Bmim]HSO4 can effectively generate a more compact surface layer on the cathode and promote an improved finer grain size. Lab-scale electrowinning of Zn was conducted from applied potentials from 3.2 to 3.5V at 90°C. Concentration polarization was confirmed in the beginning of the reaction and the diffusion coefficient was confirmed with an average value of 7.85×10-9 cm2/s at 90°C which agrees to the previous value. A gradual increase of current density was due to activation polarization in which a linear correlation between current density and over-potential was confirmed. The kinetics study of the electrowinning process was carried out in the steady-state region. Tafel plots were obtained in temperatures from 80 to 100°C. The activation energy was obtained as 3.85 kJ/mol. The effect of applied potential on microstructure was investigated. It was seen that dendrites formed in a higher over-potential of -0.65V and was beyond the calculated critical value for dendrite formation which is equal to -0.62V. Process variables such as cell voltage, temperature, and surface additive concentration were investigated to obtain higher current efficiency and lower energy consumption. An efficiency as high as 92.6% was obtained with 2.0mg/mL [Bmim]HSO4 at 100°C, 3.3V showing a promising advantage over conventional zinc electrolytic technology which owns a efficiency close to 90%. The feasibility of this novel technology was explored for large scale application, for example, in batch mode. Electrowinning duration is the main variable that was investigated under batch mode. The current efficiency was calculated with an average value of 88.8 ± 0.1% irrespective of electrowinning time. Correspondingly, energy consumption was calculated to be 3.23 ± 0.03 kWh/kg which shows reasonable agreement with results calculated in small scale electrowinning. The rate of cathodic weight gain is equal to 9.01 ×10-4 g/min at given condition. In addition, the graphite anode is proven to be stable during the long-term electrowinning as the rate of anodic weight loss is equal to 5.82×10-5 g/min A model was established to study the electrolyte fluid flow velocity distribution in the electrowinning reactor. Results were illustrated that velocity in between electrodes is higher than other domain leading to the formation of cycling flow.