The electrowinning of zinc from sodium hydroxide solutions

• We obtain zinc powder from the alkaline electrolyte of smithsonite by electrowinning.
• The potential has a great impact on the Zn ion behavior on the cathode.
• The diffusion coefficient of Zn ions is 3.22 × 10− 6 cm2/s in this solution.
• There is an energy waste stage at the beginning of electrowinning.
• The energy waste is least at the limiting diffusion current density.

Electrodeposition of zinc in the sodium hydroxide solutions on a stainless steel cathode was systematically investigated by means of cyclic voltammetry, chronoamperometry and chronopotentiometry in this paper. It was revealed that the deposition process was controlled by mass transport, and the diffusion coefficient of Zn(OH)42 − ions was 3.22 × 10− 6 cm2/s. Meanwhile, the cathode process was studied at different potentials as well. It was the charging process of electrochemical double layers at potential in a range from − 1.1 V to − 1.4 V (vs. SCE). However, nucleation and diffusion of Zn(OH)42 − ions were the dominating processes at potential from − 1.47 V to − 1.56 V (vs. SCE). The nucleation process was promoted, and the steady diffusion current improved with the increasing potential. Remarkably, the hydrogen ions were reduced fiercely on cathode at − 1.7 V and − 1.8 V (vs. SCE). Furthermore, the current efficiency was also discussed at different electrowinning times and current densities. An energy waste period was found at the beginning of electrowinning, which was caused by hydrogen evolution, and the current efficiency increased to the maximum (current efficiency peak) slowly at 500 A/m2. Nevertheless, the results showed that the current efficiency peak was highest and it was fastest to reach the peak when the applied current density was near the limiting diffusion current density, 95.7 A/m2.