Interpreting the main effects on the efficiency and morphology for establishing a procedure of electrodepositing Zn from purified chloride SPL solutions

Detailed potentiodynamic and galvanostatic methods have been used to investigate the electrodeposition of Zn and to develop a reliable recovery method from a purified and concentrated spent pickling liquor (SPL) of hot dip galvanization. Applying various combinations of Zn concentration, pH, agitation, apparent current density (c.d.) and additional NaCl concentration, a uniform initial deposit could be observed at the cathode, however the subsequent dendrite formation and the development of protrusion at the edges were targeted for a deeper investigation. The potentiodynamic experiments, showed the importance of H 2 evolution influenced by the electrolyte properties and the cathodic polarization. The cathodic current was dominated by Zn reduction in the pH > 2 range during the fast potentiodynamic runs of the 30 – 150 g/dm 3 Zn concentration range in the stationary electrolyte. Increasing the Zn concentration could considerably improve the deposit morphology. In the long-term galvanostatic experiments the current efficiency increased with the increase of pH in the examined wide Zn concentration range but the c.d. needs optimization. A current efficiency (c.e.) of ~99% can be reached with an electrolyte of pH ~5, Zn concentration ~50 g/dm 3 applying a c.d. in the 300 – 600 A/m 2 range. At low (~10 g/dm 3 ) Zn concentrations the rate of hydrogen evolution increases dramatically. The addition of NaCl can practically improve the c.e. if the Zn concentration is at least around 50 g/dm 3 . In contrast, this improvement is largely off-set by the negative effects of strong H 2 evolution at as low Zn concentrations as e.g. 10 g/dm 3 . Higher NaCl additions or too high Cl - ion concentrations, however inhibit the cathodic reaction of the electroactive species by stronger chloro-complex formation. In this case the intensive H + reduction causes hydroxide precipitation.