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

Abstract

Potentiodynamic and galvanostatic experiments have been used to investigate the electrodeposition of Zn and to develop a reliable recovery method from a purified spent pickling liquor (SPL) of hot dip galvanization. The main bjective was to find the suitable conditions to deposit high-grade Zn from the purified ZnCl2–HCl–NaCl solution in a conventional electrowinning cell holding the stationary electrolyte. The effects of pH (1.5–4.5) in and Zn concentration (30–150 g/dm3) in the stationary electrolytes were studied first by the potentiodynamic method to reveal the major characteristics of the cathodic process and to give reference for further practical examinations by the galvanostatic technique. The electrowinning experiments pointed out the effects of these major parameters, beside other practical factors, like apparent current density (c.d.) and additional NaCl concentration. The long-term galvanostatic experiments proved that the current efficiency (c.e.) increases significantly as the pH and the Zn concentration are increased. A c.e. of ~ 99% can be reached with an electrolyte of pH ~ 5, Zn concentration ~ 50 g/dm3 applying a c.d. in the 300–600 A/m2 range. Increasing the Zn concentration could considerably improve also the deposit morphology. The addition of NaCl can practically improve the c.e. if the Zn concentration is at least around 50 g/dm3. In contrast, this improvement is largely off-set by the negative effects of strong H2 evolution at low (e.g. < 10 g/dm3) Zn concentrations. Extremely high Cl− ion concentrations, however, inhibit the cathodic reaction by stronger chloro-complex formation. In this case the intensive H+ reduction causes hydroxide precipitation.