GB914515A - Process for the electrolytic production of hyper-pure zinc - Google Patents

Abstract

914,515. Electrolytic production of hyper-pure zinc. MONTEVECCHIO SOC. ITALIANA DEL PIOMBO E. DELLO ZINCO. June 30, 1961 [July 5, 1960], No. 23731/61. Class 41. Electro-deposition of hyper-pure zinc containing less than 5 p.p.m. of lead and having a purity of not less than 99.999%, by electrolysis of zinc sulphate solution, purified of impurities other than lead, using a lead or lead alloy anode with a subsidiary anode, comprising an electrolyte-impermeable diaphragm, coated with deposited manganese dioxide to render it electrically conductive and completely impermeable, surrounding the anode and isolating it from the cathode. The diaphragm separates the anolyte from the catholyte and prevents transference of electrolyte across the diaphragm ; it also prevents contamination of cathodic zinc deposits by solid anode slime products. The lead anode acts only as the current conductor ; the anodic phenomena, e.g. oxygen evolution, and current discharge being performed by the subsidiary diaphragm anode. The deposition of manganese dioxide renders the diaphragm almost completely impermeable whence the acidity of the anolyte contained in said diaphragm rises to much higher values than normal, reaching concentrations of free sulphuric acid of 250-280 gms./litre. The high acidity of the anolyte results in (i) high electric conductivity, (ii) substantially reducing the formation of PbO 2 at the anode, (iii) lowering to low limits the solubility of Pb <SP>+ </SP><SP>+</SP> ions in the electrolyte, (iv) practically zero solubility for PbSO 4 , so that lead dissolved by anodic attack precipitates completely within the diaphragm without contamination of the catholyte and concomitant deposition with the zinc as in processes hitherto. The electrolytic cell, Fig. 1, is coated with anti-acid polyvinyl chloride and comprises lead or lead-alloy (1% Ag) anodes 1 contained in diaphragms 2 of compact weft in acid-resistant vinyl resin yarn interposed with cathodes 3. Neutral zinc sulphate solution is admitted at A and spent electrolyte of low acidity (50-60 gms. of H 2 SO 4 /litre) is discharged at B. The cell operates with a current density of from 200- 250 amps./m.<SP>2</SP> at 30-35‹ C. The adherent, compact, electrically-conductive MnO 2 on the diaphragm is formed by including 1 to 5 gms./litre of Mn + + in the zinc sulphate electrolyte which should also be leadfree with Cd < 0.002 gms./litre.