Effect of Concentration of Sulfate on Diffusivity of Chloride Ion and Viscosity in Acidic Sulfate Solution

Chloride ion at concentration above 100 ppm causes serious problems in zinc electrowinning. Electrolytic oxidation method is effective for eliminating the anion. Diffusion controls the rate of such elimination. Diffusivity has been examined by voltammetry with Pt/Ti stationary and rotating disk electrodes. Current-potential curves recorded by both techniques show that the characteristic current follows the Randles-Sevcik or Levich equations. From the coefficients appearing in these equations, diffusivity of chloride ion, for example, 1.2×10-9m2·s-1 in a 10mol·m-3 ZnCl2/1.0 kmol·m-3 ZnSO4 mixture containing 1.5 kmol·m-3 H2SO4, was determined. Viscosity was also determined with a capillary type of viscometer. The product of diffusivity and viscosity assumes a constant value (1.8×10-12N). An increase in zinc sulfate concentration is accompanied by a decrease in diffusivity, and the phenomenon is attributed to an increase in viscosity. The relationship between these two parameters makes it possible to estimate the magnitude of diffusivity by measuring viscosity.