The anodic dissolution of ZnS electrodes in sulfuric acid solutions

The anodic dissolution of reagent grade ZnS and ZnS concentrate has been studied. The majority of the research was conducted in H₂SO₄ although a limited number of tests were made in HCl and KOH. Polarization studies showed that both Zn²⁺ and S were products in the acidic solutions. ZnS passivated in KOH solutions. The electrodes were fabricated by hand pressing mixtures of ZnS + pitch (5–15%) and sintering at 800°C in a N₂ atmosphere. The open circuit potentials were considered to be mixed potentials resulting from the anodic dissolution of ZnS and cathodic reduction of S or O₂. Current efficiencies and $\text{Zn}{}^{\mathrm{2+}}\text{S}$ ratios were determined at 0.5 and 0.85 V vs. SHE. The results indicated the occurrence of both electrochemical and chemical dissolution steps as well as the further oxidation of H₂S, namely,

$\text{ZnS(s)=Zn}{}^{\mathrm{2+}}\text{(aq)+S(s)+2e}$

$\text{ZnS(s)=2H}{}^{\mathrm{+}}\text{(aq)=Zn}{}^{\mathrm{2+}}\text{(aq)+H}{}_2\text{S(aq)}$

$\text{H}{}_2\text{S(aq)+4 H}{}_2\text{O(I)=HSO}{}_4\text{(aq)+9H}{}^{\mathrm{+}}\text{(aq)+8e}$

The overall dissolution appears to be mass transfer-limited, probably either by the diffusion of Zn²⁺ from the reaction interface, through the reacted layer to the bulk solution, or the dissolution of precipitated Zn(OH)₂ in the reacted layer by the diffusion of H⁺ into the layer. The concentrate anodes polarized more drastically than the reagent grade ZnS anodes, possibly due to the presence of PbS impurity which would form PbSO₄ in the anode pores.