単体硫黄の湿式酸化に関する電気化学的考察

抄録

In acidic and ammonia alkaline suspensions of sulphur particles each covered with a CuS layer, a disproportionation reaction, i. e., formation of CuS from Cu²⁺ and Cu-NH₃ complex ion, respectively, and concurrent oxidation of S to form SO₄, takes place at appreciable rates. The former is considered to be a cathodic reaction, while the latter an anodic reaction; for example, Cu²⁺+S+2e=CuS (1) and S+4H₂O=SO₄^2-+8H⁺+6e (2), respectively. For eq.(1) data of sulphidising suspension electrolysis show that the rate is very rapid. This is indicative of eq.(2) being therate-determining step. The current density i for the anodic reaction can, therefore, be calculated from the reaction rate and the total surface area of sulphur. The current densities for both acidic and ammonia alkaline suspensions are discussed as a function of the suspension potential E_m (SCE) and pH. It then is found that i is represented by
i=const.[H⁺] ^-mexp [0.87F/RT (E_m-E_o)]
where m is a pH-dependent constant and Eaatentative equilibrium electrode potential.
Sulphur in synthetic CuS is oxidized to form SO₄ at controlled suspension potentials of 0.34-0.36V and to form S at 0.42-0.55V, while that in oxidation residue of Pb concentrate to form SO₄ at 0.25-0.45V. The current densities of these SO₄-forming reactions are the same order. Explanations are given for the effects of the pH and temperature on the S-forming and SO₄-forming reactions for oxidation of metal sulphide. Data for precipitation of CuS with S in acidic solution under SO₂ pressure are compared with the results obtained. It seems that the oxidation rate of SO₂ to SO₄ is about 105 times faster than that of S.