Investigation of zinc dissolution and cadmium precipitation rates in a Cd 2+ /Zn cementation system

Abstract

In this paper, an investigation of the Cd²⁺/Zn cementation system is attempted through a semi-theoretical approach, correlating the rates of zinc dissolution and cadmium precipitation under various conditions. Two series of experiments were carried out using zinc in the form of sticks and dust, respectively, while cadmium ions were dissolved as sulphates. The parameters examined were temperature (25–50°C), additives (H₂SO₄, NH₄OH, Na₂SO₄), stirring speed (200–600 rpm), initial Cd²⁺ concentration and granulometric size of zinc dust (0·78–1·84 mm). In the second group of tests, continuous recording of pH change versus time was carried out. Total dissolved solids curves versus pH were constructed for each experiment showing solid cadmium to be the only precipitated phase, and mass balances for the two elements were evaluated for all tests. To account for the sampling position in the reactor, a new concept, the normalisation coefficient, is introduced and evaluated, correlating the species’ concentrations in the system. Also time expansion was taken into account and equilibrium constants were evaluated. Based on the above, a correlation equation between the rates of zinc dissolution and cadmium precipitation is derived theoretically and it is confirmed by the experimental results. This analysis demonstrated the important role of pH for the system kinetics and the considerable effect obtained using normalisation coefficient on the experimental data. The plots of the normalised Cd²⁺ concentrations in the solution versus Zn²⁺ dissolved, verify the proposed linear dependence between the rates of cadmium precipitation and zinc dissolution.