Circulating Flow–Electric-Field-Configuration-Enhanced Cadmium Cementation from Sulfate Systems and Its Optimization Mechanism

In this work, a novel flow–electric field coupling configuration was designed and implemented for enhancing Zn-Cd cementation. A series of tests was conducted to explore the optimization of the Zn-Cd cementation process and its mechanism. Firstly, the various characteristics of the sponge cadmium at various locations in the device were compared, and it was concluded that the optimum purity of the sponge cadmium obtained from the anode was up to 94.1%. The generation and stripping of the cadmium sponge was revealed for the first time by cross-sectional electron microscopy. The four stages of the apparent reaction in the system were analyzed in relation to the pH, cadmium concentration and cadmium sponge flaking at each flow rate. It was proved that the separation of cadmium sponge mainly occurred in the third phase. Secondly, by comparing the morphology and specific surface area of the cadmium sponge at different flow rates, the optimum flow field velocity was identified as 30 mL/s. At this point, the specific surface area reached a maximum of 1.151 m(2)/g. Six flow field configurations were compared and preferred. The results demonstrated that the LCAH (Low-Cathode-Anode-High) modulation resulted in a sparser structure of the cadmium sponge, which was more easily exfoliated from the zinc anode surface by fluid impact. This was considered to be the most beneficial flow field configuration for improving the cadmium cementation rate and reducing the cost of the reaction. Moreover, the reaction steps of the system were analyzed. Its rate-limiting step was initially empirically identified as the diffusion step and proven by calculating the activation energy of 12.6 kJ/mol. It was confirmed that the diffusion process under different flow field configurations followed the first-order kinetic principle. In addition, the system’s reaction phases were compared. Calculations confirmed that the diffusion process under various flow field configurations followed first-order kinetics. The diffusion coefficient of LACH proved to be the highest in the comparative tests, and the evident experimental results supported this conclusion.