Effective separation and recovery of Zn, Cu, and Cr from electroplating sludge based on differential phase transformation induced by chlorinating roasting

Heavy metals in electroplating sludge (ES) are usually amorphous and easily released in the environment. Especially for the ES containing multiple heavy metals, owing to the complex composition and lack of effective disposal method, it has been storage for a long time. In order to avoid environmental pollution, effective treatment methods are very urgent and necessary. Here, chlorinating roasting method was developed to enlarge the phase difference of heavy metals to fulfill the utilization of ES containing multiple heavy metals (Zn, Cr, and Cu). When CaCl2 was used as additive, Zn and Cu were volatilized to the gas phase, while Cr was oxidized to Cr(V)/(VI) and retained in the solid phase with readily leachable state. The recovery percentage of Zn, Cu, and Cr can reach 99%, 98%, and 96% respectively by chlorinating roasting for 4 h at 1000 °C with the CaCl2 addition proportion of 100%. After further extraction and purification, the purity of Cr and Zn can reach 92% and 99% respectively. Moreover, the mechanism of the differential phase transformation induced by chlorinating roasting was analyzed by the method of thermodynamics and kinetics. The kinetic reaction equation of the ZnCl2 and CuCl2 volatilization process can be described by phase boundary reaction and the function is G(α) = 1-(1-α)1/3. This work provides a simple and effective method for the treatment of ES containing multiple heavy metals.