Providing Practical Instruction for Solving Environmental Problems from Residue (cake) of Cold Purification Process in Zinc Production Process

In the waste from the cold treatment process in zinc production plants, there is about 40 to 50% of zinc and other harmful heavy metals such as cadmium and nickel. Cadmium and nickel are among the heavy metals that are harmful to human health in high concentrations. Removal of bio-pollutants from these wastes is one of the concerns of the zinc industry. The purpose of this study is to remove nickel and cadmium as impurities in the zinc production process by cementation method.
The procedure consists of three steps: cold purification cake leaching, impurity purification and electrolysis. The cake is first liquefied with sulfuric acid and the resulting solution contains zinc ions along with nickel and cadmium impurities. Optimal conditions for the removal of impurities were obtained by the surface response method (RSM), then the filtration process was performed by the cementation method and the impurities were removed and finally the solution was electrolyzed to produce zinc. Using Design Expert (DOE) test design software, the parameters affecting the filtration process such as temperature, zinc powder concentration, retention time and additive concentration were investigated.
It was found that most influences related to contact time, temperature, zinc powder amount and additive concentration, respectively. So, optimum conditions for removal of nickel and cadmium containing temperature of 85 centigrade, concentration of 6.63 mg L-1 of zinc powder, contact time of 100 min and concentration of 10.29 mg L-1 for additive were obtained. At these conditions, impurities amounts were reached to the allowed limit for electrolysis and zinc in the cake was recovery.
Using cementation method by removing impurities and then recovering zinc, in addition to solving some of the environmental problems caused by the cake, the recovery of these metals is of great economic importance.