| How to separate and recover cadmium from zinc sulfide concentrate with high cadmium content in the process of zinc hydrometallurgy with high efficiency,short flow and low consumption is one of the technical problems facing zinc hydrometallurgy.In this paper,high cadmium zinc sulfate solution produced from a high cadmium zinc sulfide concentrate in China by wet smelting of zinc was used as raw material.Considering the characteristics of high cadmium and low copper in the zinc sulfate solution,high cadmium products containing more than 80%cadmium were directly replaced by zinc powder in the purification process of neutral leaching supernatant,in order to simplify the purification process,shorten the process of cadmium recovery,reduce zinc powder consumption and improve the recovery of cadmium.This paper focuses on three aspects of reaction thermodynamics,kinetics and optimization of process parameters in the purification process of zinc sulfat e solution with high cadmium content.The main research work and results are as follows:The potential pH diagram of Cd-Cu-Zn-H2O system was drawn,and the cadmium removal reaction with zinc powder substitution had advantages.The theoretical stability zone of Cd and Zn2+is pH<5.95.High pH of solution can easily lead to hydroxide precipitation in solution.The thermodynamic calculation results of zinc powder replacement for cadmium removal in zinc sulfate solution show that the reaction can be carried out thoroughly,but it is difficult to reach the theoretical value in practical production.Therefore,the kinetics of cadmium removal in zinc sulfate solution with high cadmium content was further studied.It is found that the removal of cadmium by zinc powd er replacement in zinc sulfate solution with high cadmium content can be divided into two stages.The apparent activation energy of the first stage is 40.23 k J/mol and the second stage is 63.69 k J/mol.The diffusion control is the limiting step of zinc powder replacement reaction.An experimental study was carried out on the process optimization of zinc powder replacement for cadmium removal in simulated zinc sulfate system with high cadmium content.The influences of zinc powder addition amount,particle size,pH value,temperature,adding method and other factors on cadmium removal process of zinc powder were investigated.Further,taking zinc sulfate solution with high cadmium content actually produced in zinc hydrometallurgy plant as raw material,guided by the parameters obtained from the laboratory simulation solution process study,the process conditions of selective cadmium removal by zinc powder replacement were obtained as follows:The reaction temperature is 60℃,the stirring speed is 150 r/min,the pH control solution is5.2～5.4,the zinc powder is added in batches after water pulping,the zinc powder addition coefficient is 1.2 times,the zinc powder particle size is 40～50mm,and the reaction time is 120 min.The high efficiency purification and enrichment of cadmium in slag can be achieved.The removal rate of cadmium in zinc sulfate solution can reach 98.6%by zinc powder purification.The high cadmium zinc sulfate solution produced in zinc hydrometallurgy plant with cadmium concentration of 2.3 g/L was used as raw material to remove cadmium and prepare high cadmium products simultaneously by three-stage countercurrent zinc powder replacement purification method.The results show that the cadmium concentration is reduced from 2.3g/L to 0.48 mg/L by the replacement of tertiary countercurrent zinc powder,and the removal rate of cadmium is greater than 99.9%.The selective separation of cadmium in zinc sulfate solution is realized,and the high cadmium product containing 80.5%cadmium is produced.Furthermore,XRD and SEM-EDS analysis were conducted on the replacement products produced by single-stage replacement of cadmium and three-stage countercurrent replacement of cadmium.The results show that there are a large number of unreacted zinc powder particles coated by cadmium in the cadmium slag produced by single-stage replacement of zinc powder,leading to low utilization rate of zinc powder.However,the replacement products produced by the three-stage countercurrent zinc powder replacement are mainly cadmium,and a small number of fine zinc particles are scattered in the cadmium products,indicating that the three-stage countercurrent zinc powder replacement removes cadmium,opens the cadmium coating on the surface of the zinc powder,and the zinc powder reaction is more thorough,reducing the consumption of zinc powder,and the produced replacement products contain higher cadmium content,which lays a foundation for the subsequent resource utilization of cadmium. |
