Recycling Selenium And Tellurium From Solution Precipitated Gold And Leaching Kinetics Of Selenium

The solution precipitated gold obtained from the recovery process of precious metals in copper anode slime contains a large number of rare and noble metals. The aim of this work is at exploring an economical and environment-friendly method to recovery rare and noble metals from the solution precipitated gold and realizing the comprehensive recycling of resources. The recovery process of valuable metals such as selenium and tellurium from the solution precipitated gold by SO2reduction and the separation technology of selenium and tellurium were studied. The leaching kinetics of selenium was also investigated.The recycling rate of selenium and tellurium increase with increasing reaction temperature and increasing Cl-concentration and prolonging reaction time, the recycling rate of tellurium increases with increasing H+concentration, but H+concentration has no effect on the recycling rate of selenium when Se(Ⅳ) and Te(Ⅳ) in the solution are reduced by SO2. The optimum reducing conditions are proposed as the following:reaction temperature of85℃, reaction time of4h, H+concentration of3.3mol·L-1and Cl-concentration of0.72mol·L-1. In the optimum conditions mentioned above,99.5%Se and96.64%Te are recovered, Pt and Pd recovery rate reach100%. The mass fractions of selenium, tellurium, platinum and palladium in selenium and tellurium concentrate obtained by SO2reduction are28.06%,52.3%,0.084%and0.588%, respectively. The crystalline phase and morphology of selenium and tellurium concentrate were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The results illustrate that Se and Te exist in the form of simple substance, and that the product morphology are spheroid and columnar.The process of selective leaching Se from selenium and tellurium concentrate by Na2SO3solution was investigated. When the concentration of Na2SO3solution is less than378g·L-1, Se leaching rate increases with increasing Na2SO3solution concentration. Se leaching rate decreases with increasing Na2SO3solution concentration when the concentration of Na2SO3solution is more than378g·L-1. Se leaching rate increases with increasing reaction temperature and increasing stirring speed and decreasing particle size of selenium and tellurium concentrate. When liquid-solid ratio and reaction time are less than7:1and3h respectively, Se leaching rate increases with increasing liquid-solid ratio and prolonging reaction time. Se leaching rate decreases slowly with increasing liquid-solid ratio and prolonging reaction time when liquid-solid ratio and reaction time are more than7:1and3h respectively. The optimum leaching conditions are as follows:Na2SO3solution concentration of378g·L-1, reaction temperature of85℃, L/S ratio of7:1and reaction time of3h. Based on the optimum conditions mentioned above, the total leaching rate of selenium reaches92.70%. In addition, it can be concluded from the results of acidification of leachate containing selenium by sulfuric acid that Se recovery rate increases with decreasing pH value of leaching solution and the pH value should be controlled at less than2and Se recovery rate reaches97.68%as the solution of pH value is1.25. Purity of crude selenium obtained by acidizing leachate reaches99.61%. XRD test testifies that the phase component of crude selenium mainly is elemental Se. SEM assay illustrates that the morphology of crude selenium is spheroid.H2O2(30%) amount and reaction temperature have little effect on Te leaching rate, Te leaching rate increases with prolonging reaction time during the process of leaching tellurium from residue after separating selenium by the mixed solution of sulfuric acid and hydrogen peroxide. Te leaching rate increases rapidly with increasing H2SO4solution concentration When the concentration of H2SO4solution is less than4mol·L-1H2SO4solution concentration has little effect on Te leaching rate when the concentration of H2SO4solution is more than4mol·L-1. The optimum conditions of leaching tellurium are below:H2SO4solution concentration of4mol·L-1, reaction temperature of25℃, reaction time of150min,2times the theoretical amount of H2O2(30%) and volume of H2SO4solution to weight of residue after separating selenium ratio6:1(mL:g). In the optimum conditions mentioned above, Te leaching rate reaches98.53%, the mass fractions of gold, platinum and palladium in platinum-palladium concentrate are24.07%,2.47%and5.69%, respectively. Furthermore, Te recovery rate increases with increasing Cl-concentration during the experiment of recycling tellurium from leachate containing tellurium by SO2reduction. Te recovery rate reaches99.40%When Cl-concentration is0.72mol·L-1. Purity of crude tellurium obtained by reducing leachate reaches95.38%. XRD test testifies that the phase component of crude tellurium mainly is elemental Te. SEM assay indicates that the crude tellurium morphology is found as columnar.The kinetics of leaching Se by Na2SO3was researched. It was determined that reaction temperature was the most influential factor during the leaching process. Other influential variables included Na2SO3concentration, liquid-solid ratio, stirring speed and particle size of selenium and tellurium concentrate, which arranged in descending order according to the degree of influence on the leaching process. The leaching process is successfully described by the Avrami model and is hybrid controlled. The apparent activation energy of leaching Se and the feature parameter of Avrami equation are determined to be20.847kJ/mol and0.235, respectively. The leaching kinetics equation is:-1n(1-X)=18.739·Ca520·D-0.202·W-0.596·S0.201·exp[-20847/(RT)]·t0.235.