| Copper anode slimes are obtained from the electrorefining of copper,which are the main material to extract precious and rare metals.Generally,removal of selenium is an important pretreatment step in copper anode slimes treatment.For one thing,pretreatment can avoid selenium dispersion and recovery rate of selenium can be improved;for the other thing,through pretreatment complication of precious metals extraction can be avoid.Nowadays,hydrometallurgical and pyrometallurgical technologies are used to recover selenium from copper anode slimes.Although selenium can be recovered effectively by pyrometallurgical treatment,it faces difficulties due to rising energy costs and strict pollution regulations.Therefore,hydrometallurgical technologies have been proposed to overcome these problems.However,some problems still exist in hydrometallurgical processes,such as,low leaching efficiency of selenium,dispersion of gold,equipment corrosion and low economic efficiency.Therefore,it is necessary to explore the mechanism of selenium leaching and develop a new process to leach selenium from copper anode slimes.In this study,high nickel copper anode slimes and high lead copper anode slimes are used as materials.Thermodynamics of selenium leaching and transformation of selenium-containing phases during leaching have been studied based on the occurrence of selenium in copper anode slimes.Then,the process that leaches selenium by a nitric acid-sulfuric acid mixture has been studied and the kinetic has been researched.Based on the mechanism of selenium leaching,a process that leaches copper anode slime in a closed system has been proposed by the cyclic utilization of nitric acid.Firstly,the process mineralogy of high nickel copper anode slimes and high lead copper anode slimes,especially for the occurrence of selenium have been characterized based on particle size analysis,XRF analysis,quantitative analysis,XRD analysis and SEM-EDS analysis.The results indicate that:in high nickel copper anode slimes,main phases are gold and its alloys,nickel oxide,copper sulfate,barium sulfate,lead sulfate and copper silver selenide.Main morphology of selenide is circular pieces.The composition of selenide is approximately as CuxAgySe(x=0.17～2.60,y=0.09～1.56),with an average of Cu1.02Ag0.53Se.In high lead copper anode slimes,main phases are gold and its alloys,lead sulfate,barium sulfate,copper sulfate and copper silver selenide,which are always intergrowth or mutually inclusive.Sometimes,a small amount of gold exists in copper silver selenide.In addition,lead sulfate and barium sulfate also adhere on it.Selenide has various morphologies,such as circular pieces,tube-like and globularity.Compared with high nickel copper anode slimes,copper silver selenide has less copper and more silver.The composition of selenide is approximately as CuxAgySe(x=0.15～1.82,y=0.99～2.83),with an average of Cu0.70Ag1.85Se.Secondly,Eh-pH diagrams for Se-H2O,Se-Cu-H2O and Se-Ag-H2O systems at temperatures of 298 K(25℃)and 363 K(90℃)have been plotted using a thermos chemical computational software of FactSage.The results indicate that:it is more favorable to extract copper from selenide than silver and selenium;elemental selenium is an intermediate product in the oxidation from selenide to selenite;strong oxidant must be added to leach selenium as the potential for selenite is high.Eh-pH diagrams at elevated temperature indicate that copper leaching efficiency is independent of temperature while the elevated temperature is advantage to the leaching of silver and selenium.On the basis of thermodynamic study,leaching efficiencies of copper,silver and selenium with time have been studied and the residues produced during leaching have been characterized by XRD and SEM-EDS.Furthermore,transformations of selenium-containing phases in copper anode slimes during leaching process have been investigated.The results indicate that:leaching efficiency of selenium increases slowly to 17.65%in the first 50 min and then more rapidly to 98.34%in the following 110 min.Selenium leaching process can be divided into three stages,copper-silver selenide first transforms to silver selenide and then converts to elemental selenium.Finally,elemental selenium dissolves as selenite.The transformation could be expressed as follows:Cu-Agselenide→Ag2Se→Se0→SeO32-.Intermediate product of elemental selenium is the main reason for the first slow and then fast leaching rate of selenium.Optimal conditions for selenium leaching by a nitric acid-sulfuric acid mixture have been investigated.The results show that:the optimal conditions for high nickel copper anode slimes are HNO3 concentration of 0.5 mol·L-1,H2SO4 concentration of 2.0 mol·L-1,solid-liquid ratio of 0.25 g·mL-1,leaching temperature of 90℃ and reaction time of 3.0 h.In that case,the leaching efficiencies of copper,silver and selenium are 99.22%,79.37%and 97.79%,respectively.The results of phase analyses during leaching process indicate that selenide in high nickel copper anode slimes is firstly converted to elemental selenium and followed by selenite ions in solution.The optimal conditions for leaching high lead copper anode slimes are HNO3 concentration of 1.0 mol·L-1,H2SO4 concentration of 2.5 mol L-1,solid-liquid ratio of 0.20 g·mL-1,leaching temperature of 90℃ and reaction time of 2.5 h.In that case,the leaching efficiencies of copper,silver and selenium are 98.37%,84.36%and 96.54%,respectively.The leaching kinetics indicate:leaching rate is practically independent of stirring speed,while dependent on temperature and concentrations of HNO3 and H2SO4.The silver leaching kinetics studies show that:the activation energy is 80.4 kJ·mol-1,and chemical reaction is rate controlling step.The empirical reaction order with respect to HNO3 and H2SO4 are 0.47 and 0.36,respectively.Leaching process of selenium includes two stages.The activation energy for the first stage is 103.5 kJ mol-1,and chemical reaction is rate controlling step.It is almost independent of H2SO4 while dependent on HNO3 since the empirical reaction order with respect to HNO3 is 0.56.For the second stage,the activation energy is 30.6 kJ·mol-1,and it is controlled by a mixed of diffusion and chemical reaction.Selenium leaching is almost independent of HNO3.The empirical reaction order with respect to H2SO4 is 0.21 when H2SO4 concentration is higher than 0.75 mol L-1 and it is-1.24 when the concentration is lower than 0.75 mol·L-1.The process leaching copper anode slimes in a closed system is proposed.Cyclic oxidation of nitric acid is used in this process and selenium can be leached efficiently with a low concentration of nitric acid.Additionally,the temperature and pressure for this process is not high and the economic efficiency is good.The optimum conditions for leaching high nickel copper anode slimes are temperature of 115℃,total gas pressure of 0.1 MPa,solid-liquid ratio of 0.20 g·mL-1,HNO3 concentration of 0.070 mol·L-1,H2SO4 concentration of 2.0 mol·L-1 and reaction time of 2.0 h.In that case,the leaching efficiencies of copper,silver and selenium are 99.35%,79.13%and 99.23%,respectively.The mineralogical characterization of the residues leached with different amounts of nitric acid has been investigated.The results show that selenide is difficult to be leached without nitric acid.When nitric acid is not sufficient(0.025 mol·L-1),silver selenide and elemental selenium is found in the residue.It indicates that copper can be leached more easily from Cu-Ag selenide than silver and selenide is oxidized into the solution,undergoing the intermediate product of elemental selenium.When nitric acid is sufficient(0.070 mol·L-1),selenium can be rarely found in residue.The optimum conditions for leaching high lead copper anode slimes are temperature of 115℃,total gas pressure of 0.1 MPa,solid-liquid ratio of 0.15 g mL-1,HNO3 concentration of 0.090 mol·L-1,H2SO4 concentration of 2.5 mol·L-1 and reaction time of 1.5 h.In that case,the leaching efficiencies of copper,silver and selenium are 99.62%,77.52%and 98.68%,respectively.The leachate,which contains copper,silver and selenium,is separated and recovered by the process of silver precipitation by sodium chloride—copper removal by extraction agent—selenium reduction by sodium sulfite.The optimum conditions for copper extraction are pH of 1.25,extraction agent(LIX 984N)concentration of 20%,phase ratio(O/A)of 2:1,extraction time of 10 min.In that case,copper extraction rate is 97.31%.The optimum conditions for selenium reduction are Na2SO3 concentration of 30 g·L-1,H2SO4 concentration of 2.0 mol·L-1,temperature of 80℃,reaction time of 10 min.In that case,selenium reduction is 99.09%.The content of selenium in reduction product is 99.36%,which mainly exists as elemental selenium. |
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