Technology Of Copper Electrolyte Purification By Arsenic Valence-adjustment And Its Fundamental Theory

Abstract:The technology of removing copper and impurities by induction method is widely used for purifying copper electrolyte in industrial production, the method presents many disadvantages, such as high energy consumption, forming toxic arsine gas and producing a lot of black copper. In the paper, the new copper electrolyte purification process of decopperization by gradient current density-adjustment and dearsenication by valence-adjustment was proposed to solve these problems. The process can markedly increase the output ratio of cathode copper during copper electrowinning, and greatly reduce the black copper and recover As2O3from copper electrolyte. The electrodepositing mechanism of copper in Cu-depleted electrolyte was investigated to reveal the copper removal mechanism.Arsenic(V) in copper electrolyte was reduced to arsenic(Ⅲ) with the aid of sulfur dioxide, and n(As(V))/n(ATT) was adjusted to0.4, as well as n(Sb(V))/n(SbT), the resultant electrolyte was subject to concentrate and the ratio of initial volume and final volume was2.5time, then the electrolyte was cooled to10℃to crystallize, the removal rates of Cu, As, Sb and Bi are82%,62%,55%and85%respectively, the purifying residue includes CuSO4·5H2O and As2O3. When the purifying residue was dissolved in water, As2O3residue and copper sulfate solution were obtained respectively after filtering. The CuSO4-5H2O content is98.8%when the copper sulfate solution was purified through oxidation by H2O2, adjusting pH by Na2CO3and evaporative crystallization.Adjusting the concentrations of AsT and As(Ⅲ) to10.00g/L and5.00g/L,respectively according to the rule of arsenic valence-adjustment, after copper electrolysis carried out with current density235A/m2at65℃for168h, the concentrations of AsT, Sb and Bi were decreased2.9%,35%and18.18%, respectively. In the process of copper electrolysis, As(Ⅲ) was gradually oxidized, and the oxidation process is applicable to the first order kinetic model.The copper and arsenic in copper electrolyte were separated by multiple stage electrowinning of current density adjustment. When the Cu, As, Sb, Bi and H2SO4in the electrolyte were49.51g/L,10.75g/L,0.369g/L,0.299g/L and181g/L, respectively, the high-purity copper cathode is prepared in the first electrowinning under the conditions that current density of200A/m2, additive(with mass ratio of bone glue:gelatin: thiourea of6:4:5) dosage of40mg/L, electrolyte circulation rate of10mL/min, electrolytic temperature of55℃and Cu concentration in the electrolyte decreased from49.51g/L to29.99g/L. By adjusting the current density to100A/m2and the electrolyte temperature to65℃, when the Cu concentration decreased from29.99g/L to8.94g/L, the copper cathode(Cu-CATH-2) is prepared in the second electrowinning. In the third electrowinning with current density of100A/m2, when the Cu concentration decreased from8.94g/L to1.69g/L, the main product is black sludge. The removal rates of Cu, As, Sb and Bi are96.59%,21.30%,25.20%and75.58%, respectively.The As(V) in the Cu-depeleted electrolyte was reduced to As(III) by SO2, then the resultant solution was subjected to evaporation concentration, when the volume ratio of the reduced solution to the crystallized solution was2.65, and the concentration solution was cooled to15℃to crystallize, As2O3is obtained after filtering, then the arsenic removing solution was cooled to-20℃to crystallize and crude NiSO4is obtained. The removal rates of As, Sb and Ni are90.5%,25.38%and56.58%, respectively.By adding As2O3into the electrolyte to maintain As(III) concentration at about4.84g/L, after copper electrolysis carried out with current density235A/m2at65℃for168h, the Sb concentration decreased from0.48g/L to0.40g/L, and the Bi concentration decreased from0.45g/L to0.34g/L. Nickel removal solution was returned to electrolysis system, after copper electrolysis carried out with current density235A/m2at65℃for154h, the Sb concentration decreased from0.492g/L to0.441g/L, and the Bi concentration decreased from0.341g/L to0.234g/L, after copper electrolysis carried out with current density305A/m2for123h, the Sb concentration decreased from0.411g/L to0.375g/L, and the Bi concentration decreased from0.336g/L to0.232g/L. The quality of all the copper cathodes reaches to the Chinese standard of A grade of copper cathode (GB/T467-2010).For the electrodeposition of copper from solutions of2.5g/L Cu2++250g/L H2SO4, the reaction Cu2++e->Cu+is the rate controlling step, and which is an irreversible charge transfer reaction. There is no surface conversion in the cathodic process. The electrode reaction is controlled by both of the electrochemical and concentration polarization. The values of the apparent transfer coefficient for cathodic process (a) and anodic process(a) are0.49and1.42, respectively. The chemical measurement number for the rate controlling step, v equals1approximately. The orders of the cathodic and anodic processes are1and0, respectively. The kinetics equation of copper electrodeposition is shown as follows:When Cu2+concentration is2.5g/L and H2SO4is250g/L in the electrolyte, both As(V) and As(Ⅲ) have a depolarization effect on the process of copper deposition and increase its peak current density. When As(V) exists in the electrolyte, only one reduction peak is observed in the cyclic voltammogram, and As(V) makes the peak potential of copper deposition shift to more negative value, while when As(Ⅲ) exists in the electrolyte, two reduction peaks are observed, and oxidation peak is not observed in the presence of As(V) and As(Ⅲ).The electrode reaction is controlled by both of the electrochemical and concentration polarization.The As content in the deposit increases with the decrease of cathodic potential. In the electrolyte with Cu2+of2.5g/L, H2SO4of250g/L and As(V) of10g/L, when the cathode potential is-0.2V vs.SCE, the deposit contains Cu and Cu2O, while the deposit forms at-0.43V vs.SCE contains Cu3As, Cu2O and Cu5As2. When the cathode potential is-0.6V vs.SCE, H2and AsH3are produced. The results show that the copper and arsenic in copper electrolyte contaning As(V) can be separated by multiple stage electrowinning of current density adjustment.