Study On Synergistic Mechanism Of Tin Cadmium Ions On Indium Ion Electrodeposition

Indium is a rare metal,mainly occurring in polymetallic sulfide ores such as hematite and limonite.High-purity indium is widely used in the electronics industry,ITO film,semiconductors,CIGS solar cells and other new materials.The preparation methods of high-purity indium include electrorefining,vacuum distillation,zone melting,ion exchange and extraction.Aqueous solution electrolytic refining has attracted wide attention and research due to its simple conditions,low pollution,sustainability,and easy control of reaction conditions.In this thesis,by studying the mechanism of ion synergy and the electromigration of impurities,the electrochemical regulation of main impurities in the electrolytic refining process can be realized,which provides theoretical guidance for the electrolytic refining of crude indium.The research contents mainly include:the optimization of the main impurity factors in electrolytic refining of indium,the electrochemical mechanism of electrodeposition of metal indium,and the effect of Sn²⁺and Cd²⁺on the synergistic deposition of In during electrodeposition.The main conclusions are as follows:（1）In the In₂（SO₄）₃-H₂SO₄ electrolyte system,the effects of In₂（SO₄）₃ mass concentration,current density,Na Cl concentration and p H value on the concentration of impurity ions were studied by the single variable method.By analyzing the electromigration of impurity ions Pb²⁺,Sn²⁺,Cd²⁺and Tl⁺and combining with the characterization of cathode products,the limit concentration of impurity ions in the preparation of metal Indium system was obtained to avoid the co-deposition of In³⁺and impurity ions.The results show that when the mass concentration of In₂（SO₄）₃ is30 g/L,the current density is 60 A/m²,p H=2.5,the concentration of Na Cl is 90 g/L,the concentration of Sn²⁺dissolved in crude indium reaches the maximum value,which is 0.255 ppm.Within the limit concentration range,Sn²⁺does not co-deposit with In³⁺,which is beneficial to the electrolytic refining of crude indium.（2）The electrochemical behaviour of In³⁺on the Ti electrode was studied by cyclic voltammetry,chronoamperometry,and electrochemical impedance spectroscopy in In₂（SO₄）₃-Na₂SO₄ aqueous solution.The results show that the reduction process of In³⁺on the Ti electrode is irreversible and mainly controlled by the diffusion step.The diffusion coefficient of In³⁺is 7.31×10^-9 cm²/s.The timing current determines that the nucleation mechanism of metal Indium conforms to instantaneous nucleation.（3）To realize the synergistic impurity control of In³⁺and impurity ion Sn²⁺,the effects of different concentrations of Sn SO₄ on the electrochemical behaviour of electrodeposited In were studied by electrochemical test technology.The results show that when the concentration of Sn²⁺reaches 5000 ppm,the reduction peak of impurity Sn appears before the precipitation potential of In is-0.84 V（vs SCE）,and the reduction peak potential of Sn²⁺is-0.6 V（vs SCE）.At this limit concentration,the electrodeposition process of metal indium is irreversible,which is controlled by the diffusion step.The electrochemical reduction process of In³⁺is three steps and three electrons.The average diffusion coefficient is 5.90×10^-6 cm²/s,and the average charge transfer coefficient is 0.081.With the addition of Sn SO₄,the nucleation mechanism of metal Indium changed from continuous nucleation to instantaneous nucleation at the step potential of-0.75 V—-1.05 V.（4）To realize the synergistic control of In³⁺and impurity ion Cd²⁺,the effect of impurity ions Cd²⁺on the deposition of In was studied.Different concentrations of Cd SO₄ were added to the electrolyte of 0.06 mol/L In₂（SO₄）₃-0.07 mol/L Na₂SO₄.When the concentration of Cd SO₄ reached 500 ppm,the impurity Cd precipitation peak began to appear.The electrochemical reduction process of In³⁺at this concentration was three steps and three electrons.The average diffusion coefficient was 2.51×10^-7cm²/s,and the average charge transfer coefficient was 0.055.At the step potential of-0.75 V—-1.05 V,the metal Indium still conforms to the continuous nucleation process,and the addition of Cd SO₄does not change the nucleation mechanism of metal Indium.（5）In order to study the synergistic effect of Sn²⁺and Cd²⁺on In³⁺,different concentrations of Sn²⁺were added to the In₂（SO₄）₃electrolyte containing Cd²⁺.When the concentration of Sn SO₄ is 0.255 ppm,In³⁺and the major impurity ions Sn²⁺and Cd²⁺are co-deposited,and the co-deposition potential is-0.85 V（vs SCE）.Therefore,in the actual electrolytic refining process of crude indium,the electrolytic refining of crude indium can be further realized by purifying the electrolyte and controlling the concentration of Sn²⁺within 0.255 ppm.