Preparation of high purity indium by chemical purification: Focus on removal of Cd, Pb, Sn and removal mechanism

In this study, a wet refining process for indium purification was developed aiming at the problem that the electrolysis refining method is difficult to remove cadmium, lead (Pb), and tin in indium, so as to reduce the energy-consuming and time-consuming process of smelting at the same time. The original electrolyte made by crude indium (99%) is co-precipitated to remove lead and pre-electrified to remove tin, and then heated to activate indium ions during electrolysis refining to reduce the electrodeposition of Cd2+, and the purity of electrodeposited indium can reach 99.99% after only once electrolysis refining like this. The mechanism of the removal of Pb2+ by co-precipitation is also discussed: 1–6 ppm Pb2+ is in the metastable region (dissolution and crystallization), and cannot nucleate spontaneously, but as long as there is a surface charged solid in the solution, which can provide Coulomb force, Pb2+ can be induced to grow on solid with SO42−, regardless of crystal form, chemical configuration, and ion type. This viewpoint can explain some anomalous phenomena which cannot be explained well by the traditional co-crystallization and co-precipitation theory. In addition, it is the first observed that temperature plays an extremely important role in the separation of In3+ and Cd2+ in the electrolytic refining process. During electrolytic refining at 40 °C, even though the concentration of Cd2+ in the solution is as high as 200+ ppm, the content of cadmium in the electrodeposited Indium meets the standard of 4 N-Indium, while In3+ and Cd2+ are generally considered cannot be removed by electrolytic refining because their electrode potentials are too similar. The mechanism is also explicitly explained: the In3+ electrodepositing process is mainly controlled by activation, and the electrochemical reaction resistance decreased to 1/4 of the original with the increase of temperature, so that the cathode only needs a smaller overpotential to achieve the expected current, reducing the amount of Cd2+ electrodeposition in the cathode. •A set of simple, environmentally friendly and low energy consumption closed - loop purification process is presented.•A new view on co-precipitation of co-crystallization type is put forward, which can explain some special phenomena.•Trace tin in electrolyte was treated by pre-electrolysis.•Cadmium was removed from indium by electrochemical method for the first time and its removal mechanism was investigated.