The impurity ions discharged on the cathode are in the cathode region, and the influence of impurities on the electrolysis process mainly depends on their precipitation potential and the overvoltage of hydrogen thereon. All the ions that can be discharged on the cathode have a common point, that is, their precipitation potential is always positive than zinc , and the reduction potential of some impurities is positive, and some are negative as well as zinc, but the absolute value is relatively small. Although these impurities can precipitate at the cathode and adversely affect the electrolysis production, the effects of different impurities are not exactly the same. The main reason for this difference is different from the hydrogen bonding force of the metal there are significant differences, therefore, according to which the hydrogen overvoltage and degree of the magnitude of the stable hydride is divided into the following three groups such impurities .
A lead , cadmium , tin , antimony and other metal ions Lead and cadmium ions are often present in industrial zinc electrolytes, while tin and antimony are rare, only in some special cases will enter the solution. Impurity lead is mainly from the anode plate, while cadmium comes from the concentrate. When the concentration of these impurity ions in the solution is high, there is a tendency to increase the dissolution of zinc due to the formation of microbatteries by zinc and these impurity metals.
B. Metal ions such as cobalt , nickel , copper, etc. The commonality of these metals is that the supervoltage at which hydrogen is deposited is different. Since they are also impurities of the precipitation potential which are positive compared with zinc, according to thermodynamic regulations, they will precipitate on the cathode earlier than zinc, which is the deposition of the first group of metals such as lead, cadmium, tin and antimony. similar. However, after they are deposited on the surface of the cathode, the metallic zinc does not precipitate thereon and covers them. Only hydrogen discharge occurs where such impurities are deposited. This is because the super-voltage deposited by hydrogen here is low, and the precipitation potential is higher than the discharge potential of zinc (the absolute value of the negative number is small). If there is a certain concentration of such impurities in the electrolyte, it will cause great interference to the electrolysis process. At this time, various holes will appear on the cathode sheet to cause the burning phenomenon.
Behavior and quality control of electrowinning-impurity of zinc during electrowinning
2022 02/23