Zinc Sulfate Solution, Zinc Replacement In Addition To The Mechanism Of Cobalt

Chinese reserves and yields of zinc are the biggest in the world and Chin is the main producing and exporting country. However, China is behind in the technology and equipment of zinc metallurgy. Many overseas zinc plants have realized mechanization and automatization. Mechanized scalping zinc is the top level of zinc hydrometallurgy. To realize mechanized scalping zinc, the main impurities in zinc sulfate solution must be purified to reach adequacy low. Most researchers in domestic focused on purification technology and seldom researched purification mechanism or can't quantify it, which is the main reason that leads to low purification technology. Therefore, continuing research on impurities removal from zinc sulfate solution to obtain the cementation mechanism by zinc dust and to obtain the favorable technical parameters of impurities removal is essential to purify and electrodeposit of zinc hydrometallurgy. Of all metallic impurities in the zinc electrolyte, cobalt is the most difficult to remove and has the most detrimental effect in electrowinning. The cobalt removal process has been an important issue in the zinc metal industry and in academia for many years. Several mechanisms have been proposed for the role of copper and antimony in enhancing cobalt removal. However, the mechanism of cobalt removal from zinc sulfate solution by zinc dust cementation is still not understood well.There are many factors such as zinc ion concentration, zinc species, temperature, pH, rate of agitation influencing on cobalt removal. The mechanism of cobalt removal from zinc sulfate solution is analyzed from thermodynamics and kinetics.The experiments in this paper were carried out by zinc dust with zinc sulfate solution with high concentration. The purged liquor was vacuum filtration and was chemical analysis to determine cobalt removal rate. Residues after the cementation process from zinc sulfate solution were analyzed by XRD, SEM, and Electron probe micro-analysis (EPMA) to investigate the morphology of products. At different conditions, different species zinc dusts such as purified zinc and alloy zinc and different activators such as Cd²⁺, Cu²⁺, Sb₂O₃ and Na₂SO₃ were added to determine the morphology of products to find out the mechanism of cobalt removal by zinc dust. The reduction potential of zinc during cementation was determined to investigate the morphology of products. The stability of cemented impurities was studied in zinc sulfate solutionto obtain the cobalt redissolution rule and mechanism.The following results are obtained in this paper:1. The reduction potential of zinc during cementation was determined by ZF-10B data acquisition unit to investigate the morphology of products. It is thought that it can approximately reflect the cobalt removal process.2. The research results of cementation show that the cobalt cementation will be inhibited in the present of zinc ion. With the increasement of temperature, the cementation reaction is more easily. Different temperature will cause different structure of particles. The mechanism of cobalt removal by zinc dust is thought as follows: firstly, the cobalt ion forms Co-Zn alloy with zinc particles, and the reaction reaches equilibrium quickly; secondly, the c continues reacting to Co with Co₂₊, which is the control procedure of the whole reaction. Therefore, the whole reaction can be expressed by:Co²⁺+Zn+2e(?)Co(Zn)The Co-Zn alloy is thought to be a solid solution.The cobalt cementation by zinc dust is not a single reaction, and is not a first order reaction, which includes at least the following two reactions: forming cobalt alloy (it is not sure to form one or several cobalt alloys.); forming basic cobalt salt on the surface of zinc dust.3. It shows that copper in the absence of antimony may act both as an accelerator and as inhibitor in cobalt cementation depending on whether it precipitates as an alloy or pure copper. Alloy precipitation may be considered as a potential improver while copper precipitation may have the adverse effect. The tradition statement "it can accelerate the cementation and is benefit for cobalt removal by zinc dust with the copper added" is not imprecision and unperfected.In the presence of copper ion in zinc sulfate solution, copper ion precipitates first by zinc dust ^and formed Cu-Zn microcell, and then cobalt precipitates on copper surface. Therefore, it is benefit for cobalt removal due to positive potential on copper cathode for cobalt. Cobalt can form intermetallics with metals such as zinc and copper, and the intermetallics are thought to be solid solutions.4. Large amounts of pure cobalt, Co-Zn alloy and slight amount Co-Sb alloy are formed in the presence of antimony. The structure of particles of the purified residues from zinc sulfate solution with high cobalt ion concentration at high temperature from outer to ectonexine is cobalt metal, Co-Zn alloy, Co-Sb alloy and zinc metal. In the presence of copper and antimony, antimony and copper cements first and generates an active surface onto which a Co-Zn alloy is deposited. The zinc anode and antimony cathode form microcell of Zn∣Zn²⁺‖Co²⁺∣Co (Sb), so cobalt can deposit continually. Antimony can inhibit hydrogen evolve and make deposition invert to potential positive direction5. Study on the effect of cadmium ion on cobalt removal, the following conclusions are obtained:â‘ Cadmium is beneficial in terms of the final cobalt concentration, which can increase the rate and extent of cobalt cementation. It is an important consideration in purification circuits that removal cadmium completely prior to cobalt cementation, because cadmium can inhibit basic zinc sulfate to precipitates on the surface of zinc particles. However, the results in this research will bring forth the problem of comprehensive utilization sinter of cadmium.â‘¡Cementation by zinc dust and without any activators addition to the solution, a great deal of basic zinc sulfate is formed and it surrounds zinc. The basic zinc salt layer formed is thought to inhibit the cementation reaction and leads to low efficiency of cobalt removal. Zn-Co and Zn-Cd alloys are thought to be likely to enhance cobalt removal. Zn-Cu and Cu-Cd alloys are thought to not to be benefit for cobalt removal.6. Wishing to find a more economical way to deoxygenate the electrolyte, we investigated the possibility of adding sodium sulfite to consume the oxygen and obtained the following conclusions:â‘ Sodium sulfite is beneficial in terms of the final cobalt concentration, which can increase the rate and extent of cobalt cementation. It might be judicious to regards sodium sulfite as an activator and add it at the cobalt cementation stage.â‘¡When the amount of sodium sulfite addition is more 100g/L, what occurred instead was the reduction of sulfite to elemental sulfur, which adhered to the zinc dust surface forming a passivating layer. The reaction isSO₃^2-+2Zn+6H⁺→2Zn²⁺+S+3H₂O.7. The effect of reaction time, oxidation, zinc dust and Cu₂₊ on cobalt redissilution was experimented. Zinc size, Cu²⁺ and filtration are the main reasons of cobalt redissilution. The small particles gave a high initial cementation rate, but redissilution of cobalt was also extensive. The larger particles showed a slow cementation rate and little redissolution. The redissolution of cobalt decreased with increasing amount of added zinc dust. The redissolution of Cu²⁺ caused cobalt redissoultion. The redissolution reaction is 2Co_s+O₂+2H₂O→2Co(OH)₂, where Co(s) is simple substance Co or solid solution. The cobalt cementation by zinc dust is not a single reaction, and is not a first order reaction, which includes at least the following two reactions: forming cobalt alloy; forming basic cobalt salt on the surface of zinc dust. When kinetic models of the cobalt cementation process are made, the formation of basic cobalt salts should be included.8.According to the mechanism of cobalt removal and technical parameters mentioned above, the real solution was used to examine the mechanism and technical parameter with zinc dust, CuSO₄ and Sb₂O₃. The results showed that the impurities of As, Sb, Co, Ni, Cu, Cd can meet the purification demands and can realize zinc electrowinning for 48h.