Preparation And Properties Of Low Silver Lead Alloy Based Lead Dioxide Composite Anode

In modern world, more than 80%zinc are extracted by the hydrometallurgy methods, and the high energy consumption is an outstanding problem of the methods. The No.13 announcement (lead and zinc industry access conditions), which was conducted by the National Development and Reform Commission in 2007, indicated that the DC power consumption and the current efficiency of zinc electrowinning process in new zinc smelterys should be lower than 2900 kWh/(t·Zn) and higher than 88%, respectively. Therefore, further reduce of the power consumption in zinc electrowinning process is the need to remit the energy shortage problem in our country, and improve the independent innovation ability of hydrometallurgy enterprises.At present, Pb-(0.8?1)wt%Ag alloy is widely used as anode in the zinc electrowinning process, but this kind of anode objectively possess low electro-catalysis activity, high energy consumption, and poor corrosion resistance. In this research, the composite electrodeposition technique was employed to prepare the multicomponent composite anodes used in zinc electrowinning process, which were composed of WC, Co3O4 active particles and ?(?)-PbO2 active oxide on the substrate of low silver lead alloy (substrate is Pb-0.3wt%Ag, the interlayer and electro-catalysis surface layer are active particles reinforced ?-PbO2 and ?-PbO2 respectively). Our purpose is to reduce the cell voltage, electrowinning energy consumption and anodic corrosion rate, through the synergy of active particles and oxides and the material synergistic effect of multicomponent composite, which would improve electro-catalysis activity for oxygen evolution and corrosion resistance of the anodic materials. The topic of this paper has achieved the financial support of the National Natural Science Foundation of China, the Specialized Research Fund for the Doctoral Program of the Ministry of Education of China, and the Key Project of Yunnan Province Applied Basic Research Plan of China.First of all, the steady-state diffusion system has been established using a rotating disk electrode. The rate constant (k) and reactant's diffusion coefficient (D) of the PbO2 electrosynthesis process in alkaline solution have been calculated by hydrodynamic voltammetry methods. The ?-PbO2 electrosynthesis process in acidic solution is under the mixed control of diffusion and electrochemical, and the diffusion control plays a more important roll in the ?-PbO2 electrosynthesis process as the increase of HNO3 concentration. Thus, the change rule and regulatory mechanism of kinetics in PbO2 synthesis process were determined, and the optimized kinetic condition was obtained. The rotating ring-disk electrode was employed to carry out the collection experiment, and the results show that soluble intermediates HPbO2 or HPbO2+ were existed in ?-PbO2 electrosynthesis process. Based on the results above, we came up with the reaction mechanism of ?-PbO2 electrosynthesis process in alkaline solution. The results show that PbO2 synthesized in the alkaline solution consists of pure a phase, and the preferential orientation of growth along the (200) crystallographic plane can be observed. Meanwhile, the PbO2 synthesized in the acidic solution is mainly consists of P phase, and the grain is tetragonal crystal system.Secondly, particles reinforced ?-PbO2 composite anodes were synthesized on the Pb-0.3wt%Ag substrate using composite electrodepostion technique, and the influence of plating compositions and technological conditions on the physical and electrochemical properties of the composite anode have been investigated. The experimental data show that Pb-0.3wt%Ag substrate firstly passivated to form a PbO film. Then the ?-PbO2 electrosynthesis proceeded simultaneously with step by step oxidation of PbO (PbO?Pb3O4??-PbO2 or PbO??a-PbO2). The exsiting of WC or Co3O4 particles in ?-PbO2 plating bath did not change the mechanism of ?-PbO2 electrosynthesis process. The addition of WC particles can obviously increase the electrosynthesis rate of ?-PbO2, restrain the agglomeration of Co3O4 particles on the electrode surface, and refine the grains of ?-PbO2. The active particles reinforced ?-PbO2 composite anodes prepared under the optimal condition possess the best electro-catalysis activity for oxygen evolution and corrosion resistance.Thirdly, particles reinforced ?-PbO2 composite anodes were synthesized on the PCW substrate (PCW stands for Pb-0.3wt%Ag/?-PbO2-WC-Co3O4 substrate) using composite electrodepostion technique, and the influence of plating compositions and technological conditions on the physical and electrochemical properties of the composite anode have been investigated. The results show that the nucleation of ?-PbO2-Co3O4 coating on the PCW substrate accords with the electrochemical mechanism of nucleation, and the nucleation of ?-PbO2-WC-Co3O4 coating on the PCW substrate accords with the mechanical mechanism of nucleation. In addition, there is competition between the two kinds of particles in the adsorption process, and WC particles possess obviously advantage in the competitive adsorption. The particles reinforced ?-PbO2 composite anodes, which were prepared in the plating bath containing 30 g/L WC particles and 15 g/L Co3O4 particles, possess uniform and compact coross section, and the strong combination with a-PbO2 interlayer. In addition, the oxygen evolution overpotential at 500 A/m2 and the corrosion current density of the particles reinforced ?-PbO2 composite anode obtained in the simulation zinc electrowinning electrolyte was 0.668 V and 8.02×10-5 A·cm-2, respectively. It was 318 mV lower and one order of magnitude lower compared with pure ?-PbO2 composite anode, which shows significant improve in electro-catalysis activity and corrosion resistance.In the end, the electrochemical characteristics of the new composite anodes have been studied in a synthetic zinc electrowinning solution, and the characteristics were compared with the traditional lead-silver alloy anode (Pb-lwt%Ag). The results show that the electro-catalysis activity for oxygen evolution and the corrosion resistance of Pb-lwt%Ag in the simulation zinc electrowinning electrolyte improved gradually along with the 16 days electrowinning. The phase change of Pb-lwt%Ag in the electrowinning process showed an overall trend of step by step oxidation:Pb?PbO?Pb3O4??-PbO2??-PbO2. The electro-catalysis activity for oxygen evolution of PCW/?-PbO2-Co3O4 and PCW/?-PbO2-WC-Co3O4 composite anodes firstly reduced then improved gradually. This two kinds of new composite anodes would trace dissolved and generate PbSO4 in the electrowinning process. The PbSO4 in the anodic surface would gradually transfer to PbO2, which possess higher valence, much more stability, and better electro-catalysis activity, along with the electrowinning process. The changes of the electro-catalysis activity for oxygen evolution have the relationship with the surface phase change of the composite anodes. After 16 days zinc electrowinning, the cell voltage of PCW/?-Pb02-WC-Co304 and PCW/?-PbO2-Co3O4 composite anodes were 2.82 V and 2.77 V, which were reduced 0.3 V and 0.35 V compared with Pb-lwt%Ag (3.12 V), respectively. The current efficiencies were 93.2% and 92.7%, which were reduced 2.11% and 1.6% compared with Pb-lwt%Ag (91.1%), respectively. The power consumption were 2481.55 kWh/(t-Zn) and 2450.7 kWh/(t·Zn), which were reduced 327.28 kWh/(t-Zn) and 358.13 kWh/(t-Zn) compared with Pb-lwt%Ag (2808.83 kWh/(t-Zn)), respectively. The new composite anodes show the remarkable energy-saving effect.