Preparation And Properties Of Low Silver Lead Alloy Electrodes

Alloy anodes are widely used in industrial hydrometallurgical processes. Good anodes is beneficial for energy saving and essential to improve product yield and quality. A useful anode matrial must meet:electricial conductivity, electrocatalysis and stability. Lead anodes have been the preferred anode material for a long time. The lead-silver alloy anode in acidic sulphate bath has the following favourable features, small amounts of Ag(0.7-1.0%) alloyed with lead decrease the oxygen overvoltages, and increase the corrosion resistance of material during electrolysis. The most electrolytic zinc plants is to use Pb-Ag alloys containing 0.7-1.0%Ag as the anode material, because it has a longer life and a lower Pb content in the cathodic zinc.At the present, the research on the complex Pb alloy anode materials with low Ag content is a hotspot, but most research focus on the Ag content in Pb matrix and the addition of new elements, thereby ignoring the influence of the microstructure of Pb alloy anode on the electrochemical and mechanical properties. It is lacking that the associated research about the influence of the microstructure and structure evolution, the average grain size and the second phase distribution on the properties during the preparation of Pb alloy anode. In this paper, low Ag complex Pb alloy anodes based on the widely used optimally ratio were prepared and modified by supersonic grain refinement during solidification (SGR) with severe plastic deformation (SPD). These results provide important support for the application of high-performance, low-cost and energy-saving Pb alloy anode materials, and have important guidance to the development of this kind of materials.In this study, Pb-0.5wt.%Ag alloy with varying average grain size were successfully prepared by comprehensively utilized five process:ultrasonic, equal channel angular, temperature rolling, cold rolling and annealing. The electrical resistance, the electrode polarization curves, the cell voltage, Tafel curve, η-lg/map and microhardness were tested, the effects of process parameters on the microstructure and properties and the optimized process parameters were discussed. The influence of the average grain size of alloys and structure distribution on its properties were studied.The results have shown that ultrasound+rolling method is the best preparation process:melt temperature 643K (370℃), ultrasonic treating time 1 min, annealing temperature 343K (140℃),20% reduction rolling. Under this condition, compared with lead alloy anode by casting, the conductivity, electrochemical properties and mechanical properties are improved substantially. Which can reduce the resistivity of 44.86%,18.13% lower polarization potential, the cell voltage lower 4.82%,2.23 times the hardness can improve. The Tafel curves and η-lgi analysis indicate that using this kind of technology in favor of obtaining a lower corrosion current and over-points and improve the corrosion resistance of the anode and the electron transfer rate, with excellent electrochemical properties. At the same time, the corrosion morphologies of the anode surface obviously changed due to the dense and refined microstructures, the corrosion type is homogeneous but not previous local intergranular corrosion, thus the anode lifetime could be improved significantly.Furthermore, the results of metallomicroscopy indicate that the mechanical and electrochemical properties of anode alloys increase firstly and then decrease with the increase of average grain size and with the presence of a comprehensive optimum value, this can be explained as follows:(1) When the average grain size of Pb alloy anode materials is too small, the low electrical conductivity of anode materials can be contributed to the high defects density and the strong electron scattering effects of intragranular and intergranular, the low corrosion resistance can be contributed to the increased area and energy of grain boundary and the strong internal stress.(2) The anode materials with too large average grain size are of weak corrosion resistance and low electrical conductivity due to inhomogeneous component distribution. It’s easy to induce a large electric dipole potential and forming miniature electrochemical cell on the grains and grain boundaries of electrode surface in contact with the electrolyte, which cause the decrease of corrosion resistance. The silver component is more likely to distribute in intragrains and with the presence of the form of eutectic structure, but the electron scattering effects of grain boundaries is still strong due to low silver content, which cause the decrease of electrical conductivity.(3) For the anode materials having a suitable grain size, the Ag elemental component which electrical conductivity is much more higher than Pb distribute in the grain boundary, resulted in the electron scattering effects of grain boundaries weakened, further to improve the electrical conductivity of anode. It’s hard to induce local corrosion deeply and fastly due to the moderate grain size (about 10 μm) and the homogeneous and dense grain distribution, therefore the corrosion resistance of anode is enhanced and alloy anode materials possess good combination properties. Furthermore, the internal stress reduced and the structural thermal stability improved due to moderate grain size and make it possible to operate stably at the temperature slightly higher than the recrystallization temperature of the anode materials.Low silver content lead-silver alloy anode with 0.5wt.%Ag were prepared successfully by ultrasonic solidification associated with severe plastic deformation, rolling and annealing processing methods. The structural defects in alloy anodes were improved effectively, the mechanical properties, catalytic activity and corrosion resistance improved substantially and alloy anodes exhibit excellent combination properties. The processing method gained by this study is simple and low-cost, not only have great significance for the energy saving in the field of electrolysis, but also have important guidance to the recycling of electrode materials and yield significant economic returns.The solid liquid interfacial energy calculation model was studied, and the Warren model which can be used for the thermodynamic calculation of solid liquid interfacial energy of Pb Ag alloy two was improved. The new model can be built on Pb-Ag, Pb-Cu, Pb-Al and other two immiscible alloy system of the solid-liquid interface can be more simple and accurate estimates, and included in the Warren model without considering the temperature on the molar volume.