Modified Cation Exchange Membranes Based On Double Membrane Three-Compartment Cobalt Electrowinning Process

The ion exchange membranes application technology is a cutting-edge,sustainable and one of the significant application technology in the current research field.The centre of the application is to use the special separation capabilities of ion exchange membrane to separate some certain ions so as to achieve the function of substances separation.The specific ions exchange groups in the membrane is the core of ion-exchange membrane separation effect.The ion exchange membranes have a special feature called “ion selective permeability” which is the presence of exchange groups determines the passage of some specific ions.This kind of special function makes ion exchange membranes have a broad application values in the future.However,the chlorine ion leakage rate and some other features do not result in the current finished products of ion exchange membrane to meet the requirements of some certain processes.And problems such as chloride ion leakage or membrane fouling may occur in the separation process,which makes it difficult to achieve ideal technical outcomes.Therefore,it is necessary to conduct the modification research of ion exchange membranes.In order to complete the modification of ion exchange membrane,the experiment analyzed the mechanism of the ion selective permeability of ion exchange membrane in depth.The analysis showed that the widely recognized theory of electric double layer and Donan balance theory have many theoretical defects.The hole transport theory proposed by Wang Sanfan et al.is more applicable to the selective permeability mechanism of ion exchange membrane under the action of external electric field force.The "membrane potential repulsion-hole closure phenomenon" research could provide a theoretical basis for the ion exchange membrane modification.The main theory is that only if the range of the radical potentials affecting the both sides of the membrane pore is larger than half of the membrane pore size,the membrane could have an interception effect on the counter ions.Therefore,the modification experiment needs to start from the action potential and membrane pore size of the ion exchange membrane surface to achieve the purpose of modification.Firstly,the experiment used low-temperature plasma discharge techniques to graft dimethylaminoethyl methacrylate and 1,3-propanesulfonic acid onto a self-made sulfonated polyvinylidene fluoride grafted styrene cation exchange membrane.The lactone synthesized sulfonate zwitterionic polymer.By grafting a large number ofsulfonic acid groups to enhance the role of the ion exchange membrane surface action potential.In the mean time,the self-polymerization effect of polymer helped to seal the large membrane pore,so as to achieve the aim of reducing the ion leakage rate.In addition,the characteristics of the modified polymer could improve the hydrophilicity of the ion exchange membrane,reduce the membrane resistance,and increase the ion exchange capacity.By measuring the chloride ion leakage rate,scanning electron microscope,and using Fourier infrared analysis to analyze the ion exchange membrane before and after modification,the performance of the ion exchange membrane was characterized.Secondly,in order to save the cost of the process and make it suitable for industrial application,it usedchemical impregnation method to graft conjugated conductive polymer polypyrrole onto a polystyrene heterogeneous cation exchange membrane,by blocking the pore size of the membrane and increasing the conductivity.By measuring the chloride ion leakage rate,scanning electron microscope,and using Fourier infrared analysis to analyze the ion exchange membrane before and after modification,the performance of the ion exchange membrane was characterized.Finally,in order to verify the effectiveness of the two modified ion exchange membranes in practical applications,the original membrane and four modified ion exchange membranes were assembled in a double-membrane three-cell cobalt electrowinning process to perform parallel experiments,the chlorine reduction rate,power consumption,and stability of the device were determined by the experiment.Testing the membrane performance showed that the plasma-modified homogeneous cation exchange membrane significantly increased the number of ion-exchange active groups,the chloride ion leakage rate decreased from 11% to 2%,the hydrophilic angle decreased from 90 ° to 59°,the sheet resistance decreased from9.8? to 4.6?.The surface of the heterogeneous cation exchange membrane modified by polypyrrole was compact and flat,the leakage rate of chloride ions was reduced from 14% to 5.6%,the sheet resistance was reduced from 12? to 7.6?,and the electrolysis lifetime was increased from 336 h to 528 h.At the same time,the practical application tests showed that both of the modified membrane devices could be used in the the double-membrane three-compartment cobalt electrowinning process.The reducing effect of chlorine and electric current efficiency of plasma modified membranes had a stronger effect than the polypyrrole modified membranes.However,the stability of electrolysis of plasma modified membranes were worse than polypyrrole modified membranes.Both of the modified membranes could meet thebasic requirements of the new cobalt electrowinning process,and the modified membranes have preferable application values.