Methods And Properities Of Low Temperature Plasma Modified Cation Exchange Menberanes

As an ion-selective permeation membrane,Ion exchange membrane has been widely applied in various fields of human life and industrial production,such as material separation and purification,energy storage and transformation.In the application,the ion exchange membrane has the special property of blocking the ions of the same name that only allow the counter ions to pass through.In view of this characteristic,the research group proposed a green metallurgy process for producing metal cobalt by using ion exchange membrane to build double-membrane three-chamber electrowinning.The improved double-membrane three-chamber membrane electrowinning process utilizes acation exchange membrane to block the passage of chloride ions and avoid the generation of chlorine gas in the anode compartment.Thus,it can solve the problem of poor production environment and damage to human health,moreover,the chloride ions can be made into hydrochloric acid and used for the preparation of cobalt stock solution.At the same time,the electrolyte stability problem was solved,and the cobalt green production process was realized.In practical project,the ion exchange membrane works in a strong oxidizing environment of high-temperatureand highacid-base;and it has been found that the chlorine ions in the electrolyte of the cathode compartment can be partially penetrated due to the poor permeation performance of the cation exchange membranes,so the chlorine gas was produced in the anode,and then the hypochloric acid was produced in the anode chamber,and the ion exchange membrane would be oxidized by hypochloric acid,which further reduced theselectivity of the membrane.In order to solve the problem that the common heterogeneous cation exchange membrane in the market is easy to be oxidized by the hypochlorite produced in the anode and the membrane performance is degraded to affect the process effect in this process.Selecting the appropriate modification method to improve the chlorine resistance of the ion exchange membrane is the core technology of this process.This paper mainly studied the modification method and performance characterization of cation exchange membrane.Firstly,PVDF was used as grafting coating solution,and low temperature plasma indirect liquid-phase grafting method（Ind-PVDF method）,direct method（D-PVDF method）and combination method（C-PVDF）was used to modifiy the ionexchange membranes.After the membrane was oxidized,the leakage rate of chloride ion,the capacity of ion exchange and the breaking strength are measured,and the change rate of the original membrane is calculated as the index value.Using the uniform test design,we chose the main test factors and levels in the process of each modification,and combined the suitable uniform test table to complete the multi-factor and multi-level test program design.The regression equation is obtained by regression analysis of the data of each index value through software SPSS 23.Finally,the equation is optimized by using the inversion substitution combination method and other value line graph methods to obtain the best factor level combination.The results showed that uniform design can be used to arrange multiple factors and multiple levels with fewer trials.Ind-PVDF method used 12 trials for 7 factors and 6 levels,and D-PVDF assay used 10 trials for 5 factors and 5 levels,the C-PVDF method used 16 tests for 7 factors and 4 levels.Combining the regression equations of each index,the equation is significant（R2>0.9）,the equation was optimized by using the inversion substitution combination method and other value line graph methods to obtain the best factor level combination,the best combination of factors are: Ind-PVDF method,discharge current 5A,discharge time 10 min,oxidation time 20 min,grafting Concentration 8%,grafting time 60 min,vacuum degree 30Pa;D-PVDF method,discharge current 5.5A,discharge time 3min,grafting concentration 8%,vacuum degree 30 Pa,nitrogen discharge;C-PVDF method,discharge current I₁ is 4.5A.The discharge time t1 is 10 min,the discharge current I2 is 5A,and the discharge time t₂ is 5min.Three kinds of modified membranes were prepared by the combination of the preferred factors.The properties such as chlorine oxidation resistance,ion exchange capacity and membrane resistance of the modified membranes were tested.The surface morphology of the membrane was characterized by electron microscope（SEM）.The results are as follows: the chlorine resistance of the modified membrane is greatly improved,the leakage rate of the chloride ion after oxidation treatment C-PVDF（8.21%）< D-PVDF（9.13%）< Ind-PVDF（9.45%）< 15.14%);Membrane breaking strength（MPa）is also improved than that of the unmodified membrane,C-PVDF（1.0236）> D-PVDF（0.9563）> Ind-PVDF（0.8966）> unmodified membrane（0.8467）.However,other performance indicators of the three modified membranes have deteriorated,the ion exchange capacity of the modified C-PVDF membrane with better performance is 2.0366 mmol / g^-1,the water content is 38.53%,which is 14.61% and 8.59% lower than that of the unmodified film,and the membrane surface resistance is 11.66% higher than that of the unmodified film.The SEM photo can clearly see that the PVDF protective layer is successfully grafted.In order to improve the degradation of membrane performance caused by liquid phase graft coating of PVDF,the reprocessing（C-PVDF-AA）is performed by a low-temperature plasma liquid-phase grafting of acrylic acid（AA）on the C-PVDF modified membrane.The effects of different grafting concentration,discharge current intensity and plasma treatment time on the membrane modification were investigated.The ion exchange capacity and resistance properties of the modified membranes were tested before and after modification,as well as the surface structure of SEM and ATR-FTIR.The test results showed that the modified effect is best when the AA mass fraction is 70%,the discharge current intensity is 4A,and the plasma treatment time is 5min.Under the optimal modification conditions,the modified membrane has a significant increase in the active group,and the ion exchange capacity and water content are increased before the modificationwhich are 2.6572 mmol g^-1 and 43.89% respectively,while the membrane resistance decreased to 9.50Ω· cm2.SEM photos showed that the surface roughness of C-PVDF-AA modified membrane decreased,and a polymer membrane was successfully grafted.ATR-FTIR analysis shows that the new emerging peaks of the expansion vibration conforms to the characteristic of the PVDF characteristic band,which further illustrates the existence of the PVDF protective layer and the presence of carboxyl and hydroxyl groups on the surface of the membrane proves the successful grafting of acrylic acid.Finally,the C-PVDF-AA modified membrane was installed in a laboratory small-scale membrane electrophoresis apparatus,and the parameters were set to operate according to the actual production conditions.The results showed that the amount of chlorine generated by chloride ion passing through cation exchange membrane of C-PVDF-AA modified membrane was greatly reduced,the current efficiency reaches 93.45% and the metal production increases by 9%,which increases the output and reduces the cost and chlorine production.