Preparation, Characterization And Application Of Polymeric Ion Exchange Membranes

Ion exchange membranes (IEMs) are widely applied in the field of chemical engineering, metallurgical industry, and energy & environmental technology so on. High performance IEMs with low manufacture cost are urgently needed to meet the requirement of practical applications. Ion exchange membrane is divided into homogeneous and heterogeneous ion exchange membrane, among which heterogeneous ion exchange membrane is favored by the industry because of excellent mechanical properties, low price, convenient assembly, and cost-effective so on. Generally, IEMs are composed of robust polymer backbones and functional ion exchange groups. In this paper, we employed polyvinyl chloride (PVC) and polyvinyl fluoride (PVDF) as the polymer backbones of IEMs. Free milling sulfonated polystyrene fine resin was utilized as functional group provider for IEMs. Based on the typical method for preparation of heterogeneous IEM, the resin powder was uniformly dispersed in the polymer solution, followed by drying and hot pressing process. The resulting low-cost heterogeneous IEMs exhibit excellent mechanical properties as well as good electrical performance. The desalinating performance of PVC/PVDF heterogeneous IEMs were evaluated in the electrodialysis system composed of 0.1 M NaCl solution and coal chemical wastewater. Considering the leakage of resin powder from IEM during electrodialysis process, crosslinked no-charged IEMs were prepared employing polyvinyl alcohol (PVA) as the main matrix. Co-crosslinking reagents were investigated including tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), aminopropyl triethoxysilane (APTEOS) and methacrylate trimethoxysilyl propyl (γ-MPS), which are common and typical structure. Afterwards, the as-prepared IEMs were applied in the field of diffusion dialysis to evaluate its performance of alkaline recovery from a NaOH/Na2WO4 system.The main conclusions of this paper are listed as follows:(1) Using PVC as the binder and free milling commercial resin powder (SPSt-co-DVB powder) as the functional groups provider, the heterogeneous ion exchange membranes were prepared by uniformly distributing the resin powder in the PVC solution, followed by drying and hot-pressing process. The hot-pressing temperature 160℃ is based on the glass temperature (Tg) and the decomposition temperature temperature (Tc) of PVC. The resulting IEMs exhibit good electrical properties, high separation performance as well as excellent mechanical properties. When the weight percentage of resin powder increases,the membrane properties such as ion exchange capacity, water content, and transport number so on increase accordingly; while the area resistance decreases. When the weight percentage of resin powder increased to the maximum value of 61.5%, the prepared IEMs exhibit even better desalination performance compared with the commercial Qianqiu(?) heterogeneous membrane.(2) When using PVDF as the binder instead of PVC, heterogeneous ion exchange membranes were successfully prepared according to the above-mentioned method. The changing trend of IEMs properties with the weight percentage of resin power is in consistence with PVC based heterogeneous IEMs. Besides, ascribing to its melting point (Tm) of crystal PVDF, the hot-pressing temperature 160℃ could be also based on the critical temperature of rheological shear thinning. This enables higher dosage of resin powder (69.0%) without adding plasticizer, resulting in superior desalination performance compared with the commercial membrane.(3) Alkali recovery performance from NaOH/Na2WO4 system in tungsten mine metallurgical industry by diffusion dialysis using double coupling silane crosslinking PVA membranes shows that:at 20℃, the separation dialysis coefficient of NaOH/Na2WO4 is 0.0035-0.0076 m h-1,the separation factor is 25-110.5, while at 40℃, the separation dialysis coefficient of NaOH/Na2WO4 is 0.0056-0.010 m h-1 the separation factor is 21.7-45.0. Alkali recovery IEMs could be prepared employing co-crosslinking reagents. Considering the mechanical properties of resulting membranes, APTEOS crosslinker could be used together with TEOS or TMOS. Better performance can be achieved when using co-crosslinkers of TEOS and TMOS. γ-MPS can form co-crosslinker with TEOS or APTEOS, but is not suitable to be co-used with TMOS.(4). Compared with the commercial Qianqiu(?) membrane, PVC based heterogeneous IEMs exhibit superior performance in desalination ratio, current efficiency and energy consumption. On the other hand, the concentration ratio between the concentrated chamber and the desalting chamber is slightly lower. PVDF based heterogeneous IEMs can enhance the concentration ratio between the concentrated chamber and the desalting chamber. In order to achieve the same desalination ratio, shorter desalination time compared with the commercial membrane is observed.