Membrane technology is considered as one of the best water treatment technologies in the 26th century. In recent ten years, membrane material technology and membrane separation technology have made great progress and are widely used in the field of water treatment. The filtration and separation performance of ceramic membrane for water treatment is closely related to its pore size and distribution, porosity and surface morphology. The active separation layer of ceramic membrane is formed by random accumulation of particles, and the porosity is usually 30 ~ 35%, and the tortuosity factor is difficult to control, so the water treatment efficiency of ceramic membrane is limited. It is a research hotspot in the field of ceramic membrane water treatment to study the new technology of preparation, modification and process optimization of ceramic membrane to improve the filtration, separation and anti-pollution efficiency of ceramic membrane.
1. Preparation technology of ceramic membrane for water treatment
1. 1 preparation process of pore-forming agent
Pore-forming agent is a simple and economical method to improve the porosity of water treatment ceramics. Pore-forming agents can be divided into inorganic and organic substances. Inorganic pore-forming agents include ammonium carbonate, ammonium bicarbonate, ammonium chloride and other substances that are easily decomposed at high temperature, or inorganic carbon such as graphite and pulverized coal. Organic porogens mainly include natural fibers and high molecular polymers, such as sawdust, starch, polystyrene (PS), polymethyl methacrylate (PMMA) and so on. Yang et al. used Al2O3 as membrane matrix, bentonite as sintering aid and corn starch as pore-forming agent to prepare ceramic membrane by extrusion, crosslinking, drying and sintering. The results show that with the increase of starch content, the maximum pore size and average pore size of Al2O3 carrier increase, and the porosity of ceramic membrane increases from 24% to 38%.
Preparation technology of 1.2 template
The template can effectively control the morphology, structure and size of synthetic materials, and prepare microporous, mesoporous and macroporous materials with orderly pore structure, uniform pore size and large porosity. Template method has rich material selection and flexible adjustment means, and it is very promising to prepare ceramic membranes for water treatment by template method. Xia et al. prepared three-dimensional ordered polyurethane macroporous materials with pore size of 100nm by ultraviolet polymerization using organic polystyrene microspheres as templates. Sadakane et al. used PMMA as template to prepare metal oxide materials with three-dimensional ordered macropores, and the porosity ranged from 66-8 1%. Surfactants can form self-assemblies such as micelles, microemulsions, liquid crystals and vesicles in solution, and are often used as organic templates in self-assembly technology. Ordered mesoporous molecular sieve MCM4 1 can be prepared by using surfactant cetyltrimethyl ammonium bromide as template, which has a variety of symmetrical pores with a pore size of 2 ~ 50 nm. Choi et al. used Tween80 as a template to prepare TiO _ 2-Al2O3 ceramic membrane with gradient pore size structure, and the permeability of the ceramic membrane was greatly improved.
Preparation technology of 1.3 fiber laminate
Ceramic fiber materials can be quickly deposited on the surface of the support in the process of film formation, which obviously reduces the wettability of the film layer, and is easy to obtain higher porosity and specific surface area, which has a significant effect on improving the permeability of the film material. Ke et al. used titanium dioxide fiber as raw material to prepare ceramic fiber membrane with average pore size of 50nm by spin coating method. The rejection rate of spherical particles is more than 95%, and the membrane flux is more than 900Lm-2h- 1.
Preparation technology of 1.4 sol-gel
Sol-gel technology mainly controls the separation accuracy of ceramic membrane separation layer by adjusting the material size. Nano-scale sol can be formed by sol-gel method, and the obtained ceramic membrane has small pore size and narrow pore size distribution, which is suitable for preparing ultrafiltration membrane and nanofiltration membrane with high permeability selectivity. He et al. prepared TiO2 _ 2 nanofiltration membrane with average pore size of 0.7 ~ 2.5 nm by polymerization sol method. The molecular weight of PEG is 500～000 Da, and the rejection rate of Mg2+ is 88%.
2. Modification technology of ceramic membrane for water treatment
2. 1 chemical vapor deposition modification technology
Chemical vapor deposition is a very effective method to deposit silicon oxide or metal oxide on the surface of ceramic membrane to improve the pore structure and filtration performance of ceramic membrane. Lin et al. modified Al2O3 ceramic membrane with an average pore size of 4nm by CVD technology, and prepared SiO2 ceramic membrane with a pore size range of 0.4 ~ 0.6 nm. Generally, CVD method needs to be carried out in high temperature and vacuum environment, and the precursor is required to have certain volatility.
2.2 atomic layer deposition modification technology
Atomic layer deposition (ALD) technology can deposit substances on the surface of ceramic membranes layer by layer in the form of monatomic films, thus constructing micro-nano structures on the surface of ceramic membranes. Li et al. deposited alumina layer on the surface of ceramic membrane with an average pore size of 50nm through atomic layer, and adjusted the average pore size of the membrane by controlling the number of atomic layer deposition. After modification, the rejection rate of bovine serum albumin by ceramic membrane increased from 2.9% to 97. 1%.
2.3 surface grafting modification technology
Surface grafting technology is often used to control the surface properties of membrane materials, and the grafting process will change the pore structure of the membrane to reduce the pore size. Generally, the surface of ceramic membrane will absorb water to form a large number of hydroxyl groups, and an organic molecular layer can be modified on the surface of mesoporous membrane by grafting organosilane. By adjusting the chain length and functional groups of the grafted molecules, the pore size can be adjusted to obtain special surface properties. Singh et al. found that grafting silane coupling agent can further reduce the pore size of porous ceramic membranes. Cohen et al. grafted hydrophilic PVP on the surface of ceramic ultrafiltration membrane, the pore size of the modified membrane decreased, the interception performance improved and the anti-pollution performance improved, which can be used for oil-water separation.
3. Preparation and modification process optimization of ceramic membrane for water treatment.
3. 1 Selection of ceramic membrane materials and additives
The preparation of ceramic membrane for water treatment mainly focuses on raw materials and sintering process. Adding sintering AIDS to reduce sintering temperature, using cheap and easy-to-sinter raw materials to reduce raw material cost, and adopting advanced sintering technology to achieve low-cost control are the research focuses of ceramic membranes. In the process of preparing ceramic membranes, some liquid or solid sintering AIDS are often added to the basement membrane materials. Natural silicate clay minerals, such as kaolin and potash feldspar, can be melted at low temperature to form a liquid phase, which wets the matrix particles that wrap the membrane material under the action of capillary force between particles, and bonds the particles together, supplemented by the good mechanical strength of porous ceramic membrane. Metal oxides such as titanium oxide and zirconium oxide can form multi-element oxide solid solution with ceramic membrane matrix, which will reduce the sintering temperature and be beneficial to the preparation of ceramic membrane.
3.2 Optimization of ceramic membrane firing process
Porous ceramic membrane must be sintered many times, which has the problems of long sintering process cycle and high energy consumption. In addition to using sintering AIDS or easy-to-sinter materials to reduce sintering temperature, reducing sintering time or shortening preparation period can also achieve the purpose of reducing sintering process cost. Microwave sintering technology is a non-contact technology in shortening sintering time. Heat is transmitted by electromagnetic waves, which can directly reach the inside of the material, thus minimizing the non-uniformity of sintering and shortening the sintering time while reducing the sintering temperature. Microwave technology is mostly used to prepare nearly dense ceramic composites. Because it can improve the structure and properties of materials, it can also be used to prepare porous ceramic composites. In terms of shortening the sintering cycle, some researchers refer to the successful application of low-temperature sintering technology in the packaging field of multilayer ceramic components, and propose to adopt * * * sintering technology to reduce the sintering times, thus reducing the sintering cost.
4. Conclusion
The preparation technology of water treatment ceramic membrane aims at improving the overall performance of ceramic membrane, and the breakthrough of ceramic membrane preparation technology can be achieved by regulating the microstructure of ceramic membrane. At present, the preparation technology of ceramic membrane, such as porogen preparation technology, template preparation technology, fiber lamination preparation technology, sol-gel technology and solid particle sintering technology, has attracted more and more attention. The research on the preparation technology of ceramic membrane for water treatment will lead and promote the development of ceramic membrane technology and industry, and alleviate the bottleneck pressure of upgrading and improving water quality in waterworks.
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