| Ceramic filtration membrane system is devided into two parts – the support and the membrane layer. For the support, the difficulty lies in achieving high porosity, high mechanical strength and good corrosion resistance concurrently. Usually, a certain amount of sintering aids are employed to promote the sintering process. These sintering aids are mostly alkali metal and alkali earth metal oxides, which will inevitably result in the decrease of mechanical strength and corrosion resistance because of the generation of glassy phases. Therefore, preparation of ceramic membrane support without using these sintering aids is desirable.The common preparation methods of ceramic membrane mainly consist of sol-gel, spray, dip coating and chemical vapor deposition, etc. Of all the preparation methods, dip coating is still one of the mainstream methods. At present, ceramic membrane layers are usually obtained on the support surface via capillary force, which has the following deficiencies:(1) the requirement of high porosity, low filtration resistance and a certain thickness(1-3mm) for the support leads to largers pores(2-10μm) in the support. The particle size of the membrane-forming slurry is usually four times of the membrane pore size. For example, if the pore size of ultrafiltration membrane is about 50 nm, the average particle size of the membrane-forming slurry needs to be 200 nm or so. The membrane particles will easily penetrate into the support under the capillary force and plug the pores, which results in the large filtration resistance. Therefore, the current ceramic membranes are generally prepared on the intermediate layers. Each preparation step of the intermediate layers involves a high temperature treatment process, making the ceramic membrane fabrication extremely expensive and time-consuming.(2) because the membrane particles in the slurry have a particle size distribution, it is difficult to completely prevent the membrane-forming particles from intruding the support. As a result, reducing or eliminating intermediate layers may be an alternative solution for preparing ceramic membrane with high permeability and low filtration resistance.In this thesis, nanotechnology has been used to decrease the sintering temperature of ceramic membrane support, reducing or abandoning the use of the sintering aids which will weaken the mechanical strength and corrosion resistance of membrane support. In this way ceramic membrane supports with high mechanical strength, high porosity and good corrosion resistance have been prepared at low sintering temperature. Attempts to prepare inexpensive and high-permeability ceramic membrane without any intermediate layers via viscosity control, wet film phase inversion and pore pre-sealing have been made as well.Boehmite sols and alumina particles are chemically modified by using polyethyleneimine and polyacrylic acid respectively. At the same time, the pH values of boehmite sols and alumina slurry are adjusted to 4~5 in order to make boehmite sols and alumina particles have an opposite charge. Thus coating the alumina particles with the nanosized boehmite can be achieved via the electrostatic adsorption effect. After drying, prilling, moulding and sintering, high-purity alumina support with high open porosity, high mechanical strength, large apertures, homogeneous pore size distribution and good corrosion resistance can be obtained at low temperature.The property of the membrane-forming slurry can be improved by introducing boehmite sols and PVA into the slurry simultaneously. There exists a large amount of intermolecular and intramolecular hydrogen bondings in the structure of PVA molecules. There are also a lot of OH groups on the surface of the cubic orthorhombic crystals of boehmite nanoparticles. The hydrogen bondings of PVA molecules and boehmite nanoparticles will induce the crosslinking and incorporation of α-Al2O3 and boehmite particles with PVA. A giant network structure is finally formed with a great deal of crosslinking and interweaving, which effectively reduces the intrusion of membrane particles into the support. Thus one-step preparation of alumina microfiltration membrane without any intermediate layers on macro porous support can be achieved. The crack-free membrane fired at 1300oC for 2 h prepared using the suspension with 25% α-Al2O3, 3.01% PVA and 1.8% boehmite has a permeance of 441 L m-2 h-1 bar-1 with the resistance of 9.55×1011 m-1. The membrane has a narrow pore size distribution centered at around 0.27 μm. Its largest pore size is about 0.30 μm.Wet film phase inversion technique is an effective way to prepare highly permeable alumina microfiltration membrane in a single step. The membrane-forming slurry consists of dimethylformamide, cellulose acetate and alumina powders(d50=100nm). Deionized water is used as coagulation bath. The cellulose acetate dissolved in DMF might diffuse to the solid phase CA and grow to fibers orienting toward skin layer along with the inward phase inversion reaction. Following the phase inversion of CA alumina particles were ?solidified? in the membrane. During sintering, the orientated fibers were burned off and formed a lot of microtubular channels and the membrane with good connectivity and small tortuosity were achieved. The effects of CA and alumina quantities on pore size and permeability of the membrane have been investigated. Alumina microfiltration membrane which has an average pore size of 0.28μm and a water permeance of 1327 L m-2 h-1 bar-1 has been obtained via wet film phase inversion.Pore pre-sealing has demonstrated to be a good method to prepare highly permeable ceramic membrane without any intermediate layers in one step as well. The support is coated with a hydrogel layer resulted from the crosslinking reaction between PVA and boron acid and the support pores are pre-sealed by PVA-boron hydrogel layer. A ceramic membrane precursor is then dip coated on the hydrogel layer to obtain a sandwiched structure. After drying and sintering the membrane layer is combined with the support surface, and an alumina membrane without intermediate layers can be obtained. Because of the hydrogel barrier almost no membrane particles can penetrate into the support, which improves the permeability of membrane significantly. The alumina membrane has an average pore size of 0.24μm and a water permeance of 1410 L m-2 h-1 bar-1 via the pore pre-sealing method that has good repeatability and controllability. |
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