| With the rapid development of the global industrialization and the expansion of the population, the demand for fresh water is increasing all over the world. Obtaining fresh water from the sea has become a global consensus. More and more people pay attention to the membrane distillation desalination technology which is regarded as a new technology to solve the scarce problem of water resources. At present, membrane materials applied in membrane distillation process can be divided into two kinds: polymer membrane and ceramic membrane. As we known, the thermal and chemical stability of the polymer membrane are very bad, what is more, the membrane is very easy to be corroded when in contact with microorganisms, so its life time is very short. Compared with the polymer membrane, ceramic membrane has many advantages, such as longer life time, better high temperature resistant, stronger anti-microbial capability, better chemical stability (acid and alkali corrosion resistant, organic solvent resistant) and high mechanical strength. Therefore, ceramic membrane has attracted more and more attention in the field of seawater desalination in recent years. Now the researches of ceramic membrane mainly focus on the oxide, including alumina, titania, silica, zirconia and so on. In order to ensure that the membrane has a high permeate flux in the application of membrane distillation, the membrane must have high porosity. But the mechanical strength of the oxide ceramics is very low at high porosity, which means that the membranes are easy to break in the process of membrane assembling and sealing. What is more, the preparation process of traditional membrane is more complex and the equipment is very big, which leads a high cost and limited application scope.This thesis is based on the process of wet spinning, non-oxide nitride hollow fiber membranes have been successfully prepared by a combined phase-inversion and sintering method for the first time. This process is very simple, one-step forming, and it can simplify the process in preparation of porous ceramic membranes. Therefore, the preparation cost can be greatly reduced. The prepared silicon nitride and β-sialon hollow fibers have large porosity, high strength and excellent permeate performance. At the same time, this study is the first time to fulfill the surface modification of silicon nitride and (3-sialon hollow fibers from hydrophilicity to hydrophobicity and the application of membrane distillation. This work lays a foundation for further researches of non-oxide ceramic membranes in membrane distillation applications.The first chapter mainly introduces the dispersion of the ceramic suspension, rheological properties and the main factors influencing the suspension viscosity and fluidity. Secondly, it simply describes several traditional methods and phase-inversion process in the preparation of porous ceramic, and detailedly introduces the development, present situation, principle, classification of membrane distillation process and the use of membrane materials in this process. Finally, the the main ideas and research content of this thesis were proposed.The second chapter mainly introduces the experimental materials, preparation process and the equipments in preparation and characterization of the membrane.The third chapter mainly studies the effect of the dispersant category, calcination treatment, dispersant content, solids content of the powders and the milling time on the silicon nitride ceramic suspension. O-(2-aminopropyl)-O’-(2-methoxyethyl)-polypropylene glycol (AMPG) was selected as the best dispersant for the dispersion of silicon nitride powders by sedimentation tests. Calcination treatment at600℃for6h in air can significantly increase the oxygen amount of the powders surface, reduce the viscosity and improve the fluidity of the suspension. The test results of the rheological properties indicate that the optimal dispersant amount is4wt%, which is independent of the solids volume content of the suspension. Milling process can effectively improve the fluidity of the suspension. The optimal milling time is16h. The solids volume content of the suspension can reach50vol%with a low viscosity and good fluidity.The forth chapter is on the preparation of the silicon nitride hollow fibers by a combined phase-inversion and sintering method based on the stable dispersion of silicon nitride ceramic suspension, and the influence of ceramic powders/binder ratio, the viscosity of the suspension on the morphology, structure, strength, porosity, pore size distribution and permeability were discussed. The prepared membrane has typical asymmetric structure with a long finger-like layer near the inner and outer surface and a sponge-like region in the middle of the fiber. With increasing of the powders/PESf ratio, the viscosity of the suspension increases and the finger-like voids extending from the outer surface is greatly reduced, even disappears completely. In addition, the outer surface of the fiber is tended to densification after sintering. When the ceramic powders/binder ratio is fixed to7/1, the obtained fiber shows a good combination of gas and water flux, bending strength (290MPa), porosity (50%) and average pore size (0.74μm), which are the most suitable ceramic membranes for membrane distillation applications.The fifth chapter studies the surface modification and membrane distillation (MD) applications of the silicon nitride hollow fiber. The surface of the fiber was grafted by fluoroalkysilane. The water contact angles were changed from56°to130°, carbon and fluorine were detected on the surface of the grafted membrane by infrared spectroscopy test. Permeability test indicates that the additional FAS coating increases the membrane resistance to gas permeation but does not sacrifice too much permeability. However, the FAS grafting has a great effect on the permeance of water. The liquid water permeation was not detected for the grafted membrane until the pressure up to3.25bar. These results adequately demonstrate the hydrophobicity of the modified membrane. After surface modification, the water desalination performance of the prepared fiber was tested using two MD methods:vacuum membrane distillation (VMD) and direct contact membrane distillation (DCMD). In VMD, the influence of temperature, salt concentration, vacuum degree on the MD performance are studied. The membrane exhibits satisfactory membrane distillation performance with a high flux of679L/m2·day, a rejection rate over99%and good long-term stability for desalination of4wt.%NaCl solution at80℃when the permeate side is under a vacuum pressure of0.02bar. In DCMD, the membrane flux is only about35%compared with the flux in VMD. This is mainly due to the existence of temperature polarization effect which leads to the decrease of the driving force. What is more, the DCMD process is difficult to control due to the presence of temperature instability at both sides of the membrane. Though VMD needs provide some extra energy, the excellent MD performance can make up this deficiency.The sixth chapter introduced the preparation of P-sialon hollow fibers by a combined phase-inversion and sintering method and their application in membrane distillation process. The prepared membrane also has a typical structure with a long finger-like layer near the inner surface and a sponge-like layer near the outer surface. The prepared membrane has excellent permeability performance and mechanical strength. The fibers (Z=2) which sintered at1600℃for2h can meet all the requirements of the membrane used in membrane distillation process. The membrane has excellent hydrophobic property after grafting by FAS. The grafted fibers were successfully applied to desalination by direct contact membrane distillation process. The membrane exhibits excellent membrane distillation performance with a high flux of10.76L/m2·h (258L/m2·day) and a rejection rate over99.5%for desalination of4wt.%NaCl solution at80℃when the temperature of the permeate side was fixed at20℃. Therefore, the β-sialon hollow fiber membrane also has great potential in the application of membrane distillation for desalination.In Chapter7, a short conclusion of this dissertation has been made. The lack of this work and the direction of our future work have been discussed. |
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