Preparation And Separation Properties Study Of Asymmetric Flat-sheet Ceramic Membranes

With the increase of population,the deepening of urbanization and the rapid development of industry,the available clean water resource is becoming increasingly scarce and the accompanied sewage discharge is increasing in China.The great importance to issues related to water environment treatment has been attached by the national and local governments,and the sewage discharge standards has been raised.Due to the low energy consumption,the simple equipment,easy to operation,high efficiency and the excellent stability and durability in harsh environments containing acids,alkalis and organic solvents,ceramic membrane separation technology has attracted widespread attention among the many water treatment technologies.The asymmetric ceramic membrane is composed of a support body and a separation layer.Due to small running resistance,large flux,easy for large-scale production and easy to prepare microfiltration membrane,ultrafiltration membrane and nanofiltration membrane layer by layer according to the actual application requirements,the asymmetric ceramic membrane shows great potential in the wastewater treatment.In this paper,the support,microfiltration membrane,ultrafiltration membrane and nanofiltration membrane were prepared in order,and the separation properties of each layer was explored in detail.The main results are shown as following:(1)Al₂O₃ coarse powder with different particle size and morphology were used as raw materials.Spherical Al₂O₃ coarse powder was prepared successfully by heating treatment at 1600?.High-temperature spheroidization increased the sphericity of Al₂O₃ coarse powder from 0.63 to 0.75 and reduced the“quartile ratio”that express the spread of particle size distribution from 3.3 to 1.8 through the vacancy flow between particles of different sizes or between uneven areas on the same particle surface.While the sphericity of Al₂O₃ fine powder increased from 0.76 to 0.81 and the“quartile ratio”reduced from 2.4 to 1.8 after the super-fine grinding.(2)Spherical Al₂O₃ coarse powder(d₅₀=8.33?m)was selected as raw material to prepare the support.When the pore former addition was 10 wt%and the sintering temperature was 1350°C,the open porosity,bending strength,average pore diameter and pure water permeability of Al₂O₃ flat-sheet support were 44.3%,36.3 MPa,3.3?m and 3240 L/h m² bar,respectively.The skeleton formed by spherical Al₂O₃ coarse powder had both high strength and high porosity,which improved the strength of the support and reduced the filtration resistance.The average pore diameter and pure water permeability of Al₂O₃microfiltration membrane that prepared from spherical Al₂O₃ fine powder(d₅₀=0.41?m)were 0.12?m and 850 L/h m2bar,respectively.The membrane pores accumulated by spherical Al₂O₃ fine powder were more uniform and the tortuosity of the pores was smaller,which improved the flux of microfiltration membrane.Additionally,the elaborated microfiltration membrane was used to clarify aqueous suspension of carbon black with the maximum rejection rate of up to 99.7%,exhibiting excellent cleaning performance at the same time by completely restore the virgin permeate flux after backwash.(3)The optimized boehmite sol with a particle size of 30 nm,narrow particle size distribution and a sphericity of 0.92 was prepared by reverse micelles-mediated sol-gel process.The boehmite sol was used as raw material and ?-Al₂O₃ ultrafiltration membrane was prepared successfully after the dip-coating,drying and heat-treatment process.The as-prepared ultrafiltration membrane with a molecular weight cut-off(MWCO)of 8 k Da exhibited a narrow pore size distribution(4.8?6.8 nm),relatively high pure-water flux(30.4 L/(h m² bar))and excellent rejection for bovine serum albumin and methyl blue with the maximum rejection rate of 96.2%and 96.8%respectively.(4)?-Al₂O₃ nanoparticles with a particle size of?10 nm were prepared from aluminum alkoxide precursor chelated by oxalic acid at 500°C and added to boehmite sol as phase transition seeds in low temperature preparation of ?-Al₂O₃ ultrafiltration membrane.With the phase transition seeds doping of 2.5 wt.%,a single phase ?-Al₂O₃membrane with small pore size of 7.2 nm was prepared at 950°C.The results showed that during the heat-treatment process,the nano-scale ?-Al₂O₃ phase transition seeds would increase the nucleation density and shorten the distance between nucleation sites during the transition of Al₂O₃ from ? phase to ? phase,thereby reducing the phase transition temperature and achieving the purpose of preparing?-Al₂O₃ ultrafiltration membrane at low temperature.After long-term static and dynamic corrosion tests,the as-prepared ?-Al₂O₃ ultrafiltration membrane was found to no significant degradation in performance,displaying a stable MWCO and water permeability of 15 k Da and 85L/m² h bar,respectively.In the decolorization of vinegar fermentation wastewater,the COD removal was 63%whereas the color removal could reach 98%.Furthermore,the contaminated membrane could be reused many times after cleaning,confirming the excellent durability and practicality of ?-Al₂O₃ ultrafiltration membrane in wastewater treatment.(5)Cetyltrimethyl ammonium bromide/cyclohexane/n-butanol/water reverse micelles was introduced to modify sol-gel process.Hydrolysis of zirconium n-propanol solution was confined in the microreactors made of reverse micelles,as well as investigating the template effects of reverse micelles in particle size and shape control.It was found that there was significant particle size and shape replication between reverse micelles and ZrO₂ nanoparticles after polycondensation,indicating that the reverse micelles microreactors stabilized by surfactants could control the nucleation,growth and agglomeration process during the hydrolysis and thus the size and shape of ZrO₂ nanoparticles.Furthermore,spherical ZrO₂ nanoparticles could be customized in the range of 10?30 nm,and their roundness value was greater than 0.90.(6)8 mol% yttria-stabilized Zr O₂(8YSZ)nanofiltration membrane was prepared from size-controlled spherical ZrO₂ nanoparticles with an average diameter of?10 nm and a particle roundness value greater than 0.90.It was found that yttria doping could suppressed the tetragonal to monoclinic phase transition,ensuring the membrane integrity.The as-prepared 8YSZ nanofiltration membrane with a thickness of?260 nm exhibited high separation performances,while the pure water permeability and MWCO were 3.9?4.2 L/m²h bar and 800±50 Da,respectively.In the treatment of pesticide wastewater,the removal rate of carbofuran by 8YSZ nanofiltration membrane was more than 82%,while the maximum removal rate could reach 89%.Furthermore,the contaminated membrane could be restored as ever after alkali wash and low-temperature calcination,implementing multiple reuses.