Preparation And Properties Of The High-performance Asymmetric Alumina Ceramic Membranes And The Application In Oil-water Separation

Oily wastewater is one of the major water pollutants and its oil-water separation is of great importance.It is difficult to remove the oil droplets by conventional separation methods,due to the small particle size（<20μm）of the emulsified oil in the wastewater.In recent years,membrane separation technology has become a very economical and effective treatment technology for separating oily wastewater with oil droplet size of less than 10μm due to its excellent separation performance.Among numerous separation membranes,ceramic membrane has the advantages of good stability,high strength,acid/alkali resistance and long life.Nevertheless,it also suffers high production costs and the contradiction between high permeability and high selectivity,which limits the large-scale applications.In this work,low cost,high-performance asymmetric porous alumina ceramic membranes were prepared using a wide range of industrial grade raw materials and through the optimization of the membrane production process and the design of the membrane system structure.The main research contents and results are as follows.Porous alumina-mullite supports with low cost and excellent mechanical properties and corrosion resistance were prepared by sintering at 1550℃ using commercial colloidal silica coated industrial grade coarse Al₂O₃ micro-powder as the raw materials and cornstarch as pore former.The experimental results showed that increase of Al₂O₃/SiO₂ ratio was beneficial to the formation of mullite and the formed prismatic mullite was an alumina-rich orthorhombic crystal.Increase of sintering temperature improved the mullite content in the supports but decreased the porosity of the supports.The porosity,permeation flux and pore size of the supports increased,and the bending strength decreased with increases of Al₂O₃/SiO₂ ratio and cornstarch content.In addition,the glass phase content decreased with the increase of Al₂O₃/SiO₂ ratio,which reduced the mass loss of the supports during corrosion process.Hence,the mass loss of the supports corroded in 20 wt%H₂SO₄ and 10 wt%Na OH solutions（80℃）for 36 h was less than 0.35 wt%and 4.0 wt%,respectively.Although the corrosion weakened the grain boundary and reduced the strength,the residual bending strength（39.3-46.4 MPa）of the alumina-mullite supports still met the application requirements.The fabricated alumina-mullite support（Al₂O₃/SiO₂=4:1）possessed good corrosion resistance and thermal stability with the porosity50.5%,an average pore size of 3.1μm,a narrow pore size distribution（2-5μm）,a strength of49.7 MPa,a pure water flux of 27.7 m³m^-2h^-1 and a nitrogen flux of 2810 m³m^-2h^-1.The sacrificial interlayer method was adopted to prepare asymmetric ceramic membranes with a double-layer structure of Al₂O₃ membrane layer and the alumina-mullite support.The pre-coated sacrificial interlayer（on the surface of the support）was composed of graphene oxide-cellulose nanocrystals（C1G1）with the thickness of 0.55μm,which was thinner than other sacrificial interlayers reported in the literatures.This would reduce the cost and decrease the impact of the burning of C1G1 interlayer on the membrane layer simultaneously,which is conducive to the improvement of the bonding between the membrane layer and the support.Therefore,the ceramic membrane,prepared by using C1G1 film as sacrificial interlayer and sintered at 1300℃,possessed a smooth and intact surface morphology,a high porosity of 53.4%,an average pore size of 200 nm,a membrane thickness of 17.7μm.Meanwhile,the interfacial adhesion between the membrane layer and the support was good.Moreover,infiltration of membrane-forming particles was prevented by the sacrificial interlayer coated on the support,which optimized the structure of the ceramic membrane and greatly reduced the filtration resistance of the ceramic membrane.Thus,the permeation flux of the ceramic membrane significantly improved,i.e.,the pure water flux(4828 Lm^-2h^-1)of the prepared sacrificial interlayer ceramic membrane was 1.6 times than the pure water flux(2911 Lm^-2h^-1)of the ceramic membrane prepared on the surface of the original support without interlayer.When separating O/W emulsion（the feed）with an average oil droplet size of 180 nm,the steady-state feed flux of the membrane could reach 1050 Lm^-2h^-1 and the rejection was 97.2%.This sacrificial interlayer method effectively avoids the occurrence of increases in production cost and in membrane resistance caused by multiple“coating-drying-firing”operation.Co-pressing and co-firing method was employed to prepare high-performance asymmetric porous ceramic filtration membrane（a double-layer structure）by one step process.The results showed that the difference in shrinkage between the membrane layer and the support was small and the surface morphologies of the co-pressing ceramic membranes were uniform with low roughness in the sintering range of 1100-1300℃.Meanwhile,the interfacial adhesion between the membrane layer and the support was strong and no penetration of the membrane-forming particles into the support was observed.What’s more,the structure and performance of the ceramic membrane were further optimized by changing the sintering temperature,adjusting the Al₂O₃/SiO₂ ratio and the content of the pore former in the support.The ceramic membrane with an Al₂O₃/SiO₂ ratio of 4:1,20 wt%of pore former added in the support was prepared at 1300℃ for 2 h.The as-prepared sampels possessed a high porosity of 55.7%,a small pore size of 80nm,a narrow pore size distribution within 76～200 nm and a high pure water flux of 3640 Lm^-2h^-1.When separating O/W emulsion with an average oil droplet size of 180 nm,the steady-state feed flux of the membrane was as high as 1010 Lm^-2h^-1 and the rejection can even reach100%.Besides,the time needed to meet the discharge requirements（oil concentration in permeate≤10 mg/L）was within 5 minutes,which was much shorter than the time（60-80 min）needed to meet the discharge requirements for the ceramic membrane prepared by the sacrificial interlayer method.Furthermore,the co-pressing and co-firing method avoided the pre-firing of the support and the preparation of coating suspension and therefore eliminated the use of organic additives and longer drying and sintering times.Thus,it greatly simplifies the preparation process,shortens the membrane preparation time and simultaneously reduces energy consumption and costs.In addition,the Hermia model was used to further analyze the fouling mechanism of ceramic membranes during the separation of different O/W emulsions.The model fit results showed that the fouling mechanisms of different membranes were in accord with the cake filtration mode in the long-term（3 h）operation,although they were different in the initial stage（20-40 min）.The model fits also revealed that the fouling mechanism of a membrane usually involved intermediate pore blocking and/or standard pore blocking mode in the initial short-term separation process,but it quickly converted to cake filtration mode,which led to a sharp drop in the feed flux and a dramatic increase in the rejection of the membranes in the initial stage.The growth rate of the thickness of cake layer slowed down and gradually became dynamic and stable with the extension of the filtration time,thus,the variation of the corresponding permeation flux and rejection also slowed down and then gradually became constant.Moreover,the resistance-in-series model was used to analyze the fouling resistance ability of the ceramic membranes.It showed that reversible fouling resistance of the membrane were relatively large,especially the physically removable resistance,which was consistent with the easily removable characteristic of the foulants produced by cake filtration and was also in line with the high recovered permeation flux ratio（88.4-93.4%）resulted from membrane cleaning.These results indicated that the ceramic membranes prepared by different methods in this thesis have good fouling resistance and regeneration properties.