Preparation Of Highly Permeable Ceramic Membrane By The Transfer Film Technology

Ceramic membranes are characterized of high thermal stability,corrosion resistance and long life.Ceramic microfiltration and ultrafiltration membranes are not only widely used in food processing industry and industrial wastewater treatment,but also have the potential of large-scale treatment of municipal sewage and preparation of drinking water.However,the preparation cost of ceramic filtration membrane is relatively high,which limits its wide application.The permeance of ceramic membrane is one of its key properties.Under the same working pressure,time and other conditions,the greater the permeance of ceramic membrane is,the higher the efficiency and the lower the use cost will be.Therefore,the study of highly permeable ceramic membrane has great significance for both society and economy.Ceramic membrane is usually prepared by dip-coating method.In this method,membrane forming materials are easy to penetrate into the large pores of support,which will significantly reduce the permeance of ceramic membrane.In order to solve this problem,methods such as sacrificial intermediate layer method,hot coating method and fiber construction method were proposed in recent years.These methods improved the permeance of ceramic membrane to some extent.However,these methods still have problems,such as not easy to control the thickness of membrane,and prone to defects.In this study,a new ceramic membrane preparation method was proposed for improving the permeance of ceramic membrane by reducing the penetration of membrane forming materials into support.The author first proposed to prepare ceramic membrane by transfer film method,and successfully prepared the transfer film for ceramic membrane for the first time by functional and structural design of the transfer film.In this study,the transfer film coating solution was prepared by usingα-Al₂O₃powders(D₅₀=0.76μm)as the main material.The transfer film consisted of two layers,one was Al₂O₃/PVA layer and the other was PVB layer,both of which were prepared by an automatic film application.It can be pasted on the surface of support by wetting the transfer film.After drying and sintering,a thin and non-defective filter layer can be obtained.The Al₂O₃/PVA layer will form a filter layer after sintering.The PVB layer was designed to provide flexibility,which made the pasting of transfer film much easier.Also,the defects formed during the wetting of PVA were reduced due to the restraint of the PVB layer.The transfer film Al₂O₃/PVA layer was wetted and pasted on the surface of the support.After drying and sintering,a thin and non-defective filter layer can be obtained.The transfer film method eliminated the intermediate layer,and avoided the penetration of membrane forming materials into the large hole of support,which increased the permeance of membrane.Based on this method,the membrane with an average pore size of 0.190μm and a thickness of 16.7μm was obtained,and its pure water permeance reached 2740 Lm^-2h^-1bar^-1,which was much higher than that of the similar ceramic membrane prepared by traditional method.It was found that the thickness of the ceramic membrane was linearly correlated with the thickness of the wetting Al₂O₃/PVA layer.Based on this relationship,the thickness of the ceramic membrane can be controlled by adjusting the thickness of the fresh Al₂O₃/PVA layer.In comparison with the traditional dip-coating method,the permeance of the ceramic membrane prepared by the transfer film method was improved significantly,and the thickness can be control precisely.The pore size and permeance of the ceramic membrane can be controlled by changing the content and size of Al₂O₃powders in the transfer film coating solution.With increasing the content of D₅₀=0.76μmα-Al₂O₃powders in the transfer film coating solution from 5 wt.%to20 wt.%,the average pore size of the membrane decreased from 0.937μm to 0.186μm,and the pure water permeance decreased from above 8000 Lm^-2h^-1bar^-1 to below 3000 Lm^-2h^-1bar^-1.The reason is that with increasing the content ofα-Al₂O₃powders,the packing density of powders increases,which will refine the pore size of membrane.Under the same coating thickness,if the content ofα-Al₂O₃ powders in the coating solution is higher,the average pore size will be smaller.The pore size distribution will be narrower.The thickness of membrane will increase.The permeance will decrease.As the particle size of the powders changed from D₅₀=0.76μm to D₅₀=0.34μm（the content ofα-Al₂O₃ powders was wt.%）,the average pore size of the membrane changed from 0.223μm to 0.074μm.And the pure water permeance arrived at 940Lm^-2h^-1bar^-1,which was higher than other literature.The pore size and separation efficiency of microfiltration membrane were also controlled by adding boehmite sol into the coating solution.When increasing the boehmite sol content from 0 to 45 wt.%,the average pore size of the microfiltration membrane decreased from 0.227μm to 0.178μm,and the pure water permeance decreased from 3389 to 1691 Lm^-2h^-1bar^-1.The separation efficiency（interception rate）of 100 nm fluorescent polypropylene spheres increased from 87.4%to 99.2%.This is because the addition of boehmite sol can not make theα-Al₂O₃particles disperse more evenly in the coating solution,but also form tinyγ-Al₂O₃ particles when sintering at low temperature,which can refine the pore size of the membrane and conducive to sintering.In addition,it was proved that the ceramic membrane prepared by the transfer film method has better bonding strength and reusability.The ultrafiltration membrane with pore size less than 50 nm can be prepared using the dip-coating method,wherein the microfiltration membrane prepared by the transfer film method was used as the matrix.The microfiltration layer with an average pore size of 74 nm was used as the matrix,and the boehmite sol was used as the main material for the ultrafiltration membrane.The pore size of the ultrafiltration membrane filter layer was less than 20 nm after twice dip-coating,drying and sintering processes（the first dipping time was 16 s,the second dipping time was 8 s,and the sintering temperatures were both at 600℃）.The obtained ultrafiltration membrane had a thickness of 1.2μm.The pure water permeance was of 200～240Lm^-2h^-1bar^-1 and the permeance for filtering the 100 mg/L direct red pigment solution up to 180Lm^-2h^-1bar^-1 with the retention rate of 99.9%.