Application Of Composite Pore Former In The Preparation Of High-performance Ceramic Membrane Support

Ceramic membranes are widely used in wastewater treatment,food processing,gas cleaning,etc.,by virtue of their high separation efficiency,good chemical stability,easy cleaning and regeneration.The ceramic membranes applied in the industrial field generally consist of a macroporous support,a or more intermediate layers and a filtration top layer.In general,the support plays an important role for the preparation and application of ceramic membrane,and it should have good permeability,high mechanical strength,strong corrosion resistance and reasonable cost.Pore formers are essential for the preparation of high performance ceramic membrane supports.The removal of the pore former is a key process affecting the sintering rate and quality.A single pore former decomposes concentrated in a relatively narrow temperature range,which will lead to problems such as carbon deposition,warpage and cracks of the support.In order to solve this problem,composite pore former was selected to prepare alumina ceramic membrane support in order to broaden the decomposition temperature range of the pore former.This study primarily consists of three parts as follow:1.Effect of sintering aid and sintering temperature on the preparation of ceramic membrane supports by composite pore former.In this study,alumina powder with purity of99.9%was used as the main raw material.Sawdust,carbon black and hydroxyethyl cellulose?HEC?were chosen as a composite pore former.Polyvinyl alcohol?PVA?was used as an organic binder.Alumina supports were fabricated with the processes of ball mill mixing,granulation,pressing,drying and firing.High purity alumina support with the Al₂O₃content of99 wt.%exhibits high mechanical strength and strong corrosion resistance,and the cost of its raw materials is relatively low.Therefore,most of commercial ceramic membrane supports are made of alumina.However,the sintering temperature of high purity alumina supports is higher than 1700°C.In order to reduce energy consumption,sintering aids are utilized to improve the sintering activity.In our study,titanium oxide,nano-sized?-Al₂O₃and boehmite sol were selected as sintering aid individually or jointly to reduce the sintering temperature and meanwhile maintain the corrosion resistance of supports.The effect of sintering temperature,sintering aid and pore former content on the properties of the support were studied.The support with high flux,high bending strength and good corrosion resistance was successfully prepared,and the low-temperature sintering of high-purity alumina support was realized.When TiO₂was used as the sintering aid alone,the support fired at the same temperature had low bending strength and rough surface,and it was not suitable to be used for the ceramic membrane.When TiO₂+?-Al₂O₃or TiO₂+?-Al₂O₃+boehmite sol were used as sintering aids,the permeability and mechanical strength are greatly improved.The permeability of supports prepared with the two kinds of sintering aids are equivalent.Meanwhile,the bending strength of the support prepared by the latter was a little higher.Therefore,it is highly suitable to choose TiO₂and?-Al₂O₃as the sintering aid for this system with the consideration of preparation cost and properties of supports.We chose 1 wt.%TiO₂+5 wt.%?-Al₂O₃as the sintering aid,and the sintering temperature of the support prepared from pure alumina powder with a particle size of d₅₀=27.9?m can be reduced to 1500?.The supports fired at 1500?exhibited a pure water flux of 45.4 m³m^-2h^-1bar^-1and a bending strength of 46.2 MPa.The corrosion experimental results also indicated the supports are quite resistant to acid and alkali corrosion.2.Effect of composite pore former on the sintering and properties of alumina supports.In this study,carbon black,sawdust and HEC were chose as a composite pore former to sinter the supports rapidly.The effect of heating rate in the porocess of pore former removal on the properties of supports was studied.The results show that the properties are slightly dependent on the heating rate.The good performance stabilized even the heating rate increasing from2°C/min to 10°C/min,which revealed that the composite pore former method is a promising way to reduce the sintering time without damage to the support quality.3.Effect of composite pore formers ratio on ceramic membrane supports.We designed the ratio of sawdust/carbon black/HEC as 10/0/0?7/1/2?5/3/2?3/5/2?1/7/2 and 0/0/10.The effect of composite pore formers ratio on the permeability,bending strength,open porosity,average pore size and pore size distribution of supports was studied.The results revealed that the properties of supports are highly dependent on the composite pore formers ratio.The pore morphology,pore size and porosity of supports can be designed by adjusting the ratio of three pore formers with different particle sizes.The long fibrous sawdust is conducive to the formation of connected pores.In addition,HEC added in the form of solution can connect the small-sized pores formed by carbon black with the large-sized pores formed by sawdust.Both of them are beneficial to the improvement of the permeability of supports.The results show that the support with sawdust/carbon black/HEC ratio of 5/3/2 exhibited the best pore structure and highest matching degree of permeability flux and mechanical strength.