Preparation Of Ceramic Membranes With Stepped Pore Structure By Binder Jetting

Additive manufacturing,as a new molding technology,can produce a variety of products with complex and fine structures in a rapid and controlled manner.In recent years,this technology has received much attention in the field of membrane preparation,especially in the preparation of ceramic membranes where it can take advantage of its excellent microstructure regulation to achieve fine construction of membrane pore structure and effectively enhance membrane retention properties while ensuring the strength and flux of membrane.For this reason,using the printing technology of binder jetting in additive manufacturing in this paper controls the layered composition of embryos in order to produce ceramic membranes with stepped pore structure.Firstly,the ceramic support embryo with large pore size was printed by binder jetting,and the ceramic support with high strength and low resistance was prepared by optimizing the sintering system;then,the intermediate layer and the separation layer with different pore sizes were printed on the surface of supports after selecting ultrafine alumina powder;furthermore,in order to solve the problem of the unclear layer interface caused by the infiltration of ultrafine alumina powder during the printing process,the inverse printing method was further designed with the advantage of binder jetting to prepare ceramic membranes with clear layer interface and stepped pore structure,which were applied to the MBR process and showed good water permeability and backwash performance recovery.The main elements are as follows:(1)Preparation of high-strength ceramic support by binder jetting combined with optimization of sintering system:The powder of calcium meta-aluminate as well as the mixed glue of carboxymethyl cellulose(CMC)and polyvinyl alcohol(PVA)were selected as raw materials,and the ceramic support with high porosity was printed by binder jetting,and its flux of pure water reached 1.15×105 L·m-2·h-1;according to the fact that the pore structure of ceramic membrane is affected by sintering,the sintering system was optimized based on the sintering theory,and the optimal sintering temperature and holding time of the embryo after dipping were obtained as 1500℃and 4 h respectively,and the bending strength of the sintered support under this condition was 48.60 MPa,and the flux of pure water was 5.82×104 L·m-2·h-1,indicating that the printing technology of binder jetting can be used to produce a ceramic membrane support with low resistance to water penetration and high strength.(2)Preparation of ceramic separation membranes with stepped pore structure by the layer-by-layer printing method of binder jetting:Based on the pore formation mechanism of ceramic membranes during sintering,two kinds of alumina particles with high sphericity and different particle sizes were selected as raw materials for the intermediate layer and the separation layer respectively,and the effects on the structure and properties of membrane on the surface of the support embryo by the layer-by-layer printing of binder jetting were investigated to find out the relationship between alumina particle size as well as printing regime(printing sequence and the number of printed layers)and the morphology,pore size of membrane surface and properties of membrane.It was found that ceramic separation membranes with a certain stepped pore structure could be prepared by the method of the layer-by-layer printing,but the number of printed layers had a significant effect on the membrane structure and performance,and the phenomenon of particle infiltration appeared when only one layer of smaller particles was printed on the surface of the support or the intermediate layer,resulting in unclear layer interface and pore size reduction,which led to significant changes in water permeability;while the retention performance of the membrane did not change significantly by only changing the number of printed layers.This indicates that the pore structure of the surface membrane mainly consists of particle accumulation,and the pore size is determined by the particle size.(3)The preparation of ceramic separation membranes with stepped pore structure based on binder jetting in an integrative and inverse way and its application:In order to address the problem of unclear membrane interface caused by the infiltration of smaller particles during the layer-by-layer printing of preparing ceramic separation membranes,the adjustable advantage of binder jetting is used to design an inverse printing strategy in which the printing order of ceramic particles is from small to large.SEM and other characterization results demonstrate that printing reversely can reduce the permeation resistance and increase the flux of membrane by preventing the infiltration of small particles at the interface between the support layer,the intermediate layer and the separation layer.Applying the optimized membrane to the MBR system can obtain higher water flux(>50 L·m-2·h-1)than the commercial PVDF hollow fiber membrane,and the return rate of the flux of membrane can be higher than 90%after a simple backflush.In summary,this paper used the printing technology of binder jetting to produce highstrength and high-throughput ceramic separation membranes with stepped pore structure after one printing and one sintering,which effectively reduced the time and energy consumption of preparing ceramic membranes and showed excellent separation performance,and provided an important reference for developing new ceramic membrane preparation technology.