Preparation And Desalination Performance Of High Quality NaA Zeolite Membranes

In this paper,NaA zeolite membranes were synthesized by hydrothermal method using a Coarse macroporous alumina tube as a support.The synthetic solution was selected as follows: n(SiO2):n(Al2O3):n(Na2O): n(H2O)=5:1:50:1000.The ultra-thin NaA zeolite membranes were prepared by three different heating methods: conventional heating method,microwave assisted heating method and direct heating method.The effects of crystallization process(the number of coating seed crystals,the concentration of small seed crystals),the aging time of crystallization solution,crystallization temperature and crystallization time on the structure and properties of NaA zeolite membranes were investigated.Finally,the NaA zeolite membrane prepared by the optimum conditions was used for the desalination system by pervaporation.The effects of operating temperature,salt concentration on the water flux and ion rejection were investigated.The desalting stability of NaA zeolite membrane was also investigated.Finally,NaA zeolite membrane was used for multi-component salt solution by pervaporation,and the ion rejection of different salt ions was investigated.The results are as follows:(1)NaA zeolite membranes were prepared by conventional heating method.The effects of seed coating conditions,aging time,crystallization time and crystallization method on the morphology and ethanol/water separation performance were investigated.Preparation conditions: large seed crystal coating three times,small seed liquid concentration of 0.5wt%,the synthesis aging time is 6h,crystallization for 5h at 80?.NaA zeolite membrane with a water flux of 3.22 kg·m-2·h-1,a separation factor of 10,000 or more and a thickness of about 4-5?m was prepared.(2)On the basis of the results of(1),the ultra-thin NaA zeolite membrane was prepared by changing the heating method to microwave-assisted heating method and direct heating method.The effects of aging time,crystallization temperature and crystallization time on membrane morphology and ethanol/water separation performance were investigated.The optimum preparation condation was determined as follows: microwave method: crystallization for 35 min at 80?,the prepared NaA zeolite membrane was dense and free-defect with small grain size,membrane thickness is about 2?m,the water flux is 3.81 kg·m-2·h-1,separation factor is 17991.The Direct heating method: NaA zeolite membrane that with the best separation performance can be prepared at 80?for 1h,and the water flux is 4.12 kg·m-2·h-1,separation factor is 4906,the membrane thickness is only about 2?m.(3)NaA zeolite membranes were prepared by the optimum conditions determined in the three preparation methods(1)and(2),and applied to the desulfurization system.It was found that the water flux increased rapidly with the increase of the operating temperature,slightly decreased with the increase of the concentration of the salt of the raw material,the raw material liquid was 0.6M NaCl solution,the operating temperature was 35~85 ?,The water flux increased from 3.05 kg·m-2·h-1 to 14.57 kg·m-2·h-1,and the water flux of membrane W increased from 3.24 kg·m-2·h-1 to 15.04 kg·m-2·h-1,the water flux of both is higher than the membrane P,which is due to the preparation of film Z and film W were directly heated and microwave-assisted heating method,the heat transfer speed,crystallization time is short,The membrane thickness is only about 2?m,and the ion retention rates of the three are above 99.9%,which shows that the three NaA zeolites have good compactness.The results of the stability test show that the NaA zeolite membrane has good desalting stability at 75?,and this may be due to the adsorption of inorganic salt ions on the surface of the zeolite membrane to prevent erosion of the membrane surface.In addition,NaA showed excellent desalting performance in multi-component salt solution,and the rejection rate of all kinds of ions was more than 99.9%.and it was almost 100% for Ca2+,Mg2+ ions because they have a larger hydrated ions diameter.Therefore,the desalination mechanism of NaA zeolite membrane is mainly size sieving.