Preparation Of SiO₂ Based Ultrafiltration Membrane Using A Ceramic Microfilter As A Supporting Layer

This work has been undertaken to develop advanced techniques for preparing a silica ultrafiltration membrane suitable for separating proteins on a ceramic microfilter as a supporting layer by using water glass and sulfuric acid, which are abundant and cheap. The experimental results showed that the ultrafiltration membrane on a microporous ceramic tube as a supporting layer can be prepared by using water glass, sulfuric acid and PVA. The molecular-weight cutoff of the membrane is 44,500 Dalton and its water flux is 0.83-0.85 m3/(m2h).The conclusions are summarized as follows:1 The experiments of preparing silicalite Gel-Sol and making an ultrafiltration membraneThis experiment put emphasis upon studies on preparing conditions of silicalite Gel-Sol. The experimental results indicate that the dilution ratio can not be too high, or the time to make a membrane can not be too long, otherwise the thermal stability of the membrane made is poor. If the dilution ratio is too low, the membrane is hard to form and fragile. And selecting an appropriate amount of diluted water glass is very important. If its dosage is too low, the membrane can not maintain integrality at high temperature, while an excessive amount of diluted water glass results in a thickening and coarse membrane. Meanwhile, the concentration of sulfuric acid affects the integrity and formation time of the membrane. If it is too high, the membrane is easy to crack. On the other hand, if it is too low, it needs a long time to form the membrane. In addition, the amount and consistency of PVA should not be too large, or else the membrane is easily broken, and easy to crack. If it is too low, then the anti-shrinkage ability of the membrane is not good and it is hard to form a complete membrane.So in all, the combination of a dilution ratio of water glass being 1:5.5 or 1:6.5, the dosage of 20ml diluted water glass, the addition of 8ml sulfuric acid (2mol/L), and 10ml PVA (10%) can prepare a silicalite membrane, which is apparently good and thermally stable. This will help the follow-up experiment on coating microfiltration membrane with the silicalite Gel-Sol. Water flux of the flat membrane made by this method is 0.925m/(m2·h) and its molecular cut-off and retention rate of egg white proteins were 44500 Dalton and 91.56%, respectively.2 Formation of ultrafiltration membrane by coating a ceramic supporting layer with the silicalite Gel-Sol and the estimation of the membrane's performance From the experimental results, it can be seen that the formation of an ultrfiltration membrane on a ceramic micromembrane tube is affected by the following factors: the number of coating, the concentration of blended mixture, and aging time of the solution.The experimental results indicated that the pure water flux permeating through the membrane decreased with the increase of coating times, while all egg white proteins being not able to penetrate through the membrane increased. But after coating four times, the water permeability and retention rate of egg white protein became stable and changed little. The pure water flux after coating fourth times is 0.83-0.85m3/(m2·h) and the egg white protein retention rate is 63.8%-64.3%. When the concentration of the coating solution was high, the reduction of pure water flux resulted from the increase in the number of coating, but the retention rate of egg white proteins increased a lot. When concentration of the coating solution was low, pure water flux also decreased with the increase of coating times, and the retention rate of egg white proteins was also increased. But the retention rate of egg white proteins of the membrane prepared by using low concentrations was slightly higher than those prepared by using high concentrations. However, relatively, the comprehensive performance of the high concentration is better than another, so a dilution ratio of water glass(1:5.5) is selected for preparing an ultrfiltration membrane, its retention rate of egg white proteins is about 63.8%. In addition, the aging time before soaking ceramic tubes should be chosen appropriately. It is hard to coat the tube and the membrane on it is too thin to be fragile when the aging time is too short. However, if the aging time is too long, the Gel-Sol can easily settle down and form a solid gel, this makes it difficult to soak or coat the ceramic tube. Thus 2-2.5 hours aging time was selected. By doing so, the overall performance of the ultrfiltration membrane prepared is good, and the retention rate of egg white proteins is 63.3-63.8%. Because the content of the proteins in the egg white with molecular weight above 44500 Dalton is 71.1%, this means that 89.7% proteins (63.8%/71.1%) with molecular weight above 445000 Dalton are not able to permeate the prepared ultrafrltration membrane. Further experimental results indicated that 92.9%-93.6% bovine serum albumin (67000 Dalton) is not able to permeate the prepared ultrafiltration membrane.On the basis of these studies, an advanced procedure has been proposed for the preparation of ultrafiltration membrane by using water glass and sulfuric acid. The cost of the ultrafiltration membrane made by this method is very low and its water flux is high. This makes a great step forward in putting inorganic ultrafiltration membrane into practice. This provides the production of the inorganic ultrafiltration membrane suitable for separating and purifying proteins with theoretical and technical supports.