Colloidal filtration

The transport behavior of colloids commonly is modeled by colloid filtration theory (CFT) (Yao et al. 1971), which is based on extension of the common advection-dispersion equation. The one-dimensional advection-dispersion-filtra-tion equation is written... 

For the preparation of high-quality membranes, also high quality supports are needed. It was decided that the project would start with the development of colloidal filtrated flat supports and centrifugal cast tubular supports, which have a higher degree of homogeneity than conventional die-pressed, tape-cast and extruded supports. The development of these new supports is described in chapter 4 of this thesis. 

A more detailed synthesis procedure for the supports used in this study is provided in chapter 4 and information on the strength of these supports as a function of synthesis conditions can be found in [4], Some general information about the colloidal filtration method can be found in [5,6],... 

Stable suspensions are necessary to obtain homogeneous casts by colloidal filtration. Different kinds of stabilisation mechanisms for alumina suspensions are used in literature. Pure electrostatic stabilisation can be obtained using HNO3, while electrosteric stabilisation is obtained using polyelectrolytes like PolyMethylAcrylicAcid (PMAA) and PolyAcrylicAcid (PAA). In... 

Figure 1 Pore size distribution of AKP-30 and AKP-15 supports made by colloidal filtration.

Colloidal filtration showed to be a very convenient way of preparing high quality flat membrane supports. These supports are extremely homogeneous as can be seen from the very... 

Supported silica membranes were prepared according to the sol-gel method first described by De Lange et al. [13], In the experiments on flat membranes the supporting system consisted of a colloidal filtrated a-alumina support with a y-alumina intermediate layer. 

To realise this, first very homogeneous OC-AI2O3 supports were prepared by using colloidal techniques. Colloidal filtration was used for the preparation of flat supports, while a centrifugal casting technique was developed to prepare highly smooth and round tubular supports. 

Dip coating is analogous to a slip casting process for making ceramic parts. The membrane deposition behavior by slip casting can be described by a theory of colloidal filtration for incompressible cakes [Aksay and Schilling, 1984] and compressible cakes [Tiller and Tsai, 1986). The theory predicts that the thickness of the consolidated layer, L, is given by... 

Aksay, I.A., and C.H. Schilling, 1984, Colloidal filtration route to uniform microstrucuire, in Ultrastructure Processing of Ceramics, Glasses, and Composites (Li. Hcnch and DR. Ulrich, Eds.), Wiley, New York, USA, p. 483. 

The separation and characterization of submicron-sized particles in water is difficult, in particular because of artifacts from sampling and concentration techniques. Lead et al. (1997) have presented a critical review of the different techniques for separation and analysis of colloids (filtration, dialysis, centrifugation, but also volta-metry, gels (DET/DGT), field-flow fractionation, SPLITT). Ultrafiltration membranes have been developed with nominal cutoff sizes ranging from the thousands of daltons (Da) to hundreds of thousands of daltons, which have been used to separate the colloidal pool into several fractions. 

Fig. 6.8. Summary of principle and important parameters in the capillary colloidal filtration mode...

Capillary colloidal filtration occurs when the dry substrate comes into contact with a dispersion and the pore surface is wetted by the dispersion liquid. The capillary suction of the substrate which occurs, in effect drives particles to the interface. If the surface is not permeable for the particles, the particles concentrate at the substrate-dispersion boundary and a compact layer is formed. The thickness of the compact layer grows with time according to the well-known square root time law (see Section 6.3.1), until the substrate is saturated with dispersion liquid or, in case of a dispersion of small colloidal particles, a stationary state due to back-diffusion occurs. Figure 6.8 summcirises the capillary filtration mode of dip-coating. 

A particle packing process is a process which causes an increase in the volume fraction of a particulate material, i.e. the dispersion. The densification takes place stepwise in the colloidal filtration process and becomes more gradual... 

In principle, both colloidal filtration and film-coating can be used to prepare coatings with a thickness adjustable between 100 nm and 100 pm and pore sizes covering the micropore, mesopore and part of the macropore range. Much depends on the properties of the substrate as has been shown. 

Fig. 6.26. Attempt to apply a mesoporous Y-AI2O3 coating on a layer 2 substrate with cracks by capillary colloidal filtration of a boehmite sol without macromolecular additives. In the layer 2 crack regions no boehmite coating develops. The layer 3 coating shows pinholes (SEM picture).

Fig. 6.31. Unidirectional compact growth in capillary colloidal filtration.

Fig. 6.32. Capillary colloidal filtration (after Tiller and Tsai [52]).

In colloidal filtration it is the filtration rate which can be influenced by the addition of a thickener. Now the high shear rates at the pore openings play a role and it is the high shear viscosity which should be influenced. MC could be a suitable thickener for that purpose. 

Colloidal filtration is selected as the dip-coating mechanism for the first trials in the development path. This means that cake filtration should occur when the suspension comes into contact with the substrate. So the particle size in the suspension should not be much smaller than 1 pm (approximately 4 times less than the mean pore size in the substrate) otherwise too much penetration and clogging of the substrate occurs prior to cake build-up. This would give rise to an extra high interfacial flow resistance during application of the MF membrane. 

A. P. Philipse, B.C. Bonekamp and H.J. Veringa, Colloidal filtration and (simultaneous) sedimentation of alumina and silica suspensions Influence of aggregates. /. Am. Ceram. Soc., 73 (1990) 2720-2727. 

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