Compact Cake Structure

Capillary forces during drying minimise the large differences in packing density which exist in wet coatings from stable and coagulated or flocculated dispersions. 

The pore properties of cast bulk porous material and coating layers from the same suspension become different above sinter temperatures where intermediate stage sintering in the bulk starts (see Section 6.2.5). At lower temperatures pore properties of free casts determined with Hg porosimetry can be used to compare the pore properties of consolidated dispersion coatings. 

In Fig. 6.48 the porosity of free cast layers as a function of the suspension pH is given and in Fig. 6.49 the pore size distributions as determined with Hg porosimetry for free cast layers at pH 3.6 and 8.1 are given. It is shown that the porosity difference between consolidated coatings obtained from stable and coagulated suspensions is only 10% and the median pore diameter decreases from 210 nm to 160 nm going from instable to stable suspensions. The decrease 

6 — PREPARATION OF ASYMMETRIC CERAMIC MEMBRANE SUPPORTS BY DIP-COATING 

---

Fig. 3 shows a simplified compactible filter cake. Darcy s law and a stress balance involving the accumulated friction drag on the particulate structure (Fig. 3) are used to develop basic theory of flow through compactible porous media. 

A larger pressure drop across a compressible cake could cause more compaction, however, the time taken to compact a thicker cake is greater than that needed to compact a thinner one (Figure 6.19), and hence the filtrate flux from a thicker cake tends to be rather lower. An uneconomic flux is therefore reached when the cake is in a less consolidated state, leaving the thicker cake with a more open structure in the particular examples shown, during the consolidation phase the thickness of the 40 mm cake (per filter surface) is... 

---