Porous ceramic membranes casting

The thickness of porous ceramic membranes prepared in this manner is typically in the range of a few millimetres (Larbot, 1996), although thinner membranes have been realised (Sahibzada e/a/., 2000).Tape casting is a high volume production technique for fiat sheet membranes and is employed on both a research and commercial scale. 

A common method to slip-cast ceramic membranes is to start with a colloidal suspension or polymeric solution as described in the previous section. This is called a slip . The porous support system is dipped in the slip and the dispersion medium (in most cases water or alcohol-water mixtures) is forced into the pores of the support by a pressure drop (APJ created by capillary action of the microporous support. At the interface the solid particles are retained and concentrated at the entrance of pores to form a gel layer as in the case of sol-gel processes. It is important that formation of the gel layer starts... 

The porous structure of ceramic supports and membranes can be first described using the lUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm<pore diameter <50 nm) is deposited, most often by the solgel process [11]. Ceramic nanofilters are produced in the same way by depositing a very thin microporous membrane (pore diameter <2 nm) on the ultrafiltration layer [4]. Two categories of micropores are distinguished the supermicropores >0.7 nm and the ultramicropores <0.7 nm. 

Commercial ceramic membranes are made by the slip-casting process. This consists of two steps and begins with the preparation of a dispersion of fine particles (referred to as slip) followed by the deposition of the particles on a porous support. 

Later, Papet [58] presented an alternative process for preparing mbular ceramic cross-fiow filtration membranes. Papet s method consists of the casting of mbular mineral microfiltration membranes with titanium dioxide suspensions. The deposited particles on the porous support were then compressed and finally, the layer was consolidated by firing. 

Two main groups of lyogel film formation mechanisms can be distinguished (i) film coating, and (ii) slip casting. In the film-coating process, capillary forces in pores do not play a role and this process can be used also on dense, non-porous substrates. The slip cast process is widespread in the production of bulk ceramics but has only been recently applied to the production of membranes [3,9]. In this process capillary forces play a dominant role. 

In order to increase their selectivity, ceramic porous membranes are usually asymmetric. They usually have a multilayered structure, consisting of a support with large pores and a top made of one or more thin layers with small pores. The support could be produced by shp casting or extrusion, while the intermediate and top layers can be produced using one of several techniques. 

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