Classification of Ceramic Powders

In addition, several other two-parameter residence time distributions have been formulated [24]. With these or any other residence time distribution, the specific breakage rate, and the progeny array, the mill output can be determined ftnm the mill input thus the comminution step is mathematically described. 

---

In this chapter, the fundamentals of classification and comminution of ceramic powders have been described. Comminution is described by birth and death functions in a population balance. These birth and... 

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. 

Physical adsorption is the basis for the various techniques to measure surface area of ceramic powders. The surface area is determined in terms of the amount of the gas adsorbed by a given mass of solid powder at a given temperature, under different gas pressures p. In practice, gases with a fixed volume are used for the powder, so that the amount of gas adsorbed can be identified according to the decrease in pressure of the gas. The amount of gas adsorbed versus p, or p/po, when the gas is at pressures below its saturation vapor pressure po, can be plotted as a graph, which is known as the adsorption isotherm. Figure 4.3 shows the types of these isotherms, according to Brunauer, Emmett and Teller (BET) classification [35-38]. The Type VI isotherm is called stepped isotherm, which is relatively rarely observed, but has special theoretical interest. This isotherm offers the possibility to determine the monolayer capacity of a solid, which is defined as the amount of gas that is required to cover the surface of the unit mass of the solid with a monolayer, so as to calculate the specific surface area of the solid. 

Classification can be performed by centrifuges, cyclones, or air classifiers. In this diapter, the fundamentals of comminution and classification will be discussed in addition to a review of the different types of equipment used for these two steps. Comminution and classification are the most important methods of transformation of minerals to ceramic powders. They are also used for synthetic ceramic powders, because... 

The width of the size distribution is often measured in terms of the coefficient of variation (c.v.) of the mass distribution. Randolph and Larson [98] have shown that the coefficient of variation d the mass distribution is constant at 50% for this type of precipitator. This coefficient of variation is usually too large for ceramic powders. Attempts to narrow the size distribution of particles generated in a CSTR can be made by classified product removal, as shown in Figure 6.24. The classification function, p(R), is similar to those discussed in Section 4.2 and can be easily added to the population balance as follows ... 

Fig. 3 Classification of powder-based forming methods used to make ceramic shapes.

Air Separator. A machine for the size classification of fine ceramic powders, e.g. china clay the velocity of an air current controls the size of particles classified. 

As in the case of lyophilic colloids, the use of the adjective lyophobic does not necessarily mean that the surfaces of the colloids are uniformly liquid repelling. For example, ceramic sols such as silica and alumina powders in liquids do have surfaces with varying degrees of affinity to the liquid. Despite the ambiguities in the use of the terms lyophobic and lyophilic, such a classification is convenient. 

---