Classification on the Basis of Tackiness

A full classification of powders according to their behaviour in fluidization is complex because their behaviour depends on many particle properties. There is, however, now a widely accepted classification of powders proposed by Geldart26 which takes the two most important particle properties into account, the particle size and the particle density. Fig. 10a shows this classification, for fluidization by air at ambient conditions. 

Geldart and co-workers27 have identified parameters which can be used to classify powders into the above mentioned groups. The most obvious are density and particle size the approximate range of the product of these two particle characteristics ppdp is, for example, between 50 and 200 (if density is in g/ml and size in microns) for powders in group A, from 20 to 50 for group AC and less than 20 for group C. 

A second distinguishing property is the ratio of tapped and aerated densities (see section 5.2.4) which is less than 1.25 for group A, from 1.25 to 1.4 for group AC and greater than 1.4 for group C. 

The above-mentioned classification of powders may be useful even when fluidization as such is not of interest this is through the two related properties of bed expansion and rate of de-aeration which are of concern in the filling of containers and in the residence time needed in hoppers to avoid the powder flooding out when the discharge valve is opened. There is a striking difference, for 

In the context of gas-solid suspensions, a cohesive powder may be defined as a powder in which the interparticle forces become so large that they exceed the aerodynamic drag which can be exerted by the gas. 

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