Classification aerated powders

Other classification systems are used less frequently. Carr " also devised a system to classify materials as to their floodability. He defines the floodability of a material as its tendency to flow like a liquid because of the natural fluidization of a mass of particles by air. In order to so classify a material, the flowability is determined utilizing the method just described. This value is equivalent to a measurement Carr calls the angle of fall, angle of difference, and dispersibility. Though referred to in any of the papers mentioned here, this system is much less utilized then the flowability measurements. Geldart reported on a characterization system of powders according to their ability to aerate and later Molerus modified this system. In a more recent symposium this method of powder classification was examined. ... 

Geldart et al. (1984) in an extension of the work on the classification of fluidisable powders used the ratio of tapped density to a loosely packed density (Hausner ratio) to predict the type of particle movement that can occur with powders having various degrees of cohesiveness. Re-appraisal of this work shows that with a Hausner ratio below 1.27 (Table 1.8), all the powders investigated could be classified as aeratable. Powders with a Hausner ratio above... 

Although man s environment, from the interstellar dust to the earth beneath his feet, is composed of finely divided material his knowledge of the properties of such material is surprisingly slight. For many years scientists have accepted that matter can exist as solids, liquids or gases Although the dividing line between the states may often be rather blurred this classification has been upset by powders which at rest are solids, when aerated may behave as liquids and when suspended in a gas take on some of the properties of the gas. 

Classiflcations of Fluidization Behavior Geldart [Powder Technology, 7, 285 (1973)] and later Dixon [Pneumatic Conveying, Plastics Conveying and Bulk Storage, Butters (ed.). Applied Science Publishers, 1981] developed a classification of fluidization/aeration behavior from studies of fluidized beds and slugging in vertical tubes. 

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... 

The properties of concern in this section are to do with behaviour of powders in an aerated state this is relevant in gas fluidization, powder transport and handling. Probably the most important tests in this category are those derived from fluidization and the results of such tests are not necessarily restricted to the area of gas fluidization. It should be emphasized that the following notes apply largely to fine powders (i.e. groups A and AC in Geldart s classification). 

Figure 1.7 Geldart s empirical classification of aeratable (A), bubble (B), cohesive (C) and dense (D) fiu-idisable powders.

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