Classification of Powders

The different ways of classif)dng powders is closely related to the applications in which bulk properties play an important role. Since most applications in powder technology are relevant when considering powders as dispersion systems, they can be classified into different categories according to their particular 

In an attempt to classify the different fluidisation behaviours, Geldart16 proposed an empirical classification, which divides fluidisation behaviour 

Geldart4 distinguished these powders as those for which umb/uml- 1. At gas velocities above wmb bubbles begin to appear, which constantly split and coalesce, and a maximum stable bubble size is achieved. The flow of bubbles produces high solids and gas back-mixing, which makes the powders circulate easily, giving good bed-to-surface heat transfer. 

4 Interparticle Forces, Measurement of Cohesiveness and Flowability of Solid Particles at Ambient Conditions 

When trying to describe the fluidisation of different materials, the nature of the forces acting between adjacent particles becomes of major 

Without material bridges With material bridges 

Most obvious Cohesive, Ideal for Starts bubbling Coarse solids 

Typical solids Flour, cement Cracking Building sand Gravel, coffee 

De-aeration rate Initially fast, then exponential Slow, linear Fast Fast 

Bubble No bubbles- Bubbles split No limit to size No limit to size 

The fluidization properties of a powder in air may be predicted by establishing in which group it lies. It is important to note that at operating temperatures and pressures above ambient a powder may appear in a different group from that which it occupies at ambient conditions. This is due to the effect of gas properties on the grouping and may have serious implications as far as the operation of the fluidized bed is concerned. Table 7.1 presents a summary of the typical properties of the different powder classes. 

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Incorporation of Geldart s classification of powders in relation to fluidisation characteristics (Chapter 6). 

Identification of single substances, and classification of powder photographs. Each crystalline substance has its own set of plane-spacirfgs, which is different from those of other crystalline substances. The relative intensities of the various reflections are also characteristic. Each substance thus gives its own characteristic powder photograph, the scale of which, however, depends on the wavelength of the X-rays used and the diameter of the camera. 

Identification of single substances, and classification of powder photographs 131... 

C-E Raymond s experience in the classification of powdered materials and the success of the JetStream classifier in providing consistently sharp cut-points at full production capacities can be put to work for you to help provide the kind of quality product you demand. For a no-cost evaluation of your application, simply remove this sheet, answer the questions below, fold and mail. If you need assistance, call us at (312] 236-4044. 

TABLE 7.10 Classification of Powder Flow Properties Using the Schulze Ring Shear Tester... 

The three-digit designation affords a convenient descriptive classification of powders in terms of their bed collapsing behavior, but an additional criterion is desirable to show the entire spectrum of continuous transition of fluidized states from particulate to aggregative. 

Classification of powder grains by screening reported in France. 

C-E Raymond s experience in the classification of powdered materials and the success of... 

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

Ideally, the classification of powders would provide more than just flow ability data, such as flow rate and compaction indices. It would also include data describing the robustness and stability of the powder, for example, vulnerability to segregation, attrition, and vibration. Given this, then the two key issues of powder processing could be addressed. First, will the powder flow satisfactory— does it have flowability properties that suit the process And second, is the powder robust— will it be adversely affected by being processed ... 

The classification of powders according to their handling characteristics must, therefore, be done on the basis of direct test results. 

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. 

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... 

CLASSIFICATION OF POWDER HANDLING PROPERTIES IN PNEUMATIC CONVEYING... 

Triboadhesive enrichment or beneficiation can be used successfully in two types of applications size classification of powdered materials and the separation of mineral particles from impurities. In the first category we may mention the classification of powders such as quartz, barite, magnetite, hematite, pyrite, feldspar, coal, asbestos, graphite, periclase (crystalline magnesium oxide), pegmatite, and iron ore. As an illustration of an application in the second category we may mention the separation of asbestos fiber ( falling product) from mineral dust that adheres to the drum surface ( retained product) [327]. 

Fines n. In the classification of powdered or granular materials according to particle size, fines are in portion of the material... 

Graphical representation of classification of powders according to Jenike. 

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