Powders classification

The efficiency of particle-fluid contacting in fluidization, popularly described in terms of the quality of fluidization, has its origin not only in the physical properties of the fluidizing medium and of the solid material of which the particles are composed, but also in the particle characteristics and in the group behavior of the particles while in motion. Particle characteristics include size, size distribution, shape, and surface roughness or texture, while 

Early investigations were concerned mostly with physical properties, somewhat with particle characteristics, but little with particle group behavior. Even so, significant results were obtained—for instance, the distinction 

According to such methodology, powders can be classified in relation to the regimes in which they tend to perform in the fluidized state, as was mentioned briefly in Section I and shown schematically in Fig. 3. 

Geldart (1972, 1973) classified powders with respect to their fluidizing characteristics into four groups, as already mentioned in Section I.E, which are summarized in Table V in terms of typical particle size, particle characteristics, fluidizing behavior, and bubble characteristics. 

Group Typical dv,nm Particle Characteristics Fluidizing Behavior Bubble Characteristics 

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Glassification. Classification (2,12,26,28) or elutriation processes separate particles by the differences in how they settle in a Hquid or moving gas stream. Classification can be used to eliminate fine or coarse particles, or to produce a narrow particle size distribution powder. Classification by sedimentation iavolves particle settling in a Hquid for a predetermined time to achieve the desired particle size and size distribution or cut. Below - 10 fim, where interparticle forces can be significant, gravitational-induced separation becomes inefficient, and cyclone and centrifugation techniques must be used. Classification also separates particles by density and shape. Raw material separation by differential sedimentation is commonly used in mineral processiag. 

FIG, 17-1 Powder-classification diagram for fluidization by air (ambient conditions). [From Geldart, Powder TecbnoL, 7, 285-292 (1973).]... 

For group B and D particles, nearly all the excess gas velocity (U — U,nj) flows as bubbles tnrough the bed. The flow of bubbles controls particle mixing, attrition, and elutriation. Therefore, ehitriation and attrition rates are proportional to excess gas velocity. Readers should refer to Sec. 17 for important information and correlations on Gel-dart s powder classification, minimum fluidization velocity, bubble growth and bed expansion, and elutriation. 

However, this section pursues particle size measurement and evaluates its importance (as well as density) for the purpose of classifying the suitability of powders for long-distance pneumatic conveying applications. Initially, an appreciation of the fundamentals and the existing powder classification techniques is required. 

After determining the relevant physical properties (i.e., particle size, solids particle density and bulk density), the next step is to evaluate some of the existing techniques of powder classification. 

Powder Classification Techniques. The Geldart (1973) fluidization, and Dixon (1981) slugging classifications have been found useful in explaining ... 

Application to Long-Distance Pneumatic Conveying. From the above three classifications, there is sufficient evidence to suggest that powder classification (i.e., to select ultimately the most suitable mode of conveying for a given product and its behavioral properties) depends on the following properties ... 

Figure 6.13. Powder classification diagram for fluidisation by air at ambient conditions1 46)...

Sieving is the simplest and most widely used technique for powder classification. This method is based only on the size of the particles and is independent of other particle properties (e.g., density, optical properties, and surface roughness). 

Fig. 63. Geldart s powder classification chart, (a) Geldart s original diagram for air (b) Grace s improvement on Geidart s diagram, 1986.

Such sieves were used solely for powder classification, and the inception of test sieving did not arise until sieve aperture sizes were standardized. Standard apertures were first proposed by Rittinger [2] who, in 1867, suggested a V2 progression of aperture sizes based on 75 pm thus, similarly shaped particles passing consecutive apertures, are in a 2 1 surface ratio. Modem standards are based on a fourth root of two progression, apart from the French AFNOR series, adopted in 1938, which is based on a sieve aperture of 1 mm in a tenth root of ten progression this is known as the RIO or Renaud series. 

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. ... 

Average particle size. The average particle size (APS) of the powder is the main parameter for WC powder classification. It is obtained by Fisher Subsieve Sizer measurements and is commonly given both as as-supplied and lab-milled, according to ASTM standard procedures (ASTM B330, and 430 see also Chapter 5 Table 5.12). 

The following is a short account of powder classification in these three handling applications. 

Zinc ashes Zinc dust Zinc powder Classification Metallic element Empirical Zn... 

Synonyms Cl 77945 Granular zinc Pigment black 16 Pigment metal 6 Zinc ashes Zinc dust Zinc powder Classification Metallic element Empirical Zn... 

For some applications, in which the fines mentioned represent the product, for example, powder classification or mineral beneficiation, the net efficiency of fines is referred to as the confusing term of "recovery."... 

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