Particle shape flakiness

Source Reproduced from CEN EN 933-3, Tests for geometrical properties of aggregates - Part 3 Determination of particle shape - Flakiness index,... 

While the shape factor, introduced in the previous section, provides a quantitative definition or description of particle shape, there are other descriptors such as flakiness ratio, flakiness index, elongation index and angularity number which are also found to be in vogue. 

Depending on operation conditions and metal properties, the shapes of the atomized particles may be spheroidal, flaky, acicular, or irregular, but spherical shape is predominant. The spheroidal particles are coarse. For example, roller-atomized Sn particles exhibited a mass median diameter of 220 to 680 pm. The large particle sizes and highly irregular particle shapes suggested that the disintegration process may be arrested either by the premature solidification or by the formation of a thick, viscous oxide layer on the liquid surface. The particle size distributions were found to closely follow a log-normal pattern even for non-uniform particle shapes. 

Depolarization of one-shot seattered radiation is stipulated by deviation of the particles shape from a spherical form. The D ratio in the depolarization layer of 2.5-3.5 km is practically constant and does not exceed 0.20-0.25 at a sharp inerease of the backscattering ratio R at 3.0 km It shows that the aerosol from Iraq is represented mainly by non-spherieal particles. The vertical level-by-level cut shows a flaky stracture of aerosol particles. 

It is well known that particle shape affects many secondary properties relevant to powder handling such as the bulk density, failure properties or particle-gas interaction. For non-spherical particles, the results obtained with different methods of particle size measurement are, in general, not comparable. From the point of view of powder handling, flaky or stringy particles like wood shavings, mica or asbestos fibres are known to be difficult because they interlock and form obstructions to flow. 

Many uses of aggregates require cubical particle shapes (see sections 8.3.3 and 8.5.4). Products which are excessively flaky may be brought within specification by the use of de-flaking screens. These may have transverse, elongated or slotted apertures, and may incorporate weirs or baffles to improve the presentation of slabby particles. In some cases, such screens simply remove a size fraction which has been found to be particularly flaky. 

Part 2 Determination of particle size — Test sieves, nominal size of apertures Part 3 Determination of particle shape of aggregates — Flakiness index Part 4 Determination of particle shape of aggregates — Shape index... 

Part 3 Determination of particle shape of aggregates - Flakiness index . 

Particle shapes must not be extreme (e.g., flaky or needleUke). Smooth rounded particle shapes are preferred. 

Thakur et al. [68] studied the effect of carboxymethylation on the structure of amylopectin. The scanning electron micrographs of amylopectin and carboxymethyl amylopectin showed change in the structure of amylopectin after derivatization. Amylopectin particles are polyhedral in shape with granular surface, while carboxymethyl amylopectin particles are flaky with striated surface. 

The effect of particle shape on particle size distribution was investigated by Naito et al. (1998) with commercial particle size analyzers based on five different measuring principles electrical sensing zone, laser diffraction and scattering, x-ray sedimentation, photosedimentation, and light attenuation. The particles used are blocky aluminum oxide and barium titanate particles, flaky boron nitride particles, and rodlike silicon whisker ceramic powders. Altogether, four commer-... 

This volumetric method makes no assumptions about particle shape, and indeed is not greatly affected by particle shape except in extreme cases like flaky materials, such as some clays. To count the number of particles in a known volume of suspension, such as for particulate contamination studies or for a blood cell count, the sample volume is accurately metered by means of a calibrated "manometer". Figure 1 illustrates the original, simple, mercury siphon and metering system. 

Particle shape can be described qualitatively using standards terms such as granular, flaky, needle, etc. (BS 2955). Quantitative description, however, requires use of shape factors that enables a particle s surface area and volume to be calculated on knowing its size . 

Particle shape grains of bioclastic carbonate sand may be curved (shells), platy, thin-walled, flaky and hollow or porous (intra-particle porosity) and often have a high angularity, while oolites generally are rounded and massive (see Figure 9.21). 

Smooth jaws produce more numerous flaky pieces than corrugated jaws. Curved jaws produce less fines but more numerous flaky particles. Crusher speed has little effect. The presence of material too small to be crushed has a deleterious effect on the shape of products. Secondary crushers with a small reduction ratio can improve the shape of primaiy crushed material, but secondaiy crushers are not inherently different from primaiy crushers. Slotted screens can remove flaky particles from the product. Impact crushers produce fewer flaky particles than any other type. 

The factor iji < 1 must be determined experimentally for particles of interest. Typical values are iji = 0.77 for particles of rounded shape iji = 0.66 for particles of angular shape iji = 0.43 for particles of a flaky geometry. 

As far as conducting fillers are concerned, we have rather a wide range of choice. In addition to the traditional and long used fillers, such as carbon black and metal powders [13] fiber and flaky fillers on organic or metal bases, conducting textures, etc recently appeared and came into use. The shape of the filler particles varies widely, but only the particle aspect ratio, the main parameter which determines the probability... 

The EBRD-atomized particles may be of spherical or flaky shape. Droplet size is less than 700 pm, but much smaller droplets, 30 to 50 pm in diameter, have also been demonstrated. Cooling rates higher than 103 °C/s are achieved in fine droplets. 

Sphericity shape factor Circularity shape factor 1 W where a0 = shape factor for equidimensional particle and thus represents part of av which is due to geometric shape only av = volume shape coefficient m = flakiness ratio, or breadth/thickness n = elongation ratio, or length/breadth Sphericity = (surface area of sphere having same volume as particle) / (surface area of actual particle) Circularity = (perimeter of particle outline)2 / 4tr(cross-sectional or projection area of particle outline)... 

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