An extremely frequent practice in a lot of industries is the investigation of raw materials for particle size distribution. Particle size measures are now the standard inspection criteria for incoming materials and outgoing products that everyone uses in daily life. Particle size is the predominant method powders and other granular materials are quantized. However, how do you define “particle size”?
Challenges Faced While Analyzing Particle Size Distribution
As the majority of powder granules in the industry are shaped irregularly, the question posed does not have a straightforward answer. Industry, as a rule, circumnavigates the problem by deciding a single size, or “diameter”, for each particle, while implicitly assuming that the particles are spherical. Even when it is understood that the grains are not round, it is typical practice to use a measurement technique that produces a single size figure for each particle.
Various types of instrumentation will frequently report a different size number for the same particle, particularly if it is irregularly shaped. The more severe issue is that with many non-spherical materials, single-size number statistics are not enough to find out how the substance will function. Single-size number statistics cannot satisfactorily characterize the product. This is particularly the case if a sample contains more than one type or classification of the particle, as in the following example.
All these particles – spheres, irregular shapes, and fibers – report a single ECA diameter of about 40 μm.
The assumption that all particles are spherical can affect your process because the size statistics mix the different shapes together.
When powder particles are elongated or fibrous it is apparent that length and width measures are a necessity at the very least. But even with low-aspect ratios, that means particles are not elongated, they can still be irregular, and to predict performance it may be a requirement to obtain statistics on the minimum and maximum diameters (Feret length and width), and on the amount they deviate from being spherical.
“Circularity” is a measure of roundness of the particle silhouette, and by inference the degree of sphericity, in the range 0 to 1.0. It is computed from the perimeter and area of the actual particle silhouette. Also, it is a possibility to be able to put a number on surface smoothness from the shape of the particle outline.
Another measure, “convexity”, allows us to find out how “regular” the shape is from a bigger perspective; that is, to the extent it is free of indentations and protrusions. All these measures are possible and significant even for shapes that are approximately spherical.
Aspect ratio, circularity and smoothness inform us about non-spherical objects:
In particular industries such as abrasives, it is sometimes of value to garner further shape information; for instance, the number of polygonal sides and the interior angles.
In the case of rectangular or fiber samples, a single size number is far from enough. We need to know the distribution of widths present, or the lengths, or both, or statistics concerning the aspect ratios. Additionally, surface smoothness may be significant in these scenarios.
An especially strong characteristic of shape analysis is the ability to correlate shape statistics with size statistics to view how shape transforms with size. This is formed as a scatter diagram, with one point for each particle.
Another strong characteristic is being able to define subcategories contained in a sample, on size and shape basis simultaneously, and draw separate statistics for each class of particle.
This information has been sourced, reviewed and adapted from materials provided by Vision Analytical Inc.
For more information on this source, please visit Vision Analytical Inc.