Air classification is a technique by which dry powders of mixed particle size are separated into two distinct fractions, one above and one below a definite cut point. This is achieved by exploiting the differential forces acting upon particles of different size and mass within an air flow system. Different techniques can be required to achieve this depending on the material and particle sizes involved and British Rema has a full range of classifier options available to meet a wide range of operating parameters.

Attrition refers to wearing down or grinding by friction.

Friability refers to the ease with which a solid material can be crumbled or reduced to powder by attrition. The friability depends on the nature of the applied forces as well as the intrinsic physical properties of the material itself. There is, consequently, no absolute measure of friability but a variety of methodologies designed to provide relative measurements when applied to a specific application.

All three terms refer to the reduction of particle size by mechanical methods and are somewhat interchangeable. Although there is no fixed rule, typically, the term micronisation is used where the resulting particle size is required to be less than 10 microns and for many applications an “air-microniser” (spiral flow jet mill or opposed jet mill) would be required to achieve these particle sizes. Refer to our Equipment Selection Guide for further details.

“Mesh size” covers a number of alternative scales used for the definition of particle size, all based on identifying the smallest of a series of meshes, or sieves, of reducing grid size through which a particle will pass. The numerical value in any particular scale generally refers to the number of openings per unit of length or area and, hence, the higher the number the smaller the particle size. There are a number of different scales in use and three of the most common (US mesh, Tyler screen and BSS mesh) are set out in the Particle Size Comparison Chart with the equivalent particle sizes expressed in microns.

Micron is a commonly used alternative term for micrometer. It is one millionth of a meter (1×10−6 m) and the usual notation is “µm” (using the Greek letter mu).

Hardness measures a mineral's resistance to scratching and reflects its atomic structure. The Mohs' scale is commonly used to describe the hardness of materials and compares a material’s resistance to scratching against a standard set of reference minerals from talc (Mohs hardness 1) to diamond (Mohs hardness 10). The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material. The scale is not linear and the table below shows comparison with absolute hardness measured by a sclerometer.

There are a number of units and scales commonly used to measure particle size. These either refer to average particle diameter measured in units of length, typically microns, or refer to a smallest mesh size through which the particles will pass. The following table sets out conversions between a variety of systems:

There are a number of terms commonly used to describe particle size. Whilst there are no fixed definitions, the following table can be used as a guide: