Particle classification: making the grade; when separating powders by particle size, air-based classifiers have advantages over wet classifiers. Follow this guidance to select the best system. (Cover Story).

In general, powders -- whether raw materials, intermediate ingredients or finished products -- can be characterized by a range of parameters, such as composition, particle size, shape, surface area and electrostatic charge. To optimize processing and meet the needs of a given powder's final end use, its most significant characteristics must be specified and tightly controlled. By controlling the distribution of the desired particle attributes, engineers can, for example, produce better flow characteristics or packing density, and enhance the properties of the final product.

During powder processing, classification refers to the sorting of an initial distribution of particles to achieve a desired degree of uniformity, according to a chosen parameter, such as particle density, shape or size. Classification should not be confused with separation, which is generally used to refer to dissimilar materials. Rather, classification usually refers to the separation of different grades of the same material by the chosen parameter.

A classification step often used during powder processing serves to control or overcome the shortcomings of a previous processing stage. One example is the use of a classifier to eliminate oversized particles at the exit of a mill or spray dryer. Such unwanted particles could render a final product unusable, spoil the surface finish of a coating, cause failures in electronic components, or affect drug activity.

New needs spur development

In recent years, many types of chemical process operators have been demanding greater product selectivity, higher process capacity and yield, and increased automation. All of these end goals can be addressed, in part, through tighter control of powder-processing operations. In addition, many recent developments in powder and particulate processing have been driven by industry's demand for:

* Reduced particle sizes (especially for the manufacture of printing inks and toners, pharmaceuticals and minerals-based products)

* Tighter particle-size distributions, with greater removal of unwanted fine and coarse particles

* Improved process yield and reduced production of off-specification materials, especially during startup and shutdown

* Greater energy efficiency

* More stable operation and more precise control over unit operations

* Wider domain of operation

While classification refers to the separation of individual particles depending on any number of parameters, this article focuses on classification by particle size, since this parameter is widely important. Proper control of particle size directly impacts such things as powder flow, reactivity and film homogeneity, and has a direct impact on how a powder behaves during processing.

Particle-size analysis

For any irregular particle, there are many ways in which particle size can be defined. Many of the approaches derive particle diameter by assuming a perfect sphere with a diameter based on some particle attribute that can be measured (weight, surface area, volume, terminal velocity, sieve aperture, and maximum or minimum dimensions as measured using microscopy) [1]; e.g., if volume V is measured, the particle size would be D = [(6V/[pi]).sup.1/3].

Particle-size analysis is often not an objective in itself, but rather a means to an end -- that of correlating certain size-related properties and behavior characteristics with the preparation of a material, the process of manufacture, or the end use. For example, the ability to accurately analyze and control particle size lets manufacturers more closely design the dissolution rate of a drug, and the hydration rate and seven-day strength of a given lot of cement. It also lets them predict material-handling properties such as flowability, filter blockage, and dusting tendency, and, in doing so, to better design the process equipment.

Particle size alone is not the only important parameter defining the effective performance of a particulate material. Within a so-called monodisperse material, all particles have an identical size. However, most natural and engineered materials are polydisperse, meaning that there is a distribution of particle sizes.

The particle-size distribution (PSD) can be described as either a differential or a cumulative distribution (Figure 1). The differential PSD is the frequency of each size of particle in different size classes. The cumulative distribution can be found by integration (more details can be found in Ref. [5]):

[FIGURE 1 OMITTED]

(1) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where:

D = particle dia., [micro]m

[D.sub.i] = upper size of particle-size interval ([D.sub.i.1], [D.sub.i])

F([D.sub.i]) = cumulative PSD

[D.sub.min] = minimum particle size in differential PSD, [micro]m

F(D) = differential PSD

Figure 1 shows an example of a PSD for glass beads in air, measured using online laser diffraction, …