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Inline Particle Size Measurement System Continuous Metering of Particle Size — a Special Optical System Combines Speed and Accuracy

| Editor: Marion Henig

Laboratory analysis using graduated sieves is a tried and tested way to determine particle size distribution in the management of grinding operations. But although it is accurate, sieving is time-consuming. A new inline particle size measurement system brings the accuracy of sieve analysis to real-time measurement.

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The accuracy of the probe was tested on a HammerWitt Lab, a hammer mill used for grinding small volumes of product in the laboratory. (Pictures: Frewitt)
The accuracy of the probe was tested on a HammerWitt Lab, a hammer mill used for grinding small volumes of product in the laboratory. (Pictures: Frewitt)

Particle size distribution is often critical in determining the quality of a powder product. Minerals, salts, lactose, active pharmaceutical ingredients, proteins, pigments — all depend on particle size for some or all of their important characteristics. Because of its greater surface area, a fine powder will react faster, and can adsorb into larger volumes of other ingredients — a characteristic that is important in medicines as well as many other applications. A granular powder, on the other hand, has better flow properties and is more resistant to caking.

Milling and grinding specialist Frewitt developed its inline particle size distribution probe to allow real-time control of particle size reduction operations. This article presents experimental results showing that the probe provides comparable accuracy to that of a laboratory sieve analysis, but without the time penalty required for sieving.

Watching the shadows

The Frewitt inline measurement system uses a light source to project shadows of the particles onto an optical collector. Individual particles can be detected at a rate of up to several thousand per second. An extended spatial filter then converts the shadow signals from individual particles into size data for analysis. The system also uses optical fiber components to record the time of flight for each particle, hence calculating its speed. A computer calculates the particle size distribution in real time across the size range 50–6,000 µm, and also generates D10, D50, and D90 measurements.

In contrast to other inline methods, the Frewitt probe is not based on a particular mathematical model of particle characteristics. Instead, each particle is characterized directly from its maximum dimension. The probe then calculates particle size in terms of volumetric diameter (Dv), and the particle size distribution is presented in terms of mass.

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