Methods for Size Analysis

It goes beyond the scope of this book to give a full account of the methods available for particle size analysis. For a further study of this, we refer to the book of Allen (1990). The principles behind the methods differ widely, and they yield different measures of the particle size. 

The size that determines the behavior of the particle in cyclones and swirl tubes is the dynamically equivalent size. Using methods that measure this size avoids errors arising from such things as a varying particle density or particle shape and, for this reason, are considerably preferable to others. Unfortunately, these methods are also rather labour intensive. Below we mention the most used sizing techniques, and discuss briefly their usefulness for grade-efRciency analysis. We start with on-line methods. 

A cyclone train consists of a series of small cyclones (a few cm in diameter) with a progressively lower cut size. Like the cascade impactor, the cyclone train permits on-line measurement of the d3mamically equivalent particle size distribution. The advantage of the cyclone train is that the cyclones can collect more particles than a cascade impactor. 

A laser diffraction particle analyzer can measure, in principle, the size distribution of particles suspended in a gas. However, if one attempts to determine the size distribution of agglomerates, laser diffraction may not give a very accurate result due to the nonsphericity of agglomerates. See also the discussion of this method below. Laser diffraction does not give a dynamically equivalent particle size. 

The output from disc centrifuges has the form of turbidity versus time. The turbidity of a uniform dispersion is given by Lambert-Beer law (Devon et ah, 1991)  

---

Hydrodynamic chromatography (HDC) offers a means of obtaining size information on colloidal particles in suspension with the same ease that is characteristic of chromatographic methods for size analysis of molecules in solution. 

---