Powder segregation mechanisms

Segregation is a real problem in industry. Some mixers may not be able to form a random mixture of some powder mixes because of powder segregation mechanisms occurring within the mixer. Some mixers may even demix a mixture due to the dominance of segregation mechanisms acting on a particular mixer, for example, a... 

Johanson K. Eckert C, Ghose D, et al. Quantitative measurement of particle segregation mechanisms. Powder Technol 2005 159(11) 1-12. 

Mixing and segregating mechanisms cannot usually be separated within a powder-handling system. The final mixture quality will be determined by the relative importance of the two mechanisms but ideally segregation should be eliminated or at worst suppressed in favour of the mixing operation. 

Another technique is to change the particle size distribution. There are, however, disadvantages. If segregation is occurring by the sifting mechanism, the particles must be almost identical in size before sifting is prevented. Alternatively, the mean particle size can be reduced below 100 p.m, but this size reduction (qv) increases the probabiUty of segregation by the too fine powder mechanisms. 

To produce tablets that are uniform in weight and content and exhibit a certain degree of mechanical strength, one needs a mixture of powders with (a) good flow properties (b) a minimum tendency for segregation, and (c) the ability to be compressed. To achieve these ends, granulation with other excipients is often necessary. 

Besides theoretical prediction tools,63 commonly used devices include fluidization segregation testers and sifting segregation testers. The Jenike and Johanson fluidization tester is standardized through ASTM International.64 The tester fluidizes a powder sample (80 mL) in a column of air and allows the particles to settle in the column. The tester has a mechanism to retrieve top, middle, and bottom samples of the settled powder. The sample is then analyzed to determine if there is a property gradient within the column. However, the tester can be improved by making it material-sparing, more efficient, and more reproducible. 

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