Classification
Air classification methods are suitable for grain size analysis and classification within the range of approximately 1 - 100 µm. The importance of the method results from the facts that on the one hand the particle size distribution of numerous industrial dusts lies in this range and that on the other hand the dispersion size is determined directly by the behaviour of the particles in the gas-carried condition. The method of analysis could also be used alternatively to sedimentation, if no suitable sedimentation liquid was found for the product in which it remains insoluble and which has a smaller density.
The method of air classification can be realized favourably in a centrifugal air classifyer according to the principle of a centrifuge. The feed will be separated at an exactly defined separation limit into coarse and fine material in such an apparatus. This separation limit can be changed by change of the flow conditions in the separator, so that a particle size distribution can be determined.
The inertia forces and turbulence intensities in the air classifyer lie in the order of magnitude of most technical dust separators, so that a good comparability of the results of analysis with the results of the plant is ensured. Therefore, powdered materials with different densities, which have identical particle characteristics during analyses in the separator, will be equally separated for example by a dynamic dust separator. That is the reason why fractional separation efficiencies of dust separators can be represented by an aerodynamic particle diameter in a favourable way, which was determined by means of an air classifying analysis. If the knowledge of the grain size in form of the Stokes' diameter was important in another connection, this diameter could be calculated in accordance with the following relationship:
d0=d1/rhop^0.5
d0 - Stokes diameter
d1 - equivalent diameter for particles of the density 1 g/cm3
rhop - particle density
Air classifiers can be used effectively for the production of particle fractions. Thereby, quantities can be classified which are sufficient for small scale technical plants, too.
In-house separators:
- Laboratory centrifugal air classifier Bahco
- Laboratory zigzag air classifier Alpine