Separation processes

Under the heading ‘separation processes’ we include those procedures of particle metrology in which the feed is split into at least two fractions that ideally contain only particles below or above a certain size (cf. sections 2.3 and 4.1). Real separations are not sharp but are characterized by grade efficiency curves. The problem with analytical separation processes is to fix and determine the cut point. With separation processes the measure used for the population of particles can be freely chosen since the fractions are obtained by preparation. As a general rule the mass of the particles is used.

The most important separation processes in addition to sedimentation methods, which can also be considered in this group, are sifting and air classification.

(a) Sifting

In sifting, the solid material to be analysed is separated either into two fractions using a single sieve or into several fractions using a nest of sieves whose apertures decrease from the top to the bottom. Apart from sifting by hand, a distinction is made between dry sifting in a mechanical sieve shaker, air-jet sifting where the material is lifted off the sieve by a stream of air and repeatedly transported to another place on the sieve, and wet sifting where the solids are poured onto the sieve in a suspension. As well as the usual sieve cloths there are photomechanically manufactured sieve plates. With air-jet sifting these can be used down to apertures of about 15 pm and with wet sifting down to 5 /xm.

Sieves have a fairly wide distribution of aperture sizes. This has the effect that the cut point shifts towards a coarser level as the sifting time continues. The cut point for the sifting procedure being considered is determined after the completion of the sifting by obtaining as undersize a narrow fraction of the residue on the sieve from which, by using the counting and weighing procedure together with a knowledge of the density of the material, the diameter of the equivalent volume can be determined.

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