Powder Classification

Classification methods are used to obtain particular powder distributions or to exclude certain powder sizes from a distribution. The general process for separating dispersed materials is known as "air classification" or "fluid classification," where powder classification is based on the movement of the suspended particles at different points under the influence of a force. The fluid is usually water or air, and the field force may involve gravity or centrifugal or coriolis forces. The other forces of importance are the drag forces due to the relative flow between the particles and the flow medium, and the inertia forces due to accelerated particle movement. The classification process is defined in terms of sorting and sizing. The former includes processes such as froth flotation, where particles are separated on the basis of chemical differences and particle density. The latter, which is covered here, is based only on differences in particle size.

In an ideal system the cut size is well defined and there are no coarse particles in the fine fraction, and vice versa. In practice, however, there is always overlapping of sizes. The cut size may be predicted from theory, but this usually differs from the actual cut size due to the difficulty of accurately predicting the flow pattern in the system. It is therefore necessary to be able to predict the future performance of classifiers based on their past performance.

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