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Abstract

An apparatus and process for classifying fine particles from a particulate material, such as toner and pigment powders. The apparatus is generally a dynamic centrifugal gas (air) classifier that combines the operational advantages of centrifugal air classifiers with increased aerodynamic forces provided by a rotating member to classify powders at cutpoints of fifteen micrometers and finer. The classifier is configured to have an annular-shaped inner passage, an annular-shaped outer passage circumscribing the inner passage, and an inlet through which a gas-entrained particulate material is introduced substantially tangentially into the inner passage so as to impart a centrifugal force to the particulate material. A portion of the entraining gas is preferably separated and directed into the outer passage. Within a separation section of the classifier, the gas flowing through the outer passage is passed through the particulate material within the inner passage to separate finer and coarser fractions of the particulate material. The finer fraction is then returned to the inner passage, while a vertical outlet receives the coarser fraction from the separation section. The finer fraction continues through the inner passage to a horizontal outlet disposed at the horizontal axis of the inner passage. A horizontal rotating member is disposed within the horizontal outlet and applies additional centrifugal force to the material within the inner passage, thereby preventing an intermediate fraction of the material from entering the horizontal outlet.

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