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Modern High-Efficiency Air Classification

October 11, 2013

Particle size separation or classification can be one of the more challenging unit operations in particle technology. With average particle size distributions greater than 100 microns, screening or sieving is most commonly used. Below an average particle size of 100 microns, mechanical classification methods are difficult and the use of air classification is required. Modern air classifiers are designed for use in the fine particle separation range (i.e. sub-sieve) with particular attention given to the requirements for particle separations in the less than 15 micron range.
    The keys to good particle separation are the dispersion or separation of particles in the continuous air phase, uniform air flow, and the application of a uniform force opposing the air flow. In essence, effective separation requires the particles to be separated in a uniform energy field that provides a consistent movement of each particle for final separation. Modern classifiers accomplish each of these requirements by optimized feed systems, dispersion zones to separate the particles, and designed particle trajectories that minimize variations within the classifier.
    Classification in an air classifier is the result of opposing forces in the system. Typically, the classifier comprises a rotating disk that imparts an outward centrifugal force on the particle. Opposing that force is the inwardly moving air flow that provides an inward force on the particle. Both the outward centrifugal force and the inward air flow will interact with the particle moving the particle in a direction that depends on the particle size, particle density, and the balance of the two forces. Critical to the separation of the particles is the requirement that the particles are dispersed or separated and the force fields are uniform. Agglomerated particles or random air flows will result in random or poor separation.
    There are many designs used to achieve particle dispersion and uniform force generation. In general, the dispersion zone is created with highly turbulent air that results in particle deagglomeration. The development of the opposing centrifugal force is typically done with rapidly rotating plates with air flow counter to the centrifugal force generation. Spatially, the particles enter a zone with the air flowing toward the middle of the spinning plates. The smaller particles exit from the center hub of the spinning plates while the larger particles remain on the edges, exiting the classifier at the periphery.
    Most modern classifiers fall under the definition of centrifugal air classifiers, which utilize the opposing forces of centrifugal and drag to achieve a separation. The general theory depicting a single particle within an idealized flow field has been well documented in published literature.
    Based on the definition of a centrifugal classifier, at some point the outwardly directed centrifugal force and the inwardly directed drag force on a particle are equal. This critical particle diameter, which has an equal probability of entering either the coarse or fine fraction, is known as the "cut size" (D50).
    Air classification can be used on a wide variety of materials such as ceramics, polymers, and food products. Proper understanding and control of the process variables will result in a useful and economic product.
    Willie Hendrickson is Founder and CEO of the AVEKA Group, which is composed of AVEKA Inc., AVEKA Manufacturing Inc., Cresco Food Technologies LLC, AVEKA Nutra Processing LLC, and AVEKA CCE Technologies LLC. Prior to starting AVEKA as a 3M spin-off, Hendrickson worked at 3M as a researcher and technical manager in particle processing. He has a Ph.D. in Organic Chemistry from the University of Florida, and is currently the president of International Fine Particles Research Institute, a particle technology consortium.

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