Grinding in an air classifier mill — Part II: Characterisation of the two-phase flow

Based on the insights about the air flow within the air classifier mill presented in the first part of the paper [1] the particle laden flow in the vicinity of the grinding pins and through the impeller wheel classifier is studied. The particle motion in the size range between 29 and 573 μm is investigated using Particle Image Velocimetry (PIV). The mathematical model for the fluid flow is extended to a fully coupled two-phase flow numerical calculation.The particle motion in the grinding zone is not influenced by the peripheral velocity of the impeller wheel classifier. The grinding disk velocity doesn't affect the particle behaviour at the classification region. The impact velocity at the grinding pins is independent from the particle size. The average impact velocity normal to the pin wall amounts for the cylindrical pin geometry to 68% and for the prismatic pin geometry to 80% of the peripheral grinding disk velocity. The particles entering the classifier region move with the air velocity in radial direction while their transport in sucking direction slows down in comparison to the air motion. Because of the flow separation between the classifier blades the majority of the particles undergo impacts at the blades and consequently undesired stochastic particle motion of the rebounding particles occurs.

An in-depth study of the two-phase flow in an air classifier mill is presented. The particle motion in the grinding zone is not influenced by the peripheral velocity of the impeller wheel classifier. The grinding disk velocity doesn't affect the particle behaviour at the classification region. The figure below shows that the impact velocity at the grinding pins is independent from the particle size in the investigated particle size range (peripheral velocity of the grinding disk and the classifier 100 m/s and 30 m/s respectively, Ψ ≥ 44, vertical measurement plane across the pin centre).Figure optionsDownload as PowerPoint slideResearch highlights
► A unique normalised impact velocity distribution is found at the impact elements.
► The particle impact velocity is constant along the pin height.
► Classifier velocity doesn't influence the particle impact velocity in the grinding zone.
► Separation of particles by the investigated classifier is sub-optimal.