Measurement of Particle Impact Frequencies and Velocities on Impeller Blades in a Mixing Tank

Abstract

The impact of particles on impeller blades during solid–liquid mixing processes, such as mineral processing, has an important effect on the mechanism and rate of blade erosion. In crystallization processes, impacts between the crystals and the impeller blades lead to breakage or abrasion and the formation of secondary nucleation sites. The distribution of the impact velocities of the particles is determined not only by their mass and size, but also by the local hydrodynamic conditions within the flow adjacent to the impeller blades. A semiempirical method is proposed for the in-situ measurement of particle–blade impact velocities, and frequencies in a stirred vessel. The experimental method is a development of a technique originally proposed by Nienow and uses a thin coating of plasticine on the impeller blades to record craters formed by particle impacts. A model for the plastic deformation of the target material is described and an iterative method for calculating impact velocities from the dimensions of the craters is proposed. For a Rushton impeller blade, the impact velocities are close to the linear speed of the blade at the impact radius; the largest velocities therefore occur close to the blade tip. The highest impact rates also occur at the blade tip, and below the disc of the Rushton turbine. In contrast, for a downward pumping 458 pitched blade turbine, the most frequent impacts occur at small radii, on the lower half of the blade, where the lowest impact velocities are measured.