Use of rotary-jet mixing devices to increase the rate of the dispersion process in bead mills

Abstract

During rotation of the mixing device, the dispersed medium, under the action of centrifugal force, flows around it, taking on a directed motion along the device. The energy introduced into the dispersed medium is given up to the medium in the immediate proximity of the surface of the mixing device. In view of the great peripheral velocities of the mixing device, around it there arise considerable velocity gradients of the dispersed medium, which leads to the appearance of high shear stresses. In addition, the ~flow around the mixing device sucks out the dispersion medium located on the convex side of the working elements, setting up intense axial shear. Rotary-jet mixing devices, used as the working elements of bead mills, can be mounted on the shaft with the convex part downward or with the convex part upward, or in a combined manner (in the latter case, they can be either of identical or different diameters). On the basis of preliminary experiments, it has been established that the use of rotaryjet mixing devices makes it possible to vary over wide limits the ratio of the axial and tangential components of the force acting on a bead at the wall of the mill. As is well known [3], the correct choice of this ratio determines the most favorable conditions for collisions between the grinding bodies and the particles of the substance being dispersed. In view of this, there has arisen a need to investigate the above-mentioned devices to determine their optimal dimensions, arrangement on the shaft, and the efficiency of their use for dispersion processes in bead mills. With a study of the structure of the flow of the dispersed medium in a mill with different means of attachment of the working elements under study, it was established that microvertexes are observed to the greatest degree with a combination installation of the working elements. For a visual study of the structure of the fflow in a dispersed medium (iron oxide), tracers were introduced, i.e=, lumps of mica measuring 2  4  I mm. With illumination of the container by a slit source of light, a flat beam of light, passing through the container, is partially reflected from particles of mica moving along with the dispersed medium. Figure 2 gives the results of an investigation of the degree of grinding, characterized by the content of particles less than 2.5 ~m, on the ratio of the diameters d~/d of the working elements with their combination arrangement and on the ratio of the diameter d of the working element to the diameter D of the container. The investigated models of the working elements had a diameter d = 60, 80, I00, and 110 ram, and the container a diameter D = 120 ram. As can be seen, the optimal ratio of the ratio of the diameters of the working elements dl/ d = 0.8. With an increase in the diameter d of the working element, the degree of grinding rises and attains a maximum, irregardless of the means used for attachment of the working elements with d