KINETICS OF AVALANCHE MIXING OF GRANULAR-MATERIALS

The problem of the avalanche mixing of two fractions of granular mater ial is solved. Mixing of the fractions takes place in a cylinder that rotates slowly about its longitudinal axis, which is positioned horizo ntally. The cylinder is not filled completely and at all times mixing only occurs in the surface layer of granules. It is shown that, depend ing on the relation of the volumes of the fractions and the volume of the empty space, mixing can take place slowly, over a large number of rotations, in a diffusive regime with convection or rapidly, by the ti me the cylinder has turned through a small angle. The mixing process i s described analytically in terms of a purely geometrical approach and can, in a number of situations, be reduced to a sequence of discrete mappings. The characteristic mixing times are determined, including th e times over which one or the other of the pure fractions no longer ex ists in the regions adjacent to the surface of the cylinder. Their dep endence on the degree of filling of the cylinder and on the ratio of t he volumes of the fractions is found. (C) 1997 American Institute of P hysics.