Flow of materials in rod mills as compared to ball mills in dry systems

The rod mill product is characterized by its narrow size distribution if it is compared with that of a ball mill operating under the same conditions in open circuit. This is partly due to the bridging action of the rods over the coarse particles avoiding the fine sizes (the grinding kinetics aspect), and partly due to the nature of material flow through the mill with its rod load (the material transport aspect). This article will stress mainly on the nature of material transport within the rod mill. It was found that the presence of rods minimizes the particulate segregation which is caused by differences in particle size and/or material density of the flowing components in the system. Segregation due to differences in particle shape (spherical versus natural particle shapes) and/or surface roughness was less affected in the presence of rods as stirring media in the system. The geometric restrictions on the axial and radial movement of the rods inside the drum, in addition to the restricted particle–rod interactions affected the axial and radial dispersion of the material flowing through the rod mill. On the other hand, particulate segregation in the ball mill is almost eliminated among all material components different in size, density, shape, and/or surface roughness. Its product displays very good mixing under all operating and material conditions.

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► Material transport in rod mills.
► Material transport in ball mills.
► Segregation and mixedness in tumbling mills.
► Axial dispersion in tumbling mills.
► Comparison between material transport in ball mills and in rod mills.