THE KINETICS AND SHAPE FACTORS OF ULTRAFINE DRY GRINDING IN A LABORATORY TUMBLING BALL MILL

The kinetics of batch grinding quartz from a feed of 600 by 425-mu-m to a product of 80% less than 10-mu-m have been determined using screening and laser diffractometer sizing for size analysis. The specific rates of breakage decreased by a factor of about three when the material became less than about 100-mu-m in size, but the primary breakage distribution function also changed to give proportionately more fine material, so that the grinding efficiency expressed as the development of surface area (B.E.T.) per unit of energy input did not decrease. Analysis of the shape of the particles in the 25 x 38-mu-m size range showed that particles of this size produced by roll crushing or by 8 minutes of grinding of a 425 x 600-mu-m feed were not different but at long grinding times the particles were rounded. This suggests that the breakage mechanism changes to give more chipping and abrasion and less disintegrative fracture. As the material approached the ultrafine size range it adhered to the mill case and there was no further size reduction. However, a technique for striking the mill case to dislodge the particles was successful in allowing further grinding to 40% by weight less than 2-mu-m.