Influence of particle size and shape on the comminution of single particles in a rigidly mounted roll mill

In single-particle roll mill comminution, the degree of size reduction is controlled by the ratio of the nip size of the particle and the roll gap. However, the nip size or effective particle size of irregularly shaped particles is generally unknown. An experimental procedure is presented to assess nip size of irregularly shaped particles and to evaluate energy consumption and energy utilization in their comminution. A series of experiments in which the roll gap was successively reduced reveals that the height of a particle, that is, the particleʹs dimension normal to the plane in which it resides in its most stable position, is a satisfactory approximation of the nip size which can be conveniently measured with a slot classifier. The nip size or particle height is distributed in √2 and √4 sieve–size fraction according to a Gaussian normal distribution, the median size being roughly 80% of the geometric sieve size. The energy investment for comminuting single particles in a rigidly mounted roll mill varies with the nip size in the manner of a power function. Estimated Weibull coefficients of uniformity for dolomite, galena, and quartz are 7.5, 1.9, and 7.9. Product size distributions of comminuted nip-size fractions are self-preserving in a dimensionless size X/X50 and thus, the median size is a measure of product fineness. Grindabilities calculated from the linear reduction ratio energy relationships are 7.8, 14.0, and 4.5 t/kWh for dolomite, galena, and quartz, respectively. Single-particle grindabilities of these three minerals are power functions of the nip size. Dolomite and quartz data follow closely Rittingerʹs Law of Comminution.