Energy consumption and product size distributions in choke-fed, high-compression roll mills

Abstract

Four minerals, dolomite, limestone, quartz and hematite, were ground in a laboratory-size chokefed, high-compression roll mill, a newly invented energy-efficient comminution machine. The size distributions of the ground solids were analyzed for energy-size reduction relationships and for development of a model of grinding kinetics in terms of energy expended in the mill. The results show that the size distributions are self-similar, that log median size decreases linearly with the amount of fines generated, and that the inverse of the median size increases directly with energy input, which in turn can be controlled by adjusting the milling force on the rolls. The standard population balance model of grinding kinetics, used widely for tumbling mills, must be modified in order to account for increasing energy dissipation as the bed of particles gets packed progressively more tightly during its passage through the rolls. The resulting model simulates the product size distributions as a function of energy input quite accurately. A noteworthy observation is that, unlike in ball mill grinding, the breakage rate parameters in pressurized roll mills remain relatively insensitive to particle size, and consequently most coarse and medium size particles in the feed get broken in only one pass through the rolls. This characteristic feature is reflected in the superior performance of roll mills in energy terms.