A new approach to assessing the grindability of solids and the energy efficiency of grinding mills

Abstract

A new method is proposed for measuring the grindability of solids and the energy efficiency of grinding mills. The method is based on a similarity law of fracture mechanics for elastic materials in conjunction with the self-preserving character of the size distributions formed when feed particles are comminuted individually in compression. The single-particle grindability index reflects more closely the inherent resistance of a solid to size reduction than the grindability indices currently in vogue. The performance of grinding mills is evaluated in comparison with the single-particle breakage mode, which has the highest energy utilization or efficiency of all mechanical grinding routes. For a given set of stipulations, the performance index of the commonly used ball mill when grinding dolomite is found to be only 21%, compared to 44% for the high-pressure roll mill, a newly-invented energy-efficient mill. The difference is also reflected in the energy consumed per unit mass of fines produced when closed-loop grinding circuits are simulated, which shows that the high-pressure roll mill circuit is invariably more energy-efficient than the ball mill circuit, provided the energy input per pass through the mill does not exceed a prescribed limit. Theoretical upper bounds on the yield of the ground product in a desired size range and lower bounds on the energy investment are calculated from the single-particle breakage mode which has, apart from the highest energy utilization efficiency, a minimum dispersion in size of the comminuted particles. Data on theoretical bounds for seven minerals/ores are presented. It is argued that it is unlikely that a grinding mill can be designed and a comminution circuit constructed whose performance exceeds these bench-mark values.