Use of the attainable region analysis to optimize particle breakage in a ball mill

Abstract

Particle size reduction is one of the most widely encountered, yet least energy efficient, processes. Therefore, potentially significant energy and cost savings exist with even the slightest increase in milling efficiency. Often one would like to mill particles to a certain size, and no smaller, while minimizing energy use and milling time. We use the attainable region (AR) analysis to optimize the comminution of silica sand particles in a bench top laboratory ball mill. When the mill is loaded with a large number of grinding media ( J=volume of media/mill volume=10.7%), the breakage profiles are indistinguishable over all rotation rates investigated. However, operation at lower grinding media fill level ( J=1.5%) reveals separation between the grinding profiles for different rotation rates, suggesting more efficient breakage occurs at a lower grinding media fill level for a given rotation rate. Our results show that operation at multiple speeds, fast ( φ c = rotation rate / centrifuging rotation rate = 0.37 ) at first and then slower ( φ c = 0.03 ) , takes advantage of the initially overlapping grinding profiles and produces a similar particle size distribution with a decreased amount of processing time—less than half the time required for the single rotation rate milling. A natural extension of this idea is continuous milling, where the first mill can operate at a higher energy input for a shorter amount of time and the second mill can operate at a lower energy input for a longer amount of time.