Understanding fine ore breakage in a laboratory scale ball mill using DEM

DEM models of fine grinding in ball and stirred mills have to date almost entirely focused on the motion of the media and their interaction with the mill configuration. For SAG mills, a large fraction of the feed material can now be accurately represented in DEM models. However, for other mill types with much finer feed materials, such as the second chamber of a cement ball mill, the vast numbers of feed particles makes their explicit inclusion in the models prohibitive. However, it is now feasible to model a periodic section of a laboratory scale ball mill and include the coarser end of the ore size distribution directly in the DEM model. This provides the opportunity to better understand the effect of media on the interstitial bed of powder and of the effect of the powder on the media. The effect of the powder fill level, which is varied between 0% and 150% of the pore space in the media charge, is explored. The distribution of the powder, its effect on power draw and the way in which it contributes to the pattern of energy utilisation is assessed. The simulation results are compared with experimental results from a test at similar ball loading and rotation rate and for several size fractions of ore at a range of powder fill fractions. Tracking the collision histories of specific ore particles within the charge allows estimates of the probability (per unit time) of collision between media and ore particles (the “Selection” function) and of the intensity of each collision which can be used to estimate the severity of breakage using the JKMRC breakage model (the “Breakage” function). The energy spectra indicate that for a typical ore, only very few collisions are large enough to cause damage to the body of each particle. This provides an estimate of the energy efficiency which is less than 10% at even the best operating conditions.

Research highlights
► The analysis of ball mill behaviour using DEM is extended to include fine powder particles.
► The two way effect of the powder on the overall flow and collisional environment are understood.
► Powder filling starts at the shoulder and extends back towards the toe as the powder fill level rises.
► Mill power is high at both low and high powder fill levels and lower for intermediate values.
► The ratio of the ball–ball and powder–powder collisional energy peaks depends on powder level.