Most laboratory-scale comminution studies have been carried out by grinding narrowly sized fractions of single mineral feeds. In such experiments, the mill environment remains self-similar and invariant which is a characteristic of the mill/material system. On the other hand, in industrial practice, feed to the mill comprises oversize recycle and new feed that is highly heterogeneous and distributed in size. The objective of this investigation is to study the breakage kinetics and energetics of grinding coarse size feed in the presence of deliberately added fines. Quartz and limestone mineral systems were selected as feed in order to study these phenomena for minerals whose hardness ranges from hard to soft at different coarse/fine ratios. It was found that the cumulative breakage distribution function for this situation does not change with mixture composition. On the other hand, the initial breakage rate function of the coarse particles increases with increasing proportion of fines in the mixture. This acceleration could be due to the smaller collision cross-section of the fines. In addition, the coarse particles appear to be preferentially present at the toe of the mill where most of the grinding takes place. (C) 2010 Elsevier B.V. All rights reserved.