In this paper, the concept of conversion used in reaction engineering is applied to ball milling. The motivation is to assess the ability of a mill to produce optimally and efficiently a narrowly-sized product. To this end, a sampling campaign was initiated on a full-scale mill in open circuit. Data pertaining to the residence time distribution was collected for a range of ball filling, slurry concentration and feed flow-rate. Feed and product size distributions were also measured. After data processing, a model of the milling circuit was developed to allow for material transport and size reduction. Then, simulations were undertaken in order to determine the mill conversion of a feed material less than 850 m into particles in the range -75 + 10 mu m, termed in this work middlings. Based on the exploratory work, it was found that the middlings could be produced selectively at high ball filling and low slurry concentration. And perhaps the most important point to make is that the attainable region technique can find applications in cases where milling needs to be done under size constraints dictated by the downstream operation. (C) 2016 Elsevier Ltd. All rights reserved.