Tube mills and dynamic air classifiers are widely used to grind cement clinker to finished cement. Energy consumption is high and simulation has the potential to reduce this by increasing productivity. This paper describes the modeling and simulation of six grinding mills operated by Cement Australia Pry Ltd. Each mill circuit was sampled and data analyzed to evaluate its performance and to fit a model to it. Modeling was extended to produce equations that relate breakage rates in the first and second compartments to mill size, ball load and feed rate. The by-pass for each separator unit was low so there was scope to increase productivity in each circuit. Examples are given to show how feed rate can be increased without changing cement quality.