The breakage characteristics of low rank coals were tested in a laboratory using ECO coal from an Indonesian mine as the feed material. The grinding test results were used to fit the parameters of the breakage functions of an existing continuous hammer mill model. The mill hold-up, specific energy and projection rate to the screen were analyzed to observe the effects of operating conditions. The results indicate that for each underscreen aperture there exists a characteristic threshold point of the feed rate above which over-grinding occurs. This threshold point can be used to determine the optimal operating conditions of the breakage process. Additionally, a scale-up model of the hammer mill is established based on the energy-size relationship to predict the mill capacity as a function of the mill design and the operating parameters. A comparison between the prediction from the model and the manufacturer's data illustrates that the model based on Rittinger's theory fits the breakage characteristics of the hammer mill better than the models based on Bond's and Kick's theories. The established scale-up model agrees well with the manufacturer's test data within an acceptable degree of accuracy. (C) 2014 The Authors. Published by Elsevier B.V.