Abstract Performance data for the wet grinding of coal have been obtained in this novel, planetary ring-roller mill over a wide range of operating conditions. Results from mills of different sizes are presented, with rotational speed, feed rate, coal loading in oil and water, type of coal and residence time in the mill as the main variables. Combining empirical results with insight of mill dynamics has allowed a design capacity scale-up from 0.5 to 20 + t/h coal. The mill is able to grind coal and other materials down to the ultrafine 10 μm size range rapidly and efficiently.