LKAB has started the construction of a new concentrator at Kiruna, with a capacity of 5 Mtpy. The flowsheet of the new grinding circuit is technically innovative and critical because it entirely relies on ore breakage properties. It includes primary fully autogenous closed circuit grinding and secondary pebble milling. A critical size class of the magnetite ore is extracted from the autogenous mill and used as pebbles. In 1992, LKAB decided to adapt and use a simulator of the new concentrator. USIM PAC(C) was selected as the support software package, and a modelling team was set up, including both specialists of process modelling and simulation and specialists of the process itself. Specific model development included development of a phenomenological population balance pebble mill model, and adaptation of an existing Control International autogenous mill model. Model parameters were derived from extensive large scale pilot plant tests, and specific laboratory tests were performed to determine pebble mill breakage and selection functions for various types of pebbles and of ores. The simulator was used by the project team for comparing alternative flowsheets (one or two lines, various refinements), for predicting the influence of changes in throughput and plant feed grade, and for evaluating several options for equipment sizes. Additionally, the simulator helped understanding the process in depth and the way the unit operations interact in this particular case.