This paper describes the progress towards a general model of the crossbelt separator to be used as a process design and optimisation tool in the mineral sands industry. A technique has been developed for characterising mineral sands feedstocks in terms of the distribution of magnetic susceptibility. A Permroll laboratory separator was used to screen the feedstock into fractions of similar magnetic properties and a Satmagan magnetic weigher was calibrated to measure the specific susceptibility of each fraction. For fractions of ilmenite collected in this way the susceptibility was shown to vary with temperature according to the Curie-Weiss law. A critical susceptibility for recovery at each separator pole has been defined and a function is proposed for the probability of recovery at each pole. The resulting model for mass recovery of mineral combines the feed distribution of magnetic susceptibility and temperature relationship with the concepts of critical susceptibility and probability of recovery.