Wet high intensity magnetic separators are used in magnetic separation of minerals with low susceptibility. The dynamic process of material built-up in the matrix is influenced, among others, by the matrix geometry, gradient and strength of the magnetic field. These factors, however, do change with the built-up of magnetic material in the matrix. Detecting the built-up of magnetic material is crucial to the continuous operation of the separation process. In this study we present a numerical modelling approach for wet high intensity magnetic separation. The electromagnetic and the fluid flow field are modelled with the finite element method, while the material particles are identified and evolved using the Level-set approach. Such a framework retains the influence of magnetic particles on the surrounding magnetic field and can be used to detect and predict material build-up in the matrix. The model is validated with a benchmark example and the potential of the approach is demonstrated using a simplified magnetic separation matrix example.