A two-dimensional solution to Laplace＇s equation was used to obtain a map of the magnetic field between the two poles of a crossbelt separator. With this flux map it was possible to calculate the trajectory of a weakly magnetic particle between the main belt and crossbelt surfaces. A critical susceptibility for particle recovery at that pole was then determined using an iterative technique. This exercise was repeated for a wide range of main belt speeds and operating pole air gaps. The ratio between the critical susceptibility for a moving belt and the critical susceptibility for a stationary belt was found to be relatively constant over the normal range of pole air gaps for a given belt speed. A belt speed correction factor, gamma, was therefore defined such that the critical susceptibility for recovery, chi(c)*, at a given pole and belt speed can be calculated from: chi(c)* = gamma mu(0)g/B(z)dB(z)/dz where mu(0) is the permeability of free space, g is the gravitational constant, B-z is the maximum value of the vertical component of flux density at the main belt surface and dB(z)/dz is the vertical component of field gradient at that point.