The separation of nonmetallic particles by buoyancy in tundishes is investigated theoretically. It is shown, that the best possible separation is achieved if the projection of the region through which flow from the inlet to the outlet mainly occurs, covers as large a part of the surface as possible. In contrast, the volume of this region, its distance from the surface and the mean retention time do not influence separation directly. Numerical calculations of the flow fields and particle separation in two different tundish geometries with and without a baffle predict no significant differences in the capability of separating nonmetallic particles. This surprising result may be explained by the principle stated above