Modelling particle separation in a spiral trough is a complex process which requires knowledge of particle transport behaviour under different flow regimes. With the demonstration of the existence of secondary flow and utilizing knowledge of particle behaviour in flowing fluid films, the hydrodynamic model of particle separation in a spiral trough is, to some extent, capable of explaining the behaviour of fluid flow and that of particles at low concentrations. The recent trend in research on spiral technology is to the modelling particle behaviour in flowing films in spiral troughs. While this is essential in terms of developing a fundamental model, a question arises as to how successful such a model is in providing an adequate explanation for particle separation. Distribution of the particles of different sizes and densities across the trough, under plant conditions, has not been much reported in the literature. Therefore, a research project was undertaken to determine the distribution of coal particles across the cross-section of a spiral in a Reichert LD4 coal spiral. The results showed that using different length of spiral may be beneficial for treating coal particles at different sizes.