A mixture of two sets of similarly sized particles having different terminal velocities in a separation liquid is separated. This is done by removing either the smaller particle portion of the set having the greater terminal velocity, or the larger particle portion of the set having the lesser terminal velocity, or both, for example, by sieving. The resulting mixture is introduced into a tube in which the separation liquid is flowing upwardly. This eliminates intermixing of the fines of one set with the larger particles of the other set. The flow of liquid in the tube can be arranged to provide laminar flow separated by two regions of turbulent flow to improve separation of particles. Anion and cation exchange resin particles can be separated in this manner, regenerated and returned to service.