A multi-stage counter-current crystallization is investigated theoretically and experimentally. The process allows separating mixtures, which form complete or partial solid-solutions. An equilibrium stage model is used to describe the progress of the purification with the specific solid-liquid equilibria as main input information. Results presented in this paper concern (a) the demonstration of the process principle for two selected generic systems characterized by complete miscibility in the solid state, namely K2SO4/(NH4)(2)SO4 and KCl/KBr, (b) the scale up from a laboratory unit to a pilot plant and (c) the evaluation of the potential of the process principle for the purification of systems characterized by partial miscibility in the solid state. (C) 2013 Elsevier Ltd. All rig ins reserved.