A new melt crystallisation process based on pastillation and purification in a gravity wash column has been tested on lab scale. Analysis of the structure of the pastilles leads to the assumption that the impurities are located in pores between pure crystals. Three process parameters for the caprolactam-water system were quantified using a differential model for the column: the purification rate k(p) = 2.5 x 10(-3) s(-1), the dimensionless residual impurity content x*/x(f) = 0.35 and the axial dispersion D-ax = 10(-5) m(2) s(-1) The experimentally determined purification rate constant (2.5x10(-3) s(-1)) is transformed into a mass transfer coefficient (5.1x10(-5) m s(-1)), A comparison with the internal mass transfer coefficient (7.1 x 10(-7) m s(-1)) and the external mass transfer coefficient (2.8 x 10(-6) m s(-1)) shows the internal mass transfer indeed limits the purification of the pastilles. Design and scale-up rules for the purification of pastilles in a gravity wash column are formulated.