The purification of granules, which are polycrystalline particles, suspended in a melt has been studied experimentally. A stirred vessel was used as purification device, since the operating conditions can be manipulated easily in this configuration. The particles consist of a skeleton of caprolactam crystals. The impurities are mainly located in the interstitial space or pores between the crystals. A model that describes the purification of porous and crystalline particles is presented. The driving force for purification is the difference between the pore-averaged impurity concentration and the impurity concentration in the bulk melt. The model accounts for pore diffusion. The mass transfer coefficient has been related to the purification rate, taking the size and the shape of the particles into account. On average 94% of the impurities could be removed from the particles. The remaining 6% is attributed to inclusions (2%) and to equilibrium between the impurity concentration in the pores and the melt. The purification rate constant was in the order of magnitude of 5 x 10(-4) s(-1). The purification rate constant is affected by the overall water content of the particle, the purification temperature and the particle morphology. The presented model can be used for the design of a purifier of polycrystalline particles which may take the appearance of a stirred vessel or a gravity wash column. (C) 2004 Elsevier Ltd. All rights reserved.