Crystallization of calcium carbonate using a gas (COD-liquid (Ca(OH)(2)) reaction was conducted using a three-stage column crystallizer. In this study, the staging effect of the multistage column crystallizer and the effect of the operating parameters on the particle size distribution were experimentaily and theoretically investigated. From an economic viewpoint, the multistage column crystallizer has been developed to replace a cascade of MSMPR crystallizers, which requires a large installation area and high operating cost because some crystallizers are connected in a series. In the multistage column crystallizer, a narrower particle size distribution was obtained compared to that obtained in the standard column crystallizer. The theoretical approach to predict the particle size distribution is based on the population balance equation which takes account of the back flow into the crystallizer. The calculation results of particle size distribution fairly coincide with the experimental results, which indicates the particle size distribution in the multistage column crystallizer can be roughly predicted by this theoretical model. (c) 2006 Elsevier Ltd. All rights reserved.