A gravity wash column is a common type of continuous solidliquid separator, which can be applied in the final step in the suspension crystallisation process of purifying phosphoric acid. In this work, a mathematical model based on material balance was presented for the purification process in a gravity wash column, and the experimental data of suspension crystallisation of phosphoric acid hemi-hydrate were correlated. The developed purification model can be used to calculate concentration profiles of impurities along the column axis and to predict the effect of operating conditions on the purity of the product stream of phosphoric acid. The model parameters were also estimated by fitting the model with experimental data. (c) 2012 Canadian Society for Chemical Engineering