Based on the multivariate population balance equation (PBE) and the primary secondary particle concept a mathematical model is developed for liquid extraction columns. It is extended to include the momentum balance for the dispersed phase. The resulting model is complicated by the integral source term of the PBE. To reduce this complexity, while maintaining most of the information from the continuous PBE, the concept of the primary secondary particle method is used. The effect of the number of primary particles (PP) on the final predicted solution is investigated. Numerical results show that the solution converge fast as the number of PP is increased. The terminal droplet velocity is found to be the most sensitive model parameter to the number of PP. The predicted steady state profiles (droplet diameter, holdup and the concentration profiles) along a pilot RDC extraction column are compared to the experimental data where good agreement is achieved. (C) 2011 Elsevier Ltd. All rights reserved.