The effect of the particle size distribution (PSD) coupled with shrinking or swelling of the grains is investigated numerically by modelling the kinetic curves in batch runs and the fluid and solid phase concentration profiles in a polydispersal fixed bed. Experimental data for PSD of the strong base anionite Vofatit SBK are used, submitted to shrinking during the ion-exchange of AuCN- from waste waters. The sensibility of the obtained liquid acid solid phases concentrations to the PSD including particles size evolution is shown together with the calculated errors, due to deviation of the samples taken for analysis along the bed from the representative size distribution.