In a liquid fluidised bed system, the motion of each phase is governed by fluid-particle and particle-particle interactions. The particle-particle collisions can significantly affect the motion of individual particles and hence the solid-liquid two phase flow characteristics. In the current work, computational fluid dynamics-discrete element method (CFD-DEM) simulations of a dense foreign particle introduced in a monodispersed solid-liquid fluidised bed (SLFB) have been carried out. The fluidisation hydrodynamics of SLFB, settling behaviour of the foreign particle, fluid-particle interactions, and particle-particle collision behaviour have been investigated. Experiments including particle classification velocity measurements and fluid turbulence characterisation by particle image velocimetry (PIV) were conducted for the validation of prediction results. Compared to those predicted by empirical correlations, the particle classification velocity predicted by CFD-DEM provided the best agreement with the experimental data (less than 10% deviation). The particle collision frequency increased monotonically with the solid fraction. The dimensionless collision frequency obtained by CFD-DEM excellently fit the data line predicted by the kinetic theory for granular flow (KTGF). The particle collision frequency increased with the particle size ratio (d(p2)/d(p1)) and became independent of the foreign particle size for high solid fractions when the fluidised particle size was kept constant. The magnitude of collision force was 10-50 times greater than that of gravitational force and maximally 9 times greater than that of drag force. A correlation describing the collision force as a function of bed voidage was developed for St(p) > 65 and d(p2)/d(p1) <= 2. A maximum deviation of less than 20% was obtained when the correlation was used for the prediction of particle collision force. (C) 2014 Elsevier Ltd. All rights reserved.