This article presents a computational investigation of the dilute gas–solid flow's loss coefficient and reattachment length in a horizontally placed backward-facing step. The article's primary objective is to study the effect of the interparticle collision and three significant parameters, namely, inlet solid-phase volume fraction , particle size (dp = 70 μm and 200 μm), and solid-particle density (ρs = 1500 kg/m3 and 3000 kg/m3). The numerical procedure uses the Eulerian–Eulerian approach in conjunction with the Kinetic theory of Granular flow and the four-way coupling approach. The outcomes show that the loss coefficient and the reattachment length increase as the interparticle collision intensifies with the system. Moreover, these parameters show higher values with small particle sizes and higher solid-phase volume fractions. The reattachment length is higher with less dense solid particles and higher with heavier solid particles.