Sedimentation of particle suspensions is a relevant process to a variety of applications, such as the reclamation of land and chemical engineering systems. Direct numerical simulations of the settling of a single, two and multiple solid spheres under gravity in a square duct have been performed. The simulations involve 4 different Galileo numbers, i.e. Ga=144, 178, 200 and 250 for a fixed solid-to-fluid-density ratio Γ=1.5 and solid-volume fractions ranging from Φ=0.005 to Φ=0.3. It is shown that for these Ga the particle motions of a single particle are steady vertical, steady oblique, oblique oscillating and chaotic. In the simulations for a single particle it has been found that the observed fluctuations of the vertical particle velocity are due to particle-wall interactions. Drafting-kissing-tumbling (DKT), which is a key mechanism in the formation of particle clusters, has been observed in the two particles simulations and show that DKT initiates when the particles are almost aligned vertically and move in the same direction in the horizontal plane. DKT could occur even when the particles are several particle diameters apart.In the simulations of multiple particles only the steady vertical, steady oblique and oblique oscillating regimes have been considered and the particles are initially randomly distributed in the computational domain. It is shown for these regimes that for solid-volume fractions Φ≤0.01 an increase of the settling velocity of the suspensions compared to the value of a single particle is observed for all regimes, while for higher solid-volume fractions a hindered settling effect occurs. The statistics of a spatial parameter related to the angle of a particle and its nearest neighbour show that the particles align vertically more than a set of random distributed particles for solid-volume fractions Φ≤0.01. This indicates the formation of particle clusters. These findings are supported by multiple instances of DKT observed in the animations of the simulations. Finally, it has been observed for all regimes that the vertical particle velocity fluctuations relative to the settling velocity of the suspension increase roughly with Φ^(1/3).