Direct Numerical Simulation of collision events in flotation under the influence of gravity

Collisions between particles and bubbles are decisive for the performance of flotation processes. In this work Direct Numerical Simulations of a prototypical gravitation-driven flotation process are presented with bubbles fully resolved and modelled as rigid spheres while the solid-phase is represented as point-particles. Key bubble and particle parameters correspond to realistic setups and are varied to study their effect on the collision rate. The study addresses the main influencing parameters, such as bubble diameter, gas hold-up, particle diameter, and particle density. Locally around the bubble significant differences of the collision behaviour exist. Most collisions occur on the upper bubble half, but some also on the lower bubble half. This is caused by a high particle-bubble relative velocity towards the bubble from the upper bubble half and an accumulation of particles at these locations as they deviate around the bubble. The paper provides reference data for flotation modelling. • First DNS of three-phase multi-bubble, multi-particle flotation system with resolved bubbles • Collision frequency varies greatly around the bubble circumference. • Significant number of collisions on lower half of bubble. • Local variations of collisions influenced by particle flux and particle accumulation. • Current collision models provide sub-optimal predictions of collision kernel for cases investigated.