Abstract

Screening is the practice of separating granular materials into multiple-size fractions using an aperture for selection and is employed in most mineral processing plants. Currently, the design and scale-up of screens rely on rules of thumb and empirical methods. In previous publications, we outlined details of a mathematical model of screening developed using Discrete Element Methods (DEM). Our investigation was based on the need for fundamental research into granular dynamics in mineral processing, particularly on an inclined vibrating screen that goes beyond the current state of the art in screen modelling. We established that granular flow on vibrating screens exhibits complex phenomena such as segregation, percolation, and flow of oversize material over the separating medium. This work demonstrates a unique granular rheology for particles moving on a vibrating screen. DEM provided critical data (velocity, volume concentration, shear rate, bed depth) for developing, testing, and calibrating the granular flow models. First, a binary mixture of glass beads flowing on an inclined vibrating screen was simulated, and the subsequent continuum analysis of the flowing layer revealed a coexistence of three flow regimes- (i) quasi-static, (ii) dense (liquid-like), and (iii) inertial. The regimes are consistent with the measured solids concentrations spanning these regimes on inclined vibrating screens. The quasi-static regime is dominated by frictional stress and corresponds to a low inertial number (I). Beyond the quasi-static regime, the frictional stress chains break, and the collisional-kinetic and turbulent stresses begin to dominate. The appropriate constitutive shear stresses were then used to derive a new rheology that captures all phases of the flow transition points observed in the simulation. The model presents a fundamental understanding of the mechanisms governing the transport of granular matter on an inclined vibrating screen.

Keywords:Screens; DEM; Granular flow; Rheology