How to elutriate particles according to their density

It is well known that particles settle at a velocity dependent on both their size and density. This dependence makes it difficult to efficiently concentrate valuable minerals of a given density, unless the density range between the valuable and gangue particles is sufficient to overcome the segregation arising from the particle size range. Here we reveal a method for suppressing the effects of particle size so that particles can be routinely elutriated on the basis of their density. The particles are suspended by fluidizing with water, and then conveyed into channels inclined at 70° to the horizontal, closely spaced with a gap of 1.77 mm. Through the application of these conditions, a fortuitous coincidence arises. Firstly, a high shear rate is produced, sufficient to achieve inertial lift and hence promote the transport of the particles along the inclined surfaces, free of significant lubrication and mechanical friction forces. Secondly, the flow through the channels is laminar, with an almost linear variation in the local velocity with distance from the inclined surface. Hence a given particle resident near an inclined surface experiences a local elutriation velocity proportional to its diameter. These conditions suppress particle size segregation thus allowing the imposed hydraulic velocity to convey particles on the basis of their density.