Effects of suspension dispersity and concentration on its flocculation efficiency in static and dynamic flocculators

Abstract

Flocculation presents one of the most effective methods for enhancing separation of both anthropogenic and natural suspensions by sedimentation, filtration and flotation techniques.The flocculation effectiveness much depends on the medium shear rate in a flocculator. The objective of this research comprises the study how the suspension dispersity and concentration effect the efficiency of its flocculation in a static tubular flocculator and in a dynamic Couette flocculator. The studies used aqueous suspensions of ultra-fine calcium carbonate (<7 μm) and fine silica (<90 μm) as objects. It was established that treatment of ultra-fine calcium carbonate suspension in a static flocculator produced in the range 400-450 s-1 a pronounced primary maximum in the dependence "flocculation efficiency/shear rate". The increase of the suspension concentration to 70 g/l and above resulted in a small secondary maximum of the flocculation efficiency in the region of around 950 s-1. This can be due to a higher dissolution rate of flocculant and a corresponding increase of particles adhesion forces in flocs, which counteract viscous forces destroying them. In silicon dioxide suspension treatment, the primary and secondary peaks occur at both small and high suspension concentrations, but in a latter case, they are by far more pronounced and comparable in magnitude.