Upgrading of positively buoyant particles using an inverted reflux classifier

This paper is concerned with the separation of cenosphere particles from fly ash. Cenospheres are hollow alumina silicate micro-shells found in fly ash. They are positively buoyant in water, thus allowing gravity-separation to be used to achieve separation from negatively buoyant fly ash particles. In this study an Inverted Reflux Classifier, a combination of parallel inclined channels and a vertical fluidized bed, was used for the first time to recover and concentrate cenospheres from a real fly ash feed obtained from a coal fired power station. The effects of different operating parameters such as the feed rate, product rate, and fluidization rate were investigated. The device was fed at a solids flux of about 2600 kg/(m² h). A product grade of 76% was achieved from a feed with a grade of only 0.51%, corresponding to an upgrade of 149. Here, the recovery of the cenospheres was 42%. By increasing the overflow product rate, a significantly higher recovery of 64% was achieved, but at a reduced upgrade of 33. In both cases most of the losses were attributed to the relatively fine cenosphere particles being entrained to the underflow.