Cenospheres are hollow alumina silicate microshells found in fly ash. They are positively buoyant in water, thus allowing gravity-separation to be used to separate these low density particles from negatively buoyant fly ash particles. However, very low feed grade, of order 1%, and small particle diameter, typically less than 150 μmm, make achieving a reasonable throughput per unit vessel area difficult for conventional gravity-separation systems. In this study an Inverted Reflux Classifier (IRC) was used for the first time to recover and concentrate cenospheres from a real fly ash feed obtained from a coal fired power station. This novel, single stage, device consists of a system of parallel inclined channels, located beneath a partially enclosed vertical fluidizing chamber. The channels enhance the segregation rate of the cenospheres from the fly ash, permitting a greater downwards fluidization to aid the desliming of fine fly ash particles from the overflow product. The device was fed at a solids
flux of about 2.6 t/m2h. A product grade of 75% was achieved from a feed with a grade of only 0.53%, corresponding to an upgrade of 142. Here, the recovery of the cenospheres was 41%. By increasing the overflow product rate, a significantly higher recovery of 64% was achieved, but at a much lower upgrade of 33. In both cases most of the losses were attributed to fine cenosphere particles, less than 50 μmm, being entrained to the underflow.