Multistage Concentration of Cenospheres in Fly Ash Using the Inverted Reflux Classifier

Cenospheres are one of the most valuable components found in the fly ash waste of coal-fired power stations, consisting of hollow, aluminosilicate spherical shells and offering very low bulk density, high strength, and high thermal resistance. Although wet processing methods offer the best prospect for achieving recovery and concentration of the cenospheres, significant impediments remain for achieving satisfactory economics. This work was concerned with the recovery and concentration of cenospheres from fly ash using a novel system, the inverted reflux classifier (IRC), covering single and multistage processing. The system consists of a series of parallel inclined channels mounted underneath an inverted liquid fluidized bed. This work demonstrated a high separation performance due to the so-called Boycott effect, which arises in inclined settling. In preliminary experiments, a fly ash feed with a very low cenosphere concentration was examined. After two stages, the product grade and total recovery were unsatisfactory. However, in the main part of the study, examining a fly ash feed with about 0.9% cenosphere concentration, a three-stage IRC was used to achieve almost pure cenosphere product, with the cenosphere grade increasing to around 97 wt% (almost pure on a volume basis). In the second stage, which involved a much more dilute feed, the more neutrally buoyant cenospheres remained in the stream to tailings. Hence, with three stages, the recovery fell to 50%. It is concluded that accelerated separation of the positively buoyant cenospheres is promoted by the presence of a high concentration of the negatively buoyant fly ash, and therefore a single separation stage is preferable.