Separation of unburned carbon from coal fly ash: Pre-classification in liquid–solid fluidized beds and subsequent flotation

Abstract

Coal fly ash, which is the solid waste produced during the coal combustion and utilization, causes serious environmental problems. The preparation of concrete from coal fly ash has been proven helpful in potentially improving its value and reducing its environmental impact to a certain extent. However, the presence of unburned carbon components affects the performance of coal fly ash-based concrete. In this study, unburned carbon is separated from coal fly ash by pre-classification and subsequent optimized flotation, wherein the factors influencing are explored. The results show that under the pre-classification function of the inflatable-inclined liquid–solid fluidized bed, the underflow product is enriched in unburned carbon particles, with loss-on-ignition (LOI) and yield (Yo) values of 11.58 % and 72.87 %, respectively. Further, the underflow product is flotated for a cleaner carbon product, which is affected by the stirring rate (W) and the amount of collecting agent (C) and foaming agent (Fo). Optimized flotation (i.e., C = 18 kg/t, Fo = 6.5 kg/t, W = 1600 rad/min) results in the maximum combustible recovery rate of the flotation concentrate (89.89%), with LOI of 41.80 % and Yo of 18.51 %, yielding unburned carbon products for fuel. After mixing the flotation tailings with the pre-classified overflow production, the LOI of the formed ash-rich product is 1.73% with Yo of 81.49%, prepared into concrete. Thus, the proposed pre-classification and flotation process can improve the quality of coal fly ash. The results provide a theoretical basis for the harmless and resource treatment of coal fly ash.