Improving the A/S ratio of pretreated coal fly ash by a two-stage roasting for Bayer alumina production

Coal fly ash has been considered as a potential resource for alumina production by Bayer process. However, the implementation is always limited by low alumina-silica mass ratio (A/S) of coal fly ash. Previously, we reported an approach to realize the desilication of coal fly ash and the A/S could reach 5.34. However, this A/S was still not high enough for Bayer process. Following the previous study, herein we systematically optimize the desilication conditions to further improve the A/S, and also evaluate the applicability of desilication residue for Bayer alumina production. First, the digestion properties of amorphous silicon dioxide and γ-alumina in the two-stage roasted calcines are analyzed by thermodynamic calculation. The results indicate that their alkali-digestion properties become remarkably different at relatively high temperature, thus amorphous silicon dioxide could be digested while γ-alumina is preserved in the residue by controlling the pH and digestion temperature. The desilication ratio and the A/S ratio could reach 82.27% and 7.46 at the optimal conditions of 116 g/L Na2O, 95 °C, 15 min digestion time and 1/10 solid-to-liquid ratio. The obtained desilication residue becomes acceptable and economical high grade raw material for Bayer process, which is confirmed by the production of standard alumina (AO-2, China). This research could provide an alternative economic process for recycling alumina from high-alumina coal fly ash.