Desilication of high-silica Indian coal fly ash by alkali leaching with KOH and NaOH: A comparative study

Indian coal fly ash generally comprises 50-60% SiO2, and 25-35% Al2O3 can potentially replace bauxite as an aluminum resource. Aluminum recovery from coal fly ash is highly dependent on removing amorphous silica, activating the inactive mullite phase, and increasing the Al/Si ratio. This work thoroughly investigated and compared the alkali-desilication operation by two widely used alkalis, KOH and NaOH, based on four operational parameters: liquid-to-solid ratio, KOH or NaOH concentration, leaching time and temperature. Using SEM, EDX, and XRD, the morphology and phase of fly ash and solid byproducts were analyzed. NaOH was more effective than KOH in SiO2 dissolution at all operational parameters. A 20 wt% concentration, 5:1 liquid-to-solid ratio, 2hrs leaching time, and 100 degrees C temperature were the optimum removal conditions for silica from fly ash using NaOH. With KOH, the ideal desilication conditions were obtained at 28 wt% concentration, 3hrs leaching time, and 150 degrees C temperature. The highest Al/Si ratios were obtained under the same optimal desilication conditions. Using KOH and NaOH, the Al/Si ratio was enhanced from 0.58 to 1.26 and 1.45, respectively. The main zeolitic products obtained were Hydroxysodalite in NaOH-treated fly ash and Linde F zeolite and Kalsilite in KOH-treated fly ash.