Comparative Study on Extracting Alumina from Circulating Fluidized-Bed and Pulverized-Coal Fly Ashes through Salt Activation

Alumina was extracted from mixtures of circulating fluidized-bed fly ash (CFBFA)/pulverized-coal fly ash (PCFA) and sodium pyrosulfate (Na2S2O7) through salt-roasting activation. The effects of the sodium pyrosulfate amount, calcining temperature, and calcining time on the efficiency of extracting alumina from CFBFA(-) and PCFA were analyzed and compared. The results showed that adding Na2S2O7 to CFBFA and PCFA released alumina in the form of Na3Al(SO4)(3). Up to 93.3% of the alumina was extracted from the PCFA calcined at 420 degrees C for 2 h when n(Al2O3)/n(Na2S2O7) = 1:3; however, 92.8% of the alumina was extracted from the CFBFA calcined under the same conditions when n(Al2O3)/n(Na2S2O7) = 1:5, meaning that the CFBFA consumed more sodium pyrosulfate than the PCFA because the Al-O-Si bonds in the CFBFA were more active than those in the PCFA. A new amorphous aluminosilicate phase formed while the CFBFA was calcining with sodium pyrosulfate. Meanwhile, Na2Ca(SO2)(2) and CaAl2O4 formed at the beginning of the reaction, which prevented the alumina from dissolving and provided resistance against mass transfer.