A novel perspective process for alumina extraction from coal fly ash via potassium pyrosulfate calcination activation method

Abstract

Alumina extraction is considered as one of the promising trends for the value-added use of coal fly ash (CFA). In this study, a moderate potassium pyrosulfate (K2S2O7) calcination activation method was used to extract alumina from CFA. The calcined conditions including X1: K2S2O7/Al2O3 molar ratio (2–4), X2: calcining temperature (180–220 °C), X3: calcining time (2–4 h) was optimized by using response surface method based on 3-factor 3-degree Box-Behnken design. The effects of confirmed factors were fitted in a quadratic regression polynomial equation. The main and interacting influences between variables were studied by using the analysis of variance and 3D plots, respectively. The results show the alumina extraction rate was more influenced by K2S2O7/Al2O3 molar ratio. The optimized conditions were identified as K2S2O7/Al2O3 as molar ratio 3.55:1, calcining temperature up to 212 °C and calcining time of 3.1 h, in which the alumina extraction rate was recorded as 93.28%. The characteristics of CFA and leaching residues were studied by using XRD, SEM and 27Al and 29Si MAS-NMR. Results show that K2S2O7 removes the aluminum and effectively breaks the high polymerization units of Si–O–Al and Al–O. The mineral transformation during calcined reaction follows as K2S2O7 + CFA (mullite, corundum, and amorphous phase) → aluminum tri-potassium sulfate → leached residues rich in silicate + leached metal ions. The surface chemical reactions controlled the calcining process. The apparent rate constant was found to be 3.12 to 4.18 × 10−3 min−1 at the temperature range of 180–220 °C, respectively. The apparent activation energy was refereed to be 13.579 kJ/mol. These findings indicate that the potassium pyrosulfate calcination activation method gets a high level of the extracting rate, and is industrially applicable to extract alumina.