Removal of sodium (Na2O) from alumina extracted coal fly ash by a mild hydrothermal process

• The sodium content in the alumina extracted CFA was decreased from 20 wt.% to 0.6 wt.%.
• The mechanism of sodium removal from the alumina-extracted slag was investigated.
• Sodium in residue is mainly combined with silanol group and can be washed by water.

Sodium was removed from alumina-extracted coal fly ash (CFA) by using diluted NaOH solution. The effects of reaction temperature, reaction time, and NaOH concentration on sodium removal were studied. Result shows that the sodium (Na2O) content of the alumina-extracted CFA was decreased from 20 wt.% to 0.6 wt.% under optimal conditions: temperature of 185 °C, NaOH concentration of 50 g/L, residence time of 2 h, and liquid-to-solid ratio of 40 for water washing. The possible mechanism of sodium removal was also investigated using Fourier transform infrared spectroscopy, X-ray diffraction analysis, and nuclear magnetic resonance spectroscopy. The results indicated that NaCaHSiO4 in the residue was decomposed by NaOH to form NaSiO3 and Ca(OH)2, and then converted into tobermorite [Ca5Si6O16(OH)2·nH2O] and NaOH. The sodium obtained through the reaction in the residue was mainly combined with the silanol group and then washed with water.