Digestion reactions of paper sludge combustion ash in strong alkaline solutions at 60 °C

Alkaline reactions of paper sludge combustion ash at low temperature (60 °C) were performed using a calcite-rich paper ash (PA 1) and a gehlenite-rich ash (PA 2). Strong alkaline conditions (8, 12, 16 M NaOH) were revealed at reaction times of 1–4 h and 12–24 h. Reactions were performed with pure ashes and in the presence of NaAlO₂. The products were characterized by XRD, FTIR, SEM/EDX, gravimetry and chemical analysis. The conversion was found to proceed mainly in the period between 1 and 4 h. Portlandite and hydrogarnet were observed from PA 1 in 8 M NaOH. Onset of formation of Ca₄Al₂O₆CO ₃ ^. 11H₂O beside Ca(OH)₂ could be analyzed after reaction of PA 1 in 12 M and 16 M NaOH. Addition of NaAlO₂ favored crystallization of hydrogarnet and Ca₄Al₂O₆CO ₃ ^. 11H₂O. For PA 2 gehlenite remained stable, but a high portlandite fraction was observed. Addition of NaAlO₂ yielded hydrogarnet beside gehlenite in 8 M NaOH. Higher alkalinities favored crystallization of Ca₄Al₂O₆CO ₃ ^. 11H₂O and onset of dissolution of gehlenite. Finally transformation of Ca₄Al₂O₆CO ₃ ^. 11H₂O into sodium aluminum silicate hydrate was observed. All results were discussed with regard to heavy metal distribution of the initial PA between the alkaline digestion solution and the products. In conclusion suitable applications of the products were proposed.