Material conversion from paper-sludge ash in NaOH, KOH, and LiOH solutions

Abstract

Zeolites were synthesized from paper-sludge ash in different alkali solutions. The ash used in this study has a high-Ca content and a low abundance of Si, in part due to the presence of calcite that is used as a paper filler. The major minerals present in the ash are gehlenite (Ca₂Al₂SiO₇) and anorthite (CaAl₂Si₂O₈). Three kinds of alkali solutions (NaOH, KOH, and LiOH) at four different concentrations (1, 2, 3, and 4 M) were reacted with paper-sludge ash at 90 °C for 24 h. Powder X-ray diffraction showed that hydroxysodalite (Na₆Al₆Si₆O₂₄·8H₂O) and zeolite Linde F (KAlSiO₄·1.5H₂O) had formed in NaOH and KOH solutions, respectively, and that anorthite had dissolved in these alkaline solutions, whereas gehlenite had remained unaffected. In the LiOH solution, both anorthite and gehlenite dissolved, and various minerals, including Li-ABW zeolite (Li₄Al₆Si₆O₁₆·4H₂O), hydrocalumite [Ca₂Al(OH)₆(Cl, OH)·3H₂O], tobermorite [Ca₅Si₆O₁₆(OH)₂·4H₂O], katoite [Ca₃Al₂(SiO₄)(OH)₈], and portlandite [Ca(OH)₂] formed. The products of mineral synthesis from paper-sludge ash by reaction in alkaline solutions strongly depend on the specific alkali present.