Co-utilisation of alkaline solid waste and compressed-or-supercritical CO 2 to produce calcite and calcite/Se 0 red nanocomposite

Abstract

The coal combustion fly-ash and alkaline paper mill waste were previously used to sequester CO 2 via waste-water–CO 2 interactions. For this case, a solid mixture (calcite and un-reacted waste) was obtained after carbonation process. In the present study, we propose a solid–water separation of free lime (CaO) or free portlandite (Ca(OH) 2) contained in waste prior to carbonation experiments in order to produce pure calcite or calcite/Se 0 red composite. The calcite and carbonate composite syntheses have been also independently studied, but for both cases, a commercial powdered portlandite was used as calcium source. For this study, the extracted alkaline-solution (pH = 12.2–12.4 and Ca concentration = 810–870 mg/L) from alkaline solid waste was placed in contact with compressed or supercritical CO 2 at moderate or high temperature, leading a preferential nucleation-growth of submicrometric particles of calcite (<1 μm) with rhombohedral morphology at 90 °C and 90 bar (9 MPa), whereas a preferential nucleation-growth of nanometric particles of calcite (<0.2 μm) with scalenohedral morphology at 30 °C and 20 bar (2 MPa) were observed. When, the extracted alkaline-solution was placed in contact with supercritical CO 2 (90 bar) at high temperature (90 °C) and in presence of unstable seleno- l-cystine compound, the nucleation-growth of calcite/Se 0 red nano-composite taken place. The composite consisted predominantly of spherical, amorphous nanometric-to-submicrometric of elemental red selenium (<500 nm) deposited on the calcite matrix. Here, the calcite was constituted by nano- to microrhombohedral crystals (<2 μm) and micrometric agglomerates and/or aggregates (<5 μm). These results on the particle size and morphology of crystal faces are very similar to calcite produced using commercial powdered portlandite as alkaline reactant and calcium source. This study is a nice example of feasibility to obtain possible ecological and economical benefits from waste co-utilisation.