Synthesis of monoclinic Celsian from coal fly ash by using a one-step solid-state reaction

Monoclinic (Celsian) and hexagonal (Hexacelsian) Ba1−xSrxAl2Si2O8 solid solutions, where x = 0, 0.25, 0.375, 0.5, 0.75 or 1, were synthesized by using Coal Fly Ash (CFA) as main raw material, employing a simple one-step solid-state reaction process involving thermal treatment for 5 h at 850–1300 °C. Fully monoclinic Celsian was obtained at 1200 °C/5 h, for SrO contents of 0.25 ≤ x ≤ 0.75. However, an optimum SrO level of 0.25 ≤ x ≤ 0.375 was recommended for the stabilization of Celsian. These synthesis conditions represent a significant improvement over the higher temperatures, longer times and/or multi-step processes needed to obtain fully monoclinic Celsian, when other raw materials are used for this purpose, according to previous literature. These results were attributed to the role of the chemical and phase constitution of CFA as well as to a likely mineralizing effect of CaO and TiO2 present in it, which enhanced the Hexacelsian to Celsian conversion

Incorporation and phase separation of Cl in alkaline earth aluminosilicate glasses

Pyrochemical reprocessing of spent nuclear fuels may lead to the generation of chloride containing wastes. 36Cl wastes may also arise from the treatment of irradiated graphite. Such wastes will have limited solubility in the borosilicates currently used for waste vitrification. Despite requiring higher processing temperatures aluminosilicate glasses show promise for this application. In a series of alkaline earth aluminosilicate glasses we demonstrate that chloride solubility is related to the alkaline earth species as follows Sr > Sr+Ba > Ba > Ca > Mg, with the strontium aluminosilicate glass accommodating up to 5.92 at% Cl. Typical chloride retention rates are ~80% of the batched chloride content at 1400ºC. It has also been observed that, when Cl is present in the glass in excess, phase separation firstly occurs as formation of non-Cl crystals (mainly alkaline earth aluminosilicates, with a minority of aluminates); a segregated chloride layer is only formed at higher chlorine loadings. This indicates that chlorine solubility in glass is not only controlled by the capacity of glass network to accommodate Cl– but also by the stability of glass network after Cl– incorporation. In addition, increased incorporation of Cl– in glass results in steadily decreased glass densities and glass transition temperatures