Sintering of Metakaolin-based Na-Pollucite ceramics and their immobilization of Cs

Abstract

Metakaolin-based Na-Pollucite ceramics (high-temperature sintered Na-pollucite ceramics: HTNP), with a target composition close to Pollucite (CsAlSi2O6), were fabricated at sintering temperatures above 1000 °C to immobilize Cs as waste form material. The sintering behaviors of Metakaolin-based Na-Pollucite ceramics were investigated, in particular with respect to sintering temperature, grain size, and microstructure. The results indicate that major phases of the pollucite ceramics consist of pollucite phase and a small fraction of other mineral phases like Albite, SiO2, and Al2O3, and the microstructures greatly depends on the molar Cs/Na ratio. Furthermore, with increasing Cs content in the pollucite ceramics, the sintering temperature increases gradually, and the pollucite grain size decreases accordingly. On the other hand, to avoid the high-temperature volatilization of Cs-ions during sintering, low-temperature sintered Na-pollucite (LTNP) ceramics were prepared below 950 °C by introducing H3BO3 into the metakaolin-based Na-Pollucite ceramics. After the LTNP ceramics were sintered using a low-temperature liquid-phase process, even when the sintering temperature was decreased to 650 °C, they remained structurally as pollucite phase. In addition, the sintering-temperature and grain-size dependency on the molar Cs/Na ratio in LTNP ceramics is consistent with that in HTNP ceramics. Leaching in the PCT (Product Consistency Test) indicates that the leaching concentrations of HTNP ceramics are, in total, lower than in LTNP ceramics. However, the LTNP and HTNP ceramics samples, with the molar Cs/Na ratios 1:1.5, 1:0.67, and 1:0.25 show better immobilization abilities for the Cs ion. This indicates that, besides the pollucite phase formation, the immobilization of the Cs ion correlates also with dense microstructures, grain size, and good crystallization. More importantly, low sintering temperatures and the fabrication of Cs crystalline ceramics are the important factors to improve the Cs ion immobilization.