Optimization of the nucleation of CaMO4 powellite in molybdenum-rich aluminoborosilicate glass

Abstract

The In order to protect the humans and environment from radioactive materials dissemination during radioactive waste storage, ceramics materials offer a safe and secure solution for selected isotopes, as molybdenum. Aluminoborosilicates glass-ceramics are suitable materials for this purpose. In this study, a ceramic for Mo confinement was synthesized in Al₂O₃-Na₂O-B₂O₃-SiO₂-MoO₃alumino-borosilicate system. The material is synthesized by double melting at 1380°C for 3 h, followed by a nucleation stage at 530°C for 2 h, and a crystallization stage at 615°C. The crystallization duration is a crucial factor for the good sequestration of Mo, issued from the radioactive waste, and thus the crystallization time should be optimized. This parameter was optimized in the range from 2 to 72 h, with the aim of maximizing the occurrence of CaMoO₄powellite phase in the glass, and thus ensuring a suitable double confinement of Mo in the crystalline phase embedded in the glass. The phases’ identification performed by X-ray diffraction (XRD) analysis shows that up to 24 h of crystallization, powellite is the main formed phase in the glass, with more than 65%. Beyond 24 h, the powellite re-dissolves in the glass with a small amount crystallized in the form of phosphate. The scanning electron microscope (SEM) analysis made it possible to observe the formation of the powellite crystalline phase. Fourier transform infrared spectroscopy (FTIR)analysis of the materials shows the similarity of chemical composition between the different glass-ceramics obtained for the whole of crystallization times. The main vibrations evidenced by FTIR are those of Si-O-Si and B-O-B bonds of SiO₄ and BO₄ structural units, respectively; and O-Mo-O of MoO₄ tetrahedrons. Raman spectroscopy analysis highlights the formation of powellite crystals in the glass, by the internal vibrations ʋ1(2A1) and ʋ2(2E) of MoO₄in CaMoO₄.