Observation of nepheline crystallization in the Na₂O-B₂O₃-Al₂O₃-SiO₂ system for the vitrification of zeolites absorbing Cs

ABSTRACT

For the vitrification of zeolites used for absorbing radioactive cesium, the composition of the glass in which nepheline forms at the cooling rates of 0.5, 1, 2.5°C min⁻¹ were clarified for the Na₂O-B₂O₃-Al₂O₃-SiO₂ quaternary system to avoid crystallization during cooling of the melt. The trend of the nepheline formation composition was similar to that suggested in previous studies, i.e. the nepheline discriminator (N.D.) < 0.62 and the optical basicity (O.B.) >0.575, especially at the higher B₂O₃ content, while the criteria was eased at a lower B₂O₃ content at the cooling rate of 2.5℃ min⁻¹. The increase in the B₂O₃ content of the glasses seemed to reduce the effect of the cooling rate on the crystallization. The results obtained by adding 1 wt% Cs₂O in the quaternary system shows that the effect of Cs₂O would be negligible as long as its content in the formed glass would not exceed 1 wt%. In contrast to the quaternary system, the Cs₂O-Na₂O-Al₂O₃-SiO₂ system showed the different formation of the crystals. The fine granulous crystals were dispersed in the glass matrix and not in the region contacting the crucible as observed in the Na₂O-B₂O₃-Al₂O₃-SiO₂ system. Moreover, the number of non-crystal samples was greater than that predicted by the criteria, N.D. <0.62 and O.B. >0.575.

KEYWORDS:

• Nepheline
• zeolite
• aluminosilicate glass
• vitrification
• cooling rate
• crystal formation composition
• radioactive waste

Acknowledgments

The authors wish to thank Prof. H. Yoshida, The Nagoya University Museum, for the helpful advice on this study from the viewpoint of mineralogy, and Ms. N. Takagi, Technical Center of Nagoya University, for the kind instructions regarding the preparation of the sliced glass samples.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Chubu Electric Power Company under the [Research grant, 2019–2020].