Partitioning of rare earths in multiphase nuclear waste glass‐ceramics

Abstract

Alumino‐borosilicate glass‐ceramics (GCs) containing alkali (Na), alkaline earth (Ca), rare earth (RE) (one of La … Lu, Y), and transition metals (Zr and Mo) were formulated to test the effects of composition and RE cation size on the crystallization of simulated nuclear waste GCs. Eight‐oxide glasses were formulated in the peraluminous ([Na2Oexcess]=[Na2O]‐[Al2O3]‐[ZrO2]<0) region with 10 mol.% RE2O3 and 15 mol.% B2O3. Glasses were melted and quenched, reheated, and slow‐cooled to promote crystallization, and then characterized using quantitative X‐ray diffraction and electron probe microanalysis. Transition metal phases [powellite (Ca,Na,RE)MoO4 and zirconia (Zr,RE)O2 or zircon (Zr,RE)SiO4] always formed, while borosilicate (RE3BSi2O10) or silicate [oxyapatite (RE,Ca,Na)10Si6O26 or keiviite (RE2Si2O7)] plus borate (REBO3) phases also formed, depending on the size of the RE cation and hence its preferred coordination number. All crystalline phases were found to contain RE elements, and often phases incorporated other cations (Zr, Ca, B) which were not nominally part of their pure stoichiometry. These results suggested that the glass formulation and RE size strongly influenced the RE environment in the glass and hence the crystal phases formed. The formation of RE borosilicate, versus separate RE borate and RE silicate phases, also suggested original differences in the glass structure.

Glasses and Glass-Ceramics for Nuclear Waste Immobilization

“…Consequently, in spite of its high Fs, Zr 4+ is usually rather well incorporated in the network of nuclear borosilicate glasses if there is enough charge compensators available. Nevertheless, the crystallization of Zr-rich phases such as ZrO2 and ZrSiO4 can occur for particular borosilicate glass compositions, but these phases are well-known for their high chemical durability (Chen H., Marcial J., Ahmadzadeh M., Patil D. and McCloy J. 2020).…”

“…Nevertheless, for high waste loading the crystallization of Ln-rich phases may be observed during melt cooling such as Ca2Ln8(SiO4)6O2 (apatite) (Fig. 7), and Ln borates and borosilicates depending on glass composition (Quintas A., Caurant D., Majérus O., Dussossoy J-L. andCharpentier T. 2008, Chen H., Marcial J., Ahmadzadeh M., Patil D. andMcCloy J. 2020).…”

“…Moreover, the size of Ln 3+ cations may strongly impact the nature of RE-rich crystals and glass crystallization tendency (Loiseau P., Caurant D., Baffier N., Mazerolles L. andFillet C. 2004, Quintas A., Caurant D., Majérus O., Dussossoy J-L. andCharpentier T. 2008, Kidari A., Dussossoy J-L., Brackx E., Caurant D., Magnin M. et al 2012, Chen H., Marcial J., Ahmadzadeh M., Patil D. and McCloy J. 2020).…”

Vitrification of wastes: from unwanted to controlled crystallization, a review

“…These oxyapatites have been shown by electron microprobe to accommodate significant amounts of ZrO 2 , B 2 O 3 , and Na 2 O from the glass [McCloy et al, 2019b. There are related natural apatite family minerals which are primarily silicate (britholite) or which contain both silicate and borate in the structure (tritomite, caryocerite) [Chen et al, 2020]. Other crystalline phases can nucleate oxyapatite from these glasses, including powellite as discussed above [Chouard et al, 2016], as well as ruthenium dioxide [Kamat et al, 2020].…”

“…The role of Nd is specifically explained, in sodium aluminoborosilicate enriched in MoO 3 and Nd 2 O 3 , by the dispersion of MoO 2− 4 units in the borate network stabilized by Nd 3+ , (Nd-Mo-B-O) [Brehault et al, 2018]. Without Mo present, increasing amounts of Nd result in clustering in a borate phase that separates from silicate in borosilicate glasses [Kamat et al, 2021], an effect which can be seen in crystallization behavior as well [Chen et al, 2020]. The formation of alkalineearth molybdate phases like powellite is discussed later in this review in the context of GCs.…”