Phase composition and elemental partitioning in glass–ceramics containing high-Na/Al high level waste

Mixtures of surrogates of high level waste with high sodium and aluminum contents and sodium–lithium borosilicate frit were melted in alumina crucibles in a resistive furnace followed by quenching of one portion of the melt and annealing of the residual material in a turned-off furnace. The annealed materials with waste loading of up to 45 wt.% contained minor spinel type phase and trace of nepheline (Na,K)AlSiO4. In the annealed materials contained waste oxides in amount of 50 wt.% and more nepheline and spinel were found to be major and minor phases, respectively. At high waste loadings two extra phases: Cs-aluminosilicate (CsAlSiO4) and mixed Na/Cs-aluminosilicate were found in amount of 3–5 vol.% each. The latter phase contains of up to ∼5.7 wt.% SO3 or 0.13 formula units S (Na0.75K0.05Cs0.29Ca0.02Sr0.02Al0.99Fe0.03Si0.76S0.13O4). Sulfur incorporation as S6+ or SO42- ions into crystal lattice may be facilitated in the presence of large-size Cs+ cations. Simplified suggested formula of this phase may be represented as Na0.8Cs0.3AlSi0.8S0.1O3.95. It was also synthesized by sintering of mixture of chemicals at 1300 °C and found to be instable at temperatures higher than 1300 °C.