The presence of sulfur-containing molten salt phases during vitrification is a particular problem which can be detrimental to the melter. Borosilicate compositions were designed to explore both the effects of a fixed amount of Cr, as Cr2O3 or Na2CrO4, and the impact of Al2O3, on sulfur solubility as determined by progressively adding elemental S. The NABS series (with Al2O3) has a higher SO3 solubility of ∼3.5 mol% while the NBS series (without Al2O3) has a SO3 solubility of ∼2 mol%. Crystalline Cr2O3 formed in the NABS series upon the addition of S but is not present in the NBS series. At the highest S addition, a separate salt phase always forms, and was identified as Na2(S,Cr)O4 via electron dispersive spectroscopy, ultraviolet-visible spectroscopy, x-ray diffraction, and thermal analysis. Magic angle spinning nuclear magnetic resonance identified no change in the B coordination in either series as S is added, suggesting that the Na is scavenged from other locations, such as the Si sites, to charge compensate sulfate, whether in the glass or in the salt. The Cr precursor identity did not affect the observed overall behavior regarding sulfur, but did impact Cr redox as identified with Raman spectroscopy.
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