The Effects of Glass Doping, Temperature and Time on the Morphology, Composition, and Iron Redox of Spinel Crystals

Abstract

Precipitation of large crystals/agglomerates of spinel and their accumulation in the pour spout riser of a Joule-heated ceramic melter during idling can plug the melter and prevent pouring of molten glass into canisters. Thus, there is a need to understand the effects of spinel-forming components, temperature, and time on the growth of crystals in connection with an accumulation rate. In our study, crystals of spinel [Fe, Ni, Mn, Zn, Sn][Fe, Cr] 2 O 4 were precipitated from simulated high-level waste borosilicate glasses containing different concentrations of Ni, Fe, and Cr by heat treating at 850 and 900°C for different times. These crystals were extracted from the glasses and analyzed with scanning electron microscopy and image analysis for size and shape, with inductively coupled plasma-atomic emission spectroscopy and atom probe tomography for concentration of spinel-forming components, and with wet colorimetry and Mössbauer spectroscopy for Fe 2+ /Fe total ratio. High concentrations of Ni, Fe, and Cr in glasses resulted in the precipitation of crystals larger than 100 µm in just two days. Crystals were a solid solution of NiFe 2 O 4 , NiCr 2 O 4 , and ɤ-Fe 2 O 3 (identified only in the high-Ni-Fe glass) and also contained small concentrations of less than 1 at% of Li, Mg, Mn, and Al. INTRODUCTIONThe U.S. Department of Energy is building the Waste Treatment and Immobilization Plant at the Hanford Site in Washington State to remediate 55 million gallons of radioactive waste that is being temporarily stored in 177 underground tanks. The plan is to vitrify the waste into a durable borosilicate glass with Joule-heated ceramic melters. To do this efficiently and costeffectively, the waste loading in the glasses must be maximized.

Real‐time monitoring of crystal accumulation in the high‐level waste glass melters using an electrical conductivity method

“…The spinel crystals used to conduct Tests 2‐4 were extracted from High‐Ni‐Fe glass as described in Section 2 (Figure and Table ). The distance between probe electrodes as well as the depth of the spinel crystal layer were determined after the crucible had cool and could be cross sectioned and measured.…”

“…Table shows the composition of the High‐Ni‐Fe glass in mass fraction of oxides. The spinel crystals for all testing were produced from the High‐Ni‐Fe glass by heat‐treatment in a double crucible for 7 days at 850°C. The crystals were recovered from the layer accumulated at the bottom of the crucible through overnight treatment with heated (60°C) 20% HNO 3 to dissolve the glass, and then treated with 5% HF to dissolve the residual silica gel.…”

Spinel dissolution via addition of glass forming chemicals. Results of preliminary experiments

“…An experimental glass composition used by the Pacific Northwest National Laboratory (PNNL) to study spinel settling, identified as Ni1.5/Fe17.5, was selected for this study because of its tendency to accumulate spinel crystals during relatively short heat treatment periods. 8,9 The composition of Ni1.5/Fe17.5 is given in Table 2-1. Fluorine and chlorine were removed from the targeted composition to simplify the procedure for fabrication at SRNL.…”

“…Two batches of the PNNL-SP glass composition were fabricated using reagent-grade materials (instead of dry AZ-101 simulant 8,9 ). These batches were identified as PNNL-SP2 and PNNL-SP4.…”

Determination of ferrous and total iron in refractory spinels

“…590 Backscattered electron SEM image of crystals of Spinel 15. Crystals were isolated from 591Ni1.5/Fe17.5 glass heat-treated at 900 °C for 4 days(Matyáš et al, 2014)…”