Effect of feed melting, temperature history, and minor component addition on spinel crystallization in high-level waste glass

Spinel crystallization affects the anticipated cost and risk of high-level waste (HLW) vitrification. Spinel, (Fe, Ni) (Fe,Cr)2O4, is the primary crystalline phase that precipitates from melts containing oxides of Fe, Ni, and Cr in sufficient concentrations. This study was undertaken to help design and verify mathematical models for a HLW glass melter in which spinel crystals precipitate and partially settle. To study melting reactions, we used a simulated HLW feed, prepared with co-precipitated Fe, Ni, Cr, and Mn hydroxides. Feed samples were heated up at a temperature-increase rate presumed to be close to that which the feed experiences in the HLW glass melter. The decomposition, melting, and dissolution of feed components (such as nitrates, carbonates, and quartz) and the formation of intermediate crystalline phases (spinel, sodalite, and Zr-containing minerals) were characterized using evolved gas analysis, volume-expansion measurement, optical microscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). Nitrates and quartz, the major feed components, converted to a glass-forming melt by 880°C. A nearly chromium-free primary spinel formed in the nitrate melt starting from 520°C and eventually dissolved in the borosilicate melt by 1060°C. Sodalite, a transient product of corundum dissolution, appeared above 600°C and eventually dissolved in glass. To investigate the effects of temperature history and minor components (Ru, Ag, and Cu) on the dissolution and growth of spinel crystals, samples were heated up to temperatures below or above liquidus temperature (TL) and then subjected to different (constant or cyclic) temperature histories and analyzed. The results show that the mass fraction of spinel as well as the composition and size of crystals depend on the chemical and physical makeup of the feed and the temperature history. Small crystals result from either preventing the primary spinel from dissolving in the glass-forming melt or from the presence of nucleation agents, such as RuO2 (additions of 0.06 mass% Ag2O and 0.06 mass% CuO did not help nucleate spinel). Cyclic temperature histories with the maximum temperature above TL and minimum temperature below TL kept spinel concentration far below the equilibrium concentration, especially when a nucleation agent was absent.