Dissolution and Growth of Spinel Crystals in a High-Level Waste Glass

Abstract

Abstract A large fraction of HLW glass readily precipitates spinel. The presence of solid particles, including spinel crystals, is undesirable in a high-level waste (HLW) glass melter because the settling of solids can disrupt melter operation and shorten melter lifetime. Spinel formation in the melter can be reduced by lowering waste loading. When formulating HLW glass to maximize waste loading (thus minimizing the cost), the settling of insolubles must be considered. The rate of nucleation, growth, and dissolution of spinel crystals in a molten HLW glass was measured as a function of temperature and the presence of nucleation agents, such as noble metals. The mass transfer coefficient was calculated using the Hixson-Crowell model and expressed as an Arrhenius function of temperature that was identical for both crystal growth and crystal dissolution. The Stokes law for hindered settling and the Hixson-Crowell equation for crystal growth and dissolution were chosen as a convenient representation of spinel behavior for mathematical models of HLW glass melters. Levich’s analysis of the dissolution or growth of falling particles was used to estimate the effective diffusion coefficient (D) and the concentration-boundary-layer thickness (δ) around growing and dissolving crystals. The estimated values were in reasonable agreement with the measured concentration profile of Fe at a dissolving spinel crystal at 1200°C, determined with energy dispersive spectroscopy. The D value is comparable to that obtained by Borom and Pask for a sodium disilicate-magnetite couple. The calculated δ value agreed with the concentration distribution of Fe around falling crystals of spinel as imaged by optical microscopy. This methodology, in conjunction with mathematical modeling, provides a basis for developing optimized technology and glass formulation for HLW vitrification.