PNNL

Abstract

Noble metals (for example ruthenium) act as nucleation sites for the precipitation of spinel (crystalline) phases. The noble metals along with the spinel phases will settle to the bottom of the melter causing local viscosity increase, power fluctuations, and even potentially shorting of electrodes leading to premature melter failure. We studied the partitioning of ruthenium in a model high-level waste glass. Ruthenium oxide was chosen as this was predominantly found in melter tests with feeds containing noble metals at the bottom of the melter. A doping of 10 wt % of ruthenium oxide was selected to simulate somewhat the conditions at the bottom of the melter where noble metals accumulate. The heat-treatment conditions (temperature and duration) were chosen from reported literature, such that large crystals of trevorite (NiFe2O4) were formed in the glass. The spinel-glass interface was characterized using scanning electron microscopy (SEM) and microprobe characterization. SEM results showed the crystals distributed in the glass matrix. Microprobe measured the ruthenium concentration across and around the spinel-glass interfaces. The results did not show significant partition of ruthenium in the spinel.