Platinum group metals (PGM), including ruthenium (Ru), rhodium (Rh), and palladium (Pd) are enriched in high-level liquid waste (HLLW) during the reprocessing of spent nuclear fuels. Vitrification is currently the only practical technology to immobilize HLLW. However, the high density and low solubility of PGM in glass cause them to settle down during vitrification process, resulting in the increased viscosity and conductivity of the glass melt at the bottom of the melter. This would potentially lead to concerned issues such as blockages in the discharge port and damage of electrodes. Therefore, understanding the deposition of PGM during vitrification is quite crucial in the management of HLLW. However, few research focused on the deposition process and the spatial distribution of PGM particles in glass melts. Thus, this work took Ru as the representative of PGM and investigated spatial distribution of Ru precipitates in glass melts.

A simplified simulated waste glass with the composition of 50.00%SiO₂-18.89%B₂O₃-2.79%Li₂O- 5.00%CaO-13.30%Na₂O-6.68%Al₂O₃-3.34%ZrO₂ (mass fraction) was formulated. Batches were thoroughly mixed and placed in corundum crucibles, and then melted at 1200 ℃ for 1 h in a muffle furnace. The molten glass was quenched in air and then crushed to obtain the glass frits with the size smaller than 70 μm. Subsequently, glass frits and additional 1.00%RuO₂ (mass fraction) were then fully mixed. 3 g of mixtures was placed in a corundum crucible with a lid to prevent Ru volatilization, and remelted at 1200 ℃ for 0.5 h, 1 h, 2 h and 4 h, respectively, and then annealed at 500 ℃ for 2 h to release the thermal stress.

XRD was used to analyze the precipitated crystalline phases of the Ru-containing glasses. SEM-EDS was utilized to examine the morphology of Ru precipitates in glass samples and determine the distribution density of Ru precipitates particles in agglomerates. The glass samples were then cut into blocks with the size of 7 mm ×7 mm ×6 mm, and XRM was applied to detect the content of Ru precipitates, as well as changes in the size and quantity of agglomerates.

XRD analysis suggested that RuO₂ and Ru precipitated from the glasses in different melting durations. SEM showed both RuO₂ and Ru formed the agglomerates, but RuO₂ agglomerates exhibited higher content and the larger sizes compared to Ru agglomerates. Through calculating the distribution density of RuO₂ particles in the agglomerates, it was determined that RuO₂ particles in the agglomerates coarsened and the RuO₂ agglomerates became more compact as the melting time increased.

Results from SEM and XRM indicate that RuO₂ particles aggregated into a larger size (~20 μm) in the short-time stage (t <1 h), and the content of precipitated RuO₂ and Ru increased rapidly and reached to its added level. In the middle-time stage (1 h< t <2 h), the coalescence of the agglomerates occurred and aggregated to a much larger size (>130 μm). Consequently, the number density of agglomerates decreased, whereas the volume fraction of agglomerates remained relatively stable. In the long-time stage (2 h< t <4 h), the agglomerates turn to the denser one. This led to the increase of particles distribution density and the decrease of agglomerates size. At this stage, the number density of agglomerates remained even.

The three-dimensional images reconstructed from XRM were segmented into 12 layers from top to bottom. The analysis focused on the content of Ru precipitates in each layer of glasses melted for different durations. The samples were categorized into three regions: the surface layer (0~1.5 mm), middle layer (1.5~5 mm), and bottom layer (5~6 mm). The content of precipitated particles in the bottom layer were notably larger compared to those in the middle and surface layers, when the melting time was the same. Furthermore, the content of precipitated particles in the bottom layer increased with prolonged melting time, and approximately 50% of the precipitates settled down in the crucible bottom after 4 h melting. The rate of precipitates deposition decreased as the melting time increased.

Ruthenium primarily precipitates as the form of RuO₂ particles in the simulated waste glass. As the melting time is prolonged, the Ru precipitates undergo various processes, including formation of agglomerates, coalescence between agglomerates, and densification of agglomerates. The content of precipitates in the bottom layer was larger than those in the middle and surface layers. The volume fraction of precipitates in the bottom layer increases with extended melting time, and the rate of deposition of precipitates decreases as melting time increases.