Chemical Stability of Fine-Grained Ceramics Based on Nd0.33Zr2(PO4)3 Phosphate with Kosnarite Structure at Elevated Temperatures

The chemical stability of Nd0.33Zr2(PO4)3 fine-grained ceramics, which can be used for immobilization of rare earth elements (REE) that are part of high-level waste was studied. Single-phase Nd0.33Zr2(PO4)3 submicron powders with the structure of the mineral kosnarite were prepared by colloid-chemical synthesis. Powders were prepared by successive annealing at 600, 800, and 900°C for 6 h at each stage. Nd0.33Zr2(PO4)3 ceramic was Nd0.33Zr2(PO4)3 ceramics was produced using spark plasma sintering method (SPS). The relative density of the ceramic was 89.9%; the average grain size was 5–20 μm. The chemical stability of the ceramic in the static mode at 90°C in distilled water and mineral water, as well as in acidic media (0.1 M HCl) and alkaline media (0.01 M NaOH), was studied. The ceramics have high hydrolytic stability. The influence of the contact medium on the rate and mechanism of Nd leaching from the surface of Nd0.33Zr2(PO4)3 fine-grained ceramic samples was studied. The de Groot-van der Sloot model was used to analyze the resulting leaching rate Ri against time t of experiment. Nd leaching occurs by dissociation of Nd from the ceramic surface in acidic medium, diffusion from the inner layers in alkaline medium and mineral water, and dissolution of the ceramic surface layer in distilled water.