Hydrolysis of Uranyl‐, Nd‐, Ce‐Ions and their Mixtures by Thermal Decomposition of Urea

Speciation and Ammonia-Induced Precipitation of Neptunium and Uranium Ions

The pH evolution and corresponding changes in the UV-Vis-NIR absorption spectra of oxygenated neptunium (NpO₂⁺ and NpO₂²⁺) and uranyl ions (UO₂²⁺) in nitric acid are investigated during titration with an aqueous NH₃ solution. The speciation and precipitation regimes between acidic (pH 1.5) and alkaline (pH 10) conditions at room temperature are discussed to assess the suitability of Np(V) or Np(VI) in sol-gel conversion processes for fuel target fabrication. Under the applied experimental conditions, Np(V) hydrolyzes and precipitates into the insoluble hydroxide NpO₂OH only above pH values 7.5 and an increase up to pH 10.0 is required to precipitate quantitatively. Np(VI) displays changes in the coordination environment of NpO₂²⁺ ions in the pH interval 1.6-4.0, similar to what is observed for U(VI). Precipitation into NpO₃·H₂O or other hydroxide compounds takes place between pH 4.0 and 5.9, which overlaps largely with precipitation of ammonium diuranate species from the U(VI) solution. The use of concentrated NH₃ aqueous solution, as commonly used in the external gelation process, will allow to quantitatively precipitate both Np(V) and Np(VI) species. Internal gelation process conditions, on the other hand, seem incompatible with the high pH required to precipitate Np(V) completely. For fabricating mixed-oxide (U,Np) targets using sol-gel conversion, a feed broth containing Np(VI) and U(VI) will be required to achieve homogeneous gelation.

Homogeneous Precipitation of Lanthanide Oxalates

Oxalic acid is an important separation agent in the technology of lanthanides, actinides, and transition metals. Thanks to the low solubility of the oxalate salts, the metal ions can be easily precipitated into crystalline material, which is a convenient precursor for oxide preparation. However, it is difficult to obtain oxalate monocrystals due to the fast precipitation. We have developed a synthetic route for homogeneous precipitation of oxalates based on the thermal decomposition of oxamic acid. This work primarily concerns lanthanide oxalates; however, since no information was found about oxamic acid, a brief characterization was included. The precipitation method was tested on selected elements (Ce, Pr, Gd, Er, and Yb), for which the kinetics was determined at 100 °C. Several scoping tests at 90 °C or using different starting concentrations were performed on Ce and Gd. The reaction products were studied by means of solid-state analysis with focus on the structure and morphology. Well-developed microcrystals were successfully synthesized with the largest size for gadolinium oxalate.