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

Abstract

The hydrolysis of uranyl-, Nd-and Ce-cations, induced by thermal decomposition of urea was investigated and the impact of the urea content and the experiment temperature on the reaction kinetics was evaluated. Uranyl precipitated as ammonium diuranate (ADU) with varying stoichiometry. Nd(III) and Ce(III) showed comparable pH evolutions and lanthanide carbonate hydroxide (LnCO 3 OH) products were identified, whereas Ce(IV) hydrolysed at lower pH and formed CeO 2 . The precipitation behaviour was confirmed for mixtures of uranyl and the lanthanides. Depending on the urea content, a partial co-precipitation occurred in U/Nd mixtures. The phases formed with Ce(III) and Ce(IV) were also identified in the precipitates of binary U/Ce mixtures. In ternary U/Nd/Ce compositions, a simultaneous precipitation of Nd(III) and Ce(III) was observed and a partial incorporation of the lanthanides into the ADU phase, whereas the precipitation in the presence of Ce(IV)/CeO 2 led to the formation of three individual phases. 2 + , Nd(III), Ce(III)

Homogeneous Precipitation of Lanthanide Oxalates

“…A slight shift (∼2 nm) was found between the spectrum of Er­(III) solution before and after oxamic acid addition (Figure c). A similar shift (∼5 nm) was also found for the uranyl ion and urea (containing the −NH 2 group as well); however, it was not as strong in our case. Moreover, this shift was not observed in the case of Er­(III) (Figure c).…”

Cation-heterogeneity in internally gelated U1-Ce O2-, 0.15 ≤ z ≤ 0.3 microspheres

“…Investigating and understanding in detail the gelation process is a challenging task as it can be influenced by multiple factors happening at very small scales such as the mixing of reactants, the droplet formation, the displacement and likely start of surface interaction upon contact with the Si-oil, and finally by the heat propagation from the hot gelation medium into the broth droplet triggering HMTA and Urea protonation/decomposition [37]. This leads to a release of ammonia causing the subsequent precipitation of UO 2+ and Ce 3+ /Ce 4+ species [39], [40]. For each species, precipitation may progress on separate paths with different reported pH ranges [39] and kinetics while resulting in different products [39], [41].…”

“…For each species, precipitation may progress on separate paths with different reported pH ranges [39] and kinetics while resulting in different products [39], [41]. While the exact chemical form of the Ce-rich precipitate currently is not known [24], [37], [39], the progress and products of the former is reported to depend on temperature and R factors for HMTA and Urea as well as metal concentrations. It likely also depends on local variations of pH in time [1], [3], [5].…”

Speciation and Ammonia-Induced Precipitation of Neptunium and Uranium Ions

“…12,24−26 The hydrolysis behavior of the individual metal ions during the gelation reaction affects the quality of the microspheres. An inhomogeneous mixing of the dopant element into the uranium matrix occurs if the precipitation pH of the individual ions diverges significantly, as is, e.g., the case for Ce 4+ and UO 2 2+ ions, 27 and premature or incomplete gelation can take place. Hence, prior to fabricating oxide microspheres using gelation processes, a feasibility study is required to investigate the hydrolysis behavior of the metal ions and to evaluate how it may affect the overall gelation reaction.…”

“…Sol–gel processes can be used to produce actinide oxide microspheres, e.g., UO 2 , PuO 2 , and ThO 2 , as well as mixed-oxide or composite microspheres, such as (U,Th)­O 2 , (U,Pu)­O 2 , and MA-bearing microspheres. − The approach to fabricate such mixed oxides is to mix the uranium feed broth with a solution containing the dopant and to process this “metal blend” solution in the same manner as the conventional feed broth. ,− The hydrolysis behavior of the individual metal ions during the gelation reaction affects the quality of the microspheres. An inhomogeneous mixing of the dopant element into the uranium matrix occurs if the precipitation pH of the individual ions diverges significantly, as is, e.g., the case for Ce 4+ and UO 2 2+ ions, and premature or incomplete gelation can take place. Hence, prior to fabricating oxide microspheres using gelation processes, a feasibility study is required to investigate the hydrolysis behavior of the metal ions and to evaluate how it may affect the overall gelation reaction.…”