How To Explain the Difficulties in the Coffinite Synthesis from the Study of Uranothorite?

Abstract

The preparation of Th(1-x)U(x)SiO(4) uranothorite solid solutions was successfully undertaken under hydrothermal conditions (T = 250 °C). From XRD and EDS characterization, the formation of a complete solid solution between x = 0 (thorite) and x = 0.8 was evidenced. Nevertheless, additional (Th,U)O(2) dioxide and amorphous silica were systematically observed for the highest uranium mole loadings. The influence of kinetics parameters was then studied to avoid the formation of such side products. The variation of the synthesis duration allowed us to point out the initial formation of oxide phases then their evolution to a silicate phase through a dissolution/precipitation process close to that already described as coffinitization. Also, the uranium mole loading initially considered was found to significantly influence the kinetics of reaction, as this latter strongly slows down for x > 0.3. Under these conditions, the difficulties frequently reported in the literature for the synthesis of pure USiO(4) coffinite were assigned to a kinetic hindering associated with the coffinitization reaction.

Thermodynamics of formation of coffinite, USiO 4

“…Coffinite metastability was also inferred by Costin et al (33) in hydrothermal synthesis. They noted that the dissolution-reprecipitation process slows toward the coffinite end of the Th 1−x U x SiO 4 series, forming a correspondingly increasing amount of a Th-U dioxide phase.…”

“…Assuming water in coffinite is adsorbed on the surface with an integral adsorption enthalpy of −80 kJ/mol per mole of H 2 O, similar to the values observed for water adsorption on alumina and titania (44)(45)(46) and, through proper thermochemical cycles (SI Appendix, Table S4), a value of ΔH f,ox of coffinite (24.1 ± 3.5 kJ/mol) is obtained, in agreement with the results for coffinite-F ( Table 1). The tightly bound water could originate from metaschoepite (UO 3 ·2H 2 O), which could be formed from partial oxidation of U(IV) (47) that originally was not incorporated in the coffinite structure, but rather was embedded in the gelatinous layer of excess amorphous silica (33). This phase may be hard to detect by XRD, especially if it is fine-grained or poorly crystalline.…”

“…Coffinite-F and -G samples were prepared by hydrothermal synthesis routes, described elsewhere (33,35). Because these syntheses routes inevitably have UO 2 and amorphous silica as byproducts, further purification is needed.…”

“…It appears not to form by reacting the oxides under dry high-temperature conditions (24, 29). Synthesis from aqueous solutions usually produces UO 2 and amorphous SiO 2 impurities, with coffinite sometimes being only a minor phase (24,(30)(31)(32)(33)(34)(35). It is not clear whether these difficulties arise from kinetic factors (slow reaction rates) or reflect intrinsic thermodynamic instability (33).…”

“…Synthesis from aqueous solutions usually produces UO 2 and amorphous SiO 2 impurities, with coffinite sometimes being only a minor phase (24,(30)(31)(32)(33)(34)(35). It is not clear whether these difficulties arise from kinetic factors (slow reaction rates) or reflect intrinsic thermodynamic instability (33). Thus, there are only a few reported estimates of thermodynamic properties of coffinite (22,(36)(37)(38)(39)(40) and some of them are inconsistent.…”