Reactivity of NH4H2PO4 toward LaCl3 in LiCl-KCl Melt Flux. Step by Step Formation of Monazite-Like LaPO4.

详细信息    查看全文

• 作者：Damien Hudry ; Aydar Rakhmatullin ; Catherine Bessada ; Isabelle Bardez ; Florence Bart ; Stphane Jobic ; Philippe Deniard
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：August 3, 2009
• 年：2009
• 卷：48
• 期：15
• 页码：7141-7150
• 全文大小：855K
• 年卷期：v.48,no.15(August 3, 2009)
• ISSN：1520-510X

文摘

The synthesis of lanthanum phosphates in molten LiCl-KCL eutectic was chosen to address the preliminary treatment of chlorinated wastes containing fission products that are already present in a Li/Cl eutectic. The obtained monazite compound shows interesting properties to be considered as a good candidate to trap lanthanum for a long-time. The synthesis route based on LaCl₃ reaction with NH₄H₂PO₄ in a stoichiometric amount is a key point to obtain monazite as a pure phase. Hence, the salt composition is not modified during the synthesis reaction. The chemical reactivity of ammonium dihydrogenphosphate (NH₄H₂PO₄, hereafter abbreviated ADP) toward lanthanum chloride (LaCl₃) in molten LiCl-KCl eutectic is probed by NMR spectroscopy to follow the formation of LaPO₄. Formally, a direct transformation of the two aforementioned precursors into LaPO₄, NH₄Cl and HCl can be discarded on the basis of the low thermal stability of ADP. To shed some light on the formation of LaPO₄, in situ and ex situ NMR experiments were carried out on LiCl-KCl/LaCl₃/ADP, as well as LiCl-KCl/ADP, KCl/ADP, and LiCl/ADP mixtures. First, the reactivity of the precursors in contact with the eutectic was studied from room temperature to 600 °C by means of ³¹P, ³⁵Cl, and ¹³⁹La high temperature NMR. Second, ex situ room temperature magic angle spinning (MAS) and RadioFrequency driven recoupling (RFDR) ³¹P solid-state NMR experiments were carried out on solid samples prepared in different conditions (i.e., temperature and atmosphere) and quenched at room temperature to identify frozen intermediate species in their metastable state. On the basis of this approach, we propose a model for the LaPO₄ formation based on a multistep mechanism which highlights the strong reactivity of ADP toward the alkaline salts but without final change in the composition of the solvent.