Extraction Properties of Trivalent Rare Earth Ions from Nitric Acid Using a Triamide–Amine Extractant

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The extraction of trivalent rare earth ions (RE³⁺) from HNO₃ solution using a triamide amine, tris(N,N-di-2-ethylhexyl-ethylamide)amine (DEHTAA), was conducted, and the extraction mechanism was estimated from extraction behavior of HNO₃ and RE³⁺. A DEHTAA molecule dominantly formed a DEHTAA⸱HNO₃ at 1.0 M HNO₃ and a DEHTAA⸱(HNO₃)₂ at 6.0 M HNO₃ in the acid-equilibrated organic phase. This would provide the unique dependence of E% for the light RE³⁺ on the HNO₃ concentration, in which the E% value had a minimum and maximum at ~0.5 M and ~2 M HNO₃, respectively. The results of the slope analyses for the distribution ratios for RE³⁺ suggested that the dominant RE³⁺ complex was RE(NO₃)₃DEHTAA(DEHTAA⸱HNO₃) at 1.0 M HNO₃. The E% for RE³⁺ decreased from La³⁺ to Lu³⁺ at 1.0 M HNO₃; however, those increased from La³⁺ to Nd³⁺ at 0.25 M HNO₃ and from La³⁺ to Sm³⁺ at 6.0 M HNO₃.