A Study Of Extraction And Organic Phase’s Electrochemical Behaviors Of Uranium Based On Ionic Liquids

Ionic liquids (ILs) or room-temperature molten salts, are composed of cationsand anions. ILs has many unique properties compared with traditional solvents andelectrolytes, such as low melting points, good thermal and chemical stability, gooddissolving ability, negligible vapor pressure, excellent electrochemistry properties,recyclable and designable.In this paper, the combination of dry and wet process are used exploratorily inthe reprocessing of spent nuclear fuel. First, uranium(VI) are extracted into ionicliquid phase from water phase by water process; then uranium are electrodepositedfrom ionic liquid phase by electrochemical method. The solvent used were[C4mim][PF6],[N1116][NTf2] and [C4mim][NTf2], and the extraction andelectrochemical behaviors of uranium in these systems were systematically studied.The results demonstrate that ionic liquids are viable and implemented to be used inthe recycling of uranium.The solvent extraction of the uranyl ions (UO22+) was respectively performedusing extraction agent tributyl phosphate (TBP) and ionic liquid1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) system, tributyl phosphate(TBP) and1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide[C4mim][NTf2] system, trioctylphosphine(TOPO) and1-hexyl-3-methylammonium bis(trifluoromethylsulfonyl)imide ([N1116][NTf2]) system, N, N’-diethyl-N,N’-di(para)tolyl-dipicolinamide (Et(p)TDPA) and1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide [C4mim][NTf2] system. Electrochemical test signalsand chemical analysis techniques were studied in detail for analysis of electrodeposition. The electrochemical reduction mechanism was deduced based onthe aboved-mentioned study. The results demonstrate that the distribution ratio forUO22+in former two ionic liquids system are all higher than that in traditional solvents,two process are neutral extraction mechanism, and the extraction complex is[UO2(NO3)]·2TBP. In the former system, uranium(VI) has been reduced touranium(IV) by two electrons in single step, but without generating electrodeposition.In addition, uranium(VI) had been reduced to uranium(IV) by single electron in twosteps, with generating electrodeposition uranyl triphosphate(U3(PO4)4) in the latterone. And then turned into uranium pyrophosphate by pyrolysis, the current efficiencyis32.3%. Extraction process of uranium is cation extraction mechanism in the thirdsystem, and the extraction complex is [UO2(NO3)(TOPO)3]2+, uranium(VI) had beenreduced to uranium(IV) by single electron in two steps, with generatingelectrodeposition uranium dioxide (UO2), the current efficiency of reduction is26%.In the forth system, extraction process of uranium is cation extraction mechanism, andthe extraction complex is [(UO2)2·(Et(p)TDPA)n]4+(n=3or2and4). uranium(VI) hadbeen reduced to uranium(IV) by two electrons in single steps, without generatingelectrodeposition.Uranium(VI) can be extracted and electrochemical reduced to uraniumcompound in TOPO/[N1116][NTf2] and Et(p)TDPA/[C4mim][NTf2] systems, whichshows the feasibility of the combination of dry and wet reprocessing of spent nuclearfuel. As a solvent, ionic liquid has great potential applications in extraction andelectrochemical reduction of uranium, but also need further research to improve thecurrent efficiency.