Phosphoric acid actinides extraction

Table 4.18 Constants for Extraction of Actinide Nitrate Adducts with Phosphoric Acid Trialkylesters (Free Rotation of the P Substituents) and with A,A-Dialkylamides (Restricted Rotation Around the Amide Moiety)...

M = Am, Cm, Bk, Cf, and Es) has been investigated by solvent extraction processes using bis-(2-ethylhexyl)phosphoric acid. The stability of [M(NCS)] and [M(NCS)2] increased gradually across the series in accord with expectations based on the actinide contraction, and there was evidence for the tetrad... 

There are various types of organic proton exchangers (34, 35, 38). Diesters of phosphoric acid, (RO)2P = 0(0H), phosphonic acids, R(RO)P = 0(0H), and phos-phinic acids, R2P = 0(0H), where R represents linear or branched alkyl or phenyl substituents, are the most common cation exchangers developed in liquid-liquid extraction for the extraction of trivalent 4/and 5/elements. They were initially developed for the American TALSPEAK and the Japanese DIDPA processes and have recently been introduced in the French DIAMEX-SANEX process. As for previously described NOPCs, these organophosphorus acids present oxygen-donor atoms (hard bases) in their structures and therefore will easily coordinate trivalent lanthanide and actinide cations, but they will not allow complete discrimination of the two families of elements. However, contrary to previously described neutral organophosphorus... 

The ZEALEX Process Researchers from KRI have shown that the zirconium salt of dibutyl phosphoric acid (ZS-HDBP) was soluble in Isopar-L in the presence of 30% TBP. This super PUREX solvent, known as ZEALEX, extracts actinides (Np-Am) together with lanthanides and other fission products, such as Ba, Cs, Fe, Mo, and Sr from nitric acid solutions. The extraction yields depend on both the molar ratio between Zr and HDBP in the 30% TBP/Isopar-L mixture and the concentration of HN03 (232). Trivalent transplutonium and lanthanide elements can be stripped together from the loaded ZEALEX solvent by a complexing solution, mixing ammonium carbonate, (NH4)2C03, and ethylenediamine-N.N.N. N -tetraacetic acid (EDTA). An optimized version of the process should allow the separation of... 

A mixture of well-known extractants, di-(2-ethylhexyl)phosphoric acid (HDEHP) and CMPO, in n-paraffin was used for the study of combined extraction of different actinides (americium, plutonium, and uranium) and lanthanides (cerium and europium) and their separation from fission products (cesium, strontium, ruthenium, and zirconium).54 Combined extraction of MAs and lanthanides was studied together with group separation of MAs from lanthanides by selective stripping with a solution of diethylenetriaminepentaacetic acid (DTPA), formic acid, and hydrazine hydrate. This solution strips only MAs, leaving lanthanides in the organic phase. Subsequently, the lanthanides are stripped using a mixture of DTPA and sodium carbonate. 

Di(2-ethylhexyl) phosphoric acid (HDEHP) is an extractant molecule used for An(III)/Ln(III) separation. Used in TALSPEAK-type processes in a mixture with TBP, or in the DIAMEX-SANEX process in a mixture with a malonamide (154-157), it has also been proposed, in a mixture with TBP, to remove strontium from PUREX acid waste solution in the Hanford B plant (158). Therefore, numerous studies have focussed on the radiolytic degradation of HDEHP and its effects on the extraction of Sr(II), lanthanides(III), and actinides(III) (10, 158-163). 

Figure 1 shows the extraction behavior of the actinide elements when a HLLW solution is contacted with a solution of 1 M HDEHP (di(2-ethylhexyl)phosphoric acid) in an aliphatic diluent (Nysolvin 75A). It is evident from this Figure that a good extraction of all actinide elements can not be obtained at any single acidity. This led us to the decision to first extract Pa,... 

Extraction processes (TRUEX, PUREX, Talspeak, DIAMEX, PARC, etc.) generally involve complexation of transplutonium elements by alkyl phosphines, phosphine oxides, phosphoric acids, carbamoyl phosphonates, diamides, and thiophosphinates in aqueous/organic extractions, within derivatized solid supports, or on coated particles. There are excellent reviews of the processes and significant complexes by Mathur et al. and selected chapters in The Chemistry of the Actinide and Transactinide Elements to be published in 2003. " Work on the separation for nuclear waste management in the United States, France, and Russia have been reviewed. " ... 

The alkyl phosphoric acids were first recognized as excellent actinide extractants because dibutyl phosphoric acid existed as an impurity in TBP (8). They extract well from unsalted solutions and from systems such as sulfate in which TBP is ineffective. Organo-phosphorus acids are thus suitable for uranium hydr metallurgical applications where ore is leached with sulfuric acid. The Dapex process is an example in which bis(2-ethylhexyl) phosphoric acid (HDEHP) is used alone or in synergistic combination with neutral organophosphorus compounds for uranium and vanadium recovery (9,10). 

Alkyl phosphoric acids. Perhaps the largest single group of extractants for actinides are the alkylphosphoric acids of three main types dialkylphosphoric acids, dialkylphosphonic acids, and... 

The addition of an ether linkage in the alkyl chains of dialkylphosphoric acids has been found to increase its extractive power for trivalent actinides, resulting in an extractant that is more effective from acid solutions than is HDEHP (69). Bis(hexoxy-ethyl)phosphoric acid (HDHqEP) is a compound of this type that has been studied extensively (70,71). The ether linkage may contribute to the coordination of the metal ion, resulting in the formation of a seven-membered ring. 

When the divalent state of Md was first discovered, extraction chromatography was used to prove that the behavior of Md + was dissimilar to that of Es + and Fm + (20). The extractant, bis(2-ethylhexyl)phosphoric acid (HDEHP), has a much lower affinity for divalent ions than it does for the tri- and tetravalent ones. Thus, the extraction of Md2+ is much poorer than the extraction of the neighboring tripositive actinides as indicated by the results shown in Table 2. This became the basis for a separation method in which tracer Md in 0.1 M HC1 is reduced by fresh Jones Reductor in the upper half of an extraction column containing HDEHP absorbed on a fluorocarbon powder in the lower half. Mendelevium, in the dipositive state, is rapidly eluted with 0.1 M HC1 whereas the other actinides are retained by the extractant. The separation is quickly performed, but the Md contains small amounts of Zn2+ from the Jones Reductor and also Eu2+, which was added prior to the elution to prevent reoxidation of Md2+ by the extractant. 

Compared to the investigations described above for SLMs in processing actinides from spent fuel wastes, studies devoted to ELMs are very limited. ELM transport of U(VI) from a solution which was 0.01 M in nitric acid and 2 M in sodium chloride with an emulsion of 2.5 M phosphoric acid in 0.1 M DEHPA-dodecane is practically quantitative (Table 9) and equivalent to 66 stages of extraction with the DEHPA in dodecane (55). 

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