Stripping from HDEHP

Dapex [Di-alkylphosphoric acid extraction] A process for the solvent extraction of uranium from sulfuric acid solutions using di-(2-ethylhexyl) phosphoric acid (HDEHP). The HDEHP is dissolved in kerosene containing 4 percent of tributyl phosphate. The uranium is stripped from the organic phase by aqueous sodium carbonate and precipitated as uranyl peroxide (yellow cake). The process was no longer in use in 1988. See also Amex. 

For stripping Pu from HDEHP, Fardy recommended an organic reducing agent (6). Our experiments prove that Pu(IV) can be quantitatively stripped by oxalic acid from HDEHP, while extracted zirconium remains in the organic phase. Usually, a decontamination factor of 200-300 for Zr/Pu can be obtained. 

According to the preliminary results of a single back-extraction test performed on fully active laboratory scale, after three successive back-extraction stages, the overall Pu stripped from the loaded HDEHP/TBP solvent originated by the exhaustive extraction step (option 1) was nearly quantitative. 

The use of HDEHP for extraction of actinides from waste solutions also has several drawbacks, including extraction of trivalent ions at a pH at which tetravalent ions such as Zr(IV) and Pu(IV) are hydrolyzed, and difficulties in stripping tetravalent and hexavalent actinides. All in all, monofunctional organophosphorus reagents are vastly inferior to their bifunctional counterparts for extracting Am(III) and other actinides from strong HNO3 media. 

Thus there is considerable incentive to find extractants that could tolerate higher quantities of solids in H2SO4 leach liquors. Stripping of uranium from the Amex process extractant and subsequent regeneration of the amine solvent also consume considerable quantities of acid and base. Recovery of uranium from H2SO4 solutions would be simplified if a convenient neutral extractant could be found. An extractant with better selectivity for vanadium and molybdenum than HDEHP and long-chain amines is also desirable. 

Uranium extracted by HDEHP may be returned to the aqueous phase by contact with a strong acid, which reverses the extraction reaction. Alternatively, contact with sodium carbonate solution may be used to strip the U022+ from the organic phase as carbonate complexes. This approach is used in the DAPEX process and reactions such as that given by equation (60) may be proposed to represent the transfer process. 

In the reverse TALSPEAK process, the An(III) + Ln(III) fraction is first coextracted from a feed, the acidity of which has to be reduced to 0.1 M by denitration or nitric acid extraction. An(III) are then selectively stripped using DTPA in citric acid (1 M) at pH 3 (hence the name reverse TALSPEAK process), and the Ln(III) are finally stripped by 6 M HN03. Attempts to apply this TALSPEAK variant to the treatment of actual UREX + raffinates are reported in the literature, but they involve several steps. The problematic Zr and Mo elements are first removed by direct extraction with HDEHP (0.8 M in di-iso-propylbenzene) from the high-acidity raffinate stream arising from the UREX + co-decontamination process (238). The remaining fission products and actinides can then be concentrated by acid evaporation and denitration processes. This concentrate is further diluted to a lower acidity (e.g., [HN03] = 0.03 M) to allow the coextraction of An(III) and Ln(III) by the TALSPEAK solvent. 

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. 

An initial experiment involving the treatment of small irradiated Pu/Al targets for the production of americium 243 and curium 244 was carried out in France in 1968 (2). The chemical process was based essentially on the use of a system comparable to the Talspeak system. After plutonium extraction by a 0.08 M trilaurylammonium nitrate solution in dodecane containing 3 vol % 2-octanol, the actinides (americium, curium) were coextracted with a fraction of the lanthanides by a 0.25 M HDEHP -dodecane solvent from an aqueous solution previously neutralized by A1(N0 ) x(0H)x and adjusted to 0.04 M DTPA. The actinides were selectively stripped by placing the organic phase in contact with an aqueous solution of the composition 3 M LiN0 -0.05 M DTPA. While this experiment achieved the recovery of 150 mg of americium 243 and 15 mg of curium 244 with good yields, the process presented a drawback due to the slow extraction of Al(III) which saturates the HDEHP. This process was therefore abandoned. 

Carrier (TOPO)-mediated transport of uranium(Vl) has been studied by Akiba and Hashrmoto [94] who have observed that uranium was extracted in the liquid membrane as U02(N03)2 2TOPO and stripped into the carbonate solutions as U02(C03)3. Using TOPO and HDEHP mixmre, liquid membrane technique was apphed for the recovery of uranium from WPPA [95]. Carrier-facilitated Pu(IV) pertraction through an SLM was standardized for its decontamination from oxalate wastes employing a commercially available Cyanex-923 (TOPO analog) in dodecane as the receptor [96,97]. More than 95% of plutonium could be easily recovered from Pu oxalate wastes solution during Pu reconversion operations. 

After the denitration, the process steps sequence of option 2 is similar to the "Reverse Talspeak Process" already developed on a cold laboratory scale at the KfK laboratories of Karlsruhe (19 for the recovery of actinides from HAW. It involves the coextraction of actinides with RE by HDEHP/TBP in n-dodecane at about pH2, the selective stripping of actinides by a Na5 DTPA-glycolic acid solution and the stripping of RE by 4 M HN03. In the case of option 2, the stripping of co-extracted... 

Preconcentration by an acidic organophosphorus extractant is also effective, though the lanthanides are extracted from dilute acid solutions (10 -10 M) and stripped into acidic (>1M) solutions when HDEHP is employed as the extractant. A representative application of liquid-liquid extraction to rare-earth preconcentration is the 100 to 200-fold concentration of the rare earths in seawater prior to ICP/MS determination using a mixture of HDEHP and mono-(2-ethylhexyl)phosphoric acids (Shabani et al. 1990, Shabani and Masuda 1991). When the same extractant system was adsorbed on a Cig cartridge and used in an extraction chromatographic mode, the rare earths were concentrated 200-1000-fold (Shabani et al. 1992). 

There are also thorium recovery processes based on extraction from sulfuric acid solutions, e.g., with primary, secondary, or tertiary amines or alkyl phosphorous acids such as bis-2-ethylhexyl phosphoric acid (HDEHP) or dibutylbutyl phosphonate (DBBP). Thorium is then stripped into a nitric add solution. The alkyl phosphorous acid processes are often employed when recovering thorium as a by-product in uranium production. 

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