Lanthanide ions purification

In the various solvent-extraction circuits employed in this process, use is made of a solution of D2EHPA in kerosene as the extractant. The selective recovery of the various metals is achieved by careful control of the equiUbrium pH value of the aqueous phases in the multistage extraction and stripping operations. After the leach liquor has passed through two separate circuits, each of which comprises five stages of extraction and four of stripping, the europium product is obtained initially as a solution of europium(III) chloride. Further purification of the product is accomplished by reduction with amalgamated zinc to Eu +, which is by far the most stable of the divalent lanthanide ions with respect to the reduction of water cf. the redox potentials of the Eu /Eu and Sm /Sm + couples, which are —0.43 and —1.55 V respectively ). Sulfuric acid is added to the... 

Since pyromellitic acid sequesters heavy metals, it can also be used as an antiscaling agent(32) and as a corrosion inhibitor(33) but whether it has sufficient benefits for market applications has yet to be shewn. An unusual proposed use is to dissolve kidney stones(34). A related property is that pyromellitic acid will precipitate lanthanide ions frem solutions containing a number of divalent cations such as Mg, Ni, Mn, etc.(35). As a consequence, there may be possible applications for large scale purification of the rare earths. 

The trivalent state of americium is the stable aqueous oxidation state. Although americium is the homolog of europium, the Am " radius (0.975 A) is closer to that of Nd " " (radius 0.983 A) [76]. It is a convenient rule of thumb that the radii of the light lanthanide ions are nearly identical to the radii of the corresponding actinides shifted three elements to the right in the periodic table, e.g. r(La " ) a r(U ). In some early purification schemes, fission-product promethium accompanied americium. Am(iii) is precipitated by hydroxide, fluoride, phosphate, and oxalate ions from aqueous solution. 

Since the chemistry of actinium is confined to the Ac + ion, it can readily be separated from thorium (and the lanthanides, for that matter) by processes like solvent extraction with thenoyltrifluoroacetone (TTFA) and by cation-exchange chromatography. The latter is an excellent means of purification, as the Ac + ion is much more strongly bound by the resin than its decay products. 

This process is capable of giving individual lanthanides of sufficient purity for chemical use (99.9%), but for electronic or spectroscopic use ( phosphor grade ) 99.999% purity is necessary, and further ion exchange is used for final purification to these levels. ... 

Large-scale purification of americium, curium, and californium with pressurized cation exchange has been planned at SRP for many years (1). Initial work involved SRP batch extractions to isolate a crude actinide-lanthanide fraction followed by solvent extraction and ion exchange in the SRL high level caves (1J. For large-scale purification, a single step was substituted for batch extraction and solvent extraction. Plant Purex solvent (30 vol % tri-n-butyl phosphate in n-paraffin) was used to minimize flush time and cross-contamination of solvent. 

On the topic of lanthanide/actinide separation, few reviews have dealt in detail with the most difficult aspect of this field, separation of the lanthanides from the trivalent transplutonium actinides. Jenkins (1979,1984) reviewed ion exchange applications in the atomic-energy industry. Relatively short sections of these reviews dealt with the separation of the trivalent metal ions. Symposium volumes entitled Actinide Separations (Navratil and Schulz 1980) and Lanthanide/Actinide Separations (Choppin et al. 1985) are collections of papers from several authors covering all aspects of lanthanide/actinide separation, some of which deal with the purification of the trivalent metal ions. 

Tertiary amines are poor extractants for lanthanides and actinides from dilute nitrate media, but extract these metal ions strongly from concentrated nitrate solutions of low acidity (as was true of TBP). Similar observations have been made for extraction from chloride media. Figure 1 indicates that for 30% Alamine 336/xylene/ll M LiCl group separations are good, some interactinide separations are possible, but lanthanide separation factors are small. Weaver briefly discusses the application of the TRAMEX (tertiary amine extraction) process for the purification of... 

For the first purification of plutonium in the processing of irradiated nuclear fuels, an anion-exchange process has been widely used [202]. In this process, complex formation of plutonium(iv) with nitrate is utilized in order to remove the last traces of uranium (present as uranyl(vi)) and fission products (primarily lanthanides). In this system, the maximum sorption of plutonium (iv) occurs at a nitric acid concentration of 7.2 m. The process is run at 60°C. At lower temperatures, the sorption is too slow at higher temperatures, the distribution ratio becomes more unfavorable and the resin is more liable to deteriorate. Under the conditions chosen, neither uranyl(vi) nor lanthanides are sorbed. The elution of plutonium(iv) is readily achieved by dilute (0.7 m) nitric acid. The weak point of the process is the limited resistance of organic ion exchangers to chemical attack and to high doses of radiation, already discussed in Section 21.6.1. These difficulties can be overcome, at least partly, by careful selection of the resin to be used. 

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