Extraction of Protactinium

First identified in 1913 (the first compound, Pa20s, was isolated in 1927 by von Grosse, who isolated the element in 1931), protactinium is not generally extracted. Most of what is known about the chemistry of protactinium ultimately results from the extraction in 1960 by the UK Atomic Energy Authority of some 125 grams of Pa, from 60 tons of waste material left over from the extraction of uranium, at a cost of about 500,000 (very roughly, 1,250,000 at today s exchange rate). 

---

Ross, R. G. Grimes, W. R. Barton, C. J. Bamberger, C. E. Baes, C. F. Jr., "The Reductive Extraction of Protactinium and Uranium from Molten LiF-BeF2-ThF Mixture Into Bismuth," in "Symposium on Reprocessing of Nuclear Fuels, The Metallurgical Society of AIME, Ames, IA, August 1969, ... 

Hyde and Wolf have studied the extraction of uranium and thorium by methyl n-amyl ketone and dllsopropyl ketone as a function of total nitrate concentration In the aqueous phase. In both cases, uranium was better extracted than thorium. The extraction of thorium did not become appreciable (<55 ) until the aqueous nitrate concraitratlon was greater than 5M. Dllsopropyl ketone was found to be an excellent extractant of protactinium. ... 

However, the quantity of Pa produced in this manner is much less than the amount (more than 100 g) that has been isolated from the natural source. The methods for the recovery of protactinium include coprecipitation, solvent extraction, ion exchange, and volatility procedures. AH of these, however, are rendered difficult by the extreme tendency of protactinium(V) to form polymeric coUoidal particles composed of ionic species. These caimot be removed from aqueous media by solvent extraction losses may occur by adsorption to containers and protactinium may be adsorbed by any precipitate present. 

As the parent of actinium in this series it was named protoactinium, shortened in 1949 to protactinium. Because of its low natural abundance its chemistry was obscure until 1960 when A. G. Maddock and co-workers at the UK Atomic Energy Authority worked up about 130g from 60 tons of sludge which had accumulated during the extraction of uranium from UO2 ores. It is from this sample, distributed to numerous laboratories throughout the world, that the bulk of our knowledge of the element s chemistry was gleaned. 

An additional material based on the extractant octyl-phenyl-N,N-diisobutyl-carbamoylmethylphosphine oxide, or CMPO, (marketed under the name TRU-Spec) has also been widely utilized for separations of transuranic actinides (Horwitz et al. 1993a) but is also useful for uranium-series separations (e.g., Burnett and Yeh 1995 Luo et al. 1997 Bourdon et al. 1999 Layne and Sims 2000). This material has even greater distribution coefficients for the uranium-series elements U (>1000), Th (>10000), and Pa. As shown in Figure 1, use of this material allows for sequential separations of Ra, Th, U, and Pa from a single aliquot on a single column. Separations of protactinium using this material (Bourdon et al. 1999) provide an alternative to liquid-liquid extractions documented in Pickett et al. (1994). 

Burnett WC, Yeh CC (1995) Separation of protactinium from geochemical materials via extraction chromatography. Radioact Radiochem 6 22-32... 

Because the proportion of protactinium to its ores is of the magnitude of one part in ten milhon, it takes many truckloads of ore to extract a small quantity of the metal. About 30 years ago, approximately 125 grams of protactinium was extracted from over 60 tons of ore... 

Quantitative methods of obtaining protactinium start from the carbonate precipitate from the treatment of the acid extract of certain uranium ores. After this carbonate precipitate is dissolved, the protactinium remains 111 the silica gel residue, from the solution of which it is obtained on a manganese dioxide carrier. An alternate method effects final separation... 

The methods of piuification include the use of ion exchange resins, the precipitation of protactinium peroxide and the extraction of aqueous solutions of protactinium salts by various organic solvents. 

Protactinium can be separated from natural ore concentrates by cycles consisting of adsorption on Mn02 precipitates followed by solvent extraction of the cupferron complex with pentyl acetate.94... 

Protactinium is one of the rarest elements on Earth. It is formed when uranium and other radioactive elements break down. For many years, the only supply of protactinium of any size was kept in Great Britain. The British government had spent 500,000 to extract about 4 ounces (125 grams) of the element from about 65 short tons (60 metric tons) of radioactive waste. Relatively little is known about the properties of the element, and it has no commercial uses. 

Protactinium can be recovered from an aqueous nitrate solution of fission products and protactinium by adding sodium chromate, which brings down protactiniiun on the aluminum chromate precipitate. After dissolution of the precipitate in acid, protactinium may be recovered by solvent extraction, or it may be allowed to decay to which is more easily extracted [G4]. Protactinium can also be recovered by adsorption on powdered Vycor glass. 

Figure 10.26 compares the low-concentration distribution coefficients of uranium, thorium, plutonium, protactinium, and the principal fission products. The spread between thorium and fission-product zirconium is greatest between 1 and 2 M HNO3, the range used in the decontamination step of the acid Thorex process. Because the distribution coefficient of protactinium is close to that of thorium, it is necessary to remove protactinium or complex it with fluoride or phosphate ion to prevent its extraction with thorium. 

Protactinium as 231Pa occurs in pitchblende, but even the-richest-ores contain only about 1 part of Pa in 107. The isolation of protactinium from residues in the extraction of uranium from its minerals is difficult, as indeed is the study of protactinium chemistry generally, owing to the" extreme tendency of the compounds to hydrolyze. In aqueous solution, polymeric ionic species and colloidal particles are formed, and these are carried on precipitates and adsorbed on vessels in solutions other than those containing appreciable amounts of mineral acids or complexing agents or ions such as F , the difficulties are almost insuperable. 

To obtain significant quantities of protactinium, a separation procedure was developed for extracting protactinium from the sludge that was left after the ether extraction of uranium at the Springfields refinery. The process yielded 127 g of pure Pa from 60 tons of sludge. Protactinium metal can be obtained by reducing Pap4 with... 

---