Thorium distribution coefficient

Because of the high acidity and high sulfate and phosphate content of sulfuric acid monazite leach solutions, distribution coefficients with primary and secondary amines are lower than in Table 6.18. In monazite sulfate solutions, thorium distribution coefficients with the primary amines of Table 6.18 are still greater than 500, however. The coefficient with di(tridecyl)amine is 4.6. These are still high enough for practical processes [C5]. 

Thorium(IV), distribution coefficients between aluminum, silicon, and... 

Anion-exchange distribution coefficients for most metallic elements in sulfuric acid solution have been measured [28, 29], Uranium(Vl), thorium(IV), molybdenum(VI). and a few other elements are retained selectively from such solutions. Thorium(lV) is taken up selectively by anion-exchangers from approximately 6 M nitric acid [30]. 

When the distribution equilibrium reaction involves hydrogen ions, changing the hydrogen ion concentration will have a strong effect on the distribution coefficient. An example of this is the extraction of metal complexes of acetyl acetone (HAa) and other weakly acid complexing agents by benzene. The equilibrium reaction for extraction of thorium by this reagent is... 

Although TBP is a relatively stable organic compound, it does undergo slow hydrolysis to form di-n-butyl phosphate (DBP). Although the presence of DBP increases the distribution coefficients of uranium, plutonium, and other actinides, it interferes with the separation of plutonium from uranium, and it makes complete stripping of these elements difficult. DBP forms an insoluble compound with thorium. DBP formation is appreciable only when the... 

Attempts to separate thorium and uranium from sulfuric acid solution of monazite by solvent extraction with TBP were unsuccessful because distribution coefficients of uranium and thorium from monazite solutions were too low, as these elements are complexed by phosphate ion. Development of extractants with higher distribution coefficients for these metals has made solvent extraction a practical process for recovering uranium and thorium from monazite sulfate solutions and from sulfuric acid solutions of other thorium ores. This section describes processes tested on a pilot-plant scale by Oak Ridge National Laboratory [C5]. 

Table 6.18 summarizes distribution coefficients of hexavalent uranium, thorium, and trivalent cerium (representative of rare earths) for four different types of long-chain amines, in sulfate solution with phosphate ion absent. Primary amines have the highest coefficient for thorium and the lowest for uranium, with the converse true of tertiary amines such as were cited for uranium extraction in Chap. 5. Secondary amines extract both metals, with thorium extraction favored by branching distant from the nitrogen. Either primary or secondary amines provide good separation of thorium from cerium. 

Table 6.18 Distribution coefficients for uranium, thorium, and cerium between organic amines and aqueous sulfate solution"...

Table 6.19 Distribution coefficients for uranium between oiganic amines and aqueous monazite sulfate solution after extraction of thorium ...

Table 6.19 lists distribution coefficients for amines considered [C5] for extracting uranium from monazite sulfate solutions after removal of thorium. Except for Primene JM, all coefficients were judged [C5] to be large enough and sufficiently greater than those of the rare earths to provide efficient solvent extraction separation of uranium. 

The distribution coefficients of Pa(V) between nitric acid solutions and solvents containing TBP are less than those of uranium [C6], and are less than those of thorium except at high concentrations of HNO3 [H2]. In extraction measurements it is found that the fraction of Pa(V) that can be extracted decreases with time, due evidently to the slow polymerization of Pa(V) colloids. The more highly condensed forms caimot be depolymetized by acid treatment [K2]. 

As in the Purex process, the Thorex process uses a solution of TBP in hydrocarbon diluent to extract the desired elements, uranium and thorium, from an aqueous solution of nitrates. Thorium nitrate however, has a much lower distribution coefficient between an aqueous solution and TBP than uranium or plutonium. To drive thorium into the TBP, the Thorex process as first developed at the Knolls Atomic Power Laboratory [HI] and the Oak Ridge National Laboratory [G14] added aluminum nitrate to the thorium nitrate in dissolved fuel. This had the disadvantage of increasing the bulk of the high-level wastes, which then contained almost as many moles of metallic elements as the original fuel. To reduce the metal content of the waste, the Oak Ridge National Laboratory in the late 1950s [Rl, R2] developed the acid Thorex process, in which nitric acid is substituted for most of the aluminum nitrate in the first extraction section. The nitric acid is later evaporated from the wastes, as in the Purex process. 

VI] for equilibria at 30°C. Distribution coefficients agree with measurements of Siddall [S13] and Weinberger et al. [W6] except for thorium at aqueous concentrations below 0.06 Af within the dashed line, where the code predicts lower values than observed. 

Adequate data on distribution coefficients of uranyl nitrate between 30 v/o TBP and aqueous solutions of thorium nitrate and nitric acid are not available. Examination of concentrations of coexistent phases in Thorex process mixer-settler runs reported in references [Rll], [01], and [02] indicate that the distribution coefficient of uranium Dy when present at uranium concentrations below 0.02 M in Thorex systems at thorium concentrations above 0.1 Mis given approximately by... 

However, column separation performance in the Hanford Thorex campaign correlates better with a Z)u/ Th rstio of 14 (Prob. 10.5). Because the distribution coefficient of thorium is so much less than that of uranium, the Thorex process requires a much higher organic/aqueous flow ratio than the Purex process. 

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. 

Figure 10.24 Distribution coefficient of thorium nitrate between 30 v/o TBP in hydrocarbon diluent and aqueous nitric acid at 30°C, from SEPHIS code.

Solvent-soluble impurities are often reduced by a factor of at least 1(H (i.e. a decontamination factor of 1(H) provided their distribution coefficients are widely different from that of the main solute. Where the impurity is completely insoluble in the solvent, a small proportion may still pass with the principal solute by virtue of physical entrainment of tiny aqueous drops in the solvent phase, but decontamination factors of 10 or more have been obtained with some systems, e.g. for the separation of rare earth elements from thorium. 

Since all sources of thorium are associated with a certain amount of uranium, the first solvent-extraction cycle is designed solely to eliminate this element. The proportion of uranium to thorium can vary, but is often of the order of 5 per cent. The uranium distribution coefficient into tributyl phosphate is much higher than that of thorium under comparable conditions, e.g. 20 and 0-5 respectively for 40 per cent TBP/xylene or 6 and 0 04 respectively for 5 per cent TBP/xylene. The more highly diluted solvent is used for the uranium separation cycle in view of the higher ratio of the two distribution coefficients, or separation factor . Xylene is chosen in preference to, for example, odourless kerosene owing to the danger of formation of a third phase rich in thorium with the latter diluent. 

Several attempts have been made in order to correlate the values to physico-chemical properties of soils however, not much success has been accomplished. Carlon et al. [26] reported a correlation between pH and soil-water distribution coefficient (K ) for Pb log = 1.99-1- 0.42pH. The EPA [19] collected values for cadmium, cesium, chromium, lead, plutonium, radon, strontium, thorium, tritium and uranium in soils. The variability in values can be many orders of magnitude as shown in Table 2. 

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