Thorium separation behavior

Uranium and thorium are actinide elements. Their chemical behavior is similar under most conditions. Both are refractory elements, both occur in nature in the +4 oxidation state, and their ionic radii are very similar (U+4 = 1.05 A, Th+4 = l.lOA). However, uranium can also exist in the +6 state as the uranyl ion (U02 2), which forms compounds that are soluble in water. Thus, under oxidizing conditions, uranium can be separated from thorium through the action of water. 

Tn reviewing the chemistry of the actinides as a group, the simplest approach is to consider each valence state separately. In the tervalent state, and such examples of the divalent state as are known, the actinides show similar chemical behavior to the lanthanides. Experimental diflB-culties with the terpositive actinides up to plutonium are considerable because of the ready oxidation of this state. Some correlation exists with the actinides in studies of the lanthanide tetrafluorides and fluoro complexes. For other compounds of the 4-valent actinides, protactinium shows almost as many similarities as dijSerences between thorium and the uranium-americium set thus investigating the complex forming properties of their halides has attracted attention. In the 5- and 6-valent states, the elements from uranium to americium show a considerable degree of chemical similarity. Protactinium (V) behaves in much the same way as these elements in the 5-valent state except for water, where its hydrolytic behavior is more reminiscent of niobium and tantalum. 

Monazite ubiquitously exhibits this type of behavior. Backscattered electron images and yttrium, thorium, and uranium X-ray maps nearly always reveal complex zonation (e.g., Parrish, 1990 DeWolf et al., 1993 Zhu et al., 1997 Zhu and O Nions, 1999 Williams et al., 1999 Pyle et al., 2001 Townsend et al., 2001 Williams and Jercinovic, 2002 see Figures 27 and 28), and several studies have demonstrated significant age differences between these chemically distinct domains (e.g., DeWolf et al., 1993 Zhu et al., 1997 Zhu and O Nions, 1999 Williams et al., 1999 Townsend et al., 2001 Figures 27 and 28). Extreme compositional and age heterogeneity implies that the analysis of a bulk mineral separate or even of a single grain is not very useful... 

Many of the actinoids are also separated by exploiting their redox behavior. Thorium is exclusively tetravalent and berkelium is chemically similar to cerium, so iodate precipitation of Th and extraction of Bk(IV) with bis(2-ethylhexyl)orthophos-phoric acid (HDEHP) are used to isolated these elements. The differing stabilities of the (III), (IV), (V), and (VI) states of U, Np, and Pu have be exploited in precipitation and solvent extraction separations of these elements from each other and from fission product and other impurities with which they are found. Because of its technical importance, the process chemistry to separate U and Pu in nuclear materials has been highly developed. Extraction of Bk(IV) with HDEHP is used to separate Bk from neighbouring elements. 

The sucessful experiments for the retention of plutonium onto alumina from TTN0 -HF solution gave enough confidence to recomend the proposed method to separate traces of plutonium from waste solutions in the presence of macroamounts of uranium (VI). Of course, only macroamounts of thorium, uranium (IV) and rare earths are serious interfering ions, since they precipitate with HF. The behavior expected for neptunium in the same system should be similar to plutonium, thorium and rare earths. The retention of neptunium from HNO - HF solutions is in progress. The sorption yield for Pu was around 95%. The sorption mechanism is not well established. Figure 3 shows the proposed flowsheet for recovery of Pu traces from reprocessing waste solutions. 

The solubilities of uranium, plutonium, and thorium in magnesium at 650°C are 0.002 wt %, 55 wt %, and 44 wt %, respectively. Thus, assuming no solute interaction, uranium is essentially insoluble in magnesium, while plutonium is quite soluble and good separation may be effected. While precipitation of an insoluble phase from solution would appear to be a straightforward process, the behavior of a solute in a given metal or alloy may differ from its behavior when influenced by the inclusion of other solutes. One element may increase or suppress the solubility of another through coprecipitation or intermetallic compound formation. Such effects must be determined experimentally. 

Water. The most common thorium salt, Th(N03)4 5H20, and the chloride are soluble (forming [ThCl2(H20) ] ). Anhydrous Th(S04)2 is soluble in ice water, but it separates as a hydrate on heating. If the solution is allowed to stand without boiling, a series of hydrates will separate, their compositions depending on conditions. This behavior can separate Th qnantitatively at 0°C from the solnble Rth sulfates. 

Considerable information concerning the low-temperature chemical behavior of Pa has accumulated as a by-product of the development of chemical processes for the separation of from irradiated thorium... 

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