Other Uranyl Compounds

Uranyl ions, UO, play an important role in the extraction and concentration of uranium-containing minerals and in the purification of uranium compounds. 

Strong oxidizing acids, like nitric acid, produce stable hexavalent nranyl ions that can be extracted by suitable complex forming agents like tribntylphosphate (TBP). Uranyl compounds are generally very soluble in aqueous solutions and concentrations of several hundred grams of uranium per liter are quite common, like uranyl nitrate with solubility of 660 g L . Uranyl acetate, uranyl sulfate, uranyl chloride, and uranyl phosphate are all yellow salts and usually appear as hydrates. Uranyl fluoride, the product of hydrolysis of UFg, was discussed earlier. 

Uranyl carbonate complexes, like sodium uranyl tricarbonate, Na4[U02(C03 3], that is obtained when uranium ore is leached with sodium carbonate solutions and ammonium uranyl carbonate (AUC), (NH4)4[U02(C03)3l, that is used to precipitate the uranium in the UCF, are important in the NFC. These carbonates serve to purify the uranium from several metals (like Fe, Al, Cr, Ni, and other metals) that are precipitated as hydroxides or oxycarbonates, as well as aUcaline-earth elements. These purification methods utilize the effect of the ammonium carbonate concentration on the solubility of uranium. Upon heating of AUC to 300°C-500°C, it decomposes to UO3, ammonia, CO2, and water and at temperatures of 700°C-800°C, without air, UO2 may be formed (the ammonia serves as the reducing agent). The solubility of AUC decreases markedly in the presence of ammonium carbonate, for example, from 119.3 g L at 50°C without ammonium carbonate to 0.5 g L with 35% ammonium carbonate (Galkin 1966). The carbonate complexes also play a role in biological systems and affect clearance by the blood after exposure to uranium compounds. 

Uranyl sulfate usually appears as a lemon-yellow trihydrate (UO2SO4 3H2O) with a density of 3.28 g cm and is very soluble in 5 parts of water and 25 parts of alcohol. In geochemistry, oxidation of snlfldes would lead to formation of the sulfate, mainly in an acidic environment where carbonates are not present and cause precipitation of uranium. Uranyl sulfate plays a major role in ore processing as it is readily absorbed on anion-exchange resins and may be extracted with amines. As the uranyl sulfate is very stable, the solutions can be heated to elevated temperatures that help dissolve difficult to digest ores. 

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Nevertheless, various other uranium compounds may possibly exemplify all the known classes of satellites. We already mentioned that certain uranyl salts show shoulders of the U4/5/2 and U4/7/2 toward 3 eV higher I, which seem to be electron transfer satellites to the empty 5/ shell in analogy to the satellites char-... 

The ground state of the uranyl ion has a closed-shell electron configuration. There is a characteristic absorption 25 000 cm (400 nm) which frequently gives uranyl compounds a yellow colour (though other colours like orange and red are not infrequent). This absorption band often exhibits fine structure due to progressions in symmetric 0=U=0 vibrations in the excited state, sometimes very well resolved, sometimes not (Figures 12.1 and 12.2). 

In a much earlier patent, the removal of organics from exhaust gases by oxidation over a supported uranium oxide catalyst was reported by Hofer and Anderson [39]. The catalyst was 4% U3O8 supported on alumina spheres. The authors used the incipient wetness technique to impregnate alumina with uranyl nitrate solution. In this case the catalyst precursors were calcined at 700°C for 3 h to decompose the uranium salt. The use of other uranium compounds as starting materials was mentioned and these included uranyl acetate, uranium ammonium carbonate and uranyl chloride. The alumina-supported catalyst had a surface area of ca 400m g and further added components, such as copper, chromium and iron, were highlighted as efficient additives to increase activity. 

Bums et al. [2] developed a detailed stmctural hierarchy for inorganic uranyl compounds, and included both minerals and synthetic phases. At that time, 180 stmctures were available for inclusion. Bums [22] expanded the stmctural hierarchy and updated it in the case of minerals only. Bums [3] further developed the entire hierarchy and included coverage of 368 mineral and S5mthetic phases. The stmctural hierarchy is based upon the linkages of those polyhedra that contain higher-valence cations. In every case this includes polyhedra with U , and often it includes one or more other t5q>es of cation polyhedra. For the purposes of the hierarchy, bonds to low-valence cations and H bonds are ignored (although these bonds are important for the stability of the entire stmcture). 

The structure of PbU02(Se03)2 contains two-dimensional uranyl selenite sheets that are substantially different than those found in other uranyl selenite compounds. The selenite ligands chelate and bridge between uranyl moeities. 

Thus, the U V ratio plays a key role in the type of polymerization and on the obtained 2-D or 3-D structural architectures. However, the role of the counter ion is not obvious. It can be suggested that, as already observed for other uranyl oxoanion compounds, the counter ions have no template effects but rather act as space fillers within the uranium vanadate substructure. 

Gentry et al. (1976) have recently shown, on the basis of isotopic analyses, that uranium introduction may have occurred far more recently than was previously supposed. However, they find also that, in some instances, the uranium was introduced before coalification was complete since the haloes have been compressed with the coal as it increased in rank. These results are consistent with laboratory and field work by Szalay (1964) who showed that the insoluble humic acids in peat are capable of concentrating uranium from very dilute solutions in natural waters. Sorption occurs as uranyl humate , the process following the normal kinetics of the Langmuir adsorption equation. (Where uraninite occurs in association with peat or other carbonaceous matter, the uranium may thus have been initially sorbed as a uranyl compound which was later reduced to uraninite.)... 

This reaction was catalysed by Micrococcus latilyticus which also catalyses a number of other reduction processes. It appears to be a process which could well accompany or follow the sorption of uranyl compounds by humic matter referred to above. 

So far as is known, there is no biological component in the processes which lead to the formation of deposits such as Yeelirrie in ceilcrete (Dall Ag-lio et al., 1974). In such deposits the uranium is present in uranyl compounds, the precipitation of which appears to depend on the solubility relationships of uranyl and other ions, including complexes containing vanadium, in waters of varying composition carbonate concentration appears to have been especially important. 

The structures of the metal, hydrides, and carbides are described in other chapters, as also are the halides MX3, MX4, and MX5. Here we devote sections to certain halide structures peculiar to U, complex fluorides of Th and U, oxides of U, uranyl compounds and uranates, nitrides and related compounds, and conclude with a note on the sulphides of U, Th, and Ce. 

A variety of molecular patterns occur in uranyl oxalate compounds. Apart from the simple uranyl oxalate trihydrate, four other oxalate compounds containing the uranyl ion have been synthesized and their structures investigated. 

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