Anions uranate

How can we be sure that the U +(Q2-) complex in a mixed metal oxide is present as the UO octahedron This can be done by studying solid solution series between tungstates (tellurates, etc.) and uranates which are isomorphous and whose crystal structure is known. Illustrative examples are solid solution series with ordered perovskite structure A2BWi aUa 06 and A2BTei-a Ua 06 91). Here A and B are alkahne-earth ions. The hexavalent ions occupy octahedral positions as can be shown by infrared and Raman analysis 92, 93). Usually no accurate determinations of the crystallographic anion parameters are available, because this can only be done by neutron diffraction [see however Ref. (P4)]. Vibrational spectroscopy is then a simple tool to determine the site symmetry of the uranate complex in the lattice, if these groups do not have oxygen ions in common. In the perovskite structure this requirement is fulfilled. 

The fusion of uranium oxides with alkali or alkaline earth carbonates, or thermal decomposition of salts of the uranyl acetate anion, gives orange or yellow materials generally referred to as uranates, for example,... 

When alkali metal bases are used to raise the solution pH to moderate levels, the uranium will precipitate from the solution in the form of hydrous uranyl hydroxides or uranates, for example, Na2U207. However, through judicious choice of a base, for example, tetramethylammonium hydroxide, (TMA)OH, or tetramethylaimnoirium trifluoromethansulfonate, the study of the amphoteric behavior of uranyl hydroxides can be undertaken. Polynuclear anions of the form (U02)3(0H)7, (U02)3(0H)g, and (U02)3(OH)io are examples of soluble species in solutions where the pH < 14. When the concentration of the (TMA)OH is increased (>0.6 M OH ), highly soluble ( 0.1M) monomers ofthe form U02(0H) "(n = 3, 4,5) have been reported. These three species are in equilibrium with each other however, in solutions where the [OH ] is greater that 1M, the pentahydroxo complex predominates the speciation. 

A similar scheme is used [15, p. 312] for recovering uranium from sulfonic acid solutions. The solution containing 5 x 10" - IQ- moles per liter of uran)d sulfate at pH = 2 is treated with anion exchanger (Fig. 21). Uranium is almost completely recovered from solution by the resin. Solution containing 20 g/L of uranyl sulfate is then eluted with H2SO4. Anion exchanger in the sulfate form is transferred to column I. 

In mixed oxides, which are usually called uranates, the U(VI) atoms constitute parts of some oxoanions such as U04, UOs", 1)207, etc. In most of these anions (that, as a rule, have a polymeric structure), one can formally subdivide... 

Optimum conditions are being determined for formation of the uranate. Studies include utilizing 100% nitric acid vapor to determine uranium-plutonium separation and to investigate alternate separation methods, particularly the effects of added anions. 

Other metal oxides can also be incorporated and such ternary substances are best regarded as mixed oxides. The uranates are generally of stoichiometry M2Ua.03 c+2, but M4U05, M3U06, etc., are known. In contrast to Mo or W, there appear to be no iso- or hetero-poly anions for U in solution. A useful material obtained by addition of aqueous NH3 to U02(N03)2 solutions is the so-called ammonium diuranate. This is mainly the hydrate U02(0H)2H20. 

True. Uranic adds ccaitribute to die anionic nature of exopcdysacdiarides. 

The recently developed Excer process of the U.S.A.E.C. aims to extract uranium from low-grade ore, purify it up to nuclear specification and convert to uranium tetrafluoride ready for metal production. The process is shown in Fig. 3.15 and is based first upon a sulphuric acid leach of the ore and anion absorption of uranium from the pulp. Elution is then by 2M sulphuric acid to give a solution containing about 10 to 20 g U/1. The uranyl sulphate is then reduced by metallic iron to uranous sulphate, diluted to an acidity of 0-5M and a second cycle of anion-exchange carried out. Absorption behaves similarly to that with uranyl ion, but ferrous ion is not... 

Uranium, unlike thorium, has an accessible trivalent oxidation state. It has been shown that uranocene can be reduced by lithium naph-thalide to the bis([8]annulene)uranate(III) anion.23 The same complex can be synthesized by the reaction of two equivalents of [8]annulene dianion with one equivalent of uranium trichloride.23... 

Based on the existence of the bis([8]annulene)uranate(III) anion and the reduction of U(IV) to U(III) by [8]annulene dianion, the following mechanism has been proposed, as an alternative to simple ligand substitution, for the formation of uranocene 26... 

Table 1 List of uranyl and uranous complexes known to form with various anions...

Most uranium minerals occur in all of the several types of ore deposits. A given deposit usually has no more than two reduced minerals. The oxidized minerals that occur in the deposit depend on the Eh-pH conditions and the availability of reactive anions. In the absence of reactive anions, hydrated oxides and uranates form. The uranyl ion is, however, fairly soluble and groundwater can effectively disperse it a considerable distance from the reduced source. The uranyl minerals that are then deposited are complex compounds that employ available oxyanions. The rate of formation of these secondary minerals can be very rapid, as is evidenced by mineral formation on the walls of mine drifts in a matter of months after the drifts have been opened. In all deposits there is usually a zonation of mineralogy in which a reduced mineral... 

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