Uranyl sulfate formation

The incorporation of anions, as for example, S04 , CO2-, etc., makes leaching possible through the formation of stable uranyl (VI) oxyanions. In sulfate leaching, an observation of the potential-pH diagram for the uranium system reveals that uranium species in solution may be in the form of cations U02+, neutral species U02(S04)2 or anions U02(S04)4-. The oxidation of uraninite, U02, in acid solutions, transforming U(IV) to U(VI), yields soluble uranyl sulfate through the reaction as shown below ... 

Table 7 Thermodynamic Data" relating to the Formation of Uranyl Sulfate Complexes77...

As can be derived from the schemes (26) and (27), the reactions of complex formation will take place in the direction XVIII XVII XVI I V—>XIX—>XX. In the end, they must lead to the formation of the [1102(804)2(1120)] complex, which is the most stable and the most abundant complex in uranyl sulfates. It is of interest that it was a uranyl disulfate K2[U02(S04)2(H20)].H20, a complex of the XX t5 e, which was used by Becquerel in his discovery of radioactivity. In the structures of the XX complexes, the U atoms have CN = 7. We note that, according to the scheme (27), the most stable complexes should the electroneutral complexes XIX for which ANe = 0.25 and CNu = 7. Indeed, there are some data in the literature that provide some evidence concerning the existence of such complexes in crystals. 

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. 

Uranium(VI) is present in solution as the uranyl sulfate anion. The extraction of uranium(VI) by amines occurs in the order of tertiary > secondary > primary amine. The extraction of iron(Ill) occurs in the reverse order, so tertiary amines represent an obvious choice of extractant The tertiary alkyl amine sold as Alamine 336 or Armeen 380 is widely nsed, nsnaUy in conjunction with an alcohol phase modifier (such as isodecanol) to prevent the formation of a third phase and inhibit the formation of emulsions. 

Generally, the higher the concentration of uranyl sulfate or free acid in solution, the greater the extent of metal dissolution during film formation l)clow the critical velocity, the lower the critical velocity, and the higher the film-free corrosion rate above the critical velocity. Table 5-7 shows how the above three regions change with uranyl-sulfate concentrations at 250°C. 

The austenitic stainless steels and most other metals and alloys of practical importance in large-scale homogeneous reactors are thermodynamically unstable in aqueous solutions and depend on protective films for their corrosion resistance. Tortunately, when austenitic stainless steels are oxidized in high-temperature uranyl sulfate solutions, the steel oxidizes uniformly so that no element (or elements) is leached preferentially from the alloy. However, not all the alloying elements contribute to film formation. 

The reason why the addition of relatively large quantities of inert sulfate salts to uranyl sulfate solutions reduces the corrosiveness of the resulting solutions is not known, but may be due to one of a number of factors, such as the formation of stable complexes, reduction of acidity, changes in oxidizing power, increased viscosity and densit or changes in colloidal properties of the oxide. 

Uranyl solutions are easily prepared by dissolution of water-soluble salts the nitrate, fluoride, chloride, bromide, iodide, sulfate, and acetate. Other water-soluble uranyl salts Include those of other organic acids the formate, propionate, butyrate, emd valerate and certain double salts such as potassliun uranyl sulfate, sodium uranyl carbonate, sodium uranyl chromate, etc. Uranyl solutions may be prepared also by dissolution of a uranium(VI) compound in an appropriate solvent, by dissolution of a lower valence uranium compound in an oxidizing medium, or by oxidation of lower valence uranium ions already in solution. Uranyl solutions are yellow in color. They are the most stable of uranlvim solutions. As indicated in preceding paragraphs, the... 

The leaching reactions and the distribution of the various anionic uranyl species are very dependent on the pH value of the leach liquor and on the sulfate or carbonate concentration. Nominally, only the anionic di- and tri-sulfate or carbonate species will exchange with the functional groups of an anion-exchange resin, but the resin itself can facilitate the formation of complex anions in the resin phase because of the high concentration (approximately 0.5 M) of the co-ion on the functional group. Therefore, a complex equilibrium is established in which the resin is a participant the following reactions describe these equilibria for sulfuric acid leach liquors ... 

Indirect determinations of several types can be carried out. Sulfate has been determined by adding an excess of standard barium(II) solution and back-titrating the excess. By titrating the cations in moderately soluble precipitates, other ions can be determined indirectly. Thus sodium has been determined by titration of zinc in sodium zinc uranyl acetate, and phosphate by determination of magnesium in magnesium ammonium phosphate. Quantitative formation of tetracyano nickel-ate(II) has been used for the indirect determination of cyanide. ... 

Oxoanions promote the formation of dimeric or polymeric products. The local coordination environment in these species is that of pentagonal bipyramidal uranyl groups with two neutral ligands and three 0x0 groups from two different bridging sulfate, chromate, or acetate groups. " ... 

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