Uranous sulphate

According to Aloy, a black crystalline hydrate, of composition UO2.2H2O, is obtained when crystallised uranous sulphate is treated with hot potassium or sodium hydroxide solution. The product, when thoroughly w ashed, remains stable in air for several days. It dissolves in dilute acids, yielding uranous salts. On heating, it is completely converted to the green oxide. 

Uranium Sulphate, U(S0j)3.nH20.—The anhydrous uranous sulphate has not been prepared, but an extraordinarily large number of hydrates is known. Salts containing 1, 2, 3, 4, 5, 6, 7, 8, and 9 molecules of w-ater have been described, and of these the di-, tetra-, octa-, and nona-hydrates are stable. The actual relation of these hydrates to one another is difficult to determine, as they undergo hydrolysis to a considerable extent when in solution, and the tetra-and octa-hydrates, at least, show a marked tendency to remain in a metastable condition at temperatures far removed from the transition-point between the two phases. Solubility determinations indicate that this transition-point is in the neighbourhood of 20° C., but it has been shown that the octahydrate wffien heated in absence of air changes into the tetrahydrate at 68° to 87° C. w hen the former hydrate is in a state of metastable equilibrium. 

Uranous sulphate, even in acid solutions, is a strong reducing agent and can precipitate silver and gold from solutions of their salts. The sulphate is readily oxidised in solution by atmospheric oxygen. Both these reactions are accelerated by the presence of catalysts, especially copper salts, and in less degree platinum black or traces of iron salts. -... 

Fig. 7.—Solubility of the tetra- and oota-hydrates of uranous sulphate.

Acid uranous sulphates have been described, e.g. 112(804)4.112804, 11(804)2.112804.101120, and 1111(804)2. The latter, which separates in dark brown leaflets when a solution of uranium trichloride is added to concentrated sulphuric acid at 0° C., may contain trivalent uranium. 

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... 

For the treatment of camotite several methods are available. The method recommended by the United States Bureau of Mines2 is as follows The ore is leached with concentrated nitric acid at 100° C., neutralised with caustic soda, and barium chloride and sulphuric acid added to the solution to precipitate the radium as barium-radium sulphate. The precipitate settles in three or four days, after which time the clear liquid is decanted into tanks and is treated with excess of boiling sodium carbonate solution in order to precipitate any iron, aluminium and chromium present. The solution now contains sodium uranyl carbonate and sodium vanadate. It is nearly neutralised with nitric acid, and caustic soda is added in sufficient quantity to precipitate the uranium as sodium uranate. After filtering, the remaining solution is neutralised with nitric acid and ferrous sulphate added, whereupon iron vanadate is thrown down. By this method it is claimed that 90 per cent, of the radium, all the uranium, and 50 per cent, of the vanadium in the camotite are recovered. 

Uranium, in the uranous compounds in which the element is tetra-valent, appears to be closely allied to thorium, the terminal member of Subgroup IVa. The dioxide, UOg, is isomorphous with thorium oxide, ThOj the sulphates are also isomorphous and are only slightly soluble in water. The oxalates of the two elements are highly insoluble. Most uranous salts are readily soluble. Many other salts of tetravalent uranium and thorium are isomorphous nevertheless the two elements exhibit very considerable differences and can easily be separated. ... 

It is formed in solution by oxidising uranous chloride with nitric acid by dissolving uranic oxide in concentrated hydrochloric acid or by adding barium chloride to a concentrated solution of uranyl sulphate until precipitation is complete. Its aqueous solution on evaporation yields the monohydrate, UOaClj.HaO." The solution is unstable at ordinarj temperatures and slowly deposits uranic hydroxide, which after a time partly redissolves. The uranyl chloride may be reduced in solution to black uranous oxide by the action of magnesium or aluminium powder. The densities of aqueous solutions of uranyl chloride have been determined as follows ... 

Uranous oxide is only difficultly soluble in hydrochloric and sulphuric acids, even when concentrated. With the latter acid, insoluble uranium sulphate is formed. It readily dissolves, however, in dilute nitric acid forming uranyl nitrate it is also soluble in aqua regia. The amounts of the oxide dissohdng in these acids in a given time vary widely with the mode of preparation of the oxide. 

Ammonium Diuranate, (NH4)2U207, is obtained as a yellow voluminous precipitate when solutions of uranyl salts are treated with ammonia. It is prepared commercially (see p. 277) by boiling a solution of sodium uranyl carbonate with ammonium sulphate, or by boiling a solution of sodium diuranate with concentrated ammonium chloride solution. It is a deep yellow powder, which may be dried at 100° C. at higher temperatures it yields urano-uranic oxide. When fused with ammonium chloride, uranous oxide is formed. It is known commercially as uranium yellow (see also sodium diuranate) and is used in making fluorescent uranium glass. It is insoluble in ammonium hydroxide solution, and this fact is sometimes made use of (see p. 388) in the analytical separation of uranium. 

Potassium Hexa-uranate, K2U5O49.6H3O, results on strongly heating a nnxture of uranyl sulphate and potassium chloride j by... 

Sodium Triuranate, NagUgO g, is formed by heating together uranyl sulphate and sodium chloride, and washing the residue with water (see potassium hexa-uranate). It yields lustrous, golden-yellow, rhombic crystals, of density 6-9, insoluble in water, but soluble in dilute acids. 

Uranium Oxysulphide, U3O2S4 or UO3.2US2, is formed when uranous oxide, urano-uranic oxide, or ammonium uranate is heated in a stream of hydrogen sulphide or carbon disulphide vapour when one of the oxides is heated with a mixture of ammonium chloride and sulphur or when uranyl sulphate is heated in hydrogen or with potassium pentasulphide. It is a greyish-black powder, which is decomposed by nitric acid %vith deposition of sulphur. 

The hydrates are usually prepared by reduction of uranyl sulphate by means of alcohol and exposure to light, or directly from uranous compounds. 

Precipitation with ammonium sulphide is best performed at 80° C. in presence of ammonium chloride. Some ammonium uranate is always formed, and complete separation from the alkali metals is only obtained by repeating the precipitation. The results obtained by this method are liable to be high owing to some sulphide being oxidised to sulphate during ignition. ... 

It is usual to take advantage of the alkali addition to separate off ferric iron and other metallic ions which precipitate at a lower pH than uranium. Furthermore, this can often be done with lime. This is normally the cheapest alkali and also has the advantage that it precipitates the sulphate ion impurity at the same time as the iron. A pH of 3 -6 is used for this impurity precipitation, followed by the first-stage filtration. Then the addition of extra alkali, e.g. ammonia, sodium hydroxide or magnesia, to pH 6 7 precipitates the uranium. The second-stage filtration finally removes the uranium from solution as ammonium, sodium or magnesium di-uranates. 

Uranous complexes tend to be insoluble at low temperatures and at pH 4.5-7. At temperatures above 150°C uranous transport may become dominant. Depending on ligand concentrations, uranous fluoride, phosphate, sulphate and especially hydroxide compounds are important species under these conditions, but uranous carbonate complexes are not. Uranyl species are soluble over a wide range of conditions. In normal groundwater, at temperatures of 25°C, uranyl fluoride complexes are dominant at pH <4, uranyl phosphates at pH 4-7.5 and uranyl di- and tricarbonate complexes at pH >7.5. Uranyl silicate complexes are probably insignificant, and at temperatures near 100°C uranyl hydroxides predominate, whereas uranyl carbonate complexes dissociate. ... 

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