Uranium oxides and uranates

URANIUM OXIDES AND URANATES

The uranium oxides, which are thermodynamically most stable, play an important part in the production technology of uranium and in nuclear power. Five uranium oxides are known : The monoxide UO, the dioxide UO₂, the triuranium octoxide U₃O₈, the trioxide UO₃, and the peroxide UO₄.2H₂O, All these compound, except for the monoxide, are of practical importance.

Uranium dioxide, UO₂

The preparation of uranium dioxide may be effected in one of two waye :

• Reduction of higher oxides by gaseous reducing agents at elevated temperature.

UO₃ + H₂ → UO₂ + H₂O

1/3U₃O₈ + 2/3 H₂ → UO₂ + 2/3 H₂O

UO₃ + 2/3 NH₃ → UO₂ + 1/3 N₂ + H₂O

1/3 U₃O₈ + 4/9NH₃ → UO₂ + 2/9 N₂ + 2/3 H₂O

• Thermal decomposotion of uranium salts in the absence of air.

UO₂C₂O_{4 .}.→ UO₃ + CO + CO₂

UO₃ + CO → UO₂ + CO₂

(NH₄)₄ UO₂(CO₃)₃ → UO₃ + 3CO₂ + 4 NH₃ + 2H₂)

(NH₄)₂U₂O₇ → 2UO₃ + 2NH₃ + H₂O

3UO₃ → U₃O₈ + 1/2O₂

2NH₃ → N₂ + 3H₂

U₃O₈ + 2H₂ → 3UO₂ + 2H₂O

Triuranium octoxide. U₃O₈

Triuranium octoxide. U₃O₈is obtained by the oxidation of metallic uranium in air or by roasting of higher and lower oxides and salt of uranium in air temperature above 800 – 900 oC. Depending on the black-green or black powder. The darker sample are obtained by roasting at 900-1000 oC. U₃O₈ has a rhombic structure, its density calculated from X-ray diffraction data is 8.39 g/cm3, while the experimental density has been reported to vary between 6.97 and 8.34 g/cm3. At temperature above 100 oC, U₃O₈ decomposes with the formation of uranium dioxide. U₃O₈ may be prepared by thermal dissociation of ammonium diuranate in air 800 – 900 oC.

9(NH₄)₂U₂O₇ → 6U₃O₈ + 14 NH₃ + 15H₂O + 2N₂

Uranium trioxide
Uranium trioxide is one of the intermediate products in the manufacture of pure uranium salt. The uranium trioxide is obtained by igniting uranium peroxide at 380-400 oC, by decomposition in air of uranyl oxalate, ammonium diuranate, ammonium uranyl tricarbonate and other salt at 300-400 oC.

Reaction :

UO₂(NO₃)₂ → UO₃ + N₂O₃

(NH₄)₂U₂O₇ → 2UO₃ + 2NH₃ + H₂O

UO42H2O → UO₃ +2H₂O + ½ O₂

The density of uranium trioxide calculated from X-ray diffraction data is 8.34 g/cm3, while the experimental density has been reported to vary between 5.92 and 7.51 g/cm3.

Uranium peroxide
Uranium peroxide is one of the important compounds wich are used technology far the ultimate purification of uranium. The peroxide is obtained by acting with an axcess of hydrogen peroxide on an aqueous solution of uranyl nitrate, with stirring. As a result uranium peroxide is precipitated

UO₂(NO₃)₂ +H₂O₂ + 2H₂O → UO₄.2H₂O + 2HNO₃

This compound is not known in the anhydrous state. During the dehydration oxygen is lost as well as water :

UO₄.2H₂O → UO₂ + ½ O₂ + 2H₂O

Uranium peroxide is only sparingly soluble in acid media and only salt.

Uranates

Salt of monouranic and polyuranic acid are called urinates. Mono uranic acid H₂UO₄ has been isolated in a pure state. The salt of this acid have the general formula R₂UO₄, where R is a monovalent metal cation. Monouranates are formed in the reaction of uranium trioxide with oxides of aluminium, manganese, lead, copper, vanadium, barium, calcium, iron and magnesium. Oxides of beryllium, lanthanum, cerium, and moliybdenum MoO₂ do not react with uranium trioxide up to 600 ^oC. All urinates are insoluble in water but are readily soluble in acids, and react with alkali carbonates with the formation of readily soluble complex compounds of hexavalent uranium. When aqueous solutions of uranyl salts react with bases, yellow-orange precipitates are formed which were for a long time assumed to be diuranates.

Literature

Galkin N.P.,Sudarikov B.N.,”Technology of Uranium”,Moskva, 1964