Triuranium octoxide

Production

Triuranium octoxide is produced industrially by the calcination of ammonium uranyl carbonate or ammonium diuranate. The ammonium uranyl carbonate (AUC) method is as follows:

Uranium hexafluoride is hydrolyzed in water to form uranyl fluoride...

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... which is then precipitated with ammonium carbonate:

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The resulting ammonium uranyl carbonate is left to dry and then heated in air:

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Formation

Triuranium octoxide is formed by the multi-step oxidation of uranium dioxide by oxygen gas at around 250°C:

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It can also be formed from the reduction of compounds like ammonium uranyl carbonate, ammonium diuranate, and uranium trioxide through calcination at high temperatures (~600°C for (NH4)2U2O7, 700°C for UO3):

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Uranium trioxide can be reduced by other methods, such as reaction with reducing agents like hydrogen gas at around 500°C−700°C:

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This process can produce other uranium oxides, such as U4O9 and UO2.

Chemical properties

Oxidation state

While many studies have shown contradicting results on the oxidation state of uranium in U3O8, a study on its absorption spectrum determined that each formula unit of U3O8 contains 2 UV atoms and 1 UVI atom, without any atoms of UIV. The study used the compounds uranium dioxide and uranyl acetylacetonate as references for the spectra of UIV and UVI, respectively.

The analysis that U3O8 contains 2 UV and 1 UVI is supported by other studies.

Reactions

Triuranium octoxide can be reduced to uranium dioxide through reduction with hydrogen:

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Triuranium octoxide also loses oxygen to form a non-stoichiometric compound (U3O8-z) at high temperatures (800°C), but recovers it when reverted to normal temperatures.

Triuranium octoxide is slowly oxidized to uranium trioxide under high pressures of oxygen:

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Triuranium octoxide is attacked by hydrofluoric acid at 250 °C to form uranyl fluoride:

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Triuranium octoxide can also be attacked by a solution of hydrochloric acid and hydrogen peroxide to form uranyl chloride.

Structure

Triuranium octoxide has multiple polymorphs, including α-U3O8, β-U3O8, γ-U3O8, and a non-stoichiometric high-pressure phase with the fluorite structure.

Alpha

α-U3O8 is the most commonly encountered polymorph of triuranium octoxide, being the most stable under standard conditions. At room temperature, it has an orthorhombic pseudo-hexagonal structure, with lattice constants a=6.72Å, b=11.97Å, c=4.15Å and space group Amm2. At higher temperatures (~350 °C), it transitions into a true hexagonal structure, with space group P2m.

α-U3O8 is made up of layers of uranium and oxygen atoms. Each layer has the same U-O structure, and oxygen bridges connect corresponding uranium atoms in different layers. Within each layer, the U sites are surrounded by five oxygen atoms. This means that each U atom is bonded to seven oxygen atoms total, giving U a coordination geometry of pentagonal bipyramidal.

Beta

β-U3O8 can be formed by heating α-U3O8 to 1350 °C and slowly cooling. The structure of β-U3O8 is similar to that of α-U3O8, having a similar sheet-like arrangement and similar lattice constants (a=7.07Å, b=11.45Å, c=8.30Å [c/2=4.15Å]). It also has an orthorhombic cell, with space group Cmcm.

Like α-U3O8, β-U3O8 has a layered structure containing uranium and oxygen atoms, but unlike α-U3O8, adjacent layers have a different structure- instead, every other layer has the same arrangement of U and O atoms. It also features oxygen bridges between U and O atoms in adjacent layers, though instead of all U atoms having a geometry of pentagonal bipyramidal, 2 U atoms per formula unit have distinct pentagonal bipyramidal coordination geometries, and the other U atom has a coordination geometry of tetragonal bipyramidal.

Gamma

γ-U3O8 is formed at around 200-300 °C and at 16,000 atmospheres of pressure. Very little information on it is available.

Fluorite-type

A high-pressure phase of U3O8 with a hyperstoichiometric fluorite-type structure is formed at pressures greater than 8.1 GPa. During the phase transition, the volume of the solid decreases by more than 20%. The high-pressure phase is stable under ambient conditions, in which it is 28% denser than α-U3O8.

This phase has a cubic structure with a high amount of defects. Its formula is UO2+x, where x ≈ 0.8.

Natural occurrence

Triuranium octoxide can be found in small quantities (~0.01-0.05%) in the mineral pitchblende.

Uses

Production of uranium hexafluoride

Triuranium octoxide can be used to produce uranium hexafluoride, which is used for the enrichment of uranium in the nuclear fuel cycle. In the so-called 'dry' process, common in the United States, triuranium octoxide is purified through calcination, then crushed. Another process, called the 'wet' process, common outside the U.S., involves dissolving U3O8 in nitric acid to form uranyl nitrate, followed by calcining to uranium trioxide in a fluidized bed reactor.

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No matter which method is used, the uranium oxide is then reduced using hydrogen gas to form uranium dioxide, which is then reacted with hydrofluoric acid to form uranium tetrafluoride and then with fluorine gas to produce uranium hexafluoride. This can then be separated into uranium-235 and uranium-238 hexafluoride.

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As a reference material

Triuranium octoxide is a certified reference material and can be used to determine the impurity of a sample of uranium.

Hazards

Triuranium octoxide is a carcinogen and is toxic by inhalation and ingestion with repeated exposure. If consumed, it targets the kidney, liver, lungs, and brain, and causes irritation upon contact with the skin and eyes. It should only be handled with adequate ventilation. In addition, it is also radioactive, being an alpha emitter.

References

References

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