Triuranium octoxide
| Names | |
|---|---|
| Other names
Uranium(V,VI) oxide
Pitchblende C.I. 77919 | |
| Identifiers | |
3D model (JSmol)
|
|
| ChemSpider | |
| ECHA InfoCard | Lua error in Module:Wikidata at line 880: attempt to index field 'wikibase' (a nil value). Lua error in Module:Wikidata at line 880: attempt to index field 'wikibase' (a nil value).Lua error in Module:EditAtWikidata at line 29: attempt to index field 'wikibase' (a nil value). |
| EC Number |
|
| E number | Lua error in Module:Wikidata at line 880: attempt to index field 'wikibase' (a nil value). |
PubChem CID
|
|
CompTox Dashboard (EPA)
|
|
| |
| |
| Properties | |
| U3O8 | |
| Molar mass | 842.08 g/mol |
| Density | 8.38 g/cm3[1] |
| Melting point | 1,150 °C (2,100 °F; 1,420 K) |
| Boiling point | decomposes to UO2 at 1,300 °C (2,370 °F; 1,570 K) |
| Insoluble[2] | |
| Solubility | Soluble in nitric acid and sulfuric acid[2] |
| Thermochemistry | |
Std molar
entropy (S⦵298) |
282 J·mol−1·K−1[3] |
Std enthalpy of
formation (ΔfH⦵298) |
−3575 kJ·mol−1[3] |
| Hazards | |
| GHS labelling: | |
| GHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard | |
| Danger | |
| H300, H330, H373, H411 | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
| |
Triuranium octoxide (U3O8)[4] is a compound of uranium. It is present as an olive green to black, odorless solid. It is one of the more popular forms of yellowcake and is shipped between mills and refineries in this form.
U3O8 has potential long-term stability in a geologic environment.[5] In the presence of oxygen (O2), uranium dioxide (UO2) is oxidized to U3O8, whereas uranium trioxide (UO3) loses oxygen at temperatures above 500 °C and is reduced to U3O8.[6][7][8] The compound can be produced by the calcination of ammonium diuranate or ammonium uranyl carbonate.[9] Due to its high stability, it can be used for the disposal of depleted uranium.[10] Its particle density is 8.38 g cm−3.
Triuranium octoxide is converted to uranium hexafluoride for the purpose of uranium enrichment.
Production
[edit | edit source]Triuranium octoxide is produced industrially by the calcination of ammonium uranyl carbonate or ammonium diuranate.[9] The ammonium uranyl carbonate (AUC) method is as follows:[11]
Uranium hexafluoride is hydrolyzed in water to form uranyl fluoride...
- UF6(g) + 2 H2O(l) → UO2F2(aq) + 4 HF(aq)
... which is then precipitated with ammonium carbonate:
- UO2F2(aq) + 3 (NH4)2CO3 → (NH4)4UO2(CO3)3 + 2 NH4F
The resulting ammonium uranyl carbonate is left to dry and then heated in air:
- 3 (NH4)4UO2(CO3)3 → U3O8 + 4 NH3 + 5 CO2 + 2 H2O + ½ O2
Formation
[edit | edit source]Triuranium octoxide is formed by the multi-step oxidation of uranium dioxide by oxygen gas at around 250°C:[7]
- 8 UO2 + O2 → 2 U4O9
- 6 U4O9 + O2 → 8 U3O7
- 2 U3O7 + O2 → 2 U3O8
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):[8][9][12][13]
- 3 UO3 → U3O8 + 1/2 O2
Uranium trioxide can be reduced by other methods, such as reaction with reducing agents like hydrogen gas at around 500°C−700°C:[12][13]
- 3 UO3 + H2 → U3O8 + H2O
This process can produce other uranium oxides, such as U4O9 and UO2.[13]
Chemical properties
[edit | edit source]Oxidation state
[edit | edit source]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.[14]
The analysis that U3O8 contains 2 UV and 1 UVI is supported by other studies.[15]
Reactions
[edit | edit source]Triuranium octoxide can be reduced to uranium dioxide through reduction with hydrogen:[12][13]
- U3O8 + 2 H2 → UO2 + 2 H2O
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.[16]
Triuranium octoxide is slowly oxidized to uranium trioxide under high pressures of oxygen:[16]
- U3O8 + 1/2 O2 → 3 UO3
Triuranium octoxide is attacked by hydrofluoric acid at 250 °C to form uranyl fluoride:[17]
- U3O8 + 6 HF + 1/2 O2 → 3 UO2F2 + 3 H2O
Triuranium octoxide can also be attacked by a solution of hydrochloric acid and hydrogen peroxide to form uranyl chloride.[18]
Structure
[edit | edit source]Triuranium octoxide has multiple polymorphs, including α-U3O8, β-U3O8, γ-U3O8, and a non-stoichiometric high-pressure phase with the fluorite structure.[6][16][19]
Alpha
[edit | edit source]
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 P62m.[6][16][19]
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.[6]
Beta
[edit | edit source]
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.[6]
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.[6]
Gamma
[edit | edit source]γ-U3O8 is formed at around 200-300 °C and at 16,000 atmospheres of pressure.[16] Very little information on it is available.
Fluorite-type
[edit | edit source]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.[19]
This phase has a cubic structure with a high amount of defects. Its formula is UO2+x, where x ≈ 0.8.[19]
Natural occurrence
[edit | edit source]Triuranium octoxide can be found in small quantities (~0.01-0.05%) in the mineral pitchblende.[20]
Uses
[edit | edit source]Production of uranium hexafluoride
[edit | edit source]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.[21][22]
- U3O8 + HNO3 → UO2(NO3)2 → (heating) UO3
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.[21][22]
- U3O8 + 2 H2 → 3 UO2 + 2 H2O
- UO3 + H2 → UO2 + H2O
- UO2 + 4 HF → UF4 + 2 H2O
- UF4 + F2 → UF6
As a reference material
[edit | edit source]Triuranium octoxide is a certified reference material and can be used to determine the impurity of a sample of uranium.[2][23]
Hazards
[edit | edit source]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.[2]
References
[edit | edit source]- ^ WebElements, https://www.webelements.com
- ^ a b c d NBL Program Office, "Safety Data Sheet: Uranium Oxide (U3O8)", https://www.energy.gov/nnsa/articles/sds-uranium-oxide-u3o8
- ^ a b Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b c d e f Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b c Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ United States Nuclear Regulatory Commission, "Frequently Asked Questions about Depleted Uranium Deconversion Facilities", https://www.nrc.gov/materials/fuel-cycle-fac/ur-deconversion/faq-depleted-ur-decon.html
- ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b c Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b c d Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b c d e Cordfunke, E. H. P. The Chemistry of Uranium.
- ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b c d Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ Asghar, Fiaz & Sun, Zhanxue & Chen, Gongxin & Zhou, Yipeng & Li, Guangrong & Liu, Haiyan & Zhao, Kai. (2020). Geochemical Characteristics and Uranium Neutral Leaching through a CO2 + O2 System—An Example from Uranium Ore of the ELZPA Ore Deposit in Pakistan. Metals. 10. 1616. 10.3390/met10121616.
- ^ a b Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ NBL Program Office, "Certificate of Analysis: Certified Reference Material C123 (1-7) Uranium (U3O8) 18 Element Impurity Standard in Powder Form", https://www.energy.gov/nnsa/articles/nbl-program-office-certificate-analysis-certified-reference-material-c123-1-7-uranium