Ammonium uranyl carbonate

Ammonium uranyl carbonate (UO₂CO₃·2(NH₄)₂CO₃) is known in the uranium processing industry as AUC^[2] and is also called uranyl ammonium carbonate. This compound is important as a component in the conversion process of uranium hexafluoride (UF₆) to uranium dioxide (UO₂).^[3] The ammonium uranyl carbonate is combined with steam and hydrogen at 500–600 °C to yield UO₂. In another process aqueous uranyl nitrate, known as uranyl nitrate liquor (UNL) is treated with ammonium bicarbonate to form ammonium uranyl carbonate as a solid precipitate.^[4] This is separated from the solution, dried with methanol and then calcinated with hydrogen directly to UO₂ to obtain a sinterable grade powder. The ex-AUC uranium dioxide powder is free-flowing, relatively coarse (10 μ) and porous with specific surface area in the range of 5 m²/g and suitable for direct pelletisation, avoiding the granulation step. Conversion to UO₂ is often performed as the first stage of nuclear fuel fabrication.^[5]

Ammonium uranyl carbonate

Names
IUPAC name uranium(VI)dioxide tetra-ammonium tricarbonate
Other names uranyl ammonium carbonate AUC
Identifiers
CAS Number	18077-77-5
3D model (JSmol)	Interactive image
ChemSpider	32697822
ECHA InfoCard	100.038.156
PubChem CID	76961635
UNII	5F29TO8JCF
CompTox Dashboard (EPA)	DTXSID801350072
InChI InChI=1S/3CH2O3.4H3N.2O.U/c3*2-1(3)4;;;;;;;/h3*(H2,2,3,4);4*1H3;;;/q;;;;;;;2*-2;+6/p-2 Key: XHYJUAIOADBAKQ-UHFFFAOYSA-L
SMILES C(=O)([O-])[O-].C(=O)([O-])[O-].C(=O)([O-])[O-].[NH4+].[NH4+].[NH4+].[NH4+].[O-2]=[U+6]=[O-2]
Properties
Chemical formula	UO2CO3·2(NH4)2CO3
Molar mass	522.199 g/mol
Appearance	lemon-yellow crystalline[1]
Density	2.72[1]
Melting point	Decomposes between 165 °C and 185 °C
Solubility in water	Insoluble
Structure[1]
Crystal structure	monoclinic
Space group	C2/c
Lattice constant	a = 10·68, b = 9·38, c = 12·85 α = 90°, β = 96.45°°, γ = 90°
Lattice volume (V)	1279
Formula units (Z)	4
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

The AUC process is followed in South Korea^[6] and Argentina.^[7] In the AUC route, calcination, reduction and stabilization are simultaneously carried out in a vertical fluidized bed reactor. In most countries, sinterable grade UO₂ powder for nuclear fuel is obtained by the ammonium diuranate (ADU) process, which requires several more steps.^{[citation needed]}

Ammonium uranyl carbonate is also one of the many forms called yellowcake; in this case it is the product obtained by the heap leach process.^{[citation needed]}