Uranate

Uranium oxides are the foundation of the nuclear fuel cycle ("ammonium diuranate" and "sodium diuranate" are intermediates in the production of uranium oxide nuclear fuels) and their long-term geological disposal requires a thorough understanding of their chemical reactivity, phase transitions, and physical and chemical properties.

[2] A method of general applicability involves combining two oxides in a high temperature reaction.

However, the composition of the precipitate that forms is variable and depends on the chemical and physical conditions used.

[3] Uranates are insoluble in water and other solvents, so pure samples can only be obtained by careful control of reaction conditions.

Instead, all uranate structures are based on UOn polyhedra sharing oxygen atoms in an infinite lattice.

A particular feature is the presence of linear O-U-O moieties, which resemble the uranyl ion, UO22+.

Taking calcium uranate, CaUO4, as an example, the six oxygen atoms are arranged as a flattened octahedron, flattened along the 3-fold symmetry axis of the octahedron which also runs through the O-U-O axis (local point group D3d at the uranium atom).

Charge-balance constrains the number of oxygen atoms to be equal to half the sum of charges of the cations and uranyl groups.

Yellowcake is produced in the separation of uranium from other elements, by adding alkali to a solution containing uranyl salts.

[8] When the alkali used is ammonia, so-called ammonium diuranate, known in the industry as ADU, is the main constituent of yellowcake.

The exact composition of the precipitate depends to some extent on the conditions and anions that are present and the formula (NH4)2U2O7, is only an approximation.

The precipitates obtained on addition of ammonia to uranyl nitrate solution under different conditions of temperature and final pH, when dried, were considered as loosely bound compounds with an ammonia/uranium ratio of 0.37 containing varying amounts of water and ammonium nitrate.