Ammonium di-uranate

Comurhex A process for making uranium dioxide by reducing ammonium di-uranate by heating it in hydrogen. Operated by the French company of that name. 

Reports on truly continuous precipitation on an industrial scale are a rarity. The study by Stevenson (1964), therefore, is quite interesting not only because it reports a pilot-plant study of a continuous precipitation (of ammonium di-uranate by reacting uranyl nitrate solution with ammonia) but also because a design route for a multistage continuous unit operated with feedback is proposed. Regrettably, there does not appear to have been any follow-up to this work. 

Other deviations from a simple elution process can also be beneficial. For example, recovery of eluate is generally a practical proposition, i.e. the uranium is precipitated from sodium chloride solution by the addition of sodium hydroxide, or from ammonium nitrate solution by the addition of ammonia. After filtration of the yellow sodium di-uranate or ammonium di-uranate, the filtrates can be reconstituted as an eluting agent by the addition of hydrochloric or nitric acid respectively. 

The uranium product solution, after evaporation, is suitable for precipitation of ammonium di-uranate and conversion to uranium oxide. The major proportion at the present time is evaporated to uranyl nitrate and converted to oxide by thermal denitration, and then to fiuoride, using a fluidization process. 

The United Kingdom s process operated at the Springfields factory until recent years used a feed solution of uranyl nitrate in dilute nitric acid, from the solvent-extraction process. This contains about 130 g U/1. and is first treated batchwise with 28 per cent w/w aqueous ammonia solution to precipitate ammonium di-uranate , at a pH of about 9-5. 

Since a single furnace is used for three dryway operations, calcination, hydrogen reduction and hydrofluorination, the transfer of solids is minimized. The initial charging of trays and loading these into the furnace is relatively free from dust hazard since the ammonium di-uranate feed material is damp. Special precautions are needed, however, for removal of the final uranium tetrafluoride product from the trays, since it is a dry dusty powder and tends to adhere to the trays to some extent. This operation is therefore carried out in a small glove box , i.e. in a sealed and vented... 

The UO3 product from a fluid bed thermal denitration process tends to be less reactive than that produced by ammonium di-uranate precipitation and calcination. It can be considerably improved by the addition of a little sulphuric acid to the feed solution, corresponding to about 0-1 per cent of the uranium content.23 The product is then very suitable for the subsequent hydrogen reduction and hydrofluorination stages. 

It is usual to take advantage of the alkali addition to separate off ferric iron and other metallic ions which precipitate at a lower pH than uranium. Furthermore, this can often be done with lime. This is normally the cheapest alkali and also has the advantage that it precipitates the sulphate ion impurity at the same time as the iron. A pH of 3 -6 is used for this impurity precipitation, followed by the first-stage filtration. Then the addition of extra alkali, e.g. ammonia, sodium hydroxide or magnesia, to pH 6 7 precipitates the uranium. The second-stage filtration finally removes the uranium from solution as ammonium, sodium or magnesium di-uranates. 

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