The main task in the conversion of concentrate ("yellow cake") to UF₆ is the purification of the concentrate and its conversion into a chemically suitable form for further processing or enrichment of the ²³⁵U-isotope for the different reactor types. Isotope enrichment proceeds in the gas phase via uranium(Ⅵ) , the only uranium compound which boils at low temperatures and is stable in the vapor phase. It is advantageous that fluorine only occurs naturally in a single isotope.

Two processes are employed for the production of uranium(Ⅵ) fluoride, namely the wet and dry processes. In both processes uranium(Ⅳ) oxide and uranium(Ⅳ) fluoride are formed as intermediates. In the wet process the uranium(Ⅳ) oxide is produced from the uranium concentrate by way of uranyl nitrate, whereas in the dry process the uranium concentrate is directly reduced to uranium(Ⅳ) oxide. The methods of purification used are also different: in the wet process the purification proceeds at the uranyl nitrate stage, by solvent extraction, whereas in the dry process the end product uranium hexafluoride is itself distillatively purified.

Wet Process for Uranium(Ⅵ) Fluoride Manufacture

In the production of uranium(Ⅳ) oxide in the wet process,the uranium concentrate is first converted into a uranyl nitrate solution with nitric acid. After the purification of the uranyl nitrate by solvent extraction, it can be converted into uranium(Ⅳ) oxide by two different routes: either by thermal denitration to uranium(Ⅵ) oxide which is then reduced to uranium(Ⅳ) oxide or by conversion of uranyl nitrate into ammonium diuranate which is reduced to uranium(Ⅳ) oxide. Purification proceeds by extraction of the uranyl nitrate hydrate from the acidic solution with tri-n-butylphosphate in kerosene and stripping this organic phase with water, whereupon uranium goes into the aqueous phase.

This diluted aqueous uranyl nitrate solution is evaporated to uranyl nitrate hexahydrate, UO₂(NO₃)₂^.6H₂O, which is then calcined to uranium(Ⅵ) oxide in a tluidized bed furnace:

The temperature must not exceed 400°C, to prevent the formation of U₃O₈. The nitrous gases produced are processed to nitric acid, which is recycled. The subsequent reduction of uranium(Ⅵ) oxide to uranium(Ⅳ) oxide with hydrogen at 500°C also proceeds in the fluidized bed furnace.

The second route to uranium(Ⅳ) oxide consists of precipitation of ammonium diuranate from the solvent extraction-purified aqueous uranyl nitrate solution by adding ammonia and then reducing it with hydrogen to uranium(Ⅳ) oxide (Comurhex process developed in France).

Uranium(Ⅳ) oxide is the starting material for uranium(Ⅳ) fluoride production in which uranium(Ⅳ) oxide is generally reacted with anhydrous hydrogen fluoride. This difficult to carry out exothermic reaction proceeds either in a fluidized bed, in moving bed reactors, or in screw-reactors. To achieve as complete as possible reaction in fluidized bed reactors, two fluidized bed reactors are connected in series. Screw-reactors are also preferably connected in series. In moving bed reactors the reduction zone and the hydrofluorination are arranged above one another in a plant. The uranium(Ⅳ) oxide produced by the reduction of uranium(Ⅵ) oxide with hydrogen is very reactive and is completely reacted with HF at temperatures between 500 and 650°C to uranium(Ⅳ) fluoride.

A wet process is also utilized for the production of uranium(Ⅳ) fluoride, namely the EXCER process (Ion Exchange Conversion Electrolytic Reduction). In this process the ion exchange- or extraction-purified uranium(Ⅵ) solution is either electrolytically or chemically reduced to uranium(Ⅳ), which is precipitated with hydrofluoric acid as uranium(Ⅳ) fluoride hydrate (UF₄^.0.75H₂O). This is subsequently dehydrated at 400 to 450°C.

The conversion of uranium(Ⅳ) tluoride to uranium(Ⅵ) fluoride proceeds exclusively with elemental fluorine, either in flame-reactors or in fluidized bed reactors. The uranium(Ⅵ) fluoride formed is recovered from the reaction gases by freezing it out.

The wet process for uranium(Ⅵ) fluoride manufacture is utilized in the Kerr-McGee process, in which the reduction proceeds with a H₂/N₂-mixture from ammonia cracking and hydrofluorination is carried out in a two stage fluidized bed. British Nuclear Fuel Ltd and Eldorado Nuclear Ltd/Canada also use wet processes.

Dry Process for Uranium(Ⅵ) Fluoride Manufacture

In the dry process, introduced by Allied Chemical Corp., the uranium concentrate is pelletized and directly reduced with hydrogen to uranium(Ⅳ) oxide at temperatures between 540 and 650°C in a fluidized bed reactor. Hydrofluorination to uranium(Ⅳ) fluoride proceeds in two fluidized bed reactors connected in series. After fluorinating the uranium(Ⅳ) fluoride formed in a production unit consisting of a flame-reactor and a tluidiLed bed reactor, the uranium(Ⅵ) fluoride produced is purified in a two stage pressure distillation process. This distillative purification process is necessary, because, in contrast with the wet process, no purification is carried out in earlier stages.

The uranium conversion capacity in Western industrialized countries in 199.5 was nominally 385^.10³ t/a UF₆, of which about 73% is accounted for by wet processes. The capacity in the former States of the USSR is estimated to be ca. 14^.10³ t/a UF₆. In 199.5 the total conversion was 51.3^.10³ t UF₆.