As received by the uranium refinery, uranium ore concentrates now usually consist of uranium oxide or sodium, magnesium, or ammonium diuranate. These concentrates still contain appreciable amounts of elements other than uranium and some of uranium’s radioactive decay products present in the original uranium ore, such as radium and radon.

The first step in the conventional process for refining uranium is dissolution in nitric acid. When the concentrates have been produced by chemical leaching and are in the form of diuranates, dissolution proceeds rapidly and leaves little solid residue. When the concentrates have been separated mechanically and are in the form of the original uranium mineral, dissolution may require more concentrated acid, higher temperatures, longer times, and addition of oxidants such as Mn02. Also, filtration to remove undissolved residues is usually required. In either case, dissolution produces an aqueous solution of uranyl nitrate hexahydrate U02(N03)2’бНгО, containing excess nitric acid and variable amounts of nitrates of metallic impurities present in the concentrates.

The next step in purification is separation of uranyl nitrate from the other metallic impurities in the dissolver solution by solvent extraction. Practically all uranium refineries now use as solvent tributyl phosphate (TBP) dissolved in an inert hydrocarbon diluent. The first U. S. refinery used diethyl ether as solvent and later refineries have used methyl isobutyl ketone or organic amines, but practically all have now adopted TBP. It is nonvolatile, chemically stable, selective for uranium, and has a uranium distribution coefficient greater than unity when the aqueous phase contains nitric acid or inorganic nitrates.

Although uranium refineries use widely different types of solvent extraction contactors, their basic process flow sheets are similar, along the lines of Fig. 5.22, which illustrates the

specific process developed by the Mallinckrodt Chemical Company for the Weldon Springs refinery. A similar flow sheet is used in the Kerr-McGee plant.

Uranium ore concentrates are digested with hot 40% nitric acid. The resulting mixture is about 1 N in nitric acid and contains about 400 g uranium/liter and some suspended solids. The aqueous mixture is fed to a series of pumper-decanter mixer-settlers, where the uranyl nitrate is extracted by countercurrent flow of 30 v/o TBP in normal hexane. The flow ratio of organic to aqueous is about 13:1. Uranium concentration in the organic extract leaving the first stage is about 95 g uranium/liter and in the aqueous raffinate leaving the last stage is under 0.1 g uranium/liter. The raffinate is neutralized with lime. It contains most of the radioactive impurities in the ore concentrates, principally 230Th and 226 Ra.

In the scrubbing section all nonuranium metallic impurities and some uranium are removed from the organic phase by counterflowing dilute nitric acid, which is returned to the extracting section. In the stripping section purified uranium in the organic phase leaving the scrubbing

Makeup Nitric

HN03 acid

Recycle

HN03

Hj

Anhydrous HF

Figure 5.21 Steps in conventional uranium refining processes.

section is transferred to an aqueous phase by back-extraction with 0.01 normal nitric acid. Pulse columns are used for the scrubbing and stripping sections.

A portion of the aqueous stream leaving the stripping section is withdrawn, washed with hexane to remove dissolved and entrained TBP, and leaves the TBP-removal column as product uranyl nitrate solution (UNH).

All TBP is washed with an aqueous solution of sodium carbonate in a spray column to remove any hydrolyzed TBP and impurities that might accumulate in the TBP if it were not cleaned in this way. Sodium hydroxide is added to the aqueous sodium carbonate stream leaving the spray column to precipitate any uranium that might have been carried to this point. This impure uranium is recycled to the dissolver.

Variants of this basic process are used in other plants. For example, the Comurhex plant at Malvesi [B5] filters the output from the dissolver, uses pulse columns in the extracting section, and dilutes TBP with n-dodecane instead of л-hexane.