Thorium concentrate produced by the processes described in Sec. 8 is too impure to be used as nuclear fuel. Especially objectionable impurities, which frequently are associated with thorium in its ores, are neutron-absorbing rare earths and uranium, the latter because it would dilute isotopically 233 U formed in thorium during subsequent neutron irradiation. The objective of thorium purification is removal of these and other impurities to concentrations below a few parts per million.

Solvent extraction with TBP has become the standard procedure for purifying thorium, just as for uranium. Processes used in different countries differ, however, in details such as the solvent used to dilute TBP, its concentration, and the means used to strip thorium and coextracted uranium from TBP. Table 6.20 summarizes the main features of processes used for purification of thorium on an industrial scale in the principal thorium-producing countries. Wylie [W5] gives more detail on early pilot-plant thorium-purification runs. Most of the published U. S. work on thorium purification on an industrial scale deals with irradiated thorium rather than natural; this will be described under the Thorex process, in Sec. 5 of Chap. 10.

Here, a summary will be given of Callow’s [C2] description of a process used in England for purifying thorium concentrate and separating it from associated uranium. Figure 6.8 shows relative flow rates and nitric acid and thorium concentrations in this process. Feed is a nitric acid solution of thorium concentrates containing about 200 g Th02/liter of nitrate, a smaller concentration of uranyl nitrate, and considerable amounts of nitrates of other metals, such as iron and rare earths (RE). In the first contacting unit, consisting of five extracting stages and five scrubbing stages, one volume of feed is extracted with four volumes of recycle solvent, 40 v/o TBP in kerosine. At the 4 A nitric acid concentration of the feed, this solvent extracts effectively all of the uranium and thorium in the feed and a little of the associated impurities. Counterflow of 0.8 volume of 4 A HN03 in the scrubbing section removes these impurities from the solvent. Rare-earth content of extracted thorium is less than 5 ppm if the rare earth-to-thorium ratio of feed is less than 1:4.

In the second contacting unit, thorium is stripped from the rich solvent by 0.1 A HN03; uranium is scrubbed from the thorium product by additional solvent.

Uranium in solvent leaving the second contacting unit is stripped into an aqueous phase by 5% sodium carbonate solution. Stripped solvent is washed and reacidified with 4 N nitric acid for recycle to the process.

At the high thorium and nitric acid concentrations used in this flow sheet, two solvent phases may form, one rich in thorium and TBP and the other, lean. Callow [C2] states that formation of the two solvent phases does not interfere with operation of a mixer-settler cascade, whereas difficulty would be experienced with a pulse column. Conditions at which a

second solvent phase forms are sketched in Sec. 5 of Chap. 10. Distribution equilibrium data for thorium, nitric acid, uranium, and impurities are also given there.