Thorium extraction into organic extractants

R. Bock and E. Bock, "The Study of Thorium and Cerium Compounds by Extraction of Aqueous Nitrates Into Organic Solvents", Z. Anorg. Chem. 263. l46 (1950). 

Finely-ground monazite is treated with a 45% NaOH solution and heated at 138°C to open the ore. This converts thorium, uranium, and the rare earths to their water-insoluble oxides. The insoluble residues are filtered, dissolved in 37% HCl, and heated at 80°C. The oxides are converted into their soluble chlorides. The pH of the solution is adjusted to 5.8 with NaOH. Thorium and uranium are precipitated along with small quantities of rare earths. The precipitate is washed and dissolved in concentrated nitric acid. Thorium and uranium are separated from the rare earths by solvent extraction using an aqueous solution of tributyl phosphate. The two metals are separated from the organic phase by fractional crystallization or reduction. 

In one acid digestion process, monazite sand is heated with 93% sulfuric acid at 210°C. The solution is diluted with water and filtered. Filtrate containing thorium and rare earths is treated with ammonia and pH is adjusted to 1.0. Thorium is precipitated as sulfate and phosphate along with a small fraction of rare earths. The precipitate is washed and dissolved in nitric acid. The solution is treated with sodium oxalate. Thorium and rare earths are precipitated from this nitric acid solution as oxalates. The oxalates are filtered, washed, and calcined to form oxides. The oxides are redissolved in nitric acid and the acid solution is extracted with aqueous tributyl phosphate. Thorium and cerium (IV) separate into the organic phase from which cerium (IV) is reduced to metalhc cerium and removed by filtration. Thorium then is recovered from solution. 

Principle of Separation. Uranium forms a nitrate complex that is extractable into ethyl acetate (as well as other organic extractants). Thorium does not readily form an extractable nitrate complex. When ethyl acetate is contacted with an aqueous solution, the uranium-nitrate complex is partitioned favorably into the ethyl acetate whereas thorium nitrate is not. The distribution of the metal ion between the two phases is expressed as D = Corganic/Caqueous where C is the concentration in moles or dps per unit volume in the respective phases. The thorium remains in the aqueous phase and is precipitated as the hydroxide for counting. 

Table I. Extraction of Uranium and Thorium into Several Neutral Phosphorus-Based Organic Extractants from 2.00 M HNO. Extractant Solutions are 1.00 F in Dodecane...

The supernatant is shaken with 15 ml of 10 % tri-n-octyl amine in xylene pre-equilibrated with 8 N nitric acid. The extraction is repeated twice. Since plutonium is extracted into the organic phase together with thorium and uranium, the solvent is washed in turn with 2 portions of 15 ml of 8 N nitric acid and with 2 portions of 15 ml of 10 N hydrochloric acid in order to remove most of thorium and uranium. Then plutonium is back-extracted with 15 ml of 6 fi hydrochloric acid containing 0.2 H hydrofluoric acid or... 

Other organic spectrophotometric reagents, recommended for determination of thorium are Bromopyrogallol Red [85], morin (e = 3.3-10 at 422 nm), quercetin [86,87], phenylfluorone [87], and 8-hydroxyquinoline [88], The anionic complex of 5,7-dibromo-oxine with Th forms an ion-associate with Rhodamine B which can be extracted into benzene (e = 8.8-10 at 552 nm) [89],... 

An additional complication can arise in solution if the radioactive species in trace amounts react with trace concentrations of impurities. For example, in an investigation of the properties of pentavalent protactinium, Pa(V), it was foimd that the protactinium was extracted into pure xylene from 1 M HCIO4 solutions. Further experimentation showed that this extraction was due to the presence in the xylene of organic impurities at concentrations below the detectable limit of 0.01 %. Support for this interpretation was provided when the solution was made 10" M in thorium, which was expected to form complexes with the probable impurity, thereby preventing the reaction with the protactinium. In fact, no protactinium was extracted into xylene from this solution. The thorium in this case acts as a hold-back carrier. 

The organic phase was 1 per cent dlethyldlthlocarbamate in trichloroethylene. They extracted lead under these conditions to effect a separation from thorium which does not extract (M12). A similar separation from thorium has been used by Goreuch (g4) In which he extracts lead dlethyldlthlocarbamate Into Isoamyl acetate from pH 3.0-3.2 nitrate solution leaving the thorium In the . aqueous phase. 

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