Rare earths separation from thorium

Solvent-soluble impurities are often reduced by a factor of at least 1(H (i.e. a decontamination factor of 1(H) provided their distribution coefficients are widely different from that of the main solute. Where the impurity is completely insoluble in the solvent, a small proportion may still pass with the principal solute by virtue of physical entrainment of tiny aqueous drops in the solvent phase, but decontamination factors of 10 or more have been obtained with some systems, e.g. for the separation of rare earth elements from thorium. 

From 1940 to 1965, the principal source of these rare earth products was the mineral monazite (Th, RE orthophosphate) which fortunately or unfortunately, depending on one s point of view, contains 4-6% thorium. Today, there is essentially no market for thorium in the U.S. The expense of separating out thorium-free rare-earth products from monazite is not only excessive, but bound tightly in governmental red tape because of the mild radioactivity of the thorium. This situation does not apply in France, Brazil, or India, whose governments are wisely stockpiling all extracted thorium for future atomic energy needs. 

SooDY, F. Separation of thorium and the rare earth group from minerals. U.S. Patent 2,425, 573 (1957). 

Ce(IV) can be precipitated as the iodate or periodate and separated from the other rare earths although any thorium present will also be precipitated. 

Lanthanum is most commonly obtained from the two naturally occurring rate-earth minerals, monazite and bastnasite. Monazite is a rare earth-thorium phosphate that typically contains lanthanum between 15 to 25%. Bastnasite is a rare earth-fluocarbonate-type mineral in which lanthanum content may vary, usually between 8 to 38%. The recovery of the metal from either of its ores involves three major steps (i) extraction of all rare-earths combined together from the non-rare-earth components of the mineral, (ii) separation or isolation of lanthanum from other lanthanide elements present... 

Heating the ore with sulfuric acid converts neodymium to its water soluble sulfate. The product mixture is treated with excess water to separate neodymium as soluble sulfate from the water-insoluble sulfates of other metals, as well as from other residues. If monazite is the starting material, thorium is separated from neodymium and other soluble rare earth sulfates by treating the solution with sodium pyrophosphate. This precipitates thorium pyrophosphate. Alternatively, thorium may be selectively precipitated as thorium hydroxide by partially neutralizing the solution with caustic soda at pH 3 to 4. The solution then is treated with ammonium oxalate to precipitate rare earth metals as their insoluble oxalates. The rare earth oxalates obtained are decomposed to oxides by calcining in the presence of air. Composition of individual oxides in such rare earth oxide mixture may vary with the source of ore and may contain neodymium oxide, as much as 18%. 

Thorium sulfate, being less soluble than rare earth metals sulfates, can be separated by fractional crystallization. Usually, solvent extraction methods are applied to obtain high purity thorium and for separation from rare earths. In many solvent extraction processes, an aqueous solution of tributyl phosphate is the extraction solvent of choice. 

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. 

It is also possible, if the proper conditions are set, to dissolve selectively the rare earth hydroxides which are more basic than thorium hydroxide, see the right hand column of Figure 9. In such a case, the mixed hydroxide water slurry is brought to a pH of 3.4 by a slow and careful addition of hydrochloric acid. The undissolved thorium hydroxide is then separated from the solution by filtration. 

Cerous iodates and the iodates of the other rare earths form crystalline salts sparingly soluble in water, but readily soluble in cone, nitric acid, and in this respect differ from the ceric, zirconium, and thorium iodates, which are almost insoluble in nitric acid when an excess of a soluble iodate is present. It may also be noted that cerium alone of all the rare earth elements is oxidized to a higher valence by potassium bromate in nitric acid soln. The iodates of the rare earths are precipitated by adding an alkali iodate to the rare earth salts, and the fact that the rare earth iodates are soluble in nitric acid, and the solubility increases as the electro-positive character of the element increases, while thorium iodate is insoluble in nitric acid, allows the method to be used for the separation of these elements. Trihydrated erbium iodate, Er(I03)3.3H20, and trihydrated yttrium iodate, Yt(I03)3.3H20,... 

Assay of beryllium metal and beryllium compounds is usually accomplished by titration. The sample is dissolved in sulfuric acid. Solution pH is adjusted to 8.5 using sodium hydroxide. The beryllium hydroxide precipitate is redissolved by addition of excess sodium fluoride. Liberated hydroxide is titrated with sulfuric acid. The beryllium content of the sample is calculated from the titration volume. Standards containing known beryllium concentrations must be analyzed along with the samples, as complexation of beryllium by fluoride is not quantitative. Titration rate and hold times are critical therefore use of an automatic titrator is recommended. Other fluoride-complexing elements such as aluminum, silicon, zirconium, hafnium, uranium, thorium, and rare earth elements must be absent, or must be corrected for if present in small amounts. Copper—beryllium and nickel—beryllium alloys can be analyzed by titration if the beryllium is first separated from copper, nickel, and cobalt by ammonium hydroxide precipitation (15,16). 

Fractional crystallization (or differential crystallization) is a process whereby two chemically compounds that form crystals with slightly different solubilities in some solvent (e.g., water) can be separated by a "tree-like" process. One should remember the herculean work by Marie Curie3, who by fractional crystallization isolated 0.1 g of intensely radioactive RaCl2 from 1 ton of pitchblende (a black mixture of many other salts, mainly oxides of uranium, lead, thorium, and rare earth elements). 

Scandium can also be separated from the rare earths by using methyl acetate containing 10 per cent (v/v) water and 5 per cent (v/v) nitric acid (d 142) as solvent. The strip is subjected to a solvent run of 25 cm. Scandium is then found in a narrow strip (RF 0-17), but thorium forms a more diffuse band. 

A number of investigations on the synergistic effect of a second extractant, such as amines and oximes, on the extraction of metal carboxylates have been carried out. The utilized synergists include 8-hydroxyquinoline-2-aldoxime for Zr(IV) and Hf(IV) (122,123), various amines (25), Lix 63 (27) and nonylphenol (28) for Cu(II), dialkylphos-phoric acids for Hf(IV) (44), rhodamine B for Be(IE) (102), trioctylphos-phine oxide for U(VI) (69, 77), p-alkylphenol for Cs(I) (1), collidine for Zr(IV) and Sc(IEI) (62), and nonchelating oximes for Ni(II) and Co(II) (103). Mareva et al. (77) have successfully utilized a salicylic acid-trioctylphosphine oxide mixture for the separation of uranium from rare earths, thorium, zirconium, and iron. 

Fundamental studies have been reported using the cationic liquid ion exchanger di(2-ethylhexyl) phosphoric acid in the extraction of uranium from wet-process phosphoric acid (H34), yttrium from nitric acid solution (Hll), nickel and zinc from a waste phsophate solution (P9), samarium, neodymium, and cerium from their chloride solutions (12), aluminum, cobalt, chromium, copper, iron, nickel, molybdenum, selenium, thorium, titanium, yttrium, and zinc (Lll), and in the formation of iron and rare earth di(2-ethylhexyl) phosphoric acid polymers (H12). Other cationic liquid ion exchangers that have been used include naphthenic acid, an inexpensive carboxylic acid to separate copper from nickel (F4), di-alkyl phosphate to recover vanadium from carnotite type uranium ores (M42), and tributyl phosphate to separate rare earths (B24). 

Cerium is separated from the other rare earths based on differences in adsorption and selective elution. In a typical process, thorium free rare-earth solutions are run... 

Several hydrous oxides, such as those of aluminum, siTicon and, iron have been used to extract traces ions. Nevertheless, the sorption mechanism is not definitively established. Those oxides probably exhibit some ion exchange capacity among their properties and they can act as anionic or cationic exchangers and sometimes both. The separation of plutonium traces in the presence of HF by sorption onto an alumina column is based on its chemical similarities with thorium and lantanide elements reported by Abrao (2) In this case only thorium and rare earths are sorbed onto alumina from nitric acid-fluoride solutions while uranium remains in the effluent. 

The sucessful experiments for the retention of plutonium onto alumina from TTN0 -HF solution gave enough confidence to recomend the proposed method to separate traces of plutonium from waste solutions in the presence of macroamounts of uranium (VI). Of course, only macroamounts of thorium, uranium (IV) and rare earths are serious interfering ions, since they precipitate with HF. The behavior expected for neptunium in the same system should be similar to plutonium, thorium and rare earths. The retention of neptunium from HNO - HF solutions is in progress. The sorption yield for Pu was around 95%. The sorption mechanism is not well established. Figure 3 shows the proposed flowsheet for recovery of Pu traces from reprocessing waste solutions. 

Traces of Th can be precipitated as thorium hydroxide by ammonia (pH > 4), with Fe(III), A1 or La being suitable collectors [1-2]. Thorium is separated from rare-earth elements by double precipitation (at pH 5) of the hydroxide. 

Thorium can be separated from rare-earth- and other metals by precipitation of the iodate, Th(I03)4, from 1 M HNO3 in the presence of tartaric acid and H2O2 [4,5]. The iodate precipitate also contains any Zr and Ce(IV) present. Mercury(II) and cerium(IV) have been used as collectors. 

Strongly acid cation-exchangers have been used for the separation of Th from rare-earth elements and other metals [8,9]. From the metal cations retained on the column 3-5 A/ HCl elutes rare-earth- and most other metals except thorium. Thorium is eluted with 10 M HCI or 3 M H2SO4, as well as with 5 MHNO3 [10], ammonium oxalate [11], ammonium carbonate [1] or ammonium sulphate solutions. Cation-exchange chromatography has also been used to separate thorium with the use of media such as HBr [12], formic acid -I- dimethyl sulphoxide [13], and nitric acid -1- methanol -i-TOPO [14]. 

F. W. E. Strelow, Separation of titanium from rare earths, beryllium, niobium, iron, aluminum, thorium, magnesium, manganese and other elements by cation exchange chromatography. Anal Chem., 35,1279,1963. 

Zuo Y, Chen J, li DQ (2008) Reversed micellar solubilization extraction and separation of thorium (IV) from rare earth (iii) by primary amine in ionic liquid. Sep Purif Technol 63 684-690... 

The element cerium is inseparably connected with the rare earth group, and it is generally customary to discuss it with the members of Group III. But it differs from the other rare earth elements in forming a well defined series of quadrivalent compounds, resembling thorium quite closely. Because of this relationship, as well as its greater abundance and commercial importance, it seems best to discuss certain phases of the chemistry of cerium with Group IV. The history, occurrence, extraction, and separations are discussed in Chapter VI. 

Separation. — The separation of thorium from the rare earth metals with which it is still mixed may be accomplished by three methods (1) the carbonate separation depends on the fact that thorium carbonate is much more soluble in sodium carbonate than the carbonates of the rare earth metals (2) by the fractional crystallization of the mixed sulfates at 15°-20°, crystals of Th(S04)2 8 H20 are obtained at the insoluble end of the series (3) thorium oxalate forms a soluble double salt with ammonium oxalate, while the rare earth oxalates are almost insoluble in this reagent. Some other methods which have been suggested are fractionation of the chromates,4 of the hydrogen alkyl sulfates,5 of the acetates, by the use of sebacic add 6 and hydrogen peroxide. 

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