Thorium Hydroxides

Mona.Zlte, The commercial digestion process for m on a site uses caustic soda. The phosphate content of the ore is recovered as marketable trisodium phosphate and the rare earths as RE hydroxide (10). The usual industrial practice is to attack finely ground m on a site using a 50% sodium hydroxide solution at 150°C or a 70% sodium hydroxide solution at 180°C. The resultant mixed rare-earth and thorium hydroxide cake is dissolved in hydrochloric or nitric acid, then processed to remove thorium and other nonrare-earth elements, and processed to recover the individual rare earths (see... 

There are a number of minerals in which thorium is found. Thus a number of basic process flow sheets exist for the recovery of thorium from ores (10). The extraction of mona ite from sands is accompHshed via the digestion of sand using hot base, which converts the oxide to the hydroxide form. The hydroxide is then dissolved in hydrochloric acid and the pH adjusted to between 5 and 6, affording the separation of thorium from the less acidic lanthanides. Thorium hydroxide is dissolved in nitric acid and extracted using methyl isobutyl ketone or tributyl phosphate in kerosene to yield Th(N02)4,... 

Oxo Ion Salts. Salts of 0x0 ions, eg, nitrate, sulfate, perchlorate, hydroxide, iodate, phosphate, and oxalate, are readily obtained from aqueous solution. Thorium nitrate is readily formed by dissolution of thorium hydroxide in nitric acid from which, depending on the pH of solution, crystalline Th(N02)4 5H20 [33088-17 ] or Th(N02)4 4H20 [33088-16-3] can be obtained (23). Thorium nitrate is very soluble in water and in a host of oxygen-containing organic solvents, including alcohols, ethers, esters, and ketones. Hydrated thorium sulfate, Th(S0 2 H20, where n = 9, 8, 6, or 4, is... 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

A soluble sodium tripolyphosphate is produced as are iasoluble lanthanide and thorium hydroxides (hydrated oxides). 

The sohds are treated with hydrochloric acid at 70°C at pH 3—4. The thorium hydroxide [13825-36-0] remains iasoluble and can be filtered off. Small amounts of trace contaminants that carry through iato solutioa, such as uranium and lead as well as some thorium, are removed by coprecipitation with barium sulfate ia a deactivatioa step. The resultiag product, after SX-removal of the heavy La fractioa, is a rare-earth/lanthanide chloride,... 

Thorium dioxide, 24 761-762 Thorium fluorides, 24 762 Thorium halides, 24 762-763 Thorium hydrides, 24 761 Thorium hydroxide, 24 756 Thorium iodides, 24 763 Thorium isotopes, 24 753-754... 

The principal abiotic processes that may transform thorium compounds in water are complexation by anions/organic ligands and hydroxylation. The increase in the mobility of thorium through the formation of soluble complexes with CQs, humic materials, and other anions or ligands and the decrease in the mobility due to formation of Th(OH)4 or anionic thorium-hydroxide complexes were discussed in Section 5.3.1.2. In a model experiment with seawater at pH 8.2 and freshwater at pH 6 and pH 9, it was estimated that almost 100% of the thorium resides as hydroxo complexes (Boniforti 1987). 

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%. 

Several processes have been used to eliminate the thorium from the rare earths. The precipitated hydroxides, could, for example, be dissolved in an acid medium, then the thorium hydroxide selectively precipitated by careful and progressive addition of an alkaline solution, such as sodium hydroxide, or ammonium hydroxide, etc., see the left hand column of Figure 9. 

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. 

In 1902 Rutherford and Soddy added ammonium hydroxide to a thorium solution, filtered off the thorium hydroxide precipitate, and found that, after they evaporated the thorium-free filtrate to dryness and fumed off the ammonium salts, the residue was much more active than the original thorium salt (18). This observation led them to the discovery of a new member of the thorium series, which they called thorium X. 

There are several ways of treating the mixed hydroxide cake. The cake is either dissolved in HC1 or HNOs and thorium is removed. The original Rohden process involved a partial dissolution of the mixed hydroxide in HC1 at 70—80° C (pH 3.5—4). The crude thorium hydroxide was filtered off and the filtrate contained the rare earths practically free from thorium and phosphate. 

Some thorium is extracted along with calcium during the HOI treatment, and it is recovered by precipitation from the filtrate at pH 1.8 and returned to the process. The usual industrial practice is to attack the monazite with 60—70% NaOH at a temperature of 140—50° C for —4 hrs. Better results are obtained at 170° C under higher pressures [154]. After the reaction is complete the mixed rare earth-thorium hydroxides are filtered off while hot. The precipitates are carefully washed to remove... 

RPO4 + 3NaOH R(OH)3 + Na3P04 The thorium is converted to thorium hydroxide... 

The rare earths and thorium hydroxides are then treated with HC1 (30%) to attain a pH of 3.2 at 70°C. This step in the recovery of rare earths is schematically shown in Fig. 1.12. Rare earths are obtained as soluble chlorides leaving behind insoluble Th(OH>4. Rare earth chlorides are treated with Na2SC>4 and BaCl2 to free the rare earths from Pb, Ra and Th. It is also treated with Na2S to remove Pb, Fe, Th and U. 

Ammonia, ammonium sulphide or sodium hydroxide solution white precipitate of thorium hydroxide, Th(OH)4 or Th02.xH20, insoluble in excess of the reagent, but readily soluble in dilute acids when freshly precipitated. Tartrates and also citrates prevent the precipitation of the hydroxide. 

Sodium thiosulphate solution precipitate of thorium hydroxide and sulphur on boiling (distinction from cerium). 

Samples exhibiting high emanating power (70 to 100%) are prepared for application as emanating sources. Examples are Th or Ra coprecipitated with thorium hydroxide. Rn given off by these emanating sources may be used for chemical or physical investigations with radon. Formerly these sources have also been prepared for application of Rn in medicine. 

The ageing of thorium hydroxide and iron hydroxide in the presence of water is obvious from Fig. 18.8. Whereas the emanating power of thorium hydroxide decreases very slowly, that of iron hydroxide falls olf relatively fast, indicating faster ageing. Emanation techniques have also been applied to obtain information about surface areas and densities of porous substances. 

Figure 18.8. Emanating power of thorium hydroxide and iron hydroxide as a function of ageing. (According to O. Hahn, G. Graue Z. Phys. Chem., Bodenstein Festband 1931, S. 608.)...

Perchlorates and iodates. Thorium perchlorate forms upon dissolution of thorium hydroxide in perchloric acid and crystallizes as Th(C104)4 4H20. The precipitation of tetravalent actinides as iodates has long been used to separate these elements from lanthanides at low pH. One of the earliest forms that Pu was isolated in was that of Pu(I03)4. The structure and most properties of Pu(103)4 are currently unknown, but a remarkable feature is that it is insoluble in 6M HNO3. 

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. 

Monazite concentrate is processed either with sulfuric acid, like bastnasite, to produce a mixture of sulfates but the usual process is an alkaline treatment. The alkali process is preferred since it removes the phosphates more readily [9]. Whichever method is chosen the radioactive thorium must be completely removed. After benefication the monazite concentrate is finely ground and reacted with a hot concentrated sodium hydroxide at 140° to 150°C. Insoluble hydroxides of the rare-earths and thorium are formed while trisodium phosphate and excess sodium hydroxide remain in solution. The next step is hydrochloric acid attack on the solids portion. The thorium remains insoluble and a crude thorium hydroxide can be filtered off Trace contaminants that do carry through into solution, such as uranium and lead, as well as some thorium, are removed by coprecipitation with barium sulphate in a deactivation step. The cerium-containing product will be a rare-earth chloride differing only marginally in the proportions of the various rare- earths present, to the analogous rare-earth chloride produced from bastnasite. 

Thorium forms relatively stable tetravalent salts of many of the oxyacids. These can be prepared by reacting thorium hydroxide or basic carbonate with the appropriate acid. 

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