Monazite thorium extracted from

Because of the complications introduced by ferric iron, Audsley et al. concluded that a solvent that would be selective for thorium in the presence of ferric iron and that would not be inhibited by phosphate would be preferable to the Dapex solvent. They concurred with the conclusion of Crouse and co-workers at Oak Ridge National Laboratory [CS], that long-chain primary amines selectively extract thorium in the presence of uranyl, ferric, and phosphate ions. Compounds of this type are now the preferred extractant for thorium in such systems. Application of this so-called Amex process to thorium extraction from monazite is described in Sec. 8.6. 

The separation of basic precipitates of hydrous Th02 from the lanthanides in monazite sands has been outlined in Fig. 30.1 (p. 1230). These precipitates may then be dissolved in nitric acid and the thorium extracted into tributyl phosphate, (Bu"0)3PO, diluted with kerosene. In the case of Canadian production, the uranium ores are leached with sulfuric acid and the anionic sulfato complex of U preferentially absorbed onto an anion exchange resin. The Th is separated from Fe, A1 and other metals in the liquor by solvent extraction. 

There are several processes for commercial thorium production from monazite sand. They are mostly modifications of the acid or caustic digestion process. Such processes involve converting monazite to salts of different anions by combination of various chemical treatments, recovery of the thorium salt by solvent extraction, fractional crystallization, or precipitation methods. Finally, metalhc thorium is prepared by chemical reduction or electrolysis. Two such industrial processes are outlined briefly below. 

Th, thorium, was discovered in 1829 by Jons Jakob Berzelius, who isolated a new oxide from a recently discovered mineral which Jens Esmark had sent to him. He called the oxide thoria and the mineral thorite (ThSi04) after the Scandinavian god Thor. Berzelius subsequently made the metal by the reduction of ThF4 with Na. Th now is extracted from monazite, a phosphate of rare earths and Th. The mineral is heated in concentrated NaOH to give hydrous oxides, which are filtered out. HCl is then added to dissolve the solids and when the pH is adjusted to 3.5, Th02 precipitates and the rare earths remain in solution. The Th02 is solubilized and purified by solvent extraction. 

The principal steps in producing refined thorium compounds from thorium-bearing ores are concentration of thorium minerals, extraction of thorium from concentrates, purification or refining of thorium, and conversion to metal or the thorium compound finally wanted. This section describes the concentration of monazite, the principal source of thorium in the past the extraction of thorium from monazite and the recovery of thorium from leach liquors by solvent extraction. Purification of thorium is described in Sec. 9 and conversion in Sec. 10. 

Thorium is widely distributed in Nature and there are large deposits of the principal mineral, monazite, a complex phosphate containing uranium, cerium, and other lanthanides. The extraction of thorium from monazite is complicated, the main problems being the destruction of the resistant sand and the separation of thorium from cerium and phosphate. One method involves a digestion with sodium hydroxide the insoluble hydroxides are removed and dissolved in hydrochloric acid. When the pH of the solution is adjusted to 5.8, all the thorium and uranium, together with about 3% of the lanthanides, are precipitated as hydroxides. The thorium is recovered by tributyl phosphate extraction from >6M hydrochloric acid solution or by... 

Most commercial thorium is extracted from monazite, which is not amenable to breakdown by dilute acids. Thorite, or thorium silicate, is an alternative thorium ore which occurs in fairly large quantities in some parts of the world, for example in the tailings from the tin beneficiation processes in Nigeria. The ore contains principally zircon (zirconium silicate), but this is of much smaller value than the 5 to 10 per cent thorite content. 

Attempts have also been made to solvent-extract thorium directly from the sulphate liquor obtained by the acid breakdown of monazite. Tributyl phosphate can be used as the solvent provided a large concentration of nitric acid is added to the liquor before extraction. In order to make the process economic, a high proportion of the nitric acid must then be recovered by distillation of the raffinate liquor. Processes are also being developed which are based upon the use of higher alkyl phosphate or amine solvents to extract from sulphate solutions without the addition of nitric acid, as in the case of uranium. For example bis(l-isobutyl 1-3-5 dimethyl-hexyl) amine, di-2-ethylhexyl hydrogen phosphate and Primene JM-T have been used. 

Uranium and thorium are widely distributed in the earth s crust, with average concentrations of 2 and 8 ppm, respectively (Grainger 1958 Boyle 1982 Frondel et al. 1967 Wills 1997). Over 100 different uranium-based minerals are known, but deposits of high-grade ore (hke uraninite and pitchblende, with as much as 70% uranium by weight) are rare. Most of the world s supply of recovered uranium has been extracted from ores with a uranium content of only about 0.2%. Conversely, the mineralogy of thorium is less varied, and deposits of its principal mineral, monazite, are fairly common. The thorium content of pure monazite is typically between 5% and 10%. 

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... 

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. 

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. 

There are various modifications of the process just described. Direct extraction of thorium from monazite after conversion to the sulphate has been considered. Recovery of thorium by selective precipitation as oxalate [145, 146], sulphate [147] or fluoride [148] has been attempted. 

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... 

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 monazite from sands is accomplished 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(N03)4,... 

From the sulfuric acid solution, thorium may also be obtained by precipitation with sodium fluosilicate, sodium hypo-phosphate,1 or sodium pyrophosphate.2 An ingenious method for removing the phosphorus has been proposed by Basker-ville3 and used on a large scale. It consists in heating in an electric furnace a mixture of monazite, coke, lime, and feldspar. The phosphorus is distilled out and the mass allowed to cool. When extracted with water, acetylene is evolved from the calcium carbide formed during the heating, and the remainder crumbles to a fine powder. This is dissolved in hydrochloric acid, and the cerium earths removed. 

Solvent extraction processes for recovering thorium from monazite sulfuric acid leach liquor are described in Sec. 8.6. 

Figure 6.S Principal processes for extracting thorium from monazite acid leach liquor. R = mass ratio Th02 REj03 P2 0s Y = approximate overall Th02 yield in concentrate, a, filtered b, washed c, 10 percent excess.

Attempts to separate thorium and uranium from sulfuric acid solution of monazite by solvent extraction with TBP were unsuccessful because distribution coefficients of uranium and thorium from monazite solutions were too low, as these elements are complexed by phosphate ion. Development of extractants with higher distribution coefficients for these metals has made solvent extraction a practical process for recovering uranium and thorium from monazite sulfate solutions and from sulfuric acid solutions of other thorium ores. This section describes processes tested on a pilot-plant scale by Oak Ridge National Laboratory [C5]. 

Source D. J. Crouse and K. B. Brown, Recovery of Thorium, Uranium, and Rare Earths from Monazite Sulfate Leach Liquors by the Amine Extraction (Amex) Process, Report ORNL-2720, July 16, 1959. 

Table 6.19 lists distribution coefficients for amines considered [C5] for extracting uranium from monazite sulfate solutions after removal of thorium. Except for Primene JM, all coefficients were judged [C5] to be large enough and sufficiently greater than those of the rare earths to provide efficient solvent extraction separation of uranium. 

Figure 6.7 Separation of thorium, uranium, and rare earths from monazite by solvent extraction in Amex process. Circles, relative flow ( ), estimated. Other data from Oak Ridge Laboratory runs fC5].

---