Thorium solvent extraction

Fluoride is known to separate zirconium fission product and plutonium from solvent degradation products. (5) Since fluoride is used to speed dissolution of thoria in nitric acid, and is already present in thorium solvent extraction process feed solutions, it was the first choice as an agent to use to improve thorium-DPB separation. 

The process of rare earth recovery is based on rare-earth double-salt precipitation. However, yttrium and the heavy rare earths go with thorium. The rare earths are recoverable from the thorium fraction during the solvent extraction step used for the purification of uranium and thorium. Solvent extraction with TBP (tribulyl phosphate ), from an aqueous 8 N nitric acid solution of thorium and mixed rare earths, enables the recovery of thorium, uranium, cerium and cerium free rare earths (Gupta and Krishnamurthy 2005). Other significant processes involve precipitation of thorium pyrophosphate, or precipitation as basic salts from the leach fiquor. After that comes recovery of the rare earths from solution as double sulphates, fluorides, or hydroxides, and also selective solubilisation of thorium itself in the ore treatment stage. The sulphuric acid route does yield impure products, but it is not used anymore (Gupta and Krishnamurthy 2005). 

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. 

Singh, H. Gupta, C. K. Solvent extraction in production and processing of uranium and thorium. Min. Process. Extractive Metall. Rev. 2000, 21, 307-349. 

Thorex [Thorium extraction] A process for separating the products from the nuclear breeder reaction in which uranium-233 is produced by the neutron bombardment of thorium-232. It uses solvent extraction into tri-n-butyl phosphate. Developed at the Oak Ridge National Laboratory, TN, in the early 1960s. See also Butex, Purex, Redox. 

Np, and fission products. The Thorex solvent extraction process is generally used to reprocess spent Th-based fuels. As in the Purex process, the solvent is TBP diluted in an appropriate mixture of aliphatic hydrocarbons. Figure 12.9 shows the Thorex process flow sheet used by Kuchler et al. [41] for reprocessing high-burn-up thorium fuel. 

Precipitated Th with La-fluoride, clean-up by complexation and solvent extraction, develop color as thorium-morin complex. 

Thorium coprecipitated from acidified sample with Fe(OH)3, clean-up Th by selective solvent extraction, coprecipitated with Al(OH)3 and develop color by Arsenazo III reagent. After color development, coprecipitate Th with LaF3. 

The monazite sand is heated with sulfuric acid at about 120 to 170°C. An exothermic reaction ensues raising the temperature to above 200°C. Samarium and other rare earths are converted to their water-soluble sulfates. The residue is extracted with water and the solution is treated with sodium pyrophosphate to precipitate thorium. After removing thorium, the solution is treated with sodium sulfate to precipitate rare earths as their double sulfates, that is, rare earth sulfates-sodium sulfate. The double sulfates are heated with sodium hydroxide to convert them into rare earth hydroxides. The hydroxides are treated with hydrochloric or nitric acid to solubihze all rare earths except cerium. The insoluble cerium(IV) hydroxide is filtered. Lanthanum and other rare earths are then separated by fractional crystallization after converting them to double salts with ammonium or magnesium nitrate. The samarium—europium fraction is converted to acetates and reduced with sodium amalgam to low valence states. The reduced metals are extracted with dilute acid. As mentioned above, this fractional crystallization process is very tedious, time-consuming, and currently rare earths are separated by relatively easier methods based on ion exchange and solvent extraction. 

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. 

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. 

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. 

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. 

Processes have been developed271 for the recovery of thorium by solvent extraction into solutions of TBP, but their present status is uncertain owing to the limited market for this metal (although some countries are reportedly stockpiling thorium for future use in nuclear applications). 

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

Preston, J.S., du Preez, A.C. 1999. The influence of extractant structure on the solvent extraction of uranium(VI) and thorium(IV) from nitrate media by dialkyl sulphoxides. J. Chem. Technol. Biotechnol. 69 (1) 86-92. 

Yoshizuka, K., Shinohara, T., Shigematsu, H., Kuroki, S., Inoue, K. 2006. Solvent extraction and molecular modeling of uranyl and thorium ions with organophosphorus extractants. Solvent Extr. Res. Dev. Jpn. 13 115-122. 

Pathak P.N., Kumbhare, L.B., Manchanda, V.K. 2001. Structural effects in N,N-dialkyl amides on their extraction behavior toward uranium and thorium. Solvent Extr. Ion Exch. 19 (1) 105-126. 

Gopalkrishnan, M. Radhakrishnan, K. Dhami, P.S. Kulkami, V.T. Joshi, M.V. Patwardhan, A.B. Mathur, J.N. Determination of trace impurities in uranium, thorium and plutonium matrices by solvent extraction and inductively coupled plasma atomic emission spectrometry, Talanta 44 (1997) 169-176. 

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