Thorium refining

Thorium has a wide distribution in nature and is present as a tetravalent oxide in a large number of minerals in minor or trace amounts. Thorium is significantly more common in nature than uranium, having an average content in the earth s cmst of approximately 10 ppm. By comparison, Pb is approximately 16 ppm. Thorium has a seawater concentration of <0.5 x 10 . Thorium refined from ores free of uranium would be almost... 

Purification or refining of thorium. Thorium produced previously is too impure to be used as nuclear fuel. In fact, impurities such as rare earths and uranium, owing to their elevated thermal neutron cross sections, are objectionable. Hence, the objective of the thorium refining process is to remove these impurities until concentrations below gg/kg (i.e., ppb wt.) are reached. Solvent extraction of an aqueous thorium nitrate solution with n-tributyl phosphate (TBP) in kerosene is a common procedure to perform the refining of thorium. At the end of the purification process, the thorium is recovered in the form of an aqueous solution of thorium nitrate or crystals of hydrated thorium nitrate. 

For experimentalists, measurements of the transport properties of metals are also of great practical interest, since they are very sensitive to the purity of the studied materials. It is known that the residual resistivity ratio p(300 K)/p(4.2 K) is commonly used as a global purity test. Values as high as 1000 have been obtained for ultra-pure thorium refined by electro-transport (Peterson et al. 1967). Even in the purest lanthanides this ratio is below 300. A value of 50 has been reached for electro-refined plutonium (Arko et al. 1972) and one of 65 for electro-refined neptunium (Fournier and Amanowicz 1992). 

Commercial-scale application of solvents coming under the category of neutral reagents is largely found as applied to the nuclear industry materials, as in example, for the separation and refining of uranium, plutonium, thorium, zirconium, and niobium. A process flowsheet for extracting niobium and tantalum from various resources is shown in Figure 5.23. It will... 

Iodargyrite, natural occurrence of, 22 668 Iodates, 14 374-375 Iodate solutions, 14 362 Iodic acid, 14 375 Iodide analysis, of water, 26 41 Iodide ion, 14 367-368 25 488 Iodide-refining method, 26 149 for vanadium, 25 520 Iodides, 14 374 thorium, 24 763 tungsten, 25 379-380 uranium, 25 439... 

The light actinide metals (Th, Pa, and U) have extremely low vapor pressures. Their preparation via the vapor phase of the metal requires temperatures as high as 2375 K for U and 2775 K for Th and Pa. Therefore, uranium is more commonly prepared by calciothermic reduction of the tetrafluoride or dioxide (Section II,A). Thorium and protactinium metals on the gram scale can be prepared and refined by the van Arkel-De Boer process, which is described next. 

The efficiency of the van Arkel-De Boer process (Section II,D) for refining thorium and protactinium metals can be increased by repeating the process to achieve higher purity of product metal. 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

Methods for Determining Biomarkers of Exposure and Effect. A few authors have found elevated levels of thorium in tissues of thorium workers and these studies have been discussed in Sections 2.6 and 5.4.4. However, there are no data in the literature that correlate the concentrations of thorium in any human tissue or body fluid with its level of exposure. If a biomarker for thorium in human tissue or fluid were available, the level of the biomarker in a tissue could be used as an indicator of exposure to thorium. Analytical methods with satisfactory sensitivity are available to determine the levels of thorium in most human tissues and body fluids of exposed and background population, but the recovery of thorium by these methods needs further refinement. 

The very active chemical nature of calcium accounts tor its major uses. Calcium is used in tonnage quantities to improve the physical properties of steel and iron. Tonnage quantities are also used in the production of automotive and industrial hailerics. Other major uses include refining ul lead, aluminum, thorium, uranium, samarium, and oilier reactive metals. 

Most metals can be electrolytically deposited from water-free melts of the corresponding metal salts. It is well known that aluminum, lithium, sodium, magnesium, and potassium are mass produced by electrolytic deposition from melts. Industrial processes for the melt-electrolytic production of beryllium, rare earth metals, titanium, zirconium, and thorium are also already in use. Pertinent publications [74, 137, 163] describe the electrolytic deposition of chromium, silicon, and titanium from melts. Cyanidic melts are used for the deposition of thick layers of platinum group metals. It is with this technique that, for instance, adhesion of platinum layers on titanium materials is obtained. Reports concerning the deposition of electrolytic aluminum layers [17, 71-73, 94, 96, 102, 164, 179] and aluminum refinement from fused salts [161] have been published. For these processes, fused salt... 

Calcium is utilized in the manufacture of special metals such as zirconium, thorium, uranium and the rare earths, as a refining agent in metallurgy (steel, copper, magnesium, tantalum, lead) and in the manufacture of calcium hydride (hydrogen source). 

When a metal rod is held in an ultrahigh vacuum just beneath its melting point for 2-3 weeks in a high dc current, impurities migrate to the electrodes. Many of the lanthanoids as well as thorium are highly refined by this method, as measured by the high resistivity ratio R3ook/R. 2k ... 

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. 

As the detection technique for radioactivity has been refined, a number of long-lived radionuclides have been discovered in nature. The lightest have been motioned in 5.1. The heavier ones, not belonging to the natural radioactive decay series of uranium and thorium, are listed in Table 5.2. is the nuclide of lowest elemental specific activity ( 0.(XX)1 Bq/g) while the highest are Rb and Re (each —900 Bq/g). As our ability to make reliable measurements of low activities increases, the number of elem ts between potassium and lead with radioactive isotopes in nature can be expected to increase. 

In contrast to titanium and zirconium, the preparation of thorium metal via reduction of the oxide with calcium (method II) acquires increased importance and rivals the reduction of the tetrachloride with sodium (method I). Melt electrolysis (method III) is another possibility. Neglecting the small oxide content (up to 1%), which in any case has never been determined precisely, the metal obtained by any of the three methods is already quite pure and contains only 0.1-0.2% of other impurities. The Th prepared by the refining process (method IV), is definitely oxygen-free and should in any case yield the purest product. 

Alloys containing various elements (rare earths, zinc, thorium, and silver), but not aluminum, and containing zirconium, which provides grain refinement and improved mechanical properties these alloys provide improved elevated temperature properties compared to those in the first group. 

Barkley, D.J. Blanchette, M. Cassidy, R.M. Elchuk, S. Dynamic chromatographic systems for the determination of rare earths and thorium in samples from uranium ore refining processes. Anal. Chem. 1986, 58 (11), 2222-2226. 

The later methods of uranium refining in the United States similarly involve TBP extraction. The National Lead Company s plant at Femald came into operation during 1954. This uses a 3N nitric acid feed solution prepared from chemical concentrates or high-grade pitchblende with a uranium concentration of 200 g/1. The solvent is 33-5 per cent TBP in a purified kerosene diluent. Phosphoric acid is added in quantities up to 15 per cent on a uranium basis, in order to complex any thorium impurity and prevent its extraction into the solvent. Three separate sieve-plate pulse columns are employed for extraction into solvent, stripping of... 

A very pure grade of thorium metal can be produced in small quantities by fused salt electrolysis in a refining cell. Thorium tetrachloride or tetra-fiuoride is added to twice its weight of a lithium chloride, potassium choride eutectic (m.p. 3S2°C) and electrolysed between an anode of impure thorium metal and a molybdenum cathode, at a temperature of 400-50°C (forThF4), or 600-50°C (For ThCU). A tubular-shaped container of fused silica is employed. 

Purification or refining of rare earths. The separation of rare earths from thorium can be performed in different ways depending on the production scale. Small laboratory-scale methods used first the fractional crystalhzation of nitrates, followed by the fractional thermal decomposition of nitrates. Pilot-scale separation can be achieved by ion exchange. Large commercial-scale separation is based only on the solvent-extraction process of an aqueous nitrate solution with n-tributyl phosphate (TBP) dissolved in kerosene. 

The principal steps in producing refined thorium compounds from crude thorium-bearing ores are the concentration of thorium ores, the extraction of thorium from concentrate, the purification or refining, and finally the conversion into the desired metal or thorium compounds. 

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