Thorium metal production

Thermal dissociation of Th. Veigel et al. [VI] have prepared massive thorium metal of high purity in lots of several hundred grams each by the Van Arkel-de Boer hot-wire process, which has been used for semicommercial production of zirconium as described in Sec. 8.4 of Chap. 7. The process is less suitable for thorium because the thorium metal product is less coherent, so that batch sizes are small. In this process, ThU is evaporated at 455 to 480°C in an evacuated vessel containing a metal filament heated to 900 to 1700°C. The iodide dissociates at the higher temperature,... 

Thorium is a radioactive metal that occurs naturally in several minerals and rocks usually associated with uranium. However, it is approximately three times more abundant in nature than uranium. On average, soil contains 6 to 10 ppm of thorium. Thorium is most commonly found in the rare-earth thorium-phosphate mineral, monazite, which contains 8% 10% thorium. Current production of thorium is, therefore, linked to the production of monazite, which varies between 5500 and 6500 tonnes per year, with approximately 300 to 600 tonnes of thorium recovered (NEA/IAEA, 2006a). 

The physical and chemical properties of elemental thorium and a few representative water soluble and insoluble thorium compounds are presented in Table 3-2. Water soluble thorium compounds include the chloride, fluoride, nitrate, and sulfate salts (Weast 1983). These compounds dissolve fairly readily in water. Soluble thorium compounds, as a class, have greater bioavailability than the insoluble thorium compounds. Water insoluble thorium compounds include the dioxide, carbonate, hydroxide, oxalate, and phosphate salts. Thorium carbonate is soluble in concentrated sodium carbonate (Weast 1983). Thorium metal and several of its compounds are commercially available. No general specifications for commercially prepared thorium metal or compounds have been established. Manufacturers prepare thorium products according to contractual specifications (Hedrick 1985). 

Thorium dioxide is obtained as an intermediate in the production of thorium metal from monazite sand (See Thorium). 

Recently a simpler preparation of Thl2 has been described. A thorium metal anode is used to electrolyze a solution of iodine and tet-raethylammonium iodide in acetonitrile. Addition of ether to the electrolyzed solution precipitates Thl2(CH3CN)2 115). This adduct loses the acetonitrile at 363 K in vacuo. The product is a yellow powder. 

Use Production of thorium metal and magnesium-thorium alloys, high temperature ceramics. ThOF2 is used as a protective coating on reflective surfaces. 

Thorium metal is slowly tarnished by air at room temperature, but further attack is prevented by an adherent oxide film. At temperatures above 200 C, however, progressive attack takes place. Weight gains of 0.03, 0.43, and 8.7 g/(cm h) have been reported at 300, 400, and 500 C, respectively [W2]. The product is primarily ThOj. Finely divided thorium is pyrophoric. 

The tetrahalides are the thorium halides of greatest practical importance. The tetrafluoride ThF4 is the preferred starting material for large-scale production of thorium metal (Sec. 10.4). ThF4 has been proposed as fertile material in the fuel mixture of the molten-salt reactor. The tetraiodide has been used as feed material in the iodide process for making very pure thorium metal (Sec. 10.4). 

The more important properties of the tetrahalides, from reference [11], are listed in Table 6.10. Many of these properties, especially for ThCU, ThBr4, and TI1I4, are known only semiquantitatively. Anhydrous Thp4 is made by passing an excess of HF vapor over ThOj or ThOFj at temperatures between 550 and 600 C. The anhydrous double fluoride KThFs is precipitated from aqueous solutions of thorium nitrate by addition of an excess of KF. It has been used for electrolytic production of thorium metal. Table 6.10 Thorium Tetrahalides ... 

The principal uses of ThF4 are as intermediate in the production of thorium metal or, potentially, as a compound in the fuel mixture of the molten-salt breeder reactor. For both applications anhydrous, oxide-free ThF4 is required. 

Production of pure thorium metal is beset by all the difficulties cited for uranium metal in Chap. 5, Sec. 10.1, complicated further by the higher melting point of thorium, 1750 C. Table... 

Heat balances for Iowa process. To show the need for addition of the booster ZnCl2 tCT the charge, Table 6.23 shows that when 1 mol Thp4 and 2 mol CaCl2 at 475°C (748 K) react to produce 1 mol liquid Th and 2 mol liquid CaF2 at the melting point of thorium (1750°C or 2023 K), there is an enthalpy deficiency of 12.84 kcal/g-mol thorium. It would thus be impossible to melt the thorium product and obtain massive thorium metal free of CaF2 from these reactants preheated to 475°C. 

Table 6.26 Heat balance for production of thorium metal under conditions of Table 6.24...

Sibert, M. E., and M. A. Steinberg (Horizons, Inc.) Investigations for the Production of Thorium Metal by Fused Salt Electrolysis, Report NYO-3725, 1952. 

Hayek et al. [1951HAY/REH] obtained products with compositions close to ThCl2 and ThCh from the reaction of thorium metal with gaseous chlorine in stoichiometric quantities while Jantsch and Homayr [1954JAN/HOM] reported the formation of ThCls as a result of the reduction of ThCh with aluminium and also, which is more surprising, from the thermal decomposition of ThCh at 673 K. As noted by Rand [1975RAN], contamination by oxygen and silica may have been serious in these experiments. 

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. 

The thorium metal prepared by the above process is very pure and absolutely free of oxygen. The procedure is essentially the same as that described for Ti, except that the temperature of the glowing wire is higher (1700°C). The starting material may be any kind of crude thorium, provided it is free of metals which will also deposit on the glowing wire the product derived from the chloride is very suitable. 

The reaction takes place in a closed steel vessel, 45 in. long by 12 in. internal diameter, which is lined with electrically fused dolomite or lime, as in the American uranium metal production process. Initiation is carried out in a gas-fired furnace at a temperature of 640°C. A thorium/zinc alloy is formed, from which the zinc is removed by distillation under vacuum, between 1000°C and 1100°C in graphite pots. About 85 per cent of the zinc can be recovered for re-use. 

Thorium metal powder was produced on a small production scale for a number of years in a U.K. Atomic Energy Authority factory in Sheffield, managed by Messrs Firth Brown Ltd. o, but this has now been discontinued. The semi-continuous type of process was employed in which kilogram batches (on a thorium basis) of mixed calcium and thorium oxide are passed through a furnace in nickel boats. 

The reaction products consist of loosely aggregated cakes of thorium metal powder and calcium oxide. Each cake is individually quenched and immersed in water for 4 hr. They disintegrate to a fine powder as the excess calcium is destroyed and the calcium oxide is converted to the hydroxide. The hydroxide is then dissolved by the addition of nitric acid to the solution until an excess of about 0 3N remains. The time of exposure of the thorium metal powder to the acidic conditions is limited, to prevent its dissolution. It is fairly rapidly filtered by vacuum, rinsed with water, and dried in a vacuum oven. The dry metal powder is about 300 mesh in particle size and has the following typical analysis ... 

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