Pure thorium metal

Properties. Pure thorium metal is a dense, bright silvery metal having a very high melting point. The metal exists in two allotropic modifications. Thorium is a reactive, soft, and ductile metal which tarnishes slowly on exposure to air (12). Having poor mechanical properties, the metal has no direct stmctural appHcations. A survey of the physical properties of thorium is summarized in Table 1. Thorium metal is diamagnetic at room temperature, but becomes superconducting below 1.3—1.4 K. 

Thorium reacts with hydrogen at temperatures above 250 C to form ThH2 and Th4H,5. Thorium reacts with nitrogen at temperatures above 670 C to form ThN. For these reasons, melting of pure thorium metal must be carried out in vacuum, helium, or argon. 

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). 

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... 

The reaction mass is leached with water and dilute acid, leaving thorium metal powder. Very pure thorium metal can be prepared by the van Arkel process involving the thermal decomposition of Thl4. 

In the chemical process industry molybdenum has found use as washers and bolts to patch glass-lined vessels used in sulphuric acid and acid environments where nascent hydrogen is produced. Molybdenum thermocouples and valves have also been used in sulphuric acid applications, and molybdenum alloys have been used as reactor linings in plant used for the production of n-butyl chloride by reactions involving hydrochloric and sulphuric acids at temperatures in excess of 170°C. Miscellaneous applications where molybdenum has been used include the liquid phase Zircex hydrochlorination process, the Van Arkel Iodide process for zirconium production and the Metal Hydrides process for the production of super-pure thorium from thorium iodide. 

Thorium phthalocyanine is prepared by heating the metal (previously etched with HC1) and o-phthalonitrile, 1 25, at 270 to 300°C for 5 hr. The dark blue product is cooled to room temperature, washed with benzene, and purified twice by sublimation at 520°C and 10 4Torr. The protactinium-233 produced by w-irradiation of pure thorium phthalocyanine is separated in high purity in the residue after repeated sublimation of the thorium phthalocyanine. The thorium-231 produced by (w,2w) reaction in the thorium phthalocyanine is found to be enriched in the residue after sublimation, indicating decomposition of the phthalocyanine by irradiation. Uranyl phthalocyanine is prepared by heating a mixture of uranyl acetate and phthalonitrile at 230 to 240°C. 

Pure thorium is a silvery-white metal (melting point 1,750°C) that tarnishes upon exposure to air. Its density is 11.724 g/cm at 25°C (77°F), similar to that of lead. The best-known application of thorium is its use in incandescent mantles for gas lamps. These mantles consist of a metal oxide skeleton (99% Th02 and 1 % Ce02). Thorium(TV) oxide is used by chemists... 

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. 

J. Berzelius tried to separate pure thorium but in vain. For very long the element was known in the form of its oxide and only in the 1870 s was it prepared in the metallic form. Thus, thorium became the second radioactive element (after uranium) to be discovered by the conventional chemical analysis having nothing to do with radioactivity. 

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. 

The various chemical extraction stages from ore to metal are discussed some of the intermediates arising from these stages have major industrial uses or potential uses of their own. Pure thorium oxide, for example, has been employed for many years in the manufacture of incandescent gas mantles and this is at present probably still the major outlet for thorium in any form. It is antidpated that new uses will be found for most of these metals or their pure intermediates in the near future and as a result many of the process stages described will achieve a wider application. 

When pure, thorium is a silvery white metal. In air, it tarnishes slowly, becoming gray and finally black. Thorium has isotopes ranging in mass number from 210 to 237, all isotopes being radioactive. Much of the internal heat in the earth s crust has been attributed to thorium (and uranium). Thorium is a potential atomic fuel source, because bombardment of Th with slow neutrons yields the fissile isotope There... 

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). 

---