Thorium

THORIUM, a chemical element belonging to a group of metals contained in the oxides known as the rare earths (symbol Th, atomic number 9o, atomic weight 232.2). In 1828, Berzelius gave the name thoria (after the god, Thor) to the basic oxide extracted from the thorite of Lovon (Norway). Thorium is present as the main constituent in thorite and orangite, which are thorium silicates, ThSiO4, auerlite, calciothorite, freyalite, mackin toshite and thorianite 6o-78%, with oxides of uranium), hut the only commercial source is monazite sand, a complex phos phate of the rare (cerite) earths containing 2 to io% of thoria. This sand is found in Brazil, Travancore, North and South Caro lina, Idaho, Australia and the Malay Archipelago. The first two sources are the most important commercially, the Travancore deposits furnishing three-fourths of the world's supply. Thoria is extensively used in the construction of incandescent gas mantles (see LIGHTING AND ARTIFICIAL ILLUMINATION) this being the main industrial application of thorium.

Metallic

to its intense chemical reactivity towards hydrogen, nitrogen, oxygen, carbon and the metals, thorium is isolated only with considerable difficulty. Anhydrous thorium chloride when heated with sodium in vacuo furnishes a pure product. Thorium is also obtained (i.) by passing the vapour of its iodide over heated tungsten filaments, (ii.) by the action of calcium on thoria, (iii.) by passing thorium acetylacetone and sodium vapours through a red hot tube. It is a leaden-grey metal of specific gravity 12.16, resembling platinum in hardness and ductility. Its melting point is C. Thorium ribbon when heated to red heat continues to burn in air, giving a shower of sparks. At C the metal combines with the halogens and with sulphur, and at 65o° C with hydrogen and nitrogen. Thorium is dissolved by strong hydrochloric acid or aqua regia but other acids attack it only slowly. It is not affected by alkalis but alloys with aluminium, copper and nickel; the thorium-tungsten alloy is of industrial use for making filaments for incandescent electric lamps. Tungsten containing o.5 to 1o% of thorium, when used as the cathode of an electric discharge apparatus, gives at C an electron emission equal to that furnished by pure tungsten at C. Thorium is strongly radioactive.

Extraction of

and thorianite are dissolved in mineral acid and the solution evaporated down to render silica insoluble. Ammonium sulphide is added to the aqueous extract to precipitate such common metals as iron and lead. The thoria remaining in solution is separated through its carbonate, sulphate or oxalate.

Monazite is broken up with strong sulphuric acid, the solution diluted and partially neutralised with alkali (ammonia, caustic soda or magnesia), when thorium phosphate is precipitated. This

phosphate is dissolved in hydrochloric or sulphuric acid and the thorium reprecipitat?.c1 as oxalate. Further purification may be effected by means of (i.) thorium carbonate, which is more soluble than other rare-earth carbonates in sodium carbonate; (ii.) am monium thorium oxalate which is soluble; or (iii.) fractional crystallisation of thorium sulphate, Th (SO4)2,8H20. Finally, the purified thoria is converted into thorium nitrate, the salt employed in the mantle industry. The world's annual consumption of mona zite is about 3,00o tons, and of this, 200 tons are worked up in Great Britain, ilmenite (see TITANIUM) and mesothorium being useful by-products. Each kilogram of monazite on acid treatment evolves one litre of helium, also obtainable by heating the mineral to redness.

Gas

mantles are manufactured by im pregnating a woven stocking of combustible fabric (cotton or ramie) with the nitrates of thorium and cerium. The impregnated stocking is ignited to convert these nitrates into oxides, the mix ture of nitrates being adjusted so that the mixed oxides consist of 99% of thoria, and 1% of ceria, this mixture giving optimum luminosity at comparatively low temperatures. About 30o mil lions of incandescent gas mantles are consumed annually, of which the United States takes approximately 8o millions.

Thorium in combination is uniformly quadrivalent. Thoria (thorium dioxide), Th02, a white powder of specific gravity 10.20, is obtained by heating thorium salts of volatile acids or by igniting the hydroxide, Th(OH)4, a heavy white powder precipitated by alkali from thorium solutions. Thoria is a refractory material utilisable in making cupels and crucibles. Thorium tetrachloride is obtained in colourless prisms melting at C by heating thoria in the vapour of carbon tetrachloride or sulphur chloride. Thorium sulphate, Th(SO4)2, crystallises with 4, 8 and 9 molecules of water, and the nitrate with 5, 6 and 12 H20. Commercial thorium nitrate should con tain a minimum of 48% Th02.

Thorium acetylacetone, Th{CH(CO.CH3)2}4, a white crystalline product melting at C and soluble in alcohol or chloroform, is distillable in vacuo without decomposition. Thorium salts of organic acids have been employed medicinally for skin trouble and in cholera and dysentery on account of their radioactive character. Thoria is used as a catalyst in organic syntheses (ketones from acids). Inhalations of thorium emanation (thoron) are stated to be efficacious in diseases of the respiratory organs.