Though generally classed among the rare elements, titanium is probably at least as widely distributed in nature as most of the common metals. It occurs as the dioxide in small quantities in all the common silicate rocks and minerals, and in traces in the animal and vegetable kingdoms; the element has been identified in the sun and in many stars, and has been found in meteorites. Probably the commonest mineral in which the element occurs in quantity is ilmenite, or titaniferous ironstone, which occurs in enormous quantities in many parts of the world (see [p. 57]). The pure dioxide occurs in the three forms [Rutile], [Brookite], and [Anatase] (q.v.), in which it is said to be isotrimorphous with tin dioxide. Other important titanium minerals are Perovskite, Titanite or Sphene, the Euxenite series, and other minerals of the tantalo-columbate group (see [Part I]).
The commercial sources of titanium compounds are the minerals rutile and ilmenite. These may be opened up by fusion with alkali or alkali carbonate; the residue after extraction with water is dissolved in acid, and precipitated with ammonia; the mixture of iron and titanium oxides thrown down may be separated by one of the methods outlined on [p. 339]. Fusion with potassium bisulphate has also been employed. A very satisfactory method is that of Stähler,[437] in which the ore is fused with carbon in the electric furnace. The carbides so obtained are heated in a stream of chlorine, when the volatile titanium tetrachloride distils over, and may be obtained quite pure by redistillation; by appropriate methods, the required compounds may be obtained from this. (See also [pp. 326-7].)
[437] Ber. 1904, 37, 4405; 1906, 38, 2619.
The Metal.—The difficulty of isolating metallic titanium in the pure state is very great, on account of its great affinity for nitrogen, oxygen, hydrogen, carbon, etc., the ease with which it forms alloys with all the common metals, and the extremely high melting-point; in consequence, it is only within recent times that the element has been obtained in a state approximately approaching purity, and the accounts given of its physical properties vary very widely.
Berzelius prepared an impure titanium (Ti = 86 per cent.) by reduction of potassium titanofluoride with potassium; the method was modified by Wöhler, who heated a tube containing two boats, of which one was filled with the fluoride, the other with sodium, reduction being effected by the sodium vapour. Many authors have attempted the reduction of titanium tetrachloride by means of hydrogen. By heating the tetrachloride with sodium in a cast iron bomb, Nilson and Pettersson obtained a product containing 95 per cent. of the element. Reduction of the dioxide by means of sodium, magnesium, silicon, or aluminium has not been found to yield good results, by reason of the ease with which titanium alloys with these elements. Reduction of the dioxide with carbon yields good results only when precautions are taken to avoid the formation of the compound which the element so readily forms with carbon and nitrogen. Moissan[438] found that if temperatures high enough to decompose this compound were used, the product contained as the only impurity carbon, which could be partly removed by fusing with the dioxide; the product then contained 98 per cent. of titanium.
[438] Compt. rend. 1895, 120, 290.
The element has been obtained in the fused condition by Weiss and Kayser,[439] who pressed the amorphous form into sticks, under a pressure of 70,000 atmospheres, and employed these as pencils for the electric arc in vacuo; the metal fused, forming globules on the ends of the electrodes, which were detached after the apparatus had been allowed to cool.
[439] Zeitsch. anorg. Chem. 1910, 65, 388.
The amorphous element is a dark powder, resembling finely divided iron (Ferrum reductum), of density 3·5-3·6. The specific heat rises rapidly with the temperature, so that the atomic heat has the values 5·40 between 0° and 100°, 6·18 between 0° and 210°, 7·13 between 0° and 300°, and 7·77 between 0° and 440°. The amorphous element is said to be paramagnetic.
The fused carbonaceous product of Moissan formed an extremely brittle mass, with a shining white lustre on the fractured surface, sufficiently hard to scratch quartz and steel; its density was determined as 4·87. The product of Weiss and Kayser was also extremely hard and brittle; when rubbed against steel, it gave bright sparks. Its density was found to be 5·174, and the heat of combustion for the gram-atom, 97·79 K.