Due to its great affinity for a large number of elements, the preparation of titanium poses considerable difficulties. In particular, N, C and O dissolve to an appreciable extent in the metallic phase, and cause cold-shortness even when present in minute quantities. They cannot be removed either chemically or by sintering or melting in high vacuum. Consequently, the relatively easy conversion of TiO2 with Ca (method I below) yields only 98% pure metal, even under conditions where the highest purity of apparatus and raw materials is maintained.

Pure metal that is ductile while cold can therefore be pre - pared only by methods which use halides as the starting materials. However, these procedures, which are based on the reactions K2TiF6 (or Na2TiF6 ) + Na (method II) or TiCl4 + Na (method III below), suffer from the drawback that the deposited metal is usually porous or flaky, which leads to reoxidation during the removal of the alkali halide by-product; it is therefore used only as a crude starting material for the purification process. Nevertheless, careful adherence to a number of precautionary measures permits the preparation, even by these methods, of pure metal which can be cold-worked. The Kroll magnesium process (method IV), which utilizes the reaction between TiCl 4 and Mg, is used at present both in the laboratory and in industry.

The highest purity (0.03% C and ~0.006% N) is attained via the elegant recovery process of van Arkel and de Boer (method V below). This is based on the thermal decomposition of titanium iodide at 1100-1500°C.

(1) PREPARATION OF CRUDE METAL FROM THE OXIDE AND CALCIUM

TiO2 + 2Ca = Ti + 2CaO

PureTiO2 (770 g.), turnings of distilled Ca (770 g.), and fused and pulverized CaCl2/BaCl2 (750 g./250 g.) are mixed and pressed into briquets, which are allowed to react under 99.2% Ar in an electric furnace at > 700°C. The addition of the salts is necessary to moderate the reaction and, above all, to prevent the formation of CaTiO3, a product which does not react with Ca even on repeated reduction. The use of CaH2 in the second reduction has proved useful, since the powdery hydride mixes very readily with the other reactants while the nascent Ha it evolves is a powerful reducing agent. Thus, 348 g. of Ti (from the first reduction stage) + 400 g. of CaCl2/BaCl2 (3:1) + 50 g. of Ca + 50 g. of CaH2 gave a yield of 337 g. of metal after heating for one hour at 1000°C under 99.6% Ar. The very well-sintered product is crushed and washed with water and concentrated hydrochloric acid, yielding fairly homogeneous granules.

(2) PREPARATION OF CRUDE METAL FROM FLUORIDES AND SODIUM

Na2TiF8 + 4Na = Ti + 6NaF or K2TiF6 + 4Na = Ti + 2KF + 4NaF

(3) TiCl4 + 4Na = Ti + 4NaCl

If the reagent quantities are small, the welded steel bomb described in method I can be used. The temperatures must be very high (to start the reaction, the bomb must be red-hot) and thus the TiCl4 vapor pressure is very high. Larger quantities (500 g. of TiCl4 + 245 g. of Na) must therefore be heated in a thick-wall steel bomb, the lid of which is sealed on with a copper gasket and secured with a heavy screwed-on cap

(4) KROLL MAGNESIUM PROCESS

TiCl4 + 2Mg = Ti + 2MgCl2

Magnesium works just as well in the reduction of TiCl4 as sodium; in addition, commercial magnesium is already very pure and may be handled in air without special precautions. Thus, magnesium is the preferred reducing agent.

(5) THE REFINING PROCESS OF VAN ARKEL AND DE BOER

Til4 = Ti + 2Ι₂

The iodides are used for the preparation of small quantities (~20-30 g.) of metal; these highly hygroscopic compounds are not introduced directly as raw materials, but are produced as intermediates during the process in which they form from crude metal and iodine. The most suitable crude titanium for this process is that prepared from TiCl4 and Na. Titanium oxide, nitride or carbide are attacked by the iodine; thus, the corresponding nonmetals are left unchanged and do not incorporate into the growing metal ingot. The weak point of this refining process is that a considerable number of other metals (e.g., Zr, Hf, Th, V, B, Si, as well as Al and Fe if the filament temperature is low) are codeposited with the desired titanium; therefore, these impurities should be removed during the preparation of the crude metal, that is, prior to refining.

(6) PREPARATION BY ELECTROLYSIS OF MELTS

Crude titanium may be obtained by electrolysis of a solution of TiO or mixed TiO-TiC crystals in a CaCl 3melt at 700-850°C. The electrolytic decomposition of K3TiFe in a bath of NaCl, on the other hand, yields a very pure, coarsely crystalline metal. In this process the melt becomes enriched in NaF, according to the overall equation.

K2TiF6 + 4 NaCl = 2 KF + 4 NaF + Ti + 2 Cl2

(7) REDUCTION OF TiO2 WITH CaH2

A mixture of TiO2 and CaH2 (in 40% excess) is heated for one hour in hydrogen at atmospheric pressure (electric furnace, 950- 1075 °C); the product is treated with dilute hydrochloric acid. A fine powder, with a metal content of 96%, is obtained; the remainder is mainly H2 (3%). This process is also suitable for preparation of V, Nb and Ta from their oxides.