Extractive Metallurgy Division - Some Thermodynamical Considerations in the Chlorination of Ilmenite

Chlorination of the various constituents of ilmenite by different chlorinating agents in presence of various reducing agents, have been considered on the basis of the standard free energy and standard enthalpy changes as a function of temperature. The standard free energy change considerations show that it is beneficial to chlorinate ilmenite by chlorine in the presence of carbon and also that iron constituent of ilmenite can be preferentially chlorinated by clzlorine, titanium tetrachloride or their mixture. These findilzgs have been corroborated from the published work. METALLURGICAL processes involving the use of titanium tetrachloride have gained in importance because of the use of the latter in the manufacture of titanium metal. Since ilmenite is more abundant in nature than any other titanium mineral, the future of the metallurgical processes depends on the utilization of ilmenite for the production of titanium tetrachloride. In these laboratories, investigations have been carried out on the chlorination of ilmenite under a variety of conditions.1'2 During these studies, it was noticed that 1) preferential chlorination of iron was effected at low temperatures (400° to 600°C) and at low carbon content (6 to 7 pct), 2) carbonyl chloride retarded the chlorination of iron oxides and titania perceptibly, while 3) carbon-tetrachloride, compounds of sulphur and some other catalysts favored the chlorination. Moles3 has found that oxides of iron are chlorinated in preference to titania at high temperatures, while wilcox4 has claimed the preferential chlorination of titania between 1200" and 1500°C. It has been shown in this paper that preferential chlorination of titania claimed by Wilcox is not likely to occur. Daubenspeck and coworkers5,6 have claimed the preferential chlorination of iron by chlorine or by a mixture of titanium tetrachloride and chlorine between 700° and 1050°C in the absence of carbon. Even when plain titanium tetrachloride is employed as the chlorinating agent, pascaud7 noticed the preferential chlorination of iron and other oxides. The purpose of this paper is to explain from thermodynamical considerations, the various chlorination reactions studied so far. ILMENITE CONSTITUENTS AND THEIR CHLORINATION PRODUCTS Although the general composition of the ilmenite mineral is represented as FeTiO,, most of the ilmenites found in nature have variable quantities of TiO2 (44.6 to 64 pct), FeO (4.7 to 36 pct) and Fe2O3 (6.9 to 28 pct).8 The higher content of ferric iron in ilmenites was attributed by Millerg to the presence of arizonite (Fe2O3.3TiO2). But the X-ray studies by Overholt, Vaw, and odd" have shown that arizonite is a mixture of haematite, ilmenite, anatase, and rutile. Except for the anatase, similar views have been advanced by Lynd, Sigurdson, North, and Anderson8 from magnetic, X-ray, and optical and electron microscope studies. The ilmenite ores can, therefore, be assumed to consist of mineral aggregates of ilmenite, rutile and haematite. From the free energy of formation of ilmenite (FeTiO3), it has been shown by Kelley, Todd, and King11 that ilmenite is stable even up to its melting point (1367°C) and would not undergo decomposition into its constituent oxides. Schomate, Naylor, and Boericke12 have found that in the presence of a reducing agent the iron constituent of ilmenite is selectively reduced. The reaction of chlorine with ilmenite in presence of a reducing agent can, therefore, be synonymous with that of the reaction of chlorine with the constituents of ilmenite, viz., TiO2, FeO, and Fe2O3. Most of the reaction products of chlorination of ilmenite in the presence of reducing agents will be in equilibrium with their dissociation products, depending on the temperature. The titanium tetrachloride is, however, quite stable up to 1500°C due to its covalent nature. The equilibrium for the ferric chloride system has been investigated by Kangro and Bernstorff, 13, schafer14 and Kangro and petersen,15 and the results are summarized in Fig. 1, curves a, b, and c respectively. From these results, it is clear that the ferric chloride disociates as follows: 324° to 700°C FeaCl6(g) ?2FeCl2(c) + Cl2(g) [1] 324°to 900°C Fe2Cl6(g) =2 Fe Cl2 Reaction [I] (curve a) occurs in the forward direction to about 6 pct at 400°C but falls off very rapidly with increase in temperature and beyond 600°C, it is practically negligible, perhaps due to the formation of the stable monomer, FeC13(g). As the temperature is further increased, the amount of FeCl,(g) in-