Microstructural Changes in Several Titaniferous Materials during Chlorination Reaction Ling Zhou and Hong Yong Sohn* Department of Metallurgical Engineering and of Chemical and Fuels Engineering, University of Utah, Salt Lake City, Utah 84112-1183 Gary K. Whiting and Kevin J. Leary Du Pont Chemicals, Wilmington, Delaware 19898 The microstructural changes in titaniferous oxide materials during the chlorination reaction were studied under inert and reactive gas atmospheres (CO and Cl 2 ) using SEM and X-ray diffraction. Rutile undergoes grain growth and solid-state sintering at high temperatures, the extent of which increases with increasing temperature and decreasing partial pressure of oxygen. Chlorine, which is an electron acceptor, also promotes this process while it reacts with titanium cations, which greatly changes the surface structure with TiO and Ti 3 O 5 as final phases on the surface. The surface structures developed during these changes have significant implications for the rutile chlorination kinetics. Titanium minerals such as titania slag, beneficiated ilmenite, and ilmenite undergo similar solid-state sintering and grain growth during the chlorination reaction, but the process is more complex. Introduction Titanium and its alloys have been popular materials in aerospace and commercial industries since it was first produced in large scale in 1954 by Du Pont using the Kroll process (Barksdale, 1966). Although its reserves are diminishing and mining costs are increasing, rutile is the most favored raw material used to produce the intermediate product TiCl 4 , which is utilized to produce titanium metal or titania pigment. Titania slag and beneficiated ilmenite are increasingly in demand as the substitute for rutile (Kahn, 1984). Much effort has been devoted to finding an economical way to use the abundant low-grade minerals like ilmenite (Biswas et al., 1992; Crane et al., 1989; Elger et al., 1982; Harris et al., 1976; Kahn, 1984; Lan et al., 1991). The structure and properties of TiO 2 have been extensively studied due to its semiconductive properties (Grant, 1959). TiO 2 deviates from its stoichiometry to the metal-rich side, which results from the formation of oxygen vacancies and cation interstitials. The latter dominate the cation diffusion (Yuan and Virkar, 1988), which is a faster process than the oxygen vacancy diffusion (Kingery et al., 1976). TiO 2 occurs naturally in rutile structures as well as in the polymorph brookite and anatase, with the rutile structure being the most stable (Drobeck, 1990). Many studies have been made on the kinetics of the reactions (Morris and Jensen, 1976; Rao and Chadwick, 1988; Rhee and Sohn, 1990a-c) and the upgrading of low titanium-containing materials like ilmenite (Crane et al., 1989; Elger et al., 1982; Harris et al., 1976; Kahn, 1984; Lan et al., 1991). While global kinetics of the chlorination of particulate materials have been ob- tained, more fundamental information on the surface processes has not been elucidated. For example, the intrinsic chlorination kinetics per unit area of true gas- solid interface would require the knowledge of the true surface area. Thus, the changes in the surface micro- structure during the reaction would affect the observed kinetics. Few studies have, however, been devoted to the microstructural changes occurring during the chlorina- tion reaction of titanium-bearing materials. Thus, as part of an overall investigation of the chlorination characteristics of several different titaniferous materials (Zhou, 1994), such microstructural changes were exam- ined in detail in this work. Hot-pressed synthetic rutile and several other materials in particulate form were used to examine the effect of temperature, gas atmo- sphere, and residence time on the microstructural changes during the heating process as well as during the chlorination reaction. The reasons for examining the behavior of hot-pressed synthetic rutile are ex- plained later in the section on experimental results. The effects of different gas atmospheres were also examined because in the chlorination process the titaniferous minerals experience different gas atmospheres. Ana- lytical techniques such as scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction, and X-ray fluorescence analysis were used. Experimental Section A. Materials. The samples of natural rutile, titania slag, beneficiated ilmenite, ilmenite, and hot-pressed synthetic rutile were all provided by Du Pont Co. The synthetic rutile was flat pieces of 1 × 1 × 0.1 cm size. All other materials were 28-252 μm, with an average size of 125 μm. Their compositions are listed in Table 1. The petroleum coke used for the chlorination of titania slag and ilmenite was obtained from Unocal. The chlorine, carbon monoxide, and nitrogen gases were supplied by Air Product Co. B. Apparatus and Procedure. The chlorination reaction of titanium minerals was carried out in a fluidized bed, which consisted of a gas inlet system, quartz reactor, and product gas cooling system. The experimental procedure was to preheat the reactor to Table 1. Composition of Titaniferous Materials materials TiO2 Fe2O3 Al2O3 SiO2 MnO2 MgO synthetic rutile 99.99 natural rutile 94.44 1.12 1.67 1.52 0.0 titania slag 84.65 11.08 1.30 2.13 1.79 1.2 beneficiated ilmenite 91.71 4.28 1.31 1.09 1.19 0.4 natural ilmenite 61.29 33.12 1.0 1.23 1.1 0.3 954 Ind. Eng. Chem. Res. 1996, 35, 954-962 0888-5885/96/2635-0954$12.00/0 © 1996 American Chemical Society