Microstructural Changes in Several Titaniferous Materials during

Figure 6a is a sample chlorinated for 5 min at 1223 K. The grains of rutile more or less remained the same, although cracking and caving occurred on t...
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Ind. Eng. Chem. Res. 1996, 35, 954-962

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 Cl2) 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 Ti3O5 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

Table 1. Composition of Titaniferous Materials

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 TiCl4, 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 TiO2 have been extensively studied due to its semiconductive properties (Grant, 1959). TiO2 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). TiO2 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 obtained, more fundamental information on the surface processes has not been elucidated. For example, the intrinsic chlorination kinetics per unit area of true gassolid interface would require the knowledge of the true surface area. Thus, the changes in the surface microstructure during the reaction would affect the observed kinetics. Few studies have, however, been devoted to the microstructural changes occurring during the chlorina0888-5885/96/2635-0954$12.00/0

materials synthetic rutile natural rutile titania slag beneficiated ilmenite natural ilmenite

TiO2

Fe2O3 Al2O3 SiO2 MnO2 MgO

99.99 94.44 1.12 84.65 11.08 91.71 4.28 61.29 33.12

1.67 1.30 1.31 1.0

1.52 2.13 1.09 1.23

0.0 1.79 1.19 1.1

1.2 0.4 0.3

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 examined 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 atmosphere, 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 explained 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. Analytical 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 © 1996 American Chemical Society

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Figure 1. Surface structure of synthetic rutile at different temperatures (pN2 ) 86.1 kPa, 1 h). (a) original, (b) 1223 K, (c) 1673 K.

Figure 2. Surface structure of synthetic rutile at different heating times (T ) 1673 K, pN2 ) 86.1 kPa, the atmospheric pressure at Salt Lake City). a-d are the micrographs at lower magnification; e and f are at higher magnification. (a) 1, min, (b) 2.5, (c) 10, (d) 30, (e) 1, (f) 1 min.

the desired temperature, flow nitrogen for 5 min to remove the oxygen in the system, place the material, and, when the temperature stabilized, introduce the reactant gas. At the end of each experiment, the reactor was purged with nitrogen while cooling, and the quartz reactor was taken out and cooled to room temperature. The residues were subjected to chemical as well as morphological studies. The experimental apparatus for the study of hotpressed synthetic rutile was composed of a horizontal tubular furnace with a programmable controller and a chart recorder to trace the temperature. The cooling rate was carefully selected to ensure no cracking occurred during the cooling. The flow rates of gases were chosen so as to give a uniform gas composition without much carryover of particles, which turned out to be about 10 times the minimum fluidization velocity for rutile and beneficiated ilmenite and five times for titania slag and ilmenite. In the case of hot-pressed synthetic rutile, a sufficiently high gas flow rate (1.2 L min-1) was used in order to ensure that the gas composition at the sample surface during the chlorination reaction was the same as that in the bulk stream. Results and Discussion A. Hot-Pressed Synthetic Rutile. 1. Microstructural Changes upon Heating. The rutile grains of the hot-pressed sample experienced a solid-state

sintering and grain growth process when they were heated in a nitrogen atmosphere. The driving force for this process is the reduction in the surface energy with decreasing surface area and the elimination of solidvapor interface as well as in the grain boundary energy with a decrease in the boundary area. The movement of matter in solid state is accomplished by the diffusion of defects. The diffusivity can be expressed as

D ) DI[I] + DV[V]

(1)

where DI, DV ) the diffusivities of interstitials and vacancies, respectively, and [I], [V] ) the concentrations of interstitials and vacancies, respectively. The mechanism of solid-state sintering depends on the stage the process is in. In the early stage, surface diffusion is very important; the initial particle size plays an important role in this stage. Grain boundary and volume diffusion subsequently become more important, and thus any factors that contribute to increasing the diffusion coefficient such as high temperature and low oxygen partial pressure will enhance the solid-state sintering. The mechanism of grain growth is the grain boundary migration (Kingery et al., 1976). Due to the difference in energy, grain boundaries always migrate toward the center of curvature. a. Effect of Temperature. Figure 1 shows the effect of temperature on the microstructure of hot-

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Figure 4. X-ray pattern of synthetic rutile: (a) original, (b) after reaction with chlorine.

closer examination of the surface structure under a high magnification reveals some interesting features as shown in parts e and f which are different parts of Figure 2a. At a short time, the grain surface consists of terraces (Figure 2e), or macrosteps, which are the results of crystal growth ending at the grain boundaries (Bockris and Reddy, 1970). These macrosteps come from the consumption of the original grains by the growing large grains on which they sit (Figure 2f). 2. Microstructural Changes of Synthetic Rutile during the Chlorination Reaction. In order to study the mechanism of chlorination reaction of rutile, experiments were carried out in a CO or Cl2 atmosphere. The equation of vacancy formation is Figure 3. Surface structure of synthetic rutile under different atmospheres (T ) 1673 K, 2.5 min): (a) original, (b) N2, (c) Cl2, (d) CO.

pressed synthetic rutile. At a relatively low temperature (1223 K), solid-state diffusion is not rapid, sintering is relatively slow, and grain growth is inhibited by the remaining pores. Because the diffusion coefficient and defect concentration increase exponentially with temperature, the change is quite rapid at a high temperature (1673 K). The surface structure of the sample is totally altered: Original small crystallites of rutile are substituted by large ones with only a few pores located among the grains, and the grain size is about 40 times its original size. b. Effect of Time. As expected, the longer the residence time at a fixed temperature, the larger the grain size. This can be seen in Figure 2. At very short times (Figure 2a), some small grains did not have enough time to sinter, and they just sit on the large grains. As time increased, these grains sintered more extensively, and grain boundaries disappeared (Figure 2b). At longer times, the grain surface became smoother (Figure 2c), and the grains grew larger (Figure 2d). A

1 O0 ) O2(g) + V0 + 2e 2

(2)

and the concentration of oxygen vacancy can be obtained from (Kingery et al., 1976)

[V0] )

(41)

3

(

pO2-1/6 exp -

∆G° 3kT

)

(3)

where ∆G° is the standard free energy of reaction (2). For a detailed derivation of eq 3, the reader is referred to the original reference (Kingery et al., 1976). As can be seen from this equation, a decrease of pO2 increases the concentration of oxygen vacancies and titanium interstitials, which in turn increases the diffusion coefficient and thus enhances the solid-state sintering and grain growth. This is the case in a CO atmosphere, where the equilibrium pO2 is as low as 1.7 × 10-12 kPa (Figure 3d). It can be seen that the grain size is much larger and pore fraction is less in a CO atmosphere than in a nitrogen atmosphere [pO2 ) approximately 10-2 kPa; Figure 3b).

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Figure 6. Surface structure of synthetic rutile chlorinated for different lengths of time (T ) 1223 K, pN2 ) pCO ) pCl2 ) 28.7 kPa): (a) 5, (b) 30, (c) 60 min.

Figure 5. Surface structure of synthetic rutile chlorinated for different lengths of time (T ) 1673 K, pN2 ) pCO ) pCl2 ) 28.7 kPa): (a) 5 min, lower magnification, (b) 30 min, lower magnification, (c) 5 min, higher magnification, (d) 30 min, higher magnification.

The surface structure of TiO2 under a chlorine atmosphere is quite different from that in either a nitrogen or CO atmosphere as shown in Figure 3c. This is partly because chlorine is a very reactive gas and partly because of the structure of TiO2 in which both oxygen vacancies and titanium interstitials exist. Cation interstitials have higher energy than cations at normal positions and react more easily. It can be seen from eq 2 that the formation of oxygen vacancies is thermodynamically favored by the presence of an acceptor of oxygen vacancies and titanium interstitials and, as a consequence, increases TiCl4 formation and the rates of solid-state sintering and grain growth. The individual grains cannot be identified on the surface. The final product on the surface layer is TiO and a small amount of Ti3O5, as can be seen from the X-ray pattern in Figure 4.

The surface structure of TiO2 under Cl2 + CO + N2 is shown in Figure 5, in which solid-state sintering and grain growth are slower compared with surfaces exposed to just a chlorine atmosphere (Figure 3c). Figure 5a is the surface of rutile reacted at 1673 K for 5 min in a CO + Cl2 + N2 atmosphere. Although the reaction time is longer than under Cl2 (t ) 2.5 min) and the shape of the grains is changed, the individual grains can still be distinguished (Figure 5a), the second micrograph). As stated above, chlorine plays an important role as an electron acceptor in the initial stage of chlorination reaction of rutile, solid-state sintering, and grain growth. The lower partial pressure of chlorine in the mixture of Cl2 + CO + N2 than in pure Cl2 makes the process slower. The X-ray pattern showed the main phase on the surface is rutile and TiO. Increasing the reaction time (t ) 30 min) under the same condition eliminates the TiO and grain boundaries (Figure 5b). The main phase is now rutile. A similar investigation was conducted at lower temperatures, and the results are shown in Figure 6. Solid-state sintering and grain growth are quite slow at 1223 K, and the microstructural changes are mainly due to the chlorination reaction. Figure 6a is a sample chlorinated for 5 min at 1223 K. The grains of rutile more or less remained the same, although cracking and caving occurred on the surface due to defects where the reaction took place preferentially. Chlorination continues at these sites which form continuously, creating larger cavities (Figure 6b). Further chlorination removes the grains at the surface and exposes the underlayer where pore size decreases due to solid-state sintering, and some of the grains grow

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Figure 7. Surface structure of natural rutile chlorinated for different lengths of time (pCl2 ) 43.05 kPa, pCO ) 43.05 kPa): (a) original, (b) 15 min, 1223 K, (c) 60 min, 1223 K, (d) 120 min, 1223 K, (e) 6 min, 1423 K, (f) 15 min, 1423 K.

larger. The main phases of these three samples are TiO, Ti3O5, and TiO2. 3. Implications with Respect to Kinetics Measurement. Attempts have been made to determine the rutile chlorination kinetics per unit area using pressed polycrystalline materials, based on the assumption that the surface area can be more accurately defined and determined for such large samples than for particulate materials (Leary). The surface microstructural changes observed in this work clearly show that such an assumption needs to be very carefully examined because the microstructural changes would greatly affect the true surface area. B. Natural Rutile. The microstructural changes of natural rutile particles during the chlorination reaction in a fluidized bed are shown in Figure 7. Parts a-d and parts e and f of Figure 7 are for different lengths of time at 1223 and 1423 K, respectively. The average particle size of natural rutile was 125 µm, which is much larger than that of the synthetic rutile sample discussed above. The original surface of the natural rutile is dense and smooth (Figure 7a). When heated for 15 min at 1223 K, parts of the surface with crystal defects are reacted, creating streaks, layers, and holes (Figure 7b). Continuous chlorination results in the original surface layer disappearing (Figure 7c), leaving the surface with terraces which have more exposed and activated sites (Bockris and Reddy, 1970) than a smooth surface. Further chlorination leaves the surface with some small grains which are not properly oriented to be reacted (Figure 7d). The morphological changes at a higher temperature (1423 K) reveal a similar process, except that, once chlorination starts, it continues more vigorously and causes the layers and holes to develop deeper than those at lower temperatures (Figure 7e). Further chlorination produces a surface consisting of smooth streaks which are composed of tiny streaks (Figure 7f). It is noted that natural rutile particles are more crystalline than synthetic rutile and thus undergo fewer microstructural changes during the chlorination reaction. C. Titania Slag. The solid-state sintering and grain

growth were also observed during the chlorination of ground titania slag particles with petroleum coke in a fluidized bed. The effects of reaction time and temperature on the microstructural changes of the slag were investigated. 1. Effect of Reaction Time. The changes in the surface structure during the chlorination reaction of titania slag are shown in Figure 8. Figure 8a is the original slag particle surface, which shows no pores. Figure 8b shows a particle surface after chlorination for 15 min at 1393 K with 11% of Ti and 70% of the Fe chlorinated. The surface is covered with small crystallites about 3 µm in diameter. Figure 8c is the surface of a particle chlorinated for 30 min with 28% of the Ti and 80% of the Fe chlorinated, and Figure 8d is the surface of a particle chlorinated for 90 min with 95% of the Ti and 99% of the iron chlorinated. The EDX examination shows that these crystallites are composed mainly of rutile with small amounts of SiO2, MgO, and sometimes Al2O3, but no iron oxides. The chlorination reaction starts with the chlorination of easily chlorinated elements, such as iron exposed on the surface, leaving the particle surface with pores, crystallites of TiO2, and other chlorination-resistant oxides. Then, the chlorination proceeds toward the center topochemically. The outer shell is depleted of TiO2 and gradually enriched with impurities that hinder the reaction by inhibiting the gas diffusion. The EPMA of particle cross section showed the elemental distribution where the unchlorinated impurities containing silica collected largely at the outer shell of the particle, with the unreacted core consisting of TiO2. 2. Effect of Temperature. Increasing reaction temperature promotes solid-state sintering and grain growth. Figure 9 shows the changes in the surface structure at various temperatures. The individual grains of slag are very distinct at a low temperature (Figure 9a), but they are sintered at a higher temperature (Figure 9c). A mixed microstructure can be seen at an intermediate temperature (Figure 9b). It is apparent that, at lower temperatures, the crystallites are larger than at higher temperatures. This may be partially due to the fact that lower temperature benefits

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Figure 9. Surface structure of titania slag chlorinated at different temperatures (pCl2 ) 86.1 kPa, 15 min): (a) 1223, (b) 1273, (c) 1323 K.

Figure 8. Surface structure of titania slag chlorinated for different lengths of time (T ) 1393 K, pCl2 ) 86.1 kPa): (a) original, (b) 15 min, (c) 60 min, (d) 90 min.

the chlorination of iron relative to that of TiO2 and partially due to the fact that, at lower temperatures, the grain growth and sintering are slow. The total surface area does not vary greatly with temperature. The same results were obtained by the BET analysis. 3. Significance. The substantial amount of iron oxide contained in titania slag contributes significantly to the difference in the way the morphology of the solid changes during the chlorination reaction. When rutile is chlorinated, the generated surface morphology is largely limited to a narrow layer near the external surface. In the case of titania slag, the rapid chlorination of easily chlorinated iron oxide creates porosity that extends deeply into the interior of the particles. The remaining titanium oxide phase has a much greater surface area on which the chlorination reaction can occur. Thus, the rate of chlorination of titanium slag

per unit area of the apparent external surface is expected to be considerably higher than that of rutile. D. Beneficiated Ilmenite. The chlorination rate of beneficiated ilmenite particles is faster and the temperature at which the reaction starts is lower compared with any other titanium-bearing minerals (Zhou, 1994). The surface structure of the raw material reveals the reason. It contains numerous pores formed during the beneficiation process (Figure 11a). Furthermore, the crystal defect structure in this sample may be different from those of other titaniferous minerals. 1. Effect of Reaction Time. The chlorination of beneficiated ilmenite starts at the existing pores with those easily chlorinated elements such as iron and crystal defects on TiO2 grains (Figure 10a) and continues at these sites (Figure 10b). Further chlorination results in a special texture on the surface (Figure 10c), which is due to the solid-state sintering and crystal growth. Continued reaction from the new texture results in a surface consisting of large pores and skeleton structure, in which individual rutile grains cannot be distinguished (Figure 10d). Attempts have been made to correlate the kinetic data with some reaction models with rather limited success (Leary; Zhou, 1994). The main reason for this is due to the microstructural changes during the reaction and the fact that a single reaction model cannot incorporate the actual complex reaction mechanism. 2. Effect of Reaction Temperature. Since beneficiated ilmenite is porous, gas diffusion is easy, and the rate-controlling step should be the chemical reaction at the interface. Increasing reaction temperature will

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Figure 10. Surface structure of beneficiated ilmenite chlorinated for different lengths of time (T ) 1173 K, pCl2 ) pCO ) 43.05 kPa): (a) 5, (b) 20, (c) 45, (d) 60 min.

Figure 11. Surface structure of beneficiated ilmenite chlorinated at different temperatures (pCl2 ) pCO ) 43.05 kPa). Lower magnification: (a) original, (b) 1223 K, 5 min, (c) 1323 K, 5 min. Higher magnification: (d) original, (e) 1223 K, 5 min, (f) 1323 K, 5 min.

strongly increase reaction rate and change the surface structure. This can be seen in Figure 11. A closer look at high magnifications shows that the porosity is much higher at high temperatures (Figure 11c) than at lower temperature (Figure 11b). A magnified micrograph of the surface of an original particle (Figure 11a) is included for comparison. E. Ilmenite. Morphological changes during the chlorination of ilmenite particles with petroleum coke in a fluidized bed are shown in Figure 12, in which Figure 12a is the original particle surface, Figure 12b is the particle chlorinated for 15 min at 1393 K with 80% of the Fe and 8% of the Ti chlorinated, and Figure 12c is the particle chlorinated for 120 min with 96% of the Fe and 99% of the Ti chlorinated. Iron is chlorinated

first, followed by titanium oxide. Titanium oxide grains undergo solid-state sintering and growth (Figure 12b). At the completion of the chlorination reaction, the solid particle is composed of chlorination-resistant compounds, such as silicates (Figure 12c), which may contain some iron oxide. The effect of temperature on the microstructural changes of ilmenite chlorination is shown in Figure 13. A previous study (Rhee and Sohn, 1990a) has shown that the ilmenite phase disappeared after a short initial period of reaction, and the iron is then present in the hematite phase. In this study, the original samples in Figure 13 were found to contain about 10% Fe2O3 and 85% TiO2. The main phase is rutile. At a low temperature (1223 K), rutile grains are small and the chlorina-

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Figure 12. Surface structure of ilmenite chlorinated for different lengths of time (pCl2 ) 86.1 kPa, T ) 1393 K): (a) original, (b) 15 min, (c) 120 min.

Figure 13. Surface structure of ilmenite chlorinated at different temperatures (pCl2 ) 86.1 kPa, 15 min): (a) 1223, (b) 1273 , (c) 1393 K.

tion of iron is much faster than that of TiO2 with about 85% of iron removed and no TiO2 reacted (Figure 13a). At a higher temperature (1323 K), rutile grain sintering and growth were observed again, and the relative rate of iron chlorination is somewhat lower (Figure 13b). At a still higher temperature (1393 K), TiO2 is also chlorinated, and thus the porosity near the surface is increased (Figure 13c).

Acknowledgment

Conclusions

Literature Cited

Hot-pressed synthetic rutile samples experience a solid-state sintering and grain growth when they are heated up to high temperatures. The final grain size depends on the temperature, partial pressure of oxygen, heating rate, and time. Increasing the temperature and lowering the pO2 at a fixed heating rate and residence time greatly enhance the process. Chlorine plays a very important role in the initial stage of the chlorination reaction of rutile. Chlorine, as an electron acceptor, promotes solid-state sintering and grain growth while reacting with titanium cations. The final products on the surface layer are TiO2, TiO, and a small amount of Ti3O5. During fluidized-bed chlorination, titanium-bearing materials also experienced solid-state sintering and grain growth. The extent depends on the reaction time and temperature. The microstructural changes of titanium-bearing materials during the chlorination reaction must be taken into consideration in the analysis of their chlorination kinetics, as discussed in more detail in the text.

Barksdale, J. Titanium, Its Occurrence, Chemistry, and Technology; Renald Press Co.: New York, 1966; pp 10-46. Biswas, R. E.; Habib, M. A.; Dafader, N. C. A Study of the Recovery of Titanium from Hydrofluoric Acid Leach Solution of Ilmenite Sand. Hydrometallurgy 1992, 28, 119-126. Bockris, J. O’M.; Reddy, A. K. N. Modern Electrochemistry; Plenum Press: New York, 1970; Vol. 2, pp 1204-1207, 1176-1183. Crane, S. R.; Davidson, C. F.; Harbuck, D. D. Titanium Metal Production from Perovskite Using a Sulfate-Fluoride System. In Light Metals; TMS: Warrendale, PA, 1989. Drobeck, D. C. Kinetics of Phase Separation in the Titanium Dioxide-Tin Dioxide System. Ph.D. Thesis, University of Utah, Salt Lake City, UT, 1990; pp 4-6. Elger, G. W.; Tress, J. E.; Jordan, R. R. Utilization of Domestic Low-Grade Titaniferous Materials for Producing Titanium Tetrachloride. In Light Metals; Andersen, J. E., Ed.; TMS: Warrendale, PA, 1982; pp 1135-1147. Grant, F. A. Properties of Rutile (Titanium Oxide). Rev. Mod. Phys. 1959, 31, 646-674. Harris, H. M.; Henderson, A. W.; Campbell, T. T. Fluidized CokeBed Chlorination of Ilmenites; U.S. Bureau of Mines: Washington, DC, 1976; R. I. 8165, pp 19. Kahn, J. A. Non-Rutile Feed Stocks for the Production of Titanium. J. Met. 1984, 36 (7), 33-38.

This work was supported in part by Du Pont Chemicals through an unrestricted research grant to the University of Utah. L.Z. received during the course of this work a Graduate Fellowship from Utah MMRRI and a University of Utah Graduate Research Fellowship.

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Kingery, W. D.; Bowen, H. K.; Uhlmann, D. R. Introduction to Ceramics; John Wiley & Sons: New York, 1976; pp 449-491. Lan, Y.; Zhu, Z.; Lui, T. An Approach to Plasma Smelting Process of Luda Ilmenite. Acta Metall. Sin. 1991, 27 (4), B283-285. Leary, K. J. Du Pont Co., Wilmington, DE, unpublished results. Morris A. J.; Jensen, R. F. Fluidized-Bed Chlorination Rates of Australian Rutile. Metall. Trans. B 1976, 7B, 89-93. Rao, Y. K.; Chadwick, B. K. Chlorination of Rutile (TiO2) with COCl2-He Gas Mixtures. Trans. Inst. Min. Metall., Sect. C 1988, 97, 167-179. Rhee, K. I.; Sohn, H. Y. The Selective Chlorination of Iron from Ilmenite Ore by CO-Cl2 Mixture: Part I. Intrinsic Kinetics. Metall. Trans. B 1990a, 21B, 321-330. Rhee, K. I.; Sohn, H. Y. The Selective Chlorination of Iron from Ilmenite Ore by CO-CL2 Mixture: Part II. Mathematical Modeling of the Fluidized Bed Process. Metall. Trans. B 1990b, 21B, 331-339.

Rhee, K. I.; Sohn, H. Y. The Selective Carbochlorination of Iron from Titaniferous Magnetite Ore in a Fluidized-Bed. Metall. Trans. B 1990c, 21B, 341-347. Yuan, T. C.; Virkar, A. V. Kinetics of Spinodal Decomposition in the TiO2-SnO2 System: Effect of Aliovalent Dopants. J. Am. Ceram. Soc. 1988, 71, 12-21. Zhou, L. Fluidized Bed Chlorination of Several Titaniferous MaterialssKinetics, Morphological Changes and Mathematical Modeling. Ph.D. Dissertation, University of Utah, Salt Lake City, UT, 1994.

Received for review September 19, 1995 Accepted October 31, 1995X IE940650+ Abstract published in Advance ACS Abstracts, January 15, 1996. X