Ind. Eng. Chem. Res. 2002, 41, 4841-4847
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Separation of Titanium Tetrafluoride from a Gaseous Mixture with Silicon Tetrafluoride Using Lithium Fluoride Y. Tsaur, T. K. Pong,* J. Besida, T. A. O’Donnell, and D. G. Wood Department of Chemical Engineering, The University of Melbourne, Victoria 3010, Australia
A new process for producing pigment grade TiO2 from low grade ores such as ilmenite is being developed at the University of Melbourne. In this process, the titanium content of the ore is extracted by reaction with SiF4 at about 800 °C to produce a gaseous intermediate product TiF4. The TiF4 leaves the reactor with SiO2 fume and unreacted SiF4 gas and needs to be isolated. One method of separating TiF4 from the gas mixture is by reaction with LiF at 350-450 °C to form a stable solid, Li2TiF6. Experimental work has been performed to assess the feasibility of this separation method. It is shown that, under carefully chosen conditions, the LiF reacts selectively with TiF4 but not with SiF4. The reaction product, Li2TiF6, can be thermally decomposed to regenerate the LiF and TiF4. The pure TiF4 generated can be further treated to produce pigment grade TiO2, and the LiF can be recycled. Provided SiO2 is separated, the SiF4 gas can be recycled and returned to the fluoridation reactor. 1. Introduction A new process for producing pigment grade TiO2 from low grade ores such as ilmenite is being developed at the University of Melbourne. This process uses a fluoride route, and SiF4 is used to react with the titanium content of the ilmenite at ∼800 °C in a heterogeneous reaction and extract the titanium from the ilmenite as gaseous TiF4, an important intermediate product for producing TiO2. The advantages of this process over existing commercial process for producing TiO2 have been discussed elsewhere.1-3 In the reactor where this solid/gas reaction takes place, excess SiF4 is usually present in the gas phase. The TiF4 upon formation must be isolated from the SiF4 as well as other byproducts generated in the reaction. The aim of this study is to develop a process for separating the TiF4 from other gaseous species in the reaction mixture and subsequently regenerating the TiF4 in a pure form. In the proposed strategy, LiF is allowed to react with the TiF4 in the reaction mixture to form a stable solid, Li2TiF6, and thereby physically removing the TiF4 from the gas phase. To assess the feasibility of using LiF for separating TiF4 from the gas mixture, experimental studies have been performed, and some of the results are discussed in this paper. 2. Background When a titaniferous ore, such as ilmenite, is exposed to SiF4 at ∼800 °C, the basic reaction may be written as
TiO2(s) + SiF4(g) h TiF4(g) + SiO2(fume)
(1)
TiF4 produced is a gas at ∼800 °C, and upon cooling to temperatures 550 °C, the pressure increase may be attributed to the normal expansion of the gaseous phase. 5.2. Reactivity of LiF with SiF4. If gaseous SiF4 reacts with LiF in a heterogeneous reaction to produce a stable solid compound, Li2SiF6, a reduction in pressure of the gas phase in a reactor with a fixed volume is expected. The change in pressure of SiF4 in contact with LiF for temperature up to 800 °C, as shown in Figure 4, is found to be almost the same with or without LiF being in the reactor. The results show that there is no significant reaction between SiF4 and LiF over the range of temperature studied. The pressure increase is found to be linear, and this increase merely represents the normal expansion of the gas phase with temperature. Chemically, the reaction between SiF4 and LiF is expected to occur at some relatively low temperatures. Close examination of the pressure curves show that there may be very very low level of interaction between LiF and SiF4 at temperatures between 70 and 150 °C. However, if the reaction did occur, then the extent of conversion is rather low, and the product generated is not thermally stable.
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Ind. Eng. Chem. Res., Vol. 41, No. 19, 2002
Figure 4. Change in pressure with temperature when SiF4 alone and SiF4 in contact with LiF are heated slowly in a static reactor.
Figure 5. IR spectrum of product of reaction between LiF and TiF4 (run M17).
5.3. Kinetics of Reaction between LiF and TiF4. Having identified the appropriate temperature for the LiF/TiF4 reaction, preliminary experiments are performed to establish the reaction kinetics and to assess the effectiveness of this capturing operation in a batch reactor. Excess TiF4 is used in the present work because a significant quantity of the TiF4 in the reactor is vaporized during the initial heating period and is deposited on the lid of the reactor upon condensation. Hence, this condensed TiF4 is not available for reaction with LiF, and excess of TiF4 is needed to compensate for the loss. When fixed masses of TiF4 and LiF are heated to the reaction temperature (∼390-435 °C) and allowed to react with each other for a fixed period of time, Li2TiF6 is produced, and an increase in the mass of the solid reactant is found. This mass increase is used to determine the extent of LiF reaction, and the results are shown in Table 1. This method of determining the LiF conversion is found to be reliable because the IR spectrum of the solid residue recovered at the end of a run shows that the sample contains a significant amount of Li2TiF6. Since the LiF and the TiF4 are initially placed in separate compartments of the sample cup, the presence of titanium in the final lithium residue gives a conclusive indication that there has been active reaction between the LiF and the gaseous TiF4. Additionally, other than LiF and Li2TiF6, no other compound is found in the sample. A typical spectrum of the residue is shown in Figure 5, and a simple guide for identifying the relevant absorption peaks and bands displayed in the IR spectrum is provided in Table 4. The results show that in this experimental setup the conversion of LiF is generally 320 °C, a sample of Li2SiF6 was heated to 315 °C and maintained at this temperature for 4 h (run M6). The final pressure in the reactor was found to be ∼180 mmHg. At the end of the run, the gaseous phase in the reactor was evacuated before the reactor was allowed to cool and the residue retrieved. Analysis of the residue by IR spectroscopy showed that the residue contains mainly LiF, as shown in Figure 11. Simple calculations based on mass loss of the sample indicate that about 99% of the initial Li2SiF6 had been decomposed. 6. Application of the Separation Process The proposed technique for separating TiF4 from SiF4 can be applied to the present proposed pigment production process based on a fluoride route, and a schematic diagram of the process is given in Figure 12. The raw ilmenite is allowed to react with SiF4 at ∼800 °C in a semicontinuous reactor. The SiF4 is passed through the
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Figure 12. Schematic diagram of the proposed process for producing pigment grade TiO2.
reactor at a fixed rate. The gaseous product leaving the reactor, containing mainly SiF4, TiF4, and SiO2, is first passed through an electrostatic precipitator where the SiO2 is separated. The SiF4/TiF4 mixture is then passed through a column, containing a packed bed of LiF, in which the TiF4 is captured and separated from the SiF4. The unreacted SiF4 is mixed with the makeup SiF4, preheated, and returned to the fluoridation reactor. After a certain reaction time, the LiF in the packed bed will eventually become saturated with TiF4, and it must be renewed and the TiF4 regenerated. To implement this, a twin-column system is proposed in this design. When one column is used for capturing TiF4 at ∼400 °C, the other column is heated to ∼600 °C to bring about the decomposition of Li2TiF6 and the regeneration of LiF and TiF4. By operating the columns as an absorber and a regenerator in an alternating manner, TiF4 can be continuously removed from the gaseous mixture containing SiF4. The pure TiF4 produced in the regeneration column is cooled and dissolved in methanol. The dissolved TiF4 is allowed to react with NH4HF2 to produce a stable precipitate, (NH4)2TiF6. This is a fast reaction and can be carried out in a continuously stirred tank reactor. The precipitate can be separated from the methanol by filtration. The dried (NH4)2TiF6 is pyrohydrolyzed in the presence of moisture and air at ∼300 °C to produce gaseous NH4HF2 and HF and a solid TiO2(anatase). Upon cooling, the (NH4)HF2 condenses to form a solid which may be recycled, and the HF is collected as an aqueous solution which can be used to regenerate SiF4 from the SiO2 produced in the basic reaction. The TiO2 in anatase form is finally calcined to give TiO2 in the rutile form. 7. Conclusions The results presented show that there exists a window between 350 and 450 °C within which separation of TiF4 from SiF4 can be successfully performed. At this temperature LiF will react with TiF4 to form a stable compound, Li2TiF6, but the corresponding reaction between LiF and SiF4 will not take place because the expected product of reaction, Li2SiF6, is not stable. It has been demonstrated that at ∼1 atm the reaction between TiF4 and LiF takes place at a reasonable rate in a batch system and in a semicontinuous system. The extent of conversion is found to increase with reaction time and exposed surface area of reaction bed.
Experience from the experimental study shows that the appropriate flow rate of SiF4 and rate of TiF4 vaporization are critical for the success of the separation because these parameters directly affect the contact time for the active chemicals. A reasonably high uptake of TiF4 by LiF can only be achieved if the flow rate of the SiF4 is sufficiently low and if the TiF4 is not devolatilized and carried away too quickly. Application of a longer contact time and larger reaction surface area are beneficial for obtaining a higher LiF conversion. The proposed separation scheme can be applied to a continuous process for producing pigment grade TiO2, and a schematic diagram of the process is presented. Acknowledgment The authors gratefully acknowledge the support of the Australian Research Council. Literature Cited (1) Pong, T. K.; Besida, J.; O’Donnell, T. A.; Wood, D. G. A Novel Process for Producing TiO2 from Ilmenite. Proc. CHEMECA’94 1994, Perth, Australia. (2) Pong, T. K.; Besida, J.; O’Donnell, T. A.; Wood, D. G. A Novel Process for Producing TiO2 from Titaniferous Ore. Ind. Eng. Chem. Res. 1995, 34, 308. (3) Besida, J.; O’Donnell, T. A.; Pong, T. K.; Wood, D. G. Industrial Processes Based on Silicon Tetrafluoride. Proc. 11th Eur. Symp. Fluorine Chem. 1995, Slovenia. (4) Besida, J.; Pong, T. K.; O’Donnell, T. A.; Wood, D. G. Fluorotitanates, Pyrohydrolysis and Chemical Aspects Associated with a Novel Fluoride Process for the Production of Pigment grade TiO2. Proc. RACI 10th National Convention 1995, Adelaide, Australia. (5) Pong, T. K.; Besida, J.; Bear, D.; O’Donnell, T. A.; Wood, D. G. The Gas-Phase Equilibrium for the Fluoridation of Ilmenite Using SiF4. Proc. 7th Congress of APPChE 1996, Taipei, Taiwan. (6) Pong, T. K.; Tsaur, Y.; Besida, J.; O’Donnell, T. A.; Wood, D. G. Conceptual Design of a Continuous Process for Producing Pigment Grade TiO2. Proc. CHEMECA’2000 2000, Perth, Australia. (7) Janz, G. J.; Lorenz, M. R.; Brown, C. T. Preparation and Thermal Stability of Lithium Titanium Fluoride. J. Am. Chem. Soc. 1958, 80, 4126. (8) Deadmore, D. L.; Machin, J. S.; Allen, A. W. Stability of Inorganic Fluorine-bearing Compounds: II. Complex fluorides. J. Am. Ceram. Soc. 1962, 45, 120. (9) Nyquist, R. A.; Kagel, R. O. Handbook of Infrared and Raman Spectrum of Inorganic Compounds and Organic Salts; Academic Press: New York, 1997; Vol. 4.
Received for review October 1, 2001 Revised manuscript received April 20, 2002 Accepted April 20, 2002 IE0108245
