A New Approach to Processing Rutile from Ilmenite Ore Utilizing the Instability of Pseudobrookite
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INTRODUCTION
RUTILE (TiO2) is an important industrial raw material used in pigments, painting materials, photo catalysis, paper products, plastics, and so forth. It is also used as a raw material in the production of metallic titanium from the Kroll process. Currently, industrial-grade synthetic rutile is processed from titanium slag, sulfuric acid, and chlorine using ilmenite (FeTiO3) ore rather than natural rutile ore. Among them, titanium slag has the highest percentage of rutile, and is the main feedstock for TiO2 pigment.[1] The global demand for synthetic rutile including titanium slag was estimated to be approximately 7 million tons in 2017.[2] Despite the high demand from industry, the rutile synthesis processes currently in use are associated with
NAOKI KUMAGAI, TAKEHITO HIRAKI, and TETSUYA NAGASAKA are with the Department of Metallurgy, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. Contact e-mail: [email protected] UDAY B. PAL is with the Division of Materials Science and Engineering, Department of Mechanical Engineering, College of Engineering, Boston University, 730 Commonwealth Avenue, EMA 206, Boston, MA 02215. EIKI KASAI is with the Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan. Manuscript submitted June 26, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
significant environmental problems and challenges, including high energy consumption (approximately about 1 MWh/ton—ilmenite ore) for the titanium slag method,[3] the generation of hazardous waste (ferrosilt, which consists mainly of gypsum and iron oxide) for the sulfuric acid method,[4] the handling of corrosive and toxic gas for the chlorine method[5] and so forth. Therefore, the development of a new green process for rutile production is urgently required. In a new method for upgrading the rutile in ilmenite ore by a combination of oxidation and acid leaching,[6,7] it was reported that Australian ilmenite ore is first oxidized in air at around 1273 K according to the following reaction: 2FeTiO3 þ 1=2O2 ¼ TiO2 þ Fe2 TiO5
½1
The benefit of this method is that about half of titanium in ilmenite ore is converted to TiO2, the desired material, by simple air oxidation at a relatively low temperature. The oxidized ore is then supplied to the acid leaching process with diluted H2SO4 solution to dissolve Fe2TiO5 (pseudobrookite) into the solution. The dissolved titanium from pseudobrookite can also be recovered by hydrolysis after the leaching procedure. In this relatively simple method at relatively low temperatures, the slow dissolution rate of pseudobrookite extends the leaching time to 22 hours at 393 K. However, after oxidation at 1173 K for 40 minutes and this
extended leaching process, rutile with a purity of 90 pct is obtained. The magnetic separation technique is applied to the partly leached oxidized ilmenite ore to obtain rutile of this purity.[8] The difficulties encountered in our previous study on the synthesis of high purity rutile were largely due to the formatio
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