Kinetics and Reaction Mechanism of Soda Ash Roasting of Ilmenite Ore for the Extraction of Titanium Dioxide
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INTRODUCTION
TITANIUM dioxide is the most widely used white pigment because of its high brightness and refractive index. Anatase, brookite, and rutile are the natural forms of titanium dioxide minerals. Of the many titanates, only a few are important industrially, particularly those of iron and calcium. Ilmenite is a heavy mineral of metallic to submetallic luster, which is formed in igneous rocks.[1,2] Current beneficiation techniques for ilmenite (FeOÆTiO2) mainly involve removal of iron either via acid leaching[3] or slag making processes.[4] However, the removal of minor (Al, Cr, Mn, Mg) and trace (lanthanides and actinides) impurities is not possible in either of the two main beneficiation processes adopted worldwide. In the present investigation, a new technique for beneficiation of titaniferous minerals for removing not only iron but also the minor and trace impurities was studied. Since the supply of rich ferruginous ores is diminishing in the world, the industry is more dependent on using lower-grade ores, which have higher concentrations of minor and trace impurity oxides. The main advantage of the new technique is that it is potentially capable of minimizing waste generation by reducing the dependence on chlorination during which a majority of iron, minor, and trace impurities are removed via fractional distillation.[5] The soda-ash roasting of ilmenite involves two main steps. (a) The decomposition of ilmenite to its constituent oxides at elevated temperatures according to Reaction [1]: ABHISHEK LAHIRI, Postdoctoral Student, and ANIMESH JHA, Chair in Applied Materials Science, are with the Institute for Materials Research, University of Leeds, Leeds LS2 9JT, United Kingdom. Contact e-mail: [email protected] Manuscript submitted January 26, 2007. Article published online October 19, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
FeO TiO2 ¼ FeO + TiO2
½1
Reaction [1] can occur either in the absence of any chemical potential, e.g., the thermal decomposition, or in its presence at an isotherm. (b) Following the decomposition of the constituent oxides, TiO2 and FeO form complexes, namely, Na2TiO3 (sodium titanate) and NaFeO2 (sodium ferrite), respectively, via Reactions [2] and [3]. TiO2 þ Na2 CO3 ¼ Na2 O TiO2 + CO2 2FeO + Na2 CO3 + 0.5O2 =Na2 Fe2 O4 + CO2
½2 ½3
The complexation process with soda yields the watersoluble sodium ferrite and insoluble sodium titanate. Thus, the overall roasting reaction at elevated temperature is given by Reaction [4]: 3Na2 CO3 + 2FeTiO3 + 0.5O2 =2Na2 TiO3 + Na2 Fe2 O4 + 3CO2
½4
In the present investigation, the kinetics of overall Reaction [4] were analyzed. For explaining the mechanism of chemical reaction, stoichiometric mixtures of ilmenite and soda ash were roasted in air in the temperature range of 873 to 1173 K, and the phases formed after roasting were analyzed by X-ray powder diffraction and analytical electron microscopic techniques for chemical identification and compositional analysis. At a given time t, at an isotherm (T), the overall reaction was repres
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