Physicochemical aspects of titanium slag production and solidification
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8/28/03
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Physicochemical Aspects of Titanium Slag Production and Solidification P.C. PISTORIUS and C. COETZEE The titanium-rich slags produced by ilmenite smelting are unusual in several respects, including their low viscosity, high electrical conductivity, propensity to solidify predominantly as one phase (pseudobrookite), and reactivity (with oxidizers) in the liquid and solidified form. The low viscosity is related to the unexpectedly “basic” behavior of Ti4; the high conductivity is likely through an electron transfer mechanism involving the Ti4-Ti3 pair; the oxidation behavior is caused by the presence of a significant amount of Ti3. Possible reasons for the nearly single-phase structure of the solidified slag are considered, and it is concluded that the most likely cause is the presence of a eutectic groove close to the M3O5 (pseudobrookite) composition. This conclusion was tested by examining the compositions and microstructure of slag samples from a pilot smelter. The results show that the slags do contain a small amount of rutile, in line with predictions.
I. INTRODUCTION
ILMENITE smelting is a carbothermic process to upgrade the mineral ilmenite (nominally FeTiO3), yielding as a primary product a TiO2-rich slag (which is mainly used as feed material for TiO2 pigment production) and pig iron as a by-product.[1] This smelting process is unusual in having slag as its primary product and metal as a by-product. Another unusual aspect of the process is that no fluxes are added to control the slag properties; subsequent fluidizedbed chlorination (which is commonly used to manufacture pigment out of the slag) can only tolerate small amounts of, especially, CaO and MgO.[2] This means that the slag composition can only be changed within small limits; specifically, FeO acts as a flux in the slag, with a strong effect to reduce the melting point.[3] A typical composition of a high-titania slag is given in Table I. As indicated by the table, the carbothermic reduction process, while effective in lowering the iron oxide content of the slag, causes as a side reaction the partial reduction of TiO2 to the trivalent form (Ti2O3). Impurities in the slag (notably SiO2, MnO, Al2O3, MgO, CaO, Cr2O3, and V2O3) originate from the raw materials and are not significantly reduced to the metal. (This means that the ilmenite composition controls the impurity content of the slag; the slags referred to in this article are mostly those produced from low-impurity South African ilmenites, as used by the Richards Bay Minerals (Richards Bay, South Africa), Namakwa Sands (Saldanha, South Africa), and Ticor (Empangeni, South Africa) plants.) While the ilmenite smelting process is unusual, this article aims to show that the high-titania slag itself has several P.C. PISTORIUS, Head, is with the Department of Materials Science and Metallurgical Engineering, University Pretoria, Pretoria 0002, South Africa. Contact e-mail: [email protected] C. COETZEE, formerly Graduate Student, Department of Materials Science and
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