• PDF / 230,091 Bytes
• 7 Pages / 612 x 792 pts (letter) Page_size
• 44 Downloads / 285 Views

DOWNLOAD

REPORT

---

I. INTRODUCTION

THE carbothermic reduction of ilmenite and rutile has significant commercial importance in the formation of synthetic rutile by the Becher process[1,2] and the production of titanium carbide and nitride.[3,4,5] For ilmenite, the initial step is reduction to rutile and elemental iron. This step is performed on an industrial scale and involves heating for 8 to 24 hours at 1150 8C.[1,2] The large particles used (typically 150 mm) have been shown to give a product that consists of a core of unreduced ilmenite surrounded by elemental iron and titanium oxide, which contain an increasing fraction of Ti(III) toward the surface.[6] The elemental iron is leached away from the titanium oxide using hot ammonium chloride, leaving synthetic rutile, which typically contains 90 to 92 pct TiO2 —the remainder being ash and impurities. Rutile reduction proceeds by the removal of very small amounts of oxygen and slight crystallographic rearrangement. It has been shown that slightly reduced rutile TiO1.9986 can be formed that is crystallographically distinguishable from rutile.[7,8] The general formula for reduced rutile can be written TinO2n21, where n $ 2, which gives a series of mixed Ti(III)/Ti(IV) oxides with increasing content of titanium(III) as n decreases. Of these, only phases with n # 10 have been crystallographically characterized,[9] probably due to the difficulty in fabricating suitable standards. The mixed valence titanium oxides are becoming increasingly important in their own right as catalysts,[10] biocompatible coatings,[11] wear resistant surfaces,[12] electroconductive ceramics,[13] and electrode materials for both oxygen and hydrogen evolution.[14] Consequently, there is a need to explore low-cost routes for formation of these phases on a large scale. There is overwhelming evidence[6,15–24] that these N.J. WELHAM, Fellow, and J.S. WILLIAMS, Professor, are with the Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 2600, Australia. Manuscript submitted February 3, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B

phases form during the reduction of both ilmenite and rutile. However, the use of large particles tends to give a multiphase product ranging from an unreacted oxide center to TiC at the surface.[6] Even using ultrafine (,20 to 25 nm) TiO2 particles coated with carbon as a starting material, the mixedvalence titanium oxides were evident.[15,16,17] From all of these investigations, the general conclusion has been that the rate controlling step is solid-state diffusion. It has recently been demonstrated that milling mineral powders for extended periods of time leads to a near constant particle size by a regime of breakage and rewelding.[25,26] However, increasing the milling time gives a decreasing crystallite size within individual particles. In multiphase systems, milling causes intermixing of phases,[27] which gives increases in the interfacial surface area while shortening the diffusion paths. These enhance