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Mechanism of Titanium Sponge Formation in the Kroll Reduction Reactor CH.R.V.S. NAGESH, CH. SRIDHAR RAO, N.B. BALLAL, and P. KRISHNA RAO The Kroll process of magnesium reduction of titanium tetrachloride is used the world over the production of titanium metal in the form of sponge. Although the process has been in practice for the last five decades, there is no clear understanding of the reaction mechanism and sponge formation. The present study involved reduction experiments in a 2000 kg titanium sponge–capacity prototype reactor to develop a better understanding of TiCl4 reduction of magnesium with respect to the process parameters. Experiments were also conducted in two smaller experimental reactors to study the temperature evolution during the process as a function of the TiCl4 feed rate. Based on the results of all these experiments, a model has been proposed for the mechanism of sponge formation in the Kroll process.

I. INTRODUCTION

IN the Kroll process, titanium tetrachloride (TiCl4) is reduced with magnesium at high temperature in a closed reactor under an inert gas atmosphere.[1] The process has been widely used over the last 50 years for the commercial production of titanium metal. Many variations of the magnesiothermic reduction have been attempted, as reported in the literature.[2–6] With increasing usage of titanium in nonaerospace applications, there have been continuing efforts to decrease the energy consumption in the process and the cost of the metal. The Kroll reduction reaction TiCl4 (g)
2 Mg (l)  Ti (s)
2MgCl2 (l) H1000K  412 kJ/mole is a complex heterogeneous exothermic reaction with the possibility of the formation of lower-valence titanium chlorides (TiCl2 and TiCl3) as reaction intermediates. The smooth running of the reduction process, titanium yield, quality of the sponge, and magnesium utilization are all highly dependent on the control of the temperature, the reaction rate, and the formation of lower chlorides. In spite of the many studies reported on the process,[7–12] there is no consensus on the mechanism of sponge formation and the reaction model. In the present work, Kroll reduction experiments were conducted in a prototype reactor and in small experimental crucibles to develop a better understanding of the reduction process. II. EXPERIMENTAL WORK Reduction experiments were carried out in a prototype Kroll reactor with a capacity to produce 2000 kg of titanium sponge per batch, at three different TiCl4 feed rates. Studies

CH.R.V.S. NAGESH, Scientist “E,” is with the Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, A.P. India 500058. Contact e-mail: [email protected] CH. SRIDHAR RAO, formerly Scientist “F,” Defence Metallurgical Research Laboratory, is retired. N.B. BALLAL and P. KRISHNA RAO, Professors, are with the Department of Metal Engineering and Materials Science, Indian Institute of Technology, Bombay, Powai, Mumbai, India 400076. Manuscript submitted January 30, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B