Kinetics and Reaction Mechanisms of High-Temperature Flash Oxidation of Molybdenite
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IN the flash smelting process for copper and nickel concentrates, fine particles of dry sulfides and flux are flash smelted by injecting the gas–solid suspension through a burner located on top of a vertical reaction chamber. The particles are dispersed in the highly turbulent flow generated, igniting at some distance from the burner. The temperature at which ignition occurs under the prevailing heat, mass, and momentum transfer conditions inside the upper section of the reaction chamber determines its length. Because almost no reaction takes place before ignition occurs, it is important to determine the ignition temperature of the particles to optimize the dimensions of the reaction chamber. IGOR WILKOMIRSKY, Professor, and EDUARDO BALLADARES, Assistant Professor, are with the Metallurgical Department, University of Concepcio´n, Casilla 160-C, Correo 3, Concepcio´n, Chile. Contact e-mail: [email protected] ALFONSO OTERO, Associate Professor, is with the Mining Center, Catholic University, Casilla 306, Correo 22, Santiago, Chile. Manuscript submitted October 1, 2007. Article published online November 7, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
Previous studies on particle ignition temperature of copper, nickel, iron, and lead sulfides under flash conditions have been conducted by Jorgensen,[1,2] Otero et al.,[3] Chaubal and Sohn,[4] Tuffrey et al.,[5,6] and Morgan and Brimacombe.[7] Because molybdenum disulfide (molybdenite) is not smelted but oxidized (roasted) at lower temperatures, no study has been reported in the literature of its ignition temperature, although a process based on high-temperature flash oxidation-vaporization of MoO3 has been proposed.[8] Other more efficient roasting processes than the conventional one also have been proposed by Sohn and Kim[9] and more recently by McHugh et al.[10] Molybdenum trioxide, which is the standard product obtained from molybdenite oxidation, has a high vapor pressure with a boiling temperature of 1373 K.[11] This property, together with the fast oxidation kinetics that show molybdenite above 1000 K in pure oxygen, suggest that high-temperature oxidation to produce gaseous molybdic oxide (MoO3) rather than the solid oxide could be a better form to process molybdenite concentrates. This technological alternative could be a replacement of the traditional roasting process in multiple hearth furnaces, and it also could allow the processing of low-grade concentrates, because most VOLUME 41B, FEBRUARY 2010—63
impurities present in molybdenite concentrates are nonvolatile. The process developed[8] using these properties indeed showed that above 1000 K, the oxidation kinetics are fast and a high degree of volatilization of MoO3 can be achieved, obtaining a virtually pure condensed product of molybdic oxide.
II.
OBJECTIVES AND SCOPE
To have a better understanding of the kinetics and transfer phenomena that take place during the hightemperature flash oxidation of molybdenite particles, a basic study was conducted. The study covered the kinetics and mechanism of molybdenite par
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