OCR Text |
Show adjustable parameter, and is expressed as a multiple of the gas-side mass transfer coefficient. Oxidation and particle temperature continue to increase in this stage until all the sulfide is consumed, leaving behind an oxide melt. As the molten Fe-O droplet starts to cool, iron oxide can crystallize out of the melt. Magnetite (melting point, 1870 K) is the most thermodynamically stable oxide that can form from the melt at these temperatures, although a thermodynamically less stable form could crystallize out if its nucleation kinetics are more favorable. 1600 K Fe304(1) ~ Fe304(s) (R6) Temperature histories for oxidizing pyrite particles9 showed that supercooling by as much as 300 K occurred before the onset of crystallization. In the model, it is assumed that the droplet is supercooled to 1600 K before magnetite crystallization occurs. Once magnetite crystallization is completed, the particle temperature approaches the gas temperature. For a given oxygen concentration in the gas stream, the formation of hematite from magnetite (R7) becomes thermodynamically favorable only below a certain temperature (1600 K at 5 percent oxygen). Experimental data show that this reaction is kinetically controlled and is much slower than the oxidation of pyrrhotite to magnetite. 7 Heat and Mass Balances The rates of each of the above steps are governed by the rates of heat and oxygen transfer from the surrounding gas to the particle, and by any intra-particle resistances. The unsteady state heat balance equation is used 5 |