The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part III. The simulation of volatile reduc
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I. INTRODUCTION
PREVIOUS work with fine powders[1] and single com-
posite pellets by the authors[2] have shown that the reduction by H2 is extremely complex, and no single mechanism can explain the entire spectrum of reduction. The only realistic conclusion that could be made was a mixed-control mechanism, but even a mixed-control model did not accurately describe the actual reduction mechanism. However, useful empirical reactions rates could be obtained from the reduction curves, and predictions of volatile reduction rich in H2 could be estimated. Both the fine powders and the single pellets reduced with H2 seemed to indicate that pore mass transfer is relatively important at the later stages of reduction, beyond 50 pct. However, the reduction of iron oxides by volatiles is limited to less than 40 pct. Thus, pore diffusion is unlikely to be important in the controlling mechanism for reduction by volatiles. The reduction of 40 g of Fe2O3 powders spread over 16.5 g of high volatile (HV) coals shown in a previous publication[2] revealed that volatile reduction can reduce iron oxides to some degree. To determine the possibility of volatile reduction for pellets in multilayers similar to a multilayer rotary hearth furnace (RHF) process, three layers of composite pellets were heated to a temperature of 1000 °C on the top layer, resulting in the gradual release of volatiles from the bottom layers, which are at lower temperatures, to reduce the oxide in the top layer. To distinguish the sole effect of volatiles, the individual reactions involved at each layer were separated. In the final stage of this study, the degree of reduction for these multilayer pellets were predicted using empirical rate equations obtained from single composite pellets reduced with H2.
II. EXPERIMENTAL SETUP To approximately simulate both the heat transfer in an actual RHF where radiation is the dominant heat-transfer mode and the subsequent devolatilization from the bottom layer, a fastacting infra-red (IR) heater was used. Temperature and weight loss data are acquired at 1 Hz and the IR lamp is controlled by the surface temperature of the pellets in the upper layer using a simple proportional integral derivative (PID) program embedded into LABVIEW* 7.0. A thermal gradient of about 150 °C *LABVIEW is a trademark of National Instruments Corp., Austin, TX.
from top to bottom layer is observed at steady state. Although the RHF operates near 1200 °C 1350 °C, the evolution of volatiles and the reactions with these volatiles for multilayers are expected to occur well below 1000 °C. The Fe2O3 and coal chemistry is identical to the PAH and HV coals given in previous publications.[1,2] To obtain comparable sintering to the Fe2O3/coal composite pellets, pure Fe2O3 pellets used in the current study were thoroughly mixed with 15 wt pct of Al2O3 and rolled into 16-mm to 18-mm pellets. The induration process was subject to similar procedures described in a previous paper.[2] To remove possible water vapor in the system, the empty reactor was preheated to
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