The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part II. The reduction of iron oxide/carbo
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. INTRODUCTION
II. EXPERIMENTAL SETUP
IN a previous publication[1] on the effect of volatile reduction in a rotary hearth furnace (RHF) process, the reduction of various iron oxide powders with simulated volatiles of H2 and CO was investigated. The reduction of ore and carbon mixture powders was also studied under comparable experimental conditions, and the reduction by carbon seemed to be negligible compared to the reduction by hydrogen, even for reaction temperatures of 1000 °C. However, this can be explained by the insufficient surface contact of the carbon with iron oxides to promote the initial solid-state reaction. This contact can be maximized by using a compacted pellet geometry, which was used in this study. With the simulated volatile reduction fairly well understood for powders and pellets, the reduction with actual volatiles evolving from high volatile (HV) coals also was studied in detail. Several studies[2,3] pertaining to the RHF pellets indicated that volatiles from the coal in a specific pellet have no effect of reduction of that pellet due to the short retention time and the minimal surface contact of volatiles. However, if the volatiles are somehow retained and large amounts of H2-rich gases are generated from the volatiles in coal, there could be some contribution to the overall reduction of iron oxides. The current work also focused on the possible reduction of layered powders of iron oxide spread on top of HV coals where the volatiles evolve and may react with the iron oxides above, which are at elevated temperatures.
The apparatus and experimental technique for the thermogravimetric analyzer (TGA) were essentially the same as those used previously.[1] Pellets (diameter 16 to 18 mm) were handrolled from a homogeneous mixture of porous analytical hematite (PAH: 100 200 mesh) and graphite (100 200 mesh) or coal char (100 200 mesh), which were preheated at 200 °C for 72 hours. One of the pellets was heated to as high as 400 °C with no further apparent weight loss, indicating that chemisorbed water had been completely removed. In the initial RHF process, the top layer of the pellets will not obtain temperatures exceeding 1000 °C during the evolution of the volatiles. Based on the chemical analysis of HV coals and the work done with H2/CO mixture gases, the reduction by volatiles will be due mainly to H2. Hydrogen can exist as H2, CH4, C2H6, or tar (CnHm), which can crack into its elemental constituents of C and H2. The light hydrocarbons evolve at the early stages, with the complex hydrocarbons evolving at the later stages of heating. Although the ultimate analysis of HV coals was measured from the pyrolysis of coals[4] and is given in Tables I and II, it is difficult to accurately measure and analyze the types of gases that would evolve from the volatile matter. However, a detailed mathematical expression for the evolution of volatile gases has been developed by Merrick,[5] which can be used to estimate the volatile gas species in HV coals. Previous work by the authors found that below
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