Blast furnace burden softening and melting phenomena: Part II. Evolution of the structure of the pellets
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I. INTRODUCTION
THE performance of the blast furnace is affected by the position and shape of the cohesive zone, where ferrous burden materials soften and melt. Minimizing the size and lowering the position of the cohesive zone will improve productivity and decrease the coke rate. It is the purpose of the present research to develop a better understanding of the fundamental behavior of ferrous burden materials in the blast furnace, which affects the cohesive zone. In Part I[1] of this series of articles, an extensive literature review was presented, along with details of the experimental procedure. The results of the X-ray observation of the mechanical deformation of the pellets under load showed an interaction between an acid and a basic pellet at temperatures higher than 1100 °C. Therefore, a few selected conditions were tested under load in an inductively heated apparatus. The induction system was used so that the samples could be rapidly cooled in order to preserve the microstructure. The evaluation of the microstructure of the pellets along with the interaction between different pellets is examined. II. EXPERIMENTAL PROCEDURE The pellets chosen for the current work were an acid pellet (with small dolomite addition) and a basic pellet. Their compositions were listed in Part I of this series.[1] These pellets were sieved between 11.1 and 12.7 mm and reduced to approximately 61 or 81 pct. All reduction trials were performed in an induction furnace with an iron crucible. The temperature was controlled using a K-type thermocouple to 800 °C 3 °C. The reduction degree was measured by mass difference, attributing all mass loss to oxygen removal and considering the Fe3 and Fe2 ions. These pellets were first
PAULO F. NOGUEIRA, formerly Graduate Student with the Center for Iron & Steelmaking Research, Carnegie Mellon University, now Senior Researcher, is with Ferrous R&D, Cia. Vale do Rio Doce CURD, Victoria, ES, Brazil 29090-900. Contact e-mail: paulo. [email protected] RICHARD J. FRUEHAN, Professor, is with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted February 25, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
pre-reduced to FeO using a CO-CO2 equimolar mixture. Further reduction was obtained by using H2 for a determined time. The gas flow was set at 2000 cm3/min, controlled using a mass flow controller for each gas or gas mixture, and the time was controlled to the nearest second using a laboratory stopwatch. The time required for the desired reduction degree was determined from a reduction degree vs time plot. For 60 pct pre-reduction with H2, these times were 13 minutes and 17 minutes, 30 seconds for the basic and acid pellets, respectively, while for 80 pct pre-reduction with H2, the times were 25 and 35 minutes for the basic and acid pellets, respectively. With this procedure, it was possible to obtain a reduction degree within 1 pct of the desired level. The experimental appa
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