The Influence of calcia and magnesia in wustite on the kinetics of metallization and iron whisker formation
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1. INTRODUCTION
BOTHcalcium oxide and magnesium oxide are good fluxes for the iron blast furnace slags and are being used for making basic iron ore pellets. The presence of calcia has been reported to cause abnormal swelling in certain kinds of basic pellets or cold bonded pellets, which are made using cement as a binder. 1 7 Metallographic examination of reduced samples representing those pellets, showed that the abnormal increase of the pellets' volume, during reduction, is characterized by the formation of a network of fine filaments or whiskers of iron. lq3 Therefore, increasing attention has been given to investigating the mechanism of iron whisker formation during metallization of wustiteJ 3,6,7,9,10 The literature on the mechanism of iron whisker formation, so far, has been based on experiments which were conducted almost exclusively with hematite or magnetite specimens; lq3 however, the formation of iron whiskers takes place during the reduction of wustite. The authors believe that a stronger conclusion might be reached if the characterization of specimens were done directly on wustite, as in the present work, in comparison with that in the literature? 13 2. EXPERIMENTAL WORK Wustite powders and plates were prepared from hematite and high purity Armco iron foils, respectively. Their chemical compositions are given in Table I and Table II. A. Experimental Apparatus and Procedures The experimental apparatus assembly and procedures used in carrying out the reduction tests are described in detail elsewhere. 14A horizontal Marshall resistantheated tubular furnace which had three zone windings T. EL KASABGY is Assistant Professor, Department of Metallurgy and Materials Science, The Catholic Univerisity, Rio de Janeiro, Brazil. W-K. LU is Stelco Professor, Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario, Canada. Manuscript submitted June 1, 1979.
to adjust the temperature profile, was used to carry a quartz tube of about 4 cm ID and 100 cm long where the specimen holder (a platinum boat 2.5 / 5 cm and 1.3 high) was located, The components of the reaction gas mixtures CO, CO 2, and N 2 were purified before being mixed together outside the furnace. The flow rate of the two reaction gas mixtures used in preparing the wustite and in reduction experiments, was 500 cc (STP)/min (CO + CO 2 200 cc (STP)/min and N 2 300 cc (STP)/min). They are given in Table Ill. The accuracy of determining the reduction temperatures was checked and found to be +_5 ~ All the reduction experiments were carried out using the standard reducing gas mixture GRS. (Table III) The reduction time for the powder specimens was 30 min and for the wustite plates it was 1 or 2 h. B. Specimen Preparation Compositions of wustite powders ( - 53/~m) doped with CaO and/or MgO are listed in Table IV. Calcia and magnesia were introduced in the following ways: i) By mixing wustite powder, prepared by reducing reagent hematite (Table I) using reacting gas GW (Table Ill) for 4 h at 900 ~ with chemically pure CaO and/or MgO
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