Carbon Bonding State Has a Small Effect on Melting of Direct-Reduced Iron
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ercial direct-reduced iron (DRI) used for electric arc furnace (EAF) steelmaking typically contains 1.5 to 4.5 pct carbon and is more than 94 pct metallized.[1,2] The carbon in the DRI can be present in different forms: bound (cementite—Fe3C) and unbound (graphite or amorphous carbon). The low melting point of cementite may give faster melting and better transfer of carbon to the steel melt, resulting in an increase of power efficiency.[3,4] Most of the carbon in commercial DRI produced by the HYL ZR process is in cementite form.[5] However, it has not been tested whether DRI that contains unbound carbon melts more slowly than DRI with bound carbon. The aim of the work presented here was to compare the melting behavior of DRI pellets with carbon in cementite or graphitic form by hightemperature confocal scanning laser microscopy (CSLM), differential scanning calorimetry (DSC), and by using an infrared gas analyzer to measure the rate of
GEONU KIM, YILMAZ KACAR, and PETRUS CHRISTIAAN PISTORIUS are with the Center for Iron and Steelmaking Research (CISR), Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Contact email: [email protected] Manuscript submitted July 8, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
generation of carbon monoxide when DRI pellets react with a laboratory slag-steel melt. Commercial hematite iron ore pellets were tested; the composition is given in Table I. Pellet diameters were in the range 10 to 13 mm (pellets masses 2 to 3 g). Fully metallized DRI pellets were prepared by reduction in a mixture of hydrogen and nitrogen (flow rates H2: 0.75 L/min, N2: 0.25 L/min; temperature and time: 900 C for 1 hour 20 minutes),* and subsequently *All gas flow rates are given as equivalent volumes at 1 atm pressure and 21 C.
carburized in CH4-H2-N2 gas mixtures (flow rates CH4: 0.20 L/min, H2: 0.14 L/min, N2: 0.05 L/min; temperature: 800 C for carbidic DRI & 850 C for graphitic DRI; reaction time 8 to 16 minutes depending on the target carbon concentration), based on the approach in previous work.[6] Some carbidic pellets (with carbon present as the iron carbide, cementite) were subsequently heat-treated to decompose the cementite to obtain carbon in unbound form (flow rates H2: 0.05 L/ min, N2: 0.05 L/min; temperature and time: 910 C for 2 hours).[7] Phase compositions were quantified by Xray diffraction (Co Ka radiation: k = 1.789 A˚ at 45 kV/ 32 mA), using Rietveld quantification with X’Pert High Score+ software (PANanalytical, Almelo, The Netherlands). For high-temperature CSLM, pieces (~ 0.1 g each) of DRI pellets were individually heated to 1500 C (heating rate: 50 C/min) in an alumina crucible under argon, while capturing confocal microscope images of the sample surface. Combined thermogravimetry and differential scanning calorimetry (TG–DSC) was performed with a SETSYS Evolution TGA–DTA/DSC (SETARAM Instrumentation), using pieces of DRI (~ 45 to 53 mg) in an alumina crucible (diameter ~ 5 mm) covered with an alumina lid with a central hole, placed o
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