Kinetics of Slag Reduction in Silicomanganese Production
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DUCTION
SILICOMANGANESE is produced from mixtures of Mn-sources (ores and ferromanganese slag), quartz, fluxes, reductants as well as remelts and off-grade products, all in variable amounts. In Norway, many different ores are employed as sources of manganese, each exhibiting distinct properties.[1] As the composition of the charge has a great influence on the production of silicomanganese, a better knowledge of the ores can lead to substantial process improvements. However, silicomanganese slags are complex systems formed of many oxides, and the study of a wide range of compositions is demanding. From the MnO-SiO2 binary system to the quinary system including CaO, MgO and Al2O3, various works investigated on the thermodynamics of silicate slags, often to determine the activities of the slag components.[2–5] The thermodynamic data found experimentally was also assessed using models.[6] Zhao and
coworkers treated the phase equilibria in a pseudo-ternary ‘‘MnO’’-(CaO + MgO)-(SiO2 + Al2O3) phase diagram, broadening from the classical MnO-CaO-SiO2 system.[7] They evidenced the influence of MgO and Al2O3 on the liquidus temperatures and the primary phase fields, underlining that these oxides present in natural ores cannot be overlooked. The distribution of manganese and silicon between slag and metal phases has been studied by equilibration.[8,9] Ding et al. have evidenced the role of the R-ratio = (CaO + MgO)/Al2O3, the content of silicon in the metal increasing dramatically with increases of the R-ratio for a given silica content. In addition, they named temperature and silica content in the slag as key factors to reach low MnO content. The reduction kinetics of MnO in silicate slags has been looked upon in a few studies.[10,11] Kinetics of simultaneous MnO and SiO2 reduction has been investigated by Berg and Olsen.[12] Through their work, they could not evidence a connection between MnO and SiO2 reduction kinetics, the reaction in Eq. [1] not being at equilibrium. Besides, they suggested that gaseous SiO could act as an intermediate during the reduction reaction 2MnO þ Si ¼ 2Mn þ SiO2 :
VINCENT CANAGUIER and MERETE TANGSTAD are with the Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway. Contact e-mail: [email protected] Manuscript submitted on August 2, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
½1
A kinetic model has been established by Ostrovski et al. to describe the reduction of MnO in a ferromanganese production context.[13] The reduction reaction of MnO, following Eq. [2], was convincingly modeled by
Eq. [3]. There, ‘‘A’’ is the reaction interface area, ‘‘k’’ the reaction rate constant following the Arrhenius equation and ðaMnO aMn =Keq Þ the driving force for the reaction. MnOðlÞ þ CðsÞ ¼ MnðlÞ þ COðgÞ;
½2
R ¼ k A ðaMnO aMn =Keq Þ:
½3
This kinetic model was extended to silicomanganese by Kim and coworkers[14,15] to conform to silica reduction according to Eq. [4]. Next, the model, given by Eq. [5], was applied t
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