The Oxidation Rate of a Magnesia-Carbon Refractory under Low Oxygen Partial Pressures

Abstract

The oxidation rate of resin-bonded magnesia-carbon refractory, MgO-C, containing 5 mass % carbon, was measured continuously with a thermobalance in the temperature range from 1273 to 1823 K in N 2 -O 2 and Ar-O 2 mixed gases with 2.1 ϫ 10 4 and 3.7 ϫ 10 2 Pa of oxygen partial pressure, pO 2 , respectively. The effect of the oxygen concentration in the atmospheric gas on the oxidation rate for the MgO-C refractory has been investigated. The value of the effective diffusion coefficient D e of O 2 , which diffuses through the porous decarburized layer, decreased abruptly when the oxidation temperature went up to 1823 K under 2.1 ϫ 10 4 Pa of pO 2 in N 2 -O 2 . When the value of pO 2 in the atmospheric gas was 3.7 ϫ 10 2 Pa, the abrupt decrease of D e occurred at a lower oxidation temperature of 1673 K. At low pO 2 , a dense layer, probably MgO, formed near the surface of the refractory. It was speculated that the recession speed of the graphite phase was slower due to the formation of this dense phase.Magnesia-carbon refractories, MgO-C, used for linings of steelmaking vessels, have high thermal spalling and high corrosion resistance; however, formation of a decarburized layer in the MgO-C refractory oxidized by atmospheric gas causes degradation in the physical and chemical properties of the refractory. The oxidation mechanism of MgO-C refractories in air has been studied by previous researchers, 1,2 but it seems that there are few studies about the mechanism of the oxidation process under low oxygen partial pressures. The conventional oxidation tests of carbon-containing refractories have been made by measuring weight loss or the ratio of decarburized area to the cross-sectional area for a cubic or cylindrical specimen after firing in air. There are some difficulties in obtaining accurate and reproducible data by the conventional methods of oxidation test. 1,3 In the present study, the variations in the oxidation rate and the dense layer formation temperature, which depended on the oxygen partial pressure in the atmospheric gas, were detected by measuring the oxidation rate with a thermobalance. 4 The experimental data have been analyzed kinetically by application of a topochemical reaction model to the oxidation process in N 2 -O 2 ͑21 vol %͒ or Ar-O 2 ͑0.37 vol %͒ mixed gases. Furthermore, attempts to clarify the limiting step of the oxidation process and the conditions of dense layer formation have been made. ExperimentalSintered seawater magnesia grains of 95 mass % and natural flake graphite of 5 mass % were mixed and kneaded with the addition of phenol resin as a binder. The mixture was compacted at 197 M Pa into a shaped brick of 230 ϫ 100 ϫ 50 mm with a vacuum oil press. The maximum grain size of magnesia, MgO, was 3 mm. The shaped brick was supplied by Kurosaki Harima Co., Kitakyushu, Japan. A cubic specimen with sides 20 mm long was prepared by cutting from the shaped brick. The specimen, which was covered with activated carbon in a graphite crucible, was heated at 1273 K for 90 min in air to decompose t...

Kinetics and Thermodynamics of Carbon Isothermal and Non-isothermal Oxidation in MgO-C Refractory with Different Air Flow

“…He reported an activation energy of 48.46 kJ/mol for the diffusion mechanism through the pores for the material with a carbon content of 15 pct. Sunayama et al [8,16] determined activation energies of 36.9 and 43.3 kJ/mol for the same carbon content in the same temperature range. Ghosh et al [10] reported an activation energy of 20.9 kJ/mol for graphite oxidation by pore diffusion in MgO-C-material with a carbon content of 20 pct.…”

“…A number of authors have previously studied the oxidation of magnesia carbon refractories using conventional [3][4][5]15,[18][19][20]22,26] or continuous oxidation tests, [6][7][8][9][10][11][12][13][14]16] Table I. In the conventional oxidation test the cylindrical or cubic samples were subjected to an oxidizing atmosphere in an electric resistance furnace.…”

“…In Li's et al [6] and He's et al [7] experiments, CO generated by oxidation of the MgO-C refractory was further oxidized to CO 2 in excess O 2 , and the continuous quantitative analysis of the CO 2 concentration using an infrared spectrometer allowed for the determination of the oxidation rate. In several investigations presented in the literature [8][9][10][11][12][13][14][15][16][17] continuous measurements of the oxidation ratio. Oxidation kinetics experiments were carried out at different temperatures and for different durations, with [8,9,19] and without [10][11][12][13] air flow.…”

“…In several investigations presented in the literature [8][9][10][11][12][13][14][15][16][17] continuous measurements of the oxidation ratio. Oxidation kinetics experiments were carried out at different temperatures and for different durations, with [8,9,19] and without [10][11][12][13] air flow. The influence of the MgO grain size distribution on the rate of oxidation was evaluated by Sunayama et al [9,13] and Sadrnezhaad et al [11] .…”

Studies of the decarburisation phenomena during preheating of submerged entry nozzles (SEN) in continuous casting processes

“…Instead, it is the diffusion of O 2 gas and CO gas through the Al 2 O 3 -C RBM that controls the oxidation process. In addition, Sunayama et al [9] also found that diffusion of O 2 through the decarburised layer is the rate-determining step for the oxidation process of MgO-C RBMs during an isothermal heating of MgO-C RBMs in an Ar-O 2 atmosphere. Moreover, laboratory experiments with MgO-C RBMs which were heated isothermally showed that the decarburisation rate increased with an increased temperature in the temperature range of 800 to 1400°C.…”