No 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...

An effect of addition of 2 mass % aluminum powder on the oxidation rate of magnesia-carbon refractory, MgO-C, containing 5 mass % carbon has been investigated by measuring weight change with a thermobalance for the cubic specimens in the temperature range from 1073 to 1823 K in air. The oxidation rate of carbon was measured by analyzing the residual carbon in the specimens at some oxidation time. The variations of the weight with oxidation time at temperatures from 1273 to 1673 K showed the similar tendency. The oxidation rate of carbon in the MgO-C refractory with 2 mass % of aluminum additive was a little smaller than that of the MgO-C refractory with no additives. When the oxidation temperature increased to 1823 K, the weight change became very small. It was suggested that the gaps between MgO grains in the thin oxidation layer were bridged by MgO·Al2O3, which was identified by a X-ray diffraction method.

No abstract

The corrosion test for magnesia-carbon refractory, MgO-C, with dense layer has been carried out by a rotating cylinder method at 1673 K. The specimen, which was a cylinder of 25 mm diameter and 25mm height, was rotated in molten slag at 50 rpm. The chemical composition of slag was CaO-SiO2-FeO-MgO-CaF2. The corrosion rate was determined by the variation of radius of the cylindrical specimen against corrosion time. The decrease of radius was obtained by a calculation of dissolved MgO into the slag. In addition, the diameter of the specimen was measured with a slide calipers after the corrosion test. The cylindrical specimen was heated to form a dense layer before the corrosion test. The corrosion rate of the MgO-C refractory with dense layer was slower than that of the MgO-C refractory without dense layer. The oxidized layer, where the dense layer formed, dissolved at much the same rate as the MgO brick. The dense layer probably obstructed contact between slag and carbon in the matrix, and then oxidation of C by slag did not take place. Formation of the dense layer was effective to inhibit corrosion of MgO-C refractory by molten slag.