Degradation of MgO refractory in CaO-SiO 2 -MgO-FeO x and CaO-SiO 2 -Al 2 O 3 -MgO-FeO x slags under forced convection
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I. INTRODUCTION
TWO of the key drivers in steelmaking operations are operational security and cost reduction. Steelmaking processes rely on refractory linings to contain the hot liquids and gases involved in producing steel. Modern processes increasingly use higher temperatures and techniques such as bottom bubbling and injection that greatly increase the flow of fluids within the vessel. These conditions are highly beneficial to productivity and can increase product quality. However, they also tend to significantly accelerate the rate of degradation of the refractory lining. Higher costs associated with more frequent replacement of the refractories and the associated down time are counterproductive, and some of the benefits of the new processing methods can be lost. Accelerated wear can also reduce product quality through an increase in the number of nonmetallic inclusions in the metal. Therefore, refractory performance has a significant impact on both operational security and costs. Refractory materials based on magnesium oxide (MgO) are widely used to contain the molten metal, slag, and hot gases in steel processing vessels. However, much is still unknown about the fundamental interactions of molten steel, slag, and MgO refractories, which lead to corrosion, or about how changes in processing conditions affect corrosion and wear, particularly where there are flowing liquids or reaction products involved. Studies of MgO immersed in molten slags under static conditions have found that a dense spinel (MgOAl2O3) or magnesiowustite ((Mg, Fe)O) layer formed at the MgO surface, significantly slowing the further dissolution of MgO.[1–5] The formation of these layers involves the counterdiffusion of ions, and is at least in part controlled by mass transfer. Layer thickness depends on the relative rates of growth of the solid solution (ss) at the MgO/ss interface and the dissolution of the ss at the ss/liquid interface. Goto et al.[6] performed cup tests on MgO/MgO-Al2O3 spinel refractories
using a CaO-SiO2-Al2O3 slag. They found that the spinel grain was subject to congruent dissolution, but the MgO dissolved incongruently, resulting in the formation of secondary spinel a short distance away from the refractory/slag interface. They postulated that this occurred because there was insufficient concentration of Al2O3 at the interface due to localized enrichment with MgO. More recent studies of MgO dissolution have also reported that the solid solution layer either formed at a distance from the solid surface, or initially formed there and then became detached and moved out into the liquid slag.[7,8] Under these conditions, this layer could be easily removed by forced liquid flow. Sridhar and co-workers[9,10] used confocal scanning laser microscopy to directly observe the dissolution of small MgO particles in calcium silica aluminate[9] and calcium aluminate[10] slags at temperatures ranging from 1450 °C to 1600 °C. Convection effects were ignored. In these two studies, they reported conflicting rate controlling mechanisms f
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