Blast furnace hearth refractory and coke ash mineral interactions

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Drain, Phillp Brian
Faculty of Engineering
calcium aluminates; blast furnace health; refractory; aluminosilicate; alumina carbon; coke ash; refractory wear; CA (CaO.Al2O3; CA2 (CaO.2AL2O3); CA6 (CaO.6Al2O3)
thesis / dissertation description
The by-products of coke dissolution in iron are primarily calcium aluminate minerals known as coke ash. These minerals are unreactive with the iron bath and can therefore deposit onto the blast furnace hearth refractories when they form below the iron-slag interface. This study was focused on understanding the interactions between these coke ash minerals and blast furnace hearth refractories. Improved understanding of these interactions may have implications for the campaign life of the blast furnace hearth refractory materials.It was found that the aluminosilicate refractory followed the linear rate law and the alumina carbon refractory corresponded to the logarithmic rate law. Thermodynamic modelling was conducted and compared with the spot analysis results and micrographs to determine the phases likely to have formed. It was found that the formation of gehlenite (Ca2Al2SiO7) and anorthite (CaAl2Si2O8) were more likely in the reactions with the aluminosilicate refractory due to the higher silicon content in the refractory. This corresponds with the observation of this refractory following the linear rate law which is typical of a material which forms a non-protective reaction layer with high porosity or forms liquid phase reaction products. The rates of reaction were found to follow an Arrhenius relationship, demonstrating temperature dependence for the reactions between both hearth refractories and the calcium aluminates. The reaction rate was also observed to increase with the calcium content as predicted by Fick’s 1st law of Diffusion. The Kirkendall effect was demonstrated via an inert wire test indicating net mass transport of material into the refractory materials from the calcium aluminates. This suggests that the active species responsible for most the reactions observed was the calcium ion.Detailed analysis using thermodynamic data was carried out to determine the possible reactions between the calcium aluminates and the refractory minerals. The formation of low liquidus temperature phases (anorthite and gehlenite) and the possible formation of liquid oxide phase were determined to increase the rate of refractory wear and limit the ability of the refractories to form a protective reaction layer. The formation of grossite (CaAl4O7) and hibonite (CaAl2O19) was found to make the refractory more susceptible to structural and thermal spalling due to the increased stress in the reaction layer cause by the volume change and variation in thermal expansion coefficients of the reaction products.