Role of Ash Impurities in the Depletion of Carbon from Alumina–Graphite Mixtures in to Liquid Iron

Abstract

Due to their excellent thermal shock and slag resistance at high temperatures, alumina–graphite refractories are used extensively in the steel industry. The degradation of carbon based refractories through carbon depletion is an important issue and a fundamental understanding of refractory behaviour at high temperatures is crucially important. Natural flake graphite, with ash impurity levels ranging from 1 to 10%, is used extensively in the commercial preparation of alumina–carbon refractories. This study investigates the role played by ash impurities in the depletion of carbon from the refractory composite. Two natural graphites, respectively containing 2.1% and 5.26% ash, were used in this study. Substrates were prepared from mixtures of alumina and carbon over a wide concentration-range. Using a sessile drop arrangement, carbon pick-up by liquid iron from alumina–natural graphite mixtures was measured at 1 550°C and was compared with the carbon pick-up from alumina–synthetic graphite mixtures. These studies were supplemented with microscopic investigations on the interfacial region. Very high and similar levels of carbon dissolution were however observed from both alumina–natural graphite mixtures, with carbon pickup by liquid iron from mixtures with up to 30 wt% alumina reaching saturation. A sharp reduction to near zero levels was observed in the 30 to 40 wt% alumina range. Along with implications for commercial refractory applications, these results are discussed in terms of poor wettability between alumina and liquid iron, interactions between ash impurities and alumina, and formation of complexes in the interfacial region.