Mineralogical changes in magnesite and chrome-magnesite refractories due to variations in slag composition

Abstract

During the refining of steel by basic processes, a multitude of mineralogical reactions take place in the refractories with which the furnace is lined. The reactions are caused by an influx of material from the slag and are a function of the composition of the slag and of the refractory. The flow of material from the slag into the refractory resembles the process of metasomatism encountered in nature. The reactions between slags of different composition and magnesiteand chromite-containing refractories could be studied systematically as a result of a series of four experiments. During each of the experiments four different refractories were subjected to the influence of a slag having a specific composition for a considerable length of time. The experiments were conducted in a rotating furnace at 1700°0~ Mineralogical reactions in the refractories used in these experiments can be divided into two main categories, viz. reactions involving the oxides and reactions involving the silicates. The relevant oxides are periclase, the sesquioxides and spinel. Periclase is found to Mndergo reactions like grain growth, resorption and elongation normal to the hot face of the refractory. These reactions are dependent on the composition of the refractory and of the metasomatised components. Reaction between periclase and sesquioxides yield spinel, provided there is no excess of lime. Exsolution of bodies of spinel from periclase is governed by the crystal-chemistry of both minerals. At elevated temperature, spinel, and more specifically chromite, acts as a source of sesquioxides in general, and of ferric oxide in particular. The stability of chromite depends on the molecular ratio between lime and silica of both the refractory and the slag. This ratio determines the amount of sesquioxides present, but not in spinel or in periclase, i.e. in the matrix of the refractory. The postulation of the reactions involving the silicates is possible because of the tendency of the refractory to reach equilibrium with the composition of the slag. The silicates present in acid slags are not in equilibrium with magnesia in the refractory, and consequent reaction yields the minerals monticellite and merwinite. These minerals are not in equilibrium with the bonding silicates of magnesite refractories which have high molecular lime/silica ratios and also not with the bonding silicates of chromite- containing refractories having ver.y low molecular lime/silica ratios. Reaction between metasomatic and autochthonous silicates result in a new set of silicates. The silicates present in basic slags are in equilibrium with magnesia of the refractories as well as with the bonding silicates of magnesite refractories having high molecular lime/silica ratios. Consequently, no reaction results. Reaction does, however, take place between the silicates supplied by basic slags and the bonding silicates of chromite- containing refractories. Ultrabasic slags react mildly with magnesite and drastically with chrome-magnesite bricks. The reason is that in the former reaction, only monticellite and merwinite are not in equilibrium with the composition of the melt, but these minerals have a relatively high mobility, and migrate de~per into the refractory. In chromite-containing refractories, however, neither of the minerals chromite, monticellite or forsterite are in equilibrium with the composition of the slag. Reaction results in the formation of an association of minerals in the area of contact between the slag and the refractory which differs completely from the original minerals of the refractory. The conclusion arrived at is that chromite has a beneficial influence on refractories used in contact with less basic slags, because of the textural superiority of chromite-containing refractories over magnesite refractories. In contact with basic slags, however, metasomatism leads to the formation of components which have low melting points and low viscosities. During the refining of steel by basic processes, ultrabasic slags are preferred and magnesite refractories having high molecular lime/silica ratios are better suited for use in contact with the liquid slag.