Effect of certain transition-metal oxides on the structure and properties of periclase-chromite refractories

Abstract

The effect of iron oxides on the structure and the properties of periclase-chromite refractories has been studied extensively [1][2][3][4][5][6][7][8][9].It was established that iron oxides participate actively in the diffusional processes of oxide redistribution between the main crystalline phases (periclase and chrome-alumina spinellide) of these refractories owing to the ability to change the valency from +2 up to +3 and to occupy the tetrahedral as well as the octahedral positions in the lattice of the spinels.This paper deals with a study of the capacity of certain other elements of variable valency to participate in the diffusional processes.We selected three elements of variable valency (titanium, manganese, and vanadium) that can form (just as iron) ions having a valency of +2 and +3 and radii close to those of Fe~ + and Fe s+ ( Table I).The mineral formation processes occurring in the refractory materials with the participation of manganese oxides were studied by Babin et al. [ii]. However~ these oxides were introduced as additives into chromium spinellide (chrome spinellid) containing iron oxides. Thus, the combined effect of iron and manganese oxides was studied.In the present investigation, we synthesized iron-free chromium spinellide and introduced each of the experimental oxide additives separately.Using chemically pure raw materials, various spinellides were melted in a 'Kristall ~ induction furnace.Each spinel contained magnesium, chromium, and aluminum oxides and one of the aforementioned elements. Table 2 shows the chemical composition of the obtained fused spinellides, their microhardness, and the temperature corresponding to the beginning of melting.For the purpose of comparison, we melted two spinellides incorporating iron oxide (F-I and F-2).Microhardness was determined at a load of 1.96 N using a PMT-3 tester.The relative error in determining the microhardness amounted to • Table 2 gives the values representing the average microhardness of 10 grains in each case. Table 3 shows the content of the oxides of the experimental metals in the charge used for melting and the decrease in their content during the course of melting.In the F-I, T-l, M-I, and V-I spinellides, the ratio of magnesium oxide and the remaining oxides was maintained close to the stoichiometric ratio at which the entire quantity of magnesium oxide is expected to enter the spinel.In other spinellides, an excess quantity of magnesium oxide (with respect to the stoichiometric composition) was introduced.The phase composition of the specimens was determined on the basis of the optical properties of the constituent phases and their local qualitative chemical analysis (a 'Camebax' microanalyzer was used for this purpose).In the T-I, M-I, and V-I specimens, a spinellide having a complex composition forms the main phase.Local qualitative chemical analysis showed that manganese oxide is present in the spinellide of M-I and vanadium oxide is present in the spinellide of the V-I specimen.Titanium oxide does not virtually enter the compos...