The Effect of Spinel formation in the Ceramic Welding Fluxes on the Properties of Molten Slag

The magnesia- and alumina-based binary and ternary mixtures of the oxides as well as quaternary oxide-fluoride mixtures, which contain MgO and Al 2O3, can form the spinel MgAl 2O4 phase at high temperature treatment. The spinel phase formation and related issues were examined for these mixtures and commercial ceramic welding fluxes. Powder X-ray diffraction (PXRD), high temperature X-ray diffraction (HT-XRD), and gravimetry analysis were used to examine the phase composition of slags. For the major of the studied compositions, scanning electron microscopy (SEM) shows the formation of the prismatic microcrystallites of ca. 10-35 mkm. The energy dispersive X-ray (EDX) analysis confirms that pyramid-shaped prismatic crystals have MgAl 2O4 spinel stoichiometry. The spinel crystallites are insoluble in the slags, according to the HT-XRD and PXRD data, and so the contribution of a part of the slag components that forming the spinel should be excluded at the estimation of characteristics of the slag melts. Based on this fact, it was proposed to quantify the spinel phase formed in the molten slag. Three convenient analytical techniques were proposed for the accurate determination of the weight and the volume fractions of the spinel phase in the slags. Two of these techniques are based on the internal standard method, which is applied in the framework of the quantitative PXRD analysis. The third technique is originated from the gravimetric analysis of the products of the solid slags etching that was conducted applying strong acids. The data obtained by the methods are sufficiently agrees with each other. The results of the spinel quantification were used to estimate the molten slag basicity and viscosity.