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Magnesium oxide-based refractory materials are used industrially to contain the chemically aggressive slags present in lead smelting systems. In the present study an integrated experimental and thermodynamic modeling approach was taken to provide fundamental information on the chemical reactions taking place in these systems. New experimental phase equilibria and liquidus data were obtained for the PbO–MgO–SiO2 system in air in the temperature range 750 °C to 1740 °C. In the MgO–SiO2 binary, new experimental results were obtained at 1550 °C to 1740 °C and compared to the available thermodynamic data in the literature. The experiments were carried out using the high-temperature equilibration of oxide powder mixtures followed by rapid quenching of the samples. Electron probe X-ray microanalysis (EPMA) was used to determine the compositions of the solid and liquid phases present at equilibrium conditions. Phase equilibria and liquidus isotherms in the cristobalite and tridymite (SiO2), pyroxene (protoenstatite MgSiO3), olivine (forsterite Mg2SiO4), barysilite (Pb8MgSi6O21), massicot (PbO) and periclase (MgO) primary phase fields were measured, and the extent of the high-silica two-liquid immiscibility gap in equilibrium with cristobalite was determined. The experimental results were used to optimize the parameters in a thermodynamic database that was subsequently used to describe this multi-component, multi-phase system and predict the liquidus for the PbO–MgO–SiO2 system. The new data were used to characterize the chemical interactions of magnesia-based refractory with PbO–MgO–SiO2 slags.

Fundamental experimental investigations have been conducted to study the effect of Al2O3 on the equilibria between the gas phase (CO-CO2-SO2-Ar) and slag-matte-tridymite phases in the Cu-Fe-O-S-Si-Al system at 1573 K (1300 °C) and P(SO2) = 0.25 atm. The experimental technique used is based on equilibration, rapid quenching and electron probe x-ray microanalysis. New experimental data have been obtained for the four-phase gas-slag-matte-tridymite equilibria system for a range of alumina concentrations up to 20.4 mass% in the slag phase as a function of matte grade, including the concentrations of dissolved sulphur and copper in slag, and Fe/SiO2 ratios in slag. The results obtained for 1573 K (1300 °C) are also used to analyse the effect of temperature on phase equilibria in the range of conditions investigated to that of reported for 1473 K (1200 °C) in the literature. The results obtained show that the concentrations of sulphur, copper and “FeO” in slag decrease with increase of Al2O3 concentration in slag while it has no detectable effect on concentration of sulphur in matte for a given matte grade. The new data provided in the present study are of direct relevance to the pyrometallurgical processing of copper and will be used as an input for optimisation of the FactSage thermodynamic database for the copper-containing, multi-component multi-phase system.

Understanding the significance of magnesia as a common component in copper processing slags is essential for optimisation of the industrial copper production. Fundamental experimental studies have been undertaken to determine the effect of MgO on the equilibria between the gas phase (CO-CO2-SO2-Ar) and slag-matte-tridymite phases in the Cu-Fe-O-S-Si-Mg system at 1473 K (1200 °C) and 1573 K (1300 °C), and P(SO2) = 0.25 atm. The experimental methodology used was based on equilibration, quenching and microanalysis. New experimental data have been obtained for the four-phase gas–slag–matte–tridymite equilibria system for a range of MgO concentrations up to 3.1 wt pct in the slag phase as a function of matte grade, including the concentrations of dissolved sulphur and copper in slag, and Fe/SiO2 ratios in slag. The results are also used to analyse the effect of temperature on phase equilibria in the range investigated. The results obtained showed that dissolution of sulphur, copper and “FeO” in slag decreases with increase of MgO in slag while it has no detectable effect on concentration of sulphur in matte. Also, dissolved copper and sulphur in slag increases when temperature increases while the Fe/SiO2 ratio in slag is greater at 1473 K (1200 °C) than 1573 K (1300 °C). The new data provided in the present study are of direct relevance to the pyrometallurgical processing of copper and will be used as an input for optimization of the FactSage thermodynamic database for the copper-containing multi-component multi-phase system.

Interfacial phenomenon is critical in metal processing and refining. While it is known to be important, there are little data available for key oxide systems in the literature. In this study, the interfacial tension (σ LS) of liquid slag on solid oxides (alumina, spinel, and calcium aluminate), for a range of slags in the CaO-Al2O3-SiO2-(MgO) system at 1773 K (1500 °C), has been evaluated. The results show that basic ladle-type slags exhibit lower σ LS with oxide phases examined compared to that of acid tundish-type slags. Also, within the slag types (acid and base), σ LS was observed to decrease with increasing slag basicity. A correlation between σ LS and slag structure was observed, i.e., σ LS was found to decrease linearly with increasing of slag optical basicity (Λ) and decrease logarithmically with decreasing of slag viscosity from acid to base slags. This indicated a higher σ LS as the ions in the slag become larger and more complex. Through a work of adhesion (W) analysis, it was shown that basic ladle slags with lower σ LS result in a greater W, i.e., form a stronger bond with the solid oxide phases examined. This indicates that all other factors being equal, the efficiency of inclusion removal from steel of inclusions of similar phase to these solid oxides would be greater.

Understanding the dissolution kinetics of non-metallic inclusions in liquid slag is key in optimization of slag composition for inclusion removal. In this study, the rate of dissolution of high-precision spheres of sapphire and alumina–magnesia particles in CaO–SiO2–Al2O3 liquid slags was measured in situ using a laser scanning confocal microscope at 1500 °C. It was found that the rate of dissolution of both sapphire and alumina–magnesia particles increased when the slag basicity is increased. A layer was observed around the dissolving sapphire. This layer may be a product layer and/or indicative of a mass transfer rate-controlling system. In the case of alumina–magnesia particle, the kinetics appeared more complex and depended on slag composition. No product layer or mass transfer layer was observed around the particle dissolving in slag with low basicity, whereas for the high basicity slag, a product or stagnant layer was observed, similar to that of the sapphire particle. Assuming a mass transfer-controlled system, measured diffusion coefficients for sapphire particles in slags tested in this study ranged from 10−11 to 10−10 m2 s−1 at 1500 °C.

Inclusion type and content in steel is critical in steelmaking, affecting both productivity through clogging, and downstream physical properties of the steel. They are normally removed from steel by reacting with a slag (liquid oxide) phase. For efficient inclusion removal, the inclusions must attach/bond with this liquid phase. The strength of the attachment can be in part characterized by the wettability of the liquid oxide on the inclusions. In this study, the dynamic wetting of liquid oxides of the CaO-Al2O3-SiO2-MgO system on a solid spinel (MgAl2O4) substrate with low porosity of 1.9 pct was measured at 1773 K (1500 °C) using a modified sessile drop technique. The dynamic contact angle between the liquid and solid spinel was determined for different CaO/Al2O3 mass percent ratios ranging from 0.98 to 1.55. Characteristic curves of wettability (θ) vs time showed a rapid decrease in wetting in the first 10 seconds tending to a plateau value at extended times. A mathematical model for spreading behavior of liquid oxides by Choi and Lee was adopted and shown to provide a reasonable representation of the spreading behavior with time. The chemical interaction at the interface between spinel (MgAl2O4) and slag was analyzed by carrying out detailed thermodynamic evaluation and characterization using scanning electron microscopy/energy dispersive spectroscopy. There is evidence of liquid penetrating the substrate via pores and along grain boundaries, forming a penetration layer in the substrate. The depth of the penetration layer was found to be a function of substrate porosity and sample cooling rate. It decreased from ~350 µm for 6.7 pct-porous substrate to ~190 µm for substrate with porosity of 1.9 pct and from ~190 µm to ~50 µm for a slow-cooled liquid oxide-spinel substrate sample in the furnace to a rapidly cooled liquid cooled-spinel substrate sample, respectively.

Inclusion content minimization is key in the production high quality steels. This is typically achieved through approaches ranging from preventing inclusion formation via process control to optimising process conditions to remove any inclusions that form. Inclusion removal from steel is generally through reaction with a slag. The inclusion transfers across the steel-slag interface to dissolve in the slag. This transfer process is primarily a dynamic interfacial tension/wetting driven process. In this study, the dynamic wetting (θ) of a range of slags in the CaO-Al2O3-SiO2-(MgO) system on alumina (Al2O3), magnesia spinel (MgAl2O4) and calcium aluminate (CaO.Al2O3) substrates has been assessed using the sessile drop technique. The reactivity of selected inclusion phases was studied by evaluating the wetting results and characterizing the microstructure of the slag-inclusion interface. It was found the dependence of θ value on the composition was lower for the basic ladle type slags than the acid tundish type slags.