Liquidus- and Primary-Phase Determinations of Slags Used in the Processing of Spent Pot Lining

New phase equilibria data on complex slags containing CaO-Al2O3-SiO2FeO (x) -MgO-Na2O-NaF were obtained from high-temperature experiments using the drop-quench technique followed by characterization of the quench phases using optical and scanning electron microscopy. The results obtained show that addition of Na2O to slags of the CaO-SiO2-10% Fe2O3-20% Al2O3 system enlarged the liquid region at 1300 A degrees C to higher MgO contents and to higher CaO/SiO2 ratios compared with the soda-free system. At 5-10% Na2O, the primary phase was melilite. At lower soda contents and lower CaO/SiO2 ratios, the primary phase was anorthite at low MgO contents and spinel at high MgO contents. In general, the addition of Na2O to the slag decreased the liquidus at all MgO contents and CaO/SiO2 ratios. The addition of NaF to the CaO-SiO2-10% Fe2O3-20% Al2O3 system extended the liquid region at 1200 A degrees C to higher CaO/SiO2 ratios. The effect was the greatest at CaO/SiO2 ratio of 0.6 and at low MgO contents of slag (2%). Overall, this study showed that the Na and F from the spent pot lining (SPL) would enable the smelting process to operate at temperatures below 1200 A degrees C, which would have significant benefit, including reduced heat losses from the furnace and longer refractory life.