Phase Equilibria Study of CaO-Al2O3-SiO2-Na2O Slags for Smelting Waste Printed Circuit Boards

Phase equilibria of CaO-Al2O3-SiO2-Na2O slags were studied by thermal equilibration and quenching. The primary phases of the quenched slags were identified, and their equilibrium compositions determined by electron probe microanalysis. Liquidus temperatures of the slags were bracketed within an uncertainty of ± 10–20°C. Anorthite (CaO·Al2O3·2SiO2), gehlenite (2CaO·Al2O3·SiO2), pseudowollastonite (CaO·SiO2), and larnite (2CaO·SiO2) were observed as primary phases. Progressive doping by Na2O substantially changed the slag liquidus temperature, shifting the primary phase from anorthite to pseudowollastonite and larnite, and gehlenite to larnite. The liquidus temperature decreased significantly with increasing Na2O in slags with CaO/SiO2 (C/S) ratios of 0.3 and 0.6, while the liquidus temperature increased for slags with a C/S ratio of 1.0. The solid solubility of Na2O in the phases was quantified. Finally, the relevance of the phase equilibria study of the CaO-Al2O3-SiO2-Na2O system was discussed with regard to the optimum design of smelting with a focus on value recovery from e-waste.