Experimental Phase Equilibria and Liquidus of CaO-Al2O3-SiO2-Na2O-B2O3 Slags Relevant to E-waste Processing

Ternary master slags based on the CaO-Al2O3-SiO2 system with CaO/SiO2 (C/S) ratio 0.3–1.0 were doped with 5–20 wt pct of anhydrous borax (Na2B4O7) to study the phase equilibria of the quinary CaO-Al2O3-SiO2-Na2O-B2O3 slag system within the temperature range 1050–1350 °C. This research uses the well-known method of high temperature equilibration of slags followed by rapid quenching. The quenched slag samples were examined using a Scanning Electron Microscope (SEM) and Wavelength Dispersive Electron Probe Microanalysis (WD-EPMA) technique to determine the structure and to analyse the chemistry of the phases in equilibrium. The primary phase of 15 slag compositions was identified and the liquidus temperature of the slags were determined within an uncertainty range of ±10–20 °C by using an iterative approach. Overall, the liquidus temperature of slags decreased with increasing borax content with the highest liquidus reduction observed in slags having a C/S ratio 0.3 and the lowest in the slag series with C/S ratio of 1.0. A comparative analysis of the effect of borax, Na2O and B2O3 on the liquidus temperature of slags is discussed. Results indicated that although borax reduces the liquidus of ternary CaO-Al2O3-SiO2 slags, the addition of B2O3 individually showed more prominent effects than borax in reducing the slag liquidus. Anorthite (CaO.Al2O3.2SiO2), pseudowollastonite (CaO.SiO2) and gehlenite (2CaO.Al2O3.SiO2) primary phases were identified.