Effect of B2O3 on the Liquidus Temperature and Phase Equilibria in the CaO–Al2O3–SiO2–B2O3 Slag System, Relevant to the Smelting of E-waste

The phase equilibria in the CaO–Al2O3–SiO2 ternary system doped with around 5, 10, 15 and 20 wt% B2O3 was studied using a high-temperature equilibration followed by rapid quenching technique. Fifteen samples with CaO/SiO2 (C/S) ratios of 0.3, 0.6 and 1.0 containing 15.6–19.1 wt% Al2O3 were equilibrated at a range of temperatures close to predicted liquidus phase boundaries within the B2O3-free ternary. Quenched samples were characterised using Scanning Electron Microscopy (SEM) to reveal the equilibrium phase assemblage and Electron Probe Microanalysis (EPMA) to determine the chemistry of individual phases. The liquidus temperatures of the synthetic slags were determined within an uncertainty of ± 10–20 °C. Depending on temperature and composition, anorthite (CaO.Al2O3.2SiO2), pseudowollastonite (CaO.SiO2), gehlenite (2CaO.Al2O3.SiO2) and tridymite (SiO2) crystals were observed in equilibrium with the liquid phase at temperatures below the liquidus. Doping with successively higher amounts of B2O3 caused the boundaries of the initial primary phase fields to shift position, generally resulting in a reduction of the liquidus temperature. The lowest liquidus temperature was 900 °C for slag with initial composition having C/S = 0.6 and 18.8 wt% B2O3 representing a decline of the liquidus by 435 °C compared to the undoped slag. Only one sample (C/S = 1.0, 18.8 wt% B2O3) resulted in an increase in the liquidus temperature due to the expansion of the pseudowollastonite phase field at high B2O3 contents. Based on the experimental results, B2O3 may be a suitable fluxing agent to reduce the smelting temperature in the CaO–Al2O3–SiO2–B2O3 quaternary system. A comparison of results with liquidus data for similar experiments using Na2O flux showed that B2O3 was more effective in lowering the liquidus.