Metallurgical properties of CaO-SiO2-Al2O3-4.6wt%MgO-Fe2O3 slag system pertaining to spent automotive catalyst smelting

The metallurgical properties of the CaO-SiO2-Al2O3-4.6wt%MgO-Fe2O3 slag system, formed by the co-treatment process of spent automotive catalyst (SAC) and copper-bearing electroplating sludge (CBES), were studied systematically in this paper. The slag structure, melting temperature, and viscous characteristics were investigated by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, FactSage calculation, and viscosity measurements. Experimental results show that the increase of Fe2O3 content (3.8wt%-16.6wt%), the mass ratio of CaO/SiO2 (m(CaO)/m(SiO2), 0.5-1.3), and the mass ratio of SiO2/Al2O3 (m(SiO2)/m(Al2O3), 1.0-5.0) can promote the depolymerization of silicate network, and the presence of a large amount of Fe2O3 in form of tetrahedral and octahedral units ensures the charge compensation of Al3+ ions and makes Al2O3 only behave as an acid oxide. Thermodynamic calculation and viscosity measurements show that with the increase of Fe2O3 content, m(CaO)/m(SiO2), and m(SiO2)/m(Al2O3), the depolymerization of silicate network structure and low-melting-point phase transformation first occur within the slag, leading to the decrease in melting point and viscosity of the slag, while further increase causes the formation of high-melting-point phase and a resultant re-increase in viscosity and melting point. Based on experimental analysis, the preferred slag composition with low polymerization degree, viscosity, and melting point is as follows: Fe2O3 content of 10.2wt%-13.4wt%, m(CaO)/m(SiO2) of 0.7-0.9 and m(SiO2)/m(Al2O3) of 3.0-4.0. This work provides a theoretical support for slag design in co-smelting process of SAC and CBES.