MgO solubility in Li2O-containing iron-silicate slag at 1823 K: Pyro-processing of secondary lithium-ion battery (LIB) sources

Abstract

To simulate the slag used in pyrometallurgical processing of lithium ion battery (LIB) secondary sources, we investigated the effects of the FeO/SiO2 ratio and Li2O content on the thermodynamic behavior of MgO in the FeO-SiO2-MgO-Li2O-MnO-Al2O3 multicomponent slag system at 1823 K. Raman spectroscopy was used to observe structural changes with different slag compositions. As the FeO/SiO2 ratio (from 2 to 9) and Li2O content (up to ca. 10 wt%) increased, the solubility of MgO tended to decrease. This can be attributed to the basic character of FeO and Li2O, which increased the activity of free oxygen ions (O2−), i.e., basicity of the slag. Raman spectroscopic analysis of the quenched slags revealed that the Raman scattering intensity of the asymmetric stretching vibration of Si–O bonds in silicate network was low for all slag compositions. When the FeO/SiO2 = 2, a weak Q0 peak from a depolymerized [SiO4]–tetrahedron (monomer) was evident. However, as the FeO content increased, no Si–O stretching band in polymerized silicate network was detected, likely due to depolymerization of the silicate structure caused by the high FeO content, which reduced the degree of polymerization. Deconvolution of the intermediate frequency range Raman scattering bands showed a structural transition from [FeO4]–tetrahedron to [FeO6]–octahedron unit, driven by increased O2− ions with higher FeO and Li2O contents. Finally, a semi-empirical equation for MgO solubility in the FeO-SiO2-MgO-Li2O-MnO-Al2O3 slag at 1823 K was proposed via statistical analysis. © 2025 Elsevier Ltd and Techna Group S.r.l.