Viscosity and Its Correlation to the Ionic Structure of CaO–Al2O3–B2O3 Slags with Various La2O3 Additions

The relationship between the viscosity and ionic structure of CaO–Al2O3–B2O3 slag with different La2O3 contents was studied using a rotating viscometer, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy. The results showed that with the increase of La2O3 within 10 wt pct, the slag viscosity, activation energy, and break temperature decreased. However, when La2O3 was excessively added, the viscous behavior of slag was determined by the balance between the depolymerized structure and strong ionic interaction. Structural analysis indicated that the slag network structure was simplified due to the increment of the relative fraction of nonbridging oxygen (O−) and the decrement of bridging oxygen (O0) with the addition of La2O3, leading to the reduction of viscosity. In detail, free oxygen (O2−) released from La2O3 depolymerized the aluminate network structure. Moreover, increasing La2O3 promoted the conversion of [BO4] to [BO3] units with nonbridging oxygen, resulting in the depolymerization and loosening of the borate structure. For the present slag system, Ca2+ played the role of charge compensation due to the small cationic field strength and dominant mole fraction, while La3+ mainly interacted with more NBOs in the melt for balancing charges.