Exploration of the amphoteric transition of Al2O3 on melt structure and viscosity of silicate slag

The amphoteric oxide Al2O3 has different behaviors in different chemical environments. In an acidic environment, Al3+ breaks the network structure while in an alkaline environment, Al3+ forms a three-dimensional network like Si4+. In this paper, the amphoteric effect of Al2O3 on the structure and viscosity of a five-oxide silicate system (SiO2-Al2O3-FeO-CaO-MgO) is studied. The viscosities with different Al2O3 contents were experimented on an inclined plane and compared by the flow length of liquid slags. The study shows that the melt viscosity increases and then decreases with increasing Al2O3 content. As Al2O3 increases, the coordination number of the basic cations increases, changing from a network destruction ion to a charge balance ion. And the coordination number of Al3+ has a higher shift from four to five. The detailed reaction pathways of the structural units are revealed through the variation of oxygen types. The addition of Al2O3 leads to the replacement of [SiO4]4- by [AlO4]5-, with a decrease in the contents of Si-O-Si and Si-O-M while an increase of Si-O-Al, Al-O-Al and TO. Therefore, the network strength increases first and then decreases which is consistent with the tendency of viscosity.