Viscosity variations in the transition process of blast furnace slag towards high-Al2O3 and low-MgO content

The viscosity change of blast furnace slag has great effect on the slag granulation and equipment operation of the dry centrifugal granulation assisted thermal energy recovery. In this study, the viscosity and structure of the slag with the chemical composition transition towards high-Al2O3 and low-MgO content were studied by viscosity experiments, FactSage thermodynamic calculation, Raman spectra, X-ray photoelectron spectroscopy (XPS) and 27Al magic angle spinning nuclear magnetic resonance (27Al MAS-NMR). When the high temperature was above liquidus temperature, with the increase of Al2O3 content from 13 to 21 wt.% and the simultaneous decrease of MgO content from 14 to 6 wt.%, the viscosity increased gradually caused by the increased degree of polymerization. With the temperature decreasing, different crystals precipitation in form of solid particles resulted in the sharp increase of viscosity of the slag. As to the structure analysis of quenched slag, the mole fraction of simple Q0 and Q1 in Si-based structure decreased, while that of Q3 increased significantly. The content of bridged oxygen increased, with an opposite trend for non-bridged oxygen and free oxygen being observed. In the Al-based structure, [AlO4] behaved as network former was the predominant structure with a proportion over 80 %, with [AlO5] less than 10 %. To sum up, the degree of polymerization of slag increased in the transition process towards high-Al2O3 and low-MgO content, which supported the viscosity changes.