Anionic Effect of Chloride, Fluoride, And Sulfide Ions On Viscosity of Slag Melt

Significance Statement

The fluidity of slag in a smelting furnace is one of the most crucial rheological properties that deals with the fluid flow behavior and kinetics of pyro-metallurgical processes. Many researchers acknowledge the fact that a close relationship between the viscosity and the ionic structure of slags exists. Previous studies have established that the relaxation of viscous behavior by monovalent cations depends linearly on their ionic potential, while the effect of divalent cations show an opposite dependence. Even though the effects of silicate polymeric networks and cations on the viscosity of the slag are well established, the effects of anions on the same are still not clear. Regardless, knowledge that purport that the addition of fluoride results in a decrease in the viscosity of the slag exists despite there being reports that contradict on the same. Anions such as fluoride lead to the depolymerisation of the silicate network owing to the formation of silicon-fluoride bonds and can still predominantly coordinate with calcium cations or both. Very few reports have been presented on the effect of anions except fluoride.

Sunghee Lee and Dong Joon Min at Yonsei University in South Korea investigated the effects of anionic interactions induced by various anions on the behavior of slags. Their aim was to investigate the in-depth effects of the anions F, Cl and S2- on the structural and viscous behaviors of molten slags. Their research work is now published in the peer-reviewed journal, The American Ceramic Society.

To begin with, the researchers prepared slags by mixing the reagent chemicals for it to achieve the target composition. The slag was then pre-melted in a furnace for three hours after which they were taken out and quenched by water-cooled copper plate. The researchers then measured the viscosities of the slags while using a proportional integral differential controller to control the temperature. Once the viscosity tests were over, slag was reheated and quenched again after which its composition was analyzed by an x-ray fluorescence spectroscope. Raman spectroscopy was then employed for the structural analysis of the slag sample. Finally, the structural configuration of the silicates units in the slag were analyzed.

The authors also observed that while the viscosity of the slag was significantly influenced by the degree of polymerization, the activation energy of the viscous flow was affected by the equilibrium of the anionic structural unit. They also noted that there were three distinct compositional regions for the activation energy of the viscous flow due to the effect of the equilibrium of the polymeric silicate groups. The relaxation effect of the CaX groups on the activation energy was also noted.

The results herein provide crucial insight into the significance of the ionic interaction for understanding the structure and the viscous behavior of slags. A model for the structural behavior of the anions in the slag and the viscous behavior of the anion-containing slag have hereby proposed. The relationship between the behavior of the anions and the slag viscosity have been brought out herein and the suitability for the adoption of such anions suggested in this paper.

About The Author

Dong Joon Min, who is currently a professor in the Department of Materials Science and Engineering at Yonsei University in Korea and president of the Korean Institute of Metals and Materials, is the POSCO chaired professor and former Dean of engineering at Yonsei University in Korea.

His expertise lies in raw materials evaluation and the development of eco-friendly pyro-metallurgical extraction. He has over 200 publications and recognized by industry and academia as an expert in high temperature materials processing.


Sunghee Lee, Dong Joon Min. Anionic effect of chloride, fluoride, and sulfide ions on the viscosity of slag meltJournal of The American Ceramic Society, 2017: volume 100, pages 2543–2552.

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