Experimental and Theoretical Studies on the Viscosity–Structure Correlation for High Alumina- Silicate Melts TRINATH TALAPANENI, NATRAJ YEDLA, SNEHANSHU PAL, and SMARAJIT SARKAR Blast furnaces are encountering high Alumina (Al 2 O 3 > 25 pct) in the final slag due to the charging of low-grade ores. To study the viscosity behavior of such high alumina slags, synthetic slags are prepared in the laboratory scale by maintaining a chemical composition of Al 2 O 3 (25 to 30 wt pct) CaO/SiO 2 ratio (0.8 to 1.6) and MgO (8 to 16 wt pct). A chemical thermodynamic software FactSage 7.0 is used to predict liquidus temperature and viscosity of the above slags. Experimental viscosity measurements are performed above the liquidus temperature in the range of 1748 K to 1848 K (1475 °C to 1575 °C). The viscosity values obtained from FactSage closely fit with the experimental values. The viscosity and the slag structure properties are intent by Fourier Transform Infrared (FTIR) and Raman spectroscopy. It is observed that increase in CaO/SiO 2 ratio and MgO content in the slag depolymerizes the silicate structure. This leads to decrease in viscosity and activation energy (167 to 149 kJ/mol) of the slag. Also, an addition of Al 2 O 3 content increases the viscosity of slag by polymerization of alumino-silicate structure and activation energy from 154 to 161 kJ/mol. It is witnessed that the activation energy values obtained from experiment closely fit with the Shankar model based on Arrhenius equation. DOI: 10.1007/s11663-017-0963-3 Ó The Minerals, Metals & Materials Society and ASM International 2017 I. INTRODUCTION THE viscosity of a slag affects the permeability of the coke bed, slag-metal reactions, degree of desulphuriza- tion, refinability, and productivity of blast furnace. [1–6] Therefore, attaining a basic knowledge on viscosity and the aspects affecting this property is considerably more important to improving productivity and process opti- mization. [7,8] Usage of low-quality Indian ores and injection of pulverized coal into the blast furnace through its tuyers leads to high alumina content in slags (Al 2 O 3 > 25 pct.). PCI technique can lower the coke consumption, but increases SiO 2 (5 to 19 pct) and Al 2 O 3 (2 to 10 pct) content in the slag, also charging of low-grade ores in blast furnaces increases acidic oxides (Al 2 O 3 and SiO 2 ) in slags, which increases slag viscosity. To overcome this acidic nature, a huge amount of fluxes like limestone and dolomite are added to optimize the slag composition. The addition of basic oxides increases overall slag volume inside the furnace and decreases its productivity. CaO-MgO-SiO 2 -Al 2 O 3 quaternary slags are widely used in the metallurgical process. [9] Several experimental studies have been reported on viscosity of CaO-MgO-SiO 2 -Al 2 O 3 slag system. [10–18] Shankar et al. [10] found that viscosity of slag decreases with increase in basicity from 0.46 to 0.8. Hanna and Machin [11] showed that increase in basicity lowers viscosity of CaO-MgO-SiO 2 -Al 2 O 3 slag system. Machin and Yee [12] studied that viscosity increases with increas- ing silica content and decreasing temperature. Saito et al. [13] studied the influence of MgO, TiO 2 , and Fe 2 O 3 on the viscosity of 40CaO-40SiO 2 -20Al 2 O 3 (mass pct) quaternary slag system. It was found that the viscosity decreased with the addition of above oxides. Kim et al. [14] studied viscosities of CaO-SiO 2 -Al 2 O 3 -MgO- FeO slag, and suggested that the viscosity of bosh slag increases with Al 2 O 3 at a fixed basicity and MgO content. Scok et al. [15] studied viscosity of highly basic slags and suggested that viscosity depends more on Al 2 O 3 and FeO content than MgO content. Tang et al. [16] suggested that viscosity of slag increases with increase in Al 2 O 3 up to 20 pct, followed by the decrease with further increase in Al 2 O 3 due to its amphoteric behavior. Nakamoto et al. [17] communicated that the slag with 35 pctAl 2 O 3 -43.1 pctCaO-7.5 pctMgO-14.4 pctSiO 2 at 1673 K (1400 °C) is having better fluidity with viscosity lower than 0.6 Pa s. Xu et al. [18] analyzed that viscosity decreases with CaO/Al 2 O 3 ratio, whereas it increases at the higher side of CaO/Al 2 O 3 ratio. Furthermore, several studies used analytical models to predict the viscosity of slags. [19–26] Mills and Sridhar [19] used NPL model to find out viscosity for various iron-making and steel-making slags. Kondratiev et al. [20] used quasi-chemical viscosity model to find out viscosity for CaO-MgO-SiO 2 -Al 2 O 3 -FeO slag system. Seetharaman et al. [21] developed a relationship to TRINATH TALAPANENI, NATRAJ YEDLA, SNEHANSHU PAL, and SMARAJIT SARKAR are with the Department of Metallurgical and Materials Engineering Rourkela, National Institute of Technology, Rourkela 769008, India. Contact e-mail: yedlan@nitrkl.ac.in. Manuscript submitted July 29, 2016. Article published online March 21, 2017. 1450—VOLUME 48B, JUNE 2017 METALLURGICAL AND MATERIALS TRANSACTIONS B