Physicochemical Properties of CaO-MgO-SiO2-Al2O3 Synthetic Slag

Abstract

Blast furnace slag composition has a very important bearing on its physicochemical characteristics which affects the degree of desulphurization, coke consumption, smoothness of operation, gas permeability, heat transfer and fluidity which in turn affects the softening and melting behavior in the cohesive zone. Due to the diminution of high-quality iron ores, blast furnaces are running with iron ores having a high Al2O3/SiO2 ratio (as high as 4.17 in case of Sierra Leone ores, but the ideal Al2O3/SiO2 ratio should be in the order of 0.6-0.7) and this produce slag containing Al2O3 25-30%. Al2O3 being a refractory oxide will increase the liquidus temperature of the slag, due to this, the furnace has to operate at high temperatures, and this involves high coke consumption, which ends up with high silicon content in hot metal. Whereas, silicon pick up in the hot metal can be reduced by lowering the position of cohesive zone (CZ). This is possible with slags having a high softening temperature, coupled with a relatively low flow temperature increases the formation of a narrow cohesive zone at the lower part of the blast furnace, and this will increase the permeability in the CZ. Also, these slags are highly viscous and show less efficient desulphurization. The former is due to the network forming ability of Al2O3 and later is due to the increase in the activity of sulphur in slag. All the above factors will influence the working condition of a blast furnace and diminish the quality of hot metal. Apart from this the volume of the slag also increases. The Overall production of blast furnace slag may be estimated as equivalent to 25% to 30% of crude (pig) iron production compared to steel furnace slag as about 10% to 15% of crude steel output [217].
Therefore, the present work is focused on the study of physicochemical properties like liquidus temperature, viscosity and desulphurization ability of CaO-MgO-SiO2-Al2O3 quaternary slag system.
The effect of Al2O3, MgO and CaO/SiO2 ratio on flow characteristic temperatures are carried out using a high-temperature microscope in accordance with German standard 51730. Slag with Al2O3 (25-30 %), C/S (0.9-1.4) and MgO (4-12 %) ensures Al2O3 (30 %), C/S ratio of 1.4 and MgO (8 %) as short slag. Whereas, slags with C/S (0.8-1.6) and MgO (8-16 %) are showing Al2O3 (30 %), C/S ratio 1.2 and MgO (12%) as short slag. Also, phase diagrams and XRD analysis for slag samples reveals that all the slags are falling in Anorthite-Melilite-Spinel region.
Viscosity experiments are carried out for slag composition of Al2O3 (25-30 %), C/S (0.8-1.6) and MgO (8-16 %) using vertical rotating type (Model VIS 403 HF) high-temperature viscometer in the temperature range of 1748 K to 1848 K. A relationship between viscosity and the slag structure is intent by Fourier Transform Infrared (FTIR) and Raman spectroscopy. Chemical thermodynamic software FactSage 7.0 is used to predict viscosity of slags. It is observed that an increase in CaO/SiO2 ratio and MgO content in the slag depolymerizes the silicate structure. This leads to a decrease in viscosity and activation energy of the slag. Also, an addition of Al2O3 content increases the viscosity and activation energy of slag by polymerization of aluminosilicate structure.
Desulfurization ability of slags is determined for slag composition of Al2O3 (25-30 %), C/S (0.8-1.6) and MgO (8-16 %) at a temperature of 1773 K using slag-metal equilibrium technique. The sulphur content in the slag as well as in metal after desulfurization reaction is analyzed using XRF (X-ray Fluorescence Spectroscopy) for calculating sulphur partition ratio (Ls) of the slags. The influence of Al2O3, CaO/SiO2 ratio and MgO content on Ls has been studied. It is observed that Ls increase with an increase in CaO/SiO2 ratio and MgO content, but it reduces as the Al2O3 content increase in slag. Also, the amount of bridge oxygen (Oo) decreases with increase in basicity and MgO content. However, Oo increases with increase in Al2O3. It is witnessed that the experimental values closely fit with the values obtained from the corrected optical basicity model.