Insight into the Relationship Between Viscosity and Structure of CaO-SiO 2 -MgO-Al 2 O 3 Molten Slags
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IN recent years, the iron ore grade has been gradually declining with continuous depletion of ore reserves. It is estimated that most iron ores mined in China contain < 30 wt pct iron, which is far below the global average (60 wt pct iron).[1] In this situation, relatively highdegree iron ores from India and Australia have been considered alternative options.[2] A typical character of these ores is a higher Al2O3 content.[3,4] In addition, many changes in the operational conditions of blast furnaces have taken place in China, including the utilization of low-cost non-metallurgical coals.[5] As a result, the Al2O3 concentration in molten slags has increased to > 20 wt pct.[6] This composition change of slag will significantly affect its thermophysical properties, such as the viscosity and crystallization behaviors.
ZIWEI CHEN, HAO WANG, LILI LIU, and XIDONG WANG are with the Department of Energy and Resources Engineering, College of Engineering, Peking University, 100871 Beijing, P.R. China. Contact e-mail: [email protected] YONGQI SUN is with the School of Chemical Engineering, The University of Queensland, Brisbane QLD 4072, Australia. Contact e-mail: [email protected] Manuscript submitted April 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Slag viscosity is a fundamental thermophysical property and plays an important role in metallurgical processes.[7] It is sensitive to compositional variation of slag melt. In fact, the variational composition impacts viscosity, relying on the change in structure at the microlevel, that is, slag viscosity is directly related to the structure of the slag melt. From the micro-level, viscosity reflects the internal friction between molecules of a fluid when they flow under external force. The internal friction is determined by parameters such as the structure of the molecules and the interaction forces between the molecules. Thus, a deep understanding of the origin of slag viscosity at the micro-level is highly necessary. Generally, blast furnace slags are considered a mixture of oxides, mainly composed of CaO, SiO2, MgO, Al2O3 and other minor elements. Therefore, CaO-SiO2MgO-Al2O3 is commonly considered a basic quaternary system of slags.[8] In terms of the role of oxides in the melt structure, CaO and MgO are network modifiers; SiO2 is a network former; Al2O3 is an intermediate oxide with the ability to polymerize or depolymerize networks,[9–11] and its amphoteric behavior on the structure and viscosity is complicated, and some unclear issues remain. Many investigations[12–17] focused on the lower end concentration of Al2O3 and failed to discover its amphoteric behavior. For example, the thermophysical properties and structure of CaO-SiO2-MgO-Al2O3 slag
Table I. Samples A1 Designed XRF A2 Designed XRF A3 Designed XRF A4 Designed XRF A5 Designed XRF
Chemical Compositions of Modified Slags (in Mass Pct)
CaO
SiO2
MgO
Al2O3
Basicity
Primary Crystalline Phase
Liquidus Temperature (C)
43.5 44.39
43.5 42.94
8 7.28
5 4.78
1 1.03
Ca2MgSi2O7 or Ca3MgSi2O8
1356
41 42
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