Effect of the CaO/SiO₂ ratio on the controlled solidification of ‘Fe₂O₃’-CaO-SiO₂ melts in air

Abstract

The solidification of liquids having bulk compositions in the high iron region of the 'Fe 2 O 3 '-CaO-SiO 2 system having selected CaO/SiO 2 ratios has been investigated. The compositions investigated were selected such that the bulk compositions were within the hematite primary phase field, fully liquid at temperature (1350°C) and pass through the SFC primary phase field on cooling. Specifically, three CaO/SiO 2 ratios (wt/wt) were investigated, 2.78, 3.46 (Nicol S, Jak E, Hayes P. 2019b. Microstructure evolution during controlled solidification of "Fe 2 O 3 "-CaO-SiO 2 liquids in air. Met. Trans. B. 50:2706-2722) and 4.75 with bulk compositions comprising 69.23, 72.74 and 74.94 wt% Fe 2 O 3 respectively. The phases present and their composition was determined with electron probe microanalysis (EPMA) and X-ray diffraction (XRD). During cooling, four stages of solidification were observed at all three CaO/SiO 2 ratios, and an additional two stages at the highest CaO/SiO 2 ratio. Non-equilibrium phase and liquidus compositions were observed. Significantly, the silico ferrite of calcium phase (SFC) was not observed in any of the samples, although the formation of this phase was anticipated from phase equilibrium considerations.

Control of Interface Migration in Nonequilibrium Crystallization of Li2SiO3 from Li2O–SiO2 Melt by Spatiotemporal Temperature and Concentration Fields

“…Hence, CALPHAD model predictions of solid phase fractions might vary from experimental observations dependent on the real kinetic behavior of the phase transition. 21 The nonequilibrium models can further be classified based on the kinetics that are captured. Some of them are developed from the mass diffusion viewpoint and consider homogeneous spatial temperature fields.…”

“…However, it might be mentioned here that the prevalence of diffusion kinetics and/or kinetic barriers for liquid/solid interfacial transformations are not taken into consideration within such a model. Hence, CALPHAD model predictions of solid phase fractions might vary from experimental observations dependent on the real kinetic behavior of the phase transition …”

Iron Ore Sinter Macro- and Micro-Structures, and Their Relationships to Breakage Characteristics

“…Previous studies have shown that iron ore sinter properties depend on the bulk chemical composition of the sinters [11][12][13][14], the phases present and the volume fractions of these phases [11,15,16]. Fundamental research has established the influence of bulk composition, temperature and oxygen partial pressure on the phases formed under equilibrium [17][18][19][20] and non-equilibrium conditions [21][22][23][24][25][26]. Pownceby et al [17,18] investigated the phase equilibria in the Fe-rich region of the FeO-Fe 2 O 3 -CaO-SiO 2 system in air and in oxygen partial pressure pO 2 = 5 × 10 −3 atm from 1240 to 1300 • C. Chen et al [19] and Cheng et al [20] subsequently expanded the phase equilibria study in air to investigate the full extent of the primary phase field of SFC (silico-ferrite of calcium solid solution) which is believed to be a critical phase that enhances sinter quality [14].…”

“…Pownceby et al [17,18] investigated the phase equilibria in the Fe-rich region of the FeO-Fe 2 O 3 -CaO-SiO 2 system in air and in oxygen partial pressure pO 2 = 5 × 10 −3 atm from 1240 to 1300 • C. Chen et al [19] and Cheng et al [20] subsequently expanded the phase equilibria study in air to investigate the full extent of the primary phase field of SFC (silico-ferrite of calcium solid solution) which is believed to be a critical phase that enhances sinter quality [14]. Nicol et al [21][22][23][24][25] investigated the non-equilibrium cooling behaviour of the melt in the FeO-Fe 2 O 3 -CaO-SiO 2 system in air as an effect of bulk compositions and cooling rates. Significant undercoolings of the slag phase were identified and it was indicated that the formation of SFC(A) phase in iron ore sinters is principally controlled by kinetic processes.…”

“…Perform macroscopic (mm to cm) examination of sinter and return samples including but not limited to the breakage surfaces; • Introduce classification of sinter Macro-Types based on macro-appearances; • Perform microstructural analysis and phase identifications using scanning electron microscope (SEM) electron probe X-ray microanalysis (EPMA); • Introduce classification of sinter microstructures; • Demonstrate the relationship between the macro-scale and micro-scale structural features of the materials, with the focus on the microstructures corresponding to the breakage surfaces; • Discuss mechanisms of formation of sinter microstructures with the aid of fundamental phase equilibria [17][18][19][20] and solidifications studies [21][22][23][24][25][26];…”