A Phase Equilibrium of the Iron-rich Corner of the CaO–FeO–Fe₂O₃–SiO₂ System in Air and the Determination of the SFC Primary Phase Field

Abstract

Experimental studies of the CaO-FeO-Fe 2 O 3-SiO 2 system in air from 1 200°C to 1 260°C have been carried out in the Fe-rich region with particular focus on the phase equilibria associated with silico-ferrite of calcium (SFC) phase. The measurements have been made possible through the use of a modified experimental technique involving high temperature equilibration and rapid quenching followed by electron probe X-ray microanalysis (EPMA). Isothermal sections for 1 220°C, 1 240°C, 1 255°C and 1 260°C are reported providing information on the limits of stability of the SFC solid solution and the liquidus in the SFC primary phase field for the first time.

Effect of Gas Atmosphere on the Phase Chemistry in the CaO–FeO–Fe₂O₃–SiO₂ System Related to Iron Ore Sinter-making

“…The experimental methodology used in the present study has been described in detail previously. 1) This principally involves high temperature equilibration of synthetic chemical mixtures in selected gas atmospheres targeting two or more condensed equilibrium phases at the desired temperature, followed by direct quenching into iced water. The cross-sections of the samples were then prepared and examined using standard metallographic techniques.…”

“…An additional correction for cation ratios has been established using Fe 2 SiO 4 , Ca 2 Fe 2 O 5 , CaFe 2 O 4 , and CaFe 4 O 7 standards prepared in the laboratory. 1) No directly measured local oxygen partial pressures in the sinter bed during the iron ore sinter-making process were found in the literature. It was assumed that the gas composition during the sinter formation may vary between that in air (PO 2 = 0.21 atm), to the products of complete combustion of carbon during sinter-making, i.e.…”

“…3, including comparison to the liquidus in air. 1) Note that spinel is the stable solid iron oxide under these conditions. It can be seen that there is no SFC primary phase field appearing at the liquidus temperatures.…”

“…Experimental measurements of the phase equilibria of the pseudo-ternary system CaO-"Fe 2 O 3 "-SiO 2 in air in the iron-rich corner of the system were reported previously. 1) It was shown that under those conditions the silico-ferrite of calcium solid solution (SFC phase [2][3][4] ) was stable at the liquidus temperatures in this system from 1 192 to 1 257°C. These results are of particular significance to the sintering of iron ores 3,[5][6][7][8][9][10] since they establish for the first time the conditions for the formation of SFC solid solution from the melt.…”

“…In the present study, further detailed phase equilibria studies have been carried out; the difficulties associated with the quenching of the low-SiO 2 liquid encountered by previous researchers have been overcome by developing an open sample support method, which has been described previously by the authors. 1) The present investigation is focused on the effect of CO 2 atmosphere on the formation of the SFC phase and liquid phases in the range of bulk compositions and process conditions of direct relevance to the iron ore sinter-making.…”