Effects of the Bulk Fe₂O₃ Concentration on the Controlled Solidification of “Fe₂O₃”–CaO–SiO₂ Liquids in Air

Abstract

The solidification of "Fe 2 O 3 "-CaO-SiO 2 liquids in air at a controlled cooling rate of 2.0 K/s for a range of Fe 2 O 3 concentrations in the bulk has been investigated. The compositions investigated were selected such that the bulk compositions were within the hematite primary phase field and had a CaO/SiO 2 ratio of 3.46 wt/wt.Non-equilibrium phase assemblages were formed for all bulk compositions investigated. Specifically, the silico ferrite of calcium (SFC) phase was not formed on cooling. The microstructures and proportions of the phase assemblages formed were found to vary with the Fe 2 O 3 concentration in the bulk.

Mechanisms of Phase and Microstructure Formation during the Cooling of “Fe₂O₃”–CaO–SiO₂–Al₂O₃ Melts in Air and Implications for Iron Ore Sintering

“…total pressure has been established in previous studies by the authors. 5) Systematic investigations of the controlled solidification of alumina-free "Fe 2 O 3 "-CaO-SiO 2 melts in air [6][7][8][9] have revealed unexpected observations. In particular, it has been shown that for a range of bulk compositions (Fe 2 O 3 concentrations and CaO/SiO 2 ratios) and cooling rates, the SFC phase does not form on cooling despite the fact that this phase is predicted to be thermodynamically stable under these conditions.…”

“…A necessary precondition for SFC and SFCA formation from the liquid is that, on cooling, the liquid composition passes through the SFC and SFCA primary phase fields; this is achieved primarily through control of the CaO/SiO 2 ratio of the bulk material. It has been shown, [6][7][8][9] however, that this on its own is not sufficient to guarantee the formation of the SFC phase. For example, in the "Fe 2 O 3 "-CaO-SiO 2 system in air, hematite, magnetite, dicalcium silicate, calcium diferrite and calcium ferrite phases, all having primary phase fields in this system, have been observed to form on cooling.…”

“…The SFC-I phase is formed by nucleation directly from the melt and by heterogeneous nucleation on magnetite, but no SFC is observed in these samples. [6][7][8][9] An example of SFC-I formation on undercooled magnetite in the alumina-free "Fe 2 O 3 "-CaO-SiO 2 system in air is given in Fig. 10.…”

“…This common plane represents a potential nucleation site for SFCA minerals during the solidification process. It should be pointed out, however that magnetite has four (4) sets of close packed planes of oxygen within its crystal structure, represented by the < 111 > family of planes, (111), (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11), and (-111). 30) This being the case it would be expected that the probability of nucleation of SFCA on magnetite crystal surfaces would be greater than on hematite.…”

“…In the "Fe 2 O 3 "-CaO-SiO 2 system in air the CF 2 phase has been found to nucleate on the surfaces of hematite crystals. [6][7][8][9] Both CF 2 and hematite have fully occupied close packed oxygen planes; unlike the "almost" close packed plane of the SFCA phase, which has systematic vacancies. The CF 2 crystal is also a polysomatic mineral, containing a 'spinel' polysome and a 'Motif-F' polysome.…”