In situ X-ray Diffraction Investigation of the Formation Mechanisms of Silico-Ferrite of Calcium and Aluminium-I-type (SFCA-I-type) Complex Calcium Ferrites.

The formation mechanisms of the complex Ca-rich ferrite phase SFCA-I, an important bonding material in iron ore sinter, during heating of synthetic sinter mixtures in the temperature range 298-1623 K in air and at pO(2) = 5 x 10(-3) atm, were determined using in situ X-ray powder diffraction. In air, the initial formation of SFCA-I at similar to 1438 K (depending on composition) was associated with reaction of precursor phases Fe2O3, CaO center dot Fe2O3, SiO2, amorphous AI-oxide and a CFA phase of approximate composition 71.7 mass% Fe2O3, 12.9 mass% CaO, 0.3 mass% SiO2 and 15.1 mass% Al2O3. At temperatures above similar to 1453 K, the decomposition of another phase, gamma-CFF, resulted in the formation of additional SFCA-I. At lower oxygen partial pressure the initial formation of SFCA-I occurred at similar temperatures and was associated with reaction between similar phases as its formation in air. However, the decomposition of gamma-CFF did not result in the formation of additional SFCA-I, with the maximum SFCA-I concentration (25 mass%) lower than the values attained in air (54 and 34 mass%). Hence, more oxidising conditions appear to favour the formation of the desirable SFCA-I phase. Copyright © The Iron and Steel Institute of Japan 2013

Thermodynamic assessment and experimental study of sulphidation of ilmenite and chromite

This study provides an assessment of the high temperature thermochemical reactions of ilmenite and chromite with sulphur and uses the information to analyse the possibility of selective sulphidation of chrome-bearing spinels as a new route for chromite removal from ilmenite concentrates. The work includes both systematic thermodynamic assessment and targeted experimental investigations. Thermodynamic calculation results studying the effect of reactants composition, temperature and different sulphur sources (H2S and elemental S) showed that chromite can be selectively sulphidised at a controlled atmosphere of partial pressure of oxygen (pO2) below 10−10 atm and partial pressure of sulphur (pS2) above 10−6 atm. The addition of carbon with sulphur was found to be useful for the chromite sulphidation reaction. The optimum quantity of sulphur reactant to carbon was found to be 3 : 1 (in mole) for 1 mol of chromite. Sulphidation experiments on a mixture of natural ilmenite and chromite at 1100°C for 5 h using 5%H2S as a sulphur source showed that the ilmenite was preferentially sulphidised first, which was also in a good agreement with the thermodynamic assessment. Sulphidation of a naturally occurring chrome spinel contaminated ilmenite concentrate showed that the degree of weathering also played a role in the sulphidation of the chrome spinel. It was also concluded that H2S is not suitable for selective sulphidation of chrome spinel from the ilmenite concentrate and that tightly controlled pS2 and pO2 conditions are required

Silico-ferrite of Calcium and Aluminum (SFCA) Iron Ore Sinter Bonding Phases: New Insights into Their Formation During Heating and Cooling

The formation of silico-ferrite of calcium and aluminum (SFCA) and SFCA-I iron ore sinter phases during heating and cooling of synthetic iron ore sinter mixtures in the range 298 K to 1623 K (25 A degrees C to 1350 A degrees C) and at oxygen partial pressure of 5 x 10(-3) atm has been characterized using in situ synchrotron X-ray diffraction. SFCA and SFCA-I are the key bonding phases in iron ore sinter, and an improved understanding of their formation mechanisms may lead to improved efficiency of industrial sintering processes. During heating, SFCA-I formation at 1327 K to 1392 K (1054 A degrees C to 1119 A degrees C) (depending on composition) was associated with the reaction of Fe2O3, 2CaO center dot Fe2O3, and SiO2. SFCA formation (1380 K to 1437 K [1107 A degrees C to 1164 A degrees C]) was associated with the reaction of CaO center dot Fe2O3, SiO2, and a phase with average composition 49.60, 9.09, 0.14, 7.93, and 32.15 wt pct Fe, Ca, Si, Al, and O, respectively. Increasing Al2O3 concentration in the starting sinter mixture increased the temperature range over which SFCA-I was stable before the formation of SFCA, and it stabilized SFCA to a higher temperature before it melted to form a Fe3O4 + melt phase assemblage (1486 K to 1581 K [1213 A degrees C to 1308 A degrees C]). During cooling, the first phase to crystallize from the melt (1452 K to 1561 K [1179 A degrees C to 1288 A degrees C]) was an Fe-rich phase, similar in composition to SFCA-I, and it had an average composition 58.88, 6.89, 0.82, 3.00, and 31.68 wt pct Fe, Ca, Si, Al, and O, respectively. At lower temperatures (1418 K to 1543 K [1145 A degrees C to 1270 A degrees C]), this phase reacted with melt to form SFCA. Increasing Al2O3 increased the temperature at which crystallization of the Fe-rich phase occurred, increased the temperature at which crystallization of SFCA occurred, and suppressed the formation of Fe2O3 (1358 K to 1418 K [1085 A degrees C to 1145 A degrees C]) to lower temperatures. © 2012, Springer

Analysis of sulifdation routes for processing weathered ilmenite concentrates containing impurities

Abstract not available

Effect of oxygen partial pressure on the formation mechanisms of complex Ca-rich ferrites

The formation mechanisms of the complex Ca-rich ferrite iron ore sinter bonding phases SFCA and SFCA-I, during heating of a synthetic sinter mixture in the range 298-1623 K and at pO(2) = 0.21, 5 x 10(-3) and 1 x 10(-4) atm, were determined using in situ X-ray diffraction. SFCA and, in particular, SFCA-I are desirable bonding phases in iron ore sinter, and improved understanding of the effect of parameters such as pO(2) on their formation may lead to improved ability to maximise their formation. in industrial sintering processes. SFCA-I and SFCA were both observed to form at pO(2) = 0.21 and 5 x 10-3 atm, with the formation of SFCA-I preceding SFCA formation in each case, but via distinctly different mechanisms at each pO(2). No SFCA-I was observed at pO(2) = 1 x 10-4 atm; instead, a Ca-rich phase designated CFAlSi, formed at 1 420 K. By 1 456 K, CFAlsi had decomposed to form melt and a small amount of SFCA. Such a low pO(2) during heating of industrial sinter mixtures is, therefore, undesirable, since it would not result in the formation of an abundance of SFCA and SFCA-I bonding phases. In addition, CFA phase, which was determined by Webster et al. (Metall. Mater. Trans. B, 43(2012), 1344) to be a key precursor phase in the formation of SFCA at pO(2) = 5 x 10(-3) atm, was also observed to form at pO(2) = 0.21 and 1 x 10(-4) atm, with the amount decreasing with increasing pO(2). Copyright © The Iron and Steel Institute of Japan 2013

Selective sulfidising roasting for the removal of chrome spinel impurities from weathered ilmenite ore

With high-grade ilmenite (FeTiO3) ores becoming scarce, there is a need to process lower-grade and more weathered ilmenite ores. These alternative sources provide complexities in terms of physical characteristics, mineral compositions, and impurity levels, all of which can significantly affect the subsequent processing route. In the production of high purity white pigment from ilmenite, chromia (Cr2O3)-containing impurities such as chrome-rich spinets need to be removed from the ilmenite concentrates down to commercially accepted levels. Existing magnetising roast processes currently used in the industry do not allow a clean separation of the chrome-rich spinets from the ilmenite due to overlapping physical properties. It has been suggested that selective sulfidation of chrome-rich spinels could be a potential route for chromia separation from ilmenite. However, the detailed conditions under which the selective sulfidation can occur are not well known. This work focuses on a systematic study of selective sulfidation of chromite FeCr2O4 (one end member of the spinel series) for the purpose of chromia impurity removal from a weathered ilmenite concentrate. Detailed thermodynamic assessment and experimental studies have been carried out to determine the conditions at which selective chrome spinel sulfidation occurs. The results suggest that there are two regimes (Area-1 and Area-2) where selective chrome spinel sulfidation is possible. Area-1 is in the range of pO(2) approximate to 2.37 x 10(-09) atm to 5.01 x 10(-15) atm, while Area-2 is at lower pO(2) (<= 6.92 x 10(-19) atm) and pS(2) (<= 1 x 10(-06) atm) region relative to Area-1. Targeted experimental analyses of the two regimes revealed that selective sulfidation of chrome spinel occur only under the reaction conditions at Area-2. It is suggested that the lack of selective chrome spinel sulfidation under Area-1 conditions is the change in activity of iron (a(Fe)) due to weathering action on chrome spinet grain