Browsing by Author "Madsen, IC"

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    Effect of oxygen partial pressure on the formation mechanisms of complex Ca-rich ferrites
    (The Iron and Steel Institute of Japan Keidanren Kaikan, 2013-01-01) Webster, NAS; Pownceby, MI; Madsen, IC; Kimpton, JA
    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.
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    Fundamentals of silico-ferrite of calcium and aluminum (SFCA) and SFCA-I iron ore sinter bonding phase formation: effects of CaO:SiO2 ratio
    (Springer Link, 2014-07-22) Webster, NAS; Pownceby, MI; Madsen, IC; Studer, AJ; Manuel, JR; Kimpton, JA
    Effects of basicity, B (CaO:SiO2 ratio) on the thermal range, concentration, and formation mechanisms of silico-ferrite of calcium and aluminum (SFCA) and SFCA-I iron ore sinter bonding phases have been investigated using an in situ synchrotron X-ray diffraction-based methodology with subsequent Rietveld refinement-based quantitative phase analysis. SFCA and SFCA-I phases are the key bonding materials in iron ore sinter, and improved understanding of the effects of processing parameters such as basicity on their formation and decomposition may assist in improving efficiency of industrial iron ore sintering operations. Increasing basicity significantly increased the thermal range of SFCA-I, from 1363 K to 1533 K (1090 °C to 1260 °C) for a mixture with B = 2.48, to ~1339 K to 1535 K (1066 °C to 1262 °C) for a mixture with B = 3.96, and to ~1323 K to 1593 K (1050 °C to 1320 °C) at B = 4.94. Increasing basicity also increased the amount of SFCA-I formed, from 18 wt pct for the mixture with B = 2.48 to 25 wt pct for the B = 4.94 mixture. Higher basicity of the starting sinter mixture will, therefore, increase the amount of SFCA-I, considered to be more desirable of the two phases. Basicity did not appear to significantly influence the formation mechanism of SFCA-I. It did, however, affect the formation mechanism of SFCA, with the decomposition of SFCA-I coinciding with the formation of a significant amount of additional SFCA in the B = 2.48 and 3.96 mixtures but only a minor amount in the highest basicity mixture. In situ neutron diffraction enabled characterization of the behavior of magnetite after melting of SFCA produced a magnetite plus melt phase assemblage. © 2014, The Minerals, Metals & Materials Society and ASM International.
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    In situ diffraction studies of iron ore sinter bonding phase formation: QPA considerations and pushing the limits of laboratory data collection
    (Cambridge University Press, 2014-11-17) Webster, NAS; Pownceby, MI; Madsen, IC; Studer, AJ
    The formation and decomposition of silico-ferrite of calcium and aluminium (SFCA) and SFCA-I iron ore sinter bonding phases have been investigated using in situ synchrotron and laboratory X-ray diffraction (XRD) and neutron diffraction (ND). An external standard approach for determining absolute phase concentrations via Rietveld refinement-based quantitative phase analysis is discussed. The complementarity of in situ XRD and ND in characterising sinter phase formation and decomposition is also shown, with the volume diffraction afforded by the neutron technique reducing errors in the quantification of magnetite above ~1200 °C. Finally, by collecting 6 s laboratory XRD datasets and using a heating rate of 175 °C min−1, phase formation and decomposition have been monitored under heating rates more closely approximating those encountered in industrial iron ore sintering.© 2014 International Centre for Diffraction Data
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    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 Iron and Steel Institute of Japan, 2013-01-01) Webster, NAS; Pownceby, MI; Madsen, IC
    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.
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    An investigation of goethite-seeded Al(OH)(3) precipitation using in situ x-ray diffraction and rietveld-based quantitative phase analysis
    (Wiley-Blackwell, 2010-06) Webster, NAS; Madsen, IC; Loan, MJ; Knott, RB; Naim, F; Wallwork, KS; Kimpton, JA
    An in situ X-ray diffraction investigation of goethite-seeded Al(OH)3 precipitation from synthetic Bayer liquor at 343 K has been performed. The presence of iron oxides and oxyhydroxides in the Bayer process has implications for alumina reversion, which causes significant process losses through unwanted gibbsite precipitation, and is also relevant for the nucleation and growth of scale on mild steel process equipment. The gibbsite, bayerite and nordstrandite polymorphs of Al(OH)3 precipitated from the liquor; gibbsite appeared to precipitate first, with subsequent formation of bayerite and nordstrandite. A Rietveld-based approach to quantitative phase analysis was implemented for the determination of absolute phase abundances as a function of time, from which kinetic information for the formation of the Al(OH)3 phases was determined. © 2010, Wiley-Blackwell. The definitive version is available at www3.interscience.wiley.com
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    An investigation of the mechanisms of goethite, hematite and magnetite-seeded Al(OH)(3) precipitation from synthetic Bayer liquor
    (Elsevier, 2011-09-01) Webster, NAS; Loan, MJ; Madsen, IC; Knott, RB; Kimpton, JA
    The precipitation of Al(OH)3 from synthetic Bayer liquor at 70 degrees C seeded with goethite, hematite and magnetite particles was investigated in order to gain insight into the nucleation and growth mechanisms in the presence of these materials. A combination of characterisation techniques was employed including conductivity analysis, particle size analysis, electron microscopy and in situ synchrotron X-ray diffraction. The magnetite seed was less active for promoting Al(OH)(3) precipitation than the goethite and hematite, based on a comparison of the induction time before the onset of measurable precipitation. For each seed material, the early stages of precipitation were characterised by relatively slow deposition of gibbsite on the seed particles. Precipitation then proceeded via a two-stage mechanism, where gibbsite and small amounts of bayerite and nordstrandite precipitated concurrently. The outcomes of this investigation have implications for the nucleation and growth of scale on mild steel process equipment, and are also relevant for alumina reversion which causes significant process losses through uncontrolled precipitation. (C) 2011 Elsevier B.V.
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    New type of cubic-stacked layer structure in anthoinite, AlWO3(OH)(3)
    (Mineralogical Society of America, 2010-04) Grey, IE; Madsen, IC; Mills, SJ; Hatert, F; Peterson, VK; Bastow, TJ
    Anthoinite, AlWO3(OH)3, from the Mt. Misobo Mine, Democratic Republic of the Congo, has triclinic symmetry with cell parameters a = 8.196(1) Å, b = 9.187(1) Å, c = 11.316(1) Å, = 92.82(1)°, β = 94.08(1)°, = 90.23(1)°, space group ґI, Z = 8. The structure was solved by applying ab initio structure solution methods (Reverse Monte Carlo/Simulated Annealing) to both X-ray and neutron powder diffraction data and was refined using the Rietveld method. The structure is built up of two types of M4(O,OH)16 planar tetrameric clusters of edge-sharing octahedra, one containing predominantly Al and the other predominantly W. The Al-rich and W-rich clusters interconnect via corner sharing to form stepped layers parallel to (001). The layers are held together by strong hydrogen bonding. The structure can be described as a rocksalt derivative structure, with the close-packed anion layers parallel to (012), and with Al and W atoms ordered into one third of the octahedral sites within the cubic close-packed anion lattice. The structure is complicated by partial disorder between Al and W in the tetrameric clusters and associated disorder in the H atom sites. Infrared and 27Al MAS NMR results are also presented for anthoinite. © 2010, Mineralogical Society of America
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    Silico-ferrite of calcium and aluminum (SFCA) iron ore sinter bonding phases: new insights into their formation during heating and cooling
    (Springer nature, 2012-12-01) Webster, NAS; Pownceby, MI; Madsen, IC; Kimpton, JA
    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 Nature
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    Understanding solvothermal crystallization of mesoporous anatase beads by in situ synchrotron PXRD and SAXS
    (American Chemical Society, 2014-07-07) Xia, F; Chen, DH; Scarlett, NVY; Madsen, IC; Lau, D; Leoni, M; Ilavsky, J; Brand, HEA; Caruso, RA
    Submicrometer-sized mesoporous anatase (TiO2) beads have shown high efficiency as electrodes for dye-sensitized solar cells and are recoverable photocatalysts for the degradation of organic pollutants. The detailed mechanism for crystallization of the amorphous TiO2/hexadecylamine (HDA) hybrid beads occurring during the solvothermal process needs to be understood so that reaction parameters can be rationally refined for optimizing the synthesis. In this work, the solvothermal crystallization was monitored by in situ synchrotron powder X-ray diffraction (PXRD) and synchrotron small-angle X-ray scattering (SAXS) techniques. In situ PXRD provided crystallization curves, as well as the time evolution of anatase crystallite mean size and size distribution, and in situ SAXS provided complementary information regarding the evolution of the internal bead structure and the formation of pores during the course of the solvothermal process. By exploring the effects of temperature (140-180 °C), bead diameter (300 and 1150 nm), bead internal structure, and solvent composition (ethanol and ammonia concentrations) on this process, the crystallization was observed to progress 3-dimensionally throughout the entire bead due to solvent entrance after an initial fast partial dissolution of HDA from the nonporous precursor bead. On the basis of the kinetic and size evolution results, a 4-step crystallization process was proposed: (1) an induction period for precursor partial dissolution and anatase nucleation; (2) continued precursor dissolution accompanied by anatase nucleation and crystal growth; (3) continued precursor dissolution accompanied by only anatase crystal growth; and (4) complete crystallization with no significant Ostwald ripening. © 2014 American Chemical Society.