Showing papers on "Hematite published in 1994"
TL;DR: In this article, a calibrated chemical extraction scheme was developed for partitioning reactive Fe(III) minerals in the solid phase of marine sediments, and the following chemical extractants were used: ascorbate, oxalate, dithionite, and HC1 (0.5 M).
620 citations
TL;DR: The concentration of arsenic in natural iron oxyhydroxides and oxide minerals extracted from soils at the Ashanti mine, Ghana, and was found to vary from 2 to 35,600 mg/kg as discussed by the authors.
580 citations
TL;DR: The products of the heterogeneous oxidation of Mn(II)aq at hematite, goethite, and albite surfaces were studied, using Scanning Force Microscopy (SFM), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Micro Scopy (SEM), Auger Electron Spectrogram Spectrogram (AES), and X-Ray Diffraction (XRD).
281 citations
TL;DR: In this article, X-ray absorption fine structure (XAFS) spectra were collected on a series of ferrihydrite samples prepared over a range of precipitation and drying conditions.
Abstract: X-ray absorption fine structure (XAFS) spectra were collected on a series of ferrihydrite samples prepared over a range of precipitation and drying conditions. Analysis of the XAFS pre-edge structures shows clear evidence of the presence of lower coordination sites in the material. These sites, which are most likely tetrahedral, are believed to be at the surface and become coordination unsaturated (CUS) after dehydroxylation. With chemisorbed water molecules, the CUS sites become the crystal growth sites responsible for the phase transformation of ferrihydrite to hematite at low temperatures. On the other hand, when impurity anions such as SiO4−4 are present in the precipitation solution, the CUS sites may instead absorb the impurity anions, thereby blocking the crystal growth sites and inhibiting the formation of hematite.
209 citations
TL;DR: Information on the bonding of organic ligands to hydrous metal oxide surfaces, necessary to gain insight into the mechanisms of surface reactivity, was extracted using attenuated total reflection-Fourier transform infrared spectroscopy.
Abstract: Information on the bonding of organic ligands to hydrous metal oxide surfaces, necessary to gain insight into the mechanisms of surface reactivity, was extracted using attenuated total reflection-Fourier transform infrared spectroscopy. Various ligands were investigated in-situ on the surface of δ-Al 2 O 3 in water. Keeping the study of structure with respect to surface reactivity in the foreground, we report here specifically on the model ligand salicylate, which we also examined on the surfaces of other oxides, including corundum, hematite, lepidocrocite, and goethite. Because the direct ligand-metal bond vibrations absorb in the same region as the metal-oxygen vibrations in the bulk oxide, evidence of bonding was obtained by observing changes in the spectrum of the ligand itself
198 citations
TL;DR: In this paper, the authors report enhanced compatibility for Ru compared to previous work for spinels in an Fe-free system, but to a lesser degree, and suggest that Ru should be concentrated in spinels crystallizing in Earth's atmosphere, either within micrometeroids, or from atmospheric crystallization of impact ejecta.
159 citations
TL;DR: In this article, a shoulder near 0.63 micrometers is attributed to oxidized iron in phyllosilicates found predominantly in C- and G-class asteroids reflectance spectra.
117 citations
TL;DR: A laboratory extraction method (Ti 3+ -EDTA extraction) for the determination of the oxidation (electron accepting) capacity related to oxides and hydroxides of aquifer sediments was developed and operationally defined oxidation capacity (ORC) was determined as μmoles of electrons accepted per gram of sediment sample.
Abstract: A laboratory extraction method (Ti 3+ -EDTA extraction) for the determination of the oxidation (electron accepting) capacity related to oxides and hydroxides of aquifer sediments was developed. At room temperature, titanous ions (0.008 M Ti 3+ ) in a solution of 0.05 M ethylenediaminetetraacetic acid (EDTA) reduced oxidized aquifer species. This operationally defined oxidation capacity (ORC) was determined as μmoles of electrons accepted per gram of sediment sample. Well-described oxidized iron and manganese minerals were reduced (ferrihydrite 98%, akageneite 100%, goethite and hematite 93%,magnetite 9%, pyrolmite 99%) whereas organic matter in the sediments was not reduced significantly
116 citations
TL;DR: In this paper, the effect of morphology and other crystal properties of hematite on phosphorous adsorption and desorption was studied, and it was shown that the more platy the crystal, the lower the proportion of high-affinity sites, supporting the hypothesis that the P-adsorbing faces are the nonbasal ones.
109 citations
TL;DR: In this paper, the authors carried out transformation experiments with synthetic ferrihydrite which contained adsorbed natural uranium (predominantly {sup 238}U) and showed that the transformation of ferri hydrite to crystalline minerals substantially reduced the ability of the samples to adsorb {sup 236}U from solution.
Abstract: Uranium sorption experiments were carried out on weathered schist samples which had been treated with citrate/dithionite/bicarbonate (CDB) reagent to remove iron oxides, and the results were compared to those of similar experiments with untreated samples. Uranium sorption was greatly decreased by the CDB extraction, which reduced the surface area of the samples by about 30-40%. Chemical analysis and XRD confirmed that iron oxides were the major mineral phase extracted by the CDB reagent. To further elucidate the role of iron minerals in the natural environment, we carried out transformation (aging) experiments with synthetic ferrihydrite which contained adsorbed natural uranium (predominantly {sup 238}U). In these experiments, the ferrihydrite was partially converted to crystalline forms such as hematite and goethite. The uptake of an artificial uranium isotope ({sup 236}U) and the leaching of {sup 238}U form the samples were then studied in adsorption/desorption experiments. The results showed that the transformation of ferrihydrite to crystalline minerals substantially reduced the ability of the samples to adsorb {sup 236}U from solution. The desorption data for {sup 238}U showed that some of the {sup 238}U which was adsorbed to the samples prior to the transformation step was irreversibly incorporated within the mineral structure during the transformation process. These experiments highlight the potential importance of iron minerals both in the initial sorption of radionuclides and in isolating them from interactions with the aqueous phase. Tranformation of iron minerals from amorphous to crystalline forms provides a possible mechanism for uranium immobilisation. In considering the overall effect on U migration, this must be balanced against the reduced ability of the transformed iron oxide to adsorb U from solution. (orig.)
98 citations
TL;DR: In this article, the role of particle and carbonate concentrations on the sorption/desorption of 241 Am on montmorillonite, illite and hematite colloids is investigated.
Abstract: Actinide sorption on colloids may be described as a competition between the formation of complexes in solution and the build up of surface complexes. The role of particle and of carbonate concentrations on the sorption/desorption of 241 Am on montmorillonite, illite and hematite colloids is investigated. Since the partition coefficient (Kp) values are virtually independent of the colloid concentrations, within the range 1 to 300 ppm, no significant aggregation takes place in the sorption/desorption experiment. At pH 8, a slight decrease of Kp is observed if the concentration of total carbonate exceeds 10~ M. The formation of the carbonato(and hydroxo-carbonato-) complexes in the solution competes with the formation of surface complexes on the colloids. A relationship between the sorption coefficient and the complexation of Am in the solution has been found. This leads to the conclusion that, besides free americium cation, the hydroxo-, and carbonatoas well as the mixed hydroxo-carbonatocomplexes are sorbed. Only when the tricarbonatocomplex [Am(C03)3]~ prevails (total carbonate concentration >10~ M), a significant decrease of the distribution coefficient is observed. At pH 10 this decrease disappears because under these conditions the strong hydroxo-complexes dominate. A pragmatic and relatively simple application of surface complexation model describes the observed features.
TL;DR: In this article, a method for determining the oxidation state of Fe using the electron microprobe was described. But the sensitivity achieved with this "flank method" is about 3-4 times higher than with the conventional peak shift method or peak intensity ratio method.
Abstract: A method is described which allows the determination of Fe 2+ /Fe 3+ ratios with the electron microprobe by analyzing the FeLα and FeLβ X-ray emission spectra. We use the ratios of intensities measured on the high energy flank of the Lα and the low energy flank of the Lβ peak. The sensitivity achieved with this «flank method» is about 3-4 times higher than with the conventional peak shift method or peak intensity ratio method. The method was calibrated with the iron oxides wustite, magnetite and hematite and was then applied to a synthetic spinel solid solution series magnetite-hercynite (Fe 3 O 4 -FeAl 2 O 4 ) and to several natural olivines (Mg 2 SiO 4 -Fe 2 SiO 4 solid solutions). The results obtained by the flank method vary not only with Fe 3+ , but also with total iron content. A quantitative method for determining the oxidation state of Fe must therefore not only be based on a method with a high resolution but must also include corrections for self-absorption and matrix effects. Our method can be applied, if the bulk composition and crystal structure of the samples are known and working curves are established with similar materials
TL;DR: The dependence of phytoavailability of adsorbed phosphate on mineral type and properties has not been established as mentioned in this paper, but it has been shown that goethites usually have higher relative affinity for phosphate than hematites, suggesting that phosphate could be more available to plants in hematitic than in similar goethitic soils.
Abstract: Iron (hydr)oxides are important phosphate adsorbents in soils. However, the dependency of phytoavailability of adsorbed phosphate on mineral type and properties has not been established. To examine the influence of mineralogy, one ferrihydrite, two hematites, and two goethites were prepared. They differed widely in morphology (from euhedral crystals to granular aggregates), specific surface area (15–266 m2 g−1) and microporosity (0–87% of total surface area). Their relative affinity for phosphate, which is inversely related to the concentration exponent in the Freundlich equation, b, decreased in the order goethite (b=0.11, 0.09)> hematite (b=0.13, 0.17)> ferrihydrite (b=0.21). Phytoavailability of the adsorbed phosphate was studied by growing sunflower (Helianthus annuus L.) in pots containing suspensions of the phosphated Fe (hydr)oxides in equilibrium with a concentration of 1 mg P L−1. The fraction of the adsorbed phosphate that was phytoavailable, B
1, (1) was lower for goethite (B
1=0.43, 0.38) than for hematite (B
1=0.73, 0.49) and ferrihydrite (B
1=0.79), (2) was not negatively affected by microporosity, and (3) decreased, in general terms, with increasing relative affinity for phosphate. Similar trends were observed for the production of dry matter. These results, together with previous reports showing that goethites usually have higher relative affinity for phosphate than hematites, suggest that phosphate could be more available to plants in hematitic than in similar goethitic soils.
TL;DR: Several iron oxides and mixtures have been prepared by heating the product of a sol-gel reaction between iron(III) nitrate and ethylene glycol in different atmospheres and at different temperatures as mentioned in this paper.
TL;DR: In this paper, the second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, ferrihydrite and Fe-polymer) were used to distinguish between the Fe-oxides occurring as coatings and as occluded phases.
Abstract: Diffuse reflectance spectra of kaolins have been recorded in samples from different environ- ments. They show the systematic presence of Fe-oxides, even in bleached kaolins, with no contribution from the Fe 3+ ions substituted in kaolinite. Second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, ferrihydrite and Fe-polymer) may be differentiated by the position of a diagnostic band corresponding to the 2QA0 ~ 2(4T~(4G)) transition. The systematic com- parison of diffuse reflectance spectra of unbleached and bleached kaolins has demonstrated the differences between the Fe-oxides occurring as coatings and as occluded phases. The features observed in second derivative spectral curves are consistent with assignments of crystal field transitions to goethite, hematite, akaganeite, and aged hydrous ferric oxides. The optical determination of the Fe-phases associated to kaolins assists in the interpretation of the formation conditions of these minerals.
TL;DR: In this article, it was shown that low energy mechanical activation of the oxide surface is sufficient to effect the transformation and normal air pressure and/or application of hydrocarbons suppress the transformation.
Abstract: Total phase transformation of hematite to magnetite was accomplished at room temperature by wet magnetomechanical activation of hematite. Low energy mechanical activation of the oxide surface is sufficient to effect the transformation. Oxygen bonds on /spl alpha/-Fe/sub 2/O/sub 3/ oxide surface are apparently broken during the mechanical activation process and oxygen is released (removed) to the dispersing polar liquid. The oxygen pressure during the process as well the nature of the dispersing liquid have a critical influence on successful and fast phase transformation. Thus, all preparations performed in air, dry conditions or with nonpolar hydrocarbons (benzene, anthracene) show that the process of hematite reduction is non existent or very slow. Normal air pressure and/or application of hydrocarbons suppress the transformation. >
01 Dec 1994
TL;DR: The internal structure of monodispersed peanut-type hematite (α-Fe2O3) particles prepared by the gel-sol method has been studied through high-resolution electron microscopy on their thin films sliced with an ultramicrotome as discussed by the authors.
Abstract: The internal structure of monodispersed peanut-type hematite (α-Fe2O3) particles prepared by the gel-sol method has been studied through high-resolution electron microscopy on their thin films sliced with an ultramicrotome. The electron micrographs and electron diffraction patterns of the thin sections not only directly revealed their polycrystallinity but also disclosed the shape, size distribution, and orientation of the subcrystals in detail. The subcrystals inside the particles have a rod-like shape in the range 5 to 10 nm in width and about 70 nm in length, while those at the surfaces indicate a shape like a truncated cone. The long axis of each subcrystal has been confirmed to coincide with the c-axis of the hexagonal system. Their tilt angles against the revolutional axis of a peanut-like particle were found to be enlarged as they approached both ends of the particle, suggesting the formation process of the peanut-like shape. Also, most of the added sulfate ions, as a shape controller, were incorporated into the particles, as proved by EDX analysis. By comparing the present results with the internal structures of the pseudocubic and platelet-type hematite particles prepared by the gel-sol method, the individual formation mechanisms have been discussed.
TL;DR: In this paper, the deformation of radiogenic 4He induced by stepwise heating in a vacuum was carried out on various grain size fractions of specular and botryoidal hematite to place constraints on their 4He retentivities.
Abstract: Degassing of radiogenic 4He induced by stepwise heating in a vacuum was carried out on various grain size fractions of specular and botryoidal hematite in order to place constraints on their 4He retentivities. Samples from the Vosges, Odenwald, Saarbergland, and Cumbria were used. Below 900°C, specularite yields straight lines in Arrhenius plots. This is consistent with volume diffusion. The diffusion distances correspond with the macroscopic mean grain size. By contrast, botryoidal hematite yields curved lines which are straight only below 250°C. The actual diffusion distances appear to be commensurate with the microscopic size of the constituent crystals (0.1 to 10 μm). The diffusion coefficients of specular (D20) and botryoidal hematite (D20/r2) are 10−28 cm s−1 and 10−22 s−1, and the activation energies range from 110 to 130 and 130 to 210 kJ/mol, respectively. Helium 4 closure temperatures (Tc) of specularite grains with diameters of about 5 mm are above 200°C, and for all grain fractions in botryoidal hematite >10 μm the Tc is above 90°/160°C. On the basis of Tc, both hematite varieties should be useful thermochronometers. Low-temperature 4He losses of the order of a few percent during a few 100 m.y. are conceivable for botryoidal hematite.
TL;DR: In this paper, the concentrations of U and Th have been measured together with radiogenic 4He at four different locations inside two specularites, and the U-He concentrations co-varied sufficiently among the locations for an internal helium isochron of Late Jurassic age to be obtained from each specularite.
Abstract: Hematite, an ubiquitous iron oxide, is commonly found enriched with uranium and other elements supplied by the ore solution from which it had formed. The decay of the incorporated uranium to stable lead produces alpha particles which become 4He atoms via electron capture. In hematite this helium production may provide a convenient ‘clock’ which records the time elapsed since the iron oxide became closed enough to prevent quantitative helium escape (Wernicke and Lippolt, 1993a; 1993b). Recently, Lippolt et al. (1993) reported high retention qualities of specular hematite for 4He over geologic periods of time (activation energies above 116 [KJ/mol] and diffusion coefficients smaller than 10−30 [m2 s−1] at room temperature, and closure temperatures above 200°C).
Here the concentrations of U and Th have been measured together with radiogenic 4He at four different locations inside two specularites. The U-He concentrations co-varied sufficiently among the locations for an internal helium isochron of Late Jurassic age to be obtained from each specularite. The study suggests that specularite is a prolific chronometer and might allow a routine approach of the helium (isochron) method to obtain crystallization/cooling ages.
TL;DR: The crystallization kinetics of goethite were studied colorimetrically under highly alkaline conditions (pH 10.7 and 48.2 kJ/mol at pH 12.2) at temperatures from 40 ~ to 85~.
Abstract: The crystallization kinetics of goethite were studied colorimetrically under highly alkaline conditions (pH 10. l-12.2) at temperatures from 40 ~ to 85~ Color changes during crystallization from fresh precipitates, plotted on a*-b* colorimetric diagrams, were used to discriminate between pure goethite and mixtures of goethite and hematite. Only the b* value increased as goethite crystallization proceeded, and even a minor increase in the a* value revealed the existence of hematite. The rate of goethite crystallization, estimated from the b* value, could be modeled by a pseudo-first-order rate law. This rate depended both on pH and on temperature. Apparent activation energies for the reactions of 56.1 k.J/mol at pH 11.7 and 48.2 kJ/mol at pH 12.2 were estimated from Arrhenius plots.
TL;DR: In this paper, an iron-based catalyst precursors were generated using two novel technologies, the rapid thermal decomposition of precursor in solution (RTDS) and the modified reverse micelle (MRM) processes.
Abstract: Ultrafine nanometer-scale iron-based catalyst precursor powders were generated using two novel technologies, the rapid thermal decomposition of precursors in solution (RTDS) and the modified reverse micelle (MRM) processes. The powders were characterized according to their phase and crystallite size and were evaluated for activity toward C-C bond scission using the model compound, naphthylbibenzylmethane, in the presence of elemental sulfur and 9,10-dihydrophenanthrene. The catalytic activities of the powders were found to be strongly dependent on their crystallographic phase. RTDS magnetite, six-line ferrihydrite, and ferric oxyhydroxysulfate were found to have very high activity toward conversion of the model compound whereas two-line ferrihydrite and hematite were determined to be poor or mediocre catalyst precursors
Journal Article•
TL;DR: In this article, a comparison of sulfate/hematite and the sulfate-maghemite adsorbate systems was carried out by the catalysis on aldol and retro-aldol reactions, FTIR and XPS.
TL;DR: The properties of iron oxide concentrates of Oxisols and Alfisols on volcanic tuff in West Java, Indonesia, were studied by XRD,TGA, TEM and chemical analysis.
Abstract: The properties of iron oxide concentrates of Oxisols and Alfisols on volcanic tuff in West Java, Indonesia, were studied by XRD,TGA, TEM and chemical analysis.Goethite and hematite are the co-dominant iron oxides and are present in all samples. Iron content of the soils (dithionite-soluble Fe) ranges from 6.0 to 15.4%. Aluminium substitution in goethite and hematite ranges between 11 and 26 mol% and 0 and 9 mol% respectively. The Al substitution in hematite increases at about half the rate for goethite within the same samples. Crystal size of goethite and hematite was measured from line broadening of the 110 reflections and ranges between 7 and 18 nm for goethite with a mean value of 10 nm and between 7 and 28 nm for hematite with a mean value of 18 nm. The dehydroxylation temperature of goethite is in the range 290-320 °C and is linearly related to Al substitution by the equation y = 280 + 1.53x, (R(2) = 0.53). The goethite and hematite in these Indonesian soils are very similar to these minerals in tropical and Mediterranean soils from other regions.
01 Jan 1994
TL;DR: In this article, a process that produces an iron oxide residue with controlled properties, suitable for disposal or secondary use, knowledge of hematite solubility is necessary for the design of a process.
Abstract: Iron rejection as hematite from hydrometallurgical process solutions appears to be advantageous for a number of reasons. These include its environmental stability, low residue volume, and potential use in the iron making, ceramic, and cement industries. For the design of a process that produces an iron oxide residue with controlled properties, suitable for disposal or secondary use, knowledge of hematite solubility is necessary.
TL;DR: In this article, an adhesion method for determining the isoelectric points (IEP) of conductive metal surfaces was applied in studying the corrosion processes of stainless steel (SS).
Abstract: The recently developed adhesion method for determining the isoelectric points (IEP) of conductive metal surfaces was applied in studying the corrosion processes of stainless steel (SS). This technique allowed for characterization of the charge on the metal oxide surface as a function of pH. The magnitude of IEP was found to correlate with the structure of the oxide layer. The exposure of SS to air at 1,000°C reduced the original pHIEP from 4.15 to 2.8 as a result of the presence of magnetite. Treatment at lower temperatures yielded a significant portion of hematite in the oxide layer and pHIEP shifted to 3.3. The corrosion of SS in aqueous sodium chloride (NaCl) solution at room temperature, as examined by the proposed electrokinetic method, resulted in the formation of magnetite in the early stage of the process, while hematite and/or lepidocrocite was detected as a final product.
Journal Article•
TL;DR: In this paper, the authors studied the kinetics of magnetite magnetite oxidation in a laboratory scale reactor at constant temperatures in the range 400-850°C and found that the rate of oxidation followed the parabolic rate law, except during the initial period of 10 s or so.
Abstract: Kinetics of magnetite oxidation was studied in a laboratory scale reactor at constant temperatures in the range 400-850°C. Oxidation experiments were carried out in air for periods up to 60 s. Grain sizes in the range 74-100 μm were used. The maximum conversion of Sydvaranger magnetite concentrate was 42% after 60 s at 850 °C. Lower conversion was obtained using Kiruna and Minnesota magnetites. Topochemical growth of hematite was observed, but hematite needles were also growing ahead of the reaction front. The rate of oxidation was found to follow the parabolic rate law, except during the initial period of 10 s or so. The chemical reaction at the surface of each grain is probably rate controlling during the initial period. Otherwise, diffusion through a growing hematite layer is assumed to be the slowest step
01 Apr 1994-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this article, crystalline hematite (α-Fe2O3) particles were prepared by hydrolysis of organic iron carboxylate solutions using water at 175 °C for 30 minutes.
Abstract: Submicrometer, crystalline hematite (α-Fe2O3) particles were prepared by hydrolysis of organic iron carboxylate solutions using water at 175 °C for 30 minutes. The particle size of hematite was significantly dependent on the liquid-phase stirring speed and the organic compositions. The precipitation rate of hematite from the organic solution followed first-order kinetics. The precipitation rate increased markedly with increasing temperature, and the activation energy for the process was 94.6 kJ mol−1. At 220 °C, the hydrolysis of iron carboxylate solution led to a mixture of hematite and magnetite (Fe3O4). The iron oxides prepared at 175 °C to 220 °C were found to be free from organic contamination by the starting material.
TL;DR: In this paper, small angle X-ray scattering (SAXS) spectra obtained provided insight into structure of the hematite aggregates and the size of the primary particles.
Abstract: Small angle X-ray scattering has been utilised to study the structure of hematite aggregates. The small angle X-ray scattering (SAXS) spectra obtained provided insight into structure of the hematite aggregates and the size of the primary particles. The structural analysis results obtained by SAXS are consistent with previous results obtained from static light scattering studies. Both techniques indicate that the mass fractal dimensions of hematite aggregates are markedly higher than those obtained for other particle systems.
TL;DR: In this article, a quantitative thermogravimetric method has been developed for the routine analysis of goethite and kaolinite in a Western Australian iron ore sample, which was applied to the characterisation of an iron ore tailing and used to assess the performance of a selective flocculation process for upgrading the tailing.