Showing papers on "Hematite published in 1982"

The 5-M-NaOH concentration treatment for iron oxides in soils

Abstract: The boiling 5-M-NaOH treatment was found to aid in the identification and characterization of goethite and hematite by effectively concentrating the two Fe oxides in kaolinitic-gibbsitic soil clays. No transformations of goethite to hematite or hematite to goethite were detected, but poorly crystalline, highly Al-substituted goethite was found to dissolve and recrystallize into a more well-crystalline, less Al-substituted goethite in samples low in Si. The Si released from kaolinite was sufficient to block goethite dissolution and recrystallization in kaolinitic samples, but noncrystalline silica had to be added to samples rich in gibbsite to minimize this effect.

Aluminous hematite: A mössbauer study

Abstract: Aluminous hematites prepared in three different ways have been examined at 300K and 4.2K using the Mossbauer technique. The results indicate significant differences between the behaviour of aluminous hematites that have been subjected to high temperatures (>600° C) and those which have not. The magnitude of the room temperature quadrupole splitting of the former increases with aluminium content, approaching at ∼16 mole percent substitution the value (−0.22 mm/s) exhibited by all of the low temperature specimens. This variation may be explained qualitatively in terms of a preferential c-axis contraction of the lattice upon incorporation of aluminium, which does not occur unless a c-axis defect structure is removed by subjection of the hematite to high temperatures. The solid solubility limits of high and low temperature hematites (∼15 mol % and ≥19 mol % respectively) also differ, as do the room temperature decreases in hyperfine splitting (−0.82 kOe/mol % Al and −0.86 kOe/mol % Al). At 4.2 K only low temperature hematite exhibits a decrease in hyperfine splitting with increasing Al content (−0.40 kOe/mol % Al). The absolute values of the recoil free fractions of hydrothermally prepared aluminous hematites have been determined at 4.2 K (0.70±0.02 — pure hematite, 0.82±0.02, 14±2 mol % Al substitution) and exhibit a similar increase with Al content to that previously observed for aluminous goethites. The room temperature recoil free fraction of pure hematite has been measured to be 0.64±0.02. The effects of particle size on both hyperfine splitting and recoil free fraction have been investigated.

Influence of relative humidity on the crystallization of Fe(III) oxides from ferrihydrite

Abstract: Ferrihydrite prepared in different manners was kept under relative humidities ranging from 75 to 100% and at temperatures of 45° and 55°C for 180 days. Ferrihydrite transformed to hematite and goethite at relative humidities close to 100%, but at lower relative humidities the transformation was less pronounced and hematite was highly favored over goethite. Increasing temperature also favored hematite over goethite, and Al substitution completely prevented goethite formation. These results suggest that hematite can form in relatively dry, warm soils or sediments, although more slowly than in moister environments.

The observation of stress effects during the high temperature oxidation of iron

Abstract: Electron microscopy has been used to characterize the stress effects which occur during the oxidation of iron in the temperature range 400–700° C. Spalling and de-cohesion of the outer hematite (α-Fe2O3) layer is often observed, and analysis of the resulting scrolled oxide indicates a strong compressive stress gradient. In contrast, tensile cracks are frequently seen in the magnetite (Fe3O4) layer, while the underlying wustite (Fe1−xO) and the iron substrate are apparently able to accommodate the stresses to some extent by plastic deformation. The Pilling-Bedworth model can adequately be applied at the α-Fe2O3-Fe3O4 interface since anion diffusion occurs in the hematite. However, since cation diffusion is dominant in the other oxides, it is suggested that the anion volume ratio can be applied to the Fe3O4-Fe1−xO interface where the anion sublattice remains unchanged, in order to predict the stress state.

Quantitative determination of goethite and hematite in kaolinitic soils by X-ray diffraction

Abstract: In the past, two factors have impeded the quantitative estimate of Fe-oxides in soils by X-ray diffraction. First, Fe-oxides are still quite often considered X-ray amorphous, although numerous results, e.g. a low ratio of oxalate- to dithionite-soluble Fe, have indicated the opposite. Second, even if crystalline, the cocentration of Fe-oxides in many soils is low, thereby complicating their identification by XRD. Recently, however, more sensitive methods such as Mossbauer spectroscopy and Differential-XRD (Schulze, 1981) have been introduced, which substantially reduce the lower limit of detection. Because these two methods are not generally available and, especially in the case of Mossbauer spectroscopy, are rather time consuming, ordinary XRD should be adapted for quantitative estimation of Fe-oxides. Determination can be facilitated by using samples in which the Fe-oxides are concentrated by particle-size separation and a 5 M NaOH boiling treatment (Norrish & Taylor, 1961). The latter treatment is particularly suitable for kaolinitic soils as the Fe-oxides are unaffected―provided certain precautions are taken (Kaimpf & Schwertmann, 1982a). This paper gives details of a procedure for the quantitative estimation of goethite (Gt) and hematite (Hm) by XRD.

Gel-glass transformation in the SiO2Fe2O3 system

Abstract: Gel-glass transformation has been studied by Mossbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO 2 Fe 2 O 3 system (A: 5 wt% Fe 2 O 3 , B: 10 wt% Fe 2 O 3 , C: 20 wt% Fe 2 O 3 , D: 40 wt% Fe 2 O 3 ). The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C. Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect. In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C. The presence of hematite was confirmed by the obtained Mossbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe 3+ and Fe 2+ oxidation states was also determined, together with the attribution of the probable coordination states for Fe 3+ ions. Complex magnetic structure appeared in samples treated above 800°C.

Selective extraction of amorphous iron oxides by EDTA from selected silicates and mixtures of amorphous and crystallline iron oxides

Abstract: Amorphous iron oxides in soil are often extracted by an ammonium oxalate solution (Schwertmann, 1964). This treatment may, however, also dissolve crystalline iron oxides and iron silicates (McKeague & Day, 1966; Baril & Bitton, 1969; McKeague et al., 1971; Arshad et al., 1972; Pawluk, 1972; Schwertmann, 1973; Taylor & Schwertmann, 1974; Borggaard, 1976). It has been shown that EDTA can selectively extract amorphous iron oxides from soils (Borggaard, 1979, 1981) and a synthetic mixture of amorphous iron oxide, goethite, and hematite (Borggaard, 1976). As pointed out previously (Borggaard, 1979), the EDTA method should also be tested on selected minerals to decide if it can serve as a reference method against which other less time-consuming methods may be tested.

Fundamental Study of Size Degradation Mechanism of Agglomerates during Reduction

Abstract: The mechanism of degradation of agglomerates during reduction is studied with pellets made from powdery ingredients of chemical reagent grade, and the changes in mineral phases due to certain oxides leading to degradation, are examined quantitatively.The results obtained are summarized as follows:(1) Microscopic observations suggest that cracking occurs in the skeletal hematite and the calcium ferrite phases. Analysis by line-broadening measurements of X-ray diffraction reveals no strain both in the bulky and in the skeletal hematites regardless of the amount of alumina held in solid solution. Micropores are generated in magnetite as it is reduced from hematite to give rise to a stress when reduced at a low temperature, and stress concentration occurs around inclusions of small radius of curvature to cause cracking of skeletal hematite grains.(2) The quaternary calcium ferrite is expected to be reduced to metallic iron in the blast furnace through dicalcium ferrite or wustite. Nonferrous oxides form gehlenite and β-dicalcium silicate. Calcium ferrite is not only fragile but its amount to increase with the basicity under existence of Al2O3.(3) The amount of skeletal hematite depends mainly on the degree of supersaturation of iron ions in slag and on the dissociation temperature of hematite. The addition of TiO2 in sinters enhanced greatly the crystal growth of skeletal hematite in the case of induration at above THM, whereas bulky hematite at below THM.

Catalytic Action of Copper on the Oxidation of Structural Iron in Vermiculitized Biotite

Abstract: The ferrous iron content of two vermiculitized biotites decreased by treatment with 0.1 N salts of copper at 70°C from 9.1–14% to 1.8–2.6%. Presumably, interlayer copper ions acted as a catalyst (here, an electron carrier) for the oxidation of iron by dissolved oxygen. The oxidized iron was ejected from the structure and formed crystalline iron minerals, such as hematite and goethite. Weight loss determinations, chemical, and X-ray powder diffraction data suggest that Cu(II) ions were polymerized to hydroxy-hydrous compounds in the interlayer space. Poor exchangeability of the resultant complex is attributed to the formation of strong electrostatic attractions between OH groups of the interlayer complexes and silicate oxygens.

Microstructural investigation of the oxidation of an Fe-3 pct Cr alloy

Abstract: The microstructure and microchemistry of the oxide scale on an Fe-3 wt pct Cr alloy have been investigated after oxidation in the temperature range 700 to 800 °C. Transmission and scanning electron microscopy along with energy dispersive X-ray analysis and Auger electron spectroscopy techniques were used for the investigation. Multilayered scales are observed which vary in composition and structure; the innermost oxide is an Fe-Cr spinel of the type Fe(Fe2•xCrx)O4. The intermediate layer and the outer oxide layer are both α-Fe2O3 hematite. The outer hematite layer is nonadherent and wrinkling is observed. Spallation occurs readily at the inner hematiteJspinel interface and at the spinel oxideJalloy interface. The poor oxidation resistance of the alloy is discussed in terms of these observations.

On the Rate of the Reduction of Wustite, Magnetite and Hematite Containing Al2O3, CaO and MgO

Abstract: The effects of CaO, Al2O3 and MgO, all as the foreign oxide, on the rate limiting step of the reduction of pellets from wustite to metallic iron and on the values of rate parameters are studied.It is confirmed that the reduction behavior of those pellets may be represented either by the unreacted core model or by the uniform reaction model, depending on which kinds of the foreign oxide and the iron oxide are present at the start. The mixed rate control equation of mass transfer in the gas phase, intraparticle gas diffusion, and chemical reaction may be applied to any iron oxides that contain CaO and to hematite and magnetite both containing Al2O3. On the other hand, the reduction rates of the pure iron oxides, wustite containing Al2O3 and any iron oxides containing MgO are controlled by the oxygen diffusion in the metallic iron shell.These results agree with the conclusion of the present authors' previous studies, namely, when a complex oxide that consists of an iron oxide and any one of those foreign oxides precipitates in wustite during reduction, wustite grains are reduced nontopochemically while pellets are reduced topochemically.

Effect of heat treatment on magnetic properties and morphology of iron particles

Abstract: Acicular iron particles with high coercivity and high saturation magnetization were prepared by reduction of the hematite particles which were obtained by dehydration of the silica‐coated goethite particles in the process of heat treatment in air. Variations in specific surface area, micropore distribution, crystallite size, and magnetic properties with the dehydration temperature were measured. For the iron particles prepared by reduction of the silica‐coated hematite particles obtained by dehydration at 800 °C, a maximum coercivity of 1250 Oe was obtained, and the crystallite size reached the minimum value of 220 A which was close to the critical value of single domain particle. The highly acicular iron particles consisting of the closely packed small crystallites were obtained.

Transformation of Iron-Bearing Kaolinite to Iron-Free Kaolinite, Goethite, and Hematite

Abstract: Ferruginous clay partings in limestones of the marine, largely evaporitic, Upper Triassic Mohila Formation (Makhtesh Ramon, Israel) contain hexagonal plates and cube-like bodies up to one millimeter across. Analyses by electron microscopy, infrared and Mossbauer spectroscopy, and X-ray powder diffraction indicate that the matrix contains Fe-rich anhedral kaolinite, up to 100 µm in size; the hexagonal plates are composed of euhedral, Fe-free kaolinite covered with well-developed acicular goethite and platy hematite (0.5 to 2 µm in size), and the cubes consist of fine-grained goethite with minor amounts of kaolinite. The anhedral kaolinite appears to be detrital, the hexagonal plates to be authigenic, and the cubes to be pseudomorphs after pyrite. Oxidation appears to have altered Fe-rich kaolinite and pyrite to Fe-free kaolinite, goethite, and hematite and was accompanied by recrystallization and pseudomorphic replacement. The alteration process was slow and was probably induced by a small increase in pH and in the Al/Fe ratio, resulting from oxidation of reduced components (pyrite, ferrous carbonate, organic matter) in a semiclosed, sediment-mud system. Overlying kaolinitic flint clay deposits may be the final product of a similar process.

The effect of weight material type on rate of penetration using dispersed and non-dispersed water-base muds

Abstract: The penetration rates of dispersed and nondispersed water-base muds were studied using Mancos Shale. The full-scale wellbore simulator at Drilling Research Laboratory was utilized. Previous studies have indicated that in invert oil muds higher penetration rates can be achieved when the weighting agent is ilmenite rather than barite. This study compares barite and ilmenite in water-base systems. Two grind sizes of ilmenite (fine and standard) were used in order to determine the effects of particle size on penetration rate. In addition to these tests, hematite, fine grind ilmenite and barite were tested in a low colloid invert oil mud system as a supplement to the original study. An additional test was conducted with a conventional diamond bit and the low colloid oil mud for comparison. Highest penetration rates were observed with the standard particle size distribution ilmenite for the water-base muds. For the invert oil mud, similar penetration rates were observed for the fine-grind ilmenite and the hematite; the barite weighted mud resulted in relatively lower rates of penetration. The fine grind ilmenite/low colloid oil mud/diamond bit combination provided penetration rates equal to or greater than the barite/low colloid oil mud/ roller cone bit combination.

Iron Formations of Precambrian Age: Hematite-Magnetite Relationships in Some Proterozoic Iron Deposits — A Microscopic Observation

Abstract: Numerous specimens of metamorphosed Proterozoic iron formations in North America were studied microscopically. Textural relations exhibited by magnetite and hematite are described and interpreted.

Behaviour of α-Fe2O3 (hematite), prepared by oxidation precipitation of ferrous sulphate, on heating

Abstract: The effects of heatingα-Fe2O3 (hematite) prepared by oxidation precipitation of ferrous sulphate, at temperatures up to 700° C were studied. It was found that, in the course of heating, losses of structurally-bound water occured, accompanied by the formation and removal of pores, the lattice constants changed and the optical properties were modified, an effect which is important from the standpoint of the use of hematite as ferric pigment. With increasing annealing temperature, the complementary wavelength was shifted to higher values and the spectral purity of the pigment colour was decreased.

Deposition Rate of Suspended Hematite in a Boiling Water System under BWR Conditions

Abstract: The state and rate of deposition of suspended hematite were studied on a boiling heat transfer surface under the heat flux, temperature, and pressure similar to those in BWR power plants. ...

Iron oxide mineralogy of a hydrothermal assemblage on Santorini Island, Aegean Sea

Abstract: Basement rocks of Santorini have been locally impregnated by iron oxides and other minerals. The hydrothermal fluids which caused the mineralization have extensively altered the impregnated rocks, but left granditic garnets in these unaffected. M6ssbauer spectroscopy and differential X-ray diffrac- tion show that the iron oxides present are goethite and hematite in a ratio of about 2: 1. The presence of hematite indicates that these minerals formed, directly from a hydrothermal phase, at higher temperatures than the recent submarine iron oxide deposits of Santorini, which contain no hematite. SANTORINI is the only active volcano in the eastern Mediterranean. The volcanic activity mani- fests itself in sporadic lava eruptions and less spectacular but continuous fumarolic exhalations. The latter have locally altered the originally erupted calc-alkaline volcanics (Schorin, 1980), and are responsibe for the current production of sub- marine iron oxide deposits (Behrend, 1936; Puchelt

Fundamental Study of Size Degradation Mechanism of Agglomerates during Reduction

Abstract: The mechanism of degradation of agglomerates during reduction is studied with pellets made from powdery ingredients of chemical reagent grade, and the changes in mineral phases due to certain oxides leading to degradation, are examined quantitatively.The results obtained are summarized as follows:(1) Microscopic observations suggest that cracking occurs in the skeletal hematite and the calcium ferrite phases. Analysis by line-broadening measurements of X-ray diffraction reveals no strain both in the bulky and in the skeletal hematites regardless of the amount of alumina held in solid solution. Micropores are generated in magnetite as it is reduced from hematite to give rise to a stress when reduced at a low temperature, and stress concentration occurs around inclusions of small radius of curvature to cause cracking of skeletal hematite grains.(2) The quaternary calcium ferrite is expected to be reduced to metallic iron in the blast furnace through dicalcium ferrite or wustite. Nonferrous oxides form gehlenite and β-dicalcium silicate. Calcium ferrite is not only fragile but its amount to increase with the basicity under existence of Al2O3.(3) The amount of skeletal hematite depends mainly on the degree of supersaturation of iron ions in slag and on the dissociation temperature of hematite. The addition of TiO2 in sinters enhanced greatly the crystal growth of skeletal hematite in the case of induration at above THM, whereas bulky hematite at below THM.

The influence of surface forces of magnetic separation

Abstract: Results of the experimental investigation of the influence of surface forces on high gradient magnetic separation of iron ores will be presented. Surface characteristics of the slurries of pure hematite and hematite ores ground below 100 μm and 40 μm were varied from pH = 1.5 to 11 and these test samples were treated in high gradient magnetic separator at magnetic fields from 0.1 T to 0.7 T. The experimental data indicate that the maximum of grade and recovery of magnetic fraction of the ore can be obtained at pH corresponding to the point of zero charge, while at other pH the grade and recovery decrease. Since pH of point of zero charge for iron oxide is different from pH corresponding to water used in iron dressing plant, adjustment of pH of the slurry could lead to a substantial increase in grade and recovery. These considerable variations of grade and recovery can be attributed to a higher probability of retention of the particles on the matrix, higher resistance of the deposited particles against the shear stress and to selective magnetic and ionic flocculation of magnetic or gangue particlec.

The Influence of Preparation and Doping On the Reducibility of Hematite By Hydrogen

Abstract: Samples of hematite and of hematite doped with 1 at% lithium and 1 at% vanadium are prepared by the hot kerosene drying technique. The doped samples are also prepared by an impregnation method. The various samples are used to study the reduction with hydrogen to form iron. The results indicate that the rates of reduction are strongly influenced by the particle size of the initial samples and by the repartition of the additive in the bulk and surface layer of the hematite particles. It has been shown that both additives can retard the rate of reduction. The lithium exerts its maximum influence when present in solid solution and vanadium when concentrated in the surface layer. The importance of a careful preparation of samples to be used for reduction studies, is stressed.