Showing papers on "Hematite published in 1974"

Maghemite in soils and its origin i. properties and observations on soil maghemites

Abstract: A magnetic iron oxide is a common constituent of ferruginous concretions in highly weathered Australian soils and is genelally associated with varying proportions of hematite and occasionally with goethite. The X-ray diffraction pattern and the low Fe (11) contents (4-15~o of total Fe) of this magnetic phase identify it as maghemite which was further confirmed by IR and DTA. This maghemite is only slightly soluble in dithionite but readily in 1:1 HC1 or oxalic acid. From the Fe (11) contents and mineralogical associations in soil concretions, conditions for maghemite formation under near pedogenic conditions are suggested.

Maghemite in soils and its origin ii. maghemite syntheses at ambient

Abstract: AB S T R ACT: The products of air oxidation of mixed Fe(II)-Fe(III) chloride solutions at pH 6 and 7, at 20 and 60~ and at normal pressure contain green rust, maghemite, lepidocrocite, goethite and a paracrystalline ferric hydroxide (ferrihydrite). Among these maghemite, a cubic ferromagnetic iron oxide (Fe203) found in many soils, is favoured by slow oxidation rate, high total Fe concentration, the presence of small amounts of Fe(IlI) in the original predominantly Fe(lI) solution, higher temperature and at pH 7 rather than pH 6. The green rust is believed to be an essential precursor of maghemite. On slow oxidation it will form maghemite probably via magnetite. Fast oxidation prevents the cubic phase from being formed and lepidocrocite is the end product. At higher Fe(III) proportions ferrihydrite can be formed which under certain influences converts to goethite and/or hematite. The common iron oxides are seen to form from the same system from small variations in environment which is to be expected from their common associations in soils.

Maghemite in soils and its origin. II. Maghemite syntheses at ambient temperature and pH 7

Abstract: The products of air oxidation of mixed Fe(II)-Fe(III) chloride solutions at pH 6 and 7, at 20 and 60°C and at normal pressure contain green rust, maghemite, lepidocrocite, goethite and a paracrystalline ferric hydroxide (ferrihydrite). Among these maghemite, a cubic ferromagnetic iron oxide (Fe2O3) found in many soils, is favoured by slow oxidation rate, high total Fe concentration, the presence of small amounts of Fe(III) in the original predominantly Fe(II) solution, higher temperature and at pH 7 rather than pH 6. The green rust is believed to be an essential precursor of maghemite. On slow oxidation it will form maghemite probably via magnetite. Fast oxidation prevents the cubic phase from being formed and lepidocrocite is the end product. At higher Fe(III) proportions ferrihydrite can be formed which under certain influences converts to goethite and/or hematite. The common iron oxides are seen to form from the same system from small variations in environment which is to be expected from their common associations in soils

Significance of Color in Red, Green, Purple, Olive, Brown, and Gray Beds of Difunta Group, Northeastern Mexico

Abstract: Varicolored rocks of the Difunta Group (Upper Cretaceous-Paleocene) are composed of detritus derived from a relatively uniform terrane of volcanic rocks and deposited in fluvial, deltaic, and shelf environments. Red, green and purple rocks are restricted to delta-plain facies, whereas the dark colors are present in all facies. The color of claystone is a function of color mixing of red hematite, green illite and chlorite, and black organic matter; and possibly of grain size of hematite (purple color). Red and purple rocks owe their color to pervasive hematite grain coatings and crystals intergrown with clay; brown rocks owe their color to faint or localized iron-oxide grain coatings; and gray rocks to organic matter and authigenic iron sulfide. Green rocks owe their color to chlorite and illite and to the absence of hematite, organic matter and sulfides. Olive and yellow claystone colors are imparted by color mixing of green clay and black organic matter. Field relations and petrographic studies indicate that red and purple colors originated through post-depositional reddening of sediment, in part in soil zones on the delta plain, in a sub-humid to semi-arid climate that had seasonal wet and dry periods. Reddening occurred both by aging of hydrous ferric oxides plus staining of grains by hematite pigment formed by oxidation of detrital iron oxide and mafic grains. Some brown siltstone beds were pigmented in a manner similar to red beds, but other siltstone beds developed brown color upon weathering. Green beds formed by bleaching of red (or proto-red) beds by interstratal percolation of reducing water derived largely from fluvial channels overlying the green beds. Olive and gray claystone are present predominantly in marine facies that contain abundant organic matter and in some delta-plain facies where destruction of organic matter was incomplete. Total Fe content of claystone samples is essentially the same regardless of color, except that gray claystone has significantly less total Fe than other colors; 67% of the samples have total Fe between 3 and 4%. Iron reduced in red beds was not removed in solution but resides in chlorite in green strata, and some iron reduced in gray beds resides in sulfides.

Non-Topochemical reduction of iron oxides

Abstract: Non-topochemical behavior was studied during reduction of porous spheres of hematite by stages through the intermediate oxides and also continuously to iron by CO/CO2 mixtures at temperatures of 600 to 900°C (873 to 1173 K). The behavior became more nearly topochemical as temperature increased. Shrinking occurred during the reduction of hematite to magnetite and of magnetite to wustite, whereas swelling was observed during the reduction of wiistite to iron. Shrinking was greater, and swelling less, at higher temperatures. The total surface area of the solid decreased with increasing extent of reduction during each of the three stages. A non-topochemical model was developed which satisfies, better than previously proposed models, the reduction data for the single reactions and the three reactions occurring simultaneously. The model provides for variation in particle size and local changes in porosity and effective diffusivity. An empirical “sintering exponent” was introduced to describe changes in reacting surface area.

The association of phosphorus with iron in ferruginous soil concretions

Abstract: Ferruginous concretions from various Australian and German soils and samples of bog iron ore from various European localities were examined by X-ray diffraction and electron microprobe techniques. The association between phosphorus and iron in these samples was looked at from the point of view of differences in iron mineralogy and genesis. When surface area is considered it appears that hematite, goethite and possibly maghemite show phosphorus adsorption levels in the same order of magnitude as those obtained on synthetic preparations of these minerals. In most cases a high phosphorus content is associated with a high iron, but the converse is not true, so that although the iron minerals are definite sinks for the phosphorus in the environment a linear relation cannot be expected due to the variation in time and spatial distribution of these two elements. In the younger European soils the phosphorus/iron ratios are generally higher (average 0.015), reaching values as high as 0.05 in some bog iron ores formed in gleys, whereas in the older Australian soils the average ratio is 0.002.

Selective coagulation in quartz-hematite and quartz-rutile suspensions

Abstract: In agreement with calculations based on the DLVO theory of colloid stability, it was found possible to separate two different colloidal species by chemical control of the surface potentials. The materials used in the experiments were: (i) quartz and synthetic rutile; (ii) quartz and hematite.

Method for producing coarse powder, hardened iron oxide material from finely divided raw material substantially consisting of hematite and/or magnetite

Abstract: A method for agglomerating finely divided iron oxide material, substantially consisting of hematite and/or magnetite, preferably enriched iron ore concentrates, and hardening the agglomerates thus formed The fine material is agglomerated by rolling, micropelletizing or other granulating methods to a particle size distribution convenient fluidizing purposes, whereafter the material is transferred to a fluidized bed furnace in which the bed is heated by introducing gaseous or liquid fuel and gas containing free oxygen or by hot gases The major portion of the material is then removed from the bed in an agglomerated and hardened condition

Hydrothermal process for preparation of alpha iron oxide crystals

Abstract: Elongated, polycrystalline particles of alpha iron oxide, characterized by an open, porous structure and consisting of hematite microcrystallites containing from 2 to 4 percent of sulfate are prepared by the reaction of lithium hydroxide and ferric sulfate in weakly acidic to neutral aqueous solution under at least autogenous pressures at temperatures in the range of 150*-350*C. The particles are readily converted to gamma iron oxide of good magnetic properties and also have utility as pigments and catalysts.

Effect of low concentrations of silica, alumina, lime, and lithia on the magnetic roasting of hematite

Abstract: The conversion of hematite to magnetite was investigated in a mixture of H2, H2O, and N2. In the temperature range of 500° to 700°C, the magnetic roast reaction gives a sigmoidal kinetic curve with a finite induction time. The induction time decreases with rise in temperature and increases in the presence of alkali and alkaline earth oxides. The magnetic roast reaction was also studied in the presence of low concentrations of silica (quartz structure) and alumina (5.0 and 4.3 wt pct, respectively, which correspond to 8.4 × 10-4 g atom of Si or Al per gram mixture). The addition of SiO2 to hematite decreases the induction time. At temperatures below 550°C, alumina increases the induction time; at higher temperatures it has about the same effect as silica. For pellets containing SiO2, a maximum in the relative decrease in induction time, A, was observed at 578°C; for pellets containing A12O3, there was a steady increase in A with increasing temperature. Because the α β quartz transition occurs at 575°C, the enhanced surface activity at 578°C in the presence of quartz is attributed to the Hedvall effect of solid-state chemistry. The induction period of the magnetic roast reaction was exceptionally prolonged in the presence of lithia. Mixing of hematite with silica, alumina, and lithia (8.4 × 10-4 g atom, respectively, per gram mixture) was found to eliminate the beneficial effect of quartz by inhibiting its α β transformation.

The on-stream analysis of hematite ore fractions using radioisotopes†

Abstract: Hematite ore fractions in the form of circulating slurries of various solids content were analyzed for iron and silicon, their principal metallic constituents, to determine the relationships and factors significant to such analyses. Radioisotopes were used to excite the characteristic radiations which were measured nondispersively. Counts were also taken for the exciting radiations. The measured intensities for Fe K alpha and Si K alpha were corrected by backscatter and absorption fuctors derived by the comparison of counts from the slurries to those from water. Regression lines were used to convent all counts to a 40% solids basis. The iron content of the ore fractions was determined with a standard error of less than 2%. The determination of silicon was not satisfactory at low concentrations because all components of the slurries absorb Si K alpha strongly. (auth)