Hematite plates

Under hydrothermal conditions (150-180 °C) maghemite transforms to hematite via solution probably by a dissolution/reprecipitation mechanism (Swaddle Olt-mann, 1980 Blesa Matijevic, 1989). In water, the small, cubic crystals of maghemite were replaced by much larger hematite rhombohedra (up to 0.3 Lim across). Large hematite plates up to 5 Lim across were produced in KOH. The reaction conditions influenced both the extent of nucleation and crystal morphology. The transformation curve was sigmoidal and the kinetic data in water and in KOH fitted a first order, random nucleation model (Avrami-Erofejev), i.e. 

At the completion of the reaction, the aniline is separated from the iron oxides by steam distillation and the umeacted iron removed. The pigment is washed, filtered and dried, or calcined in rotary kilns to hematite (Plate 20.1, see p. XXXIX). Considerable control over pigment properties can be achieved in this process by varying the nature and concentration of the additives and the reaction rate the latter depends on pH, the rate of addition of iron and nitrobenzene and the type and particle size of the iron particles. Two advantages of this method are that a saleable byproduct, aniline, is produced and that there are no environmentally, harmful waste products. 

The most familiar of the C-frame, matrix-type industrial magnetic separators are the Carpco, Eriez, Readings, and Jones devices. The Carpco separator employs steel balls as a matrix, Eriez uses a combination of expanded met matrices, and the Readings and Jones separators have grooved-plate matrices. Capacities for this type of unit are reported to up to 180 t/h (in the case of Brazdian-hematite processing). 

Solenoid magnetic separators are designed for batch-type, cyclic, and continuous operation. Devices which can use matrices of expanded metal, grooved plates, steel balls, or filamentaiy metals have been designed. Continuous separators with capacities to 600 t/h for iron ores (similar to the Brazilian hematite) are commercially available (Sala International Inc.). Selection of the method of operation is apphcation-dependent, being based on variables such as temperature, pressure, volume of magnetics in the feed, etc. 

Hematite Hexagonal plates Spindles, rods, ellipsoids, cubes. 

The commonest habits for hematite crystals are rhombohedral, platy and rounded (Fig. 4.19). The plates vary in thickness and can be round, hexagonal or of irregular shape. Under hydrothermal conditions, these three morphologies predominate successively as the temperature decreases (Rosier, 1983). The principal forms are given in Table 4.1. Hematite twins on the 001 and the 102 planes. The crystal structure of hematite has a less directional effect on crystal habit than does that of goethite and for this reason, the habit of hematite is readily modified. A variety of morphologies has been synthesized, but in most cases, the crystal faces that enclose the crystals have not been identified. 

Morphologies of synthetic hematite include plates and discs, rods, spindles, spheres, ellipsoids, double ellipsoids, rhombohedra, stars, cubes and peanuts. In the absence of additives, hexagonal plates, which are often rounded, and rhombohedra predominate. Each morphology can be obtained by more than one synthesis route. Two common ways of producing idiomorphic hematite crystals in aqueous systems... 

Aluminium in the ferrihydrite system not only suppresses goethite in favour of hematite (see chap. 14) but also affects the morphology of hematite, probably by entering the structure. At temperatures of between 70 and 150 °C, a shift was noticed from rhombohedra to plates whose diameter and thickness were at a maximum at an Al/(Fe-i-Al) ratio of 0.05 (Schwertmann et al., 1979 Barron et al., 1984 Barron Torrent, 1984 Wolska Szajda, 1987). At higher levels of substitution, the plates became extremely thin and structural strain increased (Stanjek Schwertmann, 1992)... 

Fig. 4.20 Synthetic hematites grown from ferrihydrite at temperatures <100°C (Schwertmann, unpubl.) a) Hexagonal plates grown at pH 7 and RT acicular crystals are goethite b) Laths grown at pH 11 and 80 °C in the presence of 2.5 10 M citrate (see Schwertmann et al., 1968). The fine granular material is unreacted ferrihydrite c) Framboids grown at pH 6 and 70°C in the presence of 2 10 M oxalate (see Fischer, ...

Fe(OH)2 exists as hexagonal plates as do the green rusts (Feitknecht Keller, 1950 Bernal et ak, 1959). The basic morphology of wiistite is cubic, but this compound is frequently obtained as very irregular particles. It is formed as irregular rounded crystals 20-100 (xm across by reduction of hematite with H2/H2O at 800 °C (Moukassi et al., 1984). 

Synthetic 5-FeOOH has a surface area which ranges from 20-300 m g depending on the thickness of the crystals. In a series of seven synthetic feroxyhytes the surface area increased from 140 to 240 m g (EGME method) as the crystallinity decreased (Garlson and Schwertmann, 1980). 5-EeOOH displays interpartide porosity, i.e. slitshaped micro- or mesopores between the plate like crystals (Jimenez-Mateos et al., 1988 Ishikawa et al., 1992). Both TEM observations and t-plot analysis showed that 0.8 nm micropores formed upon dehydroxylation at 150 °G in vacuo. The surface area rose steeply as the temperature exceeded 100 °G and reached a value close to 150 m g at 200 °C at which temperature, the sample was completely converted to hematite. 

To the naked eye, goethite and akaganeite appear yellow-brown, lepidocrocite orange and hematite usually red (Plate 6.1). Feroxyhyte and ferrihydrite are dark reddish brown, maghemite brown to brownish red and magnetite and wiistite are black. 

Schwertmann Pfab, 1994) (see Plate 6.II). Mn substituted hematites are blackish. In case of A1 substitution, the observed shift towards redder hues is due mainly to the associated decrease in particle size (Scheinost et al. 1999). Structural A1 does not significantly influence the hue and chroma of synthetic Al-hematite, although the crystals become lighter (Munsell value increases) (Barron Torrent, 1984 Kosmas et al, 1986). 

Micaceous iron oxides are produced in a process which involves heating FeCl3 and iron at 500-1000 °C to form molten Fe complexes which are then oxidized to micaceous hematite the diameter of the plates can be varied from 5 to 75 pm depending on whether the oxide is intended for use in a primer paint or a topcoat (Carter, 1988). 

Hydrothermal processes, i. e. the heating of suspensions of ferrihydrite in alkaline media under pressure, have been used to produce large platy crystals of hematite. This process gives vell formed crystals, but is expensive. The crystals can be reduced to produce isomorphous magnetite plates. Flame hydrolysis involves burning Fe " chloride at 400-800 °C to iron oxide. Owing to the many technical difficulties associated with this process, it is not commercially important. 

Hydrothermal conversion of either ferrihydrite or goethite at 250-300 °C in alkaline media for some hours, followed by a further stage of growth at higher pH (Os-tertag, 1994). Micaceous plates of hematite result. 

Plate 16.1 a) Soil profile coloured by goethite (Ochrept, France), b) Soil profile coloured by hematite (Ultisol, Brazil), c) Soil profile coloured by lepidocrocite (Aquept, South Africa), d) Ferrihydrite formation by oxidation of Fe " in water seeping out of a Cley. 

Plate 16.1 e) Root channel in a gley soil stained by Fe oxide, f) Bleaching of the surface layer of a red soil aggregate by microbial reduction of the hematite. See root mat at the aggregate s surface supplying the biomass. 

Plate 16.111 Highway on a hematite-coloured soil betwen Concepcion and San Ignacio de Velasco, Bolivia (Courtesy Dr. P. Schad). 

Plate 19.11 Use of iron oxides as paints a) Summer house of C. F. Lnne near Upsala, Sweden, painted with hematite produced by calcination of pyrite (Courtesy Mrs. D. Schwertmann). b) House of the Lenbach Art Gallery, Munich, painted with goethite (Courtesy Dr. J. Friedel). 

Plate 19.111 Outerop of natural Fe oxide (goethite, hematite) pigments at Luberon, France (Courtesy H. Stanjek)... 

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