Unit hematite

Minerals. Iron-bearing minerals are numerous and are present in most soils and rocks. However only a few minerals are important sources of iron and thus called ores. Table 2 shows the principle iron-bearing minerals. Hematite is the most plentiful iron mineral mined, followed by magnetite, goethite, siderite, ilmenite, and pyrite. Siderite is unimportant in the United States, but is an important source of iron in Europe. Tlmenite is normally mined for titania with iron as a by-product. Pyrite is roasted to recover sulfur in the form of sulfur dioxide, leaving iron oxide as a by-product. 

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). 

The Unit of Structure.—A spectral photograph of the K-radiation of molybdenum reflected from the face (100) of hematite (planes denoted by primes refer to the axes used by Groth) gave, as shown in Table I, the value 3.682 0.010 A. for d/n. If n is one, this corresponds to a unit of structure with a = 3.70 A., and a = 85° 42. With one Fe2C>3 in this unit, the density calculated from the X-ray data is 5.25, in good agreement with the observed values,1 which range from 5.15 to 5.30. 

The Atomic Arrangement.—The same transformation of axes as for hematite is found necessary in order to account for the Laue data for corundum from the spectral data the smallest rhombohedral unit is found to have a = 5.12 0.01 A., and a = 55° 17, and to contain two AI2O3. The density from these data is 3.96 the directly determined value is 3.99. 

The structures determined for hematite and corundum show that these crystals consist of a compact arrangement of approximately, but not exactly, spherical ions of oxygen and of iron or aluminum, held together by inter-ionic forces which are prob- atoms in the units of structure of ably electrostatic in nature. No evidence hematite and corundum small cir-... 

The crystal structures of hematite and corundum have been determined through the use of Taue and spectral photographs, interpreted with the aid of the theory of space groups. The unit of structure is a rhombohedron with a = 55° 17 and a = 5.420 = = 0.010 A. for hematite, and with a = 55° 17 and a = 5.120 = = 0.010 A. for corundum. The space group underlying the atomic arrangement is D. ... 

Fig. 3.16 Schematic drawing of the MIMOS II Mossbauer spectrometer. The position of the loudspeaker type velocity transducer to which both the reference and main Co/Rh sources are attached is shown. The room temperature transmission spectrum for a prototype internal reference standard shows the peaks corresponding to hematite (a-Fe203), a-Fe, and magnetite (Fe304). The internal reference standards for MIMOS II flight units are hematite, magnetite, and metallic iron. The backscatter spectrum for magnetite (from the external CCT (Compositional Calibration Target) on the rover) is also shown...

Fig. 2.3 Basic structural units and Fe-Fe distances (in nm) for hematite, goethite, akaganeite and lepidocrocite and their associated radial distribution functions as obtained from EXAFS spectra. The first peak in the radial distribution...

The structure derived from a Rietveld fit of a neutron diffraction pattern of a 6-line ferrihydrite which showed more and sharper lines (Fig. 2.9, lower) than an XRD pattern, was in agreement with the structure proposed by Drits et al. (1993) except that it was not necessary to assume the presence of hematite in order to produce a satisfactory fit (Jansen et al. 2002). The unit cell of the defect free phase had a = 0.29514(9) nm and c = 0.9414(9) nm and the average domain size derived from line broadening was 2.7(0.8) nm. Since forced hydrolysis of an Fe solution at elevated temperatures will ultimately lead to hematite, it is likely that incipient hematite formation may occur under certain synthesis conditions. Neither these studies nor Mbssbauer spectroscopy, which showed only a singular isomer shift at 4.2 K characteristic of Fe, supported the presence of " Fe (Cardile, 1988 Pankhurst Pollard, 1992). However, the presence, at the surface, of some Fe with lower (<6) coordination, perhaps as tetrahedra (Eggleton and Fitzpatrick, 1988) which may have become unsaturated on heating, has been suggested on the basis of XAFS results (Zhao et al. 1994). 

Fig. 2.11 Structure of hematite, a) Hexagonal lined, c) Arrangement of octahedra. Note their close packing of oxygens with cations distributed face-sharing, d) Ball-and-stick model. Unit cell in the octahedral interstices. Unit cell outlined. outlined, e) 03-Fe-03-Fe-03 triplets, (a, b Eggle-b) View down the c-axis showing the distribution ton et al., 1988 with permission c, d Stanjek, of Fe ions over a given oxygen layer and the hexa- unpubl. e Stanjek, 1991 with permission) gonal arrangement of octahedra. Unit cell out-...

There are structural analogues of a number of iron oxides in the Fe-H-O system. Under certain conditions, continuous solid solutions exist between the two members of a pair. The magnetite-ulvospinel and the hematite-ilmenite pairs are well-known examples. The principle in going from the Fe oxide to the Ti-containing phase is to replace two Fe by one Fe" and one Ti , thereby increasing the unit cell size. 

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides... 

Fig. 3.9 Effect of Al-substitution in synthetic hematites on (Left) the unit cell edge length a of hematites synthesized at various temperatures (Stanjek Schwertmann, 1992, with permission), and (Right) the magnetic hyperfine field Bhf of hematites formed at 70 °C and 1000°C dotted lines indicate 95% confidence limits (Murad Schwertmann 1986 with permission).

Low levels of structural Ge" have also been observed in natural hematite from the Apex mine, Utah (Bernstein Waychunas, 1987) and to achieve charge balance, incorporation of two Fe for one Ge", i.e. similar to the two Fe" for one in ilme-nite, has been suggested. Synthetic, single crystals of Ge substituted hematite have also been grown by a chemical vapour transport method (Sieber et al. 1985). A range of elements including Zr, Ge, Hf, V, Nb, Ta, W and Pb has been used as low level dopants (2 10 - 0.2 g kg ) to improve the semiconductor behaviour of hematite anodes (Anderman Kermedy, 1988). The increase in unit cell c from 1.3760 to 1.3791 nm and in a from 0.50378 to 0.50433 nm indicated that Nd (as an inactive model for trivalent actinides of similar ionic size (Am r = 0.0983 nm Nd " r = 0.098 nm)) was incorporated in the structure (Nagano et al. 1999). 

Galvez et al. (1999) demonstrated that phosphorus up to a P/Fe mol ratio of 0.03 mol mol , can be incorporated into the hematite structure by heating P-con-taining 2-line ferrihydrite. Support for structural incorporation comes from a higher unit cell c (1.3776 => 1.3824 nm), IR-stretching bands of P-OH, a lowered intensity ratio of the XRD 104/113 lines and congruent release of Fe and P upon dissolution. 

Extensive replacement of Fe by transition metal cations and alkaline earth ions has been reported for b-FeOOH (Okamoto, 1968). Muller et al. (1979) found incorporation of up to 0.4 mol moF Ca solid solutions with the formula Fei xKxOi x(OH)i+x could be identified. Jimenez-Mateos et al. (1990) reported that Co and Mn, respectively, could replace up to 0.3 and 0.5 mol mol Fe. The unit cell parameters decreased in both cases with increasing substitution. These Mn- and Co-substituted 5-FeOOHs decomposed at 200 °C to give poorly crystalline, substituted hematites. 

The specific surface area of a solid is the surface area of a unit mass of material, usually expressed as m g . There is an inverse relationship between surface area and particle size. Massive crystals of hematite from an ore deposit (e. g. specularite) may have a surface area 1 m g". As particle size/crystallinity is governed largely by the chemical environment experienced during crystal growth, the surface area of a synthetic iron oxide depends upon the method of synthesis and that of a natural one, upon the environment in which the oxide formed. 

---