Pyrite principal forms

The mineral pyrite occurs in coal in discrete particles in a wide variety of shapes and sizes. The principal forms are [1-6] ... 

All coals contain some of the third and fifth forms of pyrites, and some coals contain all five of the principal forms [6, 8,9]. 

The principal source of sulfur dioxide, the precursor of sulfuric acid, is high-sulfur coal. Coal used in power plants can contain up to 4% sulfur, mostly in the form of minerals such as pyrite, FeS2. Combustion forms sulfur dioxide ... 

Coal contains detrital minerals that were deposited along with the plant material, and authigenic minerals that were formed during coalification. The abundance of mineral matter in coal varies considerably with its source, and is reported to range between 9.05 and 32.26 wt% (Valkovic 1983). Minerals found in coal include (Table 2) aluminosilicates, mainly clay minerals carbonates, such as, calcite, ankerite, siderite, and dolomite sulphides, mainly pyrite (FeS2) chlorides and silicates, principally quartz. Trace elements in coal are commonly associated with one or more of these minerals (see Table 2). 

Brown iron ore is found both in amorphous masses, and in crystals of a variety of forme, according as the ore may have been produced either by the decomposition of iron pyrites, or substitution and oxidation of carbonate of iron. It also occurs in small rounded pieces, either loose or conglomerated, in which state it is known as pea iron ore. When mixed with clay and other earthy matters, and of a soft texture, in which state it is found in great beds, it is termed yellow oehre. It exists in great quantities in Normandy, Berry, Lorraine, Burgundy, and constitutes the principal supply of ore for many of the French ironworks. ... 

Chalk consists almost entirely of calcite. formed principally by shallow-water accumulation of (l) calcareous tests of floating microorganisms and (2) comminuted remains of calcareous algae. The most widely distributed chalks are of Cretaceous age. as exemplified by the cliffs on both sides of the English Channel. Although an unallered deposit, chalk masses may contain nodules of chen and pyrite. 

The principal use of forms of sulfur data is in connection with the cleaning of coal. Within certain limits, pyrite sulfur can be removed from coal by gravity separation methods, whereas organic sulfur cannot. Pyrite sulfur content can therefore be used to predict how much sulfur can be removed from the coal and to evaluate cleaning processes. If the pyrite sulfur occurs in layers, it can usually be removed efficiently. If it occurs as fine crystals dispersed throughout the coal, its removal is very difficult. 

Unconformity-related deposits are found near principal unconformities. Examples include the ore bodies at Quff Lake, Key Lake, and Rabbit Lake in northern Saskatchewan, Canada, and in the Alligator Rivers area in northern Australia. Sandstone deposits are contained in rocks that were deposited under fluvial or marginal marine conditions. The host rocks nearly always contain pyrite and organic plant matter. The sediments are commonly associated with tuffs. Unoxidized deposits of this type consist of pitchblende and coffinite in arkasoic and quartzitic sandstones. Upon weathering, secondary minerals such as camotite, tuyamunite, and uranophane are formed. More information on these and other uranium deposit types is available (1). 

Fig. 6.21. Calculated dispersion of energy bands along some principal symmetry directions in (a) marcasite and (b) pyrite forms of FeS, (from the band-structure calculations of Bullett, 1982 reproduced with the publisher s permission).

Sulfur in coal occurs principally as pyritic sulfur and organically bound sulfur. Other sulfur forms (sulfate and elemental) are common trace constiments of coal. Regardless of the form, sulfur... 

Thallium (10 % of earth s crust) is recovered principally from the flue dust of pyrites burners. The soft, grey metal, which has a hexagonal close-packed structure, is rather more reactive than gallium and indium because of the ease with which it forms a unipositive ion. It oxidises in moist air, decomposes steam at red heat and dissolves readily to form thallium(I) compounds in dilute mineral acids other than HCl, because of the insolubility of TlCl. 

The principal ore of arsenic (5 x 10 % of earth s crust) is arsenical pyrites, FeAsS, but the element occurs commonly with nickel, copper and tin As40g is recovered from flue-dusts collected during the extraction of these metals. Sublimation in the presence of galena, which prevents the formation of arsenites, purifies the oxide this is reduced to arsenic with carbon in a cast iron retort. The element itself has few uses about 0.5% added to lead increases the surface tension of the molten metal and allows spherical lead-shot to be produced. The principal commercial form is the so-called white arsenic, As Og. Arsenic compounds are used mainly for their toxicity arsenical insecticides have been much used. 

It is common practice to make a distinction between the inorganic constituents of so-called "Eastern" and "Western" coals By definition. Western coals are those for which the CaO+MgO content exceeds the Fe203 content of the ash, while the reverse is true for Eastern coals [ 1 I The inorganic constituents in Eastern coals, which are principally bituminous in rank, are predominantly in the form of discrete mineral particles. Clay minerals (kaolinite, illite) are usually dominant, followed by quartz and pyrite. The range and typical values of the mineral distribution and ash chemistry of Eastern coals are shown in Table I. These data were determined from computer-controlled scanning electron microscopy (CCSEM), Mossbauer spectroscopy, and other measurements on over a hundred coals. 

Carbonization. When coal is heated to temperatures 900 to 1200°C in the absence of air, most of the volatile matter is driven off, leaving a char, or, in the case of metallurgical bituminous coal, a coke. The atmosphere in a coke oven consists principally of hydrogen and methane. Consequently, pyrite is reduced to a mixture of iron sulfide (troilite and pyrrhotite) and iron metal [ ]. The amount of iron metal formed depends on both the temperature and the composition of the coke-oven gas. The reduction of iron sulfide to iron metal is desirable since blast furnace operation is more efficient with low sulfur coke. Calcite reacts with the liberated sulfur to form calcium sulfate, thus retaining sulfur in the coke. Calcium XANES spectra of coke produced from Pittsburgh seam coal in which all calcium is initially present as calcite indicate that approximately 70 percent of the calcite is converted to calcium sulfate during coking. 

This is deadly to most forms of aquatic life. Water downstream from a mine may be contaminated by acid mine drainage, the result of microbial oxidation of discarded waste material at the mine site. Acid mine water principally contains sulfuric acid produced by the oxidation of iron pyrites (FeS2). Industrial wastes and acid rain may also contribute to the acidity of natural waters. 

The origin of sulfur in most coals is believed to be sulfate ion, derived fi om seawater. During the earliest stages of coal formation, bacterial decomposition of the coalforming plant deposits occurs. Some of these bacteria reduce sulfate to sulfide. This immediately reacts with iron to form pyrite, the principal inorganic form of sulfur in coals. It is also incorporated into the organic portion of the coal. The amount and form of sulfur in coals depend much more on the coal s depositional environment than on its age or rank. In this sense, it is largely a coal-type parameter, not a rank parameter. 

Coal desulfurization can be achieved on a commercial scale by means of physical or physicochemical methods which generally use the principal of density separation techniques or other techniques that exploit the surface properties of coals and minerals. For example, the methods exploit the difference in properties that exist between the various forms of pyritic sulfur (pyrite and/or marcasite, FeS2, and occasionally including galena, PbS) and the organic matrix of the coal. 

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