Coal pyrite

The goal of beneficiation is to remove as much sulfur from a fuel as possible before it is ever burned. When burned, fuel with lower sulfur content will produce less sulfur dioxide. Beneficiation is usually accomplished by a physical process that separates one form of sulfur, pyritic sulfur, from coal. Pyritic sulfur consists of sulfur minerals (primarily sulfides) that are not chemically bonded to coal in any way. The name is taken from the most common form of mineral sulfur usually found in coal, pyrite, or iron sulfide (FeS2). 

As the total S rises significantly above 0.5 wt%, pyrite comes to gradually account for more and more of the total S content, although even in the highest-S coals, pyritic S seldom accounts for more than 50% of total S. 

Sulfides and Sulfates. Pyrite is the dominant sulfide mineral in coal. Marcasite has also been reported from many different coals. Pyrite and marcasite are dimorphs, minerals that are identical in chemical composition (FeS2) but differ in crystalline form pyrite is cubic while marcasite is orthorhombic. Other sulfide minerals that have been found in coals, and sometimes in significant amounts, are sphalerite (ZnS) and galena (PbS). 

The dimorphs pyrite (FeS2) and marcasite (FeS2) are the dominant sulfide minerals in coal pyrite is the more abundant. Pyrite and marcasite have different crystal forms pyrite is isometric and marcasite is orthorhombic. These two minerals are readily observed and, to some degree, easily removed as well as being especially interesting because they contribute significantly to the total sulfur content that causes boiler tube fouling, corrosion, and pollution by emission of sulfur dioxide when coal is burned (Beer et al., 1992). 

A reliable method of measuring the mineral matter content of a coal is an acid demineralization procedure. The method depends on the loss of weight of a sample when treated with 40% hydrofluoric acid at 50 to 60°C (122 to 140°F). Treatment of the sample with hydrochloric acid before and after treatment with hydrofluoric acid helps prevent the retention of insoluble calcium fluoride (CaF2) in the coal. Pyrite is not dissolved in the treatment, consequently, pyrite and a small amount of retained chloride must be determined separately. Since two-thirds of the mass of the pyrite (FeS2) is accounted for by the presence of ferric oxide (Fe203) in the residual ash, the mineral matter content is then given by the formula... 

Blocks of pure pyrite embedded in rock from a hydrothermal area were hand-picked under an optical microscope. The pyrite grains were crushed, sieved and preserved in the same way as the coal. We choose mineral pyrite rather than coal pyrite because it is easier to obtain. The behaviour differences between different pyrites are due to particle size effects or matrix effects, but not to pyrite itself, which is a well defined chemical and mineralogical species, ... 

In any development of advanced physical coal-cleaning (PCC) techniques, an important consideration is the heterogeneous nature of coal and, in particular, the variable manner in which pyrite occurs in coal. This variability influences the behavior of coal with regard to cleaning (26). In some coals, pyrite is distributed throughout the coal matrix as particles only microns in size. Thus, to separate pyrite from these coals, the coal must be crushed to very fine size in order to "liberate the pyrite from the coal particles. However, conventional commercial PCC techniques cannot... 

Figures 2 and 3 contain yield curves for naphthalene and 1-methylindan as a function of reaction time for tetralin and tetralin plus coal, pyrite, or asphaltene. The asphaltene was a homogenized mixture of several samples isolated from coal liquefaction products during other work in our laboratory (9). This asphaltene sample contained essentially a negligible ash content (<0.1%). Therefore, it contains many organic structures similiar to those found in coal, but unlike coal, its reactions will be free of any complicating factors due to mineral matter. The yields of naphthalene and 1-methylindan are greater in the presence of asphaltene than in its absence, although not quite as high as in the presence of coal. This is additional evidence that these two products arise mainly from reactions associated with the presence of the organic portion of coaly matter. These reactions are quite likely free radical in nature.

For several coals, the carbon dioxide evolved during a TODS treatment has been continuously monitored by infrared spectroscopy. Under these conditions, strong carbon dioxide evolution begins at about 200° C and continues intermittantly until 650° C. Thus, the sulfur dioxide evolution at temperatures greater than 650° C in Figures 3, 4, and 5 we attribute to the decomposition of inorganic sulfates. When coal pyrite is... 

Coal Pyrite (with Montano Rosebud Oxydesulfurized Coal) 250-380... 

In a reducing atmosphere the deposit material may contain iron sulphide (FeS) formed on dissociation of coal pyrite mineral. This is likely to occur on the combustion chamber wall tubes near the burners where the reaction time is short, below one second, for oxidation of FeS residue to the oxide. It has been suggested that calcium sulphide (CaS) may also be present in the ash material deposited from a reducing atmosphere gas stream as a result of sulphidation of calcium oxide (9). 

Mineral matter has been known to enhance the conversion of coal to liquid products (1,2,3). Addition of pyrite, pyrrhotite, and liquefaction residues ( ) to coal has been shown to affect the coal conversion yields and the viscosity of the products (5.). Of all the minerals present in coal, pyrite (and marcasite) are the most important for coal utilization, especially in direct coal liquefaction (1,5). However, one has to remember that under coal liquefaction conditions pyrite rapidly transforms to a nonstoichiometric iron sulfide Fe S(0 x 0.125). It is noted that the sulfur formed as a result of the decomposition of pyrite is able to extract hydrogen from poor donor solvents. The stoichiometry of the pyrrhotite formed from FeSp depends strongly on the partial pressure of H S. 

Tucholski, D. and G. M. Colver, "Electrostatic Separation of Coal Pyrite in a Circulating Fluidized Bed, Proceedings of the 2nd International Conference on Appl. Electrostatics, Beijing China, Nov. 4-7, 1993B, pp. 412-416. 

The reduction of sulfur is one of the principal benefits of the froth flotation process. Of the two types of sulfur in coal (i.e., inorganic and organic), only the inorganic sulfur (mainly represented by pyrite) can be separated fi om coal by physical methods. The floatability of coal-pyrite is, however, somewhat different fi om that of ore-pyrite, and its separation from coal presents a difiicult problem. The research based on the behavior of ore-pyrite in the flotation process— namely, its poor flotability under alkaline conditions—did not result in a development of a coal-pyrite selective flotation process. The two-stage reverse flotation process proved to be much more successful. In... 

Coal flotation is a physiochemical process which exploits the differences in the wettability of hydrophobic clean coal and that of hydrophilic foreign particles (Arnold and Apian, 1989 Fecko et al., 2005). It is, therefore, subject to the surface properties of coal pyrite and other types of commercially worthless material present in coal which plays a major role in determining separation of such material from coal (Luttrell and Yoon, 1994 Luttrell et al., 1994). 

Sulfur is present in coal either as organically bound sulfur or as inorganic sulfur (pyrite or marcasite and sulfates) (Kuhn, 1977). The amount of organic sulfur is usually 3% w/w of the coal, but exceptional amounts of sulfur (up to 11%) have been recorded. The sulfates (mainly calcium and iron) rarely exceed 0.1% except in highly weathered or oxidized samples of coal pyrite and marcasite (the two common crystal forms of FeS2) are difficult to distinguish from one another and are often (incorrectly) designated simply as pyrite. 

Lalvani SB, Zhang G, Lalvani LS (1996) Coal pyrite passivation due to humic acids and lignite treatment. Fuel Sci Technol Int 14 1291-1313... 

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