Sulfur pyritization

Fig. 14. Coal flotation flow sheet suggested for increased sulfur removal (29). Pyritic sulfur removal from coal makes it imperative to closely control pulp...

The pyritic sulfur in coal can undergo reaction with sulfate solutions to release elemental sulfur (see Sulfurremoval and recovery). Processes to reduce the sulfur content of coal have been sought (75). The reaction of coal and sulfuric acid has been used to produce cation exchangers, but it was not very efficient and is no longer employed. Efforts have turned to the use of hot concentrated alkaH in a process called Gravimelt. 

Nitrogen, unlike pyritic sulfur, is mosdy chemically bound in organic molecules in the coal and therefore not removable by physical cleaning methods. The nitrogen content in most U.S. coals ranges from 0.5—2.0 wt %. 

Conventional coal cleaning processes can remove about 50% of pyritic sulfur and 30% of total sulfur. For northern Appalachian region coals it has been shown that a greater sulfur reduction can be achieved by applying physical coal cleaning to finer size coals (Table 3) (8). 

Oxidative Desulfurization Process. Oxidative desulfurization of finely ground coal, originally developed by The Chemical Constmction Co. (27,28), is achieved by converting the sulfur to a water-soluble form with air oxidation at 150—220°C under 1.5—10.3 MPa (220—1500 psi) pressure. More than 95% of the pyritic sulfur and up to 40% of the organic sulfur can be removed by this process. 

In an extensive study by Read et al. [93], 10 anionic surfactants were evaluated for their ability to remove pyritic sulfur and ash from ultrafine Illinois no. 5 coal by flotation processes. The authors observed that of the commercially available surfactants, sodium dodecyl sulfate was the most effective on either a weight or a molar basis, followed by a linear AOS (average molweight 272) and alkylpolyethoxylated sulfonates. Of the noncommercial surfactants tested, -(E -b-dodecene-b-suIfonate (f0) was the most effective and better than any commercial surfactant on a dosage/recovery basis. 

In both reactions, Fe is produced as a consequence of the oxidation of sulfur from the — 1 oxidation state of pyritic sulfur to the +6 state of sulfate. 

Iron in the feed concentrate is rejected either as unreacted pyrite mixed with elemental sulfur or as jarosites in the leach residue. The pyrite/sulfur mixtures said to be suitable for indefinite storage, but the well known environment effects caused by pyrite weathering are likely to make storage of this material a less than straightforward problem. Besides this, there are problems associated with the disposal of the leach residues from the pressure leach process. 

This presumably arises from easily removed organic sulfur and some of the pyritic sulfur which can be half converted thermally to H2S under the reaction conditions. 

As a second example, we constrain a fluid s oxidation state by assuming equilibrium with pyrite (FeS2). As before, direct information on this variable can be difficult to obtain, so it is not uncommon for modelers to use mineral equilibrium to fix a fluid s redox state. The choice of pyrite to buffer oxidation state, however, is perilous because pyrite sulfur, which is in the S1- oxidation state, may dissolve by oxidation to sulfate (S6+),... 

SULF-X [Sulfur extraction] A regenerable flue-gas desulfurization process in which the sulfur dioxide is absorbed by aqueous sodium sulfide in a bed packed with pyrite. Ferrous sulfate is produced this is removed by centrifugation and calcined with coke and fresh pyrite. Sulfur vapor is evolved and condensed, and the residue is re-used in the scrubber. Piloted in the mid-1980s. Not to be confused with Sulfex or Sulph-X. 

Unfortunately, pyrite sulfur makes up only half the sulfur content of coal, while the other half is organically bound. Coal gasification is the only means by which this sulfur mode can be removed. Of course, it is always possible to eliminate the deleterious effects of sulfur by removing the major product oxide S02 by absorption processes. These processes impose large initial capital investments. 

This work has demonstrated that organically bound sulfur forms can be distinguished and in some manner quantified directly in model compound mixtures, and in petroleum and coal. The use of third derivatives of the XANES spectra was the critical factor in allowing this analysis. The tentative quantitative identifications of sulfur forms appear to be consistent with the chemical behavior of the petroleum and coal samples. XANES and XPS analyses of the same samples show the same trends in relative levels of sulfide and thiophenic forms, but with significant numerical differences. This reflects the fact that use of both XPS and XANES methods for quantitative determinations of sulfur forms are in an early development stage. Work is currently in progress to resolve issues of thickness effects for XANES spectra and to define the possible interferences from pyritic sulfur in both approaches. In addition these techniques are being extended to other nonvolatile and solid hydrocarbon materials. 

Total sulfur Pyritic sulfur Sulfate sulfur Sulfide sulfur Organic sulfur... 

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). 

Benedciation may he effective in removing up to half of all the pyritic sulfur found in coal. None of the methods just described is effective, however, in removing organic sulfur intimately bound to coal particles. 

The pyrite sulfur is generally considered to be sulfur associated with iron pyrite, FeS2 In most cases only the second sulfur of the pyrite molecule can be considered to be in this class. This sulfur can be expelled from iron pyrite with moderate heating (approximately 500°C) to form iron sulfide, FeS, and elemental sulfur condensate in the cooler vapor space above the sample. 

The recent and future development of sulfur supplies in the Middle East tends to emphasize the resulting regional concept for sulfur supply/demand. However, as in North America, the rising production of involuntary sulfur has caused a drop in the amount of native pyrite sulfur produced. Table 2 shows recent past and estimated trends in several Middle Eastern countries. The future of by-product sulfur from oil and gas operations is very uncertain at the time of this writing (October 1980) because of the Iran-Iraq war and its effect on near-by countries. 

Iron pyrites -sulfuric acid from [SULFURIC ACID AND SULFURTRIOXIDE] (Vol 23)... 

Three sulfur forms recognized by ASTM are tl) sulfate sulfur, which may be in the form of calcium or iron sulfate (2) pyritic sulfur, which is sulfur combined with iron in the form of minerals pyrite and/or mareasiic and (3) organic sulfur, which is bonded to the carbon structure. 

Pyritic sulfur can be removed by froth flotation, which takes advaniage of the differences of specific gravity of the two types of sulfur. To be effective, the coal must be pulverized into particles in the micron region. The process can be enhanced by adding limestone, catalyst, und soda ash to the coal dust. After treatment, the coal is formed into briquettes for ease of handling by convenlional conveyors. 

Variations in the amount of ash arise from the retention of sulfur that originates from the pyrite. Sulfur in ash is usually determined as sulfate (ASTM D-1757 ASTM D-5016), and the method may give abnormally high amounts of sulfur. This is due to the sulfur retention from pyrite (and marcasite). If the forms of sulfur in coal are known (ASTM D-2492), the amount of pyrite retention can be estimated (see also ASTM D-3174, Note 2). Nevertheless, sulfur retention will give rise to anomalous results. 

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