Pyrite reactivity

Moisture present in the coal is known to influence spontaneous heating in a stockpile insofar as the moisture affects ventilation (air flow) and pyrite reactivity. 

Cruz R., Bertrand, V., Monroy, M., GonzAlez, I. 2001. Effect of sulphide impurities on the reactivity of pyrite and pyritic concentrates a multi-tool approach. Applied Geochemistry, 16, 803-819. 

Postma, D. (1983) Pyrite and siderite oxidation in swamp sediments. J. Soil Sci. 34 163-182 Postma, D. (1993) The reactivity of iron oxides in sediments A kinetic approach. Geochim. Cosmochim. Acta 57 5027-5034 Pourbaix, M. (1963) Atlas of equilibrium diagrams. Gauthier-Villars, republished by Per-gamon, London in 1966. 

The degree of pyritization of iron (DOP) or the fraction of reactive iron that is bound with sulfur appears to increase rapidly with increasing S Fe ratios and may approach an asymptotic value of about 75% at S Fe ratios of 3 (Figure 6). 

Figure 6. The degree of pyritization, defined as the fraction of reactive iron present as pyrite, is a measure of the extent to which available iron has reacted with sulfur (226). In lake sediments, iron monosulfides frequently are as abundant as pyrite and hence were included with pyrite in the values calculated for surface sediments from 13 lakes and presented here. Even this correction neglects Fe(II) that may have been reduced by sulfide but may be present as siderite. Availability of iron appears to be more important than bottom-water oxygenation in determining the degree of pyritization. In the right-hand graph, darkened squares represent sediments known to experience seasonal anoxia only the uppermost point experiences permanent anoxia. (Data are from references 30, 34, 56, and 61.)...

In summary, it seems meaningful to consider both the formation of pyrite from the reaction of H2S with reactive ferric oxides and sulfate recycling as a result of this process in any discussion of the early diagenesis of sulfur and iron in sediments. 

In addition to a better understanding of the reaction of sulfide with ferric oxides and its role in pyrite formation, a more exact definition of the term reactive iron is critical. Does reactive iron mean a different iron oxide fraction for bacterial dissolution (e.g., weathering products such as goethite or hematite) than for reaction with sulfide (e.g., reoxidized lepidocrocite) In other words, is there a predigestion of ferric oxides by bacteria that allows a subsequent rapid interaction of sulfide with ferric oxides ... 

Targe lumps of the ore are first crushed and ground up by very heavy machinery. Some ores are already fairly concentrated when mined. For example, in some parts of the world, haematite contains over 80% Fe2Os. However, other ores, such as copper pyrites, are often found to be less concentrated, with only 1% or less of the copper compound, and so they have to be concentrated before the metal can be extracted. The method used to extract the metal from its ore depends on the position of the metal in the reactivity series. 

Pyrite is a major sink for reduced sulfur from the JH S/S0/]- system to the extent that reactive iron is available. In sediments with limited amounts of reactive iron, reduced sulfur species in the H2S/S°/H2SX system are available for reaction with the sedimentary organic matter. 

The types of sulfur in coal, as well as their distribution and reactivity, have a profound impact on the efficiency of desulfurization processes. For practicality, coal sulfur forms are commonly classified as sulfate sulfur, pyritic sulfur, and organic sulfur. Pyritic and organic sulfur account for almost all the sulfur in coals. Sulfate sulfur is usually much less than 0.1% in freshly mined coals, and increases as the coals are exposed to the atmosphere or "weather". 

Other approaches have been used to characterize the organic sulfur in coal. Programmed temperature reduction (PTR) (9-14) as used by Attar is one such method as is programmed temperature oxidation (PTO) (15-17). Both methods rely on differences in reactivity of the different sulfur species. However, both procedures involve high temperatures and under such conditions transformation between sulfur species is possible. This reaction is highly probable if pyrite or elemental sulfur are present in the sample (18-20). 

Reaction 6.16 results in the pH of the waters, in the absence of carbonate precipitation, being buffered at higher pH values (e.g., Ben-Yaakov, 1973). It is, therefore, reasonable to expect that the effectiveness of sulfate reduction in producing carbonate dissolution or precipitation may depend in part on the availability of reactive iron. This conclusion has been demonstrated in a study of the pore water geochemistry of aluminosilicate and carbonate-rich sediments from Kaneohe Bay, Hawaii. The aluminosilicate sediments contain abundant pyrite whereas the pyrite content of the carbonate sediments in this bay is low. Pore waters collected from the aluminosilicate sediments have higher pH values than those collected from the carbonate-rich sediments. This observation is a result of the pH values in the pore waters of the aluminosilicate sediments being buffered at... 

Hoffman et al. (18) conducted a parametric study to determine the effect of bacterial strain, N/P molar ratio, the partial pressure of CO2, the coal source and the total reactive surface area on the rate and extent of oxidative dissolution of iron pyrite at a fixed oxygen pressure. The bacterial desulfurization of high pyritic sulfur coal could be achieved in 8 to 12 days for pulp densities upto 20% and particle size of less than 7 um. The most effective strains of T. ferrooxidans were isolated from the natural systems, and the most effective nutrient medium contained low phosphate levels, with an optimal N/P molar ratio of 90 1. 

Beside defects from mineral genesis, grinding of a mineral can produce roentgen amorphous states or a new crystalline phase. This leads to the formation of surfaces which differ morphologically and energetically from equilibrium surfaces. Relations were also observed between the degree of crystallinity and particle size on one side and surface reactivity with water or a surfactant on the other side. For example, the adsorption of xanthates on a very pure surface of pyrite monocrystals occurs much slower than on fine crystalline samples5. ... 

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