Pyrite bacterial

Sulfur is widely distributed as sulfide ores, which include galena, PbS cinnabar, HgS iron pyrite, FeS, and sphalerite, ZnS (Fig. 15.11). Because these ores are so common, sulfur is a by-product of the extraction of a number of metals, especially copper. Sulfur is also found as deposits of the native element (called brimstone), which are formed by bacterial action on H,S. The low melting point of sulfur (115°C) is utilized in the Frasch process, in which superheated water is used to melt solid sulfur underground and compressed air pushes the resulting slurry to the surface. Sulfur is also commonly found in petroleum, and extracting it chemically has been made inexpensive and safe by the use of heterogeneous catalysts, particularly zeolites (see Section 13.14). One method used to remove sulfur in the form of H2S from petroleum and natural gas is the Claus process, in which some of the H2S is first oxidized to sulfur dioxide ... 

In contrast to the of hydrothermal solution for the vein, that of pyrite in hydrothermally altered rocks (Shimanto Shale) varies very widely, ranging from —5%o to - -15%o. Based on the microscopic observation, pyrite with low values less than 0%o is usually framboidal in form, suggesting that low 8 S was caused by bacterial reduction of seawater sulfate. There are two possible interpretations of high 8 " S values (+10%o to - -15%o). One is the reduction of seawater sulfate in a relatively closed system. The other one is a contribution of volcanic SO2 gas. As noted already, volcanic SO2 gas interacts with H2O to form H2SO4 and H2S. value of SO formed by... 

It can be seen, therefore, that ferrous iron and chalcopyrite oxidation are acid-consuming reactions, while pyrite oxidation and iron hydrolysis are acid-producing reactions. Thus, whether the overall reaction in a dump is acid producing or acid-consuming depends on the relative proportions of chalcopyrite and pyrite and on the pH conditions. In practice, sulfuric acid additions to the leach solution applied to the dump are usually required to overcome the acid consuming reactions of the gangue minerals and to keep the pH in a suitable range, typically 2 to 2.4, to optimize bacterial activity and minimize iron hydrolysis. 

The bacterial leaching of uranium minerals is complex. This is because of the fact that uranium minerals are not sulfides and are not, therefore, directly attacked by the bacteria. However, the uranium sources usually have a substantial pyrite content which can be bac-terially oxidized to give an acidic ferric sulfate solution which is an effective leaching medium for uranium minerals. The reactions involved in the system can be shown in a simplified form as ... 

The last reaction cited above as shown is very effectively catalyzed by bacterial action but is very slow chemically by recycling the spent ferrous liquors and regenerating ferric iron bacterially, the amount of iron which must be derived from pyrite oxidation is limited to that needed to make up losses from the system, principally in the uranium product stream. This is important if the slow step in the overall process is the oxidation of pyrite. The situation is different in the case of bacterial leaching of copper sulfides where all the sulfide must be attacked to obtain copper with a high efficiency. A fourth reaction which may occur is the hydrolysis of ferric sulfate in solution, thus regenerating more sulfuric acid the ferrous-ferric oxidation consumes acid. 

Prior to gold extraction by cyanidation, refractory gold ores are either roasted or pressure oxidized to liberate the gold contained as submicroscopic particles or in solid solution in arsenopyrite and arsenic-rich pyrite. Gold extraction from such ores require roasting or pressure oxidation or bacterial oxidation prior to cyanidation to destroy the sulfide structure. 

Table 1 Important weathering reactions in order of ease of chemical weathering and solubility, which goes along with the reaction rate of the mineral dissolution, except for bacterial mediated pyrite oxidation [9, 10]...

P5Trite and calcite are mineral crystals that represent a wide Habitus and Tracht variation. Pyrite is the most persistent mineral among sulfide minerals, occurring in a wide range of modes, including inorganic processes and bacterial action, and it can also be synthesized by hydrothermal or chemical vapor transport methods. 

Pyrite is the most common mineral among sulfides. It occurs not only as a major mineral of sulfide ore deposits of base metals, such as Cu, Pb, Zn, in vein-t) e, massive-replacement t) e, kuroko-t5q3e deposits, etc., but also sporadically as an accessory mineral in volcanic, sedimentary, and metamorphic rocks. It also occurs as a precipitate in hot springs, and it may be formed by bacterial action. Pyrite itself is not an ore of Fe, though it contains iron, and at best may have economic value as an ore to obtain sulfuric acid. However, due to its occurrence in and... 

Framboidal pyrite occurs, for example, in sedimentary rocks, muddy sediments, and precipitates in hot springs two controversial origins have been suggested, one bacterial and the other relating to agitation in hydrothermal solution. Framboidal... 

Pore-water profiles are frequently interpreted according to this concept. For example, White et ah (35) described a conceptual model of biogeo-chemical processes of sediments in an acidic lake (cf. Figure 4). They discussed the numbered points in Figure 4 as follows Diffusion of dissolved oxygen across the sediment-water interface leads to oxidation of ferrous iron and to an enrichment of ferric oxide (point 1). Bacterial reductive dissolution of the ferric oxides in the deeper zones releases ferrous iron (point 2). The decrease in sulfate concentration stems from sulfate reduction, which produces H2S to react with ferrous iron to form mostly pyrite in the zone below the ferric oxide accumulation (point 3). 

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 ... 

Sulfur is widely distributed as sulfide ores, which include galena, PbS cinnabar, HgS iron pyrite, FeS2 and sphalerite, ZnS (Fig. 15.12). The mineral molybdenite, MoS2, is a soft rock with a metallic sheen and properties similar to those of graphite. Sulfur is also found as deposits of the native element (called brimstone), which are formed by bacterial action on H2S. 

This bacterial production occurs in the pore fluids of sediments and in stagnant basins (seas, lakes, rivers and fiords). At the interface between anoxic and oxic waters the H2S can be oxidized. This oxidation is frequently coupled to changes in the redox state of metals (1.2) and non-metals (2). Another major interest in the H-jS system comes from an attempt to understand the authigenic production of sulfide minerals as a result of biological or submarine hydrothermal activity and the transformation and disappearance of these minerals due to oxidation (4). For example, hydrothermally produced H2S can react with iron to form pyrite, the overall reaction given by... 

In the freshwater peat swamp, bacterial reduction of organic sulfur in plant tissues may be an important process in the formation of pyrite (93). Altschuler et al. (93) proposed that in the Everglades peat, pyrite precipitates directly by the reaction of HS or organic sulfide (produced by reduction of oxysulfur compounds in dissimilatory respiration) with ferrous iron in the degrading tissues. Pyrite formation in low-sulfur coal may be accounted for by this process. 

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. 

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