Oxidation of pyrite

Air or biological oxidation of pyrite leads to sulfate formation and dilute sulfuric acid in the mine drainage. This pollutes streams and the water supphes into which the mine water is drained. Means of controlling this problem are under study. 

The hrst successful study which clarihed the mechanism of roasting, was a study of the oxidation of pyrite, FeSa, which is not a typical industrial process because of the availability of oxide iron ores. The experiment does, however, show die main features of roasting reactions in a simplihed way which is well supported by the necessaty thermodynamic data. The Gibbs energy data for the two sulphides of iron are,... 

F. ferro-oxidants is capable of accelerating the oxidation of pyritic (FeSj) deposits at acid pH values. It is usually found in association with Thio-bacillus and was known as Thiobacillus ferroxidans before the distinction between the two organisms was appreciated. It is responsible for pollution problems arising from acid waters in gold and bituminous coal mines such waters are corrosive to pumping machinery and mining installations (see Fig. 2.20). 

Hamilton IC, Woods R (1981) An investigation of surface oxidation of pyrite and pyrrhotite by linear potential sweep voltammetry. J Electroanal Chem 118 327-343... 

Lalvani SB, Shami M (1986) Electrochemical oxidation of pyrite slurries. J Electrochem Soc133 1364-1368... 

Lowson RT (1982) Aqueous oxidation of pyrite by molecular oxygen. Chem Rev 82 461-497... 

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. 

The modern trend is to employ processes based on aqueous oxidation of pyrite and arsenopyrite, and the chemical reactions involved can simplified as ... 

Rusanen, L., Aromaa, J., Forsen, O. (2013). Pressure oxidation of pyrite-arsenopyrite refractory gold concentrate. 

The results of fixed-fugacity paths can differ considerably from those of simple titration models. Consider, for example, the oxidation of pyrite to goethite,... 

Pyrite Oxidation. The oxidation of Fe(ll) minerals by Fe3+ is also of importance in the oxidation of pyrite by 02. This process is mediated by the Fe(II)-Fe(III)system. Pyrite is oxidized by Fe3+ (which forms a surface complex with the pyrite (cf. formula VI in Fig. 9.1) (Luther, 1990). The rate determining step at the relatively low pH values encountered under conditions of pyrite dissolution is the oxygenation of Fe(II) to Fe(III) usually catalyzed by autotrophic bacteria (Singer and Stumm, 1970 Stumm-Zollinger, 1972). Thus, the overall rate of pyrite dissolution is insensitive to the mineral surface area concentration. Microbially catalyzed oxidation of Fe(II) to Fe(III) by oxygen could also be of some significance for oxidative silicate dissolution in certain acid environments. 

Pyrite and arsenopyrite have similar oxidation and self-induced collectorless flotation behavior. It is generally suggested that anodic oxidation of pyrite occurs according to reactions (2-24) in acidic solutions (Lowson, 1982 Heyes and Trahar, 1984 Trahar, 1984 Stm et al., 1991 Chander et al., 1993). The oxidation of pyrite in basic solutions takes place according to reactions (2-25). Since pyrite is flotable only in strong acidic solutions, it seems reasonable to assume that reaction (2-24) is the dominant oxidation at acidic solutions. Whereas pyrite oxidizes to oxy-sulfur species with minor sulphur in basic solutions. 

Janetski et al. (1977) used voltammetry to study the oxidation of pyrite electrode in solution at different pH in the absence and presence of ethyl xanthate to demonstrate that the oxidation of pyrite itself increases as the pH is increased. At high pH condition, the oxidation of pyrite occurs at a potential cathodic to that for xanthate oxidation and hence, only the mineral will be oxidized at the mixed potential and flotation will be depressed. 

Figure 5.4 shows the anodic oxidation of pyrite at different pH modified by CaO and NaOH. Obviously, the oxidation of pyrite is more rapid in CaO solution than in NaOH solution at the same pH. The calcium species in oxidized pyrite surface in the presence of CaO has been identified using XPS as shown in Fig. 5.5. The pyrite surface is oxidized to form Ca(OH)2, CaS04 and Fe(OH)3 in the presence of Ca lime.

Because sulfuric acid is a strong acid, it is a more potent weathering agent than carbonic acid. By serving as a source of H (aq), sulfuric acid is transformed into SO Caq). The oxidation of pyrite is responsible for 11% of the sulfete in river water, with pollution now contributing 54%. The latter is largely associated with the burning of sulfur-rich coal. 

Acid sulfate soils are an especially difficult class of acid soil formed in former marine sediments that have been drained. The acidity is generated from the oxidation of pyrite in the soil resulting in acute aluminium toxicity, iron toxicity, and deficiencies of most nutrients, especially phosphate which becomes immobilized in ferric oxide. The development and management of acid sulfate soils are discussed in detail in Dost and van Breemen (1983) and Dent (1986). 

Reductive dissolution of Fe oxyhydroxides holding sorbed As appears to explain the very large concentrations of As in water from wells drilled into alluvial sediments of the Brahmaputra and Ganges Rivers in Bangladesh and West Begal (Nickson et al 1998, 2000). Dissolved As has accumulated from the reduction of As-rich Fe oxyhydroxides formed upstream of the contaminated areas by weathering of As-rich base metal sulfides. The reduction is driven by sedimentary organic matter in the deposits. Release of As from oxidation of pyrite in shallow wells contributes little to the water contamination because any As(IV) released would be re-sorbed on Fe oxides formed in pyrite oxidation. 

Since the reduction potential for the Fe(II) / Fe(III) couple is + 0.77 V at pH 7, the energy obtainable in this reaction is small. These bacteria always oxidize reduced sulfur compounds, too. Especially interesting is their oxidation of pyrite, ferrous sulfide (Eq. 18-24). The Gibbs energy change was calculated from published data325 using a value of Gf° for Fe (OH)3 of... 

HCCV should be the only anion, except for small concentrations of Cl- and S042" from fluid inclusions in the minerals, oxidation of pyrite, and other minor sources. 

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