Oxidation pyrite

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 sulfuric acid needed to solubilize copper from chalcocite is balanced by the acid recovered from the copper electrowinning step this acid is recycled to the heaps. The overall acid requirements for the process are, therefore, dependent on the acid consumption by the gangue minerals in the ore and the acid production by pyrite oxidation. If the pyrite associated with the ore is significant and the acid consumption by the ore is low, excess sulfuric acid can be neutralized by lime. 

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. 

By this reaction, sulfur from the pyrite oxidizes to form sulfate ions, liberating protons that acidify the solution. 

Initially, pyrite oxidation in the model proceeds according to the reaction... 

In nature, at least two aqueous species, 02(aq) and Fe+++, can serve as electron acceptors during the pyrite oxidation (Moses et al., 1987). In the case of Fe+++, the oxidation reaction proceeds as,... 

Whereas pyrite oxidation in the absence of calcite produces H-Fe-S04 drainage, the reaction in the presence of calcite yields a Ca-HC03-SC>4 drainage. 

Moses, C. O., D. K. Nordstrom, J. S. Herman and A. L. Mills, 1987, Aqueous pyrite oxidation by dissolved oxygen and by ferric iron. Geochimica et Cosmochimica Acta 51,1561-1571. 

Williamson, M. A. and J. D. Rimstidt, 1994, The kinetics and electrochemical ratedetermining step of aqueous pyrite oxidation. Geochimica et Cosmochimica Acta 58, 5443-5454. 

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. 

NO 3-Reducing. Fig. 9.15 shows data on groundwater below agricultural areas. The sharp decrease of 02 and NO3 at the redox cline indicate that the kinetics of the reduction processes are fast compared to the downward water transport rate. Postma et al., 1991 suggest that pyrite, present in small amounts is the main electron donor for NO3 reduction (note the increase of SOJ immediately below the oxic anoxic boundary). Since NO3 cannot kinetically interact sufficiently fast with pyrite a more involved mechanism must mediate the electron transfer. Based on the mechanism for pyrite oxidation discussed in Chapter 9.4 one could postulate a pyrite oxidation by Fe(III) that forms surface complexes with the disulfide of the pyrite (Fig. 9.1, formula VI) subsequent to the oxidation of the pyrite, the Fe(II) formed is oxidized direct or indirect (microbial mediation) by NO3. For the role of Fe(II)/Fe(III) as a redox buffer in groundwater see Grenthe et al. (1992). 

Luther III, G. W. (1987), "Pyrite Oxidation and Reduction Molecular Orbital Theory Considerations", Geochim. Cosmochim. Acta 51, 3193-3199. 

Pyrite oxidation reactions include the reactions producing hydrophobic species ... 

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. 

Ding Dunghuang, Long Xiangyun, Wang Dianzuo, 1993. Mechanism of pyrite oxidation and flotation. Nonferrous Metals, 45(4) 4-30 (in Chinese)... 

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. 

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]...

A special case represents acid sulfate waters released from mines where metal sulfide ores and lignite have been exploited. S- and 0-isotope data may define the conditions and processes of pyrite oxidation, such as the presence or absence of dissolved oxygen and the role of sulfur-oxidizing bacteria (i.e. Taylor and Wheeler 1994). 

Nordstrom, D.K. (1982) Aqueous pyrite oxidation and the consequent formation of secondary iron minerals. In Kittrick, J.A. et al. 

Secondary U and REE minerals include autunite, Ce-phosphate, and Ld-Nd phosphates. The geochemical behaviour can be explained through pyrite oxidation that increases acidicity and releases sulphate and Fe(III), that would allow oxidative dissolution of the U ore, possibly precipitating uranopilite. When the pH increased at sites more distant from pyrite dissolution, U(VI) was hydrolysed and eventually co-precipitated with Fe3+-oxyhydroxides. 

This reaction (along with others in the overall chain of pyrite oxidation) results in this process being one of the most acid-generating in Nature (e.g., Appelo Postma 1993 Backes et al. 1993). 

Equations such as equation (1) above imply that the oxidative dissolution of pyrite is congruent, directly liberating Fe2+, SO4, and H+ to solution. However, in the common circumstance that water is insufficiently abundant to immediately transport the oxidation products away from the mineral surfaces, pyrite oxidation more commonly results initially in the accumulation of various hydroxysulphate evaporite minerals. These minerals form efflorescent crusts, typically white and yellow in colour, on the surfaces of pyrite-rich coals and mudstones (Fig. 1), and they effectively store the oxidation products in a readily soluble form until some hydro-logical event delivers sufficient water to dissolve and transport them away. Because pyrite often occurs in mudstones, where Al-bearing clay minerals are in contact with acidic pyrite oxidation waters, A1 is frequently released from the clays and is also stored in these hydroxysulphate phases. When these minerals finally dissolve, they result in abrupt and extreme increases in dissolved acidity. For this reason, they have been termed acid generating salts (AGS) (Bayless... 

Table 3. Fe and A hydroxysulphate minerals, so-called acid generating salts (AGS), which effectively store the products of pyrite oxidation in solid form until later submergence and dissolution ...

In terms of the environmental impacts of coal mine drainage, siderite plays an ambiguous role (see, for instance, Morrison et al. 1990, and works cited therein). On the one hand, the acidity released by pyrite oxidation can be... 

Pyrite oxidation transfers atmospheric 02 to the solid phase (as hydroxysulphate minerals Table 3) and or the dissolved phase (as SO4 reaction (1)), thereby reducing the atmospheric concentration of the gas. 

Backes, C. A., Pulford, I. D. Duncan, H. J. 1993. Seasonal variation of pyrite oxidation rates in colliery spoil. Soil Use and Management, 9, 30-34. 

Banks, D., Younger, P. L. Dumpleton, S. 1996. The historical use of mine-drainage and pyrite-oxidation waters in central and eastern England, United Kingdom. Hydrogeology Journal, 4, 55-68. 

Evangelou, V. P. 1995. Pyrite Oxidation and its Control Solution Chemistry, Surface Chemistry, Acid Mine Drainage. CRC Press, Florida, 293 pp. 

Rimstidt, J. D. Vaughan, D. J. 2003. Pyrite oxidation a state-of-the-art assessment of the reaction mechanism. Geochimica et Cosmochimica Acta, 67, 873-880. 

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