Dissolution of pyrite

To further illustrate how the basis-swapping algorithm can be used to balance reactions, we consider several ways to represent the dissolution reaction of pyrite, FeS2. Using the program RXN, we retrieve the reaction for pyrite as written in the llnl database 

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

The above reaction represents, in a simplified way, the origin of acid mine drainage. Streambeds in areas of acid drainage characteristically become coated with an orange layer of ferric precipitate. We can write a reaction representing the overall process by swapping ferric hydroxide in place of the ferrous ion  

Pyrite can dissolve into reducing as well as oxidizing solutions. To find the reaction by which the mineral dissolves to form H2S, we swap this species into the basis in place of the sulfate ion 

Is there a reaction by which pyrite can dissolve without changing the overall oxidation state of its sulfur To see, we return the sulfate ion to the basis and swap H2S for dissolved oxygen  

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It is found that the dissolution of zinc sulfides occurs more rapidly when they are in contact with copper sulfide or iron sulfide than when the sulfides of these types are absent. This enhancement is brought about by the formation of a galvanic cell. When two sulfide minerals are in contact, the condition for dissolution in acidic medium of one of the sulfides is that it should be anodic to the other sulfide in contact. This is illustrated schematically in Figure 5.3 (A). Thus, pyrite behaves cathodically towards several other sulfide minerals such as zinc sulfide, lead sulfide and copper sulfide. Consequently, pyrite enhances the dissolution of the other sulfide minerals while these minerals themselves understandably retard the dissolution of pyrite. This explains generally the different leaching behavior of an ore from different locations. The ore may have different mineralogical composition. A particle of sphalerite (ZnS) in contact with a pyrite particle in an aerated acid solution is the right system combination for the sphalerite to dissolve anodically. The situation is presented below ... 

In an example of a fixed fugacity path we model the dissolution of pyrite (FeS2) at 25 °C. We start in REACT with a hypothetical water in equilibrium with hematite (Fe203) and oxygen in the atmosphere... 

Fig. 14.5. Concentrations of species involved in the dissolution of pyrite, for the fixed fugacity path shown in Figure 14.4.

Figure 7.14 illustrates that in the initial stage of polarization of the pyrite electrode in xanthate solution at about 120 mV, the radius of high value capacitive reactance loop increases with the increase of the polarization potential and reaches the maximum at 320 mV, indicating that the oxidation of xanthate increases gradually and collector film on pyrite surface becomes thicker. It increases the conduction resistance and the growth of collector film is the controlled step resulting in pyrite surface hydrophobic. When the polarization potential increases from 320 mV to 400 mV, the capacitive reactance loop radius decreases, indicating the decrease of transferring conduction resistance as can be seen in Fig. 7.15. It belongs to the step of film dissolution. Capacitive reactance loop radius decreases obviously when the potential is larger than 400 mV, at where the collector film falls off and the anodic dissolution of pyrite occurs. The controlled step is the anodic dissolution of pyrite and the surface becomes...

Dissolution of pyrite can also be mediated by bacteria. Weathering reaction listed for silicates is elementary only... 

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... 

Dom nech, C., De Pablo, J. and Ayora, C. (2002) Oxidative dissolution of pyritic sludge from the Aznalcdllar mine (SW Spain). Chemical Geology, 190(1-4), 339-53. 

Mechanism of Microbial Desulfurization. The microbial dissolution of pyritic sulfur in coal by acidophilic bacteria has been thoroughly investigated (17,18,29). The pyrite is readily oxidized by oxygen or ferric ion, resulting in the ferrous state as follows ... 

Digestion. The influence of ultrasound on the dissolution kinetics of phosphate rock in HNO3 solutions [11] and variables affecting it (viz. particle size, reaction temperature, acid concentration, amplitude of US power) were studied by Tekin [12]. The term dissolution in the presence of auxiliary energy and an acid seems inappropriate in this case as the process is more like a true digestion. Another case in point is the dissolution of pyrite ores in acid and Fe2(S04)3 solutions, which is improved by 30% with respect to the absence of US energy [13]. 

Table 4 Summary of proposed rate expressions for the dissolution of pyrite in solutions containing dissolved...

Fowler T. A. (2001) On the kinetics and mechanism of the dissolution of pyrite in the presence of Thiobacillus ferrooxidans. Hydrometallurgy 59, 257-270. 

The sorption of uranium from acid sulfate leach liquors by strong base anion exchange resins is unusual since complexes of the type [U02(S04) ] " may be sorbed by both ion exchange and addition mechanisms. High concentrations of other species are present in the leach solution due to dissolution of pyritic and siliceous components of the ore, but, apart from iron, they do not interfere with the sorption of uranium as a complex anion. Iron(III) also forms an anionic sulfate complex, but is only weakly held by the resin and is displaced ahead of the uranium. The sorption of uranium may be represented by equations of the type ... 

The oxidation of small sulfide particles is rapid once soils are aerated, so that acidification is immediate and extreme when anaerobic sulfide-bearing soils are drained. Long-term low-level acidification may occur in soils containing large sulfide particles (perhaps pyrites inherited from the parent material). Measurement of the oxidative dissolution of pyrite and other sulfides has produced the following rate law ... 

Sand W, Jozsa PG, Kovacs ZM, Sasatan N, Schippers A (2007) Long-term evaluation of acid rock drainage mitigation measures in large lysimeters. J Geochem Explor 92 205-211 Sasaki K, Tsunekawa M, Tanaka S, Koimo H (1996) Supression of microbially mediated dissolution of pyrite by originally isolated fulvic acids and related compounds. Colloid Surface... 

This produces more sulfuric acid, puts additional Fe into solution, and completes the cycle for the dissolution of pyrite. The Fe(lll) ion exists in solution as the hydrated species Fe(H20)g and forms a precipitate of iron(lll) hydroxide ... 

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