Pyrite conditions

Ash is the inorganic residue that remains after the coal has been burned under specified conditions, and it is composed largely of compounds of sihcon, aluminum, iron, and calcium, and minor amounts of compounds of magnesium, sodium, potassium, phosphorous, sulfur, and titanium. Ash may vaiy considerably from the original mineral matter, which is largely kaolinite, iUite, montmoriUonite, quartz, pyrites, and gypsum. 

Several other processes for extracting Be from beryl have been patented the most feasible involves the formation of BeCl2 by direct chlorination of beryl under reducing conditions several volatile chlorides are produced by this reaction (BeCl2, AICI3, SiCl4 and FeClj) and are separated by fractional condensation to give the product in a pure state. Other methods involve the fusion of beryl with carbon and pyrites, with calcium carbide and with silicon. 

The coexisting sphalerite, pyrite, electrum, and argentite must have been at equilibrium at the time of their precipitation. Although it is difficult to evaluate this condition, it is commonly observed that these minerals are in direct contact with each other without evidence of mutual replacement texture. Therefore, it is likely that these minerals have been precipitated nearly contemporaneously. 

In the siliceous body, electrum and Cu minerals (enargite, luzonite, covelline), and native sulfur occur. The Ag content of electrum is lower (0.0-5.3 wt%) than that from epithermal Au-Ag vein-type deposits (Fig. 1.194) (Shikazono and Shimizu, 1987). Low Ag content of electrum and sulfide mineral assemblage (enargite, native sulfur, covellite, pyrite) indicate high fs2 condition (Fig. 1.194). 

The redox condition of the deep fluids may be buffered by the chlorite-pyrite equilibrium. 

It is noteworthy that bornite, chalcocite and tetrahedrite-tennantite which are common minerals in Kuroko deposits occur in gold bearing Besshi-type deposits. Although these minerals are considered to be secondary minerals, depositional environments of these minerals are characterized by higher /s, and foj conditions. It is also noteworthy that these deposits are rich in pyrite rather than pyrrhotite. Probably, Besshi-subtype deposits in Shikoku formed under the higher fo and /sj conditions than the deposits characterized by pyrrhotite (Maizuru, Hidaka, Kii, east Sanbagawa). Such typical Besshi-type deposits (Besshi-subtype deposits in Shikoku) are characterized by simple sulfide mineral assemblage (chalcopyrite, pyrite, small amounts of sphalerite). Inclusion of bornite in pyrite is also common in these deposits. 

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 ... 

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. 

This presumably arises from easily removed organic sulfur and some of the pyritic sulfur which can be half converted thermally to H2S under the reaction conditions. 

Table 7.1 Comparison of the yields of carbon-containing compounds obtained from an atmosphere of CH4, NH3, H2O and H2 using spark discharges with those obtained under hydrothermal conditions from a mixture of HCN, HCHO and NH3 at 423 K and 10 atm in the presence of pyrite-pyrrhotite-magnetite redox buffer (Holm and Andersson, 1995)...

If CO2 is replaced by CO in the pyrite/H2S system, the conditions become much more favourable, as the positive results obtained by Huber and Wachtershauser (1998) and Cody (2000a) show. 

Pyrite is not only one of the key compounds in Wachtershauser s theory, but could also have fulfilled an important function for phosphate chemistry in prebiotic syntheses. A group in Rio de Janeiro studied the conditions for phosphate sorption and desorption under conditions which may have been present in the primeval ocean. In particular, the question arises as to the enrichment of free, soluble inorganic phosphate (Pi), which was probably present in low concentrations similar to those of today (10 7-10 8M) (Miller and Keffe, 1995). Experiments show that acid conditions favour sorption at FeS2, while a weakly alkaline milieu works in an opposite manner. Sorption of Pi can be favoured by various factors, such as hydrophobic coating of pyrite with molecules such as acetate, which could have been formed in the vicinity of hydrothermal systems, or the neutralisation of mineral surface charges by Na+ and K+. 

Fig. 12.2. Redox-pH diagram for the Fe-S-H20 system at 100 °C, showing speciation of sulfur (dashed line) and the stability fields of iron minerals (solid lines). Diagram is drawn assuming sulfur and iron species activities, respectively, of 10-3 and 10-4. Broken line at bottom of diagram is the water stability limit at 100 atm total pressure. At pH 4, there are two oxidation states (points A and B) in equilibrium with pyrite under these conditions.

Respective weathering rates of sulfide minerals is pyrrhotite/sphalerite>pyrite. The buffering capacity of the tailings is low due to the lack of carbonates, allowing the rapid onset of low pH conditions. 

Each of the sulphide minerals, which are PGM carriers (i.e. pyrrhotite, pyrite, pentlan-dite, etc.) have different flotation properties under some flotation conditions. The selectivity between sulphide minerals and gangue minerals is relatively poor in principle, and in the majority of cases, a hydrophobic gangue depressant has to be used. 

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