Pyrite leaching

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 ... 

Iron in the feed concentrate is rejected either as unreacted pyrite mixed with elemental sulfur or as jarosites in the leach residue. The pyrite/sulfur mixtures said to be suitable for indefinite storage, but the well known environment effects caused by pyrite weathering are likely to make storage of this material a less than straightforward problem. Besides this, there are problems associated with the disposal of the leach residues from the pressure leach process. 

In the leaching process, bacteria such as Thiobacillus ferroxidans and those belonging to the Sulfolobus genera, play a major role in the oxidation reactions at moderate and higher temperatures respectively. The oxidation of sulfides by bacteria is typified by the reactions of pyrite, a common accessory mineral in primary copper ore bodies this reaction can be considered to proceed through two stages ... 

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. 

Longmaid-Henderson A process for recovering copper from the residue from the roasting of pyrites to produce sulfur dioxide for the manufacture of sulfuric acid. The residue was roasted with sodium chloride at 500 to 600°C the evolved sulfur oxides and hydrochloric acid were scrubbed in water and the resulting solution was used to leach the copper from the solid residue. Copper was recovered from the leachate by adding scrap iron. The process became obsolete with the general adoption of elemental sulfur as the feedstock for sulfuric acid manufacture. 

TRW Meyers [Named after the three CalTech professors who founded the company Thompson, Ramo, and Wooldridge] A chemical method for desulfurizing coal. The iron pyrites is leached out with a hot aqueous solution of ferric sulfate, liberating elemental sulfur. The resulting ferrous sulfate solution is re-oxidized with air or oxygen ... 

Sulphuric acid is the largest volume chemical in the world with an annual production of about 180 mill, t/year which is used primarily for phosphate fertilizers, petroleum alkylation, copper ore leaching and in smaller quantities for a number of other purposes (pulp and paper, other acids, aluminium, titanium dioxide, plastics, synthetic fibres, dyestuffs, sulphonation etc.). The major sulphur sources for sulphuric acid production are sulphur recovered from hydrocarbon processing in the refineries and from desulphurisation of natural gas, SO2 from metallurgical smelter operations, spent alkylation acid, and to a minor extent mined elemental sulphur and pyrites. A simplified flow sheet of a modem double-absorption plant for sulphuric acid production from sulphur is shown in Fig. 1. 

Heyes and Trahar (1984) leached pyrite with cyclohexane and compared the extract with a sulphur-containing solution of cyclohexane in a UV spectra photometer as shown in Fig. 1.4, indicating that sulphur was present at the mineral surface. Therefore, the inherent hydrophobicity and natural floatability once thought to be typical of sulphides is now thought to be restricted to sulphides such as molybdenite and other minerals or compound with special structural features. The collectorless floatability that most sulphide minerals showed came from the self-induced or sulphur-induced flotation at certain pulp potential range and certain conditions. 

Textural analysis revealed that there have been multiple leach events as recorded in previous studies (Titley 1995). For instance, in the SEM images and analysis of the samples, chalcocite can be seen as a replacement of pyrite. This is evidenced by formation of Cu mineral pseudomorphs after cubic pyrite. Evidence of replacement was seen in many of the samples. 

Chemical leaching of pyrite has been accomplished with aqueous ferric sulphate under pressure. The overall reaction is... 

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