A series of thermodynamic calculations are performed for the roasting of pyrite in changing temperatures and atmospheres. The relationship between Δ_rG^θ and temperature in the range of T = 300–1200 K shows that, depending on the atmosphere it is in, reactions of pyrolysis, oxidation or reduction can occur. Both the pyrolysis of pyrite in an inert atmosphere and its oxidation by oxygen can form pyrrhotite (mainly Fe_0.875S and FeS), but the temperature required for oxidation is much lower than that for pyrolysis. In an oxygen-containing atmosphere, the isothermal predominance areas for the Fe–S–O system indicate that a change in temperature and oxygen partial pressure can lead the pyrite to undergo desulphurization to pyrrhotite (FeS₂ → Fe_0.875S/FeS) or iron oxides (FeS₂ → Fe₃O₄/Fe₂O₃), or sulphation to iron sulphates (FeS₂ → FeSO₄/Fe₂(SO₄)₃). The presence of carbon is beneficial to the desulphurization of pyrite under an oxidizing atmosphere since iron sulphates can be converted to iron oxides at very low levels of P_CO/P_CO2. Results presented in this paper offer theoretical guidance for the optimization of roasting of pyrite for different purposes.

Arsenopyrite is a common sulphide mineral occurring in deposits of gold ore that makes the extraction of gold difficult and, thus, pre-treatment is necessary prior to gold leaching. Bioleaching pre-treatment of arsenopyrite has drawn significant attention owing to its environmental friendliness, low cost and simple operation. A critical impedance of bioleaching to its large-scale industrial application is the slow leaching kinetics. Various passivating products on the surface of arsenopyrite have been found to limit the bioleaching process. This paper reports results from an in-depth investigation into the formation process of passivating products from arsenopyrite bioleaching by Acidithiobacillus ferrooxidans in 9K culture medium including bioleaching experiments and physicochemical analyses of the materials as well as thermodynamic analyses of the leaching system. The results of phase transformation and morphological change of the solid products suggest that the passivation occurring in the bioleaching of arsenopyrite is largely attributed to an initially formed passivating film consisting mainly of realgar (As₂S₂), orpiment (As₂S₃) and elemental sulphur (S⁰) on the arsenopyrite surface. Based on the results, the paper also proposes possible passivation mechanisms to allow for a better understanding on the passivation behaviour of the bioleaching of arsenopyrite.

Arsenopyrite (FeAsS) is often associated with gold, but pre-treatment is necessary prior to gold leaching, mainly due to the gold encapsulation in the matrix of FeAsS. Bio-oxidation is attractive and promising, largely due to its simplicity, low cost and environmental friendliness. A critical problem that still impedes the large-scale applications of this green technology is its slow leaching kinetics. Some metal ions such as Ag⁺ have previously been found to expedite the bioleaching process. In this paper, the role of Ag⁺ in the arsenopyrite bioleaching by Acidithiobacillus ferrooxidans was investigated in detail by bioleaching experiments and a series of analyses including thermodynamics, X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Experimental results suggested that addition of 5 mg/L Ag⁺ to the leaching system could significantly improve the final As leaching efficiency from 30.4% to 47.8% and shorten the bioleaching period from 19 days to 15 days. Thermodynamic analysis indicates that Ag⁺ destabilises As₂S₂, As₂S₃ and S⁰ via forming Ag₂S, which is confirmed by the XRD analysis on the phase transformation during bioleaching. SEM and XPS analyses further showed that Ag⁺ removed the passivating film consisting mainly of As₂S₂, As₂S₃ and S⁰ because Ag₂S formed on the arsenopyrite surface from the start bioleaching of 36 h. In the presence of Fe³⁺, Ag₂S could easily be dissolved to Ag⁺ again, likely leading to the establishment of the Ag⁺/Ag₂S cycle. The bacteria utilised the two synergistic cycles of Fe³⁺/Fe²⁺ and Ag⁺/Ag₂S to catalyse the bioleaching of arsenopyrite.