Oxidação de calcopirita: investigação do mecanismo eletroquímico via difração de raios- X ex situ e in situ usando radiação síncrotron e quantificação do efeito galvânico da associação com pirita

Abstract

Chalcopyrite (CuFeS2) is the most abundant copper mineral. The dissolution rate of this sulfide at low temperatures is slow and typically tends to decline with time. Most of the studies ascribe the slow kinetics to the formation of an insoluble layer, which prevents further mineral dissolution. However, the formation mechanism of this product layer andits nature/effects on the process are poorly understood. Thus, the purpose of this thesis was to investigate the rate and mechanisms of CuFeS2 dissolution at low temperatures aiming at enhancing Cu extraction from low-grade, chalcopyrite ores. The findings of the presentinvestigation provide experimental, original evidences that the formation of metal-deficient sulfides under anodic prewave conditions (i.e., potentials lower than the critical potential of CuFeS2: 0.75-0.90 V vs. SHE) contributes to the slow dissolution rate of this sulfide. Results by using Raman spectroscopy and ex situ synchrotron small angle X-ray diffraction(XRD) demonstrated that CuFeS2 oxidation involves the formation of an intermediate bornite (Cu5FeS4) layer, which declined the CuFeS2 oxidation rate at 0.70 and 0.80 V vs. SHE. Elemental sulfur (S8) and covellite (CuS) were also detected at 0.80-1.00 and 0.80 V vs. SHE, respectively. While the presence of CuS led to hindered dissolution, no evidence to link the formation of S8 to the diminution of CuFeS2 oxidation rate was found. For the first time, the in situ transition of CuFeS2 to the reaction products was also investigated by in situ synchrotron time-resolved XRD. At 25 °C, no reaction product was detected up to10 h. On the other hand, the significant effect of temperature on the dissolution rate was illustrated. At 100 °C, a direct transition of CuFeS2 to CuS at anodic prewave potential (i.e., 0.75 V vs. SHE) takes place. Melted S8 was also detected under these conditions. So far evaluated qualitatively, the effect of pyrite (FeS2) mineral phase often found in copper sulfide ores on CuFeS2 dissolution was quantified in this thesis. When associated to FeS2, the dissolution current density of CuFeS2 increased by a factor of approximately 2.6 and 2.2, respectively, in solutions containing 0.001 mol.L-1 O2 and 0.05 mol.L-1 Fe3+, and by1.6 times in the presence of both oxidants. Therefore, more pronounced effect of pyrite on chalcopyrite oxidation rate was observed under the lower oxidizing conditions. In summary, this original application of electrochemical methods and ex situ and in situ synchrotron XRD techniques allowed a detailed discussion of the oxidation mechanism ofCuFeS2, which is essential to develop alternative processes for treating low-grade, copper ores.