"Caracterização geoquímica de rejeitos de mineração de ouro como contribuição à gestão ambiental e ao projeto de fechamento da mina"

Abstract

A geochemical assessment of a tailings deposit from a gold mine located in the semiarid climate was performed on this study. A total of 58 samples were collected including surface and deep tailings as well as fresh tailings from the CIL (carbon in leach) circuit. Several analyses were performed on these samples including the modified acid-base accounting, NAG test, synthetic precipitation leaching procedure (SPLP), chemical and mineralogical characterization. Kinetic tests were also performed in the fresh tailings. Both mineralogical characterization and the acid-base accounting showed that the neutralization potential in the tailings is at least two times higher than the acidification potential, confirming the current slightly alkaline conditions of the reservoir, with pH value always higher than 7. The results indicated that acid rock drainage should not occur in the short and long term. The alkalinity concentrations monitored and the gypsum presence detected in the mineralogical composition (1,3%) of the tailings collected in the reservoir suggests that neutralization and sulphate salts precipitation is occurring. In the oxidizing environment, the arsenic concentrations varied from below detection limit until 0.081 mg/L, always with a slightly alkaline pH. This concentrations can be related with the dissolution of amorphous ferric arsenate. The arsenic concentrations monitored in a slightly reducing environment (Eh= -12V) are higher. The Eh and the dissolved iron concentrations (average 3 mg/L) suggests that that the arsenic is being released by the reduction of Fe3+ to Fe2+. Based on the results from this work it was possible to establish a conceptual model of the arsenic behave in oxidizing and slightly reducing environments as well as recommend action for a stable closure of the tailings preventing the arsenic release in the long term at the environment.