Arsenic Mobility under a Neutral Mine Drainage Environment in a Gold-Mine Tailings Dam

摘要

A geochemical assessment of a lined tailings deposit from an operating gold mine located in a semiarid climate was performed for this study. A total of 58 samples were collected, including surface and deep tailings as well as fresh tailings from the carbon-in-leach circuit. Several analyses were performed on these samples, including the modified acid-base accounting, NAG test, chemical and mineralogical characterization. Kinetic tests were also performed on two samples of 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 alkalinity trends monitored and gypsum detected in the mineralogical composition (1.3%) of the tailings collected in the reservoir suggest that acid neutralization and precipitation of sulphate salts are occurring. In the oxidizing environment within the tailings, the arsenic concentrations varied from below detection limit to 0.081 mg/L/ always with a slightly alkaline pH. This concentration range can be related with the dissolution of amorphous ferric arsenate. The arsenic concentrations monitored in a slightly reducing environment (Eh=-12V) are much higher than the concentrations identified in the oxidizing environment. The Eh, pH, and dissolved iron concentrations (average 3 mg/L) suggest that the arsenic is being released by the reduction of Fe~(3+) to Fe~(2+). Based on the results from this work, it was possible to establish a conceptual model of arsenic behavior in oxidizing and slightly reducing environments within the tailings, as well as recommend actions for a stable closure of the tailings dam to prevent the arsenic release in the long term to the environment.