Yellowknife Geoscience Forum

Mining activities produce waste rock and tailings that represent a potential risk of toxic metal release into the surrounding environment. Tailings consist of fine particles with large surface area and high adsorption capacity. The presence of sulfide minerals can cause acid rock drainage issues. Fine particles of tailings are easily transported by wind, precipitation, and runoff water and deposited on land and water bodies.

Two tailings from gold and silver mines in Canada were selected and extracted using a rain mimicking procedure (synthetic precipitation leaching procedure by EPA 1312), and a sequential extraction process known as the “three-step BCR sequential extraction” that uses acid, reducing and oxidizing conditions (European Commission Community Bureau of Reference). The major metals (Al, Ca, Fe, Mg and Mn) in tailings and 16 toxic metals (including As, Be, Cd, Co, Cr, Cu, Li, Mo, Ni, Pb, Sb, Se, Tl, U, V and Zn) regulated for water quality by Environment and Climate Change Canada were analyzed using Inductively Coupled Plasma Mass Spectrometry. This study aimed to understand the mobility and geochemistry of these metals when exposed to leaching conditions simulating environmental exposure.

The tailings were kept at site in a neutral to slightly alkaline pH.  A single extraction using precipitation extraction resulted in virtually no trace metal release. However high concentrations of Ca and Mg were observed. While the three-step sequential extraction mobilized a larger proportion of trace metals from the tailings, acid extraction step 1 dissolved a large portion of Ca and Mg along with Cd, Cu, Mn, and Zn. Meanwhile Fe as a major constituent of clay particles was highly released in reducing conditions in step 2 together with Ca, Ni, and Pb.  During step 3, under oxidizing conditions, Fe (possibly from sulfide minerals) was observed in a high concentration as well as Cu, Ni, Se, U and Zn. It was observed that most of Al, Be, Cr, Li, Mo, Sb, Tl, and V remained immobile in the residue phase of both tailings.

The sequential extraction could be used as a tool to predict the potential release of toxic metals from mine wastes. The concentration of metal released from tailings depends on types of tailings and the storage conditions.  Higher concentrations of toxic metals were observed in silver tailings. Both tailings released high levels of Ca and Mg which may result from storage conditions.