Moving towards sustainable mine waste rock management through automated mineral and geochemical analysis

Abstract

Mining activities generate vast amounts of waste rock (WR); if this waste is sulphide- and sulphosalt-bearing, it can pose a risk to ecosystems due to contaminated mine drainage. Strategic long-term planning is necessary to prevent and mitigate adverse environmental impacts. This study includes a detailed characterization of WR from a mine in northern Canada and defines the diameter of physical locking of sulphides and sulphosalts (DPLS) that can be used to separate the WR into two fractions according to the geochemical reactivity. An automated mineralogy system (QEMSCAN®) and computed tomography (CT) were used to quantify the liberation degrees of acid-generating (sulphides) and neutralizing minerals (carbonates). Chemical and mineralogical analysis indicated that sulphides, mostly pyrite (FeS2) and gersdorffite (NiAsS) (dominant sulphides), were enriched in the fine- to mid-sized fractions compared to the coarse fractions. These minerals were more liberated within the fine fractions and their liberation degree was considered negligible at sizes >2.5 mm. Consequently, 2.5 mm was defined as the critical diameter of sulphide reactivity (DPLS) for the studied WR. Furthermore, kinetic tests using humidity cells were conducted to confirm this result and to assess the geochemical behaviour of the total fraction (< 10 mm), the coarse fraction (> 2.5 mm) and the fine fraction (< 2.5 mm). Sulphides associated with the coarse fraction were not significantly exposed to weathering conditions; sulphides were associated with non-sulphide gangue minerals, which was confirmed by QEMSCAN® and computed tomography. Moreover, mineral weathering occurred primarily in the fine fraction (< 2.5 mm) due to the higher available exposed surface area of the minerals. The geochemical analysis of humidity cells leachate revealed that the pH values remained within the range of 7.07 and 8.61, and that the instantaneous metal concentrations (Fe, Cu, and Ni) were consistently below environmental limits during the154- day test period. However, As concentrations from fine and total fractions exceeded environmental standards. Sample reactivity showed that the fine fraction controlled the geochemical behaviour of the studied WR, where the fine fraction was 1.5 times more reactive than the total fraction and 10 times more reactive than the coarse fraction which represents about 60 wt% of the total sample. Consequently, screening waste rock in accordance with the DPLS could prove to be an effective method for managing WR to prevent contaminated drainage. This approach may allow a reduction in the amount of WR to be stored in surface piles (which need to be managed) and will reduce the costs associated with the reclamation of waste rock piles.