Reprocessing feasibility of polymetallic waste rock for cleaner and sustainable mining

Mining operation produces significant volumes of waste rocks (WR) characterised by high heterogeneity in their physical, mineralogical and hydrogeological properties. The purposes of this study were to assess the acid-generating potential, evaluate reprocessing feasibility of polymetallic WR using desulphurisation process (i.e., gravity separation and flotation) and study the geochemical behaviour of desulphurised materials. The fresh WR was collected in the waste dump from Langlois mine at Lebel-sur-Que ' villon (Quebec, Canada). The WR sample was divided into six fractions using wet screening: D < 0.053 mm, 0.053 < D < 0.3 mm, 0.3 < D < 0.85 mm, 0.85 < D < 2.4 mm and 2.4 < D < 5 mm. Chemical and mineralogical characterisations showed that fine fractions (D < 2.4 mm) were enriched in sulphide minerals (i.e., 4.6 wt% pyrite, 3.5 wt% sphalerite, etc.) and carbonates (i.e., 3.7 wt% calcite and 5.1 wt% ankerite) compared to coarse fractions (1.2 wt% pyrite, 1.1 wt% sphalerite, 3.2 wt% calcite, 5.2 wt% ankerite). The acid-generating potentials of different fractions were evaluated using acid-base accounting (ABA) and net acid generation (NAG) tests. Only one size fraction (F2) was acid generating and others were classified as uncertain. The degree of liberation of sulphides in the size fractions greater than 2.4 mm was negligible (<16%). Consequently, for this WR sample, 2.4 mm was defined as the diameter of physical locking of sulphide (DPLS). Based on DPLS, the WR were separated into two fractions; reactive (<2.4 mm), non-reactive (<2.4 mm) and then submitted to kinetic tests. Geochemical analyses of leachates of the coarse fraction (2.4 < D < 10 mm), the total sample (D < 5 mm) and -DPLS fractions (D < 2.4 mm) and the benign desulphurised materials (D < 2.4 mm) showed that pH values remained between 6.62 and 8.11 and the instantaneous concentrations of metals such as iron (Fe), copper (Cu) and lead (Pb) were below environmental limits (D019, Quebec, Canada), except for zinc (Zn) leached from the total sample which indicates a risk of contaminated neutral drainage. The pyrite oxidation rate within the coarse fraction was low (16.6 mu mol/ kg/day) compared to fine fraction (35.8 mu mol/kg/day). A combined gravity-flotation process gave better results to decontaminate the waste rock (D < 2.4 mm), with a benign desulphurised material reaching a low sulphur content (0.23 wt%) and high sulphur recovery (89 wt%). The final concentrates from the combined gravity-flotation process showed high base metals (Zn, Cu and Pb) recovery (more than 87 wt%).

Automated summary

Mining operations produce a significant amount of waste rocks with high heterogeneity in their properties. This study assessed the acid-generating potential and reprocessing feasibility of polymetallic waste rocks, as well as studied their geochemical behavior. A combined gravity-flotation process was effective in decontaminating the waste rocks, resulting in a benign desulphurised material with low sulphur content and high base metals recovery.