Effectiveness of Organic Carbon Cover Systems on Sulfide-rich Tailings

Abstract

Acid mine drainage (AMD) occurs when sulfide minerals oxidize; generating low pH water and the release of dissolved metals. This phenomena is of primary concern in the mining industry, where the future loading of dissolved metals from inactive mine sites could continue for decades to centuries, with recent estimates of total liability related to AMD remediation exceeding $10 billion in Canada alone (Mining Watch Canada, 2017). Traditionally, remediation of AMD generally is focused on collection and treatment of water from surface-water bodies (i.e., ponds, ditches and streams) and treatment through pH neutralization using lime (CaO). While this approach improves water quality, it represents a recurring cost for the mining companies who need to continue operating these systems after mine closure. Due to this reality, numerous approaches have been developed over the past few decades to passively mitigate and remediate AMD by limiting the supply of O2 and water to tailings rich in sulfide minerals. These passive approaches can be done through the use of: 1) cover systems by limiting O2(g) diffusion and/or O2(g) consumption; and 2) waters covers by limiting O2(g) diffusion. Examples of option 1) include biosolids, wood-waste, peat, compost, geosynthetic clay liners (GCL), covers with capillary barrier effects (CCBEs), and monolayer covers with an elevated water table; while an example of option 2) is subaqueous tailings disposal.

The performance of a cover system, consisting of a 0.5 m layer of biosolids fertilizer and municipal compost underlain by a ~ 2 m layer of thickened desulfurized tailings (DST), was studied. Six locations were investigated, including four locations with a 2-layer cover system, one location with a 1-layer cover of DST only, and one location without any cover system. The DST layer had been in place for up to 12 years and the organic cover components in place between 5 and 8 years. Results demonstrated that the cover system was able to consume all atmospheric O2(g) prior to the base of the cover system, resulting in improved water quality when compared to the location without a cover system. Comparison between the locations with a 2-layer cover system and the location with a 1-layer cover revealed that the current geochemical conditions were similar, except that the organic layer consumes a portion of incoming O2(g) and leaches alkalinity to the shallow porewater; thus improving the acid-neutralizing capacity. This research shows that cover systems which use an organic layer over top of a low sulfur (S) thickened tailings layer with an elevated water table can limit oxidation of the tailings and neutralize acidity. However, in order for the organic carbon portion of this cover system to operate as an effective O2 barrier in the long-term, occasional replenishment of organic material is required.