Recycling of green liquor dregs in cover application on acid generating mine waste

Sammanfattning: The mining industry generates massive amounts of waste that without treatment and in contact with atmospheric oxygen can cause the formation of acid rock drainage (ARD). In Sweden, the most common reclamation measure for mine waste is to apply a multi-layer cover on top of the waste deposit. The access to suitable cover-materials is, however, limited and can lead to extensive costs for the mining companies. This creates a driving force for alternative solutions, e.g., bentonite amendment to till. As bentonite production is costly both economically and environmentally, recycling of industrial residues to produce sealing layer material is an attractive option. The recycling is beneficial for the industry generating the residue, the mining industry using the residue and the society in its strive for zero waste. An industrial residue that has potential to improve the sealing properties of local till is green liquor dregs (GLD), a residue from pulp production. The main objective of this study was to, by laboratory studies and field application and evaluations, investigate if green liquor dregs amended till can be used as a sealing layer material on top of acid generating mine waste. The conclusion from the laboratory study is that the hydraulic conductivity does not decrease significantly with GLD addition to the silty till. However, the water retention capacity, which can be seen as the major important feature of a sealing layer to be used on top of sulfidic mine waste by keeping the layer close to saturation, shows significant improvements with GLD addition. In the field study the GLD amended till was applied successfully from a soil mechanical point of view (e.g., the compaction degree after application). The early results from the monitoring of the sealing layer indicate results comparable to the more frequently used bentonite amendment in a sealing layer, with soil moisture values reaching close to saturation. However, this study also concludes that the great variation in GLD, especially regarding its soil mechanical properties such as water content, makes it difficult to use compared to commercial products. The material used in the laboratory investigations in the planning phase of a project might behave differently than the one that is delivered to the minesite for reclamation.