Green Liquor Dregs in Sealing Layers to Prevent the Formation of Acid Mine drainage From characterization to implementation
Abstract: One of the mining industry’s main concerns is the management of waste rock and tailings generated by sulfide ore extraction. Upon exposure to atmospheric oxygen, iron sulfides oxidize producing acidity. In the absence of alkaline minerals to neutralize this acidity, elements including harmful heavy metals may be transported with percolated water, forming acid mine drainage (AMD). A common solution to prevent oxygen and water entering sulfide-bearing mine waste, thereby minimizing the formation of AMD, is to construct a dry cover comprised of soil. In Sweden, clayey till is often used. Due to a shortage of fine-grained till close to mines, other sealant materials must be considered. This thesis presents an alternative to the traditional materials used as sealing layers in dry covers. Green liquor dregs (GLD) are a residual waste originating from the recycling of chemicals in sulfate pulp mills. The objective was to evaluate the suitability of GLD for use in sealing layers for sulfidic mine waste. This was carried out by analyzing samples both in the laboratory and in the field in a pilot-scale study. GLD was characterized physically, mineralogically and chemically to determine variations in the properties of the material. Based on the characterization results, the difference between batches was small both mineralogically and physically. In addition, it had favorable qualities with respect to reducing water percolation and oxygen diffusion, as it exhibited low hydraulic conductivity and high water retention potential. However, the shear strength was insufficient for cover applications. In an attempt to improve shear strength, but also to reduce the amount of GLD to save transportation costs, fly ash, tailings and till were blended with GLD. All of these additives increased the shear strength. Humidity cell tests were used to study the effect of GLD on the mobility of elements found in the tailings that are considered to cause major environmental issues. They showed that the alkaline capacity of the GLD raised the pH, thereby stabilizing the tailings and reducing the leaching of Zn, Cd, Cu, Ni, Co and Cr. The amount of leached elements could be directly related to pH. Mo and As leaching increased as a result of the application of GLD. This is explained by the chemical behavior of these elements and their tendency to become mobile at high pH. Because covers on mine waste should be efficient over the long-term, the aging of GLD was studied with material ranging from 0-13 years old. Aged GLD showed a hydraulic conductivity of 10-8 m/s. This is not expected to affect the function of the sealing layer to minimize oxidation because older GLD exhibited a high water retention capacity and a degree of water saturation of >91%, which would effectively limit oxygen diffusion. Leaching, mainly of S and K, was observed but had no major effect on the hydraulic properties of the material. The effects of aging are largely related to the quantity of water passing through the GLD, thereby changing its chemical composition and, in the long term, its pH. An implementation study using GLD as an additive to improve the hydraulic properties of till in the construction of sealing layers in a cover system design was carried out. The results showed that the quality of till, i.e., hydraulic conductivity and water retention capacity, could be improved by the addition of GLD. Addition of 10% GLD was found to be sufficient to take full advantage of its hydraulic properties and sufficient compaction could be achieved. Based on the results, GLD is expected to be a viable alternative to traditional cover materials. The pilot-scale study, as well as a full-scale trial that is currently in progress, will be used to evaluate further the function of a GLD-based sealing layer.
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