Green liquor dregs as sealing layer material to cover sulphidic mine waste deposits

Sammanfattning: Waste rock and tailings generated by sulphide ore mining often contains large amounts of iron sulphides which oxidize when exposed to atmospheric oxygen and may generate acidity. In the absence of alkaline minerals to neutralize this acidity, elements including harmful heavy metals become mobile. Clearly, it is a high priority for mining operations to reduce the impact of mining waste on the environment. New research aimed to take advantage of industrial rest products to inhibit acid mine drainage has yielded some reason for optimism. The recycling of chemicals in sulphate pulp mills yields green liquor dregs (GLD). The fact that GLD have a low hydraulic conductivity and are alkaline opens the possibility of using it for construction of sealing layers for sulphidic mine waste. The study consists of two parts. The aim of the first part was to characterize the GLD and evaluate their suitability for sealing layer purposes. GLD were characterized physically, mineralogically and chemically to define variationsin the properties of the material and to evaluate if they can act as a potential candidate for construction of sealing layers. Compared to lime, the pure GLD appear to have a higher porosity and surface area even though both materials have comparable particle size. The shear strength may, however, be insufficient. In cover applications, where high compaction grade and low hydraulic conductivity are desirable, mixing additives to GLD is necessary to improve the material properties. Based on the characterization results, the difference between batches is fairly small both mineralogically and physically. GLD could be an alternative to traditional sealing layers made of till since it was shown to have relatively low hydraulic conductivity and high water retention potential. The small particle size and high buffer capacity makes it a better candidate than till which usually varies in size and hydraulic conductivity. There is a chemical variation of the material but the element content does not seem to be an environmental concern. The aim of the second part of the study was to improve the performance of the GLD by blending them with other rest products. Geotechnical and geochemical investigations were performed on GLD mixed with tailings. Humidity cell tests were used to study the effect of GLD on the mobility of elements considered to be major environmental issues in the tailings. By adding tailings to GLD, the shear strength improved 2-3 times after 1 month of curing. The water retention capacity remained at a high level. Humidity cell tests revealed that a 50/50 mix of GLD and tailings has a direct effect on the metal leaching from tailings. The alkaline capacity of the pulping waste raised the pH, thereby stabilizing the tailings and reducing the leaching of Al, Cd, Cu, Ni, Co, Cr and Mn. The amount of leached elements could be directly related to the pH during the test. Elements that showed an increase of leaching as a result of the application of GLD were Mo and As. This is explained by the chemical behavior of these elements and their tendency to become mobile at high pH. In summary, GLD have the potential to both reduce the amount of leachate generated in tailings and, due to their alkaline property, improve the leachate quality by decreasing the metal mobility. However, restrictions occur when GLD are blended with tailings containing large amounts of As and Mo since an increased leaching may be expected. Further research will focus on the aging of the material and its long-term stability.