The geotechnical, chemical, and mineralogical properties of green liquor dregs (GLDs) generated as byproducts from five paper mills were investigated to assess their buffering and heavy metal immobilization capacities and their roles as water and oxygen barriers. One type of GLD was further studied to test the effects of the retrieval process and the storage, drying, and hydration of GLD. The high water retention capacity of the GLDs is valuable for limiting O2diffusion. Laboratory results showed that the GLDs had hydraulic conductivities of 3.7 × 10−9–4.6 × 10−8 m/s and varied regularly in plasticity. The chemical and mineralogical compositions of the GLDs varied greatly, reflecting the raw material used to produce paper and the process used to retrieve GLDs. Although they had high total heavy metal contents, none of the leached elements from the GLDs (L/S 10 cm3/kg) exceeded the European Union’s limits for landfills of non-hazardous waste. The GLDs exhibited high buffering capacities. In a supplementary test, the buffering capacities varied (0.0041–0.0114 M H+/g GLD) over 72 d after acid was added to the GLD. Changing the filtration process did not greatly affect the GLDs’ properties but mainly affected the hydraulic conductivity, total heavy metal contents and sulfur content. Analyzing the storage of GLDs is necessary in the mining industry because remediation measures require large amounts of material over short periods. The buffering capacity of the dried GLD decreased slightly. The effect of dewatering caused by the mixing of 2% Na-lignosulfate with GLD (w/w) was low.

Sealing layers made of two alkaline paper mill by-products, fly ash and green liquor dregs, were placed on top of 50-year-old sulfide-containing tailings as a full-scale remediation approach. The performance and effectiveness of the sealing layers with high water content for an oxygen barrier and low hydraulic conductivity for a sealing layer in preventing the formation of acid rock drainage were evaluated 5 years after the remediation. The leaching behavior of the covered tailings was studied using batch leaching tests (L/S ratio 10 L/kg). The leaching results revealed that, in general, the dregs- and ash-covered tailings released relatively lower concentrations of many elements contained in acid rock drainage compared to those from the uncovered tailings. A change in the chemical composition and mineralogical state of the tailings was observed for the tailings beneath the covers. The increase in pH caused by the alkaline materials promoted metal precipitation. Geochemical modeling using PHREEQC confirmed most of the geochemical changes of the covered tailings. Both the ash and dregs showed potential to function as sealing materials in terms of their geochemical properties. However, mobilization of Zn and Ni from the lower part of the dregs-covered tailings was observed. The same phenomenon was observed for the lower part of the ash-covered tailings. Ash showed advantages over dregs as a cover material; based on geochemical studies, the ash immobilized more elements than the dregs did. Lysimeters were installed below the sealing layers, and infiltrating water chemistry and hydrology were studied to monitor the amount and quality of the leachate percolating through

Green liquor dregs (GLD), a residual product from sulfate paper mills, was blended with tailings, fly ash, and bark sludge with the aim of improving the material’s physical properties so that it could function as a sealing layer in dry covers on sulfidic mine waste. Geotechnical and geochemical investigations, including weathering cell tests, were carried out on GLD with admixtures to assess their effectiveness. Due to its alkaline character, GLD was shown to have the potential to improve leachate quality by decreasing metal mobility when blended with tailings. The admixtures showed favorable sealing layer properties such as high water retention capacity and low hydraulic conductivity. However, caution must be exercised when the dregs are blended with tailings containing large amounts of As and Mo, since increased leaching of these elements may be expected.

One of the mining industry's main concerns is the management of waste rock and tailings generated by sulfide ore extraction. Upon exposure of atmospheric oxygen, iron sulfides oxidize generating acidity. Infiltrating water form a metal-rich acidic leachate called acid mine drainage (AMD), that can cause serious environmental problems. Green liquor dregs (GLD) is a material that resists the passage of oxygen and water and could thus be used to seal mine wastes, preventing their oxidation and AMD formation. To enable its use in dry mine waste covers, the long-term efficiency of such GLD sealing layers must be evaluated. In this study, fresh GLD and GLD aged for 3 to 13 years was collected from two sites and analysed to determine how aging affects its chemical and physical properties. Aged and fresh GLD were very similar with respect to all the properties important in a sealing layer. In particular, there was no evidence of calcite dissolution in aged GLD samples. Aged GLD also exhibited high water saturation (>91%) and chemical stability, both of which are important for effective long-term sealing. The shear strength of GLD deployed in the field increased over time but not sufficiently to ensure the long-term physical integrity of a pure GLD sealing layer. The development of hybrid materials with improved shear strength will therefore be necessary.

Dagens praxis för täckning av sulfidhaltiga gråbergsrester är att täcka med morän. Det görs som ”kvalificerad täckning” genom att lägga på ett tätskikt med lerig morän som packas tätt. I ett önskescenario finns siltig/lerig morän i anslutning till området som är tillräckligt tätt och som kan användas för att anlägga tätskikt. Erfarenhet visar dock att täta moräner förekommer sällan i gruvans närområde och funktionen som syrebarriär inte alltid uppfylls av lokalt förekommande morän. Alternativet är att använda andra tätskiktsmaterial, bentonitmatta, lera etcetera eller att modifiera lokalt förekommande morän. Ett alternativ är att modifiera lokalt förekommande morän genom inblandning av grönlutslam. Grönlutslammet har stor vattenhållande förmåga och en inblandning på cirka tio procent grönlutslam i siktad morän förbättrar moränens funktion som tätskikt avsevärt.Ett vanligt sätt att efterbehandla gruvavfall är att täcka det med ett skikt som hindrar syret att nå avfallet och därmed hindra oxidationen, även kallad kvalificerad täckning. Huvudfunktionkskravet är att minimera syretransporten och främst syre­diffusionen över tid, vilket innebär i praktiken att tätskiktets hyd­rauliska konduktivitet ska vara under 10-8 m/s. I ett önskescenario finns siltig/lerig morän i anslutning till området som är tillräckligt tätt och som kan användas för att anlägga dessa skikt.

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.

A common solution to minimize formation of acid rock drainage in sulfide-bearing mine waste is to construct a composite cover comprising till. Due to a shortage of fine-grained till close to mines, other sealant materials must be considered. Here, the feasibility of using green liquor dregs (GLD), an alkaline and inorganic residual waste, as an additive to improve low quality till in the construction of sealing layers in cover system designs was studied. GLD was blended with three types of till in a pilot scale study. 10–15 % (wet weight) GLD was found to be sufficient to take full advantage of the physical properties of the residual waste. Different mixing techniques were also evaluated. The results showed that the quality of till, i.e., hydraulic conductivity and water retention capacity, could be improved by addition of 10 % GLD. Short duration and efficient mixing was preferred as vigorous mixing released water bound in the GLD, resulting in increased w and reduced compaction efficiency, making the blended material difficult to apply and use in sealing layer constructions.

Using alternative materials such as residual products from other industries to mitigate the negative effects of acid rock drainage would simultaneously solve two environmental problems. The main residual product still landfilled by sulphate paper mills is the alkaline material green liquor dregs (GLD). A physical, mineralogical and chemical characterization of four batches of GLD was carried out to evaluate the potential to use it as a sealing layer in the construction of dry covers on sulphide-bearing mine waste. GLD has relatively low hydraulic conductivity (10−8 to 10−9 m/s), a high water retention capacity (WRC) and small particle size. Whilst the chemical and mineralogical composition varied between the different batches, these variations were not reflected in properties such as hydraulic conductivity and WRC. Due to relatively low trace element concentrations, leaching of contaminants from the GLD is not a concern for the environment. However, GLD is a sticky material, difficult to apply on mine waste deposits and the shear strength is insufficient for engineering applications. Therefore, improving the mechanical properties is necessary. In addition, GLD has a high buffering capacity indicating that it could act as an alkaline barrier. Once engineering technicalities have been overcome, the long-term effectiveness of GLD should be studied, especially the effect of aging and how the sealing layer would be engineered in respect to topography and climatic conditions.

The present study addresses the effectiveness of green liquor dregs (GLD, a residue from paper pulp-making) as a paste additive with different proportions in tailings for stabilizing mine waste by testing of uniaxial compressive strength and hydraulic conductivity. Selected samples were also investigated for freeze–thaw effect on hydraulic conductivity, and the water retention capacity was discussed based on previous results. Fly ash was also added to the paste to study the auxiliary functions such as solidification in the sealing material. GLD have the potential for use as a barrier layer material for stabilizing mine tailings by decreasing water percolation and improving water retention properties. However, the solidified paste of GLD-amended tailings possesses low uniaxial compressive strength, but the addition of fly ash to the paste increased the uniaxial compressive strength by up to 2–3 times 1 month after it was solidified, with a further two- to threefold strength increase after 3-months curing time. Although the hydraulic conductivity of the tailings paste decreased as a consequence of mixing with both GLD and FA, the difference was within one order of magnitude. The hydraulic conductivity was also reduced as the water/solid ratio of pure GLD decreased. The climatic freeze–thaw cyclic process led to 20 % increase in hydraulic conductivity. A proportion of 7:2:1 for tailings:GLD:FA was found to be a geotechnical satisfactory recipe to seal the mine waste. If porosity is reduced, improved water retention capacity and no cracking in the tailings can be assured, oxygen diffusion is limited, and oxidation of the mine waste is decelerated, thus mitigating acid rock drainage.