Acid Rock Drainage Formation Research Articles

Mathematical modeling for optimized mine waste rock disposal: Establishing more effective acid rock drainage management

Acid rock drainage (ARD), produced from sulfide-bearing mine waste (e.g., waste rock, tailings) at active and abandoned mine sites, continues to be a global concern due to the significant impacts on water, soil, biodiversity, and the creation of public health risks. Many examples demonstrate that it is technically challenging to control and manage ARD, with common methods including costly additive treatments. Instead, an improved approach to ARD management is to minimize opportunities for generation from the outset. In this paper, a new mixed-integer programming (MIP) model is proposed to optimize the placement of waste rock into waste dumps with the objective of minimizing ARD formation. The MIP model considers the net neutralizing potential (NNP) value of waste rocks and decides about the best place for each parcel of mining waste within the waste dump, regarding the target NNP value. Moreover, the waste rocks’ haulage costs obtained from the proposed model were assessed against a recently practiced model (base case). The distribution of NNP value was also simulated in the dump to evaluate neutralizing of the acidity produced by sulfide oxidation in the dump’s different parts. Finally, the capability of the model was evaluated in a real case. The results indicate that the safe threshold conditions (e.g. NNP value ≥ target NNP) are satisfied in dump cells, leading to the optimal disposal of waste rocks and ARD prevention. In addition to the neutralizing advantage, applying the proposed model results in preventing the re-handling of waste materials and reduces the haulage costs by 6.8% compared to the base case.

Characterization of an acid rock drainage microbiome and transcriptome at the Ely Copper Mine Superfund site.

The microbial oxidation of metal sulfides plays a major role in the formation of acid rock drainage (ARD). We aimed to broadly characterize the ARD at Ely Brook, which drains the Ely Copper Mine Superfund site in Vermont, USA, using metagenomics and metatranscriptomics to assess the metabolic potential and seasonal ecological roles of microorganisms in water and sediment. Using Centrifuge against the NCBI “nt” database, ~25% of reads in sediment and water samples were classified as acid-tolerant Proteobacteria (61 ± 4%) belonging to the genera Pseudomonas (2.6–3.3%), Bradyrhizobium (1.7–4.1%), and Streptomyces (2.9–5.0%). Numerous genes (12%) were differentially expressed between seasons and played significant roles in iron, sulfur, carbon, and nitrogen cycling. The most abundant RNA transcript encoded the multidrug resistance protein Stp, and most expressed KEGG-annotated transcripts were involved in amino acid metabolism. Biosynthetic gene clusters involved in secondary metabolism (BGCs, 449) as well as metal- (133) and antibiotic-resistance (8501) genes were identified across the entire dataset. Several antibiotic and metal resistance genes were colocalized and coexpressed with putative BGCs, providing insight into the protective roles of the molecules BGCs produce. Our study shows that ecological stimuli, such as metal concentrations and seasonal variations, can drive ARD taxa to produce novel bioactive metabolites.

Alkalinity generation from weathering of accessory calcite and apatite and acid drainage neutralization in an Archean granitoid waste rock

Formation of acid rock drainage is a temporal balance of acid-generating and alkalinity-generating reactions during mineral weathering. Accessory calcite and apatite are commonly present in granitoid rocks, and their weathering can be a source of net alkalinity. To evaluate the acid-consuming capability of the calcite and apatite group minerals in a granitoid waste rock, a humidity cell experiment was conducted with a mix of a high carbonate tonalite and a low carbonate pegmatite representative of the country rock surrounding a kimberlite pipe at the Diavik Diamond Mine in Canada. The alkalinity generated from the tonalite + pegmatite was evaluated through release of Ca and Sr, which indicated a primary alkalinity-generating period during the first 25 weeks of the 80-week experiment. This period represents decomposition of readily available carbonates (infillings) and phosphates (euhedral grains) that release acid-consuming CO3 and PO4 into solution. The total alkalinity—represented by the release of Ca—produced during the first 25 weeks was compared to the total acidity—represented by the release of SO4—produced during the first 25 weeks of a previous humidity cell experiment with an acid-generating (Type III) waste rock from the same site. The easily weathered carbonates and phosphates of the tonalite + pegmatite have the potential to produce sufficient net alkalinity to consume the net acid generated from the early weathering of sulfides in the Type III waste rock. Aqueous extractions performed on post-experiment samples of the tonalite + pegmatite indicate remaining carbonate and phosphate minerals that were not readily available at the experimental ≤6.3 mm size. The heterogeneity of carbonate content and availability in granitic country rock at the mine site are reflective of the variability of accessory calcite in granitoids and indicate a continued need for site-specific determination of alkalinity-generating reactions with the weathering of waste rock.

Elemental mobility in sulfidic mine tailings reclaimed with paper mill by-products as sealing materials.

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 5years after the remediation. The leaching behavior of the covered tailings was studied using batch leaching tests (L/S ratio 10L/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.

Background Conditions and Mining Pollution throughout History in the Río Tinto (SW Spain)

The Río Tinto drains the eastern part of the Iberian Pyrite Belt (IPB), an area with a huge amount of massive sulphide deposits that has been mined for the last 4500 years. This river presents extreme conditions, with very high concentrations in solution of metals and metalloids and low pH values. Mining activities in the upper part of the watershed of the Río Tinto have been documented since historical times and a huge amount of widespread acid-producing mine residues exist in this area. Nevertheless, there is no consensus among the scientific community as to whether the extreme conditions of the Río Tinto are the result of natural processes or the intense mining activity in the region. Here we show, using numerous geological, archaeological and historical records, that the present quality of the Río Tinto is the result of mining activities, especially during the period 1850–2001, while natural processes of formation of acid rock drainage can be considered negligible.

Improving Properties of Sealing Layers Made of Till by Adding Green Liquor Dregs to Reduce Oxidation of Sulfidic Mine Waste

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, ...

Isotopic signature of Cu and Fe during bioleaching and electrochemical leaching of a chalcopyrite concentrate

Bioleaching is an important process in metallurgy and in environmental sciences, either for the acquisition of metals or for the formation of acid rock drainage. In this study the implications of the processes during bioleaching of a pyritic chalcopyrite concentrate were analysed regarding its Cu and Fe isotope fractionation. The development of the redox potential during the bioleaching experiment was then simulated in an electrochemical cell in the absence of microorganisms to investigate the effect of microbial activity on the Cu and Fe isotope fractionations. The leaching experiments were performed for 28days at 45°C with a solid content of 2.5% (w/v) at pH1.5. It was found that Cu dissolution efficiency was similar in both experiments and the leaching curves were linear with no sign of passivation due to the presence of pyrite. The heavy Cu isotope (δ65Cu) was leached more easily and as a result the leachate was enriched with the heavy Cu isotope at the beginning of both experiments and as the leaching progressed δ65Cu values in the leachate became similar to the ones of the chalcopyrite concentrate, confirming an equilibrium fractionation happening in a closed system. There was no distinct difference in the Cu and Fe isotope fractionations in the absence and presence of microorganisms. Finally based on Cu and Fe isotope signatures, a simplified method is suggested for the estimation of the leaching extent during the oxidisation of sulphide materials in natural systems.

Characterization of Green Liquor Dregs, Potentially Useful for Prevention of the Formation of Acid Rock Drainage

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.

Alternative waste residue materials for passive in situ prevention of sulfide-mine tailings oxidation: A field evaluation

Novel solutions for sulfide-mine tailings remediation were evaluated in field-scale experiments on a former tailings repository in northern Sweden. Uncovered sulfide-tailings were compared to sewage-sludge biosolid amended tailings over 2 years. An application of a 0.2m single-layer sewage-sludge amendment was unsuccessful at preventing oxygen ingress to underlying tailings. It merely slowed the sulfide-oxidation rate by 20%. In addition, sludge-derived metals (Cu, Ni, Fe, and Zn) migrated and precipitated at the tailings-to-sludge interface. By using an additional 0.6m thick fly-ash sealing layer underlying the sewage sludge layer, a solution to mitigate oxygen transport to the underlying tailings and minimize sulfide-oxidation was found. The fly-ash acted as a hardened physical barrier that prevented oxygen diffusion and provided a trap for sludge-borne metals. Nevertheless, the biosolid application hampered the application, despite the advances in the effectiveness of the fly-ash layer, as sludge-borne nitrate leached through the cover system into the underlying tailings, oxidizing pyrite. This created a 0.3m deep oxidized zone in 6-years. This study highlights that using sewage sludge in unconventional cover systems is not always a practical solution for the remediation of sulfide-bearing mine tailings to mitigate against sulfide weathering and acid rock drainage formation.

Acid Rock Drainage and Rock Weathering in Antarctica: Important Sources for Iron Cycling in the Southern Ocean

Here we describe biogeochemical processes that lead to the generation of acid rock drainage (ARD) and rock weathering on the Antarctic landmass and describe why they are important sources of iron into the Antarctic Ocean. During three expeditions, 2009-2011, we examined three sites on the South Shetland Islands in Antarctica. Two of them displayed intensive sulfide mineralization and generated acidic (pH 3.2-4.5), iron-rich drainage waters (up to 1.78 mM Fe), which infiltrated as groundwater (as Fe(2+)) and as superficial runoff (as Fe(3+)) into the sea, the latter with the formation of schwertmannite in the sea-ice. The formation of ARD in the Antarctic was catalyzed by acid mine drainage microorganisms found in cold climates, including Acidithiobacillus ferrivorans and Thiobacillus plumbophilus. The dissolved iron (DFe) flux from rock weathering (nonmineralized control site) was calculated to be 0.45 × 10(9) g DFe yr(-1) for the nowadays 5468 km of ice-free Antarctic rock coastline which is of the same order of magnitude as glacial or aeolian input to the Southern Ocean. Additionally, the two ARD sites alone liberate 0.026 and 0.057 × 10(9) g DFe yr(-1) as point sources to the sea. The increased iron input correlates with increased phytoplankton production close to the source. This might even be enhanced in the future by a global warming scenario, and could be a process counterbalancing global warming.

Using sewage sludge as a sealing layer to remediate sulphidic mine tailings: a pilot-scale experiment, northern Sweden

At the Kristineberg mine, northern Sweden, sulphidic mine tailings were remediated in an 8-year pilot-scale experiment using sewage sludge to evaluate its applicability as a sealing layer in a composite dry cover. Sediment, leachate water, and pore gas geochemistry were collected in the aim of determining if the sludge was an effective barrier material to mitigate acid rock drainage (ARD) formation. The sludge was an effective barrier to oxygen influx as it formed both a physical obstruction and functioned as an organic reactive barrier to prevent oxygen to the underlying tailings. Sulphide oxidation and consequential ARD formation did not occur. Sludge-borne trace elements accumulated in a reductive, alkaline environment in the underlying tailings, resulting in an effluent drainage geochemistry of Cd, Cu, Pb and Zn below 10 μg/L, high alkalinity (810 mg/L) and low sulphate (38 mg/L). In contrast, the uncovered reference tailings received a 0.35-m deep oxidation front and typical ARD, with dissolved concentrations of Cd, Zn and sulphate, 20.8 μg/L, 16,100 μg/L and 1,390 mg/L, respectively. Organic matter degradation in the sludge may be a limiting factor to the function of the sealing layer over time as 85 % loss of the organic fraction occurred over the 8-year experimental period due to aerobic and anaerobic degradation. Though the cover may function in the short to medium term (100 years), it is unlikely to meet the demands of a long-term remedial solution.

Management of Sulfide-Bearing Waste, a Challenge for the Mining Industry

Oxidation of iron sulfides in waste rock dumps and tailings deposits may result in formation of acid rock drainage (ARD), which often is a challenging problem at mine sites. Therefore, integrating an ARD management plan into the actual mine operations in the early phases of exploration, continuing through the mine life until final closure might be successful and decrease the environmental impact. A thorough characterization of ore and waste should be performed at an early stage. A detailed knowledge of mineralogical composition, chemical composition and physical properties such as grain size, porosity and hydraulic conductivity of the different waste types is necessary for reliable predictions of ARD formation and efficiency of mitigation measures. Different approaches to prevent and mitigate ARD are discussed. Another key element of successfully planning to prevent ARD and to close a mining operation sustainably is to engage the mine stakeholders (regulators, community and government leaders, non-governmental organization (NGOs) and lenders) in helping develop and implement the ARD management plan.

Modeling the effect of stratification on cemented layer formation in sulfide-bearing mine tailings

Abstract Reactive transport simulations have been applied to investigate possible effects of stratification on the potential of sulfide-bearing mine tailings to form protective cemented layers. The simulations are based on characteristic strata found at a German tailings site, including sulfide-enriched heavy mineral layers, mica-enriched silt layers, and homogeneously mixed layers. The simulated secondary phases (jarosite, gypsum, amorphous ferric arsenate, amorphous Fe hydroxide, alunite, amorphous silica, and kaolinite) are similar to those observed in the field. Using scanning electron microscope analyses of cemented layers, it has been observed that the pore area becomes disconnected if the porosity is decreased to values below 15%, which would indicate a strong decrease in permeability. Stratification was found to play a crucial role in cemented layer formation. Cemented layers are absent or insignificant in systems with a homogeneous distribution of Fe-bearing sulfides. They are extensively developed in systems with (a) an arsenopyrite-rich layer or (b) a mica-enriched layer situated immediately below an Fe-sulfide enriched layer. The modeling results have clearly demonstrated that the key processes operating in scenario (a) are very different from the key processes in scenario (b). In scenario (a), the oxidation of arsenopyrite is followed by the precipitation of amorphous ferric arsenate, which can be solely responsible for significant pore reduction. In scenario (b), the presence of a large amount of reactive aluminosilicates (e.g. biotite and Ca-bearing plagioclase) immediately below the Fe-sulfide rich layer appears to be crucial. Key processes are extensive formation of Acid Rock Drainage (ARD) followed by enhanced (pH-driven) weathering of aluminosilicates, resulting in the accumulation of secondary phases directly below the Fe-sulfide rich layer. In both scenarios, a cemented layer is formed that effectively retards the further downward movement of the oxidation front. The presented details on the role of stratification in the formation of cemented layers could be considered in the construction of mining heaps as a possible measure to stimulate natural attenuation.

Acid rock drainage formation and treatment: a review

AbstractThe exploitation of coal and metallic mineral resources worldwide invariably results in the production of large quantities of overburden, gangue, and tailings materials containing significant amounts of sulfide minerals. These sulfide minerals, which include sphalerite, chalcopyrite, galena, and other complex sulfides, are often disseminated in pyrite, which is the most abundant sulfide mineral in the earth's crust. Once exposed to water and oxygen through mining and mineral processing operations, these sulfides become immediately susceptible to chemical and biochemical oxidation with the consequent production of highly acidic, metal‐laden leachates, which are generally referred to as acid rock drainage (ARD) or acid mine drainage (AMD). This ARD production, which can be sustained for hundreds of years, has become the single biggest environmental problem facing the mining and mineral industry. Untreated acid rock drainage leads to serious contamination of large areas of land, as well as surface and ground water resources. The seriousness of the problem has led to major research efforts to find solutions. However, effective ARD treatment and prevention solutions have eluded the scientific community over the past decades. This paper presents a detailed review of the current state of scientific knowledge with regard to the magnitude of the problem, the chemistry and mechanism of sulfide mineral oxidation and ARD formation, the role of microorganisms in ARD formation process, and the proposed approaches for the treatment, control, and prevention of ARD formation. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.

Sediment-Trap Measurements of Suspended Mine Tailings in Shallow Water Cover

Disposal of pyrites mine tailings under shallow water covers is an effective and commonly used method for reducing tailings oxidation, thus minimizing the formation of acid rock drainage. However, wind-wave-induced resuspension of tailings under the water cover could compromise suspended solids discharge criteria and increase the overall sulfide oxidation, since dissolved oxygen concentration are higher in the water cover than in the pore water. This article reports field measurements of suspended tailings collected approximately 2-10 cm above the tailings surface by means of 16 sediment traps partly buried in the tailings bed. The traps were distributed over ab area of about 600x300 m, where the water cover depth varied from 0 to 2 m. The traps were sampled on three occasions during a total study period of 93 days distributed over two field seasons. Wind speed and direction were monitored at the site during the study. Hourly average wind speeds ranged from 0 to 10 m/s. Suspended tailings were recovered in every sediment trap on each sampling event. Comparison of particle size distributions for suspended solids and bed tailings suggests that at this site and for the duration of this study, tailings erosion and resuspension occurred primarily in areas where the water cover depth was 1.0 m or less, and that suspended tailings were transported by currents to other parts of the water cover. The amount of tailings resuspension was related to the frequency of strong winds blowing in the direction of maximum fetch. Comparison of suspended solids and bed tailings compositions indicates that suspended tailings oxidized at a larger rate than bed tailings.

Large-scale experiments for microbiological evaluation of measures for safeguarding sulfidic mine waste

In the framework of a German–Romanian scientific cooperation, experiments were performed to evaluate feasible and cheap techniques for the safe storage of mine waste to prevent acid rock drainage (ARD). A large four-chamber percolator (4CP) was installed in a waste heap at Ilba Mine, Romania, to test the effect of biocides and alkaline layers on the bacteria causing acid rock drainage (ARD). The 4CP consisted of four chambers each containing 65 m 3 of sulfidic waste material. The 4CP enabled the transfer of laboratory results to a technical scale. The detergent sodiumdodecylsulfate (SDS) was proved to be active against the leaching bacteria. Organotrophic micro-organisms were not effected by the SDS application. The alkaline layers caused an increase of pH, however, a decrease of cell numbers was measured only in adjacent ore layers, but not in the whole ore body. A rapid evaluation of the effects of these countermeasures on ARD formation became possible by microcalorimetric activity measurements for bioleaching.