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This article presents an approach to investigate unsaturated water flow and geochemical transport in sulphidic waste rock piles using numerical simulations with stochastically distributed material properties. The water retention curve and hydraulic conductivity function are used to represent spatial variability for water flow, while distributed geochemical parameters are used for reactive transport. The results illustrate how the spatial correlation of hydraulic properties affects the moisture distribution and flow within the pile, which in some cases creates local preferential flow paths. The numerical simulations also show how sulphide mineral oxidation rates can be influenced by the grain size, sulphide fraction and moisture content distribution, which may explain the large spatial variations observed in the composition of drainage water. Such simulations help to better understand the long-term response of a typical waste rock pile. [ABSTRACT FROM AUTHOR]

Prediction of contaminated neutral drainage using laboratory kinetic tests designed for acid mine drainage prediction is challenging because of the low metal concentrations generated by low sulfide oxidation rates. Fresh and weathered samples from the Tio mine waste rock piles were submitted to humidity cell tests. The waste rocks were demonstrated to be non-acid generating in the long term, as interpreted by conservative oxidation-neutralization curves. The results demonstrate that even though the main neutralizing minerals react differently after 25 years of natural weathering (with regard to Ca, Mg, Al, and Si release), the response of the fresh waste rocks during humidity cell leaching was very similar to those of the weathered waste rocks, when considering all the elements related to silicate dissolution, including those implicated in secondary phase precipitation. However, Ni generation was greater in the weathered waste rocks even though sulfide oxidation rates were similar, as Ni sorption properties reach saturation. Although the Ni concentrations from the leachates of humidity cell tests remain below regulated values, they are bound to increase with continued weathering if no preventive or control measures are undertaken at the site. [ABSTRACT FROM AUTHOR]

Techniques developed for acid mine drainage (AMD) prediction might not be suitable for contaminated neutral drainage (CND) generating sites. The Tio mine waste is known to generate Ni contaminated neutral drainage in some of the piles, but humidity cell tests fail to generate the Ni concentrations observed in the field. Weathering cell tests (small-scale humidity cell tests) were performed on fresh and weathered (produced 25 years ago) waste rock samples from the Tio mine containing various levels of hemo-ilmenite ore, and results were compared to humidity cell results on similar samples. The main constituents of the waste rock are the hemo-ilmenite ore and the plagioclase gangue; these minerals were purified from the waste rocks and the purified fractions were also submitted to weathering cell tests. The fresh waste rock samples were also submitted to sorption cells (modified weathering cells), which showed that the waste rocks have an important Ni sorption potential and that the sorbed phases are stable under weathering cell conditions. Even though the Ni concentrations obtained from the laboratory tests remain significantly lower than those obtained in field conditions (from field test pads and from waste rock piles), the results from the present study give important insight into the geochemical processes implicated in CND generation. [ABSTRACT FROM AUTHOR]

Five sulfide mine tailings coming from the Joutel mine tailing ponds (Quebec, Canada) were tested by the humidity cell test (30 to 52 cycles duration) and the column test (11 to 12 cycles duration). The objectives of this study were twofold. First, there was the determination of the tailings acid generation potential for site reclamation. Second, there was the kinetic test comparison for understanding the tailings geochemical behavior under different test conditions. The samples used had a wide diversity in terms of acid-generation potential, particle size distribution, and parameters influencing reaction rates. Leachates produced remained at a near neutral pH for the duration of the tests. Evolution of the main elements involved in the dissolution processes demonstrated neutralization by carbonates as a response to the acid generated by sulfide oxidation. Depletion rates given by sulfates are higher for the humidity cell tests when compared to those obtained for the column tests. This is consistent with most studies to date, the humidity cell test being considered as more severe. However, by taking the ratio between cumulative elements coming from neutralization and the ones coming from oxidation, similar curves (named herein oxidation -neutralization curves) for all tests were obtained. These results show that overall geochemical behavior of the tailings is similar at near neutral pH for both types of tests. With this interpretation method, the acid-generation potential of the Joutel tailings were tested and compared to the static test results to constrain their uncertainty zone with regard to the studied tailings. The tailings geochemical behavior (carbonate dissolution response to sulfide oxidation) at near neutral pH condition appears slightly dependent of test conditions under certain hypothesis. [ABSTRACT FROM AUTHOR]

Abstract The former Eagle and Telbel mine site (hereafter referred to as Joutel mine), located near the town of Joutel in the Nord-du-Québec (Canada) houses a tailings storage facility (TSF) that has been inactive since 1996. Fresh, unweathered tailings (beneath 10–30 cm of oxidized horizon) are characterized by an average sulfide content of 6–7 wt% and an average Fe-Mn-carbonate content of 20–40 wt%. The oxidation of Joutel's tailings under atmospheric conditions resulted in the precipitation of secondary phases such as ferric oxyhydroxides and gypsum. Accumulation of these secondary phases throughout the TSF caused cementation and agglomeration of grains, which decreased the porosity of the material in a horizon below the surface. This horizon, which is referred to as hardpan, is frequently encountered within fine, reactive tailings. Characterizations showed that hardpans have a highly compact texture. The formation of hardpans limits vertical water infiltration and oxygen diffusion and these layers greatly affect the global geochemical behavior of underlying tailings in the Joutel TSF by protecting the unweathered material from oxidation. As a result, the water quality of the TSF is largely controlled by the reactivity of the upper oxidized tailings horizon. Joutel's oxidized tailings showed an acidic behavior early during laboratory kinetic leaching tests despite the near absence of sulfides and neutralizing minerals. However, when unweathered tailings were added under oxidized tailings, the water became neutral and metal leaching rates were reduced. After over a year of laboratory leaching tests, hardpans formed within the columns and the natural phenomenon was reproduced under laboratory conditions. Graphical abstract Unlabelled Image Highlights • Oxidized tailings showed an acidic behavior despite their sulfide depletion. • Hardpan formation requires a geochemical contrast between tow tailing horizons. • Hardpan is characterized by low hydraulic conductivity (low porosity). • Hardpan controls the overall hydro-geochemical behavior of tailings impoundment. [ABSTRACT FROM AUTHOR]

Predicting the quality of mine drainage is a very important step in the management of mine wastes. A variety of useful tools are available for acid mine drainage (AMD), but new tests must be developed to improve the accuracy of predictions for contaminated neutral drainage (CND), or metal leaching. This study was performed using 0.01 M EDTA and citric acid in the leaching solutions of kinetic tests on waste rocks and low-grade ore from the BlackRock project. Results showed an increase in metal (Co, Cu, Mn, Ni, and Zn) concentrations in the leachates, except for Zn with citric acid. Several major cations (Al, Ca and Mg) were also chelated by EDTA, but did not appear to be in competition with metal chelation because the chelating agent was present in excess in the kinetic test leachates. Ca and Mg were also chelated by citric acid, but Al was not. The release rate of iron was compared to the release rate of total sulphur in the leachates. For the waste rock samples, these release rates corresponded to the dissolution of sulphides. For the low-grade ore sample, the release rate of iron was higher and likely associated with the dissolution of iron oxides. Further studies will be needed to explore how the selection and concentration of specific chelating agents for use in modified kinetic tests must be adjusted depending on the mineralogy of the samples. [ABSTRACT FROM AUTHOR]

Abstract: A numerical model is developed for investigating the transport behaviour and geochemical evolution of acidic mine drainage (AMD) in discretely fractured porous media. The simulation approach is tested using a conceptual model of a reactive mine waste system in which an active source of AMD overlies a fractured silicate-rich porous host rock with a low but non-zero matrix permeability. Source composition is based on measured data from an existing mine tailings site. The numerical model includes groundwater flow, AMD infiltration, multi-component advective-dispersive transport, equilibrium geochemical speciation and water-rock pH-buffering reactions within a discrete fracture network (DFN). An analytical solution for parallel-fractures is used to verify the model, which is then applied to simulate the evolution of pH, the major aqueous species from the AMD source, as well as selected mineral buffers. As the acidic drainage water infiltrates into the initially uncontaminated fracture networks, high concentration gradients develop within the matrix along fracture interfaces inducing diffusion-limited pH buffering and precipitation of secondary minerals within the rock matrix. A comparison of AMD evolution in three fracture networks shows that even within a densely fractured network, AMD plume evolution can be significantly different from that obtained from assuming an equivalent porous medium (EPM). The paper also addresses issues of time scales and matrix diffusion. The results have implications for predicting environmental impacts of acid mine drainage in complex mining environments and for coupling of hydro-geochemical and geotechnical models. The model can also be applied to other hydrogeological systems including fractured clays and tills, to other contaminants including hydrocarbons or organic solvents, and to simulate geochemical evolution in natural flow systems. [Copyright &y& Elsevier]

Abstract Significant volumes of waste rock are produced in mining operations, particularly in open pit operations. Waste rock is characterized by a high heterogeneity in their mineralogical, hydrogeological, and physical properties, especially in comparison to tailings. The purpose of this study was to develop an improved method to quantify the net acid-generating potential of waste rock by incorporating the degree of liberation of acid-generating and neutralizing minerals. Three lithologies were sampled immediately after waste rock blasting. Each lithology was separated into seven fractions: D < 0.053 mm, 0.053 mm < D < 0.3 mm, 0.3 mm < D < 0.85 mm, 0.85 mm < D < 2.4 mm, 2.4 mm < D < 5 mm, 5 mm < D < 15 mm, 15 mm < D < 50 mm; particles >5 cm were not considered. Mineralogical and chemical characterizations showed that sulphides, mainly pyrite, were enriched in the fine to mid-sized fractions (between 0.053 mm and 0.85 mm), and carbonates, mainly calcite, decreased as the particle size increased. Sulphides were more liberated within the fine fractions and their liberation was considered negligible at sizes >2.4 mm. For coarser fractions (>5 mm), sulphides were associated with non-sulphide gangue minerals which was confirmed by micro-computed tomography. The waste rock samples were submitted to both acid base accounting (ABA) and net acid generation (NAG) tests to evaluate their acid-generating potentials. Sixteen size fractions were non-acid generating and five were classified as uncertain. However, standard ABA and NAG tests are based on pulverized samples that don't consider the initial textures of samples. Therefore, ABA results (acid-generating potentials and neutralization potentials) were corrected using the degrees of sulphide and carbonate liberation. The corrected classifications reduced the acid-generating potential of most of the uncertain samples due to the high carbonates content in relation to sulphides. This study also defines a parameter, the diameter of physical locking of sulphides, that can be used to separate waste rock into a reactive and non-reactive fraction based on the relationship between sulphide liberation and particle-size fraction. The reactive fraction refers to waste rock likely to require active acid rock drainage (ARD) management, whereas non-reactive fraction refers to waste rock likely not require active management. Highlights • Sulphides are mainly enriched in the fine to mid-sized fraction. • Sulphides were more liberated within the fine fractions and their liberation was negligible at size >2.4 mm. • Sulphide and carbonate liberations are used to correct AP and NP of pulverized samples to consider the initial texture of samples. • Diameter of physical locking of sulphides separates a waste rock into two fractions with different reactivities. [ABSTRACT FROM AUTHOR]

The aim of this project is to assess the efficiency of pyrrhotite rich tailings desulfurization and its capacity to make mine operations more sustainable and respectful to the environment. The Raglan mine ore concentrator produces acid-generating tailings, which contain pyrrhotite, that need to be properly managed. Partial desulphurization of the total tailings produced at the end-circuit was investigated to assess the option of using desulphurized material as a component of a cover to control acid mine drainage (AMD) generation. The weathering and contaminant generation potential was investigated through kinetic testing. The objective of Part 1 of this study is to present results in terms of: i) physical, chemical, and mineralogical characterization of the Raglan Mine tailings; ii) optimization of the pyrrhotite flotation process; and iii) kinetic testing on the desulphurized tailings to predict their acid generating potential, as well as their contaminant (particularly Ni) release rates. Desulphurization succeeded in reducing the sulphur content in the tailings to less than 0.4 wt% (mainly as pyrrhotite-lizardite intergrowths). Contaminant releases complied all with Quebec environmental regulations. The tailings were proven to be non-acid generating based on quantitative evaluation of mineralogy by scanning electron microscopy (QEMSCAN) results and the calculation of absolute reactivity. [ABSTRACT FROM AUTHOR]

Abstract: A series of laboratory column tests on reactive mine tailings was numerically simulated to study the effect of high water saturation on preventing sulfide mineral oxidation and acid mine drainage (AMD). The approach, also known as an elevated water table (EWT), is a promising alternative to full water covers for the management and closure of sulfidic tailings impoundments and for the long term control of acid mine drainage. The instrumented columns contained reactive tailings from the Louvicourt mine, Quebec, and were overlain by a protective sand cover. Over a 13–19month period, the columns were exposed to atmospheric O2 and flushed approximately every month with demineralized water. A free draining control column with no sand cover was also used. During each cycle, water table elevations were controlled by fixing the pressure at the column base and drainage water was collected and analyzed for pH and Eh, major ions, and dissolved metals (Fe, Zn, Cu, Pb, and Mg). The columns were simulated using the multi-component reactive transport model MIN3P which solves the coupled nonlinear equations for transient water flow, O2 diffusion, advective–dispersive transport and kinetic geochemical reactions. Physical properties and mineralogical compositions for the material layers were obtained from independent laboratory data. The simulated and observed data showed that as the water table elevation increased, the effluent pH became more neutral and SO4 and dissolved metal concentrations decreased by factors on the order of 102–103. It is concluded that water table depths less than or equal to one-half of the air entry value (AEV) can keep mine tailings sufficiently saturated over the long term, thus reducing sulfide oxidation and AMD production. [Copyright &y& Elsevier]

Abstract: Numerical simulations of layered, sulphide-bearing unsaturated waste rock piles are presented to illustrate the effect of coupled processes on the generation of acid mine drainage (AMD). The conceptual 2D systems were simulated using the HYDRUS model for flow and the POLYMIN model for reactive transport. The simulations generated low-pH AMD which was buffered by sequential mineral dissolution and precipitation. Sulphide oxidation rates throughout the pile varied by about two orders of magnitude (0.004–0.4 kg m−3 year−1) due to small changes in moisture content and grain size. In the fine-grained layers, the high reactive surface area induced high oxidation rates, even though capillary forces kept the local moisture content relatively high. In waste rock piles with horizontal layers, most of the acidity discharged through vertical preferential flow channels while with inclined fine grained layers, capillary diversion channeled the AMD to the outer slope boundary, keeping the pile interior relatively dry. The simulation approach will be useful for helping evaluate design strategies for controlling AMD from waste rock. [Copyright &y& Elsevier]