Atmospheric carbon sequestration in ultramafic mining residues and impacts on leachate water chemistry at the Dumont Nickel Project, Quebec, Canada

Passive carbon mineralization in ultramafic mining residues, which allows the sequestration of CO2 through carbonate precipitation, is one of the options being considered to limit the accumulation of anthropogenic CO2 in the atmosphere. The Dumont Nickel Project (DNP) will generate approximately 1.7 Gt of utramafic mining residues over 33 years of production and the mine will release about 127,700 tonnes of CO2 each year. Using two experimental cells filled with ultramafic waste rock (EC-1) and milling residues (EC-2), the impacts of carbon mineralization on leachate water quality were studied and the quantity of sequestered carbon was estimated.Hydrotalcite supergroup minerals, aragonite, artinite, nesquehonite, dypingite and hydromagnesite precipitated through atmospheric weathering, while the inorganic carbon content of the weathered mining waste increased from 0.1 wt% to 4.0 wt% which indicate active CO2 sequestration. The leachate water, sampled at the bottom of the experimental cells, is characterized by an alkaline pH (~9.5), a high alkalinity (~90 to ~750 mg/L) and a high concentration of magnesium (~50 - ~750 mg/L), which is typical from weathering of ultramafic rocks in a system open to CO2. Since 2012, the chemical composition of the leachate water has evolved seasonally. These seasonal variations are best explained by: (1) climatic variations over the year and, (2) increased carbonate precipitation between May and July. Increased carbonate precipitation decreased the alkalinity and magnesium concentrations in the leachate water and produced pore waters which were undersaturated with respect to carbonate minerals such as artinite and hydromagnesite. *Revised manuscript with no changes marked Click here to view linked References Carbonate precipitation thus self-limits carbon sequestration through a negative feed-back loop. The carbon sequestration potential of the DNP residues is also influenced by the hydrogeological properties of the residues. In cell EC-2, a high liquid/solid ratio, which limits carbonate precipitation, was maintained by the hydrogeological properties. Since 2011, an estimate of 13 kg of atmospheric CO2 has been sequestered in the milling residues (EC-2), which corresponds to a mean rate of 1.4 (+/- 0.3) kgCO2/tonne/year. Using this mean rate, the 15 Mt of tailings produced each year, during the planned 33 years of mining operation, could potentially sequester 21,000 tonne of CO2 per year by passive carbon mineralization, about 16% of the 127,700 tonnes of CO2 annually emitted by the planned mining operation

CIL Gold Loss Characterization within Oxidized Leach Tails: Creating a Synergistic Approach between Mineralogical Characterization, Diagnostic Leach Tests, and Preg-Robbing Tests

A double refractory gold ore contains gold particles locked in sulphides, solid-solution in arsenopyrite, and preg-robbing material such as carbonaceous matter, and so on. The diagnostic leach test (DLT) and preg-robbing (PR) approaches are widely used to investigate the occurrence and the distribution of refractory gold. DLT serves to qualitatively evaluate the gold occurrences within the ore. Preg-robbing, or the ore’s capacity to fix dissolved gold, is evaluated to determine physical surface interactions (preg-borrowing) and chemical interactions (preg-robbing). The objective of this project is to characterize the refractory gold in Agnico Eagle Mine’s Kittilä ore using the DLT and PRT approaches coupled with mineralogical analyses to confirm testing. The studied material was sampled from the metallurgical circuit following carbon in leach (CIL) treatment at the outlet of the autoclave in order to investigate the effect of the autoclave treatment on the occurrence and distribution of gold. Different reagents were used in the DLT procedure: sodium carbonate (Na2CO3), sodium hydroxide (NaOH), hydrochloric acid (HCl), and nitric acid (HNO3). The final residue was roasted at a temperature of around 900 ◦C. These reagents were selected based on the mineralogical composition of the studied samples. After each leaching test/roasting, cyanide leaching with activated carbon was required to recover gold cyanide. The results show that gold is present in two forms (native and/or refractory): to a small extent in its native form and in its refractory form as association with sulfide minerals (i.e., arsenopyrite and pyrite) and autoclave secondary minerals that have been produced during the oxidation and neutralization processes such as iron oxides, iron sulfates, and calcium sulfate (i.e., hematite and jarosite), along with carbonaceous matter. The results of DLT indicate that 25–35% of the gold in the tails is nonrecoverable, as it is locked in silicates, and 20–40% is autoclave products. A regrind can help to mitigate the gold losses by liberating the Au-bearing sulphide minerals encapsulated within silicates

Prediction of acid mine generating potential: Validation using mineralogy

peer reviewe

Environmental Impact of Mine Exploitation: An Early Predictive Methodology Based on Ore Mineralogy and Contaminant Speciation

Mining wastes containing sulfide minerals can generate contaminated waters as acid mine drainage (AMD) and contaminated neutral drainage (CND). This occurs when such minerals are exposed to oxygen and water. Nowadays, mineralogical work—when it is done—is independently and differentially done according to the needs of the exploration, geotechnics, metallurgy or environment department, at different stages in the mine development process. Moreover, environmental impact assessments (EIA) are realized late in the process and rarely contain pertinent mineralogical characterization on ores and wastes, depending on countries’ regulations. Contaminant-bearing minerals are often not detected at an early stage of the mine life cycle and environmental problems could occur during production or once the mine has come to the end of its productive life. This work puts forward a more reliable methodology, based on mineralogical characterization of the ore at the exploration stages, which, in turn, will be useful for each stage of the mining project and limit the unforeseen environmental or metallurgical issues. Three polymetallic sulfide ores and seven gold deposits from various origins around the world were studied. Crushed ore samples representing feed ore of advanced projects and of production mines were used to validate the methodology with realistic cases. The mineralogical methodology consisted in chemical assays and XRD, optical microscopy, SEM and EPMA were done. Five of the ores were also submitted to geochemical tests to compare mineralogical prediction results with their experimental leaching behavior. Major, minor, and trace minerals were identified, quantified, and the bearing minerals were examined for the polluting elements (and valuables). The main conclusion is that detailed mineralogical work can avert redundant work, save time and money, and allow detection of the problems at the beginning of the mine development phase, improving waste management and closure planning

Physicochemical quality of surface water: Background study prior of the Milky river sub-basin, Abitibi, Canada

In spite of its economic benefits, the mining industry can have many negative impacts on the environment, more specifically, on surface water quality. The evaluation of the impact of the mining activity must therefore be addressed adequately with a background study prior to mining operation areas. Thirteen active and past producing mines are located inside the sub-basin of the Milky. These mining sites may contribute to the water mineralization of the surface waters and sediments of the Milky river system. In the present study, nine metals including As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were measured at thirty (30) surface water locations during two years in the catchment area of the Milky river. The data collected served to calculate the Metal pollution index (MPI) and the Metal index (MI) to evaluate the surface water quality. The MI values suggest that all the samples are contaminated, while only some of the samples are considered contaminated according to the MPI values. However, it is impossible to discriminate the origin of this contamination between the natural enriched geochemical background of this sub-basin and the anthropic activities. This discrepancy between the two pollution evaluation methods demonstrate that their interpretation needs to be adapted to the context of mining districts that have both a high natural geochemical background and are affected by past and present mining activities

Three‐Dimensional Time‐Lapse Geoelectrical Monitoring of Water Infiltration in an Experimental Mine Waste Rock Pile

Open-pit mines often generate large quantities of waste rocks that are usually stored in waste rock piles (WRPs). When the waste rocks contain reactive minerals (mainly sulfides), water and air circulation can lead to the generation of contaminated drainage. An experimental WRP was built at the Lac Tio mine (Canada) to validate a new disposal method that aims to limit water infiltration into reactive waste rocks. More specifically, a flow control layer was placed on top of the pile, which represents a typical bench level, to divert water toward the outer edge. Hydrogeological sensors and geophysical electrodes were installed for monitoring moisture distribution in the pile during infiltration events. A three-dimensional (3D) time-lapse hydrogeophysical monitoring program was conducted to assess water infiltration and movement. Readings from the 192 circular electrodes buried in the WRP were used to reconstruct the 3D bulk electrical resistivity (ER) variations over time. A significant effort was devoted to assessing the spatiotemporal evolution of water ER because the bulk ER is strongly affected by water quality (and content). The water ER was used as a tracer to monitor the infiltration and flow of resistive and conductive waters. The results indicate that the inclined surface layer efficiently diverts a large part of the added water away from the core of the pile. Local and global models of water infiltration explaining both bulk and water ER variations are proposed. The results shown here are consistent with hydrogeological data and provide additional insights to characterize the behavior of the pile

Rare Earth Elements (La, Ce, Pr, Nd, and Sm) from a Carbonatite Deposit: Mineralogical Characterization and Geochemical Behavior

Geochemical characterization including mineralogical measurements and kinetic testing was completed on samples from the Montviel carbonatite deposit, located in Quebec (Canada). Three main lithological units representing both waste and ore grades were sampled from drill core. A rare earth element (REE) concentrate was produced through a combination of gravity and magnetic separation. All samples were characterized using different mineralogical techniques (i.e., quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), X-ray diffraction (XRD), and scanning electron microscopy with X-ray microanalysis (SEM-EDS)) in order to quantify modal mineralogy, liberation, REE deportment and composition of REE-bearing phases. The REE concentrate was then submitted for kinetic testing (weathering cell) in order to investigate the REE leaching potential. The mineralogical results indicate that: (i) the main REE-bearing minerals in all samples are burbankite, kukharenkoite-Ce, monazite, and apatite; (ii) the samples are dominated by REE-free carbonates (i.e., calcite, ankerite, and siderite); and (iii) LREE is more abundant than HREE. Grades of REE minerals, sulfides and oxides are richer in the concentrate than in the host lithologies. The geochemical test results show that low concentrations of light REE are leached under kinetic testing conditions (8.8–139.6 µg/L total light REE). These results are explained by a low reactivity of the REE-bearing carbonates in the kinetic testing conditions, low amounts of REE in solids, and by precipitation of secondary REE minerals

Passive Mineral Carbonation of Mg-rich Mine Wastes by Atmospheric CO2

Mg-rich process tailings and waste rocks from mining operations can react spontaneously with atmospheric CO2 to form stable carbonate minerals by exothermic reactions. Over the last decade, we have conducted a number of laboratory and field experiments and surveys on both mine waste rocks and different types of mine tailings from Ni-Cu, chrysotile, and diamond mines. The experiments and surveys cover a wide range of time (103 to 108 s) and mass (1-108 g) scales. Mine waste rich in brucite or chrysotile enhances the mineral carbonation reactions. Water saturation, but more importantly, watering frequency, are highly important to optimize carbonation. Adjusting the chemical composition of the interstitial water to favour Mg dissolution and to prevent passivation of the reaction surfaces is crucial to ensure the progress of the carbonation reactions. Preservation of the permeability structure is also critical to facilitate water and CO2 migration in the rock wastes and tailings. In field experiments, CO2 supply controled by diffusion in the mining waste is slower than the reaction rate which limits the capture of atmospheric CO2. Industrial implementation of passive mineral carbonation of mine waste by atmospheric CO2 can be optimized using the above parameters

Comparaison des essais statiques et évaluation de l'effet de l'altération pour des rejets de concentrateur à faible potentiel de génération d'acide

Les rejets de l'industrie minière -- Drainage minier acide -- Prédiction du comportement environnemental des résidus miniers -- Description générale de l'essais statique -- Description des différentes approches chimiques -- Comparaisons entre les approches chimiques -- Descriptions des différentes approches minéralogiques -- Comparaisons entre les approches minéralogiques -- Autres aspects d'intérêt -- Description des méthode et des matériaux -- Description des méthodes de caractérisation chimique -- Description des méthodes de caractérisation physique -- Description des méthodes de caractérisation minéralogique -- Description des essais utilisés -- Résultats des caractérisations des matériaux étudiés -- Comparaisons des approches de détermination du Pn -- Effet de l'altération des minéraux sur les résultats des essais statiques -- Variations minéralogiques et taux de réaction en mini-cellules d'altération