Benoît Plante

Prediction of Acid Mine Drainage: Importance of Mineralogy and the Test Protocols for Static and Kinetic Tests

AbstractStatic tests, which compare the acid-generating potential and acid-neutralizing potential for a given mine waste (tailings or waste rocks), are characterized by a wide uncertainty zone in which it is impossible to accurately predict the acid-generating potential (AGP). Then, to better assess long-term AGP, kinetic tests are usually performed to provide more information about the reaction rates of the acid-generating and acid-neutralizing minerals. The present work compares the classic Sobek static test with three mineralogical static tests to assess the importance of sample mineralogy in acid mine drainage (AMD) prediction. We also investigated how experimental procedures related to static tests can influence prediction results. We used three synthetic tailings samples made by mixing well-characterized pure minerals in calibrated proportions. Although basically different in their principles and procedures, the modified Sobek and mineralogical static tests gave similar results. These AGP predictions were then validated by the use of a kinetic test. The kinetic test protocol was also modified in this study and the results obtained correlated well with the static test results, in contrast to the standard kinetic test protocol. The present work highlights the limitations of static and kinetic test procedures, and provides recommendations for a better use of these tests for more reliable AMD prediction. ZusammenfassungStatische Versuche, welche das säurebildende und -neutralisierende Potential von Bergbauabfällen bestimmen, weisen z. T. so große Unsicherheitsbereiche auf, dass das exakte säurebildende Potential (acid-generation potential, AGP) sehr schwer ermittelbar ist. Um das langzeitlich wirksame AGP zu bestimmen, werden häufig kinetische Test durchgeführt, um so die Reaktionsraten der säurebildenden und neutralisierenden Minerale zu ermitteln. In dieser Arbeit wird der klassische statische Versuch nach Sobek mit drei weiteren statischen Versuch verglichen, um die Bedeutung der mineralogischen Zusammensetzung der Proben bei der Vorhersage der Bildung von sauren Grubenabwässern zu untersuchen. Gleichzeitig wird betrachtet, wie sich die Versuchsbedingungen auf die Resultate auswirken. Hierzu werden drei künstliche hergestellte Mischproben verwendet, welche aus gut charakterisierten reinen Mineralen bestehen. Obwohl sich sämtliche Testvarianten hinsichtlich ihrer Grundlagen und Versuchsverläufe unterscheiden, führen sie zu vergleichbaren Ergebnissen. Die auf dieser Grundlage ermittelten AGP-Prognoseergebnisse wurden im Anschluss mit Hilfe eines kinetischen Versuchs validiert. Im Gegensatz zu standartgemäßen Versuchsverlauf wurde der kinetische Test verändert, was jedoch die Vergleichbarkeit mit den statischen Versuchen erhöht. In der vorliegenden Arbeit werden die Grenzen von statischen und kinetischen Versuchsanwendungen aufgezeigt und Hinweise zur Verbesserung der Versuche abgeleitet. Dies führt zu vertrauenswürdigeren Ergebnissen bei der Vorhersage der Bildung von sauren Grubenabwässern.ResúmenLos ensayos estáticos que compararan la capacidad de generación de ácido y la capacidad de neutralizarlo para cierto residuo minero (colas o rocas residuales) están caracterizados por una amplia zona de incertidumbre en la cual es imposible predecir adecuadamente la capacidad de generación de ácido (AGP). Luego, para un mejor relevamiento del AGP a largo plazo, los ensayos cinéticos se realizan usualmente para dar más información sobre las velocidades de las reacciones de generación y neutralización de la acidez que presentan los minerales. Este trabajo compara el ensayo estático clásico de Sobek con tres ensayos estáticos mineralógicos para relevar la importancia de la mineralogía de la muestra en la predicción de AMD. También investigamos cómo los procedimientos experimentales de los ensayos estáticos pueden influir en la predicción de los resultados. Usamos 3 muestras de colas sintéticas preparadas mezclando en proporciones definidas minerales puros bien caracterizados. Aunque difieren en sus principios y procedimientos, los ensayos estáticos mineralógicos y los estudios modificados de Sobek dan resultados similares. Estas predicciones AGP fueron luego validadas utilizando un ensayo cinético. El protocolo del ensayo cinético también fue modificado y los resultados obtenidos correlacionaron bien con los resultados del ensayo estático, en contraste con el protocolo estándar del ensayo cinético. Este trabajo clarifica las limitaciones de los procedimientos de los ensayos estáticos y cinéticos y aporta recomendaciones para un mejor uso de estos ensayos para una más exacta predicción del AMD.摘要静态试验用以预测某给定废矿石(尾矿或矸石)的产酸和酸中和潜力,试验方法具有不确定性,难以准确预测废矿石的产酸潜力(AGP)。动力学试验能够为更好地评价废矿石长期产酸潜力提供更多的产酸和酸中和反应速率信息。本文对比了经典索贝克静态试验(classic Sobek static test)和3种矿物学静态试验(mineralogical static tests)结果,以评价在酸性矿井水预测中样本矿物性质的影响。同时,研究了静态试验过程如何影响产酸预测结果。3种人工合成尾矿由纯矿物按标准比例混合而成。虽然改进的索贝克(modified Sobek)和矿物学静态试验(mineralogical static tests)原理和步骤不同,但它们的试验结果相似。动态试验验证了产酸潜力(AGP)预测的可靠性。本研究中的动态试验也为改进试验,动态试验结果与静态试验结果具有很好的一致性。文章突出了静态和动态试验方法的局限性,提出了合理利用这此试验方法进行酸性矿井水预测的建议。.

Geochemical investigation of the galvanic effects during oxidation of pyrite and base-metals sulfides

Predicting the water quality at mine sites is of significant importance for developing mines with respect for the environment. Acid mine drainage (AMD) occurs when sulfides are in contact with oxygen and water, and several parameters and mechanisms influence final drainage quality. Galvanic interactions influence the reactivity of sulfide minerals, which act as semi-conductors. These galvanic interactions have been insufficiently studied in the context of AMD generation. In this study, the influence of pyrite on the reactivity of sphalerite and chalcopyrite was investigated. Five blends, comprised of free grains of quartz/pyrite, quartz/chalcopyrite, quartz/sphalerite, quartz/pyrite/chalcopyrite, and quartz/pyrite/sphalerite, were subjected to geochemical testing. Five weathering cells were monitored over a 200-day period during which they were leached twice weekly. Leachates were analyzed for pH, Eh, electrical conductivity, and sulfate and metal concentrations. The results of these analyses showed that galvanic interactions occurred between free sulfide grains. Pyrite was galvanically protected over the full testing period in the quartz/pyrite/chalcopyrite blend, and partially protected in the quartz/pyrite/sphalerite blend. Moreover, the release of Cu from chalcopyrite and Zn, Mn, and Cd from sphalerite was accelerated in the presence of pyrite. This work provides a better understanding of the influence of pyrite on chalcopyrite and sphalerite reactivity by highlighting the galvanic effects. In the future, to improve the reliability of AMD prediction tests, galvanic interactions should be considered in both the prediction of the acid generation potential and the estimation of metal and metalloid release rates.

Geochemical behavior of ultramafic waste rocks with carbon sequestration potential: a case study of the Dumont Nickel Project, Amos, Québec

The geochemical behavior of ultramafic waste rocks and the effect of carbon sequestration by these waste rocks on the water drainage quality were investigated using laboratory-scale kinetic column tests on samples from the Dumont Nickel Project (RNC Minerals, QC, Canada). The test results demonstrated that atmospheric CO2 dissolution induced the weathering of serpentine and brucite within the ultramafic rocks, generating high concentrations of Mg and HCO3− with pH values ranging between 9 and 10 in the leachates that promote the precipitation of secondary Mg carbonates. These alkaline pH values appear to have prevented the mobilization of many metals; Fe, Ni, Cu, and Zn were found at negligible concentrations in the leachates. Posttesting characterization using chemical analyses, diffuse reflectance infrared Fourier transform (DRIFT), and scanning electron microscope (SEM) observations confirmed the precipitation of secondary hydrated Mg carbonates as predicted by thermodynamic calculations. The formation of secondary Mg carbonates induced cementation of the waste particles, resulting in the development of a hardpan.

Tracer tests to evaluate hydraulic residence time in limestone drains: Case study of the Lorraine site, Latulipe, Québec, Canada

Acid mine drainage (AMD) remains one of the major environmental problems for the mining industry. When AMD is produced on a mine site, passive treatment techniques such as limestone drains (LDs) can be used to improve water quality, particularly when the effluent is relatively small. In 1999, LDs were installed in combination with a cover with capillary barrier effects to rehabilitate the abandoned acid-generating Lorraine mine site in Quebéc, Canada. However, the quality of the water exiting the LD does not meet the local regulations (even if a significant improvement has been observed). To better understand the behaviour of the drains, the hydraulic residence time (HRT) was evaluated using various tracer tests. Tests’ results indicate that the HRT in the Lorraine LDs is close to the minimum value targeted at the design stage but is significantly different than those estimated from the geometrical characteristics and porosity of the drains and the water flow discharge.

A contribution to improve the calculation of the acid generating potential of mining wastes

Mine wastes from sulfide-bearing ore extraction and processing are often stored at the surface of mine sites and could generate mine drainage. Prediction tests are completed to predict the water quality associated with the deposition of mining wastes. Static tests can quickly assess the acid-generating potential (AP) and the neutralization potential (NP). Whereas some studies recommend to take into account a mineral reactivity factor for the NP determination, the reactivity rates of acidifying minerals are not considered in the AP calculation. The aim of this study is to bring contribution to the improvement of the static test determination by adding kinetic factors in the AP determination. Eight sulfides (pyrite, Ni-pyrite, pyrrhotite, Ni-pyrrhotite, chalcopyrite, galena, sphalerite, arsenopyrite) and a sulfosalt (gersdorffite) were separately submitted to kinetic tests in modified weathering cells. This test was selected for its rapidity of results and for the low amount of material used, as it is somewhat difficult to obtain pure minerals samples. Five synthetic tailings were composed by mixing pure sulfides in various proportions and submitted to the same kinetic tests. The oxidation rates of synthetic tailings were compared with the weighted combined oxidation rates of individual pure sulfides. The oxidation rates of the synthetic tailings calculated from those of pure sulfides are within the same order of magnitude than those obtained through the kinetic experiments. The AP of synthetic tailings were calculated according to standard equations of the literature and compared with the new method.

Mobility of rare earth elements in mine drainage: Influence of iron oxides, carbonates, and phosphates

The geochemical behavior of rare earth elements (REE) was investigated using weathering cells. The influence of sorption and precipitation on dissolved REE mobility and fractionation is evaluated using synthetic iron-oxides, carbonates, and phosphates. Sorption cell tests are conducted on the main lithologies of the expected waste rocks from the Montviel deposit. The sorbed materials are characterized using a scanning electron microscope (SEM) equipped with a microanalysis system (energy dispersive spectroscopy EDS) (SEM-EDS), X-ray diffraction (XRD), and X-ray absorption near edge structure (XANES) in order to understand the effect of the synthetic minerals on REE mobility. The results confirm that sorption and precipitation control the mobility and fractionation of REE. The main sorbent phases are the carbonates, phosphates (present as accessory minerals in the Montviel waste rocks), and iron oxides (main secondary minerals generated upon weathering of the Montviel lithologies). The XANES results show that REE are present as trivalent species after weathering. Thermodynamic equilibrium calculations results using Visual Minteq suggest that REE could precipitate as secondary phosphates (REEPO4).