Acid Mine Drainage at the Abandoned Kettara Mine (Morocco): 1. Environmental Characterization

The Kettara site (Morocco) is an abandoned pyrrhotite ore mine in a semi-arid environment. The site contains more than 3 million tons of mine waste that have been deposited on the surface without concern for environmental issues. Tailings were stockpiled in a dyke and pond and in piles, over an area of about 16 ha, and have generated acid mine drainage (AMD) for more than 24 years. The mine waste and secondary precipitates from this mine were characterized using geochemical and mineralogical techniques. The Kettara wastes contain 1.6–14.5 wt% sulfur, mainly sulfide minerals (e.g., pyrrhotite, pyrite, chalcopyrite, galena, and sphalerite). The main gangue minerals were goethite, quartz, chlorite-serpentine, talc, muscovite, and albite. Carbonates occur at very low quantities (less than 1 wt%). The most abundant heavy metals were Cu, Zn, Cr, Pb, Co, As, Cd, and Ni. Acid–base accounting static test results showed that all the samples have low values of acid-neutralizing potential (NP) (0–9 kg CaCO3/t). The mine waste has high acid-producing potential (AP) (51–453 kg CaCO3/t). Abundant secondary mineralogy is present, consisting mainly of halotrichite, goethite, jarosite-hydroanion, hydroniumjarosite, starkeyite, gypsum, alunite, copiapite, butterite, and coquimbite. Hardpans, which can prevent water infiltration to fresh tailings beneath and thereby lessen the rate of sulfide reactivity, were observed during sampling of the fine tailings. Mineralogical analysis indicated that the cementitious phase of the hardpan is mainly goethite. The alteration observed in the tailings pond does not extend more than 5–15 cm.     

A Geotechnical Solution to Reduce Acid Mine Drainage Generation at Recreio Mine

Coal mining is frequently associated with acid mine drainage (AMD) generated by tailings and waste dumps containing sulphide minerals. A practical and economical alternative to minimise AMD generation is to avoid the contact between water and waste dumps using a compacted soil cover. This study evaluates the use of distinct raw materials as cover layers. The study area was an open pit coal mine in the south of Brazil. Geotechnical characterization, physical, chemical and mineralogical analyses were carried out on two different soils from this mine. Hydraulic conductivity tests were performed using Flexible-Wall Permeameter. The results obtained from the hydraulic conductivity tests for two compacted soils suggest their applicability as impermeable layers. Considering the operational aspects at the mine and the characteristics determined for the soils a new construction scheme for the dump site was suggested.     

Column Kinetic Tests Assessing Geochemical Behavior of Mine Wastes in the Jerada Coal District (Morocco)

The Jerada anthracite mine in Morocco was abandoned in 2001 after producing approximately 20 million tonnes of solid waste. The acid generating potential of these wastes was determined by performing tests on five samples collected from relatively older and more recent waste deposits. No carbonate minerals were identified. Pyrite was the only sulphide mineral observed and much of it had been completely transformed into Fe-oxides. Analysis of the waste indicated low levels (<1 %) of Ca and Mg, while Fe and Al concentrations generally exceeded 5 %. Modified Sobek static tests and column kinetic tests were conducted for 24 months. The static test results were not conclusive (20 <net neutralising potential <20 Kg CaCO₃/t). Leachates from the kinetic tests for three of the five samples showed an initially acidic pH, while those of the remaining two became acidic by the end of the tests. Sulfate concentrations (SO₄ ^2?) decreased over time from 5000 to 200 mg/L.     

A critical review of acid rock drainage prediction methods and practices

Failure to accurately predict acid rock drainage (ARD) leads to long-term impacts on ecosystems and human health, in addition to substantial financial consequences and reputational damage to operators. Currently, a range of chemical static and kinetic tests are used to evaluate the acid producing nature of materials, from which risk assessments are prepared and waste classification schemes designed. However, these well-established tests and practices have inherent limitations, for example: (i) best-practice sampling is not pursued; (ii) risk assessments rely on limited static and kinetic test data, thus compromising the accuracy of resulting ARD block models; (iii) static tests are completed off-site and do not reflect actual field measurements; (iv) kinetic test data do not become available until later stages of mine development; (v) waste classification schemes generally categorise materials as only three types (i.e., PAF, NAF and UC) with other drainage forms (e.g., neutral metalliferous or saline) not considered; and (vi) conventional testing fails to consider that reactivity of waste is controlled by parameters other than chemistry (e.g., microbiology, type and occurrence of minerals, texture and hardness). Thus, accurate prediction is challenging because of the multifaceted processes leading to ARD. Hence, risk assessments need to consider mineralogical, textural and geometallurgical rock properties in addition to predictive geochemical test data. Instead, a new architecture of integrative, staged ARD testing should be pursued. Better ARD prediction must start with improving the definition of geoenvironmental models and waste units. Then, a range of low-cost and rapid tests for the screening of samples should be conducted on site prior to the performance of established tests and advanced analyses using state-of-the-art laboratories. Such an approach to ARD prediction would support more accurate and cost-effective waste management during operation, and ultimately less costly mine closure outcomes.     

Valorization of acid mine drainage treatment sludge as remediation component to control acid generation from mine wastes,part 2: Field experimentation

The possibility of using acid mine drainage (AMD) treatment sludge as a cover component to control AMD generation from mine wastes was investigated through laboratory characterization and kinetic column testing (companion paper). The results showed that mixtures of sludge and waste rock, and sludge and tailings, may be integrated in an AMD prevention and control strategy at Doyon mine site (northwestern Quebec, Canada). In order to further investigate these scenarios in realistic climatic conditions, instrumented field test cells were installed on site to evaluate the performance of the mixtures to control AMD generation from tailings and waste rock under natural field conditions. The main findings from two seasons of monitoring are presented in the paper. The waste rock-sludge mixture placed over waste rock was able to reduce the generation of AMD from the waste rock, therefore confirming lab results, and was able to produce a neutral effluent with low concentrations of dissolved metals. The tailings-sludge mixture placed over tailings, with an evaporation protection layer, maintained a high volumetric water content and reduced sulphide oxidation from the tailings as exhibited by a neutral effluent. Monitoring of the field cells will continue to provide valuable information on the possible sludge valorization options.     

Valorisation of acid mine drainage treatment sludge as remediation component to control acid generation from mine wastes,part 1: Material characterization and laboratory kinetic testing

In operating mines, acid mine drainage (AMD) is often treated using lime treatment. This process generates a significant amount of sludge that contains metal hydroxide precipitates, gypsum, and unreacted lime. The sludge may have interesting geotechnical and geochemical properties to be used as a part of covers (oxygen barriers) to prevent AMD generation from waste rocks and tailings. The main results of a project aiming to evaluate the use of sludge from the Doyon mine site (Canada) as a material in mine site rehabilitation are presented. The first part of the project involved detailed characterization of sludge, waste rock, and tailings samples. Then, laboratory column leaching tests were performed to evaluate the performance of the mixtures to control AMD produced by tailings and waste rocks. It was found that a sludge–waste rock mixture placed over waste rock reduces the metal loads in the column effluent, which remained acidic, as well as a mixture of sludge and tailings deposited over tailings can reduce metal content in effluents from tailings.     

The process mineralogy of mine wastes

The discipline of process mineralogy developed through the recognition that metallurgical flowsheets could be optimised by thorough characterisation of the precursor ore mineralogy, mineral associations, grain size and textures. In a procedure analogous to process mineralogy it is shown here that effective characterisation of mine wastes for Acid Rock Drainage and Metal(loid) Leaching (ARDML) potential must follow a similar set of robust practices which include: (i) representative sampling; (ii) static/screening level geochemical tests and qualitative mineralogical assessment; (iii) longer-term kinetic geochemical tests and quantitative mineralogical assessment; and (iv) quantitative numerical modelling to assess source term chemistry associated with the mine facilities and thereby determine potential impacts to receptors. This process is dependent on a sufficiently robust drill core database and a detailed mine plan through which an assessment of mine wastes is possible. Such detailed characterisation may be limited by insufficient budgets, however omission of a thorough mineralogical investigation may lead to a lack of understanding of the primary geochemical controls on mine waste behaviour. In turn, this can lead to over- or under-engineering of mine facilities, which can have financial and/or environmental implications. Several case studies are presented to illustrate how mineralogy can be applied to solve problems in ARDML prediction and mitigation, specifically within waste rock assessment.     

Acid Mine Drainage at the Abandoned Kettara Mine (Morocco): 2. Mine Waste Geochemical Behavior

Weathering and humidity cell tests were used to predict the potential for acid mine drainage (AMD) and to estimate the mineral reaction rates and depletion of fine and coarse tailings from the abandoned Kettara mine, Morocco. The geochemistry of the fine and coarse mine wastes was similar and, as expected by static tests, the wastes produced significant amounts of AMD. The sulfate production rate of both fine and coarse tailings was very high (2,000–8,000 and 2,400–560 mg SO₄/kg/week, respectively) during the first weeks of kinetics tests. After 9 weeks, sulfate release became low, ranging between 600 and 78 mg SO₄/kg/week for fine tailings and 500–120 mg SO₄/kg/week for coarse tailings. Effluent water samples had low pH (2.9–4.2) and elevated concentrations of acidity, sulfate, iron, copper, and zinc. Most or all of the dissolved K, Na, Al, Mg, and Si in the AMD result from the acidic dissolution of silicates (chlorite, talc, muscovite, and albite). Fine tailings produce much higher concentrations of acidity and sulfate than coarse tailings. However, due to greater transport of oxygen and water within the coarse waste, coarse tailings could be of greater environmental significance than fine tailings. The coarse waste continued to release acid after 378 days of leaching, whereas the fine tailings naturally passivates. These laboratory results agree with field observations; the upper profile of the coarse waste rock dam is highly oxidized (75 cm) whereas oxidation in the fine tailings does not extend more than 5–15 cm beneath the surface. A comparison between weathering and humidity cell tests indicated that the general trend of dissolution of metals was essentially similar for both methods. However, sulfate depletion rates were higher for the weathering cell tests. These tests indicate that the Kettara tailings piles and dam will continue to release acid for a long time unless remedial action is taken.     

Kinetic Testing and Sorption Studies by Modified Weathering Cells to Characterize the Potential to Generate Contaminated Neutral Drainage

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.     

Evaluating waste rock mineralogy and microtexture during kinetic testing for improved acid rock drainage prediction

This study integrates detailed mineralogical and microtextural analyses of waste rock with the results of standard kinetic test procedures to identify the mineralogical changes that influence leachate chemistry over time. The integration of mineralogy and texture provides the opportunity for improved mine waste management strategies and acid rock drainage (ARD) prediction.Waste rock material from an abandoned gold mine in northern Queensland, Australia, was subjected to column leach kinetic testing over a 30 week period. The column feed comprised of a range of waste rock lithologies (porphyritic rhyolite, massive arsenopyrite, massive pyrite ± galena, and semi-massive polysulphide). In total, 12 individual columns were established to represent six lithologies prepared to two different size fractions (−10 mm and −4 mm). The mineralogy and microtextural characteristics of the column feed material was defined using quantitative X-ray diffractometry (QXRD), scanning electron microscopy and laser ablation (LA-ICPMS) at the start of kinetic tests, and at 5 week intervals during the length of the tests. These data were directly correlated with leachate chemistry (i.e., pH, SO₄ and select elements).Results of this study indicated that sulphide oxidation was strongly influenced by the morphology of sulphide minerals, their trace element contents, the presence of mineral micro-inclusions and galvanic interactions with other sulphide minerals. Waste rock with abundant arsenopyrite was consistently the most acid forming, and oxidised to scorodite (enriched in Zn, Pb and Cu). Pyrite was commonly As-rich as indicated by LA-ICPMS mapping. QXRD results indicated that the abundance of rhomboclase, jarosite, alunite and hydrous ferric oxides increased over time. Galena weathered rapidly to porous anglesite, particularly when in direct physical contact with pyrite. Sphalerite contents decreased consistently over the 30 weeks implying its oxidation, however few reaction products were directly observed. By week 30, the −4 mm fraction material generated lower pH leachate, higher mass release of elements and sulphate for the majority of samples. This indicates that the particle size used in kinetic tests can exert a significant control on leachate chemistry, especially in the absence of abundant neutralising minerals. This contribution demonstrates the value of integrating mineralogy and microtextural analyses during kinetic testwork to improve the interpretation of sulphide oxidation for better prediction of ARD.     

Use of permeable reactive barriers to treat acid mine effluents

Acid mine drainage (AMD) is one of the most serious environmental problems facing the Canadian Mineral Industry. The AMD results from oxidation of sulphide minerals (e.g. pyrite or pyrrhotite) contained in mine waste or mine tailings. It is characterised by acid effluents rich in heavy metals, which are released into the environment. A new acid remediation technology is presented in this article by which alkaline metallurgical residues (red mud) from the aluminum extraction industry are used to construct permeable reactive barriers (PRB) to treat acid mine effluents. This article describes column tests performed to simulate a PRB constructed using Bauxsol?, a chemically and physically treated bauxite refinery residue, as a reactive material. The results from these experiments show that these PRB are efficient in neutralising pH and removing metals from acidic mine effluents.     

Predicting Geochemical Behaviour of Waste Rock with Low Acid Generating Potential Using Laboratory Kinetic Tests

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.     

The effect of column dimensions and particle size on the results of kinetic column test used for acid mine drainage (AMD) prediction

Acid mine drainage (AMD) is known as one of the most important environmental problem of sulfide bearing rocks encountered worldwide. Several methods based on static and kinetic principles have been developed for estimation of AMD potential and determination of the contaminants concentration to the environment. Of these methods, kinetic column test is commonly performed due to its better representative of actual field conditions. However, the effect of main controlling factors such as column dimensions, the amount of material and its particle size on the results of kinetic column test were not investigated in details. Considering the importance of AMD and the limitations in conjunction with these main controlling factors, the objective of this study is to investigate the mechanism of the AMD generation and to perform this test by using samples in varied particle size and columns in different dimensions. For this purpose, columns were designed in various dimensions and samples were collected from Murgul Damar open-pit mining to be used in the kinetic column test. Several techniques were utilized to determine the mineralogical, physical and/or chemical composition of these samples before and during kinetic column tests. The variations of the pH and the concentration release rates of many elements and compounds were monitored during kinetic column tests. Statistically significant correlations were obtained between column dimensions and “lag time”. Similarly, it is determined that statistically significant correlations exist among column dimensions and cumulative mass release rates of SO₄ and the Ca + Mg + Mn.     

A brief survey of current surface waste disposal practices in the metal mining industry

ABSTRACT

Tailings ponds are a common method of mine waste disposal. Technical as well as environmental aspects of surface disposal of metal mine wastes are briefly reviewed The construction of tailings ponds require a broad range of parameters from geotechnical aspects to effluent treatment and reclamation procedures to be taken into consideration. Working with low grade deposits, which require intense grinding, in addition to the presence of sulphide minerals pose difficulties to the disposal of tailings. Government regulations call for reclamation procedures to be drawn-up prior to the commencement of the mining activities. Generation of Acid Mine Drainage (AMD) is one of the most important problem that a metal mine operation can face. Successful reclamation programs depend on an extensive and detailed water management procedure.

This paper reviews the environmental as well as technical aspects that need to be incorporated into the design and construction of a tailings pond facility to minimize and possibly eliminate potential environmental hazards. Current design of disposal facilities must comply with the existing guidelines and regulations which are also reviewed in this paper.     

Mitigating the generation of acid mine drainage from copper sulfide tailings impoundments in perpetuity: A case study for an integrated management strategy

Acid mine drainage (AMD) is one of the most serious and pervasive challenges facing the minerals industry. Current philosophy in sulfide tailings management takes an end-of-pipe approach which is yet to be shown to be sufficient to prevent post-closure impacts from AMD and guarantee “walk-away” status. An improved, integrated approach to tailings management and AMD mitigation is proposed, whereby conventional tailings are separated with the use of flotation into a largely benign tailings stream and a sulfide-rich product. The key features of this conceptual approach are outlined and partly demonstrated for the case of porphyry-type copper sulfide tailings. The significance of this approach is that it provides a basis for the identification of opportunities for the development of new process designs incorporating waste management systems for mitigating AMD in a manner consistent with the principles of cleaner production and sustainable development.     

Improving Management of Potentially Acid Generating Waste Rock

Managing the appropriate disposition of waste rock is a critical function of hard rock mining given the propensity of some materials to be acid-generating and the increasing environmental stewardship role of the mining industry. At most Newmont Mining Corporation mine sites, the net carbonate value (NCV) classification is used to partition mine waste according to acid generating potential; these are often further segregated into seven material types ranging from ‘highly acidic’ to ‘highly basic’. Although the NCV classification is generally a good predictor of acid-generating potential, humidity cell test (HCT) results for the NCV types ‘slightly basic’ and ‘inert/neutral’ were occasionally anomalous compared to the anticipated NCV-based acid-generating potential because acid generated by soluble aluminum and iron sulfates was not accounted for in the NCV test. This study was undertaken to develop an improved waste rock acid estimator using a battery of tests and to develop appropriate termination criteria for HCTs used to determine the potential for acid generation by waste rock. The HCTs were assessed for net alkalinity, pH, a molar ratio of (Ca + Mg/sulfate), and carbonate dissolution versus pyrite oxidation rates for HCTs over the 20 week period and/or the HCT forecast after 20 weeks. The results of HCTs compared with a combination of NCV, paste pH, and net acid generation (NAG) pH represents a simple tool to enhance screening run-of-mine waste for disposition to an appropriate storage location.     

Laboratory Evaluation of the Use of Alkaline Phosphate Wastes for the Control of Acidic Mine Drainage

Pyrrhotite tailings at the abandoned Kettara mine site in Morocco are producing acid mine drainage (AMD). We investigated the use of alkaline phosphate waste (APW) rock from a nearby operating open-pit phosphate mine to control the AMD. The neutralizing potential of the APW, using the Paktunc method, was estimated between 500 and 680 kg CaCO₃/t. In laboratory column tests, the addition of 15 wt% APW to the coarse Kettara tailings produced leachates with significantly lower acidities and metal concentrations than unamended controls. The high calcium concentration in the flushed solutions indicates that calcite was responsible for the neutralization. Dolomite dissolution seems to be negligible and fluorapatite was stable under the testing conditions. It was also observed that when the treated solution comes in contact with unweathered Kettara coarse tailings, the pH becomes acidic, although the metal concentrations remain low.     

Using permeable reactive barriers for the treatment of acid rock drainage

Acid mine drainage (AMD) is the most serious environmental problem facing the Canadian mineral industry today. It results from oxidation of sulphide minerals (e.g. pyrite or pyrrhotite) contained in mine waste or mine tailings and is characterized by acid effluents rich in heavy metals that are released into the environment. A new acid remediation technology is presented, by which metallurgical residues from the aluminium extraction industry are used to construct permeable reactive barriers (PRBs) to treat acid mine effluents. This technology is very promising for treating acid mine effluents in order to decrease their harmful environmental effects.     

Field Study of the Chemical and Physical Stability of Highly Sulphide-Rich Tailings Stored Under a Shallow Water Cover

Highly sulfide-rich (approximately 83 % pyrite), potentially acid-generating mine tailings were sub-aqueously deposited in the Don Rouyn old quarry pit from 1997 to 2000. The site covers approximately 7 ha near Rouyn-Noranda, Quebec. Various in situ measurements and laboratory tests were performed during the summer and autumn of 2008, 2009, and 2010 to: characterize tailings samples; monitor water quality in the final effluent, shallow water cover, and groundwater around the pit, and; study tailings erosion and resuspension. In situ measurements included the vertical profile and spatial distribution of pH, temperature, dissolved oxygen, electric conductivity, and redox potential. Suspended tailings, wind speed, and direction were monitored. Groundwater, cover water, and final effluent water samples were chemically analyzed and suspended sediment concentrations (SSCs) was determined. Physical, mineralogical, and chemical tailings properties were also determined. Results show that the quality of the groundwater, cover, and effluent waters complied with Canadian and Quebec regulations. SSCs were also within regulation limits. No association was found between SSC and hydrodynamic conditions (wind speed, fetch, etc.). Although theoretical calculations indicated a critical wind speed of at least 10 m/s for tailings resuspension, suspended sediment was observed for wind speeds at <10 m/s.     

Acid–Base Accounting Tests in Combination with Humidity Cells Help to Predict Waste Rock Behavior

Potentially acid-generating (PAG) waste rock is defined in Nevada, USA as material with an acid–base accounting (ABA) ratio < 1.2, based on the acid neutralizing potential (ANP) to acid generating potential (AGP) ratio. However, waste rock humidity cell tests (HCTs) used to define PAG often fail to release the anticipated acidity. For example, the pH, Fe, and SO₄ data from 150 lithology-specific HCTs run on Robinson Mine material for up to 108 weeks supported an empirical 0.3 ABA ratio PAG cut-off. At Turquoise Ridge, the cutoff for 49 HCTs representing current and prospective waste rock was 0.5; however, at Mine C, the ABA ratio cutoff for 63 HCTs was 1.6, demonstrating the need for a mine-specific analysis. The ABA ratio cutoff works at these three mines due to a wide range of ANP (e.g. 0.3–963 t of CaCO₃ per Kt of waste rock) and AGP (0.3–495 t/Kt waste rock at Robinson). At the Hycroft mine, the 28 HCTs had a much smaller range of ANP (0.3–71 t/Kt waste rock) and AGP (3–147 t/Kt waste rock), so no ABA ratio cutoff can be defined. At the Marigold Mine, 31 HCT and 605 paste pH measurements with an ABA ratio > 0.3 failed to generate sub-pH 6 leachate. In this case, while the ANP ranged from 0.3 to 230 t/Kt waste rock, the AGP was too low to develop a plausible ABA ratio cutoff. Post-mortem HCT mineralogy demonstrated pyrite encapsulation in quartz or calcite in waste rock with sub-1.2 ABA ratios, while the surfaces of the “micro-pyrite” particles exhibited secondary rinds of amorphous iron oxide, which may also thwart acidification.