MEND Report 2.22.2c
novembre 1996

EXECUTIVE SUMMARY

Production of Acid Mine Drainage (AMD) due to the oxidation of sulphidic minerals in the presence of water is probably the main threat to many natural ecosystems located close to existing mines. AMD is multi-factor pollutant that encompasses the effects of acidity, metal toxicity, suspended solids and salinization. The impact of AMD on the environment is also influenced by the buffering capacity of the receiving water and available dilution. The overall effect of AMD is reduction of living species in the affected area, leading to a simplification of the food chain and a significant loss of ecological stability.

Over the last two decades, various techniques have been studied and used to limit the environmental impact of AMD. One approach is to collect and chemically treat the acidic effluent before its discharge to the environment, to produce a final effluent that meets environmental criteria. The main disadvantage of this technique is the need to operate the treatment system for decades (or more) following mine closure. Another approach is to control the production of AMD, which is generated from tailings or waste rocks, by reducing the availability of one of the three main components of the oxidation reaction: oxygen, water or sulphide minerals. In a wet climate, an oxygen barrier is usually considered the best available option.

Oxygen barriers can be created in different ways. For instance, a water cover is sometimes used to limit the oxygen flux to the reactive materials. Alternate covers include those made of oxygen consuming materials or geosynthetic materials. Multi-layered systems can also be used to produce Covers with Capillary Barrier Effects (CCBE). This type of cover, made with soils and/or other suitable particulate media, involves the capillary barrier concept, which occurs when a fine-grained material is placed over a coarser one. The difference in unsaturated hydraulic characteristics of the two adjacent materials favors a high degree of saturation in the top, fine material layer. This, in turn, helps to reduce the oxygen diffusion flux to the lower layer, since a saturated porous media is a much better barrier than a dry one. However, CCBE and other multi-layered systems can be fairly expensive to construct.

To reduce the costs of such CCBE, the authors proposed the use of clean tailings, or tailings with very low sulphide content that do not generate AMD, as fine grain material in layered cover systems. This type of material is often available in mining areas close to problematic sites. Such clean tailings usually have favourable geotechnical properties that can enhance the durability and performance of CCBE. A preliminary laboratory study had shown that clean tailings, used as a water-retaining layer in a properly designed CCBE, efficiently limited the production of AMD from sulphidic tailings (MEND 2.22.2a). Subsequent to this investigation, experimental cells were constructed in 1995 to evaluate, on a larger scale and under more realistic field conditions, the performance of CCBE built with clean tailings.

This report presents the results of a detailed study on the behaviour of CCBE acting as oxygen barriers to reduce the production of AMD. A key feature of this cover system was that one of the layers consisted of "clean tailings". The availability of such type of fine grained materials in mining areas such as the Abitibi in northern Québec makes this concept quite interesting and feasible. In most cases, this can help reduce the costs and improve cover efficiency.

The report contains a general introduction on the project objectives and content, including a short presentation on the different steps leading to the final design of a cover system (see Figure 1.1). Chapter 2 includes a summary of the main results obtained during Phase I of this project, based on the work performed between 1991 and 1995 (MEND Report 2.22.2a). This preliminary study included the characterization of key hydro-geotechnical properties of the different materials (see Figure 2.2 to 2.6 and Table 2.2) and the development of an experimental procedure to evaluate the behaviour of CCBE. The procedure used laboratory columns in which one of the layers consisted of clean tailings with good water retention capacities (Figure 2.7 and 2.12). The results have shown that non-reactive (non-acid generating) tailings, when used in properly designed layered cover systems, can successfully create capillary barrier effects with the adjacent coarse-grained materials (e.g. sand), so that the fine material layer remains close to saturation at all times (Figures 2.8 to 2.11 and 2.14 to 2.22). This reduces the oxygen diffusion flux and hence the oxidation of the sulphidic minerals in the waste underlying the cover. The basic principles of a cover system using the capillary barrier concept are also explained, together with the mechanisms involved in oxygen flux reduction with increased saturation.

In 1995, Phase II of the project was initiated to better evaluate the performance of CCBE constructed with clean tailings, using field test plots. Six experimental cells were constructed on the Norebec-Manitou site (near Val-d'Or, Québec), to study the behaviour of engineered cover systems under more realistic conditions. The hydrogeological conditions and leachate characteristics were the main parameters monitored. A laboratory study was run in parallel, based on the work performed during Phase I. The preliminary results of the laboratory component of Phase II were presented in MEND Report 2.22.2b, and are updated in this report.

Laboratory work included a characterization of the various materials used in the field study. A series of column tests were run with these materials to evaluate the hydraulic behaviour of the different cover systems, and their relative performance with respect to sulphidic minerals oxidation and acidic leachate production. The key results are presented in Chapter 3. Additional details can also be obtained from the various progress reports produced over the course of the project (available from the MEND Secretariat) and in the Masters Thesis from Monzon (1998) and Joanes (1999).

The hydro-geotechnical and mineralogical properties of the materials are also presented in Chapter 3 (Table 3.1 to 3.8 and Figures 3.2 to 3.10). The materials included the clean tailings taken from the Sigma site located a short distance from Norebec-Manitou, a natural silty soil from the Val-d'Or area, a mixture of tailings with bentonite, and a well-graded sand. Details of the column experiments are presented, including layer stratification and column dimensions (Table 3.9 and Figure 3.11), and testing procedures with wetting and drainage cycles. The data obtained for water distribution (Figure 3.12 and Table 3.10), unsaturated modelling (Figures 3.14 to 3.20), leachate characteristics (Figures 3.21 to 3.27), and oxygen diffusion calculations (Figure 3.30) are shown and discussed.

Five of the seven columns were set-up with the same layered configuration as the five cells built in situ (the sixth cell was not covered - control). Two other columns were set-up with loose layers of sand and tailings, to evaluate the behaviour of hydraulically placed materials. The column tests lasted about two years and completed over 20 wetting and drainage cycles. The results confirmed the good performance of the different covers, although the efficiency varied according to the material characteristics. During the test period, little or no oxidation was observed from the reactive tailings (Norebec-Manitou) covered by the different systems.

Furthermore, the results also confirmed the validity of the numerical modelling completed for the prediction of the CCBE during the column tests. Because different materials and thicknesses were used, it has also been possible to make a comparison between the different configurations based on the relative effects of key parameters. The results clearly indicate that the various systems designed and tested were efficient, although some were more efficient than others.

Construction of a full-scale CCBE on an actual mine waste disposal site can be a difficult and expensive undertaking. For this project, experimental cells of intermediate sizes were constructed to validate the concepts and to compare different design scenarios. In Chapter 4, the six cells (including five with a cover) built during the summer of 1995 on ITEC Mineral Inc.'s Norebec-Manitou site, near Val-d'Or are described. The cell design is presented in Table 4.1 and Figures 4.1 to 4.6. Each cell was shaped like an inverted truncated pyramid. The sides of the cells were covered with a geomembrane (COEX 30 mil) to control exfiltration and lateral water inflow. A water collection system was built at the bottom of each cell and connected to an underground reservoir installed outside the cell area. The reservoirs and collecting systems were designed so that the water could easily flow out of the cell while air could not move inward. Each of the five CCBE cells had 3 layers of material placed on the reactive tailings. The top layer consisted of 0.3 m of well-graded sand while the bottom coarse material was made from 0.4 m of the same sand. The fine material layer, placed between the two sand layers, was either made of clean tailings with different thickness (for 3 cells), a natural silty soil (one cell), or a mixture of bentonite and clean tailings (one cell). The sixth cell was left uncovered and served as the control. The sulphidic tailings placed at the bottom of the cells were taken from the Norebec-Manitou site, located a few hundred meters from the test plots. The materials used for each cover were placed and densified to a specific porosity: about 0.44 for the different fine material layers; and approximately 0.32 for the sand layers. Photographs taken during construction of the cells are presented at the end of Chapter 4.

The same materials were used in the laboratory and field experiments; their main properties are given in Chapter 3. The natural silt (till) and the clean tailings used for the fine-grained, water- retaining layers have similar geotechnical properties. Their grain size distribution, measured saturated hydraulic conductivity k_sat (between 10^-4 and 10^-5 cm/s) and AEV (approximately between 15 and 40 kPa) are typical of silty soils. The clean tailings and the till were not acid generating. The coarse material was a typical concrete sand with a saturated hydraulic conductivity between 3 x 10^-2 cm/s to 5 x 10^-2 cm/s and an air entry value of about 2 kPa. This created the necessary contrast with the fine-grained material to have a capillary barrier effect. The sulphidic tailings, on the other hand, was a relatively coarse material with most particles corresponding in size to the sand fraction. Their saturated hydraulic conductivity was between 10^-3 and 10^-4 cm/s and their AEV was between 7 to 11 kPa. The sulphidic tailings contained about 5% pyrite and were considered acid generating, with a Net Neutralization Potential (NNP) of -88 kg CaCO₃/tonnes (see Mineralogical Analysis Report from Louis Bernier in Appendix).

The covers on the experimental cells were instrumented (Figure 4.8) and monitored between August 1995 and November 1998 (at which time the cells were disassembled). The main parameters monitored included volumetric water content q , matric suction Y , oxygen flux, and chemical composition of the leachate. The monitoring equipment used for measuring each parameter was selected after an extensive literature review and discussions with other organizations that had used such instrumentation for monitoring purposes. The time domain reflectometry (TDR) technique was used to measure the volumetric water content in the cover layers. This technique had been used successfully for the previous laboratory experiments. Three-wire probes were placed during construction at predefined locations within the covers (see Figure 4.8). The 25 probes installed were linked to a control panel and data acquisition system, which allowed regular measurements.

The matric suction was also measured in each of the three layers of the different covers, using both Watermark sensors and Jet Fill tensiometers. The measurements made over the years have shown that Watermark sensors gave results quite similar to the tensiometers. In this study, most of the matric suction results were obtained with the Watermark sensors because of the simplicity associated with using this type of equipment. Each of the sensors was located close to a TDR probe to allow a comparison between the volumetric water content and matric suction relationships obtained in the laboratory and in situ. A good correlation was obtained between field and laboratory results.

Stainless steel tubes were installed (not shown in Figure 4.8) to evaluate the efficiency of the CCBE to limit oxidation. These tubes, installed vertically at the surface of the cover, can act as gas chambers for measuring oxygen flux through the cover. This method, called Oxygen Consumption Method, consists of measuring, within a few hours, the decrease in oxygen concentration in the gas chamber and to convert the relationship between concentration of oxygen and time into an oxygen flux.

The design of the cells also allowed collection of the percolated water in the underground reservoirs. Chemical characteristics of the water were determined during and after sampling. Parameters measured were pH, conductivity and, occasionally, sulphates and metals contents.

Detailed progress reports were submitted to MEND on a regular basis throughout the project; these are summarized and presented in a concise format in this final report. For instance, the volumetric water content measurements (q vs. depth) for the five covered test cells are shown for different time periods in Figures 4.9 to 4.33. As expected, the value of q was low in the two sand layers (usually between 0.05 to 0.15) and high in the fine layer (usually above 0.33). Such distributions are typical for covers using the capillary barrier technology, and the results are comparable to those obtained from numerical calculations (Section 4.6.3 and Figures 4.96 to 4.112). For cells 1, 2, 3 and 5, the degree of saturation, corresponding to the measured volumetric water content, was usually above 85% to 90%. For cell 4, the degree of saturation was usually between 65 to 80% and was less than the other cells. This is likely due to the use of a tailings-bentonite mixture, which prevented the full hydration of the fine layers.

Suction (y ) measurements taken at the same time as the q values are presented in Figures 4.34 to 4.58. The suction values were usually less than 5 kPa in the bottom sand layers, between 5 to 15 kPa in the fine grain material layers and varied from 1 to 23 kPa in the top sand layers. The suction values in the fine grained layers were generally less than the AEV for these materials; this explains why the degree of saturation remained high in each of these layers. A high suction value was sometimes obtained in the top sand layer, mainly due to evaporation. However, the results also indicate that, as expected, the top sand layer efficiently limited the evaporation from within the fine layer. Thus, it plays a key role in maintaining cover efficiency to reduce oxygen flux.

Water quality monitoring results are presented in Figures 4.61 to 4.71. The water collected in the reservoirs from cells 1, 2, 3 and 5 stayed above pH 6 for the entire experiment. The pH of the water from bottom of cell 4 was above 6 for the first three years, but dropped to values ranging from 5 to 6 during the last year. The control cell leachate pH confirmed that the sulphidic tailings were highly acid generating, with an initial pH of 6 that dropped to values around 2 at the end of the project. The conductivity measurements are consistent with the pH measurements; high values were obtained for the seepage water of the control cell (from » 10,000 to 50,000 micro-ohms) while lower values were measured for the covered cells (between 2,000 to 3,000 micro-ohms).

Sulphates are produced by sulphide minerals oxidation. The cover performance can also be evaluated from the sulphate concentration in the seepage water. The results showed that there was a difference of about two orders of magnitude in the sulphate concentration between the control cell and the covered cells. This means that, even if the precipitation of secondary minerals (like gypsum and jarosite) in the control cell are ignored (these minerals were nevertheless observed), the reduction of the oxidation flux was about two orders of magnitude. The relative performance of covers can also be compared on the basis of metal concentrations in the leachate water from the control cell and from the covered cells. For cells 1, 2, 3 and 5, results indicate a reduction in metal concentrations of about 3 to 4 orders of magnitude for zinc and iron and between 2 to 3 orders of magnitude for copper. The reduction is lower for cell 4, and this also confirms that this cover was somewhat less efficient than the others.

A recently developed method to determine the rate of sulphide oxidation, the Oxygen Consumption Method, was investigated for the covered cells. Four series of tests were completed during the research project. The main results are presented in Table 4.3. The first series of tests were performed after construction was completed, in October 1995, by University of Waterloo personnel. These results were higher than subsequent measurements made by the authors in 1996 and 1997. The lower values measured in 1996 and1997 can be partially explained by the fact that the systems needed a certain amount of time after construction to establish the baseline profiles for moisture and oxygen concentration.

The test results from 1996 and 1997 have shown that the oxygen fluxes through the covers were usually lower than 15 moles/m²/year and often lower than 3 to 5 moles/m²/year. It is important to note, however, that the accuracy of this technique is better defined for high consumption rates than for the lower rates, such as those determined in this project. Nevertheless, these tests, combined with all other results, confirm that the oxygen fluxes were greatly reduced by the use of a properly designed CCBE.

The last component of this research project dealt with the financial aspects for the implementation of the CCBE technology. In Chapter 5, a relatively simple model, called ECR (for Evaluation of the Cost for Reclamation) is presented. The model components are introduced and explained, and further illustrated using specific Tables and Figures taken from the Excel File, which is included with the report (ECR.1). The model allows a relative comparison of the costs incurred by the application of various techniques, namely: chemical treatment (lime) of AMD (see Figure 5.1), the use of a water cover with impervious dams (Figures 5.5 to 5.6), the construction of a CCBE made of soils and/or clean tailings (Figures 5.5 and 5.8) and the inclusion of a desulphurization process to produce "non-acid generating tailings" that could be used as the fine material layer in a cover system (Figure 5.10). Each technique is explained with sufficient detail to allow the interested reader to apply the model and make preliminary calculations for site specific applications. To further help understand the model and its application, three sample case studies are presented (Table 5.2). The model aims at providing the reader with a simple tool to obtain preliminary estimates of various techniques and to help select the most promising technology for further site-specific investigations. It is shown, in the process, that CCBE made of clean tailings can be a very competitive closure technique to control AMD (Tables 5.4, 5.7, 5.10 and 5.11).

The main conclusions and recommendations for future work are presented in Chapter 6. Considering the successful results obtained with this project, and the full-scale application of this concept at Les Terrains Aurifères (MEND Report 2.22.4a - Construction and Implementation of a Multi-Layer Cover - LTA) it is recommended that further field investigations be pursued to better correlate calculated results and field observations. The ongoing project at the crown-owned (Québec) Lorraine site (Temiscamingue, Québec) is another good example of the in situ work presently underway.

The Canadian mining industry depends on the successful full-scale application of various closure technologies, including the CCBE, to help in the decommissioning of new and old mine sites. Results from these studies will provide options and direction for future applications.

Last Modified: 2003-11-26		Important Notices

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