MEND Report 2.22.2a
mars 1996

Summary

Although the mining industry provides an essential contribution to the economy of several provinces across Canada, it is recognized that mining operations can also be the source of various detrimental effects for the environment. In that regard, probably the most serious problem associated with mining activities is acid mine drainage (AMD). Such AMD can be generated when sulphuric minerals (mainly iron sulphides such as pyrite and pyrrhotite) are oxidized in the presence of water. Acid waters may contain high levels of potentially toxic elements, such as lead, cadmium, mercury and arsenic, and this constitutes a serious hazard for the local ecosystems.

The control of acidic effluents during and after mining operations is often very costly. Although water treatment is an efficient process, used with success by mines for decades, it can become a heavy financial burden on any mining company faced with the prospect of having to control water quality for tens if not hundreds of years.

One possible alternative is to control the production of AMD. This approach is often considered when one wishes to reclaim the land and return it to a productive state. Among the few techniques available for that purpose is the use of covers (or caps) installed over existing tailings ponds. It is also one of the most practical options. However, building such covers is very expensive, with most estimates above 200 000 dollars/hectare. In order to reduce these costs, the authors have proposed the use of tailings, free of acid generating minerals, to built such covers. This option would be advantageous for various reasons, including the fact that such materials are often available close to the site being rehabilitated. It is also an interesting option for mills that treat ores free of sulphides, since the resulting non-acid generating tailings could be used as a cover for the acid generating tailings. Another application is related to recent projects where mining companies have used a separation technique to produce sulphide-free tailings, as these could also be used in a cover system.

If a tailings pond is to be reclaimed, it is advisable to stop production of AMD. It is often considered that a cover that limits the flow of oxygen and/or water is one of the most practical approaches for that purpose. A cover is called "wet" when water is used to submerge the Wags, thus reducing oxygen flux to negligible levels. However, such water covers may be difficult to build and maintain over time, as topography and long term stability of the dams become key factors to the success of the project.

One could also make use of geosynthetics (geomembranes) as an impervious layer in a cover, but costs and durability are major concerns.

Because there is a great deal of experience available from the use of so-called "dry" covers built from geological materials, mostly for industrial and municipal wastes, these are often considered for reclamation projects of acid generating milling wastes. Such covers are not free of potential problems either, but they can represent the most practical solution available to mining companieswho are reclaiming their tailings impoundments.

To efficiently control the generation of AMD, it is now generally accepted that a multilayer barrier system, with each layer having its own specific function, should be used. A schematic representation of a multilayer cover is presented on Figure 1. 1. The cover layers encountered, starting with the uppermost, are as follows : a humid layer to support vegetation (layer A, thickness t > 15 cm); a coarse material layer containing a large portion of cobbles to prevent biological intrusions from roots and animals (layer B, t > 30 cm); a sandy material acting as a drainage layer (layer C, t > 30 cm); a fine grain material acting here as a moisture retention zone (layer D, t = 50 to 150 cm); and a non-capillary layer (layer E, t > 30 cm) to stop capillary rise form the underlying reactive tailings (layer F) and to prevent significant moisture drainage from the fine material layer above (layer D). Each adjacent layer of the cover should satisfy filter criteria to prevent particles migration that could affect the integrity of the barrier. In this multilayer structure, the two coarse grain material layers (layer C and E) placed adjacent to the capillary layer (D) play a double role. First, these materials (typically sands) provide a flow path for the water to the drainage zones built around the site. Second, the grain size contrast with the fine grained material produces a large difference in suction properties which minimizes moisture drainage and maintains the middle layer close to saturation. It is essential that a saturation ratio of close to 100 % be maintained in this capillary layer to provide an efficient barrier to oxygen transport into the underlying reactive tailings.

In this composite cover, the possibility exists of using various ~g wastes for the construction of the different layers. For example, the tailings fine fraction (slimes), obtained by natural segregation or by hydrocyclones, could be used to build the capillary layer (layer D on Figure 1.1). The coarse tailings fractions (sands) could then be used in layers C and E, depending on their availability and hydrogeological properties. Layer B could include cobbles found in the overburden or waste rock from the mine. Finally, humid layer A could be made with the excavated overburden soil, with the original topsoil (stacked and protected) used as the final vegetative layer.

Because the efficiency of such a cover system depends on its effectiveness to reduce water infiltration and/or oxygen flux, the most critical component is the material used for layer D. This experimental study concentrated on finding lower cost materials for this moisture retention layer. Tailings with the correct hydro-geotechnical properties may be used. Samples recovered from various sites located in the province of Québec have been studied as possible candidate materials.

This report contains six (6) Chapters. Chapter 1 summarized above, presents an introduction on the overall problem of AMD and the general principals behind the use of covers. Chapter 2 is a state-of-the-art review on cover technology that considered not only mining related projects but also other types of waste where covers have been installed, including landfills, industrial refuse piles or contaminated soils. Chapter 3 reviews the capillary barrier effects created in layered covers. Material properties, including mineralogical composition, grain size, compaction curves, consolidation characteristics, hydraulic conductivity, moisture retention curves and the effective diffusion coefficient of oxygen, are presented in Chapter 4. Chapter 5 presents the physical and numerical modelling work, and the conclusions follow in Chapter 6.

The reader is reminded that this report summarizes interim reports containing more than 600 pages already submitted to , which include all the details of the testing program. These reports are available from the Secretariat in Ottawa. Also, some of the more fundamental portions of this research were the subject of several graduate thesis and internal reports.

Cover systems are used in various waste site remediation projects, and may serve different functions. They form an essential component in the overall management of liquid and gas in and out of the disposal site. One major reason for building covers is to separate the wastes (industrial, municipal, mining, etc.) from the surface environment, to limit water infiltration and/or to control gaz flow from/to the wastes. Site specific characteristics must be considered for cover design to meet the requirements of a project. However, there are some basic principles that must be understood before undertaking any cover design. In that regard, one should be up to date on the enormous amount of experience and practical information on cover applications disseminated in the literature, and summarized Chapter 2. After presenting the basic concepts in the use of cover systems, the authors describe different configurations, including materials, thicknesses and functions, of cover systems. Advantages and limitations of the different cover systems are also given.

In composite cover systems, capillary barrier effects are created when a coarse grain material is placed below a fine grain material. The difference in moisture retention curves and hydraulic conductivity functions between these materials creates conditions that allow the fine material to remains practically saturated at all time. In Chapter 3, this phenomena is explained using continuity conditions for pressure and flux at the interface between two materials. The analysis shows that capillary barrier effects are favoured by large contrasts in grain size between two adjacent materials.

Early in the experimental program, a general testing protocol was developed to evaluate the efficiency of different materials and configurations used in cover systems. It includes the evaluation of hydro-geotechnical properties, physical modelling and numerical calculations. These components are presented in Chapters 4 and 5. Results are summarized below.

At the beginning of the project, more than 30 different tailings sites in Qu6bec (most of which being located in the Abitibi region) were sampled. After completing a series of prelimiting tests, including mineralogical analysis, grain size and Atterberg limits, five sites where selected and further sampled for more detailed studies. The grain size curve of these Wags are shown in Figure 4. 1. These are representive of average grain size curves for hard rock mine Wags. Using the Unified Soil Classification System, these materials are classifiedes sandy silts or silty sands with low plasticity.

Tailings sampled in bulk were homogenized and submitted to various laboratory tests. Tables 4.1 to 4.3 presents some basic properties of the tailings. Consolidation characteristics, were obtained using a conventional oedometer apparatus. For these tests, the densification energy for placement of the material was controlled to obtain an initial void ratio e that could be varied from 0.5 to 1.1. The required densification energy was determined from compaction tests. Figure 4.3a shows some typical results. For the different tailings, the observed compression index C_c varied from about 0.05 to 0. 15 and the coefficient of consolidation c, was found to be between 10^-3 and 10^-1 cm²/s (see Table 4.4). The consolidation properties of the homogenized tailings are well within the range of what is usually found for similar materials (i.e. sandy silts or silty sands).

The hydraulic conductivity k is one of the most important properties of any material used in a cover system. To evaluate k, three different tests were carried out on the homogenized tailings. They are : rigid wall permeameter tests with constant head and falling head conditions; permeability tests in the oedometer cells with constant total stress and varying water pressure; and flexible wall permeability tests. These tests were carried out on tailings for different void ratios. This allowed an evaluation of the effect of different factors on the k value. Among the existing relationships established to quantify the influence of these factors, it was found that the Kozeny-Carman equation (Eq. 4.1) described the observed behavior fairly well. Figure 4.4a shows a correlation between the measured and calculated values for one of the studied tailings. Other relationships have also been used.

The practicality of such type of relationship is that it allows an approximate evaluation of the hydraulic conductivity of homogenized tailings, and its evolution as a function of the void ratio and for other parameters as shown in Eq.4.1. lie measured and calculated k values are given for total saturation (S_r = 100 %). The k values are corrected for unsaturated conditions, using the moisture characteristic curve.

The moisture characteristic curve of the homogenized tailings, which gives the relationship between the volumetric moisture content and the negative pore pressure (or suction) was measured using a pressure plate apparatus and a modified Tempe cell. Typical results are shown on Figure 4.6a. The results indicate that typical Air Entry Values (AEV) range from 1.5 to 3.5 m (about 15 to 35 kPa). The results are well described by the van Genuchten model (Eq. 4.5).

The ability to control oxygen transport is among the most critical cover characteristics that play an important role in the efficiency of the system. It is considered that oxygen flux is usually controlled by Fickian type diffusion (Eq. 4.6 to 4.8) and that pressure and temperature gradient effects are negligible. In a Fickian flow, the oxygen flux is largely dependent upon the effective diffusion coefficient of oxygen D, which in turn is related to grain size, porosity, tortuosity and volumetric water content. This latter factor is very important, as the diffusion coefficient in water is about 10 000 times lower than in air. Unfortunately, the precise measurement of D_e, as a function of the above noted parameters, is not simple. A special setup shown on Figure 4.7 was created with the help of the Noranda Technology Centre (NTC). The values of D_e, obtained by comparing the evolution of oxygen concentration measured and calculated with the POLLUTE program, are shown on Figure 4.8a with predictive models (Eq. 4.9 to 4. 1 1). The results are in fair agreement with the theoretical estimate .

The behavior of the homogenized tailings materials in cover systems has also been investigated by using physical and numerical models. The hydraulic conditions in layered systems was first studied using a plexiglass drainage column with an internal diameter of 15.5 cm and a height of 1 10 cm (Figure 5. 1). The column is instrumented with tensiometer and TDR ("time domain reflectometry") probes to measure suction and volumetric water content, respectively, along its length. The column design was based on the ones used at NTC and University of Waterloo for other cover projects. Results of the drainage column tests are compared to results obtained on individual materials in capillary tests and to numerical calculations (Figure 5.3b). The results are in accordance with the project assumptions, and show that the fine layer will remain close to saturation even after long periods of drought.

The efficiency of different cover systems placed over sulphide tailings was also evaluated using plexiglass columns of 1.7m in height. Duplicate columns were prepared for each system, the first instrumented with TDR probes and thermocouples (Figure 5.2) and the second free of any instruments. The cover layers placed over a layer of sulphide containing tailings (about 20 % of iron sulphide) include a sand layer (30 cm in thickness), a fine Wags layer (60 cm in thickness) and a top layer of sand (10 to 20 cm in thickness). Concrete sand was used in the covers. The capillary layers consisted of three different sulphide free tailings. The last two columns had tailings with a small amount of pyrite in the capillary layer. Two smaller columns (called reference columns) were also built with reactive Wags without a cover and are used as controls to evaluate the relative effectiveness of the covers. In all the columns, water is added from the top periodically, and the percolating water sampled at the bottom is analyzed for electric conductivity, pH, sulphate and metal contents. These results provide indications of the possible reactions happening in the system. Temperature measurements in the columns also serve as indirect evidence of chemical reactions. The effectiveness of the cover systems is illustrated by comparing the parameters for the different columns (e.g. Figures 5.4a and 5.4b for pH, and Figures 5.5a and 5.5b for sulphates). Although some columns have shown some abnormal behavior, usually as a result of experimental problems (leaks, preoxydation, etc.), it is clearly shown that the covers can effectively prevent acid generation and the oxidation of sulphidic minerals. While looldng at the column tests results, the reader should also keep in mind that the cover configurations used in the control columns were not selected for optimizing the efficiency, but rather to verify the predictive capabilities of the experimental and numerical tools developed.

Using the available information, the efficiency of various covers was finally calculated by comparing the reduction in oxygen flux, and the results are shown in Figure 5. 1 1. This shows that if high saturation (Sr > 90 %) can be maintained in the cover through capillary barrier effects, then a one meter layer of fine material sandwiched between two sand layers will effectively reduce the oxygen flux to the reactive tailings material by a factor of about 1000 or more. The results are in accordance with calculations made by other authors for natural soils, thus showing that tailings can be used effectively as the fine material layer in cover systems.

The results presented in this report are very encouraging and warrant the continuation of the research program using more representative conditions. For that purpose, field test plots have been constructed during the summer of 1995 and new column tests were started to further analyze the practical use of non reactive tailings in layered cover systems to control AMD.

Last Modified: 2003-11-26		Important Notices

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