Mamert Mbonimpa

Oxygen diffusion and consumption in low-sulphide tailings covers

Acid-generating tailings can be reclaimed using oxygen barrier covers in humid climates, such as in Quebec. A single-layer low-sulphide tailings cover is an attractive alternative to traditional water covers to maximize the impoundment’s storage capacity. Low-sulphide tailings can serve as a moisture-retaining material to limit oxygen diffusion, and their residual sulphides can consume the diffusive oxygen that still gets through the cover. A laboratory experiment using instrumented columns was initiated to confirm the efficiency of a low-sulphide tailings monolayer cover placed over acid-generating tailings to reduce the oxygen flux reaching the reactive tailings. Oxygen concentration profiles were measured over the thickness of the cover and oxygen fluxes were calculated to evaluate the effect of three parameters on these fluxes: water table level, cover sulphide content, and cover thickness. Oxygen fluxes at the bottom of the low-sulphide tailings cover layer, calculated using oxygen gradient and estim...

Predicting Hysteresis of the Water Retention Curve from Basic Properties of Granular Soils

The water retention curve (WRC), which represents the relationship between volumetric water content (θ) and suction (ψ), is required to analyze the hydro-geotechnical response of unsaturated soils. The laboratory (or field) determination of the WRC can however be time consuming and difficult to conduct. A practical alternative, particularly useful at the preliminary stages of a project, is to estimate the WRC using a predictive model based on basic geotechnical properties that are easy to obtain. One common limitation of such predictive models is due to hysteresis effects, which are not taken into account by most of these models. The authors present in this paper an extended version of the Modified Kovács (MK) predictive model that incorporates hysteresis of the WRC along different paths, including the main wetting and drying curves and the wetting and drying scanning curves for granular soils. The model formulation is presented, and predictions are compared to experimental data obtained on different granular soils. The results show a good agreement for the main and scanning curves.

Review of the Reclamation Techniques for Acid-Generating Mine Wastes upon Closure of Disposal Sites

Acid mine drainage (AMD) remains a major environmental challenge for the mining industry. The preferred options for effectively limiting the environmental impact of AMD consist in controlling the reactions through the use of preventative techniques. Their principal objective is to exclude at least one of the constitutive elements of the chemical reactions, i.e. water, oxygen, or sulfidic minerals. The article recalls the basic principles and reviews different approaches for the prevention and control of AMD upon mine closure. The main methods include multi-layer covers, water covers, and an elevated water table (with a mono-layer cover). Their main advantages, limitations and uncertainties are addressed. Alternative approaches, such as environmental desulphurization and co-disposal of waste rock and tailings, are also discussed.

Experimental Characterization of the Shrinkage and Water Retention Behaviour of Tailings from Hard Rock Mines

Mining of hard rock ore deposits produces large amounts of tailings. The safe disposal and management of these tailings require an extensive characterization that should include their drying and desaturation behaviour. Desiccation tests have been performed to characterize the shrinkage response of low plasticity tailings having an initially loose state. The testing procedure developed for this purpose is briefly described here. The main shrinkage tests results are then presented. The experimental data are compared with those obtained from water retention tests performed in a pressure plate with volume change measurements. These two types of results are combined to define unsaturated (drying and shrinkage) relationships in six complementary planes that include the volumetric shrinkage curve and the water retention curve. Specific material characteristics are then determined, including the shrinkage limit wS, final void ratio ef, and air entry value ψa. Additional tests were also performed to define critical parameters at the initiation of cracking in terms of suction ψ, water content w, and degree of saturation Sr. The original results presented here indicate that the onset of desaturation is closely linked with the end of volumetric straining and with crack initiation. Results also show that the shrinkage limit wS is a function of the specimen initial water content w0. Other related characteristics are also presented and discussed.

A Modified Oxygen Consumption Test to Evaluate Gas Flux through Oxygen Barrier Cover Systems

The evaluation of an oxygen barrier cover performance is an integral part of many reclamation programs that aim at limiting acid mine drainage (AMD) production from sulphide tailings. The oxygen flux through the cover to the underlying reactive tailings should then be much smaller (by a factor of 1000 or more) than the flux into exposed (uncovered) tailings. Different techniques have been developed over the years to assess this flux. Given the limitations encountered when using available approaches to measure such low oxygen flux, the authors have developed a modified testing and interpretation method based on the oxygen consumption (OC) test, which is commonly used to determine oxidation rates of uncovered tailings. The proposed modifications to the OC testing procedure are presented in this paper, together with the new interpretation method. Typical tests results obtained with the modified OC method on an existing site, reclaimed using a layered cover with capillary barrier effects (CCBE), are presented and discussed. The results demonstrate that the modified oxygen consumption (MOC) test is an effective tool to assess the performance of oxygen barrier covers used to prevent AMD.

MEASUREMENT OF OXYGEN CONSUMPTION AND DIFFUSION IN EXPOSED AND COVERED REACTIVE MINE TAILINGS 1

When sulphidic tailings are exposed to atmospheric conditions, oxygen can flow in the material where it will be consumed by oxidation reactions. This paper presents results from a project studying the in situ oxygen diffusion and consumption in tailings. It provides measured values for the oxygen reaction rate Kr and the effective diffusion De coefficients based on oxygen consumption and diffusion tests that are performed under various conditions. Based on the test results, the Kr and De values determined in situ are compared to values obtained through simple predictive models. Results indicate that the value of these two parameters is influenced by factors such as the degree of saturation Sr and in situ porosity n. A lower Sr or a higher n may increase the oxygen flux toward the reactive tailings, which can increase the production rate of acid mine drainage (AMD).

Conversion of the Modified Kovács Model Parameters to the Brooks and Corey and van Genuchten Model Parameters for the Water Retention Curve of Sandy and Silty Soils

AbstractThe modified Kovacs (MK) model is a versatile and practical tool to estimate the water retention curve (WRC) of soils from their basic properties. However, the MK model has not yet been introduced in many codes or analytical solutions; most applications rather use the Brooks and Corey (BC) and/or van Genuchten (vG) formulations to express the WRC. The authors propose in this paper a direct conversion of the MK model parameters to the BC and vG equations parameters for sandy and silty materials. The proposed equations were developed using data from 15 types of sandy soils and 14 low-plasticity silts and validated using 16 other soils. The results presented in this paper indicate how the BC and vG equations parameters can be successfully predicted from the MK model parameters. The conversion procedure provides a practical means to estimate the WRC from the basic geotechnical properties of soils and to express this curve using three different mathematical formulations: MK, vG, and BC. These expressio...

A Convenient Graphical Representation of Compaction Data

The results of compaction tests are usually presented as dry density versus molding water content. In the corresponding graph, the curves of levels of saturation appear as hyperbolas. A new graphical presentation is proposed where the inverse of the dry density is plotted against the molding water content. In this convenient representation, all curves of equal degree of saturation become straight lines, which facilitates plotting and interpolation. It appears also more practical to establish a linkage with other graphs, such as those relating mechanical or hydraulic properties, to void ratio.

Comparison between the predictive modified Kovács model and a simplified one-point method measurement to estimate the water retention curve

ABSTRACT The modified Kovács (MK) model can predict fairly well the water retention curve (WRC) of different types of soils, using basic geotechnical properties. Considering its relative simplicity and its ability, a study was initiated to compare this model to an alternative predictive approach involving a one-point measurement method (OPMM). The comparison result indicates that there is generally a good agreement between volumetric water content values predicted using the OPMM and the MK model and those obtained from direct measurements. However, the OPMM typically leads to a better correlation than the predicted values obtained with the MK model for cohesive fine-grained soils because shrinkage induced by suction is not taken into account. Hence, the MK model is deemed as a valuable tool that gives realistic estimates of the WRC using easy to obtain basic geotechnical parameters, when there is no direct measurement available for a point on the WRC, and this predictive model can be particularly useful at the preliminary phase of a project.