AN OVERVIEW OF PREDICTION AND CONTROL OF AIR FLOW IN ACID-GENERATING WASTE ROCK DUMPS By:

Air movement and associated oxygen transport through waste rock dumps has the potential to significantly enhance the rate of oxidation of pyrite-bearing material. While this is a desired outcome for most heap leach operations, airflow in waste rock storage facilities can result in significant increases in generation and acceleration of acid rock drainage. Hence, a good understanding of internal airflow through waste rock dumps is required to control ARD and minimize any associated liability. The principal mechanisms contributing to airflow and oxygen transport in a waste rock pile include (i) diffusion, (ii) advection due to a thermal gradient (chimney effect) and/or wind pressure gradients and (iii) advection due to barometric pumping. While diffusion is typically limited to a near-surface zone of a few meters depth, advection and barometric pumping have the potential to move air (and oxygen) to much greater depths into the pile. In general, the more permeable the waste rock material, and the greater the height-to-depth ratio of the waste rock pile, the greater is the potential for advective air movement. The reactivity of the waste rock material as well as the coarseness (hence air permeability), and the spatial variability of these properties within a pile, have a strong influence on the magnitude of thermally induced advection. In contrast, air movement due to barometric pumping is controlled by the waste rock porosity, changes in ambient air pressure and the heterogeneity of air permeability of the waste rock dump. Results of field monitoring and numerical modeling using TOUGH AMD are presented to illustrate the concepts on air movement in waste rock piles discussed in this paper. During the design and construction phase, airflow can be controlled by judicious placement of reactive waste rock and use of selective placement techniques to control the internal structure of the waste rock facility (e.g. introduction of horizontal layering, prevention of inclined, high-permeability, channels (“chimneys”)). Several closure measures are available to minimize airflow including (i) placement of a lowpermeability cover to reduce air entry, (ii) placement of a non-reactive cover material to isolate reactive material from the zone of active airflow and/or regrading of the waste rock pile to obtain a geometry and internal structure less susceptible to advective airflow. 1 Robertson GeoConsultants Inc., Suite 640, 580 Hornby Street, Vancouver, BC, Canada, V6C 3B6, (604) 684-8072 tel, (604) 684-8073 fax, email: [email protected]. 2 INRS-Eau, Terre et Environnement, Centre Géoscientifique de Québec, 880 ch. Ste-Foy, Québec, QC, Canada, INTRODUCTION A complex interaction of physical, hydrological, geochemical and microbiological processes contribute to acid rock drainage (ARD). The oxidation of pyrite-bearing rocks can be summarized by the following exothermic reaction (Nordstrom, 1977): FeS2 + H2O + 3.5 O2 2 H + 2 SO4 + Fe In addition to pyrite, the presence of both water and oxygen is required to sustain pyrite oxidation and thus ARD. While most mine rock piles contain sufficient pore water to sustain this reaction, in many cases air movement within the pile is sufficiently slow, thus controlling oxidation by limiting the oxygen supply. Therefore, it is important to understand, and attempt to control, the mechanisms that contribute to air movement in waste rock piles as a means of mitigating ARD production. Oxygen transport, also referred to simply as airflow, occurs in waste rock piles by advection and diffusion in response to concentration, pressure and thermal gradients. The magnitude of oxygen transport is controlled by a complex interaction of various physical and chemical properties of the waste rock. Characterization of these properties along with field monitoring needs to be completed in order to adequately assess and predict ARD. The results of characterization and monitoring work can then be used to calibrate an air transport and ARD production model of the mine rock piles. This paper provides an overview of the theoretical and practical aspects of airflow in waste rock dumps. First, we provide a brief review of the key mechanisms of airflow (convection, diffusion, barometric pumping) and their fundamental controls. Second, we discuss the tools available for assessment and prediction of airflow, including material characterization, field monitoring, and numerical modeling. Examples of field monitoring and numerical modeling from well-studied waste rock piles at three different mine sites are presented to illustrate the concepts on air movement in waste rock piles discussed in this paper. Finally, airflow control options applicable both during and after pile construction are discussed. MECHANISMS OF AIRFLOW Diffusion Waste rock piles from hard rock mines are large accumulations of generally coarse-grained material containing sulphides (mostly pyrite) that remain only partially water saturated, i.e. gaseous and liquid phases are simultaneously present in the pore space between the solid grains. Initially, following the oxidation of the sulphides, a partial depletion of the oxygen present in waste rock piles occurs. Oxygen concentration gradients are thus created between the gas phase within the wastes and the atmospheric air surrounding the pile. This oxygen concentration gradient drives gaseous oxygen diffusion from the surface to the interior of the waste rock pile. Gaseous diffusion is the main process providing oxygen within waste rock accumulations after their initial placement and diffusion remains active thereafter as long as the oxidation process contributes to the depletion of oxygen concentration Co (mol/m) in the gas phase within the waste rock pile. In partially water saturated porous media, the following form of Fick’s Law describes the molar diffusive flux Jo (mol/m⋅s) of oxygen from the atmosphere to the interior of waste rock piles: l C D S n J o o g o d d ' ⋅ ⋅ ⋅ ⋅ − = τ 1 Fick’s Law states that the mass flux of oxygen is proportional to the oxygen concentration gradient and the effective diffusivity De (m/s). The magnitude of the effective diffusivity is lower than the diffusion coefficient Do (m/s) in a free fluid because of the presence of solids hindering diffusion by increasing the tortuosity of the transport pathways. In equation 1, a linear relationship is assumed between the effective diffusivity De and Do, tortuosity τ' (dim.), porosity n, and gas saturation Sg. Other commonly used effective diffusivity models are discussed in the review by Aachib (1997). A simple analytical solution allows the prediction of one-dimensional oxygen concentration profiles in materials where it is consumed by a first-order reaction, which is often applied to pyrite oxidation. In such systems, the oxygen mass loss Ro (mol/m⋅s) directly depends on its concentration Co (mol/m) and the kinetic constant Ko (s):