NUMERICAL MODELLING OF MULTIPHASE TRANSFER PROCESSES IN HETEROGENEOUS WASTE ROCK PILES

Acid mine drainage is a serious environmental problem caused by the oxidation of sulphide minerals responsible for highly acidic, sulphate and metals-rich drainage. Waste rock piles undergoing AMD production are complex multiphase and multi-component systems in which many coupled processes are interacting in an unsaturated heterogeneous coarse porous media: oxygen consumption, heat production and transfer, gas flux by advection and diffusion, water infiltration and dissolved mass migration. Few natural or man-made systems involve the interaction of so many coupled physical and geochemical processes as waste rock piles undergoing AMD production. Waste rock have a heterogeneous nature and present contrasting physical properties and behaviours that put them in some case at opposite of the spectrum of physical processes involved in AMD. The purpose of this paper is to numerically model the effect of heterogeneity, anisotropy and mode of deposit on the conditions and transfer processes in heterogeneous waste rock piles, emphasizing gas transfer and vapour water transport. Simulations show the degree of preferential flow of fluid in the system and its influence on the other processes. Simulations also show that the preferential flow of water and gas seems to be the rule more than the exception. Keywords: Acid Mine Drainage; Waste Rock Piles, Heterogeneity; Anisotropy; Pyrite Oxidation; Heat transfer, Gas convection, Gaseous diffusion; Water vapor transport; Numerical simulation