Matthew Otwinowski

This brief presentation reviews some recent results obtained during the development of a Waste Rock model.

By the research project agreement our recent modelling effort was limited to the analysis of acid Rock drainage at pH values above 4.5 in the absence of bacterial oxidation¹.

The geochemical and thermodynamic behaviour of Waste Rock Piles is significantly different from that of tailings. Unlike tailings, waste rock piles are not saturated with water. For this reason oxygen diffusion in waste rock piles is much faster than in tailings. At the same time, the thermal conductivity of waste rock piles is much smaller than the thermal conductivity of tailings. This causes a relatively slow transport of the heat generated by sulphide oxidation. Fast oxygen transport and slow heat removal are often responsible for fast acid generation rates due to the fast oxidation of sulphide minerals when high temperatures (often exceeding 40°C) develop inside waste rock piles. Rock reactivity, water flow, oxygen transport, and heat dissipation to the surroundings, are the main physical factors which control acid rock drainage for given mineral composition of waste rock. Both mass and energy transport depend on pile size, pile geometry and the ambient conditions.

Our previous scaling analysis², and the numerical results presented here show that large-scale thermodynamic effects (on a length scalę greater than one metre) depend in a strongly nonlinear way, on the geochemical and physical properties at both the micro-scale (length scale less than 0.1) and meso-scale (1 cm to 1 m). The prediction of discharge values requires the understanding and proper description of the interplay between the geochemical and physical processes at all scales. The proper characterization of waste rock at all scales is very important for scaling-up laboratory data, and to enable one to address site-specific situations.

The geochemical, biological and physical processes responsible for acid rock drainage are strongly nonlinear.

► Sulphide oxidation rates accelerate with increasing temperature. The experimental data show a strongly nonlinear acceleration of the reaction rates in the temperature range observed in waste rock piles.

► Waste rock reactivity depends in a strongly nonlinear manner on particle size. We have performed a detailed geostatistical analysis of particle size distribution and its effect on rock reactivity and acid generation rates. In a typical ensemble of rock particles there are two important physico-chemical effects responsible for the increasing rock reactivity when particle size decreases:

• Small particles have a large surface area per unit rock mass and the amount of sulphide exposed to oxygen dissolved in water film on rock surface increases with decreasing particle size.

• The distance which oxygen has to travel through the micropores and cracks to reach reactive sulphide sites inside the rock particles, is short for small particles. 

The particle size distribution is characterized by a fractal distribution function.

► Field data show that the total discharge of contaminants is a strongly nonlinear function of the water flux.

► Waste rock piles may exhibit thermodynamic catastrophes associated with thermodynamic instabilities³. Such strongly nonlinear behaviour poses a serious difficulty for ARD prediction. 

The strongly nonlinear features of acid rock drainage impose high requirements on the numerical and analytical methods used in a realistic model. We have used the finite element method with an adaptive grid generator.

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¹ A detailed analysis of the available experimental data on bacterial oxidation and chemical kinetics at pH below 4.5, is presented in our report Quantitative Analysis of Chemical and Biological Kinetics for the Acid Mine Drainage Problem, MEND 1994.

² M. Otwinowski, Scaling Analysis of Acid Rock Drainage, MEND 1995.

³ A thermodynamic catastrophe occurs when a small change of parameters such as pile size, ambient temperature, geochemical properties of waste rock, etc. causes a large change in overall acid generation rates.