FURTHER REFINEMENT OF THE DAVIS-RITCHIE MODEL OF PYRITE OXIDATION IN GEOLOGIC MATERIALS ASSOCIATED WITH MINING

The Davis-Ritchie (DR) model (1987) is frequently used to predict the rate and extent of pyrite oxidation in mine waste rock and tailings, and along mine pit walls and floors. The model requires input information on rock porosity and percent pyrite.  A distribution function is used to describe particle sizes. The model assumes that O₂ is the limiting reactant, and that oxygen transport involves O₂ diffusion through pore spaces, followed by its diffusion into reaction sites within the particles. The oxygen diffusion coefficient in pore spaces (D₁) is assumed equal to its value in normal air.  Davis and Ritchie assumed that D₂, the coefficient for O₂ diffusion into reaction sites within the particles, equaled its value in water (2.6 x 10^-10 m²s^-1). The water content of rock pores was not considered in their model.

Fennemore et al. (1998) argued that water content was an important variable, particularly in arid climates, and so they revised the DR model to account for changing humidity. They also assumed that D₂ was a variable and solved for it by modeling data from humidity cell tests.  Resultant D₂ values ranged from 2x10^-13 to10^-10 m²s^-1.

We have developed a numerical code that includes all the refinements suggested by Fennemore et al., but also allows the input of measured, variable particle size distributions of rock materials.  Data from humidity cell testing of rocks from metal mines in Washington state and Nevada was modeled assuming the measured particle size distributions of the rock materials used in the tests. Values of D₂,which ranged from 10^-14 to 4 x 10^-12 m²s^-1, and averaged 5.45x10^-13 m²s^-1 were obtained by matching the mass of sulfate leached in the tests and model simulations. This study suggests that D₂ values derived from Davis-Ritchie modeling of humidity cell test results may differ significantly depending on whether rock particle sizes are input to the model in a distribution function or as measured size distributions.