Measuring Intra-Particle Diffusion Coefficients for Mine Waste Rock Fragments

Sulfide mineral oxidation in mine waste materials is one of the principal contributors to poor quality drainage. The oxidation process is generally limited by the diffusion of O₂ into the pore space of waste piles and rock fragments. In this study, a non-destructive method that couples laboratory measurements and modelling techniques is developed to measure the diffusive properties of common sulfide-mineral-bearing waste rock fragments. X-ray imaging is used to acquire time-series images of iodide tracer diffusion through the oxidized rim and into the unaltered core of the rock fragments. The distribution of diffusion coefficients is determined by fitting results from two-dimensional numerical diffusion simulations to the measured spatial distribution of iodide tracer. In addition, one dimensional experiments were conducted to determine pore diffusion coefficients normal and parallel to waste rock foliation/bedding planes in the oxidized and unaltered domains. The average pore-diffusion coefficient (Dₚ) values measured parallel to foliation in the unaltered core and normal to foliation in the oxidized rim of quartz-sericite schist are 3.30 × 10⁻¹⁰ ± 5.00 × 10⁻¹¹ m²/s and 8.27 × 10⁻¹¹ ± 1.46 × 10⁻¹¹ m²/s, respectively; 2.29 × 10⁻¹⁰ ± 3.07 × 10⁻¹¹ m²/s and 1.89 × 10⁻¹⁰ ± 8.82 × 10⁻¹¹ m²/s parallel to foliation in the unaltered core and oxidized rim of quartz-biotite schist, respectively; 2.34 × 10⁻¹¹ ± 3.70 × 10⁻¹² m²/s and 4.79 × 10⁻¹¹ ± 1.89 × 10⁻¹¹ m²/s in the unaltered core and oxidized rim of felsic meta-volcanic rock, respectively; and 6.75 × 10⁻¹¹ ± 5.00 × 10⁻¹² m²/s and 3.82 × 10⁻¹¹ ± 3.72 × 10⁻¹² m²/s parallel to foliation in the unaltered core and normal to foliation in the oxidized rim of quartz-muscovite schist, respectively. The Dₚ values for solutes such as O₂ and SO₄ are calculated using the known relationship between free-water diffusion coefficients for iodide and the solutes of interest. Coupled with inter-particle diffusion coefficient estimates, these data will aid in the development of numerical models that estimate oxidation rates and duration of hazardous leachate drainage from mine waste materials.