Integrated 3D geo-environmental assessment of acid-forming materials in historic coal waste piles for sustainable management.

Abstract

Coal mining produces coal mine waste rock (CMWR), posing significant environmental risks, including acid mine drainage (AMD) if unmanaged. The Jerada Mine in eastern Morocco has accumulated CMWR since it began operations in 1936, with no rehabilitation efforts until 2001. This study assessed the stability of the T08 pile, which has been deposited over five decades across various oxidation zones. More than 400 samples from 13 drill holes were thoroughly analyzed, including particle size distribution, X-ray fluorescence (XRF), and other advanced techniques, culminating in a 3D model to identify potentially acid-forming (PAF) zones. Particle sizes (D₃₀ and D₉₀) ranged from 16.3 to 16.5 μm in low-oxidation zones to 353.3-409 μm in highly oxidized areas, respectively. Sulfur content varied from 0.32 to 2.05 wt% for sulfide and from 0.0013 to 0.17 wt% for sulfate, with an acidification potential ranging from 14.42 to 29.2 kg CaCO₃/_t and negative net neutralization potential (NNP) from -35.12 to -11.14 kg CaCO₃/_t. NAG tests revealed a low pH of approximately 4 and acidity levels exceeding safety thresholds, with low neutralizing minerals content. Pyrite was the dominant sulfide, alongside ankerite, hematite, and goethite. Highly oxidized zones exhibited larger particle size distributions, increasing porosity and airflow. Thereby, enhancing oxidation and converting iron into different oxidation states. This process affects sulfur speciation, leading to sulfate formation. The 3D model estimated 3.8 Mt of PAF material in the upper pile, highlighting a heterogeneous distribution linked to porosity and oxidation levels, underscoring the necessity for further kinetic testing to evaluate long-term AMD risks.