Experimental and numerical evaluation of the critical degree of saturation and critical exposure time of acid generating filtered tailings

Abstract

Tailings filtration has many advantages (e.g., water recirculation, progressive reclamation, reduction of geotechnical risks and of environmental footprint) and is increasingly used by mining companies to improve the geotechnical stability of tailings storage facilities (TSF). However, low water content also exposes filtered tailings to oxidation thus increasing the risk for acid mine drainage (AMD) generation. Tailings management techniques must therefore be adapted to prevent contamination during operations. More specifically, tailings degree of saturation and exposure time should be controlled so they do not remain exposed too long to oxygen before reclamation work starts. The main objective of this research was therefore to evaluate the combined influence of the mineralogy and the degree of saturation on the time before filtered tailings start generating AMD. This work included (i) physical, chemical, hydrogeological and mineralogical characterization of different tailings, (ii) laboratory kinetic tests, (iii) calibration and validation of reactive transport numerical simulations, and (iv) numerical extrapolations and analysis. Results showed that AMD started when carbonates were depleted and that the critical time (i.e., the time tailings could be left exposed before AMD starts) strongly depended on the mineralogy. Maintaining the degree of saturation above 90% could contribute to delay the generation of AMD, and the greater the degree of saturation, the longer the critical time. However, the critical time became independent of the degree of saturation and depended only on the mineralogy when S_r< 90%. The results of this study tend to show that it should be possible to plan the deposition of reactive filtered tailings to limit the generation of AMD, for example by mixing reactive tailings with other tailings containing a greater neutralization potential and/or by regularly disposing of a new layer of tailings on the surface, with the maximum delay between two layers being shorter than the critical time.