Assessment of hydrogeochemical behavior of layered waste rock stockpiles: A meso-scale laboratory experiment

Abstract

Large quantities of waste rocks are removed during ore extraction particularly in open pit mining and stored at the surface in piles. When these materials contain reactive minerals and are exposed to surface environmental conditions, they can generate contaminated mine drainage. A new approach of construction of waste rock piles using inclined compacted benches was developed in order to control water infiltration and minimize contamination. An experimental waste rock pile was built on the Lake Tio mine site to evaluate the performance of this approach under field conditions. To address all concerns related to the limitation of the field experiment, laboratory infiltration tests were conducted inside a steel and plexiglass box filled with three layers of different materials including a flow control layer (consisting of two compacted sand and crushed anorthosite sub-layers) and a layer of coarser materials (hemo-ilmenite waste rock containing sulphides and the non-reactive anorthosite waste rock). These infiltration tests were carried out testing different rates of precipitation (8 mm/day, 17 mm/day and 27 mm/day) at different slopes (0, 3, 5 and 7%). The effects of precipitation rate and box slope on the flow and quality of the infiltrated water in the system were studied in detail. Results show that the inclination angle of the box and the applied rainfall rate largely govern the diversion of water down the slope of the infiltration box. The results also show that the distribution of infiltration within the system was dependent on rainfall intensity. Based on the results of infiltration tests, it was found that nickel and sulfate loadings are influenced by the applied rainfall rates and a better diversion of water by the flow control layer improves the drainage water quality. These results suggest that nickel and sulfate loadings are mainly controlled by the capillary barrier effects (CBEs), flow paths and the flow rate. The findings are useful for improving the use of a flow control layer as a waste rock management and reclamation method.