Predicting pyrite oxidation and multi-component reactive transport processes from an abandoned coal waste pile by comparing 2D numerical modeling and 3D geo-electrical inversion

Abstract

In this paper, pyrite oxidation process resulted from an abandoned coal waste pile in northeastern Iran was evaluated by a combination of three-dimensional (3D) geo-electrical data inversion and two-dimensional (2D) numerical modeling of long-term pyrite oxidation and multi-component reactive transportation of the oxidation products. To achieve the goal, 42 solid samples were obtained from different depths of two trenches excavated over the pile. The samples were analyzed for the fraction of pyrite remaining within the waste particles. Besides, the mole fraction of oxygen at various depths of the pile was measured in situ. The results reveal that pyrite oxidized considerably at the shallower depths where oxygen diffused. Simultaneously, 2D geo-electrical surveys were conducted on four parallel profiles over the waste pile. Afterward, the measured apparent resistivity values were assembled into a 3D data set and further modeled by inverting these data. The results show that, horizontal slices of the 3D resistivity model almost correspond with the geochemical data. Besides, a computational fluid dynamics (CFD) software called PHOENICS was modified to numerically solve the governing equations of oxygen diffusion, pyrite oxidation and solute transport applying a finite volume technique and considering all sources and sinks related to the chemical reactions. The results show that the oxidation occurred in the upper layers of the pile which almost correspond with the geo-electrical data interpretation. Moreover, the predicted results show that further pyrite oxidation would generate more products that reach the pile base in the future. Eventually, a reclamation scenario was suggested by covering an impermeable cap on the pile surface. The results reveal that this process would prevent oxygen diffusion to the pile which in turn limit pyrite oxidation process and acid mine drainage generation, in the waste particles and it would lead to termination of the transportation of oxidation products within the pile by surface recharge.