A model of long-term oxidation and leaching processes in pyritic coal cleaning wastes

Abstract

This study presents a numerical model to simulate a long-term pyrite oxidation process and subsequent multi-component reactive transportation of the oxidation productions in an abandoned coal waste pile in Northeast of Iran. For this purpose, 240 solid samples of wastes were obtained from different depths of 10 vertical trenches excavated in the pile. Then, geochemical analyses, including determination of pyrite content remaining in the waste particles, paste pH tests, sulfate concentration, were conducted on the collected samples. In addition, oxygen concentration profile associated with each trench was determined. The results show that the oxidation zone is limited to the shallower depths of the pile where oxygen was readily available. Furthermore, governing mathematical equations describing oxygen transport, pyrite oxidation, and transport of oxidation products were formulated based on the fact that the oxidation process follows a shrinking core concept, and gaseous diffusion is the main process for oxygen supply within the waste materials. The transport equation incorporates physical processes of advection and dispersion. Eventually, an existing one-dimensional numerical finite volume model was slightly modified, and the governing equations were simultaneously solved using a commercial computational fluid dynamics software called PHOENICS by providing extra codes for those terms of equations which are not included in PHOENICS governing equation. The results of simulation for 15 years of abandoned of the pile showed close agreement with the geochemical data. The simulations were further performed to predict the pyrite oxidation and subsequent transportation processes for years 5, 10, and 20 from now. The results revealed that more load of the oxidation products would be expected than those are generated now, and the products would reach the pile base. It will result in environmental problems for soil, surface, and groundwater of the study area and nearby lands in future.