The description of physico-chemical processes in coal mine spoils and associated production of acid mine drainage

[摘要] English: In the first part of this study the typical physico-chemical processes involved in Acid Mine Drainage (AMD) generation were discussed. This included a detailed description of mineralogical reactions, oxygen migration processes and heat conduction typically found in coal mine wastes. In the second part of the study the mineralogy as well as the AMD generation potential of coal mine spoils and discard in South Africa was assessed. A geochemical model of the AMD generation at a rehabilitated backfilled open-cast mine was constructed that simulated the oxygen diffusion process.Coal mine wastes produce Acid Mine Drainage (AMD) due to the ingress of oxygen and the subsequent oxidation of pyrite in the waste rock. As coal mine wastes consume oxygen, oxygen molecules will spontaneously diffuse into the coal mine waste rock along the concentration gradient. Oxygen migration into coal mine wastes also occurs through the passive process of advection. Mechanisms for advection include 1) air convection due to heat differences or changes in gas composition, 2) barometric pumping or 3) changes in water saturation.Heat is generated in coal mine waste mostly through pyrite oxidation and spontaneous combustion of coal. Heat flow occurs by means of conduction, convection, radiation and latent heat. Heat has an indirect influence on every aspect of Acid Mine Drainage (AMD) including microbial activity, gas migration, the rate of chemical reactions etc.Coal discard analysed in this study showed a high net acid generation potential, whether subjected to spontaneous combustion or not. Overall it can be concluded that coal discard has a high potential to generate AMD in South Africa. In spoil material it was found that some trace to dominant minerals were present that were also identified in other studies. These minerals include quartz, kaolinite, calcite, dolomite, mica, K-feldspar, plagioclase, siderite, pyrite, illite/smectite, smectite and anatase. The average pyrite content decreased from coal samples to carbonaceous clastic rocks to non-carbonaceous clastic rocks. The net acid generation potential was found to increase upwards from the No. 1 to the No. 4 coal seam.In the case study, one dimensional modelling of oxygen diffusion through waste material was performed using the PYROX 3 model of the University of Waterloo. The interaction between the gas, mineral and water phases was modelled using The Geochemist's Workbench 6 (GWB).Consumption of oxygen by pyrite resulted in a decrease in oxygen concentration towards the bottom of the pit. Oxygen was however present in the saturated zone at the bottom of the pit and oxygen migration into the unsaturated zone was therefore not seen as the rate limiting step for the overall pyrite oxidation process.The pyrite oxidation rate was determined in PYROX after calibrating the oxygen concentration in the model to field conditions. By adjusting the reactive pyrite surface area in GWB the pyrite oxidation rate was calibrated to the rate modelled in PYROX.In the geochemical mass model seven scenarios were run that modelled closed, open and intermediate systems with regard to the oxygen and CO2 buffer. It was found that the geochemical system was nearly open to atmospheric oxygen and CO2, which can be attributed to the shallowness of the pit (average depth of 12 m).During the first few years a near neutral stage (pH 5.5 - 7.5) existed where calcite and dolomite buffered the solution. After depletion of the carbonate minerals an acidic stage (pH 3.0 - 4.0) followed where silicate minerals consumed some of the acidity. These two stages had a significant influence on the mineral reactions in the waste material and the ion concentrations in the pit water.The abiotic rate of pyrite oxidation by oxygen is positively dependent on the dissolved oxygen activity and negatively dependent on the hydrogen activity. As the pH decreased over the model time, the pyrite oxidation rate also decreased.The concentrations of all the major parameters and of iron in the pit water were modelled. It was observed that sulphate reached its peak concentration during the near neutral stage just before the acidification of the pit water and thereafter it decreased steadily before reaching a long-term concentration.During the neutral stage Fe(OH)3 had the highest activity of all iron species. Fe(OH)3 became undersaturated while Fe(OH)2+ and Fe(OH)2+ were the dominant species during the acidic stage. Oxidation of pyrite by ferric iron did however not occur as the ferric iron activity became only high enough at pH levels of below pH 3.This study showed that the AMD process can be successfully modelled using numerical modelling. In South Africa the number of operating coal mines have declined significantly over the past two decades and numerous coal discard dumps are present throughout coalfields. Modelling of AMD at coal mining sites will become increasingly more important for management decisions as more mines close in the Witbank and Highveld coalfields.

[发布机构] University of the Free State