Institute for Groundwater Studies (IGS)
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Browsing Institute for Groundwater Studies (IGS) by Subject "Acid mine drainage"
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Item Open Access Composition and performance of multi-layer liner systems to inhibit contaminant transport in a fly-ash dump(University of the Free State, 2013-07) Mokhahlane, Lehlohonolo; Steyl, G.English: The engineering properties of a South African class F fly ash were studied as a potential base liner for a dry coal ash dump. In order to increase the unconfined compression strength, lime and gypsum were added to the fly ash while also aiding in reducing the hydraulic conductivity. Lime was added in the range of 1 to 10% while the gypsum amounts were varied at 1% and 3% per specimen. The constant head method was used to determine the hydraulic conductivity of compacted specimens in the laboratory. Gypsum was observed to have more influence in reducing the hydraulic conductivity as specimens with 3% gypsum had a more reduced hydraulic conductivity that those with 1%. The variations in lime percentages did not appear to reduce the hydraulic conductivity but rather displayed higher values than fly ash specimens without additives when higher percentages of lime were used. A fly ash admixture of 3% lime and 3% gypsum was found to have the lowest hydraulic conductivity of 2.27 x10-9 m/s after 60 days of percolating with brine water. The unconfined compression strength also appeared to be more influenced by gypsum than lime percentages as specimens with 3% gypsum obtained higher strength values than those with 1% gypsum added. Unreacted lime was observed in specimens with higher percentages of lime added and these specimens also presented lower strength values. The addition of lime and gypsum was observed to have limited the release of some trace elements from fly ash. The secondary mineral ettringite was detected and could have possibly precipitated and captured out these toxic elements. An attempt was also made to increase the plasticity index of fly ash using lignosulphonate and values recommended by the South African legislative guidelines for liner materials were obtained. The plasticity was however not retained with subsequent leaching. Two multi-layer liner systems were loaded under different compaction rates in permeameter cells with fitted inflow and outflow points. The primary liners of both systems were able to contain over 95% of leachate that percolated through a waste layer. Compaction rate was found to affect the liners performance as primary liners with a higher compaction rate had less seepage than primary liner compacted at a lower rate. An addition of lime and gypsum improved the overall engineering properties of fly ash to levels accepted by the South African legislative guidelines for a liner material that is able to line hazardous waste. Even though concentrations of some trace elements in fly ash were reduced by addition of lime and gypsum the level of some of these trace elements remain above the threshold set by South African legislative guidelines and therefore remains a health and environmental concern.Item Open Access The description of physico-chemical processes in coal mine spoils and associated production of acid mine drainage(University of the Free State, 2007-05) Fourie, Petrus Johannes; Usher, B. H.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.Item Open Access An investigation into the temporal and spatial mobility of leachate production in a fly-ash dam(University of the Free State, 2013-07) Marais, Izak Lukas; Steyl, G.