Evaluation of the leachate chemistry and contaminants attenuation in acid mine drainage by fly ash and its derivatives
Gitari, Wilson Mugera
MetadataShow full item record
The mining industry in South Africa has a huge potential to impact negatively on the environment. Negative impacts include generation of reactive tailings and acid mine drainage (AMD). AMD is highly acidic (pH 2-4), sulphate-rich and frequently carries a heavy metal burden. South Africa uses more than 100 million tonnes of low grade bituminous coal annually to produce cheap electricity. The associated mining operations result in millions of tonnes of polluted water and in turn coal burning power stations produce vast amounts of waste ash such as fly ash. The highly soluble CaO occurring as sub-micron fragments on the fly ash particles is highly reactive and can be utilized in the neutralization of acid mine drainage. Acid mine drainage (AMD) was reacted with two different South African fly ashes in a batch setup in an attempt to evaluate their neutralization and inorganic contaminants removal capacity. The concentrations of major constituents in the AMD were found to determine the final pH attained in the reaction mixture and the reaction time of breakthrough to circum-neutral and alkaline pH. Efficiency of elemental removal in the AMD by the FA was directly linked to the amount of FA in the reaction mixture and to the final pH attained. Most elements attained ≈ 100 % removal only when the pH of minimum solubility of their hydroxides was achieved. In the second part of the study, Acid mine drainage (AMD) was reacted with coal fly ash in a 24 hour equilibration time using 1:3 and 1:1.5 FA: AMD ratios by weight to produce neutral and alkaline process waters. The capacity of the fly ash to remove the major inorganic contaminants from AMD was examined with time. The geochemical computer software PHREEQC and WATEQ4 database were used for geochemical modeling of the process water chemistry at selected reaction times. The collected solid residues were analyzed by X-ray diffraction, scanning electron microscopy (SEM) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX). At both ratios the reaction mixture was at saturation or oversaturated with alunite, basaluminite, jurbanite, boehmite, gibbsite, diaspore, gypsum, barite, K, Na-jarosites, ettringite, amorphous Fe (OH)3 and goethite at specific contact times. The precipitation of the many inorganic contaminants was established in terms of the mineral phases at saturation or over-saturation. Sequential extraction revealed the amorphous fraction to be the most important in retention of the major and minor inorganic contaminants at pH > 6.32 which implies that the concentration of total Fe and Al in the AMD being treated has a direct effect on the clean-up efficiency of the process. In the third part of the study, a column leaching of the solid residues (SR) blended with varying amounts of fly ash (5 %, 25 %, 40 %) and 6 % Ordinary Portland Cement (OPC) was carried out to assess the contaminant attenuation with time. The columns were drained with synthetic acid mine drainage (SAMD) over a period of 165 days. In addition the solid residues were modified with 1-6% OPC and their strength development monitored over a period of 365 days. The column solid cores were observed to acidify in a stepwise fashion, exhibiting three buffer zones. The SR alone and SR blended with fly ash exhibited strong buffering capacity at pH (7.5-9) for an extended period of time (97-110 days). Encapsulation of solid residue particles by the calcium silicate hydrate gels (CSH) in OPC blended solid residues obscured the appearance of the sustained buffering at pH 7-9.5. The fly ash and OPC blend solid residues exhibited decontamination efficiencies of (82-99 %) for Al, Fe, Mn and SO4 2- over the study period. However the OPC blend SR exhibited high attenuation efficiency even as the pH dropped to below 4. SR + 6 % OPC core was observed to be the most efficient interms of retention of highly mobile elements such as B and Mo. pH was observed to be the main determining factor in contaminants attenuation. Geochemical modeling results revealed that pH and SO42- concentrations in the leachate had a significant impact on the mineral phases controlling Fe and Al concentration in the leachates. In the SR + 6 % OPC solid cores, EDX analysis revealed that CSH gels and calcium aluminate hydrate gels were being precipitated. These gels were either incorporating Fe, Mg, Mn in their matrix or encapsulating the solid residue particles that were rich in these elements. Sequential extractions of the leached solid cores revealed the amorphous fraction to be the most important in retention of the major contaminants and were most enhanced in the OPC blend solid residues. The OPC blend solid residue slurries developed unconfined compressive strength (UCS) (2-3 Mpa) comparable to paste formulated from sulphidic rich mine tailings confirming that the solid residues can be used for backfilling. Therefore the solid residues (SR) can successively be applied for a dual purpose in mined out areas namely, to remediate acid mine drainage waters and also provide support for the overburden. Keywords: Acid Mine Drainage; Fly Ash; Neutralization; Sulphates; Metal ions; Solid Residues (SR); Column Leaching; Geochemical Modeling; Sequential Extraction; Buffering.