Passive neutralisation of acid mine drainage by fly ash and its derivatives: A column leaching study

Abstract

Active treatment of acid mine drainage using fly ash (FA) generates solid residues. This work investigated the suitability of using the solid residues (SR) generated from the process for passive treatment of acid mine drainage in a backfill set-up. A column leaching study of the solid residues blended with varying amounts of fly ash (5%, 25%, 40%) and 6% ordinary portland cement (OPC) and fresh fly ash was carried out to assess the neutralisation and contaminants attenuation in AMD with time. All the solid cores acidified in a stepwise fashion, exhibiting three buffer zones (12–10, 9–7.5 and 6–4). Dissolution of CaO in fly ash and solid residues blended with fly ash impacted high pH in the initial leachates while hydrations reactions in OPC blend solid residues contributed to high initial pH. Dissolution of SiO 2(amorphous) and mullite in the solid residues contributed to sustain buffering at pH 7–9.5 for SR and FA blended solid residues. Encapsulation of the solid residue particles by the calcium silicate hydrate gels (CSH) in OPC blended solid residues reduced interaction of particles with SAMD hence the buffering at pH 7–9.5 was not observed. Eventual clean-up of the AMD for all solid cores was dependent on the increase of the pH to alkaline and circum-neutral levels and sustainability of the buffering capacity at circum-neutral pH. It is hypothesized that the duration of the effectiveness of generation of alkalinity and AMD clean-up by the solid residues will greatly depend on the quality of the AMD percolating through. The SR and SR + 6% OPC were observed to be superior in removal of Al, Fe and SO 4 2 - compared to FA and SR + 25% FA. Lowest efficiency was observed in FA and SR + 25% FA cores for all contaminants except for Mn in FA core. This study shows that the solid residues (SR) can successively generate alkalinity over an extended period of time and passively treat AMD. This has positive implications upon the application of the solid residues for backfilling of mine voids. Different solid cores exhibited varying degrees of leachability for various toxic elements. B was the most leached element and the leaching was more pronounced in the SR and SR + FA blends. The SR + OPC core was observed to be superior in retention of Mo and B. Calculation of % toxic trace elements leached based on total composition from XRF data indicated insignificant amounts are leached over the experimental period for SR core. However the pH regime generated will depend on backfill scenario which in-turn will dictate the extent of leaching of toxic trace elements. It is recommended that more studies be carried out on the leachability of the toxic trace elements on a long-term basis before suitability of the SR and SR + FA blends can be fully evaluated.