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Acid mine drainage (AMD) has been reacted with two South African fly ashes in a batch setup in an attempt to evaluate their neutralization and major, trace elements removal capacity. Different fly ash: acid mine drainage ratios (FA: AMD) were stirred in a beaker for a set time and the process water analyzed for major, trace elements and sulphate content. The three factors that finally dictated the nature of the final solution in these neutralization reactions were the FA: AMD ratio, the contact time of the reaction and the chemistry of the AMD. Efficiency of the elements removal was directly linked to the amount of FA in the reaction mixture and to the final pH attained. Most elements attained approximately 100% removal only when the pH of minimum solubility of their hydroxides was achieved (i.e., Mg = 10.49-11.0, Cu(2+) = 6, Pb(2+) = 6-7). Dissolution of CaO and subsequent precipitation of gypsum and formation of Al, Fe oxyhydroxysulphates, Fe oxyhydroxides with subsequent adsorption of sulphate contributed to the sulphate attenuation. Significant leaching of B, Sr, Ba and Mo was observed as the reaction progressed and was observed to increase with quantity of fly ash in the reaction mixture. However B was observed to decrease at high FA: AMD ratios probably as result of co-precipitation with CaCO(3(s)).

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Acid mine drainage was reacted with coal fly ash over a 24 h reaction time and species removal trends evaluated. The evolving process water chemistry was modeled by the geochemical code PHREEQC using WATEQ4 database. Mineralogical analysis of the resulting solid residues was done by X-ray diffraction analysis. Selective sequential extraction was used to evaluate the transfer of species from both acid mine drainage and fly ash to less labile mineral phases that precipitated out. The quantity of fly ash, volume of acid mine drainage in the reaction mixture and reaction time dictated whether the final solution at a given contact time will have a dominant acidic or basic character. Inorganic species removal was dependent on the pH regime generated at a specific reaction time. Sulphate concentration was controlled by precipitation of gypsum, barite, celestite and adsorption on iron-oxy-hydroxides at pH > 5.5. Increase of pH in solution with contact time caused the removal of the metal ions mainly by precipitation, coprecipitation and adsorption. PHREEQC predicted precipitation of iron, aluminium, manganese-bearing phases at pH 5.53-9.12. An amorphous fraction was observed to be the most important in retention of the major and minor species at pH > 6.32. The carbonate fraction was observed to be an important retention pathway at pH 4-5 mainly due to initial local pockets of high alkalinity on surfaces of fly ash particles. Boron was observed to have a strong retention in the carbonate fraction.

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