Acid drainage is an environmental problem, not exclusive, but commonly associated to coal mines. It arises from the oxidation of sulfides, like pyrite, in overburden and mining tailings. The use of ashes from coal combustion is still limited in Brazil, but such residues have a potential use as neutralizing agent for acid mine drainage. This study intended to evaluate the interaction between the use of coal ash and CaCO3 to neutralize acid drainage from pyrite oxidation in coal mining overburden from Candiota (RS) - Brazil. Samples of overburden material containing pyrite were treated with increasing CaCO3 doses and coal ash in leaching flasks. The treatments were disposed in a completely randomized design in 4 x 4 factorial scheme (4 doses of CaCO3 and 4 doses of coal ash) with three replications. The flasks were submitted every two weeks to leaching with distilled water during eight months. The leached solutions were analyzed for pH, free acidity and S, Si and Fe contents. Results showed that the use of ash was impractical not only due to its low neutralizing capacity, but also because it reduced the CaCO3 efficiency. After eight months of simulated weathering only 11% of the sulfides were oxidized in the absence of ash or carbonate. The use of CaCO3 and, to a lesser extent of ash, increased the pyrite oxidation rate. Pyrite in coal overburden oxidized faster in the first two months of simulated weathering, thus it is recommended that lime be applied just after overburden exposition in order to minimize acid drainage. Further investigation aiming to optimize the CaCO3 doses and test alternative neutralizing materials in long term experiments should be encouraged.
Sulfide oxidation; acid drainage neutralizers; pyrite; coal
