Potencial de acidificação e de neutralização em materiais geológicos da jazida de carvão de Candiota, sul do Brasil

Abstract

In Candiota (RS) coal mining is carried out in surface strip mining by Companhia Riograndense de Mineração (CRM). The soil and geological layers above the coal seams are removed and the topographic recomposition is done by replacing the geological layers in the previously opened pit when the next strip is excavated; Afterwards, the overburden piles are reconformed and on the leveled surface is placed a layer of surface soil (topsoil), composing a "constructed" minesoil. Pyrite, found in coal and associated rocks, in the “constructed” minesoils, reacts rapidly to sulfuric acid, triggering acidification of soil and water and causing solubilization of metals, a process known as acid mine drainage (AMD). The objective of this work was to estimate the probable composition of the constructed soil and its predisposition to generate AMD through the chemical characterization and estimation of potential acidity of the materials of the geological column of areas that will be mined in the coal mining area of Candiota (RS), thus providing subsidies to prevent / mitigate the generation of acid drainage in future areas. Six drill holes were selected and the main lithologies that will compose the future constructed minesoil profile (topsoil and overvurden layers) were sampled. The core sample materials were ground and treated as soil, and the basic characterization of the exchangeable complex was carried out, determining the following attributes: water and CaCl2 pH, basic cations (Ca, Mg, K, Na), Al and potential acidity (H+Al). The acidification potential (AP) and neutralization potential (NP) of the geological materials were estimated using the hydrogen peroxide method adapted for NP determination. Data on the chemical composition of the exchangeable complex of these materials indicate that practically all the coal layers will provide the overburden layer with a strongly acidic reaction. This is relevant because, generally, some layers are not explored, due to their lower thickness and lack of continuity, as the BL layer (Louco Seam), that are incorporated into the overburden materials of the constructed minesoil profile. In future mining, if the same type of action is carried out with the S2, S3, S4 and S5 coal layers, the problems of acidification will be intensified. In addition, some layers of carbonaceous shale and sandstone above the BL coal seam also exhibit acidic reaction, which means that even by exploiting all the coals, the acid reaction in the overburden minesoil profile layer could persist. On the other hand, other layers present alkaline reaction, which can neutralize part of this acidity. The estimation of the acidification and neutralization potentials of these materials makes it possible to predict the process balance and the trend of the system to evolve depending on the decisions taken regarding the unmined coal layers and the controlled or uncontrolled disposal of the geological materials that will compose the constructed minesoil profile.