A utilização de uma camada de solo compactado como revestimento impermeabilizante de fundo de bacias de disposição de lama vermelha produzida em Barcarena-PA

Abstract

International technical specifications and the study of mass transport through porous media for environmental control point out to the need of bottom liners, usually composed of a geomembrane associated to a compacted clay liner (CCL), for waste disposal in soil. In 1995, an industrial unit of Alumina of Brasil S/A started activities in the state of Para, Brazil, to produce alumina, Al2O3, for Albrás Alumínio do Brasil S/A. The wastes of this industry, known as "red mud", are generated by the digestion of bauxite by solutions of sodium hydroxide (Bayer process) and disposed in basins lined by geomembranes. The solid phase of red mud is constituted by hematite, anatase and goethite (residues of bauxite) and sodalite (formed during the industrial process); the liquid phase, initially at pH 12, is rich in soluble sodium. This research investigated the technical feasibility of the utilization of a local soil in the construction of a CCL for the disposal basins, considering its impermeabilization capacity and compatibility with the red mud. The experimental investigation consisted of the physical, chemical and mineralogical characterization and determination of geotechnical (compaction, permeability and collapse) and environmental properties (leaching and diffusion) of the soil. The study showed that the used soil is equivalent to Yellow Latosols of Amazonia: it is silt-sandy, yellow, composed predominantly of quartz, kaolinite and smaller concentrations of goethite/hematite and anatase, and therefore rich in SiO2 and Al2O3 besides Fe2O3 and TiO2. The compacted soil, submitted to seepage of water and solutions of sodium hydroxide at concentrations of 1.23% and 5%, presented permeability coefficient smaller than 10-9 m/s. SEM/EDS analyses of the compacted soil in contact with red mud for over a year identified the diffusion of Na, which content in the soil increased 1950%, however without reaching limit contamination levels. Oedometric collapse tests revealed that the soil compacted in the optimum water content under Proctor standard effort becomes remarkably compressible when saturated with NaOH solutions, which was confirmed by zeta potential, rheometer, and characterization tests with NaOH solutions: solid particles are dispersed and consequently the geo-mechanical behavior of the soil alters by contact with NaOH solutions. The results lead to the conclusion that infiltration of caustic solutions can cause a decrease of the safety factor of dams and CCLs in red mud disposal areas, when geomembranes present defects or holes.