ABSTRACT
The exposure of pyrite (FeS2) to atmospheric conditions during mining activity causes a series of complex oxidation reactions, resulting to acid generation and the subsequent release of toxic heavy metals in the surrounding aquatic and terrestrial ecosystems. The produced acidic mine waters, known as acid mine drainage (AMD), constitute one the major environmental problems of both operating and abandoned mixed sulphide, coal and other mine sites where sulphidic minerals are encountered. A sustainable approach to the environmentally safe pyrite-bearing extractive waste management is related to the prevention of oxidation by developing artificial coatings on the pyrite surfaces. In this study, experiments performed to study the conditions of the silica coating formation on the FeS2 particles contained in a pyrite concentrate are presented. Batch tests involving the treatment of pyrite samples with a coating solution, consisting of Na2SiO3, H2O2 and buffered pH, were performed under a liquid to solid ratio (L/S) 100 l/kg. The effect of parameters including SiO2 concentration (5 - 50 mM), pH values (5.0 - 8.0) and contact time up to 24 hours, was investigated. Parameters examined to monitor the silica coating formation process include analysis of Fe, Si,
1. Introduction Acid mine drainage (AMD) constitutes one of the most severe environmental problems of the mining industry. AMD which is a low-pH aqueous phase enriched primarily in iron (Fe) and sulfate anions, but also containing many metals and metalloids, such as lead (Pb), copper (Cu), zinc (Zn), manganese (Mn), nickel (Ni), cadmium (Cd), mercury (Hg) etc., depending on the mineralogy of the ore/waste. These acidic waters are produced by the oxidation of sulphide minerals, contained in the mine wastes and rocks, under the action of atmospheric air and water and catalyzed by the presence of bacteria. Singer & Stumm (1970) described the pyrite oxidation, which may occur via direct and indirect mechanisms, according to the following reactions:
Once generation of acidic waters commences, the sources of acidity may remain active for a long time after the mine closure. So, the effects of AMD can be both pernicious and far-reaching since contamination of surface and/or underground water can carry the pollution considerable distances from the mining area. The acidic drainages are treated by conventional reclamation techniques, such as addition of alkaline materials (mainly, limestone and lime), covering with water to prevent oxygen access or formation of low permeability soil covers to minimize oxidizing agents transfer to the sulphides. Apart from the conventional prevention and remediation methods, an emerging and appealing technique for the inhibition of acid generation from sulphidic wastes involves the development of artificial coatings on sulphide grain surfaces (particularly on pyrite grains due to the abundance to all geological environments). This process is known as microencapsulation and was first described by Evangelou (1995a, 1995b), investigating the formation of phosphate coating on pyrite surfaces.
Other published studies refer to the establishment of coatings using inorganic materials, FeOOH (Huminicki & Rimstidt, 2009), silicates (Zhang & Evangelou, 1998; Evangelou, 2001) and magnesium and manganese compounds (Glover, 2007) and organic coatings such as humic acid and other organic ligands (Lalvani et al., 1996; Elsetinow et al., 2003; Kargbo et al., 2004).
The methodology, which was developed for the formation of silicic coating includes partial leaching/oxidetion of the sulphidic waste using a solution containing hydrogen peroxide (H2O2), a silica source and adjusted to pH 4.0 - 6.0 (Evangelou, 1996).
According to the study of Evangelou (2001), pyrite treatment with a solution consisting of H2O2 0.145 M, Si 0.0018 M, CH3COONa 0.01 M, NaCl 0.1 M and adjusted to pH 6 at room temperature results in the formation of a silica coating composed of an inner layer of Fe3+ oxy-hydroxides and an outer layer of Si hydroxides. The coating process reduced by 85% the amount of oxidized pyrite compared to the control test. Bessho et al. (2011) performed batch-type tests and an effective silica coating was established on the pyrite surfaces, after treatment with a solution of Si concentration higher than 2500 mg/L and pH: 7.0.
However, coating technology needs further investigation. In this study, batch-type tests were performed in order to investigate the effect of pH and silica concentration on the formation of silica coating on the grains of a pyrite concentrate material.
more:http://www.scirp.org/journal/PaperInformation.aspx?PaperID=47770
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