Alumina Extraction from Mexican Fly Ash
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Mater. Res. Soc. Symp. Proc. Vol. 1243 © 2010 Materials Research Society
Alumina Extraction from Mexican Fly Ash Jorge López-Cuevas, David Long-González, Carlos A. Gutiérrez-Chavarría, José L. RodríguezGalicia and Martín I. Pech-Canul CINVESTAV-IPN Unidad Saltillo, Ramos Arizpe, 25900 Coah., México ABSTRACT Two alternative chemical methods are studied for the extraction of Al2O3 from Mexican Fly Ash (FA). Reaction of FA with H2SO4 at high temperature allows extracting ~37% of the total Al2O3 contained in the FA as Al2(SO4)3, regardless of H2SO4 concentration, treatment time and temperature employed. This is partly due to the high chemical resistance of mullite (Al6Si2O13) contained in the FA. In contrast, reaction of FA with a CaCO3-Na2CO3 mixture at 1300ºC/1h, followed by lixiviation with a Na2CO3 aqueous solution and precipitation of bohemite [AlO(OH)] by addition of either H2O2 or NH4HCO3, allows extracting ~80% of the total Al2O3 contained in the FA as θ-alumina, after calcination of bohemite at 1200°C/1h. INTRODUCTION The totality of the alumina (Al2O3) consumed by the Mexican ceramic industry is imported, due to a lack of suitable bauxite (Al2O3⋅nH2O) mineral deposits in this country. Fly Ash (FA) constitutes an attractive alternative raw material for the extraction of alumina. FA is a solid and fine silicoaluminous byproduct of the combustion of powdered coal in power generation plants. It is formed by evaporation of some inorganic components of powdered coal at high temperature. Once these vapors are cooled down, a large number of fine particles is formed with a size ranging from 5 to 75 μm; they can be collected by means of electrostatic precipitators [1]. Chemically, FA is composed by SiO2, Al2O3, FeO + Fe2O3, alkali and alkali earth oxides, plus some heavy and transition metal oxides. Mineralogically, FA is composed mainly by mullite (Al6Si2O13), quartz (SiO2), magnetite (Fe3O4) and/or hematite (α-Fe2O3), plus some glassy phases [2]. FA contains about 60-80 wt.% of glassy spheres, the rest of it corresponds to crystalline granules. The spherical particles are constituted by glassy bubbles (cenospheres), solid spheres and cenospheres filled with smaller spheres (plenospheres). The total annual production of Mexican FA is not accurately known, however, in the year 2002 the “José López Portillo” power generation plant, located in the northern state of Coahuila, reported a daily production of ~4,600 tons, from which only ~4-5% is used, mainly by the construction industry. The remaining FA is disposed in landfills. In this work, we present the results obtained from a study carried out on the extraction of Al2O3 from Mexican FA, by using acidic and basic chemical methods. EXPERIMENTAL DETAILS FA produced by the “José López Portillo” power generation plant has been employed. Iron oxide impurities are separated from the FA by using a high intensity Indiana General dry magnetic separator, with a magnetic field intensity of 5000 Gauss. Two magnetic separation cycles are given to the employed FA batch. The part of
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