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Preparation of Ultrafine Aluminum Hydroxide from Coal Fly Ash by Alkali Dissolution Process

A new two-step alkali dissolution technique was used to prepare ultrafine aluminum hydroxide from coal fly ash in this study. The first step alkali dissolution was to leach SiO2 from the coal fly ash using sodium hydroxide solution for obtaining the desilicated product with higher Al2O3/SiO2 ratio. The leaching efficiency of SiO2 reached to 40% when the coal fly ash was leached with 8 M sodium hydroxide solution at 95°C for 150 min. The second step alkali dissolution was that the mixtures of the as-desilicated product and white lime were leached with 20 M sodium hydroxide solution at 260°C for 60 min for extracting alumina from the desilicated product. The alumina extraction efficiency was up to 89%. The sodium aluminates solution obtained from the second alkali dissolution was used to synthesize ultrafine Al(OH)3 after treating by lime and NaHCO3 solution respectively. The results of XRD, SEM and chemical analysis revealed that the prepared ultrafine aluminum hydroxide with monoclinic structure was in the form of spherical particles with the average size of 80∼150 nm.