Conversion of coal fly ash to Na-X zeolites

Abstract

Coal ash is formed by combustion of coal in coal-fired power station as a waste product. In the world and Japan, the amounts of coal ash formed are approximately 500 and 5 million tons per year, respectively, and are predicted to increase [1]. The coal ash can be separated into two forms: one is fly ash (Fa) and another is clinker ash. The Fa is collected at the top of burner using cyclones, electric precipitators, or mechanical filters. Whereas, the clinker ash is obtained from bottom of furnace as a mass of fused rock. The proportion of Fa generated is occupied 84% of total coal ash [2]. In the world, only 15% of the Fa are used as a raw material of cement and concrete, and the remaining Fa is disposed of for reclamation [1]. Although, Fa contains various hazardous heavy metals, for instance, As, Pb, Sn and Cd [3]. Therefore, resource recover of Fa is of concern at present. It has been known that Fa is mainly composed of amorphous aluminosilicate with some crystals such as α-quartz (SiO2), mullite (2SiO2 · 3Al2O3), hematite (α-Fe2O3) and magnetite (Fe3O4) [4] and can be converted to zeolites in alkali solutions by hydrothermal treatment [5–10]. Since the faujasite possesses large pore openings and large cavities, this material is used as catalysts, washing builder, cation exchangers, adsorbents and so forth. The conversion of Fa to faujasite has been studied by Henmi [5] and LaRosa et al. [8]. They treated Fa in NaOH aqueous solutions at 80–90 ◦C to produce faujasite. However, not only faujasite but also other type zeolites such as zeolite P and phillipsite were formed. Recently, a single phase of faujasite has been synthesized from Fa fused with NaOH at 550 ◦C [11–13]. The disadvantage of this technique is that fusion of Fa requires a high temperature. In this study, we report the synthesis of a single phase of faujasite from Fa by treating in NaOH aqueous solutions below 100 ◦C without fusion process. The chemical composition of Fa supplied from Nippon Steel Co. was determined by a Rigaku X-ray fluorescence spectrometer (XRF) and is listed in Table I.