Preparation of Ultrafine -Al2O3 Powder from Fly Ash by Ammonium Sulfate Roasting Technology

Abstract

(NH4)2SO4 roasting technology was used for extracting alumina from fly ash which located in Shandong Zibo power plant. The roasting temperature, the mole ratio of Al2O3 to (NH4)2SO4 and roasting time on the effect of the extraction rate of alumina was studied. The optimal roasting technology condition is 380°C for 120min with mole ratio of Al2O3 to (NH4)2SO4 of 1:6. Under the optimal roasting condition, the extraction rate of Al2O3 can reached 82% and a new phase NH4Al(SO4)2 was formed in clinker. NH4Al(SO4)2 in clinker was dissolved by distilled water and then separated from silicon-rich residue by filtration. Iron in NH4Al(SO4)2 solution was removed by goethite process, and then, NH4Al(OH)2CO3 was synthesized by adding (NH4)2CO3 in NH4Al(SO4)2 solution. Ultrafine α-Al2O3 powder was prepared by calcining NH4Al(OH)2CO3 in 1200°C for 120min, which was characterized by XRD and SEM. Introduction Fly ash is generated during high temperature combustion of coal in coal-fired power plants [1-2]. Currently, about 800 million tons of coal fly ash has been generated in the world [3-4]. Since coal is still the major energy resource in China, the output of fly ash has increased continuously over the past several years, reaching 480 million tons in 2010 [5]. The annual fly ash discharge is more than 4×10 8 tons in China [6−7]. Presently, fly ash is consumed on a large scale in cement and concrete fillers as well as roadway and pavement utilization [8−11]. However, despite these positive uses, the production rate of fly ash is much greater than its consumption. There is still a proportion which is disposed of in ponds or landfill, so fly ash has become the main waste of power plant [12]. Fly ash contains significant amounts of alumina, typically about 20-50%, which presents an exciting new alternative source of alumina other than bauxite. Thus, extraction of aluminum from coal fly ash is environmentally and scientifically significant for disposing and utilizing waste materials and exploring new aluminum source, and has attracted extensive attention recently [13]. The Bayer process is the dominating method of refining alumina from bauxite ores throughout the world. The Bayer process mainly involves the bauxite digestion, solid-liquid separation, gibbsite precipitation and calcination [14]. In fly ash, the main form of alumina is mullite, and the alumina-silica ratio (mass ratio of alumina to silica) is approximately 1-1.5. With the low alumina-silica ratio and the stable mineral phase of mullite, a traditional Bayer Process is not suitable for the extraction of alumina from fly ash [15-18]. By using the acid method, alumina is dissolved in acids at high temperature. However, the acid method has not been used in industry because acid-resistant equipment is expensive and iron oxide had has to be separated from the alumina prior to treatment [19]. In the alkali method, fly ash is calcined with soda and lime at 1200°C, and the silica is converted into stable calcium silicate, thus separating the silica from the alumina. The alkali method is a relatively mature process, but the high sintering temperature consumes vast amounts of energy [17]. Recently, a new process for extracting alumina from coal fly ash has been established; where ammonium sulfate is used to roasting with coal fly ash [20]. Compared with acidic methods and alkali methods, this process results in fewer residues and less corrosive to equipment because ammoniums sulfate as a raw material. Ultrafine α-Al2O3 powder has received increasing attention due to its wide applications in transparent and electronic ceramics, single crystals, abrasives and catalysts [21]. Presently, the traditional methods for preparing α-Al2O3 powder are thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) or ammonium aluminum sulfate hydroxide, Bayer process [22]. This research is therefore focused on the preparation of ultrafine α-Al2O3 powder from fly ash by ammonium sulfate roasting technology. The technology includes the extraction of Al2O3 from fly ash by roasted with (NH4)2SO4, Fe removal of solution, the synthesis of AACH and the preparation of ultrafine α-Al2O3. The effects of roasting process conditions on the extraction rate of Al2O3 were investigated and the product was characterized.