The Research Of Extracting Alumina From Fly Ash By Lime Autoclave-Low Temperature Calcining And Coal Powder-Lime Burning Process

This paper includes two parts, one is the research of extracting alumina from fly ash through lime autoclave - low temperature calcining process, the other is the research of producing the alumina clinker by-product through coal powder -lime burning process.The extracting alumina from fly ash by lime autoclave -low temperature calcining process is that fly ash and lime were mixed with water in a certain proportion, then hydration calcium-aluminium silicates were got by lime autoclave reaction, after low pemperature calcining, the hydration calcium-aluminium silicates were disassembled and the alumina clinker was got, which the major components were 12CaO·7Al₂O₃ and 2CaO·SiO₂. Through alkaline leaching and carbonation process, finally alumina was extracted from fly ash. The experimental results shown that the best technologic parameters are: autoclave temperature is l80℃, heattreatment time is 12h, calcination temperature is 900±50℃, calcination time is 4h, weight ratio of lime to fly ash is 2.2:1, weight ratio of water to ash is 2:1-2.2:1, the concentration of Na₂CO₃ solution is 6%(wt), temperature and time of alkaline leaching are 75℃and 15min respectively, and carbonation temperature is 50℃, residue from alkaline leaching contained CaCO₃ and 2CaO·SiO₂, which could be used in cement producing.For getting alumina clinker by-product, the lime and high-alumina coal were mixed and milled as coal powder mixture, which were calcined at high temperature in muffle furnace. Coal ash and calcium oxide directly reacted and formed alumina clinker during coal powder mixture combustion process. The results shown that CaO reacted with Al₂O₃ and SiO₂ respectively, and CaO·Al₂O₃ and 2CaO·SiO₂ were formed. And then 2CaO·Al₂O₃·SiO₂ was formed by CaO·Al₂O₃ reacted with 2CaO·SiO₂ directly. With the combustion temperature increased, 3CaO·3Al₂O₃·CaSO₄ were formed by CaO·Al₂O₃ and CaSO₄ reacted with each other. The best percentage of lime in coal powder mixture was 29.23-31.54%, the best combustion temperature was 1200℃and the alumina clinker crystal phases was 2CaO·SiO₂, 3CaO·3Al₂O₃·CaSO₄ and 2CaO·Al₂O₃·SiO₂. In addition, from the thermo-gravimetry analysis results, the combustion properties of the coal powder mixture was better than pure coal powder.