Study On Extraction Of Al₂O₃ And SiO₂ From Fly Ash

Fly ash is rejects residue of coal combustion in power plants. The large amount of rejects fly ash occupies farm field, causes environment pollution, creates harm to residents nearby and restricts local economic development. Therefore, the study on fly ash utilization has significance both in practice and in strategy in the long run.This paper studied on extraction of Al2O3 and SiO2 from fly ash emission from a power plant in Shanxi province. The process was designed and studied. Al2O3 was extracted from fly ash leached by concentrated sulfuric acid 98%, and SiO2 was extracted from fly ash residue leached by 60% concentrated NaOH solution.The conclusion of the research goes as the following:1. Study on Al2O3 Extrated From Fly Ash by Leached with Concentrated Sulfuric Acid 98%In this method Al2O3 was leached by concentrated sulfuric acid 98% from fly ash. Al2(SO4)3·18H2O was decomposed to Al2O3. The processing scheme is proposed as followed:Fly ash→Mechanical activation→Acid leaching→Baking→Dry residue→Dissolving→Aluminium sulfate solution→Crystallizing→Al2(SO4)3·18H2O→Iron-removal recrystallizing→High-purity Al2(SO4)3·18H2O→Baking→Al2O3.The determined acid-leaching processing conditions are:fly ash D50 of 10μm, sulfuric acid concentration of 98%, temperature of 260℃, leaching time of 120min, and the ratio of liquid to solid of 4:1.Dissolving condition is:aluminium sulfate solution is obtained by dissolving acid-removal dry residue in water (500ml water/100g fly ash) for 40 minutes. The Al2O3 extraction yield could reach 87%.Primary iron-removal Al2(SO4)3·18H2O measured up to chemical industry Standard; Al2O3 product prepared by three time iron-removal recrystallizing and baking at 1100℃measured up to nonferrous metals industry Standard. The thermal decomposition of Al2(SO4)3·18H2O involves a two-step dehydration and a Al2(SO4)3 decomposition. The three equations of reaction rate above were determined from activation energy, reaction order, and preexponential factor which were figured out based on Doyle-Ozawa Method and Kissinger Method. Rate equation of first dehydration: Rate equation of second dehydration: Rate equation of decomposition:2. Application Study on Extracting Process of Al2O3The acid-extract process performed on low-grade bauxite acquired 88% Al2O3 leaching rate, which showed acid-extract process of Al2O3 is applicable to low-grade bauxite.3. Study on Extracting Process of SiO2 From Fly Ash Residue Leached by 60% Concentrated NaOH SolutionPrecipitated silica was made using fly ash residue as silica source. 60% concentrated NaOH solution dissolved the residue in very low ratio of liquid to solid condition to produce sodium silicate solution, that maked precipitated silica by means of carbonation method. The processing scheme was proposed as followed:Fly ash residue→Alkali leaching→Dissolving→Sodium silicate solution→Carbonation→Precipitated silica. Secondary Carbonation was designed to prepareing high purity precipitated silica. Causticization treatment on sodium silicate solution enabled the recycling use of all materials and realized clear production.Leaching and dissolving Conditions are:temperature of 110℃, leaching time of 90min, NaOH concentration of 60%, the ratio of liquid to solid of 2.2:1, Dissolving temperature of 80℃, water addition of 1000ml/100g residue, dissolving time of 60min. The SiO2 extraction yield reached 86%.Secondary carbonation condition are:the carbonation temperature of 60℃～80℃, sodium silicate solution concentration and modulus of 70 g·L-1 and 0.6, air flow rate of 20ml·min-1～30ml·min-1, the first carbonation termination of at pH 11, the second carbonation termination of at pH 9.5, and the reaction rate of above 90%. The Precipitated silica was washed and then dried to get precipitated silica with purity of over 99.7% up to Chemical Industry Standard.The method of iron-removal by hydrochloric acid was adopted to prepare silicon powder from fly ash residue. The iron-removal conditions are:temperature of 80℃, liquid-solid ratio of 6:1, HCl concentration of below 6mol·L-1 and the iron-removal rate of 75.5% after two hours' reaction. The quantity of iron-removal product accorded to Metallurgy Industry Standard.4. Viscosity Behavior Analysis of Sodium Silicate SolutionThe viscosity of sodium silicate solution and the concentration of SiO2 and NaOH have linear relationship in high temperature and deviate from the linear in low temperature. The viscosity and viscous flow activation energy of sodium silicate solution increase with the increase in concentration of SiO2 and NaOH. The viscous flow activation energy of sodium silicate solution has a quadric relation with the concentration of SiO2 and has linear relation with the concentration of NaOH. The formulas for calculation respectively are as follows: E=6.8825-0.0829×CSiO2+0.00143×CSiO22 (20g·L-1≤CSiO2≤80g·L-1) E=6.4862+0.0696×CNaOH (0g·L-1≤CNaOH≤200g·L-1)5. Economy Analysis and Environmental AssessmentMaterial balance, energy balance, economy analysis and environmental assessment were carried out on the process of Al2(SO4)3·18H2O from coal fly ash and precipitated silica from fly ash residue.