Study On High Value-added And Green Utilization Of Fly Ash

Fly ash is a fine ash produced from coal burning. By 2020, China’s total amount of fly ash will reach more than 3 billion tons. The contents of Al2O3 and SiO2 in fly ash are very high. However, a large proportion of fly ash is used by the building industry, highway road bases and grout mixes, which cause huge waste of aluminum resource. So it will be necessary to improve value-added utilization of fly ash.In this paper, a novel method was proposed for extracting aluminum and silicon from fly ash. First, aluminum from fly ash is treated by NH4HSO4 roasting, and then, silicon from Al2O3 extracted slag is recovered by alkali leaching. The main contents and conclusions of this paper are as follows:1. Based on the results of thermodynamic calculation, the feasibility of extracting Al2O3 from fly ash by NH4HSO4 roasting process is determined.2. The technology process for extracting Al2O3 from fly ash by NH4HSO4 roasting method is studied. This technology mainly includes five parts:roasting process, aluminum precipitation, alkali dissolution, carbonation decomposition and calcination.1) The effects of roasting temperature, Al2O3-to-NH4HSO4 mole ratio and roasting time on aluminum extraction rate are investigated in NH4HSO4 roasting process. The optimal conditions are as follows:roasting temperature 400℃, Al2O3-to-NH4HSO4 mole ratio 1:8 and roasting time 60min. The aluminum extraction rate is up to 90.95% under these conditions. The element of silicon is enriched in Al2O3 extracted slag, and the content of SiO2 is improved from 45.41% to 90.00%.2) The effects of temperature, pH value and holding time on aluminum and iron precipitation rates are studied in aluminum precipitation process. The optimal conditions for this process are temperature 80 ℃, pH value 6 and holding time 40min. Under these conditions, the precipitation rates of aluminum and iron are up to 99% and 98%, respectively.3) The effects of temperature, Al(OH)3-to-NaOH mass ratio and holding time on aluminum dissolution rate and deironing rate are examined in alkali dissolution process. The optimal conditions for this process are temperature 80 ℃, Al(OH)3-to-NaOH mass ratio 1:4 and holding time 30min. Under these conditions, the dissolution rate of aluminum and the deironing rate are up to 99% and 96%, respectively.4) The effects of temperature, CO2 gas-flow rate and holding time on aluminum precipitation rate are investigated in carbonation decomposition process. The optimal conditions for this process are temperature 25℃, CO2 gas-flow rate 40mL/min and holding time 25min. Under these conditions, aluminum precipitation rate is up to 98%.5) The conditions of calcination process are temperature 1200℃ and holding time 2h. The α-Al2O3 product is prepared, and than characterized.3. The kinetics of NH4HSO4 roasting process is studied. This process follows the shrinking unreacted core model, and the inner diffusion through the product layer is the rate-controlling step. The activation energy is calculated to be 16.627kJ/mol, and the kinetic equation is expressed as: 1-2/3x-(1-x)2/3=0.0374×exp(-16627/RT)t4. The digestion kinetics of sintered clinker to extract alumina from fly ash with NH4HSO4 roasting process is studied. The optimal conditions for digestion process are temperature 90℃, stirring intensity 300r/min, liquid-to-solid mass ratio 9:1 and holding time 60min. The digestion process is divided into two stages:0-20min for the first stag; 20-60min for the second stage. The digestion rate of aluminum in the first stag is faster than that in the second stage. The rate-controlling steps of the two stages are both the outer diffusion of no product layer. The activation energies of the first and second stages are calculated to be 8.013kJ/mol and 4.973kJ/mol, respectively, and the kinetic equations are expressed as: The first stage:1-(1-x)2/3=2.3672×exp(-8013/RT)t The second stage:1-(1-X)2/3=0.0381×exp(-4973/RT)t5. The technology of extracting Al2O3 from fly ash with NH4HSO4 roasting process is popularized. And fly ash F2 is used. This technology mainly includes four parts:roasting process, deironing, aluminum precipitation and calcination.1) The effects of roasting temperature, Al2O3-to-NH4HSO4 mole ratio and roasting time on aluminum extraction rate are investigated in NH4HSO4 roasting process. The optimal conditions are as follows:roasting temperature 400℃, Al203-to-NH4HS04 mole ratio 1:8 and roasting time 120min. The aluminum extraction rate is up to 73.96% under these conditions.2) The effects of 30% H2O2 addition, pH value, temperature, holding time, Fe(OH)3 seed addition and water addition on deironing rate and aluminum loss rate are investigated in Fe(OH)3 deironing process. The optimal conditions for this process are 30% H2O2 addition 3.6mL, pH value 3.3, temperature 90℃, holding time 1.5h, Fe (OH)3 seed addition (wet seed, moisture content 67%) 12g and water addition 100mL. Under these conditions, the deironing rate is up to 98%, and the aluminum loss rate is only 10%.3) The effects of temperature, pH value and holding time on the precipitation rates of aluminum and iron are investigated in aluminum precipitation process. The reaction solution is filtered immediately at 90℃ and pH value 5. Under these optimal conditions, the precipitation rates of aluminum and iron are up to 99.23% and 35.2%, respectively.4) The conditions of calcination process are temperature 1200℃ and holding time 2h. The a-Al2O3 product is prepared, and then characterized.6. The effects of basic properties of six fly ashes on aluminum extraction rate are investigated, such as chemical component, phase composition, fineness and microstructure. The aluminum extraction rate increases with increasing the contents of AI2O3, glass phase and fine ash.7. The kinetics of SiO2 leaching from Al2O3 extracted slag of fly ash with NaOH solution is studied. The effects of leaching temperature, NaOH-to-SiO2 mass ratio and stirring intensity on SiO2 leaching rate are investigated. The SiO2 leaching rate is up to 95.66% under the conditions of leaching temperature 90℃, NaOH-to-SiO2 mass ratio 2:1, stirring intensity 300r/min and leaching time 30min. There are two stages for the leaching process. 1-5min is the first stage, and 5-30min is the second stage. The leaching reaction is very rapid in the first stage but quite slow in the second stage. The whole leaching process follows the shrinking unreacted core model, and the outer diffusion of no product layer is the rate-controlling step. The activation energies of the first and second stages are calculated to be 8.492kJ/mol and 8.668kJ/mol, respectively. The kinetic equations of the first and second stages can be expressed as: The first stage:1-(1-χ)2/3= 0.5409 × exp﹝-8492/RT﹞t The second stage:1-(1-χ)2/3=0.046×exp﹝-8668/RT﹞t8.The products of Si02·nH2O and CaO·SiO2 are prepared from Na2O·nSi02 solution, and the two products are characterized.