Study On Preparation Of Al - Si Alloy By Carbon Reducing Fly Ash

In 2014, the output of fly ash reached above 0.5 billion tons, and the annual output of fly ash will increases with the development of power industry. The improper treatment of massive fly ash will impose negative influence on the quality of air, water and soil, which will consequently do damage to human health. High aluminum content fly ash has high content of aluminum oxides and silicon oxides, which is a kind of valuable secondary resource which can be exploited. The aluminum and silicon alloy extracted from fly ash with the method of carbothermic reduction can be used as reductant substituting silicon and iron alloy to smelt magnesium, which can not only broaden the utilization channels of fly ash, improving the high additional value utilization of fly ash, prolonging the coal industry chain as well as keeping the sustainable development of coal and power industry.The thermodynamics of Al-Si-O-C system was calculated at 100KPa, 100Pa, 50Pa by HSC chemistry software, and experiment research was performed under the condition of atmospheric pressure and vacuum. Three different sources of coal fly ash as raw materials, and their chemical compositions, phase constitute and microstructure were studied. Investigation on the phase change law of three kinds of fly ash with great differences between their constituents has been conducted during their reduction process in high-frequency induction furnace, and the characteristic of these three kinds of fly ash is that they are of high content of aluminum, high content of iron and high content of calcium, respectively. The technological conditions of carbothermic reducing high aluminum content coal fly ash,such as the option of reducing agent, smelting temperature, smelting time and carbon charging rate,that were investigated in detail. Simultaneously, production of magnesium was studied used the coarse Al-Si alloy as reducing agent which was generated during carbothermic reducing high aluminum content coal fly ash, by thermal reduction process.The chemical reaction thermodynamics of Al-Si-O-C system was investigated under ordinary pressure, and the experimental results indicate that it was easier for silicon oxides to react with carbon than aluminum oxides, the temperature at which silicon carbide was produced is lower than the one at which monatomic silicon was formed, the initial reaction temperature of aluminum and carbon substance is lower than the temperature at which aluminum was produced, gas state Al2O and SiO were produced under high temperature, and only when the temperature was higher than 2330K can aluminum and silicon alloy be obtained by smelting aluminum oxides and silicon oxides together with the method of carbothermic reduction. Alumina and silica react with carbon which was a competing reaction. Silica more easily reacts with carbon, result that small amount carbons react with alumina. The reducing slag and condensation product during vacuum carbothermaic alumina and silica mostly were aluminum carbide and SiC.The reduction process of carbothermic reducing high aluminum content coal fly ash mainly were that decomposition of mullite, silica react with carbon formed SiC. The primary source of Al and Si were the reaction between SiC and Al2O3, the coarse alloy was consisted of Si, Al, SiC, and Al9FeSi3. the reduction process of carbothermic reducing high iron content coal fly ash in relatively low temperature was similar with the high aluminum content coal fly ash,the phase of FeAl2.?Si2.3 was emerged at 2173K was indicate that the presence of iron can reduce the generated temperature of metal aluminum,the phase of coarse alloy were SiC, FeAl2.7Si2.3, Feo.42Si2.67, Si. Phase transformation of carbothermal reduction high calcium coal fly ash was different with the above two coal fly ash. CaSO4 react with C produced CaS at 1873K,then CaSO4 react withC produced CaO at 1973K, CaO, SiO2 andAl2O3 were generated Ca2Al2Si07 and CaA104, it was not CaAl2Si2, FeAl3Si2, Al and SiC generation until the temperature reachs 2273K. Coarse alloys of three kinds coal fly ash at 2273K which were consist of dark gray phase, gray phase, and silver phase, the kinds of elements in each phase almost were the same,but the content of elements were different.Three repeat experiments were performed at a better technological condition that was charcoal as reducing agent,2323K,30min, carbon charge rate (0.92; mass fraction). The average chemical composition of coarse alloy were Al (29.71%),Si (48.62%),Fe (7.27%),Ti (1.42%),Ca (2.17%),C (2.06%),others (8.75%) The mean recovery rate of Al, Si and Fe were 60.19%,72.66%,111.92% respectively, the production rate of SiC was 5.36%.Production of magnesium by thermal reduction process was conducted, magnesium oxide as raw material,the coarse Al-Si alloy as reducing agent, vacuum degree ranges of 5-15Pa.The content of Mg was 97.00 wt.% in the coarse Mg that was generated during thermal reduction. With the increase of reduction temperature magnesium recovery rate increased, magnesium recovery rate was 65.12% at 1473K. Due to the coarse al-si alloy with magnesium oxide generated MgAAl2O4 and Mg2SiO4 against reaction continues to go smoothly, so that the magnesium recovery rate is low.