Effects of calcium oxide and ferric oxide on the process of alumina extraction of coal fly ash activated by sodium carbonate

Abstract

The efficient utilization of aluminum and silicon resources simultaneously is expected to reach by the process of alumina extraction from coal fly ash via Na2CO3 activation-acid leaching method. The impurities in the system, CaO and Fe2O3 especially, would affect the process. The effects of CaO and Fe2O3 on the “Na2CO3 activation-acid leaching” process were studied systematically using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR) and Inductively Coupled Plasma (ICP). The possible influencing mechanisms in the process were proposed. The results showed that soda-nepheline ((Na2O)0.33NaAlSiO4) was the main product in FA-Na2CO3 system. The presence of CaO, however, would react with Al2O3 and SiO2 and form Ca3Al2O6 and Ca2SiO4. The presence of Fe2O3 would limit the formation of (Na2O)0.33NaAlSiO4 by capturing small amount of Na2O to form Na2Fe2O4, leading to the formation of NaAlSiO4. Overall, the dissolution of Al2O3 was predominately affected by CaO, but less by Fe2O3. The compositions of Al-extracted residue would not be significantly affected by merely increasing the amount of CaO or Fe2O3. In the presence of CaO and Fe2O3 together in the FA-Na2CO3 system, an insoluble Ca2Fe2O4 was formed preferentially during the calcination process, and therefore decreased the content of SiO2 in Al-extracted residue. The further study showed that both the dissolution of Al2O3 and the content of SiO2 in Al-extracted residue could reach >90% if the contents of CaO and Fe2O3 were controlled within 6.33% and 15.27% respectively, by which the synergistically utilization of Al2O3 and SiO2 could be realized.