Role of additives in improved thermal activation of coal fly ash for alumina extraction

The extraction of alumina from coal fly ash is a good direction for its value-added utilization. The presence of the inert matters with high degree of polymerization, such as mullite and other aluminosilicates, makes the reactivity of coal fly ash very poor. The activation of coal fly ash is necessary before its utilization. The thermal activation calcination with the addition of NaOH and Na2CO3 was carried out in this research. The results showed that the addition of NaOH and Na2CO3 improved the alumina extraction evidently. The maximum alumina extraction reached ~ 60% when calcination at 600–900 °C with the addition of NaOH and that could reach 82% at 900 °C with the addition of Na2CO3. Detailed analysis and characterization was carried out by using thermal gravimetric and differential scanning calorimetric analysis (TG-DSC) and X-ray diffraction (XRD). The results indicated that NaOH and Na2CO3 facilitated the decomposition of the polymeric phases. At lower temperatures (< 600 °C), NaOH played a main role while Na2CO3 did at higher temperatures (>  700 °C). As a result, the mixed additives containing NaOH and Na2CO3 made alumina extraction attain 95% at 700 °C.

► NaOH or Na2CO3 greatly improved the extraction of Al2O3 from coal fly ash.
► NaOH and Na2CO3 improved the disaggregation of mullite at lower and higher temperatures.
► The mixed additives containing NaOH and Na2CO3 obtained better activation effect on coal fly ash.
► The mechanisms about the additives on the effect of activation of coal fly ash were discussed.