As shown in Fig. 4C, a proportion of the F1, F2 and F3 fractions occurred from 600 to 1000 °C in the S3 samples, possibly because of the Nexturastat A process. Some of the F4 fraction was transformed into other fractions through the high-temperature oxidation process in the presence of oxygen, chlorine and sulfur. However, when the temperature was greater than 800 °C, these newly generated fractions were evaporated off or converted into the F4 fraction again, indicating that the F4 fraction is stable under heat treatment in temperatures near or above 1000 °C. This result is consistent with the low evaporation of HM in SA.
Therefore, these results indicate that different speciations in fly ash have different evaporation characters. The F1 fraction was easily evaporated below 800 °C the F2 fraction from 800 to 1000 °C the F3 and F4 fractions in three metals are relatively stable at a temperature of approximately 1000 °C.
3.4. Relationship between the chemical speciation and evaporation of HMs