Comparison of microwave and radiofrequency low-temperature ashing of coal using Fe Mössbauer spectroscopy of sulphur forms

Low-temperature ashing of coal was carried out by the normal radiofrequency method and a new microwave technique to compare the effects of the two procedures on the Fe-S compounds in coal. Results from ⁵⁷Fe Mössbauer spectroscopy and chemical analyses indicate that the microwave technique, like the normal LTA procedure, effectively ashes the coal without the oxidation of pyritic sulphur. Dehydration of iron sulphates and oxidation of Fe²⁺ to Fe³⁺ in sulphates occurs in both ashing procedures under the conditions tested, the sequence being FeS0₄.4H₂O → FeS0₄.H₂0→Fe³⁺. Dehydration is caused by sample heating rather than by reaction with the activated oxygen. The combination of the Mössbauer method and chemical analysis has allowed the determination of the recoilless fraction ratio f(sulphate)/f(pyrite) = 0.55 ±0.15, which puts the Mössbauer results on a more quantitative basis than before. This is thought to be the first quantitative work on coal and coal products that has accounted for differences in f-values for different Fe compounds.