Desulphurisation of coal using low power microwave energy

Abstract

The application of microwave radiation at 500 w power and a frequency of 2.45 GHz for the preferential heating of dielectric inorganic pyrite in high sulphur coals to form a more magnetically susceptible phase and allow the low gradient magnetic separation of the FeS 2 is described. However, although pyrite was selectively heated under these conditions compared with clean coal, the heating rates were not enough to reach the temperatures necessary for conversion of pyrite to pyrrhotite, owing to the low power of the microwave in this work. When the coal was mixed with strong caustic (KOH or NaOH) solutions (of concentration 300 g/l) at 20% solids which are excellent microwave absorbers, a degree of heating was obtained that enabled desulphurisation to take place. The powdered −212 μm coal after exposure to microwaves of 2.45 GHz frequency for periods up to 60 seconds was not significantly altered in character apart from a maximum of 70% reduction in the total sulphur content, which varied with the degree of exposure and the concentration of alkali. The mechanism of this reduction was by magnetic separation of a magnetically enhanced pyrite and also the chemical dissolution of both pyrite and organic sulphur which was washed along with any remaining caustic after treatment.