An experimental study into pyrite transformation during pyrolysis of Australian lignite samples

Transformation of pyrite in an Australian lignite during pyrolysis was studied using a thermogravimetric analyser (TGA) and a fixed-bed reactor. Samples of a lignite with high pyrite content, a pyrite-free lignite, acid-washed lignite, a pyrite mineral, and pyrite-free lignite blended with various amounts of the pyrite minerals were employed in the experimentation. It was shown that in nitrogen the pyrite mineral (S/Fe = 2) decomposes to troilite (S/Fe = 1) at ca. 1200 K and above this temperature, some of the troilite further decomposes to form elemental iron (S/Fe < 1). However, when blended with the demineralised lignite, the pyrite mineral can be completely decomposed to troilite at 873 K as confirmed by FTIR, SEM-EDS and XRD analyses on the resulting chars. During the TGA pyrolysis of the pyrite–lignite blends, two DTG peaks were observed, indicating two major decomposition events occurring. The first DTG peak was related to the devolatilisation of the lignite, while the second peak was attributed to the pyrite decomposition. Using a mass spectroscopy coupled with the TGA, it was revealed that hydrogen sulphide and a small quantity of sulphur dioxide were released during the pyrolysis of the pyrite–lignite blends. However, only sulphur dioxide was detected during the pyrolysis of the lignite with high pyrite content. This is attributed to the reactions of the inherent pyrite with oxygenates in the lignite structure. By analysing sulphur retention in the chars, it was shown that inherent inorganic matter in the raw lignite retains part of the sulphur in the solid phase. For the pyrite–lignite blends, pyrite initially decomposes to form FeS, some of which further decomposes to elemental iron at a much slower rate.