An experimental study of sulphate transformation during pyrolysis of an Australian lignite

The transformation of sulphate minerals during pyrolysis of an Australian lignite has been studied using pure sulphates (CaSO₄, FeSO₄ and Fe₂(SO₄)₃), a high mineral (HM) lignite sample and a low mineral (LM) lignite sample collected from different locations of the same deposit, and samples of acid-washed LM doped with sulphates (CaSO₄+ LM and FeSO₄+ LM), respectively. Thermogravimetric analysis and fixed-bed reactor techniques were used for the pyrolysis experimentation and the lignite samples and their chars were analysed using FTIR and XRD. The TGA experiments showed that CaSO₄ decomposes between 1400 and 1700 K in nitrogen and a 50/50 N₂/CO₂ mixture, while in air CaSO₄ decomposes between 1500 and 1700 K. Using a TGA-MS it was found that only a small fraction of CaSO₄ in CaSO₄+ LM decomposed at 653 K, releasing SO₂. CaSO₄ was still observed in the char recovered at 1073 K as confirmed by the FTIR and XRD analysis. FeSO₄·7H₂O released the bound water below 543 K and the remaining FeSO₄ decomposed between 813 and 953 K. FeSO₄ in FeSO₄+ LM decomposed at 500 K to release SO₂. The inherent sulphates in HM were dominated by iron sulphates which started to decompose and release SO₂ at around 500 K and all sulphate had been decomposed at 1073 K. It was observed that during the fixed-bed pyrolysis at 1073 K in nitrogen, approximately 36% of the total sulphur in the CaSO₄+ LM decomposed, 88% of the total sulphur in the FeSO₄+ LM decomposed and around 76% of the total sulphur in HM decomposed. It was also confirmed that FeSO₄+ LM produced more volatile sulphur than CaSO₄+ LM during pyrolysis.