Poor management of pyrite tailings and waste gypsum could lead to acid mine drainage. The simultaneous treatment of pyrite and gypsum to produce valuable products, such as CaO, S, SO2 and Fe, will generate additional commodities from mining residues. The recycling process includes the decomposition of pyrite (FeS2) under a controlled atmosphere to obtain pyrrhotite (Fe1-xS) and elemental sulfur (S) which can be collected by condensation. The produced pyrrhotite will be reduced by bio-carbon in the presence of lime to produce metallic iron (Fe), calcium sulfide (CaS) and carbon monoxide gas (CO). The reduction product will be separated by magnetic separation to a magnetic part (Fe-rich) and a non-magnetic part (CaS-rich). Finally, the produced CaS will be recycled by reaction with gypsum (CaSO4.2H2O) to produce CaO and sulfur dioxide (SO2) gas. It is worth mentioning that the CaO and SO2 produced during the recycling process can be further returned to cement and sulfuric acid production, which is the right step towards the circular economy.

In this study, the thermochemical conversion process of pyrite tailings (from Aitik Boliden mine, Sweden) and gypsum into valuable products is assessed. This assessment is conducted through thermodynamic modeling and simultaneous thermal analyses coupled with mass spectroscopy.