Recycling of pyrite and gypsum mining residues through thermochemical conversion into valuable products

Abstract

Pyrite-containing tailings pose an environmental challenge due to the risk of Acid Mine Drainage (AMD) upon storing if not properly managed. AMD can, however, be neutralized with alkali materials like lime which leads to the generation of gypsum that also ends up in the landfill. Instead of landfilling this residue, it can be recycled into potentially useful products. A proposed process involves decomposing pyrite, reduction of pyrrhotite, and finally conversion of gypsum to lime. This study evaluates the recycling/thermochemical conversion process of pyrite tailings and gypsum residues from the Boliden Aitik mine using thermodynamic modeling, simultaneous thermal analyses coupled with mass spectroscopy, and mineralogical composition identification. It was observed that pyrite can undergo complete decomposition into pyrrhotite and release elemental sulfur below 750 °C in an inert atmosphere. Furthermore, successful reduction of pyrrhotite was achieved through the application of biochar in the presence of lime at 950 °C in an inert atmosphere. The study also reveals that gypsum can be effectively converted to lime at 1200 °C in an inert atmosphere when accompanied by calcium sulfide (CaS). Moreover, the energy and material balance of the process was analyzed, considering the conversion of 1 ton of pyrite tailings. The products include 0.4 tons of iron powder, 2.7 tons of SO2 gas, 0.4 tons of CO2 gas, and 1.21 tons of excess active lime. The calculated energy consumption for this process was determined to be 2893 kWh.