Effect of storage time on the pressure oxidation enthalpy of pyrite

The information on heat of oxidation-reduction reactions is important for the heat balance and optimization of the autoclave design in the hydrometallurgical industry in ore processing. Pyrite (FeS2) is a gangue mineral that presents with nickel-containing pentlandite, and copper-containing chalcopyrite minerals. The presence of pyrite impacts to the overall heat of leaching process. This study has been performed on a differential scanning calorimeter (DSC80, Setaram) with a commercial mixing cell to study the thermal behaviour of pure pyrite FeS2 (Valdenegrillos, Spain) mineral particles during oxidative pressure leaching at 150 A degrees C and partial oxygen pressure of 3.4 MPa. A calorimetric method for determining the enthalpy of leaching of sulphide minerals at high temperatures and oxygen pressures has been used to evaluate the enthalpies of oxidation of freshly ground pyrite and pyrite stored for a year in contact with air (stored pyrite) under conditions relevant to pressure oxidation operations. Ground pyrite stored over time has long since been known to result in greater heat evolution during oxidative leaching. A likely mechanism for this phenomenon was uncovered: formation of ferrous sulphate and sulphuric acid during storage in contact with air influencing greater heat evolution at the outset of the reaction. Two mass loss steps on TG curve of stored pyrite, attributed to the elimination of atmospheric moisture and H2O molecule from FeSO4 center dot H2O, is absent on TG curve of freshly ground pyrite.