A thermogravimetric study into the effects of additives and water vapor on the reduction of gypsum and Tunisian phosphogypsum with graphite or coke in a nitrogen atmosphere

The paper reports an investigation on the reduction of synthetic gypsum and a Tunisian phosphogypsum sample in pure nitrogen or loaded with H2O vapor, using graphite or natural coke. Thermogravimetry analysis in non-isothermal conditions showed that H2O gas has no effect on the onset temperature of reduction (T (red)), whereas the use of coke as a carbon source lowers the T (red) of about 50 A degrees C but the reduction rate and the conversion degree into CaS are not significantly affected by the coke proportion. The activation energy and the reaction order were determined using the direct Arrhenius plot and the Coats-Redfern kinetic model. These models led to the same reaction order and to close values of activation energies. The FWO isoconversional method shows that the reduction is more complex than a one step process and leads to lower values of activation energy. Addition of 5% mass of various oxide lowered the activation energy; however, the use of the Co2O3/Co3O4 as additive seems to enhance more the reduction rate.