EVOLUTION OF POROSITY PROFILES OF MAGNETITE PHASE DURING HIGH-TEMPERATURE REDUCTION OF HEMATITE

The reduction of initially nonporous hematite to porous magnetite by C O+CO2 (3:97) mixture was monitored thermogravimetrically at 850 degree s C. For the series of six kinetic runs the grains of diameter ca. 1.5 mm were used. The final reduction degree varied from 13 to 100%. Afte r each kinetic run the microscopic observations of the central cross-s ection of grains were done in quantitative way. The observations yield ed the values of local porosity. The empirical equations were found de scribing the continuous exponential decrease of local porosity with th e distance from the external surface of the grain. The value of total porosity obtained by mercury porosimetry agrees in a reasonable way wi th microscopic data. The classical shrinking core model (SCM) was fitt ed to kinetic data. The model took into account the gas-solid reaction occurring at sharp defined interface as well as the pore diffusion ph enomena occurring inside the magnetite layer. The model was also modif ied. The local value of porosity was introduced to the definition of e ffective diffusivity (D-eff). In this way D-eff was allowed to vary wi th the distance from the external surface of the grain. The corrected three parameter SCM yielded slightly worse results. It implies that ap art from the spatial variation of D-eff the temporal one should be als o considered. Indeed, it was found that the local porosity of the alre ady reduced layer varied also with time. However, the data are not acc urate enough to permit the temporal variation of D-eff to be included in the model calculation.