Date of Award
Doctor of Engineering Science in Chemical Engineering
C. L. Mantell
Saul I. Kreps
Jui Sheng Hsieh
Samples of synthetic pyrolusite were reduced in hydrogen at various partial pressures in the temperature range of 200° to 500°C. Most reduction experiments were conducted using a Pyrex vertical-tube reactor, in which small beds of porous particles (0.07 to 0.21 mm.) or single porous pellets of the oxide were suspended. The reaction kinetics were followed by recovering and weighing the water product formed during measured time intervals, and the intermediate reduction products were identified by X-ray diffraction analysis.
The data show that reduction proceeds through the sequence MnO2->Mn2O3->OMn3O4->O-MnO. Below 250°C no MnO was detected, and the reaction practically terminated with the formation of Mn3O4. Above 250°C the Mn3O4 became progressively more protective, and the reduction of Mn3O4 to MnO became appreciable. Above 300°C all four oxides were detected in the partially reduced products, although the MnO2 and MnO phases were usually the major components. The kinetic data were conveniently divided into two temperature regimes.
Below 25O°C the reaction was found to practically subside with the formation of Mn3O4. The apparent activation energy for pellets and particles was, respectively, 26.8 and 22.2 kcal./mole at a hydrogen pressure of 800 mm. Hg and in the temperature range of 200° to 240°C. The rate at 226°C increased nonlinearly with hydrogen partial pressure and was sharply retarded by the presence of water vapor.
Above 300°C the reduction was always characterized by an exceedingly high initial rate; this is attributed to a rapid build-up of layers of Mn2O3 and Mn3O4. The Mn3O4 became protective but was itself further reduced to MnO at measurable rates. The apparent activation energy (for particles) at conversions above 20%, at hydrogen partial pressures between 80 and 800 mm. Hg, and in the temperature range of 325° to 425°C, was approximately 26 to 30 kcal./mole. The rate increased nonlinearly with hydrogen partial pressure, and at 375°C was sharply retarded by the presence of water vapor.
The low-temperature pellet data were correlated with a core reaction model, in which the reacting solid was assumed to approximate the system, MnO2(core)-Mn3O4(product layer). The thickness of the intermediate Mn2O3 layer was neglected, and the solid-gas reaction at the Mn2O3/Mn3O4 interface was taken as the rate-controlling step. Similarly, the high-temperature particle data were correlated by a core model in which the reacting solid was assumed to approximate the MnO2(core)-MnO(product layer) system. The intermediate oxides were assumed to be of negligible thickness, and the solid-gas reaction at the Mn3O4/MnO interface was taken as the rate-limiting step. The kinetic data in both regimes were consistent with the concept that the solid-gas reactions involve adsorption of hydrogen, surface reaction, and desorption of water, the surface rearrangement being rate-controlling.
Reduction experiments were also conducted using a natural pyrolusite ore. These data showed characteristics similar to those of the synthetic pyrolusite.
Barner, Herbert Eugen, "Kinetics of hydrogen reduction by manganese dioxide" (1967). Dissertations. 1329.