Der einfluss der thermischen bearbeitung von verschiedenen Braunsteinen auf ihre adsorptions- und Depolarisationseigenschaften

Abstract

Two kinds of manganese dioxide (electrolytic γ-MnO2 and natural β-MnO2) were heat-treated over a temperature range of 105 to 700°C.The annealed manganese dioxide was examined for structural changes by radiographic and thermogravimetric methods, the changes in adsorption properties being determined by the powder-electrode method. The depolarization properties of the annealed samples were tested by means of a depolarization meter, both for continuous and intermittent discharge. In addition, the BET method was used to determine the intrinsic surface.Our investigations proved that electrolytic manganese dioxide (γ-MnO2) is converted into β-MnO2 above 300°C. This transformati on from γ-MnO2 into β-MnO2 always took place up to 400°C, after which the next transformation from β-MnO2 to α-MnO2 occurred.Natural manganese dioxide of Caucasian origin (β-MnO2) is stable up to 5OO°C, but on the other hand, at temperatures over 5OO°C it experiences a change into α-MnO2 and γ-Mn2O3. Electrolytic manganese dioxide (γ-MnO2) slowly loses its depolarizing properties after heat treatment up to 400°C. After the change from β-MnO2 into α-Mn2O3, complete loss of depolarization capacity becomes noticeable. On the other hand, in the case of β-MnO2, annealed at 5OO°C, there was an increase in depolarization capacity. As soon as the lower manganese oxide appears in appreciable quantities, the capacity for depolarizing disappears.The values of the adsorption potential of the annealed manganese dioxide sample become smaller between 200 and 450°C, whilst above 450°C the potential values rise.Initially, the BET-surface of the annealed manganese dioxide sample diminishes with the potential changes, but rises, however, at 350–450°C.For natural manganese dioxide (β-MnO2) there is a heavy drop in potential between 250 and 350°C, and above 350°C there follows a rise in the potential value. The BET-surface changes on annealing natural manganese dioxide (β-MnO2) in a similar way to the potential values.