Effect of firing atmosphere on extraordinary expansion of sintered brick from incinerated ash of sewage sludge

The extraordinarily large expansion observed during sintering of bodies fro m incinerated ash of sewage sludge was investigated in connection with chem ical states of iron in the bodies. The ash was pressed into a rectangular b ody under 100 MPa, and the body was fired at 1000 degrees C for 3 h in flow ing Ar-O-2 (PO2:0.1-1.0 kPa) atmosphere. The bulk density of the bodies fir ed in various atmospheres decreased with a decrease in oxygen partial press ure. The formation of porous black core accompanied by extraordinary expans ion was observed in the bodies fired at P-O2 = 0.1-0.7 kPa. Reflectance spe ctra and the ratio of Fe2+ to total Fe measured by chemical analysis indica ted that iron oxide in the black core was in the form of FeO or Fe3O4. The evolution rate of CO2 during heating of the body was measured by using a qu adruple mass spectrometer. In the case of P-O2 = 0.5 kPa, the evolution rat e of CO2 at 1000 degrees C was higher than the rate of O-2 supply through t he atmospheric gas how. The fact indicated that the ferric oxide was reduce d to ferrous oxide by the residual carbon and the evolution of CO2 occurred inside the body under lower oxygen partial pressure.