Investigations into shell corrosion of rotary cement kilns

The corrosion of 12 suspension calciner kilns with tertiary air duct was studied, and corrosion scales from the kiln transition, zone examined. A common feature of the scales was the layered structure in which high-oxygen low-sulphur layers alternate in a variable manner with high-sulphur low-oxygen, layers. Varying amounts of chlorides may be present and are distributed quite homogeneously. Oxygen-deficient layers are less frequent where larger quantities of alkalis are present. The scales are acidic, water extracts showing pH congruent to 3.5. Water extracts from mechanically cleaned kiln shells contain iron and chloride. The dominant crystalline components of the scales are haematite, magnetite, pyrite and pyrrhotite. The observations may be understood as the combination of ash-deposit corrosion. and hot corrosion seen on, the fire side of boiler walls. Thermodynamic calculations were used to suggest why calciner kilns with tertiary air ducts are so vulnerable to this type of corrosion. It is tentatively concluded that the main reason is the delayed evaporation of chlorides from the kiln feed which causes the evaporation to occur at temperatures where hydrolysis of alkali chlorides dominates, the latter leading to release of hydrogen chloride gases. One of the recommended steps to counter this phenomenon is to increase the kiln valve for chlorides by introducing a by-pass or significantly reducing the burning zone temperature.