ALKALI BYPASS

Alkali compounds, particularly argillaceous components, enter the rotary kiln system from raw materials, contributing to concrete deterioration under service conditions. This alkali-silica reaction (ASR) is widespread in highway construction and has led to the promulgation of standards and regulations, including limiting cement alkali content to 0.6%. The build-up problem in preheater and precalciner systems is exacerbated by the presence of molten salts, particularly alkali sulfates and chlorides. The increase in alkali concentration inside the kiln is caused by cycles of volatilization and condensation of alkali, chlorine, and sulfur compounds. The intimate contact between raw materials and kiln gas results in nearly complete condensation of volatilized salts, leading to excessive concentrations. Precalcination has a significant advantage in this respect, as it requires only the completion of calcination and the maintenance of an exothermic clinkering operation. In bypass systems, the necessary portion of the gas stream exiting the kiln is diverted into a separate gas train, aiming to remove the bulk of gases and as little dust as possible.