Staged characteristics of red mud dealkalization by CO2 and SO2

Abstract

As a by-production of alumina industry, red mud has caused serious environmental issues such as water, soil, and air pollution. The disposal and comprehensive utilization of red mud are restricted by the alkalinity seriously. The dealkalization of red mud by acid neutralization has been widely investigated. Nevertheless, the involved issues concerning reaction mechanisms are not fully understood, which questions the feasibility of the practice. This study focuses on and compares the dealkalization behaviors of red mud by CO2 and SO2 to provide a full insight into the neutralization. The differences are analyzed by simulating state changes of structural alkali by Phreeqc. Results show that the removal of structural alkali is the point of improving the dealkalization efficiency of red mud since structural alkali is the main form of alkali in red mud, especially sodalite, accounting for 83.0% of the total alkali content. CO2 mainly removes soluble alkali and has a limited effect on structural alkali, thus exhibiting a low dealkalization efficiency, only at 18.4%, which is similar to the effect of water. SO2 removes both the soluble and structural alkalis resulting in 95.5% dealkalization efficiency. The saturation index simulated by Phreeqc shows that the dissolution of sodalite requires a pH environment between 7 and 4, where CO2 is too weak to reach, and the dealkalization of red mud can be divided into three stages based on the development of pH and temperature: the neutralization of solution alkali at a pH range from 10 to 7, the neutralization of structural alkali at a pH range from 7 to 4 and the continuous dissolution of the excess SO2 at a pH lower than 4.