Study on phase transformation and dealkalization of red mud in NaOH-Ca(OH)2 system and CO2 adsorption performance of dealkalization residue

Abstract

Red mud is a kind of solid waste produced in extracting alumina from bauxite, and its high alkalinity poses a severe threat to the environment and land resources. This work investigates the removal of alkali metals from red mud through a hydrothermal reaction under NaOH and Ca(OH)₂ conditions. Hydrothermal slag can be further utilized for CO₂ adsorption, thereby enhancing its value and utilization. The optimal conditions for alkali metal removal were determined through systematic experiments and theoretical analysis: a reaction temperature of 150°C, a Ca/Si molar ratio of 1.0, a reaction time of 2 hours, and a NaOH concentration of 15 %. Under these conditions, the alkali metal removal efficiency reached 97.32 %. In the hydrothermal process, the aluminum-silicon component dissolved and recrystallized to form new mineral phases, such as calcium aluminum garnet and calcium aluminum silica. These phase transitions contribute to the removal of alkali metals. Besides, the CO₂ adsorption experiment showed that the CO₂ adsorption capacity of the sample at 0.15 bar and 1 bar was 1.02 mmol/g and 2.97 mmol/g, respectively. This work proposes an effective method for removing alkali metals from red mud and obtains silicon-based materials with good CO₂ adsorption performance. This study not only increases the resource utilization value of red mud but also provides new material choices for carbon dioxide capture, which has significant environmental and economic significance.