Dual-value utilization of calcium-silica slag: Sequestration of CO2 to recover high-purity vaterite with silica fertilizer as a by-product

Calcium-silica slag (CSS) is a highly alkaline residue, rich in calcium, generated during the chemical pre-desilication process of medium- and low-grade bauxite. Mismanagement of CSS can result in detrimental environmental impacts. This study developed a novel method that combines CO 2 sequestration with CSS utilization, thereby achieving simultaneous dual-value enhancement of CSS and eliminating waste discharge. Under optimized Ca 2+ leaching and carbonation conditions, a remarkable Ca 2+ extraction efficiency of 96.93 % was attained, while the carbonation efficiency reached 99.37 %. Consequently, this process sequestered 301.7 kg of CO 2 per ton of CSS. The leaching process of Ca 2+ in NH 4 Ac solution was realized based on the unreacted shrinking core model, which accounts for internal diffusion and chemical reaction as factors influencing leaching. The resulting carbonated product exhibited outstanding characteristics: high-purity vaterite composed of 97.43 wt% CaCO 3 with a whiteness level of 95.2 %. By regulating the carbonation temperature, the crystal particulate size distribution of CaCO 3 was tailored to meet specific requirements for various applications. Furthermore, silicon content was enriched in the leaching residue, increasing the effective SiO 2 content from 25.2 wt% to 53.12 wt% and reducing the pH from 12.23 to 7.34, rendering it a suitable silica fertilizer for agricultural production.