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

Abstract

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 CO2 sequestration with CSS utilization, thereby achieving simultaneous dual-value enhancement of CSS and eliminating waste discharge. Under optimized Ca2+ leaching and carbonation conditions, a remarkable Ca2+ extraction efficiency of 96.93 % was attained, while the carbonation efficiency reached 99.37 %. Consequently, this process sequestered 301.7 kg of CO2 per ton of CSS. The leaching process of Ca2+ in NH4Ac 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% CaCO3 with a whiteness level of 95.2 %. By regulating the carbonation temperature, the crystal particulate size distribution of CaCO3 was tailored to meet specific requirements for various applications. Furthermore, silicon content was enriched in the leaching residue, increasing the effective SiO2 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.