Integrated acid-alkali recovery of high-purity silica from alumina-extraction residues derived from Mongolian coal ash as a secondary silicon resource
Year: 2026
Type: article
Abstract: Coal-derived alumina-extraction residues represent a largely underutilized secondary silicon resource. In this study, silica (SiO 2 ) was recovered from alumina-extraction residues generated during alumina extraction from coal ash collected from the Erdenet, Darkhan and Ulaanbaatar Fourth thermal power plants in Mongolia via an integrated alumina extraction–alkali extractionacid precipitation route. The raw coal ashes and the final silica products were subjected to physicochemical characterization (XRD, XRF, SEM and BET), whereas the alumina-extraction residues were not independently characterized, but served solely as the chemically activated intermediate for silica recovery. The original ashes were classified as Class F according to ASTM C618–08a and radiological assessment confirmed compliance with the Mongolian standard MNS 5072:2001, supporting their safe utilization. Under optimized conditions (ash-to-acid ratio of 1:6, 20 wt% HCl, 200–250 rpm and 75–95 °C), high-purity silica was consistently obtained from the alumina-extraction residues, with SiO 2 contents ≥99.9 wt% and reaching up to 99.999 wt% in selected samples, while total residual impurities were controlled below 0.1 wt%.The recovered silica exhibited specific surface areas of 32.8–352.1 m² g⁻¹ with well-developed mesoporosity, and SEM revealed porous aggregated morphologies. A preliminary mass-balance analysis indicates that 2.46–4.44 t of coal ash can yield 1 t of high-purity silica via the alumina-extraction residue pathway. This study demonstrates alumina-extraction residues, rather than raw coal ash, serve as an effective intermediate feedstock for silica recovery, enabling a coupled alumina–silica utilization route. The integrated acid–alkali process provides a chemically selective and potentially scalable approach for converting coal-derived residues into high-purity, mesoporous silica, thereby supporting the concept of coal ash as a secondary silicon resource within a circular-economy framework.
Source: Results in Chemistry
Institution: Mongolian University of Science and Technology
Language: English
Topic: Coal and Its By-products
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,870
OpenAlex ID: W7128527451