Enrichment and modes of occurrence of gallium and germanium in coal combustion fly ash and bottom ash

Recovering strategic dispersed metals such as Ga and Ge from coal ash has attracted increasing attention, and understanding their occurrence characteristics is essential for efficient recovery. In this study, Ga–Ge-rich coal ash was investigated to elucidate the enrichment behavior, particle-size distribution, and modes of occurrence of Ga and Ge in coal-fired fly ash and bottom ash. The results show that Ga and Ge are significantly enriched in fly ash relative to bottom ash. Their concentrations increase with decreasing particle size. Pearson correlation analysis indicates that Ga and Ge are positively correlated with MgO and CaO, suggesting that Mg- and Ca-bearing phases may be associated with their enrichment behavior during coal combustion and subsequent cooling processes. Sequential extraction results reveal that Ga and Ge in bottom ash are predominantly present in the residual fraction, indicating that aluminosilicate minerals such as mullite are the dominant final host phases of these elements. In contrast, although the residual fraction remains dominant in fly ash, its proportion decreases, whereas the organic matter and sulfide-bound fractions become more prominent. The TOF-SIMS elemental mapping results further support the occurrence characteristics of Ga and Ge in fly ash.