Comprehensive characterization of solid waste incineration fly ash before and after salt washing: evaluating analytical methods for sustainable waste management

• Salt washing led more homogenous particle size distribution. • The amorphous content in the ash samples varied, ranging between 22-76 wt.%. • Minor metals are unevenly distributed in discrete or embedded phases. • PXRF showed high correlation for Al, Ca, Cu, Fe, P, and Zn. Despite extensive research on municipal solid waste incineration (MSWI) fly ash (FA) and air pollution control ash (APCA), comprehensive studies employing multiple characterization techniques remain limited. This study evaluates an extensive suite of analytical methods to characterize MSWI FA and APCA before and after water washing. Washing effectively removed soluble salts (e.g., NaCl, KCl), increasing stable phases like CaCO₃ and silicates, with variations dependent on ash origin and phase solubility. Washing also reduced particle size variability, yielding more uniform distributions. Chemical analyses demonstrated portable X-ray fluorescence (PXRF) as a reliable, cost-effective alternative to ICP-MS for elements including Al, Ca, Cu, Fe, P, and Zn, though weaker correlations for Pb, Si, and S suggest the need for more precise methods. Automated mineralogy showed sample-specific phase distributions: rotary kiln (RK) ash contained abundant ultrafine, unclassified matrix that increased post-washing, while grate furnace (GF) samples were dominated by lime, gypsum, and Ca-Si-Al phases, with washing reducing Cl-bearing and gypsum phases and slightly increasing reacted silicates. The circular fluidized bed (CFB) sample predominantly contained lime, and Ca-Cl-O and related reacted phases, showing minimal changes after washing. Minor metals such as Zn and Cu appeared heterogeneously distributed, bound within discrete or embedded phases, whereas bulk elements (Al, Ca, Mg, Si, Na) were uniformly integrated into the matrix. The findings emphasize the importance of combining multiple analytical techniques to achieve comprehensive characterization of MSWI ashes, bridging rapid screening with detailed analysis.