Magnetic and chemical evaluation of coal ashes from Zonguldak and Soma power plants

Coal combustion residues are increasingly regarded as valuable secondary resources rather than inert wastes, particularly due to their iron-bearing and aluminosilicate phases; accordingly, this study presents an integrated chemical, magnetic, and image-based characterization of 74 coal ash samples collected from two major coal-fired power generation regions in Türkiye, namely the Zonguldak and the Soma Thermal Power Plant. Bulk chemical analyses performed by X-ray fluorescence reveal distinct compositional differences between the two ash groups, with Zonguldak ashes (3.26–8.36 wt.%, average 4.38 wt.%) compared to Soma ashes (3.50–4.70 wt.%, average 4.11 wt.%), Consistent with these chemical trends, mass-specific low-frequency magnetic susceptibility values are generally higher for Zonguldak ashes (709–902 × 10−5 m3 kg−1), whereas Soma ashes display a wide susceptibility range (22.9–2508.2 × 10−5 m3 kg−1) reflecting variations in iron mineral speciation and grain size. Moreover, frequency-dependent susceptibility reaches up to 14.52%, indicating the presence of fine-grained and superparamagnetic iron oxide phases, particularly in Soma samples. In parallel, quantitative image analysis based on RGB and HSV color parameters reveals systematic relationships between optical appearance and iron enrichment, such that increasing Fe2O3 content is associated with a progressive color shift from grayish to yellow – reddish tones, while strong correlations are observed between Fe2O3 concentration and both red channel intensity (R2 = 0.7735) and hue (R2 = 0.8054). Overall, Zonguldak ashes for magnetic separation – based recovery of iron-rich magnetospheres and, conversely, the suitability of Soma ashes for aluminosilicate-based applications such as geopolymers and construction materials.