Coal fly ash (CFA) is a heterogeneous secondary resource containing aluminosilicate, iron-rich, and carbonaceous fractions, together with rare earth elements and other valuable elements. This critical narrative review examines the evidence required to select feed-specific beneficiation and matrix-opening routes. Part I addresses representative sampling, mineralogical and chemical characterization, element occurrence, physical separation, acid leaching, alkaline treatment, roasting, hydrothermal processing, and field-assisted intensification. The reviewed studies show that enrichment, dissolution, solution concentration, and final product recovery describe different endpoints and cannot be ranked without consistent material balances. Mineral hosts and process conditions govern both target-element accessibility and the accompanying impurity, reagent, water, and residue burdens. Physical pretreatment is beneficial only when reduced treatment demand or co-product value compensates for target-element losses. Laboratory reuse and larger-batch demonstrations provide useful development evidence but do not alone establish industrial feasibility. The resulting qualitative framework links feed characterization to route selection and identifies the measurements required for credible comparisons. Part II will address solution purification, selective recovery, final products, residue valorization, and integrated environmental and economic assessment.

Flue gas desulfurization (FGD) gypsum is increasingly considered a secondary raw material, but gypsum purity and bulk mineralogy alone do not fully describe the environmental behavior of associated trace elements. Three Serbian FGD gypsum materials were investigated: FGD-A and FGD-B from two wet-FGD operating streams of the Nikola Tesla A Thermal Power Plant and FGD-C from Kostolac B. Bulk characterization, X-ray diffraction, sequential extraction, and aqueous leaching were combined. FGD-A and FGD-B contained >98.6 wt.% CaSO₄·2H₂O, whereas FGD-C contained 87.47–92.06 wt.%; gypsum nevertheless remained the predominant crystalline phase in all three materials. Operationally defined non-residual fractions (F1–F3) accounted for 74.4%–82.8% of As, 91.9%–95.4% of Cd, and 89.9%–96.8% of V, whereas 81.5%–88.3% of Sb was retained in the residual fraction. Aqueous release was generally low but element- and material-dependent: Zn reached 5.25 µg L⁻¹ in FGD-B, V reached 0.98 µg L⁻¹ in FGD-C, and Pb remained below the validated detection limit in all leachates. The non-residual fraction was not directly proportional to aqueous release, showing that sequential fractionation and water leaching provide complementary information. The results demonstrate that FGD gypsum materials with broadly similar gypsum-rich characteristics can nevertheless exhibit distinct trace-element partitioning and short-term aqueous release.