Coal Fly Ash as a Secondary Resource: Recent Advances in Critical-Element Recovery and Material Valorization—Part I: Characterization, Beneficiation, and Matrix-Opening Strategies

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.