Reactive Transport Modeling to Predict Leaching of Coal-Derived Fly Ash

In this study, a reactive transport model was developed using the CrunchFlow code to predict the long-term leaching of coal fly ash in a laboratory-scale column. The heterogeneous composition of coal fly ash was introduced to the model via primary and secondary host phases, determined by characterization techniques. The model considered dissolution/precipitation reactions with thermodynamic and kinetic data (i.e., solubility constant, rate constant, and specific surface area) and transport parameters (i.e., advection and dispersion). The calibrated model predicted the concentration of major components and trace elements (As, Mo, Se, and B) within 10 years of leaching. The washout phase was the result of easily dissolvable mineral dissolution, and was known by the release of high contents of Na, K, Mg, Ca, S, B, and Mo. The model predicted the kinetically controlled reactions to occur within 100 days by depletion of gypsum and calcite, and the formation of secondary minerals such as clay and iron oxyhydroxides. Congruent dissolution of simulated glass phase continuously released Si, Al, Fe, K, Mg, Se, and As to the leachate in the long term. Glass dissolution was also controlled by precipitation of tobermorite. Formation of ferrihydrite with high specific surface area controlled the iron concentration. The porosity of fly ash column increased from 28 to 45% during 30 years of leaching, which can significantly affect the transport properties of fly ash deposits.