A variety of leachates derived from the acid leaching of a unique Fe-rich fly ash, namely Yallourn from the Latrobe Valley of Australia at different temperatures, have been processed to achieve two key goals: synthesis of Fe-rich precipitate and mineral carbonation of alkaline earth metal cations for the storage and utilization of carbon dioxide (CO2). The behavior and interference of unprecipitated Fe-bearing cations on the carbonation stage was for the first time examined by us. The research findings are also applicable to the leachate derived from other industry wastes and even natural minerals, which also contain varying amounts of iron as one impure metal. To precipitate Fe out of the leachate, sodium hydroxide (NaOH) was added to adjust the pH to 4. Subsequently, the pH of the resultant supernatant was further increased to ∼13 and bubbled with CO2 to precipitate the remaining cations. As has been confirmed through the characterization of solid products by synchrotron Fe Kedge X-ray adsorption spectroscopy (XAS), quantitative X-ray diffraction (Q-XRD), and scanning electron microscopy (SEM), Fe was precipitated out as a mixture that was predominantly ferrihydrite with a nanoscale size for its primary nuclei. Its composition and size also varied largely with the leachate, i.e. the acid leaching temperature. For the unprecipitated Fe2+ that is predominant in the high-temperature leachate (i.e., ≥150 °C), it was preferentially oxidized and converted into nanoscale magnetite during the carbonation process, providing seed/nuclei for the crystallization and growth of magnesian calcite (Ca0.2Mg0.8CO3). Accordingly, the carbonation of Mg2+ was enhanced remarkably and reached completion in 20 min at room temperature. In addition, all the resulting products, Fe-precipitate (i.e., ferrihydrite), magnetite, and magnesian calcite, are relatively pure, presenting cheap and suitable precursors for a variety of value-added environmental applications.