Enhancement of fly ash carbonation by mechanical ball milling: From lab-scale characterization to pilot‑scale...

Direct wet carbonation is an effective method for utilizing fly ash while reducing carbon emissions but suffers from slow kinetics. Mechanical ball milling pretreatment enhances reactivity, yet previous studies focused on physical effects, lacking insight into mechanochemical activation and scalability. Three typical fly ash specimens were adopted for laboratory-scale experiments. Results indicate that fly ash carbonation efficiency first rises then declines with the extension of reaction time and increase of liquid-solid ratio, and presents a continuous growth trend with elevated temperature and pressure. Mechanical ball milling significantly reinforces carbonation reactivity: dry milling elevates the average carbonation efficiency by 2.49%, and wet milling achieves a more remarkable increment of 4.38%. The better performance of wet milling is ascribed to the inhibition of fine particle agglomeration by water, which exposes more active sites. The enhancement mechanism of ball milling on fly ash carbonation is interpreted as involving two core aspects based on multi-scale characterization: particle crushing creates more internal active sites, and mechanical energy input is associated with a crystal-to-amorphous phase transition inferred from XRD peak attenuation. Direct evidence of bond-level mechanochemical changes remains to be further investigated. A pilot-scale reactor with daily treatment capacity of 1 ton fly ash was built to explore carbonation behavior under scaled-up conditions. Upon scale‑up to pilot scale, the absolute carbonation efficiency declines because of deteriorated gas‑liquid‑solid mixing. Interestingly, the relative enhancement provided by ball milling becomes even larger at pilot scale, because the larger reactor volume promotes dispersion of fine milled particles and reduces sedimentation.