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[en] Highlights: • A HAc/NaOH/ultrasonication-assisted joint process was applied to extract SiO2 from FA. • This joint process was feasible to separate Si/Al effectively. • High purity (98.65%) and high yield (51%) of SiO2 were obtained. • The extracted SiO2 exhibited better adsorption prospect to dyes. • This research extended the application and recycling of FA solid waste. Silica extracting from fly ash (FA) becomes a key technology for its comprehensive utilization of waste. Here, high-purity SiO2 was extracted from FA via organic acid/inorganic alkali/ultrasonication-assisted joint process and characterized by scanning electron microscopy, transmission electron microscopy, Brunauer-Emmet-Teller, X-ray fluorescence spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The optimal extracting conditions were optimized: with acetic acid (HAc) leaching at 25 °C for 1 h and FA/HAc of 1:3; then with 15% NaOH dissolution at 100°C for 2 h and FA/15% NaOH ratio of 1:4.5; cooled and introduced water (5.42 g/100 mL); ultrasonication-assisted for 20 min. The final yield and purity of SiO2 were 51% and 98.65%, respectively. This joint method could be applied in several various FA raw materials and the extracted SiO2 exhibited better adsorption prospect to cationic dyes of methylene blue and malachite green, which extended the application and recycling of FA.