Two-step hydrothermal synthesis of nano-kaolinite from fly ash: Thermodynamics and mechanism

Realizing the comprehensive utilization of fly ash is beneficial to reduce the environmental hazard. Herein, we propose a new route for synthesizing nano-scale kaolinite by a two-step hydrothermal method using fly ash as raw material. Above all, the feasibility of each step was determined by calculating the Gibbs free energy of the reaction. In 3 mol/kg sodium hydroxide solution at 240 °C, the glass sphere, quartz, and mullite in fly ash raw materials decomposed wholly and transformed to sodalite first and then hydroxycancrinite eventually. The intermediate product, hydroxycancrinite, dissolved in the HNO3 solution quickly and precipitated to boehmite as a precursor, which combined with Si4+ from the solution and transformed to flaky kaolinite with a diameter of around 200 nm ultimately. This method has a potential in industrial-scale manufacturing of nano-size kaolinite and provides a new route for the green recycling of fly ash. [Display omitted] •A cleaner two-step hydrothermal synthesis of nano-kaolinite.•A Gibbs free energy analysis is performed to determine reaction conditions.•Explain the relevant formation mechanism from fly ash to zeolite.•Boehmite is formed firstly before the formation of kaolinite.