Synthesis of zeolite 4A from low-grade kaolin containing muscovite and quartz via ultrafast Joule heating route: A comparison with conventional calcination

This study introduced an innovative ultrafast Joule heating route for synthesizing zeolite 4 A from low-grade kaolin and made a comparison with the conventional calcination route. Comprehensive characterization was conducted using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), mineral liberation analysis (MLA), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and thermogravimetric analysis (TGA). The results indicate that the raw kaolin is primarily composed of kaolinite, muscovite, and quartz, with the latter two minerals intimately intergrown with kaolinite as impurities. Conventional calcination failed to convert muscovite and quartz, leaving them as residual impurities in the zeolite 4 A product. While precursors (sodium aluminosilicate and orthoclase) were rapidly generated by ultrafast Joule heating under the action of Al(OH)₃ and Na₂CO₃, and the muscovite and quartz impurities were effectively converted and utilized at 1300 °C for 20s, the precursors were then crystallized through hydrothermal treatment to synthesize zeolite 4 A, incorporating their Si and Al components into the zeolite framework, and the partial replacement of Na⁺ in the zeolite framework by K⁺ released from muscovite via ion exchange, but the overall purity and phase uniformity of zeolite 4 A were significantly improved. This research provides a novel and efficient pathway for the resource utilization of low-grade kaolin and the synthesis of zeolites.