Temperature-Controlled Synthesis of Na A Zeolite from High-Alumina Coal Gangue: Mechanism and Optimization

To realize high-value utilization of high-alumina coal gangue, this work investigates a temperature-regulated alkali fusion-hydrothermal route for synthesizing Na A (LTA) zeolite. Calcination temperature strongly governs the phase evolution, dissolution behavior, and subsequent crystallization of Na A. Below 800 °C, alkali fusion is incomplete: layered/irregular fused products persist, reactive aluminosilicate species are insufficiently generated, and the melt shows limited solubility in alkaline leaching, resulting in weak Na A diffraction features and suboptimal crystal development. At 800 °C, the gangue-NaOH reaction becomes the most effective, producing abundant soluble sodium silicate and sodium aluminate/aluminosilicate species. This temperature window enhances Si–O bond depolymerization (NMR evidence) and maximizes the availability of Si and Al in the filtrate, thereby providing an optimal chemical environment for LTA nucleation and growth. Consequently, the synthesized Na A exhibits high crystallinity with a well-defined cubic morphology and achieves the highest calcium ion adsorption capacity (296 mg/g). When calcination exceeds 800 °C, reactive phases tend to recondense into less soluble, more stable crystalline products, decreasing dissolution and diminishing Na A crystallinity and adsorption performance. Overall, 800 °C is identified as the optimal calcination temperature for producing highly reactive fusion products and high-quality Na A zeolite.