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.