Synthesis of zeolite from fly ash with red mud and its adsorption performance

In order to enrich and utilize the valuable elements of silicon and aluminum in red mud and fly ash, zeolite was synthesized by the alkali melting-hydrothermal method. By integrating X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), the effects of the molar ratio of silicon-to-aluminum (n(Si)∶n(Al)), hydrothermal crystallization temperature and duration on the synthesis of red mud-fly ash-derived zeolite were systematically investigated. The BET specific surface area and pore size distribution of the samples were determined by the low-temperature nitrogen adsorption static capacity method. Combined with Cu²⁺ adsorption experiments, the adsorption performance of the synthesized zeolite was systematically evaluated. The results showed that with the n(Si)∶n(Al) range of 1∶0.70 to 1∶0.60, increasing the silicon-aluminum ratio was conducive to the generation of P1 zeolite. The temperature of hydrothermal crystallization should be strictly controlled. A temperature that was too low hindered the formation of crystal nuclei, whereas an excessively high temperature might lead to coarse grain development and induce phase transformation. The duration of crystallization significantly influenced both the crystallinity and grain size of the synthesized zeolites. Under the conditions of n(Si)∶n(Al)=1∶0.65, a crystallization temperature of 100 ℃, and a duration of 10 h, well-crystallized P1 zeolite, faujasite zeolite and X zeolite were simultaneously obtained. The synthesized zeolite exhibited excellent nitrogen adsorption performance and Cu²⁺ adsorption capacity. When the amount of zeolite was 3.0 g/L, the adsorption efficiency increased with temperature, and the adsorption rate for Cu²⁺ at 60 ℃ reached 98.04%. This indicated that the material could achieve high-efficiency removal of heavy metals under mild conditions, providing a feasible path for the high-value utilization of red mud and fly ash.