Green synthesis of zeolite 4A using fly ash fused with synergism of NaOH and Na2CO3

A high-purity zeolite 4A was synthesized by the hydrothermal method using fly ash as the raw material. The effects of sodium hydroxide (NaOH) or/and sodium carbonate (Na2CO3) on the activation of fly ash were studied, and the removal efficiency of Cu2+ in aqueous solution was also investigated for the synthesized zeolite. The formation process of the zeolite from fly ash was surveyed by ex situ techniques such as X-ray diffraction (XRD), thermogravimetric analysis and differential scanning calorimetry (TG-DCS), scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDX) and Fourier transform infrared spectroscopy (FTIR). The important influential factors of energy and water consumption were analysed by orthogonal tests, and the best conditions for green synthesis were accurately determined through batch tests. The factors affecting the green synthesis of zeolite are the alkali mixture ratio, alkali melting temperatures, solid-to-liquid ratios, crystallization times and crystallization temperatures. The alkali mixture ratio was indicated to be an important factor for green synthesis according to the results of the orthogonal test. Compared with the use of alkali alone, when NaOH and Na2CO3 were mixed at a mass ratio of 1:2.8, the alkali melting temperature (760 °C) and solid-to-liquid ratios (1:5) were both lower, the crystallization time (4 h) was shorter in the zeolite synthesis process, and the relative crystallinity was the highest at 75.8%. The removal rate of 100 mg/L Cu2+ solution from pH 3 to pH 7 by 0.18 g of zeolite synthesized for 60 min was close to 100%, and the adsorption capacity was 55.5 mg/g. After the zeolite was desorbed and reused 4 times, the removal efficiency of Cu2+ was maintained at 73% at a pH of 3.