Synthesis of zeolite X from fly ash in an impinging stream reactor and its mechanisms

Abstract

The conventional hydrothermal method with magnetic stirring for the synthesis of zeolite X from fly ash has some problems, such as low micromixing efficiency, low mass transfer rate, and long synthesis time. In this study, impinging stream reactor is used to improve the mass transfer and micromixing and the mechanism how the nucleation and crystallization of zeolite is accelerated is explored. The results show that premixing in the impinging stream reactor can reduce the stirring and aging time from 4 h to 20 min because the hydroxyl free radicals resulting from the cavitation effect can accelerate the crystallization of zeolite. After collision, the viscosity of the system is increased from 0.22 Pa·s to 3 Pa·s, which can increase the residence time of cavitation bubbles in the gel solution and effectively inhibit the aggregation of crystal nuclei. As a result, the crystal nuclei in the system are well dispersed. The impinging stream reactor can also accelerate the mass transfer between liquid and solid phases and improve the micromixing of raw materials, so that the polymerization between Si and Al is increased and the nucleation of zeolite X is accelerated. The degree of order of zeolite X samples obtained by premixing in the impinging stream reactor is much higher than that obtained by magnetic stirring. The relative crystallinity after crystallization for 12 h (100 %) is higher than that obtained by magnetic stirring (94.93 %); and the n(Si/Al) is lower than that obtained by magnetic stirring. It is concluded that premixing in the impinging stream reactor can improve the polymerization of aluminosilicate and accelerate the nucleation and crystallization of zeolite X. Under the same conditions, zeolite X synthesized by premixing in the impinging stream reactor has better CO2 adsorption capacity than those synthesized by magnetic stirring and industrial synthesis.