Optimization of zeolite LTA formation from kaolin using fusion/hydrothermal method: Crystallization parameters and Box-Behnken experimental design

Abstract

Using the fusion/hydrothermal approach, the effects of NaOH/kaolin ratio, the crystallization temperature and the crystallization time of the reaction system on the synthesis of zeolite A from natural kaolin have been methodically investigated. The study aimed to enhance crystallinity of the obtained zeolite by employing optimization curves generated from a three-level Box-Behnken Design (BBD) simulation. Using XRD, SEM/EDS, FTIR, XPS and TEM/SAED, the synthesis products' phase, composition and morphology were described. The morphological analysis and elemental characterization confirm that the NaOH/kaolin treated at 550 °C has the necessary Si/Al ratio and structural transformation to support LTA zeolite synthesis. The XRD analysis reveals that optimal conditions for synthesizing high-purity Zeolite A include a NaOH concentration of 3 M, 5 h of agitation, 12 h of crystallization, and a crystallization temperature of 90 °C. Higher NaOH concentrations and crystallization temperatures can lead to unwanted phases like sodalite, while longer times do not significantly improve crystallinity. From the statistical results, the optimal conditions identified 90 °C, a NaOH/kaolin weight ratio of 0.9, and a crystallization duration of 12.50 h resulted in a significant increase in crystallinity, reaching 88.60 %. These findings underscore the importance of carefully controlling synthesis parameters to achieve high-quality zeolite products through the fusion/hydrothermal process. In addition, the validation of the response surface methodology (RSM) model confirmed its effectiveness in optimizing the synthesis parameters for Zeolite A, achieving a crystallinity of 91.10 % under the ideal conditions of a NaOH/kaolin weight ratio of 1.06, crystallization temperature of 92.99 °C, and crystallization time of 12.69 h. The results, supported by SEM analysis, demonstrated improved crystallinity and uniform particle size, aligning closely with predicted values. This successful validation emphasizes the model's reliability and potential for enhancing production efficiency while maintaining high-quality zeolite A. Its economic viability for industrial-scale applications was demonstrated by the projected $0.95/kg cost of producing zeolite A from Algerian kaolin using the fusion/hydrothermal technique.