Surface zeolitization of millimeter clay particles

C. RIVERA ENRIQUEZ; MAXIMILIANO GONZALEZ; BARRAQUÉ, FACUNDO; ANDREA M. PEREYRA; E. BASALDELLA

CABA

Congreso; 11th World Congress of Chemical Engineering.; 2023

Asociación Argentina de Ingenieros Químicos

This work presents a novel and cost-effective procedure that improves the reactivity of kaolinitic clay particles toward the generation of millimetric hard-core/zeolitized-shell units. Granulated clay from kaolinitic deposits, located in Tandil, Argentina, was used as raw material. The starting solid was submitted to two sequential steps: calcination and alkaline activation. In the first instance, the clay fractions were heat-treated in static conditions for 2 h at different temperatures: 800°C to 1200°C. Then, the mixtures of calcined clay and Na2CO3 were heat-treated at 800°C for 2 h [1]. The synthesis mixture was prepared by adding the appropriate amounts of reagents such as NaOH, NaAlO2 and deionized water to each previously activated sample. The synthesis mixtures were aged at room temperature for 48 h without stirring and hydrothermally treated in a conventional air oven at 100ºC. The starting clay, pretreated clays, and products obtained after hydrothermal synthesis were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray microanalysis (EDX).The first calcination produced a collapse of the lattice order that increased the amorphous fraction or a topotactic transformation of the natural clay to a new crystalline configuration. Then, the samples previously heat-treated at relatively low temperatures (800°C and 900°C) showed the formation of nepheline and low-carnegieite after activation with Na2CO3. The samplespretreated at higher temperatures (1100°C and 1200°C) produced low-carnegieite as a new phase, coexisting with nepheline, mullite and cristobalite. For all samples, the background halo became less intense after alkaline activation. Besides, Na2CO3 calcination at 800°C and 960°C reduced the XRD intensities corresponding to quartz. FAU and LTA zeolite structures were successfully obtained on the surface of hardened particles, reaching conversions close to 90%. The clay surface was covered with cubic zeolitic crystals, corroborating that the applied pretreatments facilitated the conversion of intermediate phases into zeolites.The procedure presented in this work could be considered as a general methodology for obtaining hardened millimeter-sized clay particles covered by an adsorbent zeolitic coating. The pretreatments applied to the clay take advantage of different transformation routes belonging to these kaolinite-based materials, which comprise dehydration, generation of metastable phases, and T-O- bond rearrangement, including structural collapse. The results suggest that high conversions of zeolite NaA and NaX can be achieved under suitable conditions. The obtained particulate material could be used as ion exchanger beads in a fixed adsorption bed.