Upscaling of Zeolite Synthesis from Coal Fly Ash Waste: Current Status and Future Outlook

Abstract

Coal fly ash has been recognised as suitable feedstock for zeolite synthesis, and with this discovery, there has been much literature produced over the years on various synthesis processes with a view to address the disposal problems associated with fly ash wastes. The different synthesis processes can be divided into a pre-synthesis activation with an alkali solution and a hydrothermal synthesis of heating the resulting aluminosilicate gel. However, the challenge lies in up scaling the synthesis to allow production on a commercial scale. The most common pre-synthesis involving hightemperature fusion of fly ash with sodium hydroxide may not be feasible in an economic context, given that a large-scale furnace that would be needed. In this review, the successes recorded in the application of acoustic cavitation by using ultrasonication technique were discussed as a possible alternative to the fusion process on one hand. On the other hand, jet-loop reactor system provides hydrodynamic cavitation, which may be relatively scalable compared to ultrasonication and may offer an economic advantage, if systematically explored, over fusion process for large-scale synthesis of zeolites. It is also recommended that agitation is critical during the crystallisation process and the suitability of an impeller type that offers a minimum shearing to be explored during the hydrothermal process with respect to quality and yield of the zeolite produced. The understanding gleaned from these recommendations may be useful in designing an appropriate scale-up operation for zeolite synthesis from coal fly ash.

Synthesis and Characterization of Zeolites Produced from Low-Quality Coal Fly Ash and Wet Flue Gas Desulphurization Wastewater

“…Zeolite X was present in the samples prepared from both distilled water and wastewater NaOH solutions ( Figure 2 b,c), however, the XRD patterns indicated that the synthesis was more efficient in distilled water. This can be explained by the complex chemical composition of wastewater [ 49 ] and high Cl − ion concentration. As proven in previous research [ 46 ], crystallization of the zeolite phase is favored by the absence of Cl − ions in the solution.…”

“…For all the conditions, an increase in the intensity of characteristic peaks at subsequent samples was observed, indicating further progress of the zeolitization process. Depending on the reaction conditions, different and mixed products of synthesis are described in numerous reports [ 49 , 50 ]. Extending the heating time was identified as a sufficient factor to change the dominant zeolite phase in the collected samples, as well as to speed up the zeolitization process.…”

“…The amorphous substance was noticed due to the rise of the background line in the XRD pattern. Many factors affected the type and crystallinity of synthetic zeolite formed from CFA during hydrothermal processes [ 49 ], including treatment time, temperature, chemical composition of raw ash, type of alkaline hydroxide, alkaline concentration, and liquid-to-solid ratio [ 13 ]. The results obtained in the experiment proved that even a minor temperature applied for a limited amount of time is a crucial factor for zeolitization from raw CFA, regardless of the applied alkali concentration.…”

Processing of high-grade zeolite nanocomposites from solid fuel combustion by-products as critical raw materials substitutes

“…CFA is specified in two classes F and C according to the international standard ASTM C618 regarding its chemical composition from the point of view of the suitability of this raw material for production of cement [9]. Smart utilization of coal fly ash is the processing of zeolites by alkaline conversion processes [10,11]. CFA-based zeolites with a controllable size of the constituent crystallites of the order of nanosizes and with a uniform distribution of the metal oxide phases transferred into the zeolite matrix from the raw ash as nanoparticles can be considered as nanocomposites unlike their pure synthetic analogies.…”

Upscaling of W zeolite direct synthesis from coal fly ash and its water adsorption capacity

“…The main methods used to obtain a specific zeolite as a unique crystalline phase from coal FA are alkaline fusion and two‐stage synthesis that involves silica extraction, but only few studies have reported a unique crystalline phase product by a direct zeolitization method. The current challenge lies in scaling up the synthesis to allow production on a commercial scale …”