Scale-up development for the synthesis of coal fly ash based zeolite beta and ZSM-5 using a controlled bench pressure reactor

Abstract

Coal fly ash (CFA) is the major industrial waste material produced during the combustion of coal to generate electricity. Poor management of this waste causes serious environmental and health problems globally, including contamination of surface and ground waters, air pollution, degradation of land and quality of life of aquatic species, and several diseases to humans. In South Africa, approximately 40 million tons of CFA is produced annually, while only 5.5% is reused in cement and concrete production at a commercial scale, and the rest is disposed of in holding ponds, lagoons, and landfills. The disposal of CFA not only requires a large area of valuable land but is also a major source of environmental pollution. Therefore, more research on the use and recycling of CFA is required to alleviate the environmental burden caused by its disposal. Due to its high Si and Al content, CFA has been used as an inexpensive feedstock in the synthesis of zeolites. Although the synthesis of CFA-based zeolites using various synthesis techniques is well researched, these techniques were limited to laboratory scale. The synthesis of high-silica zeolites such as ZSM-5 from CFA requires the addition of alumino-silicate sources or treatment with a chelating agent to adjust the Si/Al ratio prior to the hydrothermal synthesis. This process leads to a synthesis of low-quality zeolites. The synthesis of high-quality high-silica zeolites requires the extraction of silica followed by treatment with a chelating agent such as oxalic acid. The introduction of an additional chelating agent during the conversion of CFA into zeolites may not be economically and environmentally sustainable. In addition, these processes generate a huge amount of secondary solid and liquid waste, which needs further treatment and disposal, the volume of which may be prohibitive if large-scale synthesis is desired. This study aimed to synthesise pure phase high-silica zeolite ZSM-5 from CFA silica extract without treatment with oxalic acid or the addition of external aluminium or silicate sources and to use the resultant solid waste in the synthesis of sodalite zeolite, with the view to minimise or completely eliminate the solid waste generated in the process. This aim was achieved by applying a three-step alkaline leaching process, which was comprised of (i) removal of the magnetic fraction at room temperature; (ii) alkaline extraction of silica from CFA (150˚C for 24 h); and (iii) treatment of the silica extract with oxalic acid or water (80˚C for 6 h). The solid residue from the alkaline extraction of silica was transformed into sodalite zeolite. The silica extracts treated with either oxalic acid or water were used as feedstocks in the preliminary synthesis of ZSM-5 zeolite (160˚C for 72 h). The results showed for the first time that a pure phase ZSM-5 zeolite can be synthesised from the water-treated silica extract without the need for the oxalic acid pre-treatment step. The optimum molar composition of 1 Si: 0.003 Al :0.612 Na: 0.190 TPABr: 95.766 H2O (water-treated silica extract) was used as a basis for further optimisation studies, which investigated the effect of NaOH, TPABr, water content, hydrothermal synthesis time and temperature on the morphology and crystal size of ZSM-5 zeolite. The variation in concentration of NaOH and water content had a significant impact on the morphology, crystallinity, and crystal size of ZSM-5 zeolite. It was also shown that TPABr within the range 0.3 ≤ 0.15 ≤ 0.075 g resulted in the incomplete conversion of the amorphous material during the hydrothermal synthesis and led to impure ZSM-5 zeolite. Furthermore, the study showed for the first time that the previously recorded ZSM-5 hydrothermal synthesis time of 72 h can be reduced to 3 h. This shorter processing time would have significant implications on the economics of the production of CFA-based ZSM-5 zeolites. Recycling protocols to manage the liquid waste generated during the silica extraction treatment step were also explored in this study. It was shown that the liquid waste can be recycled up to four times without compromising the quality of the ZSM-5 products. However, after each treatment cycle, an increase in concentration of Na was observed in the resultant silica extract. This indicated that the resultant liquid waste was highly saturated with Na salts, and that recycling beyond the fourth treatment cycle could compromise the quality of the silica extracts. A technology economic analysis (TEA) of the synthesis of sodalite and ZSM-5 zeolites from the same CFA batch was also conducted in order to predict their cost of production at a larger scale. The results showed for the first time that the synthesis of ZSM-5 zeolite from CFA-based silica extract was economically viable, with a net present value of approximately R300 M, and a payback period of five years over a 20-year period. This study provides a holistic approach and strategy to using CFA to synthesise sodalite and pure ZSM-5 zeolite with limited to zero solid waste, which in addition to addressing the environmental problems associated with its disposal, presents CFA as a useful resource that can be commercialised.