SYNTHESIS OF ZEOLITE FROM COAL FLY ASH AS WATER ADSORBENT

Abstract

This dissertation comprised two main parts. The first part is to evaluate the technical feasibility in the conversion of CFA to zeolite, which is widely employed in the ethanol purification unit. The synthesis began with the pretreatment of CFA to remove impurities (e.g., Fe2O3, CaO, etc.) under various acid types (HCl, H2SO4 and HNO3) and acid/CFA ratios (5 to 25 mlacid/gCFA). Due to the high purity (up to 97%), the high relative of zeolite X crystallinity (up to 88%), and the high specific area (461 m2/g), the use of 20%wt HCl with an L/S ratio of 20 mlHCl/gCFA to wash the CFA for 2 hr at 80°C were found to be the most suitable conditions for the acid-washing pretreatment. Furthermore, to reduce the environmental impact of the treatment process, the reuse of the acid was studied and found that most impurities in CFA could be removed effectively, and the products exhibited high purity. After acid-washing, the treated CFA was then converted to zeolite by fusion reaction at 550ºC with a NaOH/CFA mass ratio of 2.25 and a Si/Al molar ratio in the range of 0.54 to 1.84. The Si/Al ratio was adjusted by mixing Al2O3 to the treated CFA. The fused product was further crystallized at 80ºC for 4 hr. Zeolite type A was obtained with a yield in the range of 63-73% when the Si/Al molar ratios were lower than 1, whereas zeolite type X was formed with a yield in the range of 28-33% when the Si/Al molar ratio were higher than 1. The second part is to investigate the water adsorption performance of the synthesized samples compared to the commercial grade molecular sieve. The zeolite synthesized using the most suitable conditions of acid-washing and zeolite synthesis (NaOH/CFA mass ratio of 2.25 and Si/Al molar ratio of 0.82), had a higher water adsorption performance than the commercial grade molecular sieve and under ten adsorption-testing cycles at 90ºC, a high ethanol purity (>99.5%wt) can still be achieved without deactivation. Zeolite from the large scale production was always found to take the form of zeolite type X whereas that from the small scale was type A. This discrepancy occurred due to the inherited long filtration time in the large reactor which allowed further development of zeolite (from A to X). This gave a yield in the range of 71-74%. Regarding water adsorption performance, the maximum adsorption capacity of the sample treated with 20%wt using the large scale production were lower than the commercial molecular sieve type 4A, the maximum adsorption capacity of these samples were higher than the commercial molecular sieve type 3A. However, an economic analysis demonstrated that the total expense per batch of zeolite synthesized by large scale is relatively low when compared with the cost of commercial grade molecular sieve.