Chemical conversion of coal waste to micro/meso porous aluminosilicate materials
Authors:   , Chang Hsiao-Lan , Shih Wei-Heng 
Published: July 2024

The main objective of the present research is to develop chemical synthesis processes to produce useful materials from the class-F type fly ashes in coal wastes. There are two kinds of useful materials studied: one is the microporous aluminosilicate material that has a zeolitic structure with a pore size <1.0 nm; the other is the mesoporous aluminosilicate material that has pores range from 2.0 nm to 4.0 nm and arrange in an orderly hexagonal array. The general applications of micro/meso porous materials include catalysts, catalyst supports, molecular sieves, and adsorbents. In this research, the microporous aluminosilicate materials are used for stabilization of nuclear waste whereas the mesoporous aluminosilicate materials have the potential to be used for SO2 emission control. The primarily chemical components of the class-F fly ash are aluminosilicates, which is more than 80 wt%. By controlling the curing temperature and the bulk Si/Al molar ratio in a precursor mixture containing sodium hydroxide solution and class-F fly ash, three types of zeolites including zeolite A, faujasite and zeolite P have been synthesized during a hydrothermal process. The nucleation and growth of zeolites depend on the chemical composition in the liquid phase and the curing temperature. Fusing the sodium hydroxide powder and class-F fly ash at 550[degrees]C for 1 hour before the hydrothermal process, the yield of a faujasite type zeolite type produced from fly ash can be increased. Furthermore, faujasites can be crystallized from different fly ashes with a wide range of chemical compositions. This yield increase by the fusion method is due to the fact that quartz and mullite in fly ash react with the melted sodium hydroxide to form the soluble sodium silicate and aluminosilicate, and as a result, the Si and Al species in the liquid phase are increased. The application of the treated fly ashes containing zeolites have been tested for their ion-exchange property with nonradioactive Cs+ and Co2+ ions. The highest ion exchange capability of treated ashes for Cs+ at the concentration of 0.1 N CsCl was 75.6% of that of the commercial zeolite; the highest ion exchange capability of treated ashes for Co2+ at the concentration of 0.1 N CoCl2 was 75.7% of that of the commercial zeolite. The fusion method has also been successfully applied in the synthesis of mesoporous aluminosilicate materials. Two synthesis systems were developed to produce mesoporous aluminosilicate materials. The first system included sodium hydroxide, class-F fly ash and surfactant (C16H33(CH3)3NBr). The second system contained calcium hydroxide, class-F fly ash and surfactant. The mesoporous aluminosilicate materials synthesized from fly ashes have 2 uniform pore size, high surface area (>700 m2/g) and strong acid resistance. The mesoporous aluminosilicate materials synthesized from NaOH system have hexagonally arranged pores with a uniform size of 2.74 nm. The Si/Al molar ratio of the mesoporous aluminosilicates was found to be 13.4 but no Na was found. On the other hand, those synthesized from Ca(OH)2 system have larger pores with a uniform size of 3.09 nm. However, the pores are disorderly arranged. The Si/Al ratio and the Ca content were found to be 13.9 and 1.6 wt%.