(84c) Phillipsite Synthesis from Fly Ash Prepared by Hydrothermal Treatment with Microwave Heating

1. INTRODUCTION

In Japan, over 7.5 millions tons of fly ash are generated per year by thermal power plants and so on as industrial wastes. The amount of discharged fly ash is expected to increase monotonously in the years ahead. Therefore, effective ways to reuse fly ash should be quickly developed. One new way to reuse fly ash by synthesizing phillipsite, which can be used as adsorbents, catalysts and so on, with hydrothermal treatment method has been proposed.

On the other hand, the microwave can heat an object up rapidly, directly and selectively. For this reason, the microwave heating is widely used to synthesize fine powder materials in recent years. However, effects of the microwave irradiation on phillipsite synthesis from fly ash was not discussed sufficiently. It is the aim of this work to clarify which process of the phillipsite synthesis from fly ash is intensively affected by the microwave irradiation and to propose the effective irradiation method newly.

2. EXPERIMENTAL

A coal fly ash supplied from Shin-Onoda thermal power plant was used for a raw material. This fly ash has relatively high silica content and the crystalline phase of quartz.

2.45 GHz, 300 W microwave generated by the magnetron attains to the vessel by way of the wave guide. The slurry, which contains 50.0 ml of NaOH aqueous solution and 2.0 g of fly ash, is irradiated and heated up by the microwave. The temperature of the slurry is controlled by varying the output of the microwave with PID controller. With using this equipment, the fly ash was treated hydrothermally at 373 K. After the hydrothermal treatment, the slurry was cooled down to the room temperature. The product powder was separated from the hydrothermal solution by Buchner funnel, washed with distilled water thoroughly and dried at 393 K for 48 hours.

As an control experiment, the fly ash was treated hydrothermally under the same conditions with the conventional electric heater.

The properties of obtained product powder were evaluated by the crystal structure, ammonium ion adsorption capacity, chemical components and morphology.

3. RESULTS AND DISCUSSION

Figure 1 shows SEM images of these product powders. Although the surface of fly ash is smooth, in the cases of these product powders, phillipsite crystal particles deposit on the surface of the fly ash. Moreover, it can be observed that phillipsite synthesized with microwave heating has relatively smaller particle size than that with the conventional heating. Hence it is thought that the microwave heating reduces the size of the synthesized material.

Then, the crystal growth rate and the change in the adsorption capacity of product powder was investigated. The crystal growth was evaluated by [024] peak intensity of phillipsite of the product powder. Figure 2 shows the change in [024] peak intensity of product powder with treatment time. In any heating cases, the peak intensity can not be detected by XRD at first and after some waiting time for crystallization, the peak intensity increases with treatment time. After that it keeps almost constant. The microwave heating has the much shorter waiting time until the beginning of crystallization than the conventional heating. It is thought that the dipole rotation of phillipsite nuclei by the microwave induces the reduction of the waiting time when the crystal growth begins. Besides, the microwave heating require the shorter time to complete the crystallization than the conventional heating. However, the increasing rate of the peak intensity, which corresponds to the crystal growth rate, of the conventional heating is slightly larger than that of the microwave heating.

Figure 3 shows the change in the ammonium ion adsorption capacity of product powder with treatment time for microwave and conventional heating. In any heating methods, adsorption mass decreases to minimum value at 30-60 minutes, later it increases to constant. However, the microwave heating raises the increasing rate and the constant value of the adsorption capacity, though it gives no difference in the required time until the adsorption capacity attains to the constant. Moreover, it is found that the adsorption capacity also increases while the phillipsite crystal can not detected by XRD in Fig.2. This means that phillipsite nuclei which can not be detected by XRD are generated during the waiting time for crystallization. Therefore it is thought that the microwave heating promotes the nucleation of phillipsite.

The concentration of silicate and aluminate ions in NaOH aqueous solution was measured in order to acquire the information about effects of the microwave heating on the phillipsite formation. The change in the concentration of silicate ion and aluminate ion with treatment time is shown in Figure 4. In both heating methods, the concentration of silicate ion increases with treatment time. On the other hand, the concentration of aluminate ion increases considerably and acquires the maximum value at 45 minutes. After that it decreases monotonously. Furthermore, it can not be found that the change in ion concentrations depends on the heating method. This result suggests that the microwave heating does not enhance the dissolution of silicate ion and aluminate ion from fly ash and the formation of the precursor aluminosilicate gel. Accordingly it can be said that the microwave heating effects on the nucleation and the crystal growth of phillipsite mainly.

4. CONCLUSION

Influences of the microwave irradiation on phillipsite synthesis from fly ash by hydrothermal treatment were investigated. The results obtained in this work can be summarized as follows.

1) The microwave heating reduces the particle size of phillipsite synthesized from fly ash.

2) The product powder treated hydrothermally with microwave heating has the almost same crystal structure and chemical composition as that with conventional heating.

3) The microwave heating makes the waiting time for the crystallization of phillipsite much shorter than the conventional heating, thought it reduces the crystal growth rate slightly.

4) The microwave heating enhances the increasing rate and the constant value of the adsorption capacity of the product powder.

5) The change in the concentrations of silicate ion and aluminate ion does not depend on the heating method.