Original Research Paper
Synthesis of zeolite from coal fly ash by microwave hydrothermal treatment with pulverization process

https://doi.org/10.1016/j.apt.2016.12.006Get rights and content

Highlights

  • Zeolite was synthesized from coal fly ash using a microwave heating method.

  • Pulverization promoted elements dissolution from coal in early stage of synthesis.

  • Promotion of elements dissolution promoted phillipsite generation.

  • Timely and proper addition of an aluminum source improved the yield of phillipsite.

Abstract

Coal fly ash was hydrothermally treated with a NaOH aqueous solution at 373 K using microwave heating with zirconia beads. We investigated the effect of the pulverization of coal fly ash on the generation rate and crystalline phase of synthesized zeolite. As a result, it was found that the pulverization process increased the generation rate of phillipsite in the early stage of the hydrothermal treatment because it enhances the dissolution of aluminate and silicate ions from coal fly ash and the generation rate of the aluminosilicate-gel precursor. On the other hand, prepulverization before the hydrothermal treatment and long-term pulverization during the hydrothermal treatment promote the formation of hydroxysodalite rather than phillipsite. In addition, we examined the effect of the addition of an aluminum source on the yield of synthesized phillipsite. This examination revealed that the excess addition of aluminate ions promotes the generation of hydroxysodalite and that timely and proper addition improves the yield of synthesized phillipsite. From these results, it was concluded that pulverization in the early stage of the hydrothermal treatment is effective for the generation of phillipsite from coal fly ash. Moreover, the timely and proper addition of an aluminum source improves the yield of synthesized phillipsite.

Introduction

Coal is the most abundant and widely distributed fossil fuel around the world. Coal fuels are used for most of the electricity production in many countries. Moreover, the growing energy needs of the developing world are likely to ensure that coal remains a key component of power generation, regardless of the climate-change policy [1], [2]. In Japan, the energy self-sufficiency rate as of 2012 declined to 6.0% after nuclear power plants were shut down owing to the Great East Japan Earthquake and huge tsunami, resulting in the increase in fossil-fuel imports as alternatives to nuclear energy. In this situation, coal has been re-evaluated as an important base-load power supply because it has the lowest price per unit of heat energy among all fossil fuels [3].

Coal-fired power plants excrete coal fly ash as byproduct of combusted coal. The amount of discharged fly ash is expected to increase in the future. Approximately half of the discharged coal fly ash is used as the raw material of cement and so on [4], [5], but the rest of the coal fly ash is disposed of in landfills. Recently, this practice of landfilling has become less attractive because of environmental concerns. Furthermore, the disposal of coal fly ash may soon be too expensive owing to stricter legislative requirements. Therefore, the reuse of coal fly ash can have important economic and environmental implications. As a consequence, considerable research has been conducted on the reuse of coal fly ash [6], [7], [8].

As an effective usage of coal fly ash, its conversion to zeolite has been receiving much attention. Zeolites are crystals consisting of aluminate and silicate frameworks and have the ability to act as adsorbents, catalysts, and so on. As a consequence of their properties, zeolites have many potential applications in the fields of radioactive-waste immobilization [9], petrochemical reactions [10], water purification [11], [12], and the purification of gasses [13], [14]. Many researchers have reported zeolite formation from coal fly ash using hydrothermal treatment methods [15], [16], [17], [18], [19], [20], [21], [22]. Previously, we have reported the effects of the size and composition of coal fly ash on the growth rate and crystal structure of the generated zeolite [23], [24]. In our previous series of studies, we focused on the generation of phillipsite, which is a kind of zeolite and has a high cation adsorption capacity. Furthermore, we have reported the effect of microwave irradiation on phillipsite synthesis from coal fly ash and have also proposed a method to improve the purity and yields of synthesized phillipsite [25], [26]. However, shortening the synthesis time and further improvements in the cation adsorption capacity of the products are required for practical use.

Zeolite, as well as phillipsite, is generated on the coal-fly-ash surface, and it is thought that the dissolution rate of the silica and aluminum components decreases in accordance with the growth of zeolite. Therefore, the pulverization of the fly-ash during the synthesis treatment would suppress this decrease in the dissolution rate and maintain the generation rate of zeolite, thereby improving the cation adsorption capacity of the products.

In this study, we investigated the effect of pulverization on phillipsite synthesis from coal fly ash by a hydrothermal treatment with microwave heating. The pulverization could increase the specific surface area of the fly ash by breaking it. It is expected that this increase of the specific surface area enhances the dissolution rates of the silica and aluminum components from the fly ash. Moreover, we examined the effect of the addition of an aluminum source, which is considered the limiting component of phillipsite synthesis, on the yield of synthesized phillipsite.

Section snippets

Experimental methods

Coal fly ash supplied from Shin-Onoda thermal power plant (Chugoku Electric Power) was used as the raw material. Fig. 1 shows the XRD peak charts of this tested fly ash. The properties of this tested fly ash are listed in Table 1. This fly ash has a relatively high silica content and small median diameter than typical coal-fly-ash. These characteristics are suitable for the synthesis of phillipsite by a hydrothermal treatment [23], [24].

A schematic of the experimental equipment is shown in Fig. 2

Crystalline phases of the product powders

Fig. 3 shows the XRD diffractograms of the product powder obtained for the four different experimental conditions (i.e., nonpulverized, prepulverized, pulverized, and partially pulverized). In all cases, peaks corresponding to quartz in the unreacted fly ash and those of the newly generated zeolites can be observed. However, for the prepulverized and pulverized conditions, hydroxysodalite (Na4Al3Si3O12OH) was remarkably generated as a by-product along with phillipsite (Na6Al6Si10O32·13.5H2O).

Conclusions

The influences of the pulverization of the slurry and the addition of an aluminum source to the slurry on the phillipsite synthesis from coal fly ash by a hydrothermal treatment with microwave heating were investigated. The results obtained in this work are summarized as follows:

  • (1)

    Pulverization of the slurry before the thermal treatment remarkably promoted the generation of hydroxysodalite as a by-product than phillipsite.

  • (2)

    Pulverization during the thermal treatment for the first hour promoted the

Acknowledgment

This work was partially supported by JSPS KAKENHI Grant Number 26420763. SEM images were obtained using FE-SEM (Hitachi S-5200) at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University. The authors acknowledge valuable discussions with Prof. Hideto Yoshida.

References (30)

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