Original Research PaperSynthesis of zeolite from coal fly ash by microwave hydrothermal treatment with pulverization process
Graphical abstract
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)
- et al.
Conversion of coal fly ash to zeolite utilizing microwave and ultrasound energies: a review
Fuel
(2015) - et al.
Coal fly ash utilization: low temperature sintering of wall tiles
Waste Manage.
(2008) - et al.
Utilizing coal fly ash as a landfill barrier material
Waste Manage.
(1996) A review on the utilization of fly ash
Prog. Energy Combust. Sci.
(2010)- et al.
Environmental-benign utilisation of fly ash as low-cost adsorbents
J. Hazard. Mater.
(2006) - et al.
Conversion of fly ash into zeolites for ion-exchange applications
Mater. Lett.
(1996) - et al.
Novel zeolite Na-X synthesized from fly ash as a heterogeneous catalyst in biodiesel production
Catal. Today
(2012) - et al.
Removal of Mn from aqueous solution using fly ash and its hydrothermal synthetic zeolite
J. Environ. Manage.
(2014) - et al.
Effective utilization of waste ash from MSW and coal co-combustion power plant—zeolite synthesis
J. Hazard. Mater.
(2008) - et al.
Preparation and characterization of nano-NaX zeolite by microwave assisted hydrothermal method
Adv. Powder Technol.
(2014)
Synthesis of zeolite from Italian coal fly ash: differences in crystallization temperature using seawater instead of distilled water
Waste Manage.
Zeolite formation from coal fly ash and heavy metal ion removal characteristics of thus-obtained zeolite X in multi-metal systems
J. Environ. Manage.
Microwave-assisted two-step process for the synthesis of a single-phase Na-A zeolite from coal fly ash
Adv. Powder Technol.
Microwave-assisted zeolite synthesis from coal fly ash in hydrothermal process
Fuel
A two-step process for the synthesis of zeolites from coal fly ash
Fuel
Cited by (54)
Preparation of hierarchically porous carbon ash composite material from fine slag of coal gasification and ash slag of biomass combustion for CO<inf>2</inf> capture
2024, Separation and Purification TechnologyHydrothermal synthesis of high-purity zeolite X from coal fly ash for heavy metal removal: Kinetic and isotherm analysis
2023, Advanced Powder TechnologyZeolites synthesized from industrial and agricultural solid waste and their applications: A review
2023, Journal of Environmental Chemical EngineeringHierarchical porous zeolite synthesis from coal fly ash via microwave heating
2023, Colloids and Surfaces A: Physicochemical and Engineering Aspects