How-to, Tips and Articles: Investigation the Proper Conditions to Form Various Types of Zeolites From Coal Fly Ash

Abstract

Coal fly ash generated from power plants is continuously increasing each year in Korea. Only small portion of the fly ash is utilized, and the rest of them is disposed as landfills which causes a serious environmental problem due to toxic trace elements. In this study, the conversion of coal fly ash to various type of zeolite (cancrinile, X, P) was investigated as an other application area of coal fly ash utilizations.

The conversion conditions from coal fly ash to the various types of zeolites depend on the many parameters, such as chemical composition of coal fly ash and curing time. Therefore, in this study the proper conditions necessary to form various types of zeolites from coal fly ash were investigated.

INTRODUCTION

Large amount of coal fly ash are generated from coal-fired power plant throughout the world. Over 3 million tons of coal fly ash is produced in Korea every year. Only a small portion of total coal fly ash generated is recycled such as an additive to cements in a building material industry, and the remainder is disposed as landfills. The disposal of the large amount of coal fly ash causes serious environmental problems due to toxic trace elements such as As, Cd, Cr, Pb. Therefore, much effort should be made to find an alternative and meaningful applications for this waste. Since coal fly ash contains mainly amorphous alumi nosilicates (glassy phase) and some crystalline minerals (quartz, mullite, haematite, etc) having major compositions of SiO2 and Al2O3. Thus, It can be used as a raw material with the synthesis of zeolite-like materials.

Zeolites are a class of aluminosilicates having three dimensional polyhedral structures. The molecular framework structures in zeolites are open and contain channels and cavities. The openings in zeolites are normally occupied by alkali cations and water molecules, and both of them can be desorbed/adsorbed reversibly. Also these openings of such size are able to take up certain molecules selectively into their porous structure, known as the properties of molecular sieving. Such properties are able to use zeolites in water treatments, agriculture, paper manufacturing, gas adsorbers and catalysts.

Several groups around the world have attempted to develop zeolite-like materials from a fly ash and progress has been achieved in the synthesis of zeolite P, A, X, Y, analcime, and chabazite, etc. However, the fmal products are generally a mixture of some of the above mentioned zeolites, mainly Na-X zeolite with low crystallinity and surface area. The conversion conditions from fly ash to the various types of zeolites are complex and depend on many parameters, such as chemical composition of fly ash, and curing time.

Therefore, the objective of this study is to develop the proper method to form zeolites with high crystallinity and surface area. Emphasis was made to investigate the dissolving effets of coal fly ash.

EXPERIMENTAL PROCEDURE

Material

The coal fly ash was obtained from the Boryung power station. The chemical composition analyzed by X-ray fluorescence, and main component was alumino silicate (92.2%). The fly ash was used without further purifica tion. Other chemicals used in this study were sodium hydroxide pellets (Junsei Chemicals), aluminum hydroxide (Aldrich), and distilled water (laboratory made).

Synthesis of Various Zeolites from Coal Fly Ash

In this research, we have tried 3-different methods to prepare zeolites from the coal fly ash.

Treatment of coal fly ash by strong basic solution: By weight, one part of coal fly ash was mixed with 1.2 parts of 3M NaOH solution and vigorously stirred for 24h, and then this mixed solution was transferred to an autoclave (460 mL) and heated to 373K for 12h.

Treatment of coal fly ash by strong acidic solution: First, coal fly ash was dissolved in strong acid. By weight, one part of coal fly ash was mixed with 0.6 parts of 6M HCL solution with vigorous stirring for 12h and then slowly added 2.4 parts 3M NaOH solution and stirred for 12h This mixed solution was transferred to an autoclave (460 mL) and heated to 373K for 12h.

Treatment of coal fly ash by fusion method: For fusion, a mixture of one part coal fly ash and 1.2 parts NaOH was milled and then transferred to a high temperature furnace and heated to 773K for 1h. The fused mixture was cooled to room temperature and mixed with 10 parts 1120 in a teflon beaker. The resulting mixture were transferred into an autoclave and heated at 373K for 4h.

Prepared zeolites were washed three times with double distilled water, and dried at 383K for 24h and calcined at 773K for 8h.

Characterizations

Specific surface areas of prepared zeolites were determined by nitrogen adsorption iso therms at 77 K with a Micrometrics ASAP 2400. Powder X-ray diffraction patterns of all samples were recorded by a 2155 D6 X-ray diffractrometer with a nickel filtered Cu K𝞪 radiation. Scanning Electron Micrographs were obtained by a JEOL JSM-840A microscope operating at 20 KV and magnification values of X 10,000. The samples were covered with gold by sputtering method.

RESULTS AND DISCUSSION

The coal fly ash is agglomates of spheres having cenospheres of 1~100 nm in diameter (Figure 1(a)). X-ray diffraction patterns of the coal fly ash show that it is amorphous with some crystaling structure such as 𝞪-quartz (SiO2), mullite (2Al₂O₃ • 2SiO2), haematite (𝞬- Fe2O3) and magnetite (Fe3O4) (Figure 1(b)) The major elements of the coal fly ash are silicon and aluminium as Table I.

Table 1. Chemical analysis of coal fly ash

Figure 1. (a) SEM photograph and (b) XRI) pattern of coal fly ash (M, mullite; Q, quartz; H, haematite; C, corundum; Ma, magnetite).

Figure 2. [a} SEM photographs and [b] XRI) patterns of zeolitic adsorbents obtained at various temperatures; (a) 80°C, (b) 120°C, (c) 160°C (M, mullite; Q, quartz; C, corundum; Ma, magnetite; P, zeolite-P).

Among these components, 𝞪-quartz can easily dissolve in a NaOH solution, but mullite could not. However, 40% of coal fly ash is mullite. Therefore, coal fly ash has to be converted to a soluble form, and this is the most important procedure to synthesis of zeolite successfully. Therefore, we have tried 3-different trials to synthesize zeolites from the coal fly ash.

Treatment of Coal Fly Ash by Strong Basic Solution

The coal fly ash was treated at different temperatures with a strong basic NaOH solution and used in the preparation of zeolite. Since heating temperature plays an important role in a certain zeolite formation, the effects of heating temperature on the synthesis of zeolites from the coal fly ash were examined. Figure 2 shows the results of the X-ray diffraction patterns of zeolites which were synthesized at various temperatures. Final products obtained at 80°C (Figure 2(a)) and 120°C (Figure 2(b)) showed the coexistence of a zeolite-P and the coal fly ash. However, the product obtained at 160°C showed no XRD pattern of 𝞪-quartz and mullite phase, but almost similar XRD patterns to a pure zeolite-P (Figure 2(c)). The crystallinity of the prepared zeolite-P from the coal fly ash was increased with the increase of heating

Figure 3. Crystallinity of zeolite-P with respect to ageing temperature for 24h with out the presence of seeding.

Treatment of Coal Fly Ash by Strong Acidic Solution

As a second trial, the coal fly ash was mixed with a strong acid, and then NaOH solution was added to this mixture. After this, above solution was hydrothermally treated. The final product was a needle-type cancrinite zeolite as shown in Figure 4. However, there is a strong peak at 26° (2𝞱) which indicates that mullite was not completely dissolved.

Treatment of Coal Fly Ash by Fusion Method

As a third trial, the coal fly ash was treated by a fusion method. A alkali fusion is a conventional method for the chemical analysis to decompose materials containing silicon and/ or aluminum. Firstly, coal fly ash was fused with NaOH powder at different temperatures, and then this was hydrothermally treated. As shown in Figure 5, XRD patterns and SEMs of synthesized products showed a very large difference to each other depending on fusion temperatures.

Figure 4. (a) SEM photograph and (b) XRD pattern of cancrinite (M, mullite; C, cancrinite).

Figure 5. [a] SEM photographs and [b] XRD patterns for various fusion temperature; (a) 300°C, (b) 400°C, (c) 500°C (M, mullite; Q, quartz; A, analcime; X, zeolite-X).

In case of fly ash treated below 300°C, mixed analcime and coal fly ash were obtained as major product, and analcime was obtained at 400°C. However, cubic shape zeolite X (average diameter: 1𝞵m) was obtained as a major product at 500°C (surface area: 710 m2/g).

The optimum ratio of NaOH/coal fly ash weight (Na/ash) to obtain the best crystal was also investigated. The best crystal was obtained from 1~1.2 of NaOH/ash (Figure 6).

CONCLUSIONS

The most important procedure to synthesize of zeolites from the coal fly ash is to convert it to a soluble form. We have developed 3-different dissolving methods to prepare of zeolites from the coal fly ash. As a results, first, zeolite-P was synthesized by hydrothermal crystallization of mixed solution of NaOH and the coal fly ash. Apparently coal fly ash was completely dissolved and converted to zeolite-P. Second, cancrinite was synthesized by a hydrothermal crystallization of the solu tion of coal fly ash which was prepared by the mixing with strong acid and then slowly added NaOH solution.

Figure 6. XRD patterns for various NaOH/ash ratio; (a) 1.8, (b) 1.5, (c) 1.2, (d) 0.6 (M, mullite; A, analcime; X, zeolite-X).

Third, zeolite-X was synthesized by a hydrothermal crystallization of fused coal fly ash with NaOH powder. This zeolite has cubic (average diameter: 1𝞵m) shape. The best crystal of this was obtained from 1~1.2 of NaOH/ash.

Source: Sang-Sung Nam*, Myung-Woo Lee, Seong-Bo Kim, Kyu-Wan Lee, and Moon-Kyu Ko'

Chemical Technology Division I, Korea Research Institute of Chemical Technology,

'Department of Chemical Engineering, Konyang University, Korea

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Tuesday, March 17, 2020

Investigation the Proper Conditions to Form Various Types of Zeolites From Coal Fly Ash