Environ. Sci. Technol. 1997, 31, 2527-2533
A Fast Method for Recycling Fly Ash: Microwave-Assisted Zeolite Synthesis X A V I E R Q U E R O L , * A N D R EÄ S A L A S T U E Y , ANGEL LO Ä PEZ-SOLER, AND FELICIA ` PLANA Instituto de Ciencias de la Tierra “Jaume Almera”, CSIC, C/ Martı´ i Franque`s S/N, 08028 Barcelona, Spain
Ho¨ller and Wirsching (1) investigated zeolite formation after alkaline activation of fly ash as a function of temperature, solution composition, and concentration in open and closed systems for long activation periods (8-40 days). A fusion with sodium hydroxide prior to hydrothermal reaction has been applied by Shigemoto and co-workers (5) to improve the conversion of fly ash into Na-X zeolites. Recent studies (7, 8, 13, 14) have obtained high Na and K-zeolite synthesis efficiencies after alkaline activation of fly ash in closed systems during relatively short activation periods (8-100 h). The
TABLE 1. Chemical Composition, Content of Major Inorganic Phases, and Grain-Size Distribution of Teruel Fly Ash
J O S E M . A N D R EÄ S , R O B E R T O J U A N , PEDRO FERRER, AND CARMEN R. RUIZ Instituto de Carboquı´mica, CSIC, C/ Poeta Luciano Gracia, 5, 50015 Zaragoza, Spain
Zeolitic material was synthesized from fly ash by conventional and microwave-assisted hydrothermal alkaline activation experiments. The zeolite synthesis was studied as a function of temperature, time, and activation solution concentration. K+-Na+/NH4+ exchange properties of the zeolitic material synthesized were studied as a function of time and zeolite type. The zeolitic material synthesized from the same fly ash by changing the synthesis parameters contained: NaP1, hydroxysodalite, hydroxycancrinite, analcime, tobermorite, and nepheline hydrate using NaOH as an activation agent and F linde zeolite, kalsilite, and phillipsite-KM zeolite from KOH activation. Synthesis yields and zeolite types obtained from the microwave and conventional experiments were very similar, but the activation time needed was drastically reduced by using microwaves (from 24-48 h to 30 min). Consequently, the industrial application of the synthesis process is enhanced by the microwave-assisted method. From this point of view, the most interesting zeolites synthesized are NaP1, KMphillipsite, and F linde zeolites since the NH4+ retention capacities obtained for the activation products were close to 20 and 30 mg of NH4+ g-1. The experiments performed also showed that high NH4+ retention capacities are attained after a few minutes of equilibrium with NH4+-rich solutions.
Introduction Zeolites have been synthesized by hydrothermal treatment of fly ash (1-14). This synthesis process has an important background, given the intensive research on zeolite growth in geological materials such as volcanic rocks and clay minerals (15-21). The similarity in chemical composition of fly ash and some volcanic rocks has led to the testing of zeolite growth. Fly ash is a suitable starting material for zeolite synthesis given the high content of the reactive phases, such as aluminosilicate glass, and given the high specific surface area of fly ash particles. Fly ash has a low Si/Al ratio, which allows the synthesis of low-Si zeolites with a high ion exchange capacity, a high selectivity for polar molecules, and a large pore volume (1). Consequently, these minerals have important industrial applications, mainly as sorbents for the removal of ions and molecules in different solutions and as a replacement for phosphates in detergents (22-24). * Corresponding author e-mail:
[email protected].
S0013-936X(96)00937-6 CCC: $14.00
1997 American Chemical Society
Chemical Composition % wt SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O MnO TiO2 P2O5 SO3
47.21 25.57 16.58 5.63 1.17 0.24 1.64 0.04 0.85 0.23 0.92
mg kg-1
B V Cr Co Ni Cu As Mo Cd Th U
509 208 134 29 88 72 60 15 1.3 22 23
Inorganic Phases % wt glass mullite quartz
62 17 7
% wt
magnetite anhydrite anorthite
13 0.7 0.3
Grain-Size Distribution undersize (µm) cumulative % wt undersize (µm) cumulative % wt 2.5 4.2 8.2 18.3
5 10 25 50
37.2 67.0 275
75 90 100
TABLE 2. Zeolitic Material Synthesized from Fly Ash and JCPDS Files for XRD Identification and List of Major XRD Reflections (Å, in Decreasing XRD Intensity) for Zeolites zeolitic product NaP1 zeolite analcime phillipsite-KM zeolite F linde zeolite tobermorite nepheline hydrate hydroxysodalite hydroxycancrinite kalsilite
NaP1 3.17 7.10 4.10 2.68 5.02 1.97 1.72 1.67 1.77 1.63
JCPDS Na6Al6Si10O32‚12H2O NaAlSi2O6‚H2O K2Al2Si3O10‚H2O KAlSiO4‚1.5H2O Ca5(OH)2Si6O16‚4H2O Na2Al2Si2O8‚H2O Na1.08Al2Si1.68O7.44‚1.8H2O Na14Al12Si13O51‚6H2O KAlSiO4
10 Major XRD reflections (Å) analcime KM F hy-sodal. 3.43 5.60 2.93 2.23 4.85 1.74 2.69 2.51 1.90 3.67
3.25 3.19 2.97 5.07 8.34 7.14 5.38 4.47 4.31 3.66
3.00 6.97 3.08 3.07 2.81 3.47 4.18 2.35 6.55 5.93
3.67 6.36 2.59 2.11 1.76 1.59 2.11 2.74
39-0219 19-1180 30-0902 25-0619 19-1364 10-0460 31-1271 28-1036 33-0988
hy-canc. 3.26 4.71 3.68 2.76 2.44 6.40 2.60 4.18 2.13 2.64
VOL. 31, NO. 9, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2527
FIGURE 1. SEM microphotograph showing synthesized NaP1 on partially activated fly ash spheres. The fly ash used in this experiment is from the Teruel power station in NE Spain, and the reaction parameters used were as follows: synthesis method, thermal treatment; time, 24 h; solution/fly ash ratio, 18.2 mL g-1; alkali concentration, 1.0 M NaOH; and temperature, 150 °C.
FIGURE 3. Evolution of synthesized zeolites as a function of molar concentration of the alkaline activation solutions in conventional synthesis experiments. The normalized XRD intensity (without units) is the normalized integrated intensity of the highest hkl reflection for each zeolite (proportional to their concentration in the sample). (Upper) Synthesis experiments carried out with KOH at 150 °C and 48 h. (Middle) Synthesis experiments carried out with NaOH at 150 °C and 24 h. (Lower) Synthesis experiments carried out with NaOH at 200 °C and 24 h. KM, phillipsite-KM zeolite; F, F linde zeolite; tob, tobermorite; kal, kalsilite; NaP1, NaP1 zeolite; ca, hydroxycancrinite; an, analcime; sod, hydroxysodalite.
FIGURE 2. XRD patterns of (A) Teruel power station fly ash; (B) product of the alkaline activation of the Teruel power station fly ash during 30 min by means of 1.0 M NaOH using microwaves (1000 W); and (C) product of the alkaline activation of the same fly ash during 24 h by means of 1.0 M NaOH using conventional thermal activation. A solution/fly ash ratio of 10 mL g-1 and a temperature of 175 °C were used for both conventional oven and microwave synthesis experiments. Notice the similarity of NaP1/magnetite ratios obtained from both experiments, which reflect a similar synthesis efficiency.
2528
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 9, 1997
results of our previous work on this subject (10, 11, 25) demonstrated that different zeolites can be synthesized from the same fly ash by changing the activation parameters (mainly temperature, activation agent, concentration of the activation solution, and time of activation). However, the industrial application of this process has certain limitations given the long activation periods needed for the synthesis of these materials. The novelty of the present study concerns the acceleration of this process (from hours or days down to minutes) by means of microwave-assisted synthesis, as it has been done in prior works on zeolite synthesis from pure products (26-28).
Materials and Methods Materials. Fly ash from the Teruel power station (ENDESA, 1050 MW) in NE Spain was collected from the electrostatic precipitators. The chemical composition of the Teruel fly
TABLE 3. Experimental Conditions Used for Synthesis Experiments and Zeolites Obtaineda 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
solution
t (h)
T (°C)
NaOH 0.1 M NaOH 0.5 M NaOH 0.5 M NaOH 0.5 M NaOH 0.5 M NaOH 0.5 M NaOH 1.0 M NaOH 1.0 M NaOH 1.0 M NaOH 1.0 M NaOH 1.0 M NaOH 3.0 M NaOH 5.0 M NaOH 0.5 M NaOH 0.5 M NaOH 0.5 M NaOH 1.0 M NaOH 1.0 M NaOH 1.0 M NaOH 0.1 M NaOH 0.1 M NaOH 0.5 M NaOH 0.5 M NaOH 1.0 M NaOH 1.0 M NaOH 1.0 M NaOH 3.0 M NaOH 5.0 M KOH 0.5 M KOH 0.5 M KOH 0.5 M KOH 1.0 M KOH 3.0 M KOH 5.0 M KOH 0.5 M KOH 1.0 M KOH 1.0 M KOH 3.0 M KOH 5.0 M
15 8 15 24 48 96 8 15 24 48 96 24 24 24 48 98 24 48 98 15 24 15 24 15 24 48 24 24 15 24 96 24 24 24 24 24 48 24 24
150 150 150 150 150 150 150 150 150 150 150 150 150 175 175 175 175 175 175 200 200 200 200 200 200 200 200 200 150 150 150 150 150 150 200 200 200 200 200
zeolites P1* P1** P1** P1** P1*** P1** P1*** P1*** P1*** P1***, An* S**, P1**, T* S***, T P1**, An* P1**, An*, S*, T* P1**, An**, T* P1**, An**, T* P1**, An**,S*, T* P1**, An**, T* P1, An P1** P1*, An**,S*, T* P1*, An, S*, T* P1*, An**,S*, T*,Nep* An, S*, T*,Nep** S**, C**, T* S**, C***, T*
KM F***, T K***, F*, T KM** KM** KM*** K***, T K***
original minerals Q***, Mu***, Mt*** Q***, Mu***, Mt*** Q***, Mu***, Mt*** Q*,Mu**, Mt*** Mu*, Mt*** Mt*** Q,Mu*, Mt*** Q,Mu*, Mt*** Mu, Mt*** Mt*** Mt*** Mt*** Mt*** Mu, Mt*** Mt*** Mt*** Mt*** Mt*** Mt*** Q***, Mu***, Mt*** Q***,Mu***,Mt*** Mu,Mt*** Mt*** Mt*** Mt*** Mt*** Mt*** Mt*** Q***, Mu***, Mt*** Q***, Mu***, Mt*** Q***, Mu***, Mt*** Q***, Mu***, Mt*** Mt*** Mt*** Q*, Mu***, Mt*** Q*, Mu*, Mt*** Mt*** Mt*** Mt***
a P1, NaP1; S, hydroxysodalite; C, hydroxycancrinite; An, analcime; Nep, nepheline hydrate; T, tobermorite; KM, phillipsite-KM zeolite; F, linde F zeolite; K, kalsilite; Q, quartz; Mt, magnetite; Mu, mullite.***, high content; **, middle content; *, low content; without *, trace amounts.
TABLE 4. Experimental Conditions Used for Microwave-Assisted Activation and Zeolites Obtaineda 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
solution
t (min)
T (°C)
NaOH 1.0 M NaOH 1.0 M NaOH 5.0 M NaOH 5.0 M NaOH 3.0 M NaOH 1.0 M NaOH 1.0 M NaOH 5.0 M NaOH 5.0 M KOH 1.0 M KOH 1.0 M KOH 5.0 M KOH 5.0 M KOH 3.0 M KOH 1.0 M KOH 1.0 M KOH 5.0 M KOH 5.0 M
10 30 10 30 20 10 30 10 30 10 30 10 30 20 10 30 10 30
175 175 175 175 200 225 225 225 225 175 175 175 175 200 225 225 225 225
zeolites P1*** S**, T* S***, T* S***, C* S** S**, An***, T* S***, C*, T* S***, C*** F F** F F**, K F***, K***
original minerals Q***, Mu***, Mt*** Q*, Mu**, Mt*** Q*, Mu**, Mt*** Mu*, Mt*** Mu**, Mt*** Q**, Mu***, Mt*** Mt*** Mt*** Mt*** Q***, Mu***, Mt*** Q***, Mu***, Mt*** Q***, Mu***, Mt*** Q**, Mu*, Mt*** Q***, Mu***, Mt*** Q***, Mu***, Mt*** Q***, Mu***, Mt*** Q**, Mu**, Mt*** Mt***
a P1, NaP1; S, hydroxysodalite; C, hydroxycancrinite; An, analcime; T, tobermorite; F, linde F zeolite; K, kalsilite; Q, quartz; Mt, magnetite; Mu, mullite.***, high content; **, middle content; *, low content; without *, trace amounts.
ash (determined by means of ICP-AES and ICP-MS), the content of major inorganic phases (determined by quantitative XRD), and the particle-size distribution are shown in Table 1. Synthesis Experiments. Bulk fly ash samples without prior treatment were used for zeolite synthesis experiments. The
activation of fly ash was performed by means of NaOH and KOH solutions in a closed system with and without microwave assistance. Classical hydrothermal activation experiments were carried out using closed steel vessels lined with PTFE (45 mL) and conventional ovens. The amount of fly ash used in the
VOL. 31, NO. 9, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2529
TABLE 5. Zeolitic Material Synthesized as a Function of Reaction Parameters alkali concn (M) 0.5-3.0 3.0-5.0
temp (°C)
zeolitic product
175 150-200
NaOH NaP1 analcime, hydroxysodalite, tobermorite, nepheline hydrate hydroxysodalite, hydroxycancrinite, tobermorite KOH
conventional activation 0.5-1.0 3.0 5.0 3.0-5.0 microwave activation 0.5-1.0 3.0-5.0 3.0-5.0 M
150-200
