Fuel 103 (2013) 533–541

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Fuel

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Zeolites formation by hydrothermal alkali activation of coal fly ash from thermalpower station ‘‘Maritsa 3’’, Bulgaria

Annie Shoumkova ⇑, Valeria StoyanovaRostislaw Kaischew Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 11, Sofia 1113, Bulgaria

h i g h l i g h t s

" Successful short-term zeolitization of fly ash, obtained from low-rank coals." SEM–EDX analyses of individual fly ash particles, gel aggregates, zeolite crystals." Zeolitic materials, containing hydroxy sodalite, NaA, or Linde F as main a product." The dominating zeolite phase was designed during the initial intensive nucleation.

a r t i c l e i n f o

Article history:Received 27 March 2012Received in revised form 25 July 2012Accepted 27 July 2012Available online 3 September 2012

Keywords:Coal fly ashZeolitizationHydrothermal activationSEM–EDXXRD

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.07.076

⇑ Corresponding author. Tel.: +359 29792598; fax:E-mail address: ashoumkova@ipc.bas.bg (A. Shoum

a b s t r a c t

Fly ash from lignite coal burning was subjected to a hydrothermal activation with NaOH and KOH(1.5 � 12.5 m) under different heating regimes to investigate zeolites formation. Products morphology,elemental and mineralogical composition were studied by means of SEM, EDX, and XRD. The activationwith KOH (>6 m) and NaOH (>3 m) for 7 h at 373 � 393 K produced zeolite Linde F and hydroxy sodalite,respectively. Blend of zeolites A and P was obtained at treatment with 3.1 m NaOH at 353 K. It was foundout, that in all types of zeolites Na+ had been partially replaced by Ca2+. The other impurities (e.g. Fe, Mg,and S) were detected only in hydroxy sodalite crystals and in smaller amounts. Some experiments indi-cated that the dominant zeolite phase in the product was determined in the first couple of hours of iso-thermal treatment, i.e. before the appearance of micron-sized zeolite crystals. Zeolites crystallizedpreferably on particles having glassy surfaces and had quite a constant composition, unrelated to thatof the surface.

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1. Introduction cations exchange, etc. Some of these processes go on simulta-

Bulgarian coal-burning thermal power stations (TPSs) generateover 6 million tons of fly ash (FA) annually, three fourths of whichare being disposed of in opened landfill sites, creating problemswith air, groundwater and soil pollution with harmful elements[1–3]. On the other hand, the significant content of silica and alu-mina in FA makes them suitable row materials for the synthesis ofvalue-added zeolitic products used in various environmental appli-cations, mostly in the field of industrial wastewater treatment[4–8]. The aluminosilicate ingredients in FA could be partiallyconverted into zeolites by alkali hydrothermal treatment, usuallycarried out at temperatures in the range 373–473 K, conversiontime 3–48 h, alkali concentration 0.5–5 M and liquid/solid ratio1–20 ml g�1 [5]. This technologically simple process is character-ized with a complicated mechanism [9], involving dissolution ofAl and Si from FA, precipitation of aluminosilicate gel on undis-solved FA particles, nucleation and growth of crystalline zeolites,

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+359 29712628.kova).

neously, but are affected oppositely by the treatment conditions.As a result, the quality and the yield of zeolite product are stronglydependent on the selection of process parameters [5], as it wasdemonstrated by Querol et al. [10], who obtained 13 different zeo-lites from the same FA, varying the activation conditions. On theother hand, it is known that the type of zeolite produced is de-signed by the ratio Si/Al in the solution at the early stage of reac-tion [5,11], which means that under the same activationconditions materials with similar bulk Si/Al ratios may produce dif-ferent zeolites [5,12], owing to the specific distribution of Si and Alamong the glass and crystallite phases in them. The unicity of FAcompositions and the complex influence of the treatment condi-tions, insist reliable assessment of the zeolitization potential ofany particular FA to be based on experimental data. Such data forBulgarian FA are absent, but a preliminary study [13] with FA from6 TPSs in the country has indicated that FA from ‘‘Maritsa 3’’ TPS isa suitable material for alkali activation experiments.

The aim of the present study was to investigate the zeolite for-mation at alkali hydrothermal treatment of FA from ‘‘Maritsa 3’’TPS under different conditions in order to evaluate the potential

534 A. Shoumkova, V. Stoyanova / Fuel 103 (2013) 533–541

of this solid waste as a raw material for the synthesis of valuablezeolitic products. A special accent in the work was put on theinvestigation of individual particles of different morphologicaltypes. Such data were scarce in the literature, although they couldcontribute for the elucidation of some important details of zeoliti-zation mechanism on a micro-level.

2. Materials and methods

2.1. Materials

FA was taken from the electrostatic precipitators for exhaustedgas cleaning in ‘‘Maritsa 3’’ TPS (Dimitrovgrad, Bulgaria), burning

Fig. 1. SEM micrographs and compositions (averaged, normalized) of typical particlessolutions.

local lignite coals. Detailed information about FA compositionand physicochemical properties is published elsewhere [14,15].Nevertheless, for the purpose of the present study it should bepointed out that the material consisted exclusively (>99.9%) of par-ticles less than 10 lm in diameter and had a relatively high specificsurface (15–16 m2 g�1). The content of the main zeolite constitu-ents, Si and Al, in the FA was respectively 42 wt.% (as SiO2) and20 wt.% (as Al2O3) [14]. These elements were included in two crys-talline phases – quartz (SiO2) and –calcium aluminum silicate(Ca0.88Al1.77Si2.23O8), and in glass phase, containing also differentquantities of K, Na, Ca, Mg, and Fe. Iron oxides, namely hematite(a-Fe2O3), maghemite (c-Fe2O3) and/or magnetite (Fe3O4), and cal-cium sulfates were also present in the FA. The material was quite

, observed in samples synthesized at different concentrations of KOH and NaOH

Table 1Parameters of the alkali hydrothermal activation process.

Sample no. Alkaline activator Temp. K (±5 K)a Isothermal treatment (h)

1 3.1 m KOH 393 (375) 72 6.2 m KOH 393 (383)3 12.5 m KOH 3934 1.6 m NaOH 393 (375)5 3.1 m NaOH 393 (377)6 6.2 m NaOH 393 (381)7 12.5 m NaOH 393 (391)

8 3.1 m NaOH 393 (377) 0b

9 110 211 312 413 514 615 7

16 3.1 m NaOH 403 717 373 718 353 719 333 720 298 64

21 3.1 m NaOH 393 (377) 7

a Measured in the oil bath; the boiling temperature (when reached) is given inbrackets (error ± 1 K).

b Sample 8 was only heated to the boiling temperature (with duration of 40 min).

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inhomogeneous in particles composition and morphology [14]. Theoverview of the microstructure of FA is shown in Fig. 1.

For the alkali solutions technical grade (purity >98%) NaOH andKOH were used.

2.2. Methods

2.2.1. Alkali hydrothermal activationThe reaction mixtures, prepared by dispersing of 10 g of FA in

100 ml KOH or NaOH solution, were subjected to a thermal treat-ment in a closed vessel, sealed in oil bath with controlled temper-ature and equipped with water-cooled reflux condenser andmagnetic stirrer. Sample 16 was synthesized in a Teflon-coatedautoclave (120 ml), heated in an oven. After that, the suspensionswere aged for 16 h at 298 K, with the exception of samples 8–15,

Table 2Elemental composition (in wt.%, normalized) of the initial FA and alkali activated samples

Sample O Na Mg Al Si

Initial FA 53.8 1.0 1.2 10.0 17.5Sample 1 54.4 0.5 1.5 9.4 16.5Sample 2 54.0 0.0 1.2 9.9 15.6Sample 3 52.2 0.0 1.2 10.2 15.0Sample 4 54.9 1.6 1.7 10.2 16.4Sample 5 53.2 6.3 0.9 10.5 16.2Sample 6 53.7 9.1 1.2 9.2 14.8Sample 7 52.1 9.5 1.2 9.1 14.4Sample 8 53.8 1.0 1.7 9.9 17.6Sample 9 54.6 1.0 1.8 9.4 16.8Sample 10 55.0 2.0 1.7 9.4 15.8Sample 11 54.7 2.1 1.7 9.5 16.0Sample 12 55.0 5.5 1.4 9.7 15.2Sample 13 55.0 4.7 1.4 9.8 15.5Sample 14 54.3 3.7 1.4 10.1 15.6Sample 15 53.8 4.5 1.5 10.1 15.6Sample 16 51.4 9.0 1.4 8.2 15.5Sample 17 53.7 6.5 1.3 10.2 15.6Sample 18 56.3 4.1 1.2 10.0 15.6Sample 19 55.4 0.8 1.8 10.0 15.5Sample 20 54.5 0.9 1.8 9.6 17.0Sample 21 53.5 6.7 0.9 11.1 15.9

which were not aged, and sample 21 that was aged for 1 month.The obtained solid products were separated by vacuum filtering,washed with hot distilled water to neutral pH, dried at 378 K,and analyzed. The parameters of the hydrothermal activation arepresented in Table 1.

2.2.2. Samples characterizationMorphology and elemental composition of the initial FA and the

products were examined by scanning electron microscope (SEM),model JEOL JSM6390, coupled with energy-dispersive X-ray(EDX) analyzer, Oxford Instruments. Average elemental composi-tion of the samples (Table 2) was estimated on the base of so called‘‘integral spectra’’, taken at low magnification on a surface com-prising several thousand particles. Data for the average composi-tion of particles/crystals of given morphological type, werecalculated on the base of at least 10 analyses of different individualparticles/crystals.

Powder X-ray diffraction (XRD) data for phase identificationwere collected by Philips PW 1050 automatic powder diffractome-ter, using Cu Ka filtered radiation and Bragg–Brentanogeometry.Step-scan data were recorded in the angle interval 7–93� (2h) witha step of 0.04� (2h) and accounting time of 2 s per step. Equal massof powdered samples and identical sample holders were used in allmeasurements.

3. Results and discussion

3.1. Effect of the alkali activator

Data for the elemental (Table 2) and phase (Fig. 1) compositionindicated some common trends for both activators. The treatmenthad led to complete dissolution of calcium sulfates and partial dis-solution of the glass phase. With the increase of the concentrationof the base, the content of the respective alkali metal in the productincreased, while Si/Al ratio slightly decreased. At the more concen-trated solutions (samples 3, 6 and 7) calcium aluminum silicatedissolved and the characteristic peaks of quartz and iron oxides be-came smaller than those of the respective newly formed zeolites(Fig. 2). The comparison of the pairs of samples, synthesized atequal molal concentrations of NaOH and KOH (for examplesamples 1 and 5), showed that both, the mass yield and the contentof the respective alkali metal were higher in the sodium samples,

.

S K Ca Ti Fe Mass change (%)

1.8 0.6 6.0 0.2 7.9 –0.0 3.9 6.2 0.5 7.1 +0.70.0 7.1 5.8 0.4 6.0 +9.70.0 9.2 5.9 0.4 5.9 +6.20.3 0.3 7.0 0.4 7.2 +7.90.0 0.0 6.7 0.0 6.2 +9.90.9 0.0 5.2 0.0 5.9 +10.00.9 0.0 6.4 0.0 6.4 +11.10.4 0.6 5.8 0.5 8.7 �7.50.0 0.5 6.7 0.5 8.7 �7.00.0 0.2 7.1 0.4 8.4 �5.40.0 0.3 7.5 0.5 7.7 �5.00.6 0.2 6.0 0.4 6.0 +7.20.0 0.2 6.3 0.5 6.6 +7.50.4 0.2 6.8 0.5 7.0 +7.50.5 0.2 6.6 0.4 6.8 +7.70.9 0.4 6.0 0.5 6.7 +11.41.2 0.0 5.3 0.0 6.2 +7.00.7 0.3 5.4 0.4 6.0 +3.30.0 0.3 8.1 0.7 7.4 �7.50.3 0.5 6.3 0.5 8.6 �10.60.0 0.0 6.5 0.0 5.4 +13.9

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i.e. that NaOH was more efficient zeolitization agent, which was incompliance with other studies [4,5,9,16,17]. This conclusion wasconfirmed during the observation of samples microstructure: insample 1 only corroded particles and amorphous gel presented,while in sample 5 the particles were almost fully covered by zeolitecrystals (Fig. 1). These crystals usually appeared as crystallites ofrandomly oriented rods or intersecting semi-discs (called ‘‘blade-shaped’’ crystals), often forming small sphere agglomerates ontothe parent particles surface. They were found to have quite a con-stant content of Na (11.2 ± 1.6 wt.%), Al (12.1 ± 1.4 wt.%), and Si(15.5 ± 1.3 wt.%). In addition to these elements, Ca was found topresent in almost all of the analyzed particles; in some of themtraces of Fe and S (up to 2 wt.%) were detected. In the literature,crystals with similar morphology have been associated with twozeolites – Na–P [9,18,19] and hydroxy sodalite (HS), [2,20,21]. Insample 5 zeolite Na–P was detected only as a trace phase, thus itwas assumed that blade-shaped crystals belonged to the semi-dehydrated HS (Na6Al6Si6O24�4H2O), which was the prevailingphase, identified by XRD (Fig. 2). Some other types of crystals that

Fig. 2. XRD patterns of initial FA and alkali activated samples. Legend: A – Na12(Si12Al12

0503) and/or CaSO4�2H2O (gypsum) (PDF 01-072-0596); C1 – Ca0.88Al1.77Si2.23O8 (calci(PDF00-038-0216); H – (a-Fe2O3 (hematite) (PDF 01-073-0603 or PDF 01-089-8103); M0315 or PDF 01-089-6466); P – Na3.552Al3.6Si12.4O32(H2O)10.656 (zeolite type GIS) (PDF 01-0SOD) (PDF 00-042-0216); S1 – Na8Al6Si6O24S.4H2O (ultramarine, structure type SOD) (P

were occasionally observed in sample 5 are discussed in the nextsection.

Samples 6 and 7 synthesized using more concentrated NaOHsolutions had similar bulk composition and contained significantquantity of prismatic crystals (Fig. 1), generally of larger size insample 6. These crystals were also identified to be SOD structuretype minerals (HS or ultramarine) (Fig. 2). They had the same Si/Al ratio as the blade-shaped crystals and a higher content of non-structural metals (Ca and Fe) (Fig. 1).

Although some researchers reported successful FA zeolitizationusing 2–3 M KOH and 1–2 M NaOH solutions under comparabletreatment conditions [5,9], the activation of the studied FA with1.56 m NaOH and with 3.12 m KOH did not produce noticeablequantities of crystalline zeolites, but only amorphously lookinggel (Fig. 1, samples 1 and 4). This fact indicated that short-termactivation could be achieved only when higher alkali concentra-tions and/or higher temperatures are applied, as demonstrated in[16,22]. Despite of its generally lower efficiency, KOH was consid-ered as an effective activator when used at higher concentrations

O48)H2O (zeolite type LTA) (PDF 01-080-0699); C – CaSO4 (anhydrite) (PDF 01-072-um aluminum silicate) (PDF 00-052-1344); F – KAlSiO4�1.5H2O (zeolite type EDI)– (c-Fe2O3 (maghemite) (PDF 01-089-5894) and/or Fe3O4 (magnetite) (PDF 01-088-80-0699); Q – SiO2 (quartz) (PDF 03-065-0466); S – Na6Al6Si6O24�4H2O (zeolite typeDF 00-038-0515).

Fig. 3. SEM-micrographs of the microstructure of samples, synthesized at different treatment times.

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(samples 2 and 3), because it produced a valuable zeolite, Linde F(KAlSiO4�1.5H2O). This zeolite crystallized as typical tetragonalcrystals (Fig. 1), forming a compact, firmly adhered [23] layer onFA particles. Traces of Bicchulite (Ca2Al2SiO7�H2O, SOD) were de-tected by XRD in samples 2 and 3, as well.

3.2. Influence of treatment duration

In the untreated FA, spherical particles with relatively smoothsurface texture predominated (Fig. 1). The initial heating of thereaction mixture to the boiling temperature led to the dissolution

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of about 7.5 wt.% of FA, mainly on the expenses of S, Ca, Si and Al,without noticeable changes in particles morphology (Fig. 3, sample8). During the first hour of the isothermal treatment the dissolu-tion of FA was progressing and caused roughening of particles sur-face (Fig. 3, sample 9). At the same time, an amorphousaluminosilicate gel, having highly variable composition, startedto precipitate onto particles surface and to impend the dissolutionof Si and Al. The active gel formation continued in the next 3 h andthen slowed down, due to the depletion of Si and Al in the solution.Simultaneously, self-arrangement of the amorphous gel into rag-ged-fiber structures (Fig. 3, sample 10) and spontaneous formation

Fig. 4. Evolution of the elemental composition of particles su

of spherical gel aggregates, most likely linked with the develop-ment of zeolite nuclei, were observed. The critical concentrationof sodium in the gel, which drove zeolite nucleation and growth[9], was found out to be about 4.5 wt.%, the value, reached at theend of the 2nd hour (Fig. 4, sample 10). Further thermal treatmentled to transformation of the ragged-fibers into randomly orientedrod-shaped crystals placed on parent particles or forming ‘‘rods–ball’’ aggregates (Fig. 3, sample 12). This result was in conformitywith the findings of other authors that, at similar conditions, theincubation period of zeolites was about 3 h [24]. The significantmass yield at sample 12 (Table 2) indicated that after the 3rd hour

rface, gel, and crystalline zeolite species with the time.

Fig. 5. SEM-micrographs of zeolite crystals of different morphological types, co-existing in samples 5 and 21: (a) blade-shaped, (b) fir cone-shaped, (c) cubic, and (d)octahedral.

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the rate of dissolution became negligible when compared to therate of gel precipitation. In the next 3 h sample mass was increas-ing slowly. The quantity of gel obviously decreased, while rods–balls became larger in size, and, in sample 15, parent particles werealmost fully covered with HS crystals (Fig. 3). Independently of thegreater variations in the gel composition, the composition of HScrystals, analyzed in samples 12–15, varied in quite narrow limits(Fig. 4) at constant Si/Al ratio (1.35 ± 0.1) and progressivelyincreasing (from 0.41 to 0.53) Na/Si ratio. HS showed higher affin-ity for sorption of Ca than for Mg and Fe (Fig. 4).

In the non-aged samples, HS crystals presented predominantlyas ball-shaped aggregates of randomly oriented rods, (Fig. 3, sam-ples 12 and 15). During the post-treatment ageing (samples 5 and21), they transformed into well-walled intersecting semi-discs(‘‘blades’’) (Fig. 5a). Although HS have been often observed to formboth types of crystals aggregates, the reason that caused its crystal-lization in one preferred shape is still not fully clarified even whenproducts are being synthesized from clear reactants [25,26]. Theobservations in the current study denoted that blades were themore stable crystal shape at room temperature, while the thermaltreatment promoted the formation of elongated rod-shaped crys-tals. The longer period of ion exchange with the solution throughthe post-treatment ageing led to negligible increase in Fe and Mgconcentrations (Fig. 4).

In addition to blade-shaped crystals, in samples 5 and 21 cubic,octahedral, and ‘‘fir cone’’ – shaped crystals (Fig. 5) were occasion-ally detected. Among these three types, fir cones were the mostabundant. They appeared for the first time in the 4th hour and pre-sented in all later samples. Crystals of similar shape were found tocorrespond to just a few previous reports [21,27]. Our furtherobservations (see Section 3.3) indicated that the fir cone crystal-lites could be associated with zeolite P, which was detected byXRD as a trace phase in sample 5. Fir cones were found to be veryselective to Ca2+: they contained only small amounts of Na and donot contain any other non-structural elements (Fig. 4).

Unlike fir cones, octahedral and cubic crystals were obtained bymany researchers and were clearly recognized as zeolite X (FAU

type [18,19,23]) and zeolite A (LTA type [28–30]), respectively.They both appeared in the 7th hour and were found to grow di-rectly onto particles surface (i.e. below HS crystals), most probablyfrom nuclei formed in the shallow outer layer of particles that wereintensively attacked by the base. In this layer, the maximum Naconcentration of about 3.7 wt.% (see Fig. 4) was achieved in the5th hour (sample 13) and remained steady until the end of thetreatment. Since zeolite A crystals were much more often observedin sample 21, it was supposed that zeolite A nucleated predomi-nantly during the cooling period (at lower temperature) and grewup during the post-treatment ageing of the product.

The detailed SEM–EDX observation of the treated samplesshowed that FA particles covered with iron oxide crystals, as wellas, iron microspheres, quartz crystals, and carbon residues moreoften remained uncovered by zeolite crystals, i.e. zeolites preferredto crystallize on aluminosilicate glassy surfaces. The elementalanalyses did not show any correlation between the content ofimpurities (Ca, Mg, Fe, etc.) in the particles and in the zeolite crys-tals growing on them.

3.3. Influence of temperature

As expected, sample 16 (synthesized in autoclave) had greatermass yield, respectively higher degree of transformation of FAmaterial into zeolite (SOD), coming from the intensified dissolutionof Si and Al. An evidence for the almost complete digestion of theglass phase in this sample was the presence of the typical, needle-shaped crystals of mullite, a resistant aluminosilicate mineral(3Al2O3�2SiO2) that originally presented in the core of FA particles,covered by glass phase. Hence, higher temperature was proved tohave a positive effect on the extraction of Al and Si. On the otherhand, it seemed that the increased (autogenic) pressure and the ab-sence of stirring had led to the formation of thick aggregates ofirregularly - shaped crystals (Fig. 6), instead of well-walled ones.

The composition and microstructure of sample 17, synthesizedat 373 K, i.e. just below the boiling point, was very similar to that ofsamples 5 and 15, treated at boiling conditions. The only difference

Fig. 6. SEM-micrographs and compositions (averaged and normalized) of typical morphological species, observed in samples synthesized at different temperatures.

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noticed, was in the shape of HS crystals: in sample 17 rods–ballspredominated over blade-shaped crystallites, while in sample 15,and especially in sample 5, it was on the opposite.

Thermal treatment at 353 K (sample 18) produced noticeablechanges in both, sample microstructure and phase composition,without significant difference in the elemental composition. Themain zeolite phase in this sample was Na–A (Na12(Si12Al12O48)H2-

O) (Fig. 2), crystallizing as cubic crystals, 1–3 lm in size (Fig. 6).

Zeolite Na–P (Na3.552Al3.6Si12.4O32(H2O)10.656) also was detectedby XRD (Fig. 2). This zeolite could be associated with the sphericalaggregates of irregularly shaped crystals, growing onto cubes(Fig. 6) that were quite similar in morphology and compositionto the fir cones in samples 13–15. Ca was detected in both typesof zeolite crystals, but more often in zeolite P. In sample 18, thepeaks of quartz and iron oxides were quite intensive, which fact,in a combination with the small mass gained (3.3%), testified to a

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relatively low degree of zeolitization. This result is in a compliancewith the conclusion of Querol et al. [5] that in the direct conversionof FA into zeolites higher temperatures are needed to dissolve, par-tially or totally, the Al–Si phases in FA before the growth of zeolitecrystals. In the case of FA from ‘‘Maritsa 3’’ TPS, the higher temper-atures promoted the formation of the SOD zeolites, which hadsmaller pore size and lower cation exchange capacity than LTAtype. For this reason and according to an idea of Wałek et al.[31], in an additional experiment, the thermal treatment was car-ried out at boiling temperature (377 K) in the dissolution stage(until the 2.5th hours) and at reduced temperature (353 K) in thecrystallization stage (2.5th–7th hour). The aim was to obtain zeo-lite A at increased yield; however, again HS was the prevailingphase in the product (which was very similar to sample 15). Thisresult proved that the main zeolite phase in the final productwas designed in first couple of hours of activation.

The negative mass changes in samples 19 and 20 indicated thatthe predominant process at lower temperatures was the dissolu-tion, authough gel aggregates or thin gel fibers with low Na content(Fig. 6) were often observed in these samples. Yet, an activation atroom temperature could be applyed as a pre-treatment in order toenhance the extraction of Si and Al.

4. Conclusions

This study demonstrated that FA from ‘‘Maritsa 3’’ TPS could betransformed into zeolitic materials, containing Linde F, blend of Aand P, or HS, by means of a short-term (7 h) activation with KOH(>6 m) or NaOH (>3 m) solutions at relatively low temperatures(353–383 K). The treatment with 3.1 m NaOH at 377 K led to theformation of aggregates of intersecting blade-shaped HS crystalswith visibly larger specific surface (i.e. better adsorption proper-ties) than those of the typical prismatic HS crystals, obtained athigher alkali concentration or temperature. The activation at353 K for 7 h produced valuable material, containing zeolites Aand P in sodium–calcium form that could be used for wastewaterpurification from ammonia, heavy metals, and phosphates [8]. Inall types of zeolites obtained, Ca2+ replaced partially Na+ in somecrystals, while the other impurities (e.g. Fe, Mg, and S) were de-tected only in HS crystals. Some experiments indicated that thedominant zeolite phase in the product was determined in the firstcouple of hours of isothermal treatment, i.e. before the appearanceof micron-sized zeolite crystals. Zeolites preferred to crystallize onparticles having glassy surfaces and had quite a constant composi-tion, unrelated to that of the surface.

Acknowledgements

The expert technical assistance of our colleagues Mrs. TsenkaTsacheva (SEM–EDX operator) and Mr. Georgi Avdeev (XRD opera-tor) is highly appreciated.

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