Synthesis of zeolite Na-P1 under mild conditions using Brazilian coal fly ash and its application in wastewater treatment

Fuel 139 (2015) 59–67

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Synthesis of zeolite Na-P1 under mild conditions using Brazilian coal fly ash and its application in wastewater treatment Ariela M. Cardoso a,b,⇑, Alexandre Paprocki a,b, Lizete S. Ferret c, Carla M.N. Azevedo a, Marçal Pires a,b a Post-graduation Program in Engineering and Technology Materials, Pontifical Catholic University of Rio Grande do Sul, Av. Ipiranga 6681, Prédio 30, 90619-900 Porto Alegre, RS, Brazil b Faculty of Chemistry, Pontifical Catholic University of Rio Grande do Sul, Av. Ipiranga 6681, Prédio 12B, 90619-900 Porto Alegre, RS, Brazil c Science and Technology Foundation (CIENTEC), Rua Washington Luiz, 675, 90.010-460 Porto Alegre, RS, Brazil

h i g h l i g h t s  The molar Si/Al ratio for raw ash and its fine fractions varies from 2.5 to 2.9.  Despite the similar composition of the ashes, differences were observed in zeolite production.  Pre-treatment of ash by decreasing particle size and undesirable material.  Zeolites can be synthesized in a glass reactor under milder temperature conditions.  The synthesized zeolite was used to remove contaminants from acid mine drainage.

a r t i c l e

i n f o

Article history: Received 21 June 2014 Received in revised form 7 August 2014 Accepted 7 August 2014

Keywords: Coal Coal fly ash Zeolites Acid mine drainage

a b s t r a c t Zeolites may be easily obtained from coal fly ash by conversion processes based on the similar essential composition of both. Among the existing zeolites, we highlight the Na-P1 which is normally produced by hydrothermal treatment, this zeolite has been synthesized in pressurized reactors at high temperatures. The purpose of this study was to synthesize Na-P1 zeolites under mild conditions using Brazilian coal fly ash. Zeolite Na-P1 was produced simultaneously in pressurized reactors at 150 °C and a glass reactor at 100 °C. Complementary synthesis parameters were also evaluated, such as concentrations of alkali, crystallization time, previous separation of magnetic material from the ashes and effects of seeding on synthesis. The zeolite synthesized in the optimized process (77% Na-P1) was used to remove contaminants from acid mine drainage (AMD). Treatment of this wastewater with 10 g L1 of synthetic zeolite for 30 min presented high removal of arsenic, nickel, calcium, copper, iron and manganese ions and the partial yet significant removal of ammonium, magnesium, potassium and zinc ions. This performance indicates that the synthetic zeolite has excellent adsorption characteristics and can be an alternative for the treatment of DAM. As such, synthesis under mild conditions in a glass reactor is practical, reducing energy costs and minimizing environmental impacts, with the possibility of reproducing zeolite synthesis on an industrial scale. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Thermoelectric power plants using coal as fuel to generate electricity are considered a major source of pollution and are responsible for generating a substantial amount of pollutants, including solid waste (coal fly ash). World production of ash is over 750 million tons/year [1,2], whereas in Brazil seven thermoelectric ⇑ Corresponding author at: Faculty of Chemistry, Pontifical Catholic University of Rio Grande do Sul, Av. Ipiranga 6681, Prédio 12B, 90619-900 Porto Alegre, RS, Brazil. Tel.: +55 51 33534305; fax: +55 51 33203549. E-mail address: [email protected] (A.M. Cardoso). http://dx.doi.org/10.1016/j.fuel.2014.08.016 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.

plants in the southern states alone generate approximately 3 million tons/year (65–85% fly ash and 15–35% bottom ash), which are partially used, largely in the production of cement and concrete, due to its pozzolanic properties [1–4]. Other emerging uses of this residue have been reported, including soil amelioration agent, adsorbent, catalyst and in the manufacture of ceramic materials [2,5–7]. However, an increase in the use of coal and consequently ash generation (6 million tons/year) in Brazil is expected in the coming years, along with greater restrictions on sulfur emissions through the implementation of desulfurization systems. Particularly prominent among SOx removal technologies is the direct addition of double carbonate of calcium and magnesium

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(dolomite), modifying the composition of ash and further restricting its use [4]. In this respect, there is a need for new alternatives in the sustainable use of this waste. Coal fly ash consists of both amorphous aluminosilicate material and crystalline phases, mainly a-quartz (SiO2), mullite (3Al2O32SiO2), hematite (a-Fe2O3) and magnetite (Fe3O4) [8,9]. This composition allows the conversion of these ashes into zeolites through hydrothermal treatment, for example [10–14]. Zeolites are hydrated, crystalline and microporous aluminosilicates structured into three-dimensional networks of TO4 (T = Si, Al) tetrahedra joined at the corners by oxygen atoms. This structural configuration means zeolites have a significant number of intermolecular channels and cavities, with molecular dimensions that allow the transfer of matter between intercrystalline spaces [15]. As a result, these materials are widely used in a vast array of industrial applications, primarily as sorbents for the removal of ions in liquid and gaseous effluents [13,16–20], mercury removal from flue gases [21] and also as material for capturing carbon dioxide [22]. The use of zeolites synthesized from ashes is particularly interesting in that it allows undesirable waste to be converted into high value added products. Zeolite Na-P1 (Na6Al6Si10O3212H2O) is a synthetic material with a high ion exchange capacity (0.72–3.9 meq g1) [8,22], due to substitution of Si (IV) by Al (III) in its structure, which results in the overall negative charge, leading to applications as ion exchange or molecular sieve. Consequently, this material presents significant potential for industrial and environmental applications [2,8,14]. This zeolite has an affinity with some metal ions, generally found in acid mine drainage (AMD) effluents, and is an alternative for correcting this problem [11–13,23–25]. Although some studies address the synthesis of Zeolite Na-P1, the zeolite produced generally exhibits low purity and a low efficiency. Typically, metal reactors and high synthesis temperatures are used, raising production costs [26,27]. A recent study has shown that zeolite Na-P1 can be prepared using coal fly ash and mine waters obtained from coal mining operations in South Africa. The experiments were performed using a two-step synthesis: a preliminary aging stage followed by hydrothermal treatment in Parr bomb at 140 °C for 48 h [28]. Sommerville and co-workers synthesized zeolite Na-P1 using 4.0 mol L1 NaOH (10 g L1 ratio solid/liquid), with the addition of sodium aluminate in the crystallization step and heating at 100 °C for more than 12 h [29]. The aim of this study was to produce zeolite Na-P1 from coal fly ash originating in Candiota (southern Brazil), under mild synthesis conditions. The resulting zeolites were applied in the treatment of acid mine drainage (AMD) produced in the exploitation of this coal. 2. Experimental 2.1. Materials The present study used fly ash collected from the electrostatic precipitators of the Presidente Médici Thermoelectric Power Plant (UTPM, Candiota, Brazil). This facility has two combustion plants (Units A and B) and the ashes collected were denominated UA and UB. Commercial grade zeolite Na-P1 (Industrias Quimicas del Ebro – IQE, Spain) was used as reference material for comparison with the zeolites produced. The zeolites were dried in an oven at 105 °C for 2 h and stored in a desiccator. Wastewater treatment analysis used AMD samples from surface water at a mine located in southern Brazil; AMD-RS in Candiota, Rio Grande do Sul state. NaOH solutions (Merck 99.5%) at concentrations between 0.5 and 3.5 mol L1 were used for alkaline hydrothermal treatment of the ashes. An NH4Cl solution (Merck 99%) with a concentration of 0.1 mol L1 was used in cation-exchange capacity (CEC) tests. All the solutions were prepared using deionized water (Milli-Q Plus, Millipore, resistivity >18 MX cm1).

2.2. Characterization of the ash and zeolites The chemical composition of the ashes was determined by X-ray fluorescence (XRF) using a Rigaku RIX 2000 spectrometer and rhodium as a source of radiation. Particle size distribution of the ash was obtained by continuous delivery with a laser spectrometer (CILAS 1180, detection limit of 0.04–2500 lm). Particle morphology was analyzed by scanning electron microscopy (SEM, Philips XL 30) and elementary analysis was performed by energy-dispersive X-ray spectrometry (EDS) using EDAX software. Crystalline phases were characterized via X-ray diffraction (XRD) using an X-ray diffractometer (SIEMENS model D5000) with Ka radiation in a copper tube (40 kV and 25 mA). Minerals present were identified using the JCPDS (Joint Committee on Powder Diffraction Standards) database. Amorphous phases Al and Si in the coal fly ash were obtained using data from XRF (total amount) decreased values of the crystalline phases using data from XRD [4]. The cation-exchange capacity of the zeolites was determined by saturation with ammonium ion, following the methodology of the International Soil Reference and Information Centre [30]. In addition to establishing cation mobility within the structure of the solid, CEC can also be used to estimate semi-quantitative data of the zeolite content in the synthesized material. This is achieved by comparing the CEC values of pure zeolite (100%) with those of the synthesized material, as per Eq. (1) [8], where Sx and Sr are the CEC values of synthetic zeolite and pure zeolite (reference), respectively. This estimate considers that CEC values for non-zeolitic material of the product are negligible, based on the low CEC of the ash used in synthesis.

Zeolite content % ¼

Sx  100 Sr

ð1Þ

2.3. Pre-treatment of ash Pre-treatment processes were limited to those that can be used industrially. The first involved separating fractions according to particle size, using vibrating sieves of 60, 100, 150, 200, 270 and 400 Mesh in a mechanical shaker (BERTEL). In the second process, the magnetic particles in raw fly ash (UAr and UBr) and nonmagnetic fractions (UAr/nm and UBr/nm) were separated manually on a permanent magnet (Magnetos Gerais Ltda., 6000 Gauss). The magnetic particles of the sub-samples of the fine fly ash fractions (<38 lm) (UAf and UBf) were also removed, generating two new fractions (UAf/nm and UBf/nm). To perform manual magnetic separation, the permanent magnet was coated with low-porosity paper. The approximately 1.0 g of the coal fly ash was weighed and added on the system. With the aid of a brush, the coal fly ash was carefully separated from the magnetic fraction. 2.4. Synthesis experiments Raw fly ash and its fractions were used in zeolite synthesis, applying methodology adapted from the literature [4,12,26,27]. All fly ashes were activated by NaOH solutions in three different closed systems: (A) Polytetrafluoroethylene (PTFE) reactor cased in steel, with an internal volume of 180 mL; (B) Perfluoralkoxy (PFA) reactor with an internal volume of 180 mL and (C) a borosilicate glass reactor with an internal volume of 500 mL. Replicate the best synthesis conditions were performed to verify the reproducibility of the process. 2.4.1. Synthesis in reactors A and B Zeolitization was performed with 1 g of fly ash in 18 mL of NaOH solution (0.5–3.5 mol L1), producing a solution/fly ash ratio of 18 mL g1. Tests were conducted at constant temperature

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(150 °C) and different reaction times (16–48 h), in the presence of a promoter (0–8% of Na-P1 seed). Tests were performed in a laboratory oven and solid content was filtered (glass membrane, Millipore, 0.22 lm) and washed with deionized water until residual NaOH was removed (filtrate pH < 10). Solids and post-reaction solutions were stored in polyethylene flasks at ambient temperature and protected from light until quantification of Si and Al in the filtrate with a view to its reuse and subsequent characterization of the solid.

Table 1 Chemical composition, molar ratio Si/Al, cation exchange capacity (CEC), humidity and average diameter of coal fly ash and its fine fractions (<38 lm). Parameter

2.4.2. Synthesis in reactor C Zeolitization was performed in the glass reactor with 10 g of coal fly ash in 18 mL of NaOH solution (1.0–3.0 mol L1). Syntheses were carried out in an oven for 24 h at 100 °C. The solid and postsynthesis solution received the same treatment as the material formed in zeolitization in the steel reactor, as described in item Section 2.4.1. 2.5. Treatment of Acid Mine Drainage The efficiency of zeolite Na-P1 in removing the ions in an AMD sample was evaluated in two stages: (1) The AMD-RS sample (from CRM) was treated with commercial grade zeolite Na-P1 (IQE) at different doses of 10 and 20 g L1. The experiment was conducted under conditions similar to those applied for CEC, at ambient temperature with 30 min of agitation. (2) The AMD-RS sample was treated with the synthesized zeolite in reactor B (15° zeolitization test) at a dose of 10 g L1, under the same conditions used in the treatment with the commercial zeolite. This zeolite was chosen for its high CEC value, which is representative of the synthesized samples. Soluble metal concentrations were measured in the AMD by flame atomic absorption spectrometry – FAAS (Varian SpectrAA 55 spectrometer) before and after contact with zeolite. Cations and anions were determined using ion chromatography (Dionex 500). 3. Results and discussion 3.1. Characterization of fly ash and pre-treatment The chemical composition, molar Si/Al ratio, cation exchange capacity (CEC) and mean diameter of the UA and UB fly ash, as well their fine fractions (<38 lm), are shown in Table 1. There are no significant variations in composition between the two types of ash or between the raw and fine fractions. SiO2 and Al2O3 are the main components of all the samples, accounting for >90% of the material. Other important components, albeit at lower concentrations, are Fe2O3 (5%) and CaO (1.5%). As a result of this composition, the ashes are classified as sialic, with high alumina and silica content and low levels of impurities such as Fe, Ca and S [9,26]. Both the unburned material (LOI < 0.60%) and moisture (0.14%) contents are low for all the samples. However, the lower LOI of UB raw ash (0.27%) compared to UA raw ash (0.41%) is likely related to the lower burning efficiency of Unit A, which has been in operation for 40 years and is less efficient than Unit B [31]. The molar Si/Al ratio for raw ash and its fine fractions is similar and varies from 2.5 to 2.9. These values point to the potential use of this ash in the synthesis of zeolites with intermediate silica content (1.5–3.8) such as Na-P1, which is suitable for wastewater treatment [15]. In addition, raw fly ash showed very low cation exchange capacity (CEC 0.03–0.04 meq g1), while fine fractions exhibited higher CEC (0.23–0.72 meq g1). Nevertheless, these cation exchange capacities are still too low for the application of this residue in wastewater treatment processes based on ion exchange. Granulometry results (not shown) for UA and UB raw ash indicated

Fly ash UA

a

UB

Total (%)

Fine (%)

Total (%)

Fine (%)

SiO2 Al2O3 TiO2 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO4 LOIa Total Si/Al

67.53 20.54 0.76 4.88 0.02 0.87 1.53
67.89 21.94 0.78 4.49 0.02 0.86 1.38
66.63 22.18 0.76 5.05 0.03 0.85 1.65
64.51 22.47 0.75 5.72 0.03 0.80 1.38
Humidity (%) CEC (meq g1) D50 (lm)

0.14 0.04 32.7

nd 0.72 nd

0.14 0.03 18.4

nd 0.23 12.1

LOI: loss on ignition; nd = not determined, LD: detection limit.

average sizes of 32 and 18 lm, respectively, whereas the UB fine fraction showed a mean diameter of 12 lm. These values corroborate those reported in other studies [9,33]. The fine particle size observed for all the samples positively influences the synthesis process. Semi-quantitative analyses of the crystalline (XRD) and amorphous (XRF) phases, raw ash and the UB fine fraction are shown in Table 2. The most important crystalline phases detected in these samples were quartz SiO2 and mullite (3Al2O32SiO2), as well as small amounts of hematite (Fe2O3). UA raw ash displayed a larger amount of crystalline phases (27.4%) compared to UB raw ash (18.2%), likely due to the different combustion conditions at the two plants [4]. UB ash also showed a slight increase in crystalline phases (21.3%) of the fine fraction when compared with the whole sample. The amorphous content of Si and Al, expressed as oxides, was estimated based on the quartz and mullite contents in Table 2 and chemical analysis of the ash by XRF described in Table 1. Whereas 81.8% SiO2 and Al2O3 are present in the amorphous phases of UB raw ash, these levels fall to 71% and 67%, respectively, for UA raw ash. A slight decrease in amorphous phase content was also recorded in the UB fine fraction (73–78%) when compared to raw ash. High amorphous Si and Al levels indicate easier dissolution of these elements and therefore favor the production of zeolites. The pre-treatment processes aimed to obtain raw material with smaller particle sizes and low contaminant levels. Table 3 shows the complete particle size distribution of raw ash and the percentage of unburned material in each of the seven grain size fractions obtained. The magnetic particle content (denominated the magnetic fraction – MF) was also determined for raw ash and its finer fractions (<38 lm). Although the unburned material content in raw ash is low (0.27% and 0.41%), it shows significant segregation. This behavior is more significant for UA ash (0.41%), where the unburned fraction reaches 6.5% of coarse particles (>250 lm). This behavior was also observed for other fly ashes [34]. It is reported that unburned material interferes in zeolite synthesis because it is deposited on the surface of ashes preventing their conversion and reducing CEC of produced zeolite. For this reason, the unburned carbon must occasionally be removed [35]. This is not the case for raw ash and its finer fractions (<38 lm), whose unburned material content is very low. Magnetic separation resulted in different behavior between the two types of ash. For UB raw ash (MF 3.3%) a significant increase was observed

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Table 2 Quantitative analysis of crystalline phases present in the coal fly ash UA and UB global and fine fraction (UB < 38 lm). Ashes

Phases (wt%, bs)

Si and Al

Crystalline

UA UB UB

Amorphous

Amorphous (%)

Quartz

Mullite

Hematite

Total

Total

SiO2

Al2O3

16.7 10.7 11.6

9.3 6.4 8.4

1.4 1.1 1.3

27.4 18.2 21.3

72.6 81.8 78.7

71 81 78

67 79 73

fines bs: Dry basis.

Table 3 Distribution unburned material and magnetic in the fly ash with particle size. Diameter (lm)

Fly ash UA Mass

Unburned carbon (%)

MF

Mass

Unburned carbon (%)

MF

>250 200 128 91 64 46 <38 Raw

2.8 4.0 6.7 21.1 16.9 25.4 23.0 100.0

6.52 2.07 0.82 0.59 0.42 0.35 0.22 0.41

nd – – – – – 1.9 5.1

1.4 5.3 9.7 37.4 25.3 11.5 9.4 100.0

0.85 0.33 0.22 0.14 0.19 0.14 0.13 0.27

nd – – – – – 9.8 3.3

Fly ash UB

MF: magnetic fraction; nd = not determined.

in the magnetic fraction of the finer particles (9.8%), whereas UA raw ash showed a decline of 5.1% for 1.9% of the magnetic fraction. The literature reports an increase in magnetic particle concentration in the finer fractions of coal fly ash [32,36], as observed for UB ash. The unexpected behavior of UA ash may be related to a number of factors, including the different burn conditions and emission control used. These results indicate that the use of UB raw ash is more favorable for the production of zeolite Na-P1.

3.2. Zeolite synthesis Table 4 shows the results of zeolite synthesis under the different conditions studied. This table displays the characterization of the products (XRD analysis, SEM, CEC) and an estimate of zeolite content in the synthesized material through Eq. (1). In the first syntheses (Tests 1–13) both ashes in reactor A were evaluated at 150 °C. For all the tests the use of 1.0 mol L1 NaOH resulted in greater yields, reaching 50% and 60% for UA and UB ash, respectively. These tests also indicated that alkali concentration exerts a more significant influence on zeolite formation when synthesis is performed with raw ash. This behavior was more evident for UB ash, with a 16% increase (Tests 6 and 7) in raw ash and only 4% (Tests 9 and 10) in the fine fraction. A further increase in alkali concentration (3.5 mol L1) did not lead to Na-P1 formation, but rather to undesired zeolitic phases (cancrinite, sodalite and X). These zeolites exhibit lower CEC values compared to Na-P1 and are not suitable for adsorption processes. The chemical composition of raw ash and its respective fine fractions, displayed in Table 1, show no significant differences in term of total Al and Si content, whereas the molar Si/Al ratio varies from 2.9 to 2.5, with higher values for UA ash. However, different values are observed for amorphous Si and Al content (Table 2), which are effectively dissolved during the first phase of zeolitization. Raw and fine UB ash fractions exhibit 82% and 79% of amorphous Si and Al oxides, respectively. These values are higher than those recorded for UA ash (73%). These results may justify the lower yields observed for the UB fine fraction and UA raw fraction when compared to UB raw ash. Table 3 shows a significant reduction in unburned material in fine particles, although absolute values are too low (0.27–0.41%) to influence the reaction. Magnetic material content in UA (5.1%) and UB (3.3%) raw ash is consistent with the larger yields observed in zeolites synthesized from the latter. Lower reactivity of the UB fine fraction, significantly enriched in magnetic material (9.8%), reinforces the possible influence of this parameter in zeolitization. However, testing conducted on ash (Tests 5 and 11), from which the magnetic material is previously removed, resulted in negligible yield increases (62%). The addition of crystallization seeds (pure Na-P1) into the reaction mixture was tested.

Table 4 Conditions of synthesis and basic characterization of zeolite species (ratio = 18 mL g1). Synthesis conditions

Characterization of the products

ID

Ash

Size

Reactor

T (°C)

[NaOH] (mol L1)

t (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

UA UA UAa UA UAa UB UB UB UB UB UBa UB UB UB UB UB UB UB UB

Whole sample Whole sample 638 lm 638 lm 638 lm Whole sample Whole sample Whole sample 638 lm 638 lm 638 lm Whole sample Whole sample Whole sample Whole sample Whole sample Whole sample Whole sample Whole sample

Steel Steel Steel Steel Steel Steel Steel steel Steel Steel Steel Steel Steel Polymeric Polymeric Glass Glass Glass Glass

150 150 150 150 150 150 150 150 150 150 150 150 150 150 150 100 100 100 100

0.5 1.0 0.5 1.0 1.0 0.5 1.0 3.5 0.5 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 3.0 3.5

24 24 24 24 24 24 24 24 24 24 24 24 24 24 16 24 24 24 24

Seedb (%)

4 8

SEM

XRD

CEC (meq NH+4 g1)

Contentc (%)

Na-P1, An Na-P1, An Na-P1, An Na-P1, An Na-P1, An Na-P1, An Na-P1, An Can, Sod, X Na-P1 Na-P1, An Na-P1, An Na-P1, An Na-P1, An Na-P1, An Na-P1, An nd nd Na-P1, Chab Na-P1, Sod

Mu, Qtz, Na-P1 Mu, Na-P1 Mu, Na-P1 Mu, Na-P1 Mu, An, NaP1 Mu, Na-P1 Can, Sod, X – Mu, Na-P1 Mu, An, Na-P1 Mu, Na-P1 – Mu, Na-P1 Mu, Na-P1 Q, Mu, Hem Q, Mu, Hem, Na-P1 Q, Mu, Na-P1, X, Sod –

2.1 2.4 1.9 2.1 2.1 2.1 2.9 1.0 2.0 2.2 2.3 3.3 3.2 3.7 3.0 0.9 0.8 2.2 1.7

44 50 40 43 44 44 60 21 42 46 48 69 67 77 63 20 17 48 35

Qtz = quartz; Mu = mullite; HEM = hematite; An = analcime; Can = cancrinite; Chab = chabazite; Sod = sodalite; X = zeolite type X; CEC = cation exchange capacity; nd = not determinate. a Removed magnetic phase. b Na-P1 pure. c Conversion ash product.

A.M. Cardoso et al. / Fuel 139 (2015) 59–67 Table 5 Residual concentrations of aluminum and silicon in the reaction solutions after synthesis. Test

6 7 18

Al

Si

Si/Al

mg L1

%

mg L1

%

1.50 1.10 7.60

0.3 0.2 1.5

238.4 199.8 466

15.2 12.8 29.7

59.9 68.5 61.3

Higher yield was observed with 4% addition, corresponding to an increase of only 5% when compared to no seed addition (Test 7, 60%). An additional seed dose (8%) shows no further effect on zeolite formation (67%). In light of the results obtained, subsequent tests were conducted under better synthesis conditions, using UB

63

raw ash and 1.0 mol L1 of alkali solution without seed addition. During the investigation with encased PTFE reactors (reactor A), the autogenous pressure of the system reached a maximum of 3.24  104 Pa. At this pressure polymeric PFA reactors (reactor B) can be used without metal shielding, but equipped with safety valves. In addition to being low cost, B reactors display heat exchange differences and are transparent, enabling the reaction to be visually monitored. Under optimized synthesis conditions, the yield of reactor B (77%, Test 14) was 17% higher than that of reactor A (Test 7). In light of the good results, the effect of a shorter reaction time (from 24 to 16 h) on the formation of zeolite Na-P1 was assessed. A significant decline was observed in zeolite production (63%, Test 15), albeit higher than that obtained with the steel reactor over longer reaction times, these syntheses were done in replicates and the variation of conversion of coal fly ash

Fig. 1. SEM image of zeolite structures obtained from fly ash: (a) Na-P1; (b) analcime; (c) cancrinite; (d) chabazite and (e–g) morphological structures not found in the literature.

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Fig. 2. SEM imagem of typical structures of zeolites with their respective EDS spectra (a) Na-P1 and (b) analcime.

Fig. 3. SEM image of structures obtained in the process reactor zeolitization C (a and b) ash maintained its initial morphology without the formation of zeolites, (c) Na-P1 and (d) chabazite.

into zeolites was around 3.0%. This behavior may be related to faster heating and cooling rates, which seem to favor zeolite Na-P1 formation under the conditions studied. Given the easy handling of PFA reactors, they are most suited to zeolite production in laboratory. Milder synthesis temperatures (100 °C) were tested using glass reactors (reactor C) under previously optimized conditions (NaOH 1.0 mol L1; 18 mL g1). There was no formation of zeolitic phases (SEM and XRD analyses), likely due to lower Si and Al dissolution at the temperature applied. However, the reaction products showed low CEC values (0.9 meq g1), though higher than the preceding ash (0.03 meq g1). This behavior may be related to the formation of amorphous aluminosilicates, which exhibit

ammonium adsorption properties. Higher alkali concentrations (2.0–3.5 mol L1) were tested in order to increase Si and Al dissolution, maintaining mild temperature conditions. As observed in the previous test, traces of zeolite Na-P1, formed (XRD) at a concentration of 2.0 mol L1 NaOH, exhibited low CEC (0.8 meq g1). However, the increase in alkali concentration to 3.0 mol L1 (Test 18) resulted in the formation of zeolite Na-P1, with slight contamination of other zeolitic phases (Sodalite, type X). A higher CEC value of 2.2 meq g1 was obtained under these conditions, with zeolitic content of 48%, 12% lower than under optimal conditions for the steel reactor (60%). Despite the lower purity of synthesized zeolite, the reduced production costs in relation to processes using

A.M. Cardoso et al. / Fuel 139 (2015) 59–67

65

Fig. 4. Diffractogram of commercial zeolite NA-P1 (a), of a typical sample synthesized in a glass reactor obtained in this study (b) and a sample of zeolite synthesized in a glass reactor (c).

other reactors favor production on a larger scale. Although an undesirable alkali attack on the glass reactor can occur, significant contamination of the reagent solution was not observed after the completion of the reaction. 3.2.1. Analysis of the post-synthesis alkaline solution Over time (2–3 weeks), post-synthesis solutions stored at ambient temperature showed the formation of precipitates identified as Na-P1 (SEM), indicating the presence of significant amounts

of residual Si and Al in these solutions. Some of the post-synthesis extracts analyzed (Table 5) exhibited high Si content, accounting for up to 30% of the initial available mass of this element in the ash displayed in Table 1. The lower levels recorded for aluminum (0.2–1.5%) were expected given its lower solubilization and greater consumption in zeolitization. This result points to the possible reuse of these alkaline solutions in the zeolitization process, improving the conversion efficiency of ash and minimizing the volume of wastewater to be treated.

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3.3. Characterization of zeolites

3.4. Treatment of AMD with zeolite

3.3.1. Morphological analysis Fig. 1 shows scanning electron microscope (SEM) images of the zeolites produced using reactor A. Some zeolites, such as Na-P1 (Fig. 1a), analcime (Fig. 1b), cancrinite (Fig. 1c) and chabazite (Fig. 1d), displayed structures characterized by well-defined crystal planes, as previously reported in the literature [4,26,34,38]. The formation of other unidentifiable morphological structures was also noted. Rod-shaped crystals (Fig. 1e) may indicate the structure of sodalite [38]. The image shown in Fig. 1f is characterized by fine crystals with unique morphology not found in the literature, whereas Fig. 1g depicts crystals resembling cotton wool, characteristic of sodalite [35]. The images in Fig. 2 are structures typical of zeolite Na-P1 (Fig. 2a) and analcime (Fig. 2b) and their respective spectra. As expected, both structures contain silica and aluminum. Fig. 3 shows the zeolites produced in reactor C. For tests with NaOH at concentrations of 1.0 mol L1 (Fig. 3a) and 2.0 mol L1 (Fig. 3b), ash maintained its initial morphology without the formation of zeolites. On the other hand, testing conducted using a larger alkali concentration (3.0 mol L1 NaOH) showed zeolites Na-P1 (Fig. 3c), sodalite (Fig. 3d) and chabazite (Fig. 3e), structures previously reported in the literature [4,27,37,38].

Table 6 shows the concentrations of anions, cations and metals present in AMD-RS from Candiota, before and after treatment with different doses of commercial and synthetic zeolite Na-P1 (Test 15). AMD exhibits high electrolyte content, moderate acidity and a significant presence of calcium and sulfate ions, as well as significant levels of metals such as manganese and iron. Treatment of this wastewater with 10 g L1 of commercial zeolite for 30 min resulted in complete removal of arsenic and nickel, virtually all the calcium (>98%), copper (>96%), iron (>98%) and manganese ions (>98%) and the partial yet significant removal of ammonium (52%), magnesium (60%), potassium (82%) and zinc ions (79%). There was also significant removal of sulfate (85%), probably through precipitation as a function of alkalinization of the medium and the presence of calcium. On the other hand, an increase in sodium ion content was observed, which is expected given that these ions compensate the zeolite framework. Increased sulfate levels are related to leaching from zeolite. This behavior was confirmed through contact of the zeolite with deionized water, under the same testing conditions (Test 6). The larger 20 g L1 dose of commercial zeolite showed a significant increase in the removal of the analytes under study, as well as greater leaching of the undesirable compounds present in zeolite. In light of the better results achieved with the 10 g L1 dose of commercial zeolite, synthetic zeolite was tested using an intermediate CEC value (2.9 meq NH+4 g1), which is more representative of the syntheses obtained. More efficient removal was recorded for fluoride, chloride, nitrate and magnesium than that observed with commercial zeolite. Synthetic zeolite was as efficient as commercial zeolite in removing sulfate; however, sodium leaching levels were higher than those recorded for commercial zeolite. With regard to metals, synthetic zeolite displayed highly significant results rivaling those of commercial zeolite, maintaining high and in some cases superior removal percentages, such as those observed for manganese (99.8%) and zinc (81%). It is important to note that the CEC value of synthetic zeolite is approximately half that of commercial zeolite. These results indicate the potential use of low-cost synthesized zeolites in the treatment of AMD from coal mining.

3.3.2. XRD analysis Fig. 4 depicts three diffraction spectra for the following samples: commercial grade Na-P1 (Fig. 4a), zeolite synthesized (Test 11) in reactor A (Fig. 4b) and zeolite synthesized (Test 18) in reactor C (Fig. 4c). In addition to the presence of peaks characteristic of zeolite Na-P1, smaller quantities of analcime are also observed as well as residual mullite for the product obtained in reactor A. The product in reactor C, however, shows peaks characteristic of Na-P1, mullite and residual quartz, as well as traces of zeolite X and sodalite. All the remaining samples analyzed exhibited similar characteristics and the diffraction patterns obtained were used to identify the presence of crystalline phases (see Table 4).

Table 6 Concentrations of anions, cations and metals present in AMD-RS from Candiota, before and after treatment with different doses of commercial (IQE) and synthetic zeolite Na-P1. ADM

Na-P1(IQE)

Na-P1(synt.)

H2O

ADM

10 g L1

10 g L1

20 g L1

ADM 10 g L1

pH Conductivity (MS cm1)

3.3 1,429

10.5 267

10.2 351

10.5 490

10.0 598

Anions (mg L1) Fluoride Chloride Nitrate Sulfate

1.2 12.4 3.8 406


1.2 12.4 3.9 60

0.3 4.2 1.5 61

0.3 6.4 1.8 61

Cations (mg L1) Sodium Ammonium Potassium Magnesium Calcium

21.6 9.6 7.4 26.8 97

908
93 2.3 1.3 10.9 1.6

193 2 0.6 0.2 0.9

132 2.7 3.1 0.3 2.2

Metals (lg L1) Arsenic Copper Iron Manganese Nickel Zinc

40 220 5,620 4,640 60 130





LD = detection limit.

4. Conclusion The results demonstrated that fly ash from coal combustion in Candiota (Brazil) can be used as raw material in the synthesis of zeolite Na-P1, employing hydrothermal conversion under mild conditions. Despite the similar chemical composition of the ashes studied (UA and UB), significant differences were observed in zeolite production. This behavior may be related to the higher amorphous Si and Al content in UB ash. Pre-treatment of ash by decreasing particle size and undesirable material (unburned material and iron oxide – magnetic fraction) showed no significant improvement in the zeolitization process. A comparison between the standard system for hydrothermal synthesis (reactor A) with alternative PFA (reactor B) and glass reactors (reactor C), showed advantages in terms of reaction yield (+17%) for reactor B, as well as milder conditions and lower cost for reactor C. Use of reactor B is preferable when the aim is to obtain zeolites with high purity levels and synthesis yield (>75%), using UB ash without pretreatment, such as 1.0 mol L1 NaOH at 150 °C for 24 h. A product with lower specifications (50% purity) can be synthesized in a glass reactor under milder temperature conditions (100 °C), but with higher alkali content (3.0 mol L1). These processes can also benefit from the reuse of residual Si and Al present in the postsynthesis solutions and the resulting decrease in the volume of the wastewater to be treated. The zeolite synthesized in the

A.M. Cardoso et al. / Fuel 139 (2015) 59–67

optimized process (77% Na-P1) was used to remove contaminants from acid mine drainage (AMD) generated in the mining of coal from Candiota. In addition to high removal of cations, manganese, zinc and arsenic, significantly lower sulfate and neutralization of AMD acidity were recorded. These results are comparable and competitive in relation to removal rates obtained with high purity commercial zeolite Na-P1, under the same conditions (10 g L1, ambient temperature, 30 min of contact). Acknowledgements The authors are grateful to the CNPq and FAPERGS for their financial support and Master’s scholarships awarded. Special thanks to Mr. Franisco Cacho (Ind. Quimica del Elbro, Spain) for donating zeolite Na-P1 and to the CGTEE for the ash samples provided. References [1] Kalhreuth W, Levandowski J. Chemical and petrographical characterization of feed coal, fly ash and bottom ash from the Figueira power plant, Paraná Brazil. Int J Coal Geol 2009;77:269–81. [2] Blissett RS, Rowson NA. A review of the multi-component utilisation of coal fly ash. Fuel 2012;97:1–23. [3] González A, Navia R, Moreno N. Fly ashes from coal and petroleum coke combustion current and innovative potential applications. Waste Manage Res 2009;27:976–87. [4] Ferret LS. Zeolites from coal ash: synthesis and use. Thesis, Doctorate in Mining Engineering, Metallurgical and Materials-PPGEM, UFRGS; 2004. p. 139 [in Portuguese]. [5] Varga IDL, Castro J, Bentz D, Weiss J. Application of internal curing for mixtures containing high volumes of fly ash. Cem Concr Compos 2012;34:1001–8. [6] Sumer M. Compressive strength and sulfate resistance properties of concretes containing class F and class C fly ashes. Constr Build Mater 2012;34:531–6. [7] Majchrzak-Kuce˛ba I, Nowak W. Characterization of MCM-41 mesoporous materials derived from polish fly ashes. Int J Miner Process 2011;101(1– 4):100–11. [8] Querol X, Moreno N, Umaña JC, Alastuey A, Hernández E, López-Soler A, et al. Synthesis of zeolites from coal fly ash: an overview. Int J Coal Geol 2002;50:413–23. [9] Moreno N, Querol X, Andrés JM, Stanton K, Towler M, Nugteren H, et al. Physico-chemical characteristics of European pulverized coal combustion fly ashes. Fuel 2005;84:1351–63. [10] Wu D, Sui Y, Chen X, He S, Wang X, Kong H. Changes of mineralogical-chemical composition, cation exchange, capacity, and phosphate immobilization capacity during the hydrothermal conversion process of coal fly ash into zeolite. Fuel 2008;87:2194–200. [11] Juan R, Hernández S, Andrés JS, Ruiz C. Synthesis of granular zeolitic materials with high cation exchange capacity from agglomerated coal fly ash. Fuel 2007;86:1811–21. [12] Querol X, Umaña JC, Plana F, Alastuey A, López-Soler A, Medinaceli A, et al. Synthesis of Na zeolites from fly ash in a pilot plant scale. Example of potential environmental applications. Fuel 2001;80:857–65. [13] Nascimento M, Soares PSM, Souza VP. Adsorption of heavy metal cations using coal fly ash modified by hydrothermal method. Fuel 2009;88:1714–9. [14] Ryu TG, Ryu JC, Choi CH, Kim CG, Yoo SJ, Yang HS, et al. Preparation of Na-P1 zeolite with high cation exchange capacity from coal fly ash. J Ind Eng Chem 2006;12:401–7. [15] Auerbach SM, Carrado KA, Dutta PK. Handbook of zeolite science and technology. New York: Marcel Dekker Inc.; 2003. p. 585.

67

[16] Endres JCT, Ferret LS, Fernandes ID, Hofmeister LC. The removal of Fe, Zn, Cu, and Pb from wastewater using chabazite zeolites produced from southern Brazilian coal ashes. In: International ash utilization symposium, 2001. Proceedings of the international ash utilization symposium; 2001. [17] Moreno N, Querol X, Ayora C, Pereira CF, Jassen-Jurkovicová M. Utilization of zeolites synthesized from coal fly ash for the purification of acid mine waters. Environ Sci Technol 2001;35:3526–34. [18] Otal E, Vilches LF, Moreno N, Querol X, Vale J, Fernández-Pereira C. Application of zeolitised coal fly ashes to the depuration of liquid wastes. Fuel 2005;84:1440–6. [19] Penilla RP, Bustos AG, Elizalde SG. Immobilization of Cs, Cd, Pb and Cr by synthetic zeolites from Spanish low-calcium coal fly ash. Fuel 2006;85:823–32. [20] Oliveira CR, Rubio J. New basis for adsorption of ionic pollutants onto modified zeolites. Miner Eng 2007;20:552–8. [21] Wdowin M, Wiatros-Motyka MM, Panek R, Stevens LA, Franus W, Snape CE. Experimental study of mercury removal from exhaust gases. Fuel 2014;128:451–7. [22] Wdowin M, Franus W, Panek R. Preliminary results of usage possibilities of carbonate and zeolitic sorbents in CO2 capture. Fresenius Environ Bull 2010;21(12). [23] Querol X, Moreno N, Juan R, Andrés JM, López-Soler A, Ayora C, et al. Synthesis of high íon exchange zeolites from coal fly ash. Geol Acta 2007;5:49–57. [24] Peñapenilla R, Bustos AG, Elizalde SG. Immobilization of Cs, Cd, Pb and Cr by synthetic zeolites from Spanish low-calcium coal fly ash. Fuel 2006;85:823–32. [25] Yanxin W, Yonglong G, Zhihua Y, Hesheng C, Querol X. Synthesis of zeolites using fly ash and their application in removing heavy metals from waters. Sci China Ser D 2003;46(9):967–76. [26] Querol X, Plana F, Alastuey A, López-Soler A. Synthesis of Na-zeolite from fly ash. Fuel 1997;76:793–9. [27] Paprocki A. Synthesis of zeolites from coal ash aiming its utilization in the decontamination of acid mine drainage. Thesis, Master of Engineering Materials and Technology. Faculty of Engineering, Physics and Chemistry, PUCRS. Brazil (Porto Alegre); 2009. p. 156 [in Portuguese]. [28] Musyoka NM, Petrik LF, Fatoba OO, Hums E. Synthesis of zeolites from coal fly ash using mine waters. Miner Eng 2013;53:9–15. [29] Sommerville R, Blissett R, Rowson N, Blackburn S. Producing a synthetic zeolite from improved fly ash residue. Int J Miner Process 2013;124:20–5. [30] ISRIC. Procedures for soil analysis, technical paper 9. International soil reference and information centre. FAO-UN; 1995. p. 9.1–.13. [31] Company Thermal Electricity Generation (CGTEE). [accessed May 2008, in Portuguese]. [32] Sakar A, Rano R, Udaybhanu G, Basu AK. A comprehensive characterisation of fly ash from a thermal power plant in Eastern India. Fuel Process Technol 2006;87:259–77. [33] Pires M, Querol X. Characterization of Candiota (South Brazil) coal and combustion by-product. Int J Coal Geol 2004;60:57–72. [34] Gross M, Soulard M, Caullet P, Patarin J, Saude I. Synthesis of faujasite from coal fly ashes under smooth temperature and pressure conditions: a cost saving process. Microporous Mesoporous Mater 2007;104:67–76. [35] Wang C-F, Li J-S, Wang L-J, Sun X-Y. Influence of NaOH concentrations on synthesis of pure-form zeolite a from fly ash using two-stage method. J Hazard Mater 2008;155:58–64. [36] Hollman GG, Steenbruggen G, Janssen-Jurkovicova MA. Two-step process for the synthesis of zeolite from coal fly ash. Fuel 1999;78:1225–30. [37] Palmerola NM. Valorisation of fly ash for zeolite synthesis by extraction chromatography and direct conversion, environmental applications. Thesis, Doctorate in Mineral and Natural Resources Engineering-UPC. Barcelona; 2002. p. 251. [38] Umanã Peña JC. Synthesis of zeolites from fly ash from coal fired power plants. Thesis, Doctorate in Mineral and Natural Resources Engineering-UPC. Barcelona; 2002. p. 254.