Characterization of Candiota (South Brazil) coal and combustion by-product

International Journal of Coal Geology 60 (2004) 57 – 72 www.elsevier.com/locate/ijcoalgeo

Characterization of Candiota (South Brazil) coal and combustion by-product Marcßal Pires a,*, Xavier Querol b a

Faculty of Chemistry, Pontifical Catholic University of Rio Grande do Sul, Av. Ipiranga 6681 Predio 12B, 90619-900 Porto Alegre-RS, Brazil b Institute of Earth Sciences ‘Jaume Almera’, CSIC, c/Martı´ i Franque`s s/n, E-08028 Barcelona, Spain Received 8 October 2003; accepted 30 April 2004 Available online 28 July 2004

Abstract Elemental composition and mineralogy of a high ash feed coal (ash: 49.7 wt.%), and its bottom and fly ash from a Brazilian power plant (Presidente Me´dici Power Plant or UTPM-446 MW) was determined using ICP-MS, ICP-AES, X-ray diffraction (XRD) and scanning electron micrography (SEM). Most trace elements in coal fall in the usual range determined for world coals. However, concentrations of some elements were higher than the expected for coals, including Cs Rb and heavy rare earth elements (REEs). This might be due to the high content of detrital minerals of the studied coal, given that these elements are usually associated with clay minerals. Elements were classified into three groups based on the analysis of trace element concentrations in fly and bottom ashes, and enrichments or depletions of these concentrations in relation to the coal: Group I (volatile elements with subsequent condensation): As, B, Bi, Cd, Ga, Ge, Mo, Pb, S, Sb, Sn, Tl and Zn; Group II (no volatile elements enriched in bottom ash vs. fly ash): Ca, Fe, Mn, P, Ti and Zr; Group III (low volatile elements with no partitioning between fly and bottom ashes): Al, Ba, Be, Co, Cr, Cs, Hf, K, Li, Mg, Na, Ni, Rb, Sr, Th, U, W, Y and most of REE. The mass balance for trace elements obtained demonstrated that the volatile emission of the trace elements studied is very low. According to the leachable proportion obtained, the elements may be classified as follows: B (40 – 50%)>Mo>Cu>Ge = Li = Zn = As>, Ni, Sb, Tl, U>Ba, Cd, Sr, V (0.3 – 2%). For the other elements studied, the leachable fraction is in most cases < 1% of the bulk content. D 2004 Elsevier B.V. All rights reserved. Keywords: Candiota coal; Fly ash; Trace elements; Leaching; Brazil

1. Introduction Coal was first mined in Brazil 140 years ago (BRAZIL, 1987), but the sector has not developed as fast as other segments of Brazil’s economy because Brazilian coal has low caloric value and high ash

* Corresponding author. Fax: +55-51-3320-3612. E-mail address: [email protected] (M. Pires). 0166-5162/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2004.04.003

contents, thus requiring expensive processing treatment hampering its competitiveness. These limitations may become less significant with the development and introduction of new technologies which favor direct burn, thus dispensing with phases of processing which had to be used in the past when the coal used for thermoelectric power generation was a by-product of the coal for steel production. Until 1975, coal contributed no more than 3.2% of Brazil’s energy requirements, and its main use (80%

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of the total) was in iron and steel production (Carrisso and Possa, 1995). Since 1975, industrial use of coal increased as a consequence of the price advantage it offered in comparison to fuel oil and also as a result of transport subsidies, which were reduced after 1986 with the reduction in cost of fuel. Currently (2000), coal provides 5.3% of Brazil’s energy requirements, of which 1% is Brazilian coal and 4.3% imported coal and coke (Gomes et al., 1998). In 1997, 78% of the over 6 Mt of nationally produced coal was consumed by thermoelectric plants and 22% by industry (BRAZIL, 2001a). In the year 2000, coal-consuming power plants with capacity of 1.4 GW, produced 7.4 TWh of electricity (85% increase in relation to 1996), using 6.1 Mt of coal. Current plans for the electrical sector foresee an increase in capacity of 1.1 GW (four plants, two with 350 MW and two with 200 MW capacity) by the year 2005. The prospects for thermoelectric generating plants in Brazil will most certainly receive a boost from privatization of the Electric Power Sector; however, competitiveness dictates that in order to be successful they must use clean-coal technologies with locations close to the mines. Although such technologies have proven to be effective, even for the combustion of low-rank coal, none of them has been applied in Brazil. Under the Expansion Plan 1998/2007 of the Brazilian Government (BRAZIL, 2001b), the country’s electricity production will grow from 59.3 to 95.7 GW. The contribution of thermoelectric power generation will grow from 8% to 17% during this period, with a corresponding decline of hydroelectric power (BRAZIL, 2001a). The projected power plants will consume mainly natural gas, but coal will also be used at a large scale. 1.1. Candiota area The region of Candiota, located 380 km from Porto Alegre in the southwest of Rio Grande do Sul, has an area of 430 km2. The largest coal reserves in Brazil, as well as a coal-fired power station (Presidente Me´dici Power Plant or UTPM-446 MW) are located in this area approximately 50 km from the border with Uruguay. The Candiota coal is Permian –Gondwanan, belonging to the Tubara˜o subgroup of the Parana´ Basin (Alves and Ade, 1996). This coal, as most of the Brazilian coals, is a high bituminous class C

volatile coal with high ash content, as classified according to ASTM (1996a). Environmental aspects from the combustion of fossil fuels, mainly coal, have been reported in certain areas of Rio Grande do Sul, the southernmost state of Brazil (JICA, 1996). Thus, Teixeira (1997) detected contamination problems due to the disposal of coal wastes and combustion by-products in area of Baixo Jacuı´, affecting the quality of surface and underground water, as well as of air quality, mainly from suspended particulate matter (Sanchez et al., 1995). The purpose of this work is the physico-chemical, mineralogical, and morphological characterization of Candiota coal and combustion by-products produced at UTPM. The aim of this characterization is to assess further studies on the environmental impact due to coal combustion (atmospheric emissions) and the consequent waste disposal (fly ash and bottom ash leaching).

2. Methods 2.1. Sampling and sample preparation The present work was performed with a sample of feed coal ( f 50 kg) used at UTPM, located in Candiota, state of Rio Grande do Sul in the south of Brazil (446 MW, Fig. 1). The combustion by-products from this power station were sampled simultaneously with feed coal. To this end, samples of 30 kg of bottom ash and 50 kg of fly ash, from the hoppers of electrostatic precipitator (EP), were collected following ABNT (1983) and ASTM (1996b) procedures. The coal and bottom ash samples were ground to pass a < 63-Am sieve. The fly ash, due to its homogeneity and finer grain size, was used as received in most of the subsequent analysis. The only exception was the grain size separation with the cascade impactor, which needed the separation of the coarser fraction (>63 Am). Subsamples of coal and ashes were dried at 105 jC, and used in subsequent characterization analysis. 2.2. Immediate and elementary analysis of the coal The moisture, ash (HTA), volatile matter and fixed carbon contents of the coal were measured according

M. Pires, X. Querol / International Journal of Coal Geology 60 (2004) 57–72

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Fig. 1. Location of the Candiota area, in the southern state of Rio Grande do Sul, Brazil (adapted from Migliavacca, 2001).

to ASTM standards (ASTM, 1996c). Major and trace elements content in the coal, the fly and bottom ashes, and the different grain size fractions of the fly ashes

obtained in the cascade impactor were determined using ICP-MS and ICP-ES according to methodology of Querol et al. (1995). After acid digestion (HF/

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Table 1 Characterization of Candiota coal Proximate analysis Moisture (%) Ash (%) Volatile matter (%) Fixed carbon (%) Gross calorific value (MJ kg
1) Sulphur (%)

Mineral content (wt.%) 16.4 49.7 23.1 27.2 14.3 0.96

Quartz Kaolinite Illite K-feldspar Calcite Pyrite/marcasite

30.9 16.0 2.0 0.5 < 0.5 0.3

HNO3/HClO4) of bulk samples, the contents of major and trace elements were determined by ICP-MS and ICP-AES. Reference materials of coal (SARM 19) and ashes (NIST 1633b) were analyzed with the same procedure to check the analysis quality. The mineralogical composition of the bulk coal and ashes, as well as of ashing products, was characterized by X-ray diffraction (XRD) using similar methods to those described by Ward (2002) and Vassilev and Tascon (2003). The grain size distribution of fly ash sample was characterized by a laser analyzer using ethanol as dispersing agent. A cascade impactor was used to obtain seven grain size fractions (0.4 – 0.8; 0.8 – 1.7; 1.7– 3.3; 3.3 –6.6; 6.6 –13; 13 – 26; and 26– 63 Am) from the < 63-Am fractions of the bulk fly ash. The subsamples were recovered from the impactor units in an ultrasound bath. The insoluble fraction was recovered by filtration (membrane filter 0.45 Am) and analyzed by ICP-MS and ICP-AES. The efficiency of the grain size cut off was checked by SEM. 2.3. Leaching tests Room temperature leaching tests using open and closed systems were carried out for the fly ash. For bottom ashes, only closed leaching was performed. Three different procedures were used in the closed system: (a) DIN-38414-S14 procedure with a fly ash/ water ratio of 100 g/l with mechanical agitation for 24 h, followed by centrifugation; (b) a 20 g/l ratio with mechanical agitation for 24 h; (c) a 20 g/l ratio with mechanical agitation for 2 h, followed by centrifugation. Conductivity and pH were measured immediately, and the major and trace elements contents were determined in the leachates using ICP-AES and ICP-MS. Previously cleaned polyethylene flasks were used in all tests.

The open system leaching test is based on the passage of a continuous flow of water through a column containing the ashes. In the first test, a water flow of 50 ml h
1 was put through the column containing 2 g of ash. Samples of leachates were collected at intervals of 10 ml ( f 8 min) and the pH and conductivity were determined. The test was performed for 20 h, with total water volume of 1000 ml, when low (10 AS cm
1) and constant conductivity values were observed. In a second test, performed in the same conditions, continuous 10 ml leachate samples were periodically collected and analyzed by ICPMS and ICP-AES.

3. Results and discussion 3.1. Chemical and mineralogical characterization of coal Tables 1 and 2 show the results of ultimate, proximate, chemical and mineralogical analyses of the studied coal and combustion by-products. The high ash yield and the relative low sulfur content obtained for this coal confirm literature data (BRAZIL, 1987; Fiedler, 1987). The main mineral phases present in the coal are quartz, kaolinite, illite, Kfeldspar and pyrite. The mineralogy of HTA (750 jC) ash consists of meta-kaolinite, hematite and microcline. The fly and bottom ashes were characterized by relatively high contents of SiO2 (56 – 59%) and Al2O3 Table 2 Major oxide contents in Candiota coal HTA, fly ash and bottom ash, on dry basis (wt.%)

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2 O P2O5 TiO2 MnO SO3 SiO2/Al2O3

HTA

Fly ash

Bottom ash

70.6 24.3 2.9 0.7 0.2 0.03 0.50 0.01 0.8 0.01 4.7 2.9

56.7 38.4 2.5 1.1 0.2 0.04 0.6 0.02 0.5 0.02 0.2 1.5

58.9 36.0 2.4 1.3 0.2 0.04 0.6 0.02 0.6 0.02 < 0.1 1.6

M. Pires, X. Querol / International Journal of Coal Geology 60 (2004) 57–72 Table 3 Trace and major elements contents in coal and in fly and bottom ashes (% from Al to Ti, other elements in mg kg
1 on a dry basis) Coal % Al Ca Fe K Mg Na P S Ti mg kg
1 Li Be B V Cr Mn Co Ni Cu Zn Ga Ge As Rb Sr Y Zr Nb Mo Cd Sn Sb Cs Ba La Ce Nd Sm Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl

Fly ash

Bottom ash

3.24 0.42 2.01 0.92 0.21 0.06 0.01 0.96 0.23

10.16 0.77 4.63 1.74 0.46 0.12 0.02 0.08 0.41

9.54 0.90 7.68 1.67 0.47 0.12 0.025 0.01 0.49

14 2.5 9.2 55 26 410 10.1 17 20 45 9.1 2.1 4.4 50 42 24 128 16.1 1.4 0.5 3.2 1.4 9.1 152.3 24.9 53.7 25.4 4.2 6.8 0.8 5.3 0.7 3 0.31 3.2 0.9 4.4 7.4 3.2 0.8

29 4.9 23.9 78 51 614 16.2 29 33 77 19.3 3.8 11.5 119 103 49 215 22 4.8 1.2 5.4 3.2 16.8 283.9 52.7 113.3 55.2 9 12.1 1.8 10.5 1.7 5.7 0.74 6.3 1.3 10.2 13.9 4 2.1

28 4.3 15.3 93 47 991 17.5 30 32 47 11.8 1.3 1.8 107 100 46 218 25.1 3.6 0.3 3.5 2.6 14.8 278.9 49.7 109.4 53.6 8.9 10.7 1.6 10.5 1.6 5.2 0.78 6.3 1.3 9.1 14.3 3.4 0.4

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Table 3 (continued ) Coal mg kg
1 Pb Bi Th U

20.5 0.8 9.5 3.3

Fly ash 41.1 1.6 21.0 6.0

Bottom ash 19.2 1.1 22.0 6.1

(36 – 38%) and low contents of alkaline oxides (Table 2). This fact has great influence on the leaching processes discussed later, and on the potential application of these materials. For comparison, most of the European fly ashes have < 55% SiO2 and < 30% Al2O3 (Moreno et al., 2002). The main components of the fly ash are the glassy aluminium –silicate matrix, mullite, quartz and magnetite. A similar composition was obtained for bottom ash, with a higher content of magnetite. Calcination (1000 jC) of fly ash resulted in the oxidation of the magnetite into hematite. 3.1.1. Concentration of trace elements in coal Table 3 shows the concentrations of major and trace elements in the feed coal and fly and bottom ashes from UTPM. Table 4 shows typical concentration values of several trace elements of subbituminous coals of different countries and the worldwide concentration range for coal. The results of 17 studies on Brazilian coals, among them the coal of Candiota, compiled by Pires et al. (2002) are also presented in this table, together with more recent data on the coals from Santa Catarina, feed coal of the Jorge Lacerda Power Plant (Pereira, 1996), the largest coal-burning utility in Brazil (860 MW). In general, the concentration of (a) Cu, Mo, Se and Zn is lower than other Brazilian coals; (b) Be, Bi and Ba are within the range, and similar to the coal from Santa Catarina; (c) concentration of Mn is greater than the other Brazilian coals. The concentrations of most of the trace elements in the Candiota sample are within the range for world coals (Swaine, 1990). 3.2. Concentration of the trace element in fly and bottom ashes As indicated previously, the measured concentrations of major and trace elements present in fly and

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Table 4 Typical trace element concentrations (mg kg
1) in the world coals (Swaine, 1990) and coals from selected countries’ Worlda

Specific coals USAb

Canadab

Australiab

Brazilc JL-SCd

As B Ba Be Bi Cd Ce Cl Co Cr Cs Cu Dy Er Eu F Ga Gd Ge Hf Hg Ho La Li Lu Mn Mo Nb Nd Ni P Pb Rb Sb Sc Se Sm Sn Sr Tb Th Tl Tm U V W Y Yb Zn Zr

0.5 – 80 5 – 400 20 – 1000 0.1 – 15 2 – 20 0.1 – 3 2 – 70 50 – 2000 0.5 – 30 0.5 – 60 0.3 – 5 0.5 – 50 0.5 – 4 0.5 – 3 0.1 – 2 20 – 500 1 – 20 0.4 – 4 0.5 – 50 0.4 – 5 0.02 – 1 0.1 – 2 5 – 300 1 – 80 0.03 – 1 5 – 300 0.1 – 10 1 – 20 3 – 30 0.5 – 50 10 – 3000 2 – 80 2 – 50 0.05 – 10 1 – 10 0.2 – 10 0.5 – 6 1 – 10 15 – 500 0.1 – 1 0.5 – 10 < 0.2 – 1 no data 0.5 – 10 2 – 100 0.5 – 5 2 – 50 0.3 – 3 5 – 300 5 – 200

0.1 – 420

4 – 53 5 – 32

0.05 – 32

< 0.1 – 36 1 – 14 < 0.5 – 12

0.03 – 3.7

1.3 – 12 36 – 230 30 5 – 73 9 0.024 – 2.2

– 0.06 – 70 0.54 – 70

1.1 – 31 5.7 – 64

< 5 – 321 < 5 – 117

100 – 8000 6 – 43 9 – 74

0.16 – 120

14 – 80

< 1 – 741

9 – 71

126 – 1287 23 – 28

0.01 – 8.0

0.02 – 0.44

35.14 49.48 98.13 3.49 1.41 2.6 110.73 10.29 73.2 10.59 33.39 6.13 3.53 1.64

Candiota-RS Rangec

This work

Ratioe

1.3 – 2.6 36 30 73 9 2

4.4 13.7 152 2.7 0.8 0.1 54 na 10.1 24 9.1 4.4 5.3 3.0 ni na 9.1 6.8 2.3 4.4 0.1 0.6 25 14.2 0.9 452 1.4 16.1 25 17.1 166 20.5 50 ni ni ni 4.2 3.4 42 0.8 8.5 0.7 0.3 3.3 50 3.2 23.7 3.2 11.4 128

0.05 0.03 0.15 0.18 0.04 0.04 0.77 0.00 0.34 0.40 1.81 0.09 1.33 1.01 0.00 0.00 0.46 1.70 0.05

169 – 1287 6 – 21 27 – 73 24 – 71

110 – 560 18.35 8.46 9.49 8.87

0.05 – 0.8

0.1 – 0.8 1.24 56.22 42.2 0.49

1.4 – 3500 0.13 – 41

1.6 – 1000 < 0.8 – 9

0.32 – 69

< 2 – 667 < 0.1 – 6

2 – 161

0.7 – 76

1.9 – 19

0.04 – 43

0.1 – 2.4

–

0.10 – 16

0.5 – 2.0

–

0.08 – 54

< 0.1 – 9

< 1 – 595

31 – 240 2.6 – 10

15 – 64 2 – 95

1.5

< 0.4 – 26

0.06 – 76 0.14 – 370

0.2 – 2.4 23 – 300

< 1 – 58 2 – 279

0.88 – 910

7.6 – 83

< 2 – 273

23 – 86

30 – 217 110 – 127

5.9 20.57 42.05 24.05 49.64 69.59 5.88 19.44 2.57 8.51 8.79 56.32 1.16 8.9 4.21 3.8 187.25 6.59 37.25 3.64 473.53 857.82

31 – 240 4

22 – 34 4 – 50

1.5

23 – 60

80 – 98 110

0.06 0.30 0.62 0.18 0.90 1.50 0.14 0.80 0.85 0.34 0.06 0.26 1.00 0.00 0.00 0.00 0.70 0.34 0.08 0.84 0.85 0.67 0.33 0.50 0.63 0.47 1.08 0.04

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Fig. 2. Ratio of major, minor and trace elements content in fly and bottom ashes. Reproduced with permission of the editor from: Pires, M. et al. Caracterizacßa˜o do carva˜o de Candiota e de suas cinzas. Geochimica Brasiliensis, 15 (1 – 2): 113 – 130.

bottom ashes are shown in Table 3. The contents of trace elements in fly and bottom ashes depend on several factors related to the concentration and the geochemical distribution of the elements in the coal and to the combustion and pollution control technologies used (Davidson and Clarke, 1996; Meij, 1994; Ratafia-Brown, 1994; Davidson, 2000; Huggins, 2002; Huggins et al., 2004). Fig. 2 shows element concentration ratios in fly ash (FA) and bottom ash (BA). Through the analysis of these results, it is possible to classify the elements to the following groups: (a) Elements enriched in fly ash (FA/BA 1.3 –9.7): As, B, Bi, Cd, Ga, Ge, Mo, Pb, S, Sb, Sn, Tl and Zn. (b) Elements enriched in bottom ash (FA/BA < 1): Ca, Fe, Mn, P, Ti and Zr.

(c) Elements present in similar concentrations (FA/ BA f 1,0): Al, Ba, Be, Co, Cr, Cs, Hf, K, Li, Mg, Na, Nb, Ni, Rb, Sr, Th, U, V, W, Y and most of rare earth elements (REEs). The first class of elements (a) partially volatilise (especially As, Cd, Tl, Pb and mainly S) during coal combustion, condensing as temperature decreases, mainly on the finest and coolest particles, or are emitted, in different proportions, as gaseous species. The relatively low proportion of volatile B is due mainly to the association with detrital minerals instead of the organic matter (Boyd, 2002). On the other hand, the low-volatile elements (c) present a weak segregation and the non-volatile elements (b) may be found in higher concentrations in bottom ashes. In general, it is observed that the segregation of the trace elements in fly ash/bottom

Notes to Table 4: a Swaine (1990). b Swaine and Goodarzi (1995). c Pires et al. (2002). d Pereira (1996). e Ratio between the element concentrations in Candiota coal and maximum observed level by Swaine (1990) for the World coals. ni—lower than detection limit. na—element not quantified.

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ash is not very strong at UTPM. This fact could be related to the low efficiency of the burning conditions, due to the high mineral matter contents of the Candiota coal (>50%). To elucidate the behavior of the trace elements during the coal combustion, their concentrations in fly and bottom ashes can be normalized using a nonvolatile element, whose concentrations both in the coal and in the ashes are accurately known. Such procedure allows calculating an enrichment factor EF (Gordon and Zoller, 1973) as follows: EF ¼ ð½XS =½YS Þ=ð½XC =½YC Þ where [X]S and [X]C are the concentrations of the X element in the fly ash or bottom ash, and in the coal, respectively. [Y]S and [Y]C are the contents of a nonvolatile element taken as reference. The non-volatile

elements most frequently selected for calculation of EF are Al, Ce, Fe, La, Si and Ti (Ratafia-Brown, 1994; Smith, 1987). Most REEs, such as Ce and La, present the additional advantage of being present in constant concentrations in all particle sizes of the fly ashes. Consequently, cerium was chosen as the reference element in the calculation of the enrichment indices of elements. Considering that around 80% of the produced waste is fly ash and the remaining 20% is bottom ash, the total enrichment factor based on mass can be calculated as: EFtotal = EFFA  0.8 + EFBA  0.2. It is assumed that this is an approximation and not an accurate calculation of the flow mass balance into the power plant. Fig. 3 shows the Ce-normalized EF for studied elements, calculated for the fly and bottom ashes, in relation to Ce. The partially volatile elements such as As, B, Cd, Ga, Ge, Mo, Pb and Tl (EFFA>1) are enriched

Fig. 3. Enrichment factors for major and trace elements in fly (EFFA), bottom ash (EFBA) and total EF (EFtotal = EFFA  0.8 + EFBA  0.2.) normalized to Ce and coal.

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in the fly ash samples. These elements are associated with organic and sulfide affinities, and are similar to the elements classified previously as group (a). Ba, Be, Co, Cr, Cs, Hf, K, Li, Mg, Na, Ni, Rb, Sr, Th, U, W, Y and most of REE presented similar EF f 1 in both fly and bottom ashes. Although a precise mass balance is difficult to obtain with the data obtained in this study, the calculation of EFtotal demonstrated that the volatile emission of the trace elements studied is very low since most EFtotal are close to 1 (Fig. 3). The data show that volatilization of many elements occur (EFfly ash>EFbottom ash) but most of them condense on fly ash particles (Fig. 3).

65

3.3. Grain size separation of fly ash Fig. 4 shows the results of the grain size classification of the fly ashes. Two different characterization methods are used: laser analyzer (LA) and cascade impactor (CI). The CI results were recalculated, for the sample of the analyzed fly ash was previously fractioned through sieving ( < 63 Am). In spite of the differences between these two methods, the results obtained are similar, as it can be observed by the accumulated distribution of particle sizes (Fig. 4B). The continuous interval of particle sizes estimated in the LA was between 0.5 and 600 Am, and an average diameter of 49.30 Am was obtained. These values are within the expected diameter for the Candiota fly ash

Fig. 4. (A) normal and (B) accumulated fly ash particle distribution measured by a laser analyser (bulk sample) and a cascade impactor ( < 63 Am fraction, recalculated). Reproduced with permission of the editor from: Pires, M. et al. Caracterizacßa˜o do carva˜o de Candiota e de suas cinzas. Geochimica Brasiliensis, 15 (1 – 2): 113 – 130.

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(Andrade, 1995). Ninety percent of the particles have diameters smaller than 232 Am, while the finest particles ( < 5 Am) correspond to f 1% of the total mass of the sample. Fig. 5 shows scanning electron micrographs of the grain size fractions separated from the fly ash with the

CI. It is observed that CI presents good separation efficiency, with most of the particles of each fraction being within the range of expected aerodynamic diameters. It also observed different morphologies of ash particles size, with the predominance of cenospheres in the intermediate fractions (1.7 – 13 Am).

Fig. 5. SEM photomicrographs of size fractions of Candiota fly ash. Reproduced with permission of the editor from: Pires, M. et al. Caracterizacß a˜o do carva˜o de Candiota e de suas cinzas. Geochimica Brasiliensis, 15 (1 – 2): 113 – 130.

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Fig. 6. Mass percent of major and trace elements during ultrasound water extraction, of grain size fractions of fly ash sample ( < 63 Am).

The percent solubility of the elements in the different grain size fractions is shown in Fig. 6. Some elements present remarkable increasing solubility (>40%) as grain size decreases (As, Co, Sr, Ba, Zn and Ni). The same behavior was observed to the elements Zr, Cr, V, Tl and Hf but with small solubility (5%). On the other hand, the elements Fe, Nb, REE, Cs, Li and W showed low solubility in all grain size fractions. Sulfur and Mo presented very different solubility profiles with higher values in coarse grain size fractions. 3.4. Leaching tests 3.4.1. Closed system Table 5 shows that the fly ash leachate is slightly acidic pH in all tests, while bottom ash yielded a slightly alkaline leachate. This behaviour was also pointed out for the Candiota coal ashes by other authors (Sanchez et al., 1994; Fernande´z-Turiel et al., 1994), and it is possibly due to the low

concentration of alkaline oxides in these ashes (CaO < 1.3%), especially in the light fraction, Low conductivity values ( < 100 AS/cm) are observed when compared to other leachates for fly ash (Querol et al., 2001), reflecting the low solubility observed for most major species. The highest conductivity value was measured in the test with fly ash with agitation of 2 h. This may be due to the re-precipitation of slightly soluble elements in the Table 5 pH and conductivity of the leachates measured for the closed system using fly and bottom ashes

Fly ash

Bottom ash

Test code

L/S ratio (ml g
1)

Agitation time (h)

pH

Conductivity (AS cm
1)

1 2 3 4 5 6

10 50 50 10 50 50

24 24 2 24 24 2

3.90 3.94 4.03 8.30 8.42 7.90

105 105 130 30 30 30

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24-h test. Any influence was detected on the percentage of element leached by changing the liquid/solid ratio from 10 to 50 ml g
1. The concentration of trace elements in the leachates from bottom ash in closed system showed very low concentrations for all elements. As shown in Table 6, B, Ni, Cu, Zn, Mo and Sb were leached from fly ash using ultra sound in appreciable proportions (55%, 8%, 9%, 13%, 18% and 8% of the bulk content, respectively), and only B and Ge were leached in appreciable levels using a shaker in a closed system (57% and 8%, respectively). The rest of extractable yields for the other elements in this table reached values < 5%. Although these second group reach extraction yields are close to the analytical error determined for these elements in the bulk sample (Finkelman et al., 1990; Gentzis and Goodarzi, 1999; Seames et al., 2002; Wang et al., 2004), the leachable proportion is clearly lower than the one obtained for the first group of elements. 3.4.2. Open system In the open system, it was possible to follow the time evolution of the pH and conductivity as well as the concentrations of the leachable elements. Fig. 7 shows the profiles of the pH and the conductivity obtained in the leaching tests of fly ash using a L/S ratio of 50 ml g
1. A rapid increase of the pH from 4.2 to 7.0 is observed in the first 100 ml water, with stabilization and slow decrease with the increase of the L/S ratio, reaching a pH of f 6.0 after a 1000 ml leaching. This behavior is associated to the fast initial extraction of acidic components, concentrated in the surface of the particles (Swaine, 1990), with subsequent slower extraction of alkaline elements (Querol et al., 1996, 2001), less soluble or linked to the matrix of the ash. The initial pH, measured in the first aliquot of 10 ml, is similar to that found in the closed system test (pH 4.0), performed in the same conditions. Recalculating pH values after the passage of 100 ml, using the identical volume in the closed system, a relatively higher value is obtained (pH 4.9), which can indicate that the processes/kinetics of solubilization of the two systems are different. The time evolution of the conductivity corroborates with a rapid extraction of the mobile elements, predominantly with acidic character in that ash,

Table 6 Trace element extraction yields (% of bulk concentration) obtained in open and closed leaching tests applied to fly ash Closed

Li Be B Ti V Cr Mn Co Ni Cu Zn Ga Ge As Rb Sr Y Zr Nb Mo Cd Sn Sb Cs Ba La Ce Nd Sm Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Tl Pb Th U

Open

Ultrasound, 30 ml

Shaker, 100 ml

100 ml

1000 ml

4.6 < 0.1 55.2 < 0.1 0.2 1.1 1.5 0.5 8.3 8.7 13.2 < 0.1 3.6 1.4 0.5 1.8 0.8 0.1 1.0 18.1 < 0.1 < 0.1 8.1 < 0.1 2.0 0.4 0.4 < 0.1 1.4 1.0 < 0.1 < 0.1 1.1 < 0.1 < 0.1 1.2 < 0.1 2.5 2.1 2.6 4.5 1.1 0.8

3.8 0.2 57.3 < 0.1 2.2 0.4 0.9 1.0 2.9 3.4 1.4 0.1 7.8 2.5 0.4 1.5 0.5 < 0.1 < 0.1 3.3 2.1 < 0.1 1.0 0.5 0.4 0.4 0.3 0.3 0.1 0.6 < 0.1 0.3 < 0.1 0.1 < 0.1 0.4 1.7 0.2 0.6 1.4 1.4 0.0 0.6

3.9 0.4 42.9 < 0.1 0.2 0.2 0.7 0.6 0.9 3.5 2.7 0.1 5.0 2.4 0.4 1.4 0.6 < 0.1 0.1 20.2 0.3 0.6 2.5 0.3 0.4 0.4 0.4 0.4 0.5 0.4 0.7 0.5 0.7 0.4 2.1 0.5 0.8 0.1 0.1 1.1 0.3 0.1 1.2

5.2 0.5 43.0 < 0.1 0.2 0.6 0.9 1.2 12.4 9.3 18.9 0.6 9.3 9.6 1.0 2.2 0.6 0.2 0.7 52.4 0.3 8.4 14.3 1.0 2.1 0.4 0.4 0.6 0.9 0.8 2.0 1.2 1.2 0.4 16.0 0.9 6.2 0.1 0.1 11.9 2.5 1.9 9.5

Experiments made using 0.08 g (ultrasound) and 2.0 g of sample ash (mechanical agitation) and different water volumes (30 and 100 ml for closed system using ultrasound and mechanical agitation, respectively; 100 and 1000 for open system).

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Fig. 7. Temporal evolution of pH and conductivity of the leaching tests using Candiota fly ash in open system.

reaching high ionic concentrations ( f 500 AS/cm) in the first 10 ml of water, and accentuated decrease with the increase of the L/S ratio (30 AS/cm to 100 ml and 10 AS/cm to 1000 ml). Recalculating these values, as was previously done with pH values, a conductivity of 117 AS/cm for an accumulated leachate volume of 100 ml is obtained. This value is within the range obtained for the conductivity in closed system tests performed with the same L/S ratio (105 –130 AS/cm). In fact, the pH and conductivity profiles observed are similar to those obtained by Querol et al. (2001) for the ashes from Puertollano, which had similar composition to the Candiota ashes (high SiO2, Al2O3 and low CaO contents). Fig. 8 shows the time evolution of the concentration of some trace elements (Li, Cu and Mo) in leachates from fly ash. These elements represent typical profiles of three time evolution trends (elements in between brackets, less evident): A: As, Li, Mn, Sb (Ba, Co, Cs, Dy, Eu, Ge, Ho, Nb, Pr, Rb, Sm, Sr, Tb, Tm and Yb) B: B, Cu, Lu, Ni, Pb, Ti, Tl, U, (Zn V, Y, Be, Ce, Cr, Er, Gd, La, Nd, Sc, Th and Zr) C: Mo and Sn (Ga) It has to be pointed out that the results on a number of elements in brackets are of a lesser significance due to the fact that the extractable fraction is < 5%. The elements classified in groups A and B present the highest concentrations in the first aliquot of leachate (10 ml), with a rapid decrease with the increase of the solid to liquid ratio. The differences between these groups can be seen in leachate volumes

>50 ml. While the elements of Group A present low concentrations (100 times lower than the initial one) but measurable until the end of the test (1000 ml), Group B elements present behavior quite similar to Group A elements, differing only for a small concentration increase in the last leaching stage. The elements from Group C (Ga, Mo and Sn) presented atypical behaviors and could not be classified in any of the previous groups. The highest Mo concentrations are not observed in the first leachate fractions, but after the passage of some milliliters. This maximum is followed by a slight concentration decrease up to 800 ml when a pronounced concentration decrease is evident in the last leached fractions. Swaine and Goodarzi (1995) classify the behavior of trace elements during the leaching processes in three categories: Type I—Elements that dissolve immediately and do not form slightly soluble compounds. In closed system tests, the concentration of these elements reaches a plateau that corresponds to the total dissolution of the soluble fraction on the ash particle surface. In the open system, these elements are present in high concentrations in the initial stages of the leaching with rapid decline with the increase of the L/S ratio. Type II—Elements present in constant leaching concentrations with the increase of the L/S ratio in open system. In this case, there is dissolution of slightly soluble superficial phases and/or desorption of the element linked to the matrix phases. This type of behavior is also the result of the dissolution of the matrix itself and of the migration,

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proposed by Swaine and Goodarzi (1995). On the other hand, Group B presents a concentration increase with the increase of the L/S ratio, like Type III elements, but with high initial concentrations. 3.5. Comparison among leaching systems

Fig. 8. Temporal evolution of the A, B and C class types, using as example the concentrations of the Li, Cu and Mo, respectively. Concentrations in the leachates of Candiota fly ash in open system are expressed in microgram per liter versus water volume, in ml.

limited by the diffusion, of the element contained inside the fly ash particles. Type III—Corresponds to low initial leaching concentrations, followed by a concentration increase with the increase of the L/S ratio. This behavior occurs due to solubility limitations mainly due to the pH. The behavior of the elements classified in Group A could be correlated to Type I leaching categories

Table 6 shows the results obtained from the closed system fly ashes leaching tests with mechanical agitation or ultrasound assistance, and from open system in a column with a constant water flow. The results are expressed in percentage values of the extraction in relation to the total content of each element in the ashes in order to facilitate a comparison between the different tests performed. Comparing the tests performed in closed systems, ultrasound and shaker, it is observed that the ultrasound stirring contribute to a more efficient leaching of most elements (25 of 47 studied). However, these differences may be in part attributed to the fact that the sample submitted to the ultrasound stirring was the < 63-Am fraction of the bulk sample. Unfortunately, data on ultrasound leaching was not found to compare with our results. The tests performed in open system best simulate the mobilization of the trace elements in the environmental conditions. Comparing the data from the open and closed systems, similar extraction rates are obtained for most elements for the same L/S of 50 ml.g
1. However, some elements, such as Mo and Sb, among others, presented notably higher extraction yields in the open system when compared with the closed one (20% and 3%, and 3% and 1% for the open and closed systems of Mo and Sb, respectively). B, V, Ge, Cd, Pb and Ni yielded lower or similar extraction rates in the open systems. For some elements that are present in concentrations close to the detection limit, this behavior could be attributed to a low analytical precision. With the increase of the L/S ratio (50 – 500 ml g
1) in the open system, a extraction yield increase is verified for the following elements: Li, Cu, Zn (3 –19%), Ge, As (2 –10%), Mo (20 –52%), Sb (2 –14%), Sn, Ni (1 –12%), Cs, Ba, Th, Tl, Pb, V, U, and most REEs. Thus, higher extraction yields for Mo, Sb, Ni and As can only be reached with the use of large water volumes. According to the leachable proportion obtained in open and closed systems, the elements may be clas-

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sified as follows: B (43 –57%), Mo (3 –52%), Cu (3– 9%), Ge (4– 9%), Li (4 –5%), Zn (2– 19%), As (1– 10%), Ni, Sb, Tl, and U (1– 14%). For the other elements studied, the leachable fraction is in most cases < 1% of the bulk content.

4. Conclusion The present results indicate that: 

The elemental concentration of the Candiota high ash coal is within the range of the world coals. However, the concentration of some elements, such as Cs, Rb and heavy REEs, are higher than those element concentrations indicated in the literature. This might be due to the high content of detrital minerals of the studied coals.  Three groups of elements can be recognized based on the partitioning between coal and fly and bottom ashes: Group I (volatile elements with subsequent condensation): As, B, Bi, Cd, Ga, Ge, Mo, Pb, S, Sb, Sn, Tl and Zn; Group II (no volatile elements enriched in bottom ash vs. fly ash): Ca, Fe, Mn, P, Ti and Zr; Group III (low volatile elements with no partitioning between fly and bottom ashes): Al, Ba, Be, Co, Cr, Cs, Hf, K, Li, Mg, Na, Ni, Rb, Sr, Th, U, W, Y and most of REE.  The leachability of elements is as follows: B (43 – 57%), Mo (3 –52%), Cu (3 –9%), Ge (4 – 9%), Li (4– 5%), Zn (2 –19%), As (1– 10%), Ni, Sb, Tl, and U (1– 14%),). For the other elements studied, the leachable fraction is in most cases < 1% of the bulk content.

Acknowledgements The present study was supported by FAPERGS, AECI and IJA-CSIC. We would like to express our gratitude to Dr. J. Alastuey and Dr. F. Plana for their valuable comments and expert technical assistance and to CGTE and Dr. E.C. Teixeira for supplying the samples. We are especially grateful to Ms. Silvia Rico, Ms. Merce´ Caban˜as and Mr. Josep Elvira for their invaluable collaboration in the analytical work, and to C.A. Palmer and another anonymous reviewer for their valuable comments and suggestions.

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