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Characterization of Candiota (South Brazil) coal and

combustion by-product

Marcal 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/Martı́ i Franquè s s/n, E-08028 Barcelona, Spain

Received 8 October 2003; accepted 30 April 2004Available 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 Médici 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

contents, thus requiring expensive processing treat-

ment 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%

0166-5162/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.coal.2004.04.003

* Corresponding author. Fax: +55-51-3320-3612.

E-mail address: [email protected] (M. Pires).

www.elsevier.com/locate/ijcoalgeo

International Journal of Coal Geology 60 (2004) 57–72

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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 theBrazilian Government (BRAZIL, 2001b), the coun-

try’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 Médici

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 Tubarão subgroup of the Paraná

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 certainareas of Rio Grande do Sul, t he 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

Jacuı́, 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 ( f50 kg) used at UTPM, located inCandiota, 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 follow-

ing 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 homo-

geneity and finer grain size, was used as received in

most of the subsequent analysis. The only exceptionwas the grain size separation with the cascade impac-

tor, which needed the separation of the coarser frac-

tion (>63 Am). Subsamples of coal and ashes were

dried at 105 jC, and used in subsequent characteriza-

tion 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) 57–7258

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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/

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

M. Pires, X. Querol / International Journal of Coal Geology 60 (2004) 57–72 59

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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 charac-

terized by X-ray diffraction (XR D) 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. Thesubsamples were recovered from the impactor units in

an ultrasound bath. The insoluble fraction was recov-

ered 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

f low of 50 ml h 1

was put through the columncontaining 2 g of ash. Samples of leachates were

collected at intervals of 10 ml (f8 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 sam-

ples were periodically collected and analyzed by ICP-

MS 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 (BRA-

ZIL, 1987; Fiedler, 1987). The main mineral phases

present in the coal are quartz, kaolinite, illite, K-feldspar 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 1

Characterization of Candiota coal

Proximate analysis Mineral content (wt.%)

Moisture (%) 16.4 Quartz 30.9Ash (%) 49.7 Kaolinite 16.0

Volatile matter (%) 23.1 Illite 2.0

Fixed carbon (%) 27.2 K-feldspar 0.5

Gross calorific

value (MJ kg 1)

14.3 Calcite < 0.5

Sulphur (%) 0.96 Pyrite/marcasite 0.3

Table 2

Major oxide contents in Candiota coal HTA, fly ash and bottom ash,

on dry basis (wt.%)

HTA Fly ash Bottom ash

SiO2 70.6 56.7 58.9

Al2O3 24.3 38.4 36.0

Fe2O3 2.9 2.5 2.4

CaO 0.7 1.1 1.3

MgO 0.2 0.2 0.2

Na2O 0.03 0.04 0.04

K 2O 0.50 0.6 0.6

P2O5 0.01 0.02 0.02

TiO2 0.8 0.5 0.6

MnO 0.01 0.02 0.02

SO3 4.7 0.2 < 0.1

SiO2/Al2O3 2.9 1.5 1.6


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(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 appli-

cation 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 mag-

netite. 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 concentra-

tion values of several trace elements of subbituminous

coals of different countries and the worldwide con-centration range for coal. The results of 17 studies on

Brazilian coals, among them t he 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 thanthe 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 concentra-

tions of major and trace elements present in fly and

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 Fly ash Bottom ash%

Al 3.24 10.16 9.54

Ca 0.42 0.77 0.90

Fe 2.01 4.63 7.68

K 0.92 1.74 1.67

Mg 0.21 0.46 0.47

Na 0.06 0.12 0.12

P 0.01 0.02 0.025

S 0.96 0.08 0.01

Ti 0.23 0.41 0.49

mg kg 1

Li 14 29 28

Be 2.5 4.9 4.3

B 9.2 23.9 15.3

V 55 78 93

Cr 26 51 47

Mn 410 614 991

Co 10.1 16.2 17.5

Ni 17 29 30

Cu 20 33 32

Zn 45 77 47

Ga 9.1 19.3 11.8

Ge 2.1 3.8 1.3

As 4.4 11.5 1.8

Rb 50 119 107

Sr 42 103 100Y 24 49 46

Zr 128 215 218

Nb 16.1 22 25.1

Mo 1.4 4.8 3.6

Cd 0.5 1.2 0.3

Sn 3.2 5.4 3.5

Sb 1.4 3.2 2.6

Cs 9.1 16.8 14.8

Ba 152.3 283.9 278.9

La 24.9 52.7 49.7

Ce 53.7 113.3 109.4

Nd 25.4 55.2 53.6

Sm 4.2 9 8.9

Gd 6.8 12.1 10.7Tb 0.8 1.8 1.6

Dy 5.3 10.5 10.5

Ho 0.7 1.7 1.6

Er 3 5.7 5.2

Tm 0.31 0.74 0.78

Yb 3.2 6.3 6.3

Lu 0.9 1.3 1.3

Hf 4.4 10.2 9.1

Ta 7.4 13.9 14.3

W 3.2 4 3.4

Tl 0.8 2.1 0.4

Table 3 (continued )

Coal Fly ash Bottom ash

mg kg 1

Pb 20.5 41.1 19.2Bi 0.8 1.6 1.1

Th 9.5 21.0 22.0

U 3.3 6.0 6.1


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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

USA

b

Canada

b

Australia

b

Brazil

c

JL-SCd Candiota-RS

Rangec This work Ratioe

As 0.5 – 80 0.1 – 420 4 – 53 < 0.1 – 36 1.3 – 12 35.14 1.3 – 2.6 4.4 0.05

B 5– 400 5 – 32 1 – 14 36 – 230 49.48 36 13.7 0.03

Ba 20 – 1000 30 98.13 30 152 0.15

Be 0.1 – 15 0.05 – 32 < 0.5 – 12 5 – 73 3.49 73 2.7 0.18

Bi 2 – 20 9 1.41 9 0.8 0.04

Cd 0.1 – 3 0.03 – 3.7 0.024 – 2.2 2.6 2 0.1 0.04

Ce 2 – 70 110.73 54 0.77

Cl 50 – 2000 – 100 – 8000 169 – 1287 na 0.00

Co 0.5 – 30 0.06 – 70 1.1 – 31 < 5 – 321 6 – 43 10.29 6 – 21 10.1 0.34

Cr 0.5 – 60 0.54 – 70 5.7 – 64 < 5 – 117 9– 74 73.2 27 – 73 24 0.40

Cs 0.3 – 5 10.59 9.1 1.81

Cu 0.5 – 50 0.16 – 120 14 – 80 < 1 – 741 9 – 71 33.39 24 – 71 4.4 0.09

Dy 0.5 – 4 6.13 5.3 1.33

Er 0.5 – 3 3.53 3.0 1.01

Eu 0.1 – 2 1.64 ni 0.00

F 20 – 500 126 – 1287 110 – 560 na 0.00

Ga 1 – 20 23 – 28 18.35 9.1 0.46

Gd 0.4 – 4 8.46 6.8 1.70

Ge 0.5 – 50 9.49 2.3 0.05

Hf 0.4 – 5 8.87 4.4

Hg 0.02 – 1 0.01 – 8.0 0.02 – 0.44 0.05 – 0.8 0.1 – 0.8 0.1 0.06

Ho 0.1 – 2 1.24 0.6 0.30

La 5 – 300 56.22 25 0.62

Li 1 – 80 42.2 14.2 0.18Lu 0.03 – 1 0.49 0.9 0.90

Mn 5 – 300 1.4 – 3500 1.6 – 1000 < 2 – 667 31 – 240 31 – 240 452 1.50

Mo 0.1 – 10 0.13 – 41 < 0.8 – 9 < 0.1 – 6 2.6 – 10 5.9 4 1.4 0.14

Nb 1 – 20 20.57 16.1 0.80

Nd 3 – 30 42.05 25 0.85

Ni 0.5 – 50 0.32 – 69 2 – 161 15 – 64 24.05 22 – 34 17.1 0.34

P 10 – 3000 166 0.06

Pb 2 – 80 0.7 – 76 1.9 – 19 < 1 – 595 2 – 95 49.64 4 – 50 20.5 0.26

Rb 2 – 50 69.59 50 1.00

Sb 0.05 – 10 0.04 – 43 0.1 – 2.4 – 5.88 ni 0.00

Sc 1 – 10 19.44 ni 0.00

Se 0.2 – 10 0.10 – 16 0.5 – 2.0 – 1.5 2.57 1.5 ni 0.00

Sm 0.5 – 6 8.51 4.2 0.70

Sn 1 – 10 8.79 3.4 0.34Sr 15 – 500 56.32 42 0.08

Tb 0.1 – 1 1.16 0.8 0.84

Th 0.5 – 10 0.08 – 54 < 0.1 – 9 < 0.4 – 26 8.9 8.5 0.85

Tl < 0.2 – 1 4.21 0.7 0.67

Tm no data 0.3

U 0.5 – 10 0.06 – 76 0.2 – 2.4 < 1 – 58 3.8 3.3 0.33

V 2 – 100 0.14 – 370 23 – 300 2 – 279 23 – 86 187.25 23 – 60 50 0.50

W 0.5 – 5 6.59 3.2 0.63

Y 2 – 50 37.25 23.7 0.47

Yb 0.3 – 3 3.64 3.2 1.08

Zn 5 – 300 0.88 – 910 7.6 – 83 < 2 – 273 30 – 217 473.53 80 – 98 11.4 0.04

Zr 5 – 200 110 – 127 857.82 110 128


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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 technol-

ogies 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 f1,0): 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 ele-

ments (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

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.

Caracterizacão do carvão de Candiota e de suas cinzas. Geochimica Brasiliensis, 15 (1–2): 113–130.

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 Can-

diota 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 non-

volatile 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=½YSÞ=ð½XC=½YCÞ

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 non-

volatile 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 inconstant 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 approxima-tion 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 associ-

ated 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

EFf1 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 st udied is very low

since most EFtotal are close to 1 (Fig. 3). The data

show that volatilization of many elements occur

(EFfly ash>EF bottom a sh) but most of them condense

on fly ash particles (Fig. 3).

3.3. Grain size separation of fly ash

Fig. 4 shows the results of the grain size classifi-

cation of the fly ashes. Two different characterizationmethods 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 obser ved 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. Caracterizacão do carvão 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 f1% of the total

mass of t he 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 ceno-

spheres 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. Caracterizac

ão do carvão de Candiota e de suas cinzas. Geochimica Brasiliensis, 15 (1–2): 113–130.


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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; Fernandéz-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 con-

ductivity 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

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

Table 5

pH and conductivity of the leachates measured for the closed system

using fly and bottom ashes

Test

code

L/S ratio

(ml g 1)

Agitation

time (h)

pH Conductivity

(AS cm 1)

Fly ash 1 10 24 3.90 105

2 50 24 3.94 105

3 50 2 4.03 130

Bottom ash 4 10 24 8.30 30

5 50 24 8.42 30

6 50 2 7.90 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 leach-ates 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%, respective-

ly). 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 conductivityobtained 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 f6.0 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 subse-

quent 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 corrobo-

rates 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 Open

Ultrasound,

30 ml

Shaker,

100 ml

100 ml 1000 ml

Li 4.6 3.8 3.9 5.2

Be < 0.1 0.2 0.4 0.5

B 55.2 57.3 42.9 43.0

Ti < 0.1 < 0.1 < 0.1 < 0.1

V 0.2 2.2 0.2 0.2

Cr 1.1 0.4 0.2 0.6

Mn 1.5 0.9 0.7 0.9

Co 0.5 1.0 0.6 1.2

Ni 8.3 2.9 0.9 12.4

Cu 8.7 3.4 3.5 9.3

Zn 13.2 1.4 2.7 18.9

Ga < 0.1 0.1 0.1 0.6

Ge 3.6 7.8 5.0 9.3

As 1.4 2.5 2.4 9.6

Rb 0.5 0.4 0.4 1.0

Sr 1.8 1.5 1.4 2.2

Y 0.8 0.5 0.6 0.6

Zr 0.1 < 0.1 < 0.1 0.2

Nb 1.0 < 0.1 0.1 0.7

Mo 18.1 3.3 20.2 52.4

Cd < 0.1 2.1 0.3 0.3

Sn < 0.1 < 0.1 0.6 8.4

Sb 8.1 1.0 2.5 14.3

Cs < 0.1 0.5 0.3 1.0

Ba 2.0 0.4 0.4 2.1La 0.4 0.4 0.4 0.4

Ce 0.4 0.3 0.4 0.4

Nd < 0.1 0.3 0.4 0.6

Sm 1.4 0.1 0.5 0.9

Gd 1.0 0.6 0.4 0.8

Tb < 0.1 < 0.1 0.7 2.0

Dy < 0.1 0.3 0.5 1.2

Ho 1.1 < 0.1 0.7 1.2

Er < 0.1 0.1 0.4 0.4

Tm < 0.1 < 0.1 2.1 16.0

Yb 1.2 0.4 0.5 0.9

Lu < 0.1 1.7 0.8 6.2

Hf 2.5 0.2 0.1 0.1

Ta 2.1 0.6 0.1 0.1Tl 2.6 1.4 1.1 11.9

Pb 4.5 1.4 0.3 2.5

Th 1.1 0.0 0.1 1.9

U 0.8 0.6 1.2 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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reaching high ionic concentrations ( f500 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 leach-

ate 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 ar e 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 concentra-

tion of some trace elements (Li, Cu and Mo) in

leachates from fly ash. These elements represent

typical profiles of three time evolution trends (ele-

ments 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 concen-

tration increase in the last leaching stage.

The elements from Group C (Ga, Mo and Sn)

presented atypical behaviors and could not be classi-

fied 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 concentra-tion 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 elementsare 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 desorp-

tion 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,

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


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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 in-

crease 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

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

Table 6 shows the results obtained from the closed

system fly ashes leaching tests with mechanical agi-

tation 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 ultra-

sound 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

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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. Mercé Cabañas 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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