Geochemical s udy o p oduc s associa ed wi h spon aneous oxida ion
o coal in he Ce o Pelado Fo ma ion, Venezuela
M. Ma ínez
a
, G. Má quez
b
, F.J. Alejand e
c,*
, J.J. Del Río
c
, A. Hu ado
a
a
Ins i u o de Ciencias de la Tie a, Uni e sidad Cen al de Venezuela, Ca acas, 3895, 1010-A, Venezuela
b
Depa men o de Ingenie ía Mine a, Mecánica y Ene gé ica, Uni . de Huel a, Palos de F a., 21819 Huel a, Spain
c
Depa men o de Cons uc iones A qui ec ónicas II, Uni e sidad de Se illa, Reina Me cedes n°4, 41011 Se illa, Spain
a icle in o
A icle his o y:
Recei ed 28 Sep embe 2007
Accep ed 22 Decembe 2008
Keywo ds:
Spon aneous oxida ion
Ammonium hiosul a e
Clinke ma e ial
Bi uminous coal
Fuma olic mine aliza ions no hwes e n
Venezuela
abs ac
The aim o his esea ch wo k is a geochemical, mine alogical, and ex u al cha ac e iza ion o spon ane-
ously smoulde ed coal-de i ed p oduc s in no hwes e n Venezuela (Ce o Pelado Fo ma ion, some
10 km om Ped egal ci y). Se e al solid samples we e collec ed om his o ma ion, six o unwea he ing
coal, an o he six o esul ing unmel ed ocks o ming on a su ace coal bed, and he las ou o mine -
aliza ions ound accumula ing a ound gas en s. The esh coal and he unmel ed ma e ial we e analysed
by scanning elec on mic oscopy, X- ay di ac ion, and p oxima e echniques. P oduc s such as magne-
i e and chabazi e-K we e iden ified in he al e a ion ocks. Likewise, bo h ma e ials we e also s udied in
o de o de e mine he mobiliza ion o 17 elemen s in o he en i onmen ; such elemen s we e analysed
h ough induc i ely coupled plasma a omic emission spec oscopy on ex ac s ob ained by a sequen ial
ex ac ion me hod: each sample was fi s ly ex ac ed wi h MilliQ wa e and hen he esul an esidue
was washed. This and he subsequen ly esul ing esidues a e ex ac ed acco ding o he men ioned p o-
cedu e by using, espec i ely, ammonium ace a e, chlo hyd ic acid, pe oxide and chlo hyd ic acid, ni ic
acid and fluo hyd ic acid, and ni ic acid. The s udied elemen s a e classified as highly mobile (Na, Ni,...),
nea ly immobile (Ti, P) and pa ially mobile (Mg, Fe, K,...). In ega ds o mine aliza ions a ound uma oles
associa ed wi h smolde ing coal seams, Fou ie - ans o m in a ed spec oscopy and X- ay di ac ion
analyses ha e e ealed he p esence o salammoniac, mascagni e and o he solid combus ion compounds
o med by eac ion o gas emi ed om coal oxida ion, in addi ion o p e iously non- epo ed sul u - ich
by-p oduc s associa ed wi h gas fissu es, pa icula ly ammonium hiosul a e, a phase fi s ob ained only
syn he ically in he labo a o y. Ano he objec i e o he esea ch was o collec and analyse gases escap-
ing om su ficial en s. Rela i ely high concen a ions o se e al a oma ic compounds we e de ec ed in
he gas collec ed a he s udied coal ou c op, as well as alipha ic hyd oca bons including e hane, p opane,
bu anes, among o he s. High con en s o ca bon monoxide, me hane and ca bon dioxide we e also mea-
su ed o gas samples.
Ó2009 Else ie L d. All igh s ese ed.
1. In oduc ion
This s udy desc ibes and analyses he p incipal oxida ion p od-
uc s o med by smolde ing coal beds in he Ce o Pelado Fo ma ion
(Venezuela). The p ima y objec i es o his esea ch we e o cha -
ac e ise esh ma e ials, clinke s and uma olic mine aliza ions
p oduced by spon aneous oxida ion in an ou c op o sul u -ni o-
gen- ich coal, along wi h 17 chemical elemen s (mino and ace
cons i uen s o coal; Finkelman, 1999) eleased om coal in di e -
en g ades. The beha iou o hese elemen s in he p ocess o sel -
oxida ion is s udied based on hei concen a ions in he ex ac s
de i ed om bo h esh coal and clinke samples a e he sequen-
ial a ack by a ious eagen s. Ano he objec i e was o de ec
new and unclassified uma olic mine aliza ions o med as eac ion
p oduc s o gas emi ed om he men ioned ou c op o coal in o -
de o be e unde s and o he e ec s o high sul u and ni ogen
con en s on he geochemis y o smolde ing coal beds. This wo k
also con ibu es o documen he main compounds p esen in gas-
eous e fluen s associa ed wi h ho uma oles.
Unde ce ain na u al condi ions, a coal bed in con ac wi h ai
may spon aneously oxidise, esul ing in an exo he mic p ocess
e med spon aneous combus ion. This phenomenon can u n he
bed o esh coal in o a sui e o he mally me amo phosed ocks
– known as clinke s – o med by hea ing and chemical al e a ion
o sedimen a y ocks du ing oxida ion o unde lying coal beds
(Sa necki, 1991; Coa es and He e n, 2000). This p ocess is also
cha ac e ized by he emission o di e en gases and pa icula e
ma e (S ache and Taylo , 2004). Likewise, some mine aliza ions
associa ed wi h uma oles ha e esul ed om he in e ac ion o gas
0895-9811/$ - see on ma e Ó2009 Else ie L d. All igh s ese ed.
doi:10.1016/j.jsames.2008.12.001
*Co esponding au ho . Fax: +34 954556691.
E-mail add ess: [email p o ec ed] (F.J. Alejand e).
Jou nal o Sou h Ame ican Ea h Sciences 27 (2009) 211–218
Con en s lis s a ailable a ScienceDi ec
Jou nal o Sou h Ame ican Ea h Sciences
jou nal homepage: www.else ie .com/loca e/jsames
je s and mine al phases along wi h su ace wa e and su ounding
ocks. The bi uminous coal is inclined o spon aneous bu ning, and
any ou c op o esh coal is likely o bu n spon aneously i exposing
o he ai by landslide, o apid e osion (Lyman and Volkme ,
2001). This p ocess is induced ei he by coal fines, o by exo he -
mic eac ions ca alyzed by oxygen ci cula ing h ough high-sul u
coal seam join s (An hony e al., 1977).
The mechanisms o spon aneous oxida ion o coal ha e been
ex ensi ely s udied (Kaymakci and Dida i, 2002; S ache e al.,
2004; Cha e jee, 2006; among o he s), and a e shown he e. The
in e ac ion o coal wi h ai and/o mois u e depends on a se ies
o adso p i e, abso p i e and chemical p ocesses (Hudak, 2002)
and he physico-chemical cha ac e is ics o he coal (Cegla ska-
S e anska e al., 1998). The main low empe a u e p ocesses lead-
ing o sel -oxida ion o coal a e physical and chemical abso p ions
o oxygen o o m di e en uns able compounds known as pe oxy-
complexes and gene a e hea as a by-p oduc o he modified su -
ace ene gy o he ma e ial (Rosema e al., 2001). These p ocesses
cause an ini ial ise in empe a u e i condi ions a e jus igh o
adequa e ai flow o he coal, nei he oo high, as he hea gene -
a ed would be dissipa ed, no oo low, which would impede he
coal-oxygen in e ac ion (Chak a o y and Kolada, 1988). A he
sel -hea ing poin o minimum empe a u e – app oxima ely
70 °C – a which he ma e ial bu ns spon aneously, he pe oxy-
complexes decompose a an accele a ing a e o p o ide oxygen
o he oxida ion p ocess (Bane jee, 1985). As he empe a u e o
coal ises, he pe oxy-complexes decompose a a a e g ea e han
hey o m (Wang e al., 2003) and gaseous p oduc s o exo he mic
eac ion appea . Finally, when empe a u e exceeds 150 °C, he
oxida ion a e inc eases apidly in he la e s ages o he bu ning
p ocess un il spon aneous combus ion occu s (S ache e al.,
2004).
Coal-fi e beds a e ela i ely common in opical a eas, whe e
high a mosphe ic humidi y means dense ai s ays low o he
g ound, main aining con ac wi h he coal bed. In hese condi ions,
mois u e fixa ion and finely di ided py i e o ma casi e is e y
impo an (León, 1992). Fe ous py i e oxidises a ambien empe -
a u es in humid ai , p oducing e ous sul a e and sul u ic acid
h ough he ollowing eac ions ha exempli y he ac ha mois-
u e is also leading o spon aneous bu ning (Limache , 1963):
2FeS
2
þ7O
2
þ2H
2
O¼2H
2
SO
4
þ2FeSO
4
þHea
The e ous ion has a ca aly ic e ec on spon aneous coal oxida-
ion (Ca as and Young, 1994). Whe e seams a e sca cely o no a
all py i ic, sel -oxida ion may be he esul o he hyg oscopic na -
u e o he coal, which ends o eadso b o gain humidi y om he
ai as a mosphe ic apou . This humidi ying p ocess can be e en
mo e exo he mic han he esul s o py i e oxida ion. Humidi y
also has he e ec o ac u ing he coal and hus g an ing be e
access o ai ; u he mo e, oxygen dissol ed in wa e is mo e ac i e
(Walke , 1999).
In he s a e o Falcon in Venezuela is an in e es ing ou c op o
coal uns udied un il now (Fig. 1a). I has been spon aneously smol-
de ing o se e al yea s, emi ing ho gases accompanied by he
p ecipi a ion o unusual al e ed ma e ials. Mo eo e , he coal
ma e ial does no unde go apid sel -oxida ion; a he i cons an ly
dissipa es he hea p oduced (Glasse and B adshow, 1990). This
coal bed, app oxima ely 60 cm hick, is loca ed in ‘‘La Cues a” mine
(coo dina es equal o 11°04
0
43
00
N, 70°12
0
41
00
W) nea he ci y o
Co o in he Ce o Pelado Fo ma ion (Fig. 1b). The ou c op, abou
40 m
2
, con ains en en s, h ee o e 50 cm in diame e and he
o he s less han 20 cm in diame e . On he edges o he en s
and in hei icini y appea uma olic mine aliza ions.
The coal igni es and smoulde s ou side in o a slope, o as long
as i is able o d aw in ai , hen goes ou when ac u es ail o each
he su ace o he fi e eaches he le el o he wa e able in he
coal ou c op. The on pa o he clinke mass is a acked and e-
mo ed by e osion, pe manen ly exposing mo e coal, which igni es
and o ms mo e clinke ma e ial (He e n and Coa es, 2004). As a
consequence, he clinke ma e ial is highly ac u ed, which allows
ai and mois u e o en e and gas je s o escape, so ha he oxida-
ion p ocesses con inue (Coa es, 1991). Gases emi ed om en s
in coal ou c ops consis o a complex mix u e o g eenhouse gases,
hyd oca bons, hyd ogen sulphide, a gon, ca bon monoxide, wa e
apou , oxides o sul u and ni ogen, and o he s ha a e associ-
a ed wi h nega i e e ec s on en i onmen and on human heal h
(Finkelman, 2004). The p esence o hese p oduc s in he gas sam-
ples may be explained by he he mal deg ada ion du ing spon a-
neous oxida ion p ocess o biopolyme s bu ied in he coal seams
(Pu mann e al., 1991).
2. Geological se ing
The Falcón Basin is loca ed in no hwes e n Venezuela, com-
p ising se e al s a es o his coun y: he whole o Falcón and pa -
ially Zulia, La a and Ya acuy (Audema d e al., 1999). This is a
Te ia y basin, wi h a sedimen s olume app oxima ely o
161,000 km
3
and an ex ension o 36,000 km
2
(González de Juana
e al., 1980). The Falcón Basin has also a sui able posi ion in no h-
wes e n Sou h Ame ica, occu ing in he in e ac ion zone be ween
se e al majo pla es – Ca ibbean, Nazca and Sou h Ame ica – and
mino li hosphe ic blocks: Ma acaibo, Bonai e and Wes e n
Colombia. I s sedimen a y eco d, excep a ious uncon o mi ies
o egional ex en , is almos con inuous since he la e Eocene
(Audema d, 2003).
The Falcón Basin is he Venezuelan basin wi h he smalles ol-
umes o liquid hyd oca bons, bu i shows significan gas ese es
and coal deposi s. In his basin, se e al s a ig aphic uni s con ain
o ganic ma e (Boesi and Godda d, 1991). One o hem is he Ce o
Pelado Fo ma ion (Miocene) deposi ed du ing a ma ine ansg es-
sion, and made o an al e na ion o hin laye s o lu i es wi h lime-
s ones and sands ones ep esen ing a u bidi ic sedimen a ion
(Díaz de Game o, 1977).
The Ce o Pelado Fo ma ion, a sedimen a y uni a ound 1500 m
hick, ex ends h oughou he sou h-cen al pa o Falcón S a e in
Venezuela (González de Juana e al., 1980). The uni p ima ily con-
sis s o li hic sands ones in beds up o 20 m hick in e cala ed wi h
lu i e and limoni e beds wi h a maximum hickness o 2.7 m (Díaz
de Game o, 1977).
The bo ommos pa o he uni is g ey o eddish-b own e u-
ginous sands ones wi h c oss-bedding. Upsec ion a e calca eous
sands ones wi h bedding planes con aining coal beds up o 2 m
hick (Díaz de Game o and Lina es, 1989). Lu i es a e some imes
ca bonaceous and con ain g eyish-blue limes one nodules; in addi-
ion, hey con ain ossils p ima ily ypical o b ackish wa e s om
which he o ma ion has been da ed as Miocene (Hambalek e al.,
1994).
These coal beds we e deposi ed in a ma ine sedimen a y en i-
onmen wi h lagoons and ma shlands ha de eloped due o del a
ad ancemen , and he coal (sub-bi uminous o bi uminous) is
ound in he middle and uppe pa o he Ce o Pelado Fo ma ion
(González e al., 1985). Ino ganic ma e ial occu s in a iable p o-
po ions and o al sul u con en s a e high, gene ally o e 2% (Gon-
zález e al., 1985).
3. Ma e ials and me hods
Random sampling was ca ied ou o esh coal, clinke s, and
mine aliza ions in he icini y o se e al gas en s. Sampling
enches (Glei , 1986) we e dug o a leng h o 25 m wi h 1 m be-
ween adjacen enches. We hand-collec ed he ollowing sam-
212 M. Ma ínez e al. / Jou nal o Sou h Ame ican Ea h Sciences 27 (2009) 211–218
ples: a ound 600 g o esh, b igh black coal wi h an a e age spe-
cific g a i y o 1.5 and an a e age Mohs ha dness o 0.4; abou
500 g o unmel ed ma e ial ha was e y iable and powde y;
and app oxima ely 200 g o mine aliza ions ound accumula ing
a ound gas en s. Six samples o clinke s, an equal numbe o unal-
e ed coal, and ou o uma olic mine aliza ions we e homoge-
nised hen pu h ough a Jones jaw c ushe o ob ain 100 g o
each. An E ex GPS was used o eco d coo dina es o he s udy
ou c op.
The clinke and esh coal samples we e analysed o mois u e,
ola ile ma e , ashes, and fixed ca bon (in acco dance wi h s an-
da ds ASTM D-3172-89; ASTM In e na ional, 2002). The fi s h ee
pa ame e s we e de e mined by weigh di e ence by hea ing 10 g
o sample; he fixed ca bon was calcula ed by sub ac ing he sum
o he o he h ee pa ame e s om he o al (100%). Mois u e was
de e mined by hea ing he sample o 1 h a 110 °C (s anda d
ASTM 3173-03). Vola ile ma e was hea ed o 7 min a 950 °C
(s anda d ASTM 3175-02) and ash o 4 h a 750 °C (ASTM 3174-
02). Las , he pe cen age o o al sul u was calcula ed on 0.5 g o
sample in a LECO SC-432 analyse a 1350 °C o 6 min (s anda d
ASTM 2492-02). Quan ifica ion o ca bon, hyd ogen, and ni ogen
con en s we e ca ied ou using a CARLO ERBA 1106 elemen al
analyse (Ma ínez e al., 2001).
Chemical analyses o 17 mino and ace elemen s we e de e -
mined by induc i ely coupled plasma a omic emission spec os-
copy echnique (ICP-AOS) on aqueous ex ac s ob ained by he
sequen ial a ack o a se ies o eagen s on 10 g samples o clinke
and esh coal. A Pe kin Elme Op ima 3000 sequen ial spec om-
e e equipped wi h an au oma ic sample was used. Specifically,
he ollowing sequence o ex ac an s was used o 30 min a
ambien empe a u e: 50 ml o MilliQ wa e , 50 ml o 1 M aque-
ous solu ion o ammonium ace a e, 50 ml o 1 M chlo hyd ic acid,
50 ml o an aqueous solu ion o 11% hyd ogen pe oxide and 0.7 M
chlo hyd ic acid, and 15 ml o 70% ni ic acid. Finally, 15 ml o an
aqueous solu ion o 30% fluo hyd ic acid and 27% ni ic acid was
used o 24 h a 100 °C. This six-s ep me hod is a modifica ion o
p e ious p ocedu es (Palme e al., 1993; McCa y e al., 1998;
among o he s). Accu acy o he sequen ial me hod o unal e ed
coal analysis was es ed wi h SARM 19 s anda d. In con as , no
ce ified ma e ial compa able he clinke examined in his s udy
could be ound.
T ace and mino elemen composi ion o he esh coal and clin-
ke we e de e mined h ough X- ay fluo escence spec oscopy
using an ene gy-dispe si e X- ay Panaly ical spec ome e , model
Axios, equipped wi h a digi al signal p ocesso and dual mul i-
channel analyse .
The su ace ex u e was examined by using a scanning elec on
mic oscopy. A Jeol JSM 6460-LV mic oscope was used. SEM images
in back-sca e ed elec on mode (BSE) we e acqui ed using se e al
gold-pla ed pieces o sample.
The mine alogical composi ion was s udied in he solid samples
by powde X- ay di ac ion (XRD) using a B uke -AXS D8 Ad ance
di ac ome e equipped wi h a coppe filamen , Cu K
a
adia ion,
ube condi ions o 40 kV and 30 mA, fixed slo and spa kle de ec-
o . The di ac og ams we e ob ained using he powde echnique.
XRD pa e ns in 2h ange 10–70°we e acqui ed applying a 0.05°
s ep scan wi h a 1 s s ep ime. Subsequen ly, a sample o unal e ed
coal was analysed in a 10–70°2hscanning a ea using a 0.05°s ep
scan wi h a 3 s s ep ime.
Fou ie - ans o med in a ed spec oscopy (FTIR) was used in
o de o de e mine he mine alogical composi ion o he uma olic
mine aliza ions h ough a Bomen spec opho ome e , model MB-
120, using 4 cm
1
esolu ion and spec a eco ded be ween 4000
and 400 cm
1
.
Gas samples we e collec ed om he s udied coal ou c op using
acuum s ainless s eel canis e s. The gas was pumped in o a con-
aine o s o age and anspo ed o he labo a o y o analysis.
In addi ion, he en empe a u e was measu ed using a digi al
he mocouple connec ed o a 1.0 m pla inum he mop obe.
Gas ch oma og aphic analyses o he gas samples we e pe -
o med h ough a specific me hod (Colman e al., 2001; Ba le a
e al., 2002). The analy ical echniques in ol ed he c yogenic
p e-concen a ion o each sample wi h liquid ni ogen (196 °C).
The p e-concen a ed gas was la e apou ised using a ho wa e
ba h and spli in o di e en de ec o s connec ed o wo sepa a e
Hewle -Packa d 5890 gas ch oma og aphs (GC). A mass spec om-
e e de ec o (MSD) and a flame ioniza ion de ec o (FID) we e
bo h used o analyse hyd oca bons. Ca bon monoxide and me h-
ane we e analysed using a GC equipped wi h FID. Fo ca bon diox-
ide, a he mal conduc i i y de ec o was used. Wa e apou was
no quan ified because o i s p opensi y o condense in he canis e
and i s a iabili y in coal-fi e gas (Pone e al., 2007).
Fig. 1. (a) Loca ion o se e al bu ning coal si es in Venezuela (Ce o Pelado, Cas illo and Ma celina o ma ions); (b) Si ua ion (black ci cle) o he s udy si e in he Falcón Basin.
M. Ma ínez e al. / Jou nal o Sou h Ame ican Ea h Sciences 27 (2009) 211–218 213
4. Resul s and discussion
4.1. Chemical and p oxima e analyses
The unmel ed subs a e samples comp ised 95.2% ash by
weigh and 9.5% sul u by weigh ; bo h alues a e well abo e he
a e age o esh coal om he Ce o Pelado Fo ma ion, which
gi es a e ages o 1.9% and 2.1%, espec i ely. In con as , he clin-
ke ma e ial con ains 2.0% fixed ca bon as opposed o 54.0% by
weigh in he unal e ed coal. Mois u e, on he o he hand, is 1.8%
o esh coal and 2.8% o clinke s (see Table 1 o all hese esul s).
This able also shows a e age alues o elemen al analyses o
s udied samples.
As can be seen, a compa ison o he clinke ma e ial wi h he
da a o he esh coal clea ly shows ha , no su p isingly, he o -
me has almos no ola ile ma e and a e y high pe cen age o
ash. This subs an ial d op in he pe cen age o fixed ca bon in
he clinke s is an unequi ocal indica ion ha oxida ion des oys
nea ly all o he o ganic ma e ial in he coal. In addi ion, a se ies
o ola ile componen s a e eleased (ca bon monoxide, anhyd ous
ca bon, me hane, e hane, and o he s) wi h a consequen en ich-
men in he mine al ac ion in he unmel ed subs a e.
The concen a ions o 17 elemen s (Si, Al, Ti, Na, Mg, Ca, K, C ,
Zn, P, Mn, V, Cu, Ni, S , Ba, and Fe) we e de e mined in he ex ac s
de i ed om bo h unal e ed coal and clinke ma e ial. The p es-
ence o a specific elemen in each ex ac se es o ela e i o i s
co esponding compounds. Thus, he soluble sal s occluded in
he o ganic ma e a e associa ed wi h an a ack by MilliQ wa e ;
he exchangeable ionic species adso bed wi h he aqueous dissolu-
ion o ammonium ace a e: labile subs ances (ca bona es, and o h-
e s) p esen in he coal wi h chlo hyd ic acid; he oxida i e
compounds associa ed wi h o ganic ma e a e ela ed o he dis-
solu ion o pe oxide and chlo hyd ic acid; he species de i ed om
he diges ion o e ac a y phases (sul u and o he s) wi h ni ic
acid; and finally, he subs ances de i ing om silica e diges ion
a e ela ed o he dissolu ion o fluo hyd ic acid and ni ic acid.
Tables 2 and 3 summa ise he concen a ions o he 17 ele-
men s men ioned in he aqueous ex ac s ob ained o he esh
coal and o he clinke samples. Significan di e ences in elemen
concen a ions we e demons a ed (P< 0.04) by using - es o
bo h ypes o samples; hus i is no accep able o hypo hesise ha
he wo se s o alues a e he same. This s a is ical analysis was
pe o med using he SPSS 13.0 package o Windows.
The esul s show ha spon aneous bu ning in he s udy coal
ou c op causes a significan lowe ing in he concen a ions o Na,
S , Si, Cu, Ni, and Zn, sugges ing hese elemen s a e lixi ia ed du -
ing oxida ion. These elemen s a e p ima ily associa ed wi h clay
mine als and oxidizing compounds in unal e ed coal (Bouska,
1981), bu no in clinke ma e ial. This shi indica es ha he
majo i y o hese mine als pass o an aqueous solu ion, almos en-
i ely mobilising o o m hyd osoluble species (Pone e al., 2007).
The e is also a ela i e en ichmen in o he mine als (Fe, C , Mg,
Ba, Ca, Mn, Al, K, and V) in he aqueous solu ions when compa ing
esh coal wi h clinke , indica ing ha hey a e only pa ially lixi -
ia ed du ing sel -oxida ion. Howe e , he las ou elemen s (Mn,
Al, K, and V) also inc ease se e al old in silica es and e ac o y
phases in he ans o ma ion om unwea he ed coal o clinke
ma e ial (Escoba and Ma ínez, 1993).
Finally, he e is nea ly a 50- old inc ease in he P and Ti concen-
a ions om esh coal o clinke (app oxima ely he same a io by
which he coal is educed in mass due o he spon aneous oxida-
ion) as a esul o en ichmen due o loss o o ganic ma e . In bo h
unal e ed coal and clinke , P and Ti a e p ima ily associa ed o sil-
ica es and e ac o y phases (Escoba and Ma ínez, 1993).
Fig. 2 displays he pe cen ages o o al ex ac ed om esh coal
and clinke samples o each elemen du ing he six-s ep sequen-
ial me hod. In o de o de e mine he e ec i eness o he me hod
o ex ac ing elemen s, a o al pe cen eco e y alue was calcu-
la ed. Fo each elemen , he o al concen a ions ob ained om
bulk analyses o clinke ma e ial and esh coal we e compa ed
o he o al concen a ions ex ac ed by he me hod; eco e ies
we e abou 90% and 80%, espec i ely.
4.2. Scanning elec on mic oscopy
SEM analysis in back-sca e ed elec on (BSF) mode o he
clinke ma e ial e eals a se ies o needle-like and pneuma oly ic
Table 1
P oxima e and elemen al analyses o esh coal and clinke samples. All da a
exp essed in pe cen ages (%).
Sample F esh coal Al e ed ma e ial
Ash weigh 1.9 95.2
Vola ile ma e 42.3 0.0
Fixed ca bon 54.0 2.0
Mois u e 1.8 2.8
Sul u 2.1 9.5
Ni ogen 1.9 0.1
Ca bon 82.5 2.1
Hyd ogen 5.2 0.4
Table 2
A e age Si, Ti, Na, Mg, Ca, K, C , Zn, P, Mn, V, Cu, Ni, S , Ba and Fe con en s (ppm) o aqueous ex ac s ob ained om unal e ed coal.
MilliQ Ace a e HCl HCl-H
2
O
2
HNO
3
HF-HNO
3
Si 0.00 0.00 0.00 50 ± 6 0.00 1364 ± 210
Al 121 ± 20 45 ± 7 32 ± 5 52 ± 7 280 ± 31 32 ± 6
Ti 0.00 0.00 0.70 ± 0.10 3.0 ± 0.4 0.00 61 ± 7
Na 52 ± 8 21 ± 3 38 ± 5 163 ± 22 139 ± 19 191 ± 25
Mg 281 ± 29 52 ± 6 42 ± 5 23 ± 3 104 ± 11 103 ± 10
Ca 86 ± 9 59 ± 8 106 ± 13 64 ± 8 120 ± 14 173 ± 20
K 25 ± 3 72 ± 8 144 ± 16 74 ± 9 160 ± 19 796 ± 90
C 2.0 ± 0.3 0.21 ± 0.03 0.00 0.80 ± 0.11 1.1 ± 0.2 29 ± 3
Zn 28 ± 3 6.0 ± 0.8 10 ± 1 7.0 ± 1.1 137 ± 20 33 ± 4
P 0.00 0.00 0.00 0.00 7.0 ± 1.2 42 ± 6
Mn 57 ± 6 2.0 ± 0.2 0.60 ± 0.09 0.50 ± 0.07 0.20 ± 0.03 6.0 ± 0.5
V 0.46 ± 0.07 0.34 ± 0.05 0.00 0.20 ± 0.04 0.00 40 ± 6
Cu 2.3 ± 0.3 0.83 ± 0.14 3.0 ± 0.4 10 ± ± 1 10 ± 1 6.0 ± 0.9
Ni 30.3 ± 3.8 1.2 ± 0.2 1.0 ± 0.1 1.0 ± 0.2 6.0 ± 0.6 16 ± 2
S 4.0 ± 0.5 1.0 ± 0.2 8.0 ± 1.0 15 ± 2 65 ± 6 31 ± 4
Ba 0.70 ± 0.09 1.24 ± 0.17 3.0 ± 0.4 9.0 ± 1.2 53 ± 9 52 ± 8
Fe 416 ± 42 33 ± 4 76 ± 8 223 ± 25 107 ± 12 124 ± 13
Range (±).
214 M. Ma ínez e al. / Jou nal o Sou h Ame ican Ea h Sciences 27 (2009) 211–218
s uc u es p oduced by apid agmen a ion and des uc ion o
he unal e ed ca bon s uc u e du ing sel -oxida ion (Fig. 3a).
In con as , he sample o esh coal displays a compac ex u e
in BSE mode (Fig. 3b). Bo h images show a uni o m chemical
composi ion o he wo samples. Since he sys em loses almos
all he ola ile ma e and much o he fixed ca bon du ing sel -
oxida ion, his loss o mass su ely accoun s o he ex u al
changes obse ed (Ben o e al., 1981). The end p oduc o he
spon aneous oxida ion is a clinke ma e ial wi h a p edomi-
nan ly ounded shape and possible sca cely mel ed ex u es, as
compa ed o he low po osi y ha is ypical o esh coal (Cosca
e al., 1989).
4.3. XRD analyses
The mine alogical composi ion o he clinke ma e ial and esh
coal samples has been analysed using XRD (see Fig. 4a o he e-
sul s). An ini ial di ac og am co esponding o unal e ed coal
(acqui ed using a 0.05°scan s ep and a 1 s ime s ep) did no allow
he c ys alline phases p esen o be easily dis inguished. The e o e,
a subsequen di ac og am was un wi h a 0.05°scan s ep and a
3 s ime s ep. In nei he di ac og am is i possible o dis inguish
any o he mos common c ys alline phases o unwea he ed coal
(i.e., py i e, dolomi e, calci e, and o he s).
Fu he XRD analysis ob ained di ec ly om he he mally al-
e ed ma e ial showed a ai ly complex composi ion. The majo
mine als iden ified we e: qua z (85-0794); po assium hyd ogen
sul a e (46-0496), K(HSO
4
); sodium hyd ogen phospha e (09-
0100), Na
2
H
2
(PO
3
)
4
; calci e (86-2341); and aluminum sul a e hy-
d a e (49-1096), Al
2
(SO
4
)
3
16H
2
O. Mo eo e , he ollowing c ys al-
line phases we e also obse ed as mino compounds: magne i e
(89-6466); sodium magnesium i anium oxide (37-0545), Na
0.7-
Ti
1.7
Mg
0.4
O
4
; and chabazi e-K (44-0250), KAlSiO
4
xH
2
O. The p es-
ence o chabazi e-K indica es ha he en om which he
sample was aken eached empe a u es no highe han 230 °C,
he empe a u e a which his zeoli e decomposes. The occu ence
o magne i e and se e al sul a es ha e been p e iously epo ed in
coal fi es (Sokol e al., 1998; He e n and Coa es, 2004). In ega ds
Table 3
Composi ion da a (ppm) o 17 chemical elemen s in aqueous ex ac s ob ained om clinke samples.
MilliQ Ace a e HCl HCl-H
2
O
2
HNO
3
HF-HNO
3
Si 0.00 0.00 10 ± 1 67 ± 10 0.00 165 ± 20
Al 249 ± 29 137 ± 16 134 ± 20 115 ± 14 97 ± 12 227 ± 24
Ti 178 ± 22 21.7 ± 3.4 43.7 ± 5.5 32 ± 4 8.1 ± 1.4 1841 ± 233
Na 79 ± 11 12.5 ± 2.1 9.0 ± 1.5 27 ± 3 4.8 ± 0.5 6.0 ± 1.0
Mg 1474 ± 211 74 ± 11 5.2 ± 0.6 1.2 ± 0.2 17 ± 2 468 ± 39
Ca 473 ± 41 105 ± 13 144 ± 15 49 ± 7 102 ± 11 151 ± 19
K 502 ± 45 278 ± 28 245 ± 26 371 ± 33 226 ± 22 1385 ± 170
C 14 ± 2 0.66 ± 0.08 2.0 ± 0.3 1.1 ± 0.2 0.5 ± 0.1 24 ± 3
Zn 10 ± 1 3.0 ± 0.4 4.4 ± 0.5 0.00 63 ± 8 16 ± 2
P 68 ± 8 14 ± 2 0.00 319 ± 30 57 ± 6 230 ± 25
Mn 253 ± 26 23 ± 3 0.7 ± 0.1 4.0 ± 0.5 0.3 ± 0.1 12 ± 2
V 11 ± 1 3.1 ± 0.5 2.0 ± 0.3 0.8 ± 0.1 0.3 ± 0.0 13 ± 1
Cu 0.94 ± 0.15 0.13 ± 0.02 0.11 ± 0.01 0.40 ± 0.05 4.0 ± 0.6 3.1 ± 0.5
Ni 0.5 ± 0.1 0.41 ± 0.06 0.00 0.3 ± 0.0 0.6 ± 0.1 2 ± 0
S 6.4 ± 0.7 9.5 ± 1.0 2.1 ± 0.2 0.3 ± 0.0 0.9 ± 0.1 8.0 ± 1.1
Ba 9.1 ± 1.2 19 ± 2 27 ± 3 8.0 ± 0.9 4.0 ± 0.4 57 ± 6
Fe 505 ± 43 386 ± 35 86 ± 9 61 ± 7 60 ± 6 247 ± 26
Range (±).
Fig. 2. Scanning elec on pho og aphs o esh coal and clinke ma e ial: (a) SEM
mic og aph in BSE mode o clinke ma e ial shows lens-shaped nodules, (b) BSE
image displays he mo phology o unal e ed coal.
Fig. 3. Rela i e pe cen s o o al ex ac ed om esh coal and clinke ma e ial o
indi idual elemen s eleased du ing sequen ial p ocedu e.
M. Ma ínez e al. / Jou nal o Sou h Ame ican Ea h Sciences 27 (2009) 211–218 215

o he uma olic mine aliza ions, X- ay di ac ion da a show he
ollowing mine alogical composi ion (Fig. 4b): qua z (86-1560);
po assium ammonium aluminum sul a e hyd a e (48-0243),
K
0.5
(NH
4
)
0.5
Al(SO
4
)
2
H
2
O; sul u (78-1889); K- eldspa (79-0349),
H
0.9
K
0.1
(AlSi
3
O
8
); magnesium calci e (89-1306), Ca
0.9
Mg
0.1
CO
3
; sal-
ammoniac (07-0007), NH
4
Cl; mascagni e (76-0579), (NH
4
)
2
SO
4
;
and, finally, ammonium hiosul a e (31-0067), (NH
4
)
2
S
2
O
3
.
The dominan mine als de i ed om smolde ing coal beds in
he Ce o Pelado Fo ma ion a e simila o phases de ec ed a he
p incipal bu ning coal seams in Pennsyl ania (USA) and Gau eng
(Sou h A ica), among o he s (S ache , 1995; Pone e al., 2007).
Pa icula ly wo hy o no e is he impo an p esence o sul u -
bea ing mine als and ammonium condensa ion by-p oduc s a
he gas en s in he s udy ou c op o coal; his mine al assemblage
sugges s he mobiliza ion o di e en componen s o he coal ma-
ix, which we e ecombined wi h sul a e and o he ions in he
sys em.
I is e ealed (Sokol e al., 1998) ha salammoniac, di e en
eldspa s, magnesium phases, ee sul u and mascagni e a e some
widesp ead ma e ial- o ming mine als o p oduc s associa ed wi h
oxida ion o coal. The p esence o salammoniac occu s as a subli-
ma ion p oduc o gas je s and may, in pa , be dissol ed in o he
su ace wa e (Pone e al., 2007). Elemen al sul u and o he sul a e
compounds a e also in e p e ed o ha e been sou ced om gas-
eous exhala ions associa ed wi h al e a ion o coal; hey all ul i-
ma ely de i e om sul u oxides and a e o med by in e ac ion
o gases exhaled om en s wi h su ace wa e , and su ounding
ocks (S ache , 1995).
The h ee ypes (Calkins, 1994) in o which sul u in coal can be
di ided – py i ic sul u , o ganic sul u and sul u in sul a e mine -
als – unde go a ans o ma ion du ing coal oxida ion o p oduce
sulfi e and, hence, sul a e and hiosul a e (Mills e al., 1999). Mo e-
o e , mascagni e and ammonium hiosul a e could be de i ed om
eac ion be ween ammonium ions associa ed wi h salammoniac
and sul u -con en ions in he aqueous phase. Specifically, ammo-
nium hiosul a e could well ha e o med by he selec i e oxida ion
o he sul u -con en ions, a ou ed by ce ain me al oxides (Cho
e al., 2002). None heless, in c ys alline o m, ammonium hiosul-
a e is only s able a ce ain empe a u es, a neu al o sligh ly
alkaline pH, and low Eh alues; i decomposes by oxida ion o p o-
duce in e media e p oduc s comp ising me a-s able sul u com-
pounds such as e a hiona es o i hiona es and end p oduc s
consis ing o elemen al sul u and ammonium sul a e (Wan, 1997):
4S
2
O
2
3
þO
2
¼4SO
2
4
þ4S
In addi ion, a g ea e amoun o ammoniacal subs ances inhibi s he
decomposi ion o ammonium hiosul a e (Aylmo e and Mui , 2001),
whe eas a mosphe ic oxygen a ou s he decomposi ion o he
hiosul a e ion (Li and Mille , 1996):
5S
2
O
2
3
þ2S
3
O
2
6
þ10NH
4
þO
2
¼10NH
3
þ5H
2
Oþ4S
4
O
2
6
Finally, ammonium hiosul a e and po assium ammonium alumi-
num sul a e hyd a e canno be conside ed as alid mine als since
he combus ion p oduc s o coal beds a e no gene ally ega ded
as mine al species (CNMMN, 1998). In addi ion, nei he ammonium
hiosul a e no po assium ammonium aluminum sul a e hyd a e
a e ac ually o med as mine als by o he geological p ocesses. How-
e e , bo h compounds ha e been epo ed as syn he ic c ys alline
phases (Ele man e al.,1978; Smi h, 1979; T oemel, 1997).
4.4. FTIR analysis
Ammonium hiosul a e was iden ified in a uma olic mine ali-
za ion sample using he XRD powde echnique, and i s IR abso p-
ion spec um was analysed (Fig. 5). Se e al abso p ion bands we e
ela ed o di e en compounds in ag eemen wi h e e ence da a
in he li e a u e (P e sch e al., 2000). The in a ed spec um o
he analysed sample was use ul in iden i ying ammonium sul a e,
which shows abso p ion bands a 1088, 670, 614, and 464 cm
1
,
cha ac e is ic o he sul a e and hiosul a e g oups ( ib a ional
modes o sul u -oxygen bonds).
In pa icula , one band o e 3000 cm
1
(3207 cm
1
) can be e-
la ed o he ammonium ion (N–H s e ching ib a ions). The S–S
cha ac e is ic abso p ion was also ound o he sample wi h a
peak a 532 cm
1
; his abso p ion can be due o a mode co e-
sponding o hiosul a e g oup. The significan band a ound
1400 cm
1
can be assigned o RO–SO
2
–SR de o ma ions; hence,
his band indica es he p esence o ammonium hiosul a e. Wi h
calcium ca bona e, he posi ion o he 1675 cm
1
band could co -
espond o a ib a ional mode o ca bon-oxygen double bonds
belonging o he ca bona e g oup. Finally, se e al dis inguishable
bands we e also obse ed (979 and 798 cm
1
). These abso p ion
bands and o he o e lapping ones (1087 and 464 cm
1
) a e due
o silicon-oxygen bond s e ching ib a ions ela ed o qua z.
Fig. 4. (a) X- ay di ac ion pa e ns showing he peaks co esponding o clinke
ma e ial; (b) XRD da a o he mine aliza ions, acqui ed applying a 0.05°s ep scan
wi h a 1 s s ep ime. No e: Q z, qua z; Als, aluminum sul a e hyd a e; Cal, calci e;
Shp, sodium hyd ogen phospha e; Mgn, magne i e; Phs, po assium hyd ogen
sul a e; Sm , sodium magnesium i anium oxide; Chk, chabazi e-K; Sal, salammo-
niac; Sul, sul u ; Mas, mascagni e; San, sanidine; A h, ammonium hiosul a e; Mca,
magnesium calci e; Pas, po assium ammonium aluminum sul a e hyd a e.
216 M. Ma ínez e al. / Jou nal o Sou h Ame ican Ea h Sciences 27 (2009) 211–218
Rega ding he es o he uma olic mine aliza ion samples, no con-
side able changes in FTIR spec a we e obse ed.
4.5. Gas ch oma og aphic analyses
A e age alues o all alipha ic hyd oca bons, e hene and he
main a oma ic compounds analysed om gas samples a e shown
in Table 4. O he analyses o gas samples e ealed he p esence
o significan concen a ions o me hane (1345 ppm), ca bon mon-
oxide (480 ppm) and ca bon dioxide (23,945 ppm). Finally, em-
pe a u es in he ange om 180 °C o 210 °C we e measu ed in
gases escaping om en s.
The esul s o g eenhouse and o he gases indica e a he low
concen a ions when compa ed o hose obse ed in gaseous e flu-
en s emi ed om he Wi bank coalfield, Sou h A ica (Pone e al.,
2007). Benzene and oluene we e he majo VOCs emi ed om
smolde ing coal beds in he Ce o Pelado Fo ma ion; bo h a oma ic
compound con ibu ion was app oxima ely 95% o o e all VOCs
emi ed. In con as , he concen a ions o alipha ic hyd oca bons
we e mo e egula han ha o a oma ic compounds; e hane con-
ibu ion was 65% o he o e all alipha ic hyd oca bons, ollowed
by p opane (13%) and e hene (8%). Finally, i was es ima ed ha
o oxida ion o coal in ‘‘La Cues a” mine, app oxima ely 1.8 onnes
o ca bon dioxide pe onne o coal was p oduced.
5. Conclusions
The in eg a ion o he geochemical, mine alogical, and ex u al
esul s e eal ha he sel -oxida ion o coal is qui e a complex geo-
chemical p ocess. Since he sys em is open, gene ally immobile
elemen s such as i anium and aluminum lixi ia e a low pH, and
elemen s such as phospho ous and ni ogen a e concen a ed. This
p ocess eleases con amina ing me als in o he en i onmen , such
as C , Cu, Ni, and Zn, as well as he elemen s Mg, K, Ca, and Na.
The chemical analyses o he di e en ex ac s ob ained by he
sequen ial a acks on clinke and coal samples has e ealed h ee
basic g oupings o he elemen s s udied, based on hei geochem-
ical pa e ns du ing sel -oxida ion: (1) hose ha become qui e
mobile (Na, S , C , Ni, e c.); (2) hose ha unde go pa ial en ich-
men (Mg, Fe, V, K, e c); and (3) hose ha emain nea ly immobile
(Ti, P).
Spon aneous oxida ion a ou s a ious c ys alline phases as
well, mainly sul a es, hiosul a es, phospha es, elemen al sul u ,
and hyd a e aluminosilica es. The p ocess also causes he coal o
change om a compac ex u e o a ounded, much mo e po ous
ex u e. Consequen ly, he fi s na u al occu ence o ammonium
hiosul a e, a non-mine al c ys alline phase p e iously ob ained
only syn he ically, which decomposes a ela i ely low empe a-
u es, can be explained by ci cums ances on he sel -hea ing o
he coal such as he en i onmen al condi ions, he high sul u con-
en and he inhibi o y e ec o la ge amoun s o ammoniacal
p oduc s.
Gas eadings in se e al emissions om en s showed significan
a e age con en s o g eenhouse gases, BTX and alipha ic
hyd oca bons.
Acknowledgmen s
We a e g a e ul o CITIUS Cen e o he expe imen al analyses.
We also hank Ch is ine Lau in o e ising he English in he ex .
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Concen a ion (ppm)
E hane 87.6
P opane 16.4
i-Bu ane 1.5
n-Bu ane 3.8
i-Pen ane 0.9
n-Pen ane 1.7
n-Hexane 1.0
n-Hep ane 0.6
n-Oc ane 0.3
E hene 10.9
Benzene 5.0
Toluene 3.9
m/p-xylene 0.2
o-xylene 0.2
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