Fuel 89 (2010) 2126–2132

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Fuel

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Distribution of inorganic and organic substances in the hydrocyclone separatedSlovak sub-bituminous coal

Anton Zubrik a,*, Slavomír Hredzák a, Ludmila Turcániová a, Michal Lovás a, Ingo Bergmann b,Klaus Dieter Becker c, Mária Lukcová a, Vladimír Šepelák a,c

a Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, SK-043 53 Košice, Slovakiab VOLKSWAGEN AG, 38436 Wolfsburg, Germanyc Institute of Nanotechnology, Karlsruhe Institute of Technology, Herrmann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 November 2008Received in revised form 1 March 2010Accepted 3 March 2010Available online 16 March 2010

Keywords:Slovak sub-bituminous coalHydrocycloneFe-bearing mineralsDiterpanesHumic acids

0016-2361/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.fuel.2010.03.010

* Corresponding author. Tel.: +421 55 7922601; faxE-mail address: zubant@saske.sk (A. Zubrik).

A low-rank Slovak sub-bituminous coal from the Handlová deposit was physically treated by washing ina water-only cyclone with the goal to find the separation effect for inorganic (mainly Fe-bearing miner-als) and organic substances (humic acids, diterpanes). A high-quality coal product with the ash content inthe dry matter of 9.02% and carbon content of Cd = 68.12% at a mass yield of 29.51% was obtained usingthe water-only cyclone processing. At first, the physically treated coal samples were detailed character-ized by XRD, 57Fe Mössbauer spectroscopy, FT-IR and HR-TEM. In addition to non-crystalline organic coalcomponents, inorganic compounds belonging to silicate minerals (kaolinite, muscovite and quartz) aswell as to Fe-bearing sulphide minerals (pyrite) were identified in the sub-bituminous coal by XRD.57Fe Mössbauer spectroscopy detected the presence of iron carbonate (siderite), iron-containing claymineral and two sulphur-containing minerals (pyrite, jarosite) in the untreated coal. On the other hand,only one Fe-bearing mineral, (pyrite) was found in the washed coal. Effect of the physical separation isalso demonstrated in FT-IR spectra, where the peak at 1040 cm�1 representing the silicate componentin the untreated sample is not detectable in the washed coal sample. Presence of extractive organic sub-stances, i.e. humic acids and tetracyclic diterpane (16a(H)-phyllocladane), in the hydrocyclone productsis also evidenced. It was confirmed that the isolated diterpenoic compound is attendant in the washedproduct with the lowest ash content and it is assimilated with the organic part of coal. Surprisingly,humic acids were found in the highest concentration in the slurry that has the highest content of ash(63.14%).

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1. Introduction

Coal continues to be one of the major fossil fuel for generatingelectricity all over the world. Moreover, coal contains the largeamount of organic substances, which are valuable for industrialproduction and utilization. Quality of coal plays an increasinglysignificant role in global use in clean coal technologies [1–3]. Theoccurrence of mineral impurities and the presence of sulphur incoal are indicators of several industrial and environmental prob-lems [4–6] related with mining, preparation, purification and com-bustion of coal. Sulphur in coal is represented by both organic(fixed to the organic matrix) and inorganic form [4,7,8]. The mainquantity of inorganic sulphur is bonded via Fe-bearing sulphidesand sulphates (mainly – pyrite, marcasite, melanterite, jarosite,szomolnokite) [4,9,10]. The other Fe-bearing minerals belong tothe group of clay minerals (represented by kaolinite, illite and

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montmorillonite) and carbonates (calcite, dolomite, ankerite andsiderite). Finally, quartz, opal and chalcedone are also consideredas significant inorganic impurities in coal [11–16]. In order to im-prove the quality of coal, especially low-rank coals with a highamount of ash, the inorganic impurities should be removed usinga cleaning process [4,17], which is the primary step followed bythe coal combustion, conversion, mechanical activation and/orextraction of biologically active rare organic substances (tetracyclicditerpanes, humic acids) [18,19]. Several procedures of physicaltreatments have been used to beneficiate the coal. One of the mostconvenient procedures is the coal washing in a water-only cyclone(WOC) as it is referred in [20–22] for the case of Slovak sub-bitu-minous coal. Excepting the ecological aspect it is well known thatthe Fe-bearing minerals have beneficial effect as catalysts for thegasification, liquefaction [23,24] and conversion of coal [25]. Fur-thermore, from the viewpoint of mechanical activation of carbonmaterials, it has been reported that the presence of small fraction(<1 wt.%) of iron stabilizes the carbon nanocrystallites formed dur-ing milling [26,27].

A. Zubrik et al. / Fuel 89 (2010) 2126–2132 2127

Several techniques, e.g., Fourier transform infrared spectros-copy (FT-IR), X-ray diffraction (XRD), scanning electron microscopywith the energy dispersive X-ray spectroscopy (SEM-EDX), differ-ential thermal analysis/thermal gravimetry (DTA/TG) andMössbauer spectroscopy, have been applied to identify and quan-tify the minerals in the coal obtained after physical, chemical andbiological treatments [10,28–31]. The individual advantages andlimitations of methods used for characterization of coal mineralmatters are summarised elsewhere [13,17,32]. 57Fe Mössbauerspectroscopy is one of the mostly used and the mostly precisemethod for the detection of iron-containing minerals in coal. Thistechnique can detect iron compounds with concentrations as lowas 1% [10] and also small phase changes caused by the coal pro-cessing, e.g., microwave heating, oxidation and conversionprocesses.

To the best of our knowledge, information on the effect of phys-ical treatment of coal on the extraction of humic acids and diter-pane substances as well as the detail characterization of Fe-bearing minerals in the products of hydrocyclone separation, ismissing in the literature. The main goal of this work was to findthe separation effect of physical treatment of coal to inorganicand organic compounds in particular: (i) to identify the inorganicphases, mainly Fe-bearing minerals in the Slovak sub-bituminouscoal and in the products of its washing by means of Mössbauerspectroscopy, XRD and FT-IR spectroscopy, (ii) to extract the rareorganic substances from the coal, and (iii) to investigate the distri-bution of diterpanes and humic acids in the physically treated coalproducts.

2. Materials and methods

2.1. Coal washing in WOC and analysis of products

A sub-bituminous (brown) coal from Handlová Colliery (centralpart of Slovakia) was washed in the WOC. During the experiments,the WOC was installed in bench scale hydrocyclone station (Fig. 1)[33]. The WOC is characterized by the following parameters: theinner diameter of cylindrical part: 150 mm; the height of this part:200 mm; the diameter of inlet pipe 45 mm. The vortex finder andapex diameters were 68 and 14.6 mm, respectively. The cone con-

Fig. 1. Scheme of bench scale hydrocyclone station [33]. 1 – Hydrocyclone, 2 –agitating tank, 3 – pump, 4 – valve of reverse circuit, 5 – valve for regulation ofinput press, 6 – pressure gauge, 7 – hydrocyclone input, 8 – heavy product –underflow, 9 – light product – overflow, and 10 – circulating charge.

sisting of three angles (135�, 75� and 20�) was applied. The exper-iments have been carried out at the input pressure of 10 kPa. Theconcentration of solids was 100 g/l. The basic products of washingwere collected using sieves with a mesh size of 0.5 mm. Finally, cir-culating slurry was collected and included into the total materialbalance. The basic coal sample – feed (F), the obtained treatmentproducts, i.e. washed coal – overflow (O), tailings – underflow(U) and circulating slurry (S), were dried, ground below 0.1 mmand characterized by standard analytical methods. Elemental anal-ysis for carbon, hydrogen, and nitrogen was realized using elemen-tary analyzer Carlo Erba Model 1106 equipped with themicroprocessor and recorder. Helium (purity 99.998%) was usedas the carrier gas; oxygen (99.999%) was used as the oxidativeagent and argon created an inert atmosphere during measurement.The chromatographic column was filled with the Porapak QS withgranularity 80–100 mesh. The cyclohexanone – dinitro-phenylhyd-razone (content of N = 20.14%, C = 51.79% and H = 5.07%) was usedas the standard material. Net heating value was measured accord-ing to [34]. Ash content was determined by ignition in muffle ovenat 815 �C to constant value of weight (after 3 h) [35]. The analysisof ash was performed as follows. The SiO2 content was assayedgravimetrically. Other elements have been determined by atomicabsorption spectroscopy using the device VARIAN with accesso-ries: Fast Sequential AAS AA240FS, Zeeman AAS AA240Z with Pro-grammable Sample Dispenser PSD120, Graphite Tube AtomizerGTA120 and Vapor Generation Accessory VGA-77. Total sulphurand its forms were analyzed using ESCHKA method [36] and theprocedure described in [37], respectively.

2.2. Mössbauer spectroscopy

The Mössbauer spectra were taken in transmission geometryusing a 57Co/Rh c-ray source at room-temperature. The mass of ab-sorber (powdered coal) was 200 mg. The velocity scale was cali-brated relative to 57Fe in Rh. Recoil spectral analysis software[38] was used for the quantitative evaluation of the Mössbauerspectra. The Voigt-based fitting method provided distribution ofhyperfine parameters for multiple sites in a spectrum. Lorentzianline half width at half maximum was 0.156 mm/s.

2.3. Infrared spectroscopy

FT-IR spectroscopy was used for characterization of organicstructure and identification of inorganic substances in the coal.The measurements were performed using an FT-IR spectrometerEQUINOX 55, Brucker Optik (absorbance range: 4000–400 cm�1).One milligram of the coal sample was homogenized with drypotassium bromide (299 mg) and continuously pressed to createpellets.

2.4. X-ray diffraction

XRD patterns were collected using a PHILIPS PW-1820 diffrac-tometer with Cu Ka radiation. The XRD data were analyzed usinga computer program STOE WinXPOW utilizing the JCPDS PDF data-base [39].

2.5. Transmission electron microscopy

The morphology of coal was studied using a combined field-emission (scanning) transmission electron microscope (S)TEM ofthe type Jeol JEM-2100F. Prior to high-resolution transmissionelectron microscopy (HR-TEM) investigations, powders werecrushed in a mortar, dispersed in ethanol, and fixed on a coppersupported carbon film.

Table 1Results of chemical analysis of Handlová sub-bituminous coal and of hydrocycloneseparated products.

Characteristics/WOC product Overflow Underflow Slurry Feed

Mass yield (%) 29.50 35.30 35.20 100.00Ad (%) 9.01 21.04 63.14 36.80cmd (%) 90.99 78.96 36.86 63.20

Qdi (MJ/kg) 26.03 21.93 8.41 17.56

Wa (%) 7.55 7.59 7.63 9.02Vd (%) 30.07 25.81 12.48 18.17Cd (%)) 68.12 59.44 23.92 46.11Hd (%) 4.49 4.43 1.93 3.04Nd (%) 0.62 0.60 0.30 0.32Od (%) 16.03 12.14 9.52 11.78SiO2 (%) 4.14 10.80 36.80 20.42Al (%) 1.01 2.24 7.28 4.14

2128 A. Zubrik et al. / Fuel 89 (2010) 2126–2132

2.6. Extraction of humic acids

The Grinding Aqueous Caustic Leaching (GACL) procedure wasused for the extraction of humic acids (HA). The coal samples (F,O, U, S) were subjected to mechanochemical activation under thefollowing conditions: mass of the grinded coal: 20 g, input granu-larity of coal: under 0.5 mm; volume of leaching agent: 200 ml,concentration of leaching agent: 1% NaOH, leaching temperature:60 �C, grinding period: 60 min, revolutions: 400/min.

Mechanochemical wet grinding of coal was performed in a Mol-inex attritor (type 075, Netzch, Germany) using a grinding cham-ber with the volume of 500 ml and the grinding balls (2 mm indiameter; balls made of glass) with the total weight of 685 g. Afterthe grinding tests, the solid phase was separated from the liquidproduct in the centrifuge (JANETZKI T23, Leipzig, Germany). Afterthorough washing of the leached coal extraction in distilled waterand its drying, the humic acids were determined by the standardmethod [40].

2.7. Isolation of tetracyclic diterpane

All WOC products (O, U, S) and untreated coal sample (F) (20 g)were extracted by microwave-assisted extraction with dichloro-methane (250 ml) in microwave oven (Wirlpool AVM 434,500 W, 2.45 GHz) in a distilling flask with the reversing reflux sys-tem (according to [41]) at boiling point (20 min). After the extrac-tion, the mixture was filtered and the solvent was evaporated ona rotary evaporator. The crude extracts were re-extracted on thesolid phase extraction (SPE) columns (Chromabond SiOH – 6 ml/500 mg, Macharey – Nagel GmbH + Co., KG) with n-hexane(100 ml). The solvent was evaporated and the purified extractwas further separated by column chromatography on silica gel(5 g of Kieselgel 60, granularity 0.06–0.2 mm, Carl RothGmbH + Co., KG) with 4 � 20 ml of hexane to four fractions denom-inated as F1–F4. All fractions were analyzed by thin layer chroma-tography (TLC) and gas chromatography/mass spectrometry (GC/MS). Tetracyclic diterpene is distributed in the F1 fraction as anon-polar, saturated hydrocarbon. TLC analysis of F1 showed pres-ence of a single spot with RF = 0.91, that contained four com-pounds: the most abundant component (82.5%) is a tetracyclicditerpene – 16a(H)-phyllocladane; than isopimarane (11.4%); 18-norisopimarane (4.0%) and finally abietane (2.1%). This spot wasnot observed in other fractions (F2–F4). Unambiguous identifica-tion as 16a(H)-phyllocladane was achieved by coinjection of thesample and standards in the laboratory of B.R.T. Simoneit. The de-tails of isolation and structure elucidation of 16a(H)-phyllocladane(Fig. 2) were published previously [42,43]. The final isolate (F1)containing 82.5% of 16a(H)-phyllocladane was evaluated.

2.8. Gas chromatography/mass spectrometry

GC/MS analyses were performed at the Institute of OrganicChemistry and Biochemistry v.v.i., Academy of Sciences of theCzech Republic using a 6890 N gas chromatograph (Agilent, USA)

Fig. 2. The structure of 16a(H)-phyllocladane (according to [42,43]).

coupled to a 5975B quadrupole mass spectrometer. A fused silicacapillary column HP-5MS (30 m � 0.25 mm, 0.25 lm, Agilent,USA) was used for separations. The carrier gas was helium at1 ml/min. The injector was operated in the splitless mode at230 �C. The temperature program used was: 45 �C (2 min), then15 �C/min to 150 �C, then 4 �C/min to 320 �C (5 min). Standard70 eV mass spectra were recorded in the range of 25–800 Da, anda solvent delay of 4 min was used. Temperatures of the ion sourceand quadrupole were 230 and 150 �C, respectively. The ion opticswas tuned and calibrated with the automatic standard spectra tuneprocedure using perfluorotributylamine.

3. Results and discussion

3.1. Separation and characterization of inorganic components

The basic proximate and ultimate analyses of Handlová sub-bituminous coal and its washed WOC products are listed in Table1. Recoveries (see Table 2) of selected components into productsof washing were calculated on the basis of mass yields and chem-ical analysis. The washed coal (O) contained lower amount of ash(Ad = 9.01%) in comparison to the untreated sample (Ad = 36.80%)and it points to a good washability in the WOC process. The de-crease of the content of iron and clay formed components (SiO2,Al) in the washed coal is observed. Chemical analysis also demon-strates that the amount of inorganic sulphur in the coal after wash-ing decreased, but the organically bonded sulphur is fixed inorganic coal matrix. An amount of arsenic for all samples of theHandlová sub-bituminous coal is relatively low (for example, theamount of arsenic in the Nováky lignite from the same deposit isabout 600–800 ppm [2,22]). In the washed coal (O), the contentof carbon, which reflects the quality of coal, was much higher(Cd = 68.12%) comparing to raw coal – F (Cd = 46.11%).

3.2. Identification of Fe-bearing minerals in the hydrocyclone products

From the XRD analysis of the F, S and U samples (Fig. 3), inorganicphases belonging to silicate minerals group, namely phyllosilicates

Ca (%) 0.31 0.46 0.66 0.48Mg (%) 0.12 0.24 0.87 0.47Fe – total (%) 0.59 1.25 1.94 1.49Fe2+ (%) 0.16 0.93 1.68 0.95S – sulphide (%) 0.75 1.57 0.78 1.21S – sulphate (%) 0.03 0.04 0.03 0.11S – organic (%) 0.95 0.74 0.38 0.63S – total (%) 1.73 2.35 1.19 1.95As (ppm) 66.00 73.00 40.00 62.00

A – ash, cm – combustible matter, Qi – net calorific value, V – volatile matter,W – moisture.Superscripts: d – dry basis, a – analytical specimen, Od – by difference – accordingto 100 – (Ad + Cd + Hd + Nd + Sd

Total).

Table 2Recovery of selected components into products of washing.

Component Recovery (%)

Overflow Underflow Slurry Feed

Ad 8.22 22.94 68.84 100.00cmd 39.68 41.13 19.20 100.00SiO2 6.80 21.20 72.00 100.00Al 8.16 21.61 70.23 100.00Ca 18.81 33.34 47.84 100.00Mg 8.30 19.83 71.87 100.00Fe – total 13.41 33.93 52.66 100.00Fe2+ 4.76 33.83 61.41 100.00S – sulphide 21.09 52.72 26.19 100.00S – sulphate 26.41 42.05 31.54 100.00S – organic 41.63 38.57 19.81 100.00S – total 29.02 47.17 23.82 100.00As 32.84 43.39 23.77 100.00

10 20 30 40 50 60 70 80

P

P PP PP

P

(M)K

K

M Q

Q

QQQM

MK KKMK

(M)

PP P P

(F)

(O)

(U)

(S)

Cou

nts

2 Theta (deg)

Fig. 3. XRD patterns of Slovak sub-bituminous coal and its WOC products.M – muscovite-3T, Q – quartz, P – pyrite, K – kaolinite.

A. Zubrik et al. / Fuel 89 (2010) 2126–2132 2129

muscovite-3T (mica group), kaolinite (clay mineral) and quartzwere identified to be present in the coal. Fe-bearing mineral pyritewas also detected by this technique in all studied samples.

It should be mentioned that especially in the XRD pattern of thewashed coal (O), in addition to the sharp diffraction peaks of pyrite

Fig. 4. HR-TEM image of washed coal presenting the non-crystalline nature of coal.

there is a broad diffraction ‘‘hallo” in the 2Theta-range of 15–30�.This should be assigned to non-crystalline compounds in the coalthat are visible by HR-TEM (Fig. 4).

The detailed study of iron-containing compounds encounteredin the Slovak sub-bituminous coal was performed by 57FeMössbauer spectroscopy. The room-temperature Mössbauer spec-tra of the raw coal sample (F) and of the WOC products (O, U, S)are presented in Fig. 5. The hyperfine parameters and the relativeintensities of the spectral components are shown in Table 3.Mössbauer spectrum of raw coal consists of four different doubletscharacteristic of paramagnetic state of the coal structure. The mainiron-containing phase in the sample is pyrite with the parametersof IS = 0.19 mm/s, QS = 0.58 mm/s and I = 74.02%. Note that marca-site has the same chemical formula as pyrite, i.e. FeS2, but it pos-sesses different crystalline structure; pyrite is cubic andmarcasite is orthorhombic [44]. Since the values of quadrupolesplitting of pyrite and marcasite are different (QSpyrite � 0.6 mm/s, QSmarcasite � 0.5 mm/s), Mössbauer spectroscopy can differenti-ate between these phases [45]. Thus, only pyrite is occurring inthe raw coal sample. The next Fe-bearing mineral in the coal is rep-resented by sulphate with the hydroxyl group, i.e. jarosite, whichcontains Fe3+ ions (IS = 0.27 mm/s, QS = 1.16 mm/s, I = 9.88%). Thissulphate can be associated with the weathering process of sulp-hides in coal, and its possible origin is the oxidation of pyrite[46,9] (pyrite is oxidized to iron sulphates and oxy-hydroxides dur-ing the coal stockpiling). There are also non-sulphur minerals inthe coal sample, namely, siderite (IS = 1.11 mm/s, QS = 1.83 mm/s, I = 10.19%) and a clay mineral (IS = 0.97 mm/s; QS = 2.86 mm/s,I = 5.91%), which naturally occur in coals. Based on both, the valuesof hyperfine parameters of muscovite-3T [47] and on the fact thatthis mineral was detected in the coal also by XRD, it should be sta-ted that this component is also present in the sample. Since theother non-sulphur minerals, mainly clay minerals like illite andkaolinite (also detected by XRD, see Fig. 3) have similar hyperfineparameters (IS = 0.9–1.2 mm/s; QS = 2.6–2.9 mm/s) [10,47]; theirsub-spectra are superimposed. We suppose that illite is presentin the coal, because it was identified during the previous experi-ments [48].

The Mössbauer spectrum of washed coal (O) shows only onespectral component corresponding to inorganic iron phase – pyrite.We expect that pyrite is fine distributed and intergrown in coal or-ganic matrix. In general, the physical treatment of coal with theaim to remove fine intergrown impurities like pyrite is very diffi-cult, because the release of such fine grains from the coal material

-3 -2 -1 0 1 2 3

Clay mineral

JarositeSiderite

Pyrite

95

95

95

95

100

100

100

100

(U)

(S)

(F)

(O)

Tran

smis

sion

(%)

v (mm/s)

Fig. 5. Mössbauer spectra of Slovak sub-bituminous coal (F), washed coal –overflow (O), middlings – underflow (U) and slurry (S).

Table 3Mössbauer parameters of Slovak sub-bituminous coal and WOC products.

WOC product IS (mm/s) QS (mm/s) I (%) Mineral

Feed 0.19 0.58 74.02 Pyrite1.11 1.83 10.19 Siderite0.27 1.16 9.88 Jarosite0.97 2.86 5.91 Clay mineral

Overflow 0.19 0.61 100.00 Pyrite

Underflow 0.19 0.60 84.90 Pyrite1.08 1.80 7.60 Siderite0.30 1.10 7.40 Jarosite

Slurry 0.20 0.57 68.70 Pyrite1.10 1.79 11.36 Siderite0.31 1.13 9.10 Jarosite0.98 2.83 10.80 Clay mineral

2130 A. Zubrik et al. / Fuel 89 (2010) 2126–2132

needs ultrafine grinding and it is economically unacceptable forapplication under operation conditions. It means that in our casenot only organic sulphur but also pyritic sulphur cannot be entirelyremoved by WOC. Minerals such as illite, ankerite or ferrous sul-phate do not always appear in spectra of float (O) and sink (U) sep-arated coals, despite the fact that these minerals occur in the rawcoals [10]. These mineral phases could be also associated withthe organic matter and they are not liberated in the comminutionprocess [49]. On the other hand, it is evident from Table 2 thatapproximately 92% of ash and 71% of total sulphur have been re-moved from the raw coal by washing. Note that the product ob-tained at underflow of WOC can be considered as the middlings,because of its relatively low ash content (21.04%) (Table 1).

4000 3500 3000 2500 2000 1500 1000 5001.0

0.8

0.6

0.4

0.2

0.0

538 cm-1

1040 cm-1

(F)

Abs

orba

nce

Wavenumber (cm-1)

4000 3500 3000 2500 2000 1500 1000 500

1.0

0.8

0.6

0.4

0.2

0.0

3400-2400 cm-1

-OH

C-O-R1300-1100 cm-1

C=0, C=C1720-1500 cm-1

KBr HAr

900-700 cm-1

Hal

2850 cm-12920 cm-1

(O)

Abs

orba

nce

Wavenumber (cm-1)

Fig. 6. FT-IR spectra of untreated Slovak sub-bituminous coal (F) and its WOCproduct (O).

Fig. 6 displays FT-IR spectra of both untreated sub-bituminouscoal (F) and washed coal (O). FT-IR spectra for tailings – underflowand slurry – are very similar to the spectrum of the sample of un-treated coal and therefore they are not shown here. Strong absorp-tion is observed at 1040 cm�1 for the raw coal sample caused bythe presence of clay minerals and quartz. In the washed coal, thisabsorption band is not detected. This is in accordance with the re-sults of XRD and Mössbauer analyses. This absorption is not pres-ent in the washed coal (product O). The other absorption bands forproducts of O and F can be allocated to organic groups; the strongabsorption in the broad spectral range up to 2400 cm�1 originatesmainly from vibration of hydroxyl functional groups of carboxylacids and water. The peaks at 2920 and 2850 cm�1 can be assignedto the existence of aliphatic structure in the coal. Between 1720and 1500 cm�1 there are several vibration frequencies of oxygenin the C@O functional group for aliphatic and aromatic aldehydes,carboxyl acids, esters and ketones. In the frequency range from1660 to 1600 cm�1, there are C@C bonds of alkenes. In the intervalfrom 1600 to 1475 cm�1, there are vibration of C = CAr (in aromat-ics). Moreover, vibrations of hydrogen in the aromatic compoundsare found in the range from 700 to 900 cm�1.

3.3. Extraction of humic acids from WOC products

Table 4 presents the total content of humic acids in the WOCproduct after and before GACL procedure. The highest concentra-tion of HA in the non-leached samples is detected in the slurry(S) with the highest content of ash, i.e. 63.14%. Other WOC prod-ucts (O, U) and feed have the lover content of humic acids com-pared to slurry.

The GACL treatment led to an increase of the humic acids con-tent, which is the consequence of the destruction of coal matrixand changes in the chemical structure in the treated coal [2,50].The greatest effect of mechanochemical activation is reached inthe case of overflow (O) and feed (F). For these samples, duringthe mechanical activation (by GACL) humic acids content increases1.31 times. The accumulation of humic acids is not observed in therest of WOC samples (S, U).

The concentration of HA in Handlová sub-bituminous coal isrelative low in comparison to the other Slovak coals (mainly lig-nites) [2] where the accumulation effect of humic acids by GACLwas significant. For example, the total humic acids content inNováky coal (central Slovakia) increased from 29.3% in untreatedsample to 40.3% ion the sample treated by GACL process for90 min (the increase is 1.38 times) [50]. Our results are similarfrom the viewpoint of increase by GACL, although the content ofHA in untreated Novaky and Handlová sub-bituminous coal arevery different (29.3% and 7.2%, respectively). It was also confirmedthat GACL has positive effect on the desulphuration and dearseni-fication of coal. Dearsenification up to nearly 96% recovery was de-tected in the GACL-treated samples of Nováky coal [50].

Table 4Results of the extraction of humic acids by GACL process carried out with 20 g of coal.

cHA (%)a mEXT (g)b mHA (g)c YHA (%)d

WOC productOverflow 9.2 18.9 2.4 12.0Underflow 7.1 15.8 1.3 6.5Slurry 15.3 10.3 3.0 15.0Feed 7.2 14.4 1.9 9.5

a Content of humic acids before the GACL process.b Mass of crude GACL extract.c Mass of humic acids after the GACL process.d Overall yield of humic acids after the GACL process related to the original

weight of coal: YHA = mHA � 100/mcoal.

Table 5Results of microwave extraction of diterpane fraction carried out with 20 g of coal.

mEXT (mg)a mF1 (mg)b YF1 (%)c

WOC productOverflow 366.5 36.7 0.184Underflow 329.8 24.7 0.124Slurry 272.7 19.8 0.099Feed 295.0 24.4 0.122

a Mass of crude extract.b Mass of diterpene fraction F1.c Overall yield of diterpenes related to the original weight of coal:

YF1 ¼ mF1 � 100=mcoal .

A. Zubrik et al. / Fuel 89 (2010) 2126–2132 2131

It was found that also dry mechanical activation of coal causesan increase of HA [19,51]. According to [51], the HA increase in theinert atmosphere is 15–25%, and in the air, only 10% increase of HAwas observed. Moreover, the field tests showed that humic acidsextracted from the mechanically treated coal samples possesshigher physiological activity [51]. It should be noted that the yieldof HA and generally the change in the content of organic com-pounds in the coal during the physical, chemical and mechanicaltreatments depends on the nature and genesis of initial coal sam-ple [2,18,43].

3.4. Presence of diterpanes in physically treated coal

This part presents results on the distribution of diterpanes tothe individual WOC products. Table 5 shows that the highestamount of tetracyclic diterpane is present in the washed overflow,where we obtained the most deal of crude extract (366.5 mg) withdifferent organic compounds and also the highest quantum of dit-erpane fraction (36.7 mg; overall yield is 0.184%). Comparing to thebasic coal sample (F), the increase of diterpane fraction is morethan 50%. Consequently, the effect of hydrocyclone treatment isevident and diterpenoic compounds are assimilated with the coalorganic part and they are attendant in the sample with the lowash content. It is worthy of remark that the slurry contains15.8 mg of the diteprane fraction (overall yield is 0.099%). Com-mentary, the biologically active tetracyclic diterpenes are usuallyextracted from plants, but the gained quantity and efficiency ofextraction in comparison to our results is relatively low. For exam-ple: from the extraction of 10 kg of Oryza sativa, 10 mg of phyllo-cladane diterpene (orizaterpenol) was obtained (overall yield was0.0001%) [52]. Tetracyclic diterpene (7.8 g) (ent-16-kauren-19-oicacid) and 6.2 g of ent-3b-hydroxy-16-kauren-19-oic acid was iso-lated from 21 kg of Pseudognaphalium vira vira (Asteraceae) [53].Overall yield was 0.037% and 0.0295%, respectively. Clearly, fromthe practical viewpoint, the extraction of tetracyclic diterpene fromSlovak sub-bituminous coal is very effective and simple. By themicrowave extraction, at the short time period, large amount ofditerpane was obtained. The WOC separation has the great effecton the diterpane yield, thus there is an open way for the synthesisa new phyllocladane diterpane derivates from the basic hydrocar-bon skeleton. The next step is their biological activity determina-tion, where the first results have already been accomplished[43,54].

4. Conclusions

Distribution of inorganic and organic substances in the hydro-cyclone separated Slovak sub-bituminous coal was studied. Wash-ing of the coal in the water-only cyclone resulted in the obtainingof the high-quality coal product (at overflow) with the ash and car-bon content of 9% and 68.12%, respectively, and also with the sig-nificant decrease of the sulphur content. Presence of silicates and

Fe-bearing minerals in the coal was evaluated by XRD and by57Fe Mössbauer spectroscopy in untreated coal, slurry and under-flow. In the case of washed coal, only pyrite and non-crystallinecarbon phases were identified. The highest content of humic acidswas recorded in the case of slurry. The increase of humic acids byGACL was observed in all WOC products (about two times for F, U,O and tree times for S). Distribution of isolated diterpanes in theWOC products after microwave extraction was also evaluated. Itwas found that the washed coal contains the highest amount ofditerpane substances. Thus, physically treated Slovak sub-bitumi-nous coal can be used not only in coal-fired power with higher per-formance, but also for special purposes, i.e. the preparation ofbiologically active diterpenoic substances and humic acids withhigher yields.

Acknowledgements

We are grateful to Dr. A. Feldhoff for HR-TEM studies of the coal,to Prof. B.R.T. Simoneid, to Dr. J. Cvacka for mass analytics of hydro-carbons as well as to an anonymous reviewer for his commentsthat improved the manuscript. One of the authors, A. Zubrik,thanks the Deutscher Akademischer Austausch Dienst (DAAD) forsupporting his work at the Institute of Physical and TheoreticalChemistry, Braunschweig University of Technology, within DAADresearch fellowship. This work was supported by the Slovak Re-search and Development Agency under the Contract No. APVV-51-035505, SK-CZ-0097-07 and by the Slovak Grant Agency for Sci-ence VEGA (Grant No. 2/0141/10).

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