- Email: [email protected]
Adsorption of o-cresol from aqueous solution on coal
7
Colloids ar~J Surjoces, 64 ( ! 992) 7- I3 Elscvicr Science Publishers B.V.. Amsterdam
adsorption
of o-cresol
from. aqueous solution
on coal
Ahmet Giirses”, Samih Bayrakqeken” and M. Sahin Giilaboglub 3Deuartmertt of Chemistry. Atatiirk Universitv, K.K. Eiitim Fakiiltesi. 25240 Erzurwn. Turkev bDe~artrnertt df Chernicai Engitteering, Atatiiik (Received 21 February
199 1; accepted
University,
26 November
Miikendislik Fakiiltesi, 25240 Erzuhn,
Turkey
199 1)
Abstract of o-crcsol, which is commonly used in coal flotation as frother. on two lignites and a bituminous coal The adsorption has been studied. Coal samples used for the adsorption experiments were: fresh; oxidized; demineralized; demineralized then oxidized. The oxidation proccsscs wcrc conducted by dry (in air at 100, I50 and 200°C) and wet (with 15% HZO1) methods. The extent 0: the coal oxidation was quantified by determining the amounts of the two principal oxygcncontaining functional groups, carboxylic and phcnolic, in the coal samples. The UV spectlophotometric technique was used for the analysis of o-cresol in solution. The adsorption for the lignites was found to increase by oxidation, but to dccrcasc by demincralization, implying that n-crcsol is adsorbed by hydrophilic interactions at the coal surface. In contrast, for the bituminous coal, the adsorption was increased by demineralization while it was decreased by slight oxidation, indicating that the adsorption occurs predominantly through hydrophobic interactions bctwecn the brother molecule and the coal surface. Howcvcr, further oxidation of the bituminous coal resulted in an increase in the extent of adsorption 111comparison with the fresh coal sample. Among the coal samples used, the adsorption on the bituminous coal samples conformed to the Langmuir equation, and adsorption free energies for thcsc samples were found to be in the range -22.6 to -29.3 kJ mol-‘. KCJWW~S: Adsorption;
coal; coal demineralization;
coal oxidation;
Introduction
Coal is a heterogeneous mixture c-f the degradation products of pIants and mineral matter. The appearance and properties of each individual coal is determined by the nature of the original vegetation, and the extent of the physical and chemical changes occurring after deposition. Coal is a very important energy resource and the most common limitations on its use result from sulfur and mineral matter impurities. The increase in pollution of the natural environment forces the use of refined coal. This situation, and the increase in amount *Jffine coal during coal production, increases the importance of froth floCorresporrderrce lo: S. Bayrakceken, Dept. of Chemistry. Atatiirk University, K.K. Egitim Fakiiltesi, 25240 Erzurum, Turkey. Ol66-6622/92/SO5.00
0
1992 -
Elsevier
Science
Publishers
o-cresol;
frothcr;
Langmuir
isotherm.
tation as a cleaning process f’or coal [l-3]. Froth flotation is one of the most effective methods of separating fine inorganic mineral matter and pyrite from coal in a relatively fine size rang’-: (up to 0.5 mm). Coal flotation requires the use of various reagents to obtain the desired recovery and grade. There are three general classes of reagent: frothers, collectors or promoters, and modifying agents. The most commonly used frothers are amyl and butyl alcohols, terpinol and cresols (cresylic acids) [3]. Of these frothers, cresols have been very popular in the coal industry, largely because of their proximity and low cost as coke-oven by-products. However, it has been reported that they adsorb in substantial quantities on coal and this action reduces the residual c:>llcentration in solution available for frothing [2]. The frother concentration is an important variable in coal flotation B.V. All rights
reserved.
A. Giirses et al./Colloi&
in terms of the flotation rate in addition to ?he recovery and grade of coal [2,4]. This paper is concerned with the adsorption of o-cresol from its aqueous solution by lignite and bituminous coals. Fuerstenau and Pradip [S) have investigated the adsorption of three frothers, namely methyl isobutyl carbinol (MIBC), a-terpino1 and o-cresol. on a bituminous coa! and an anthracite, and have shown that both coal samples adsorb a considerable amount of the frother. They found that adsorption of o-cresol by the demineralized anthracite reduced as the extent of oxidation of the coal increased. The authors have suggested that adsorption occurs through hydrophobic interactions between the frother molecules and the coal surface. According to the results obtained by Miller et al. [6], the adsorption density of MIBC on a bituminous coal was not particularly sensitive to surface oxidation of the coal. Miller et al. [7] found that the extent of adsorption of dextrin by a bituminous coal decreased with an increased level of oxidation, while it increased by demineralization; according to the authors, the adsorption occurred by hydrophobic bonding. In a study of the adsorption of phenol and o-cresol onto activated carbon and fly ash, Kumar et al. [S] indicated that fly ash, in which the major constituents are alumina and silica, had an appreciable adsorptive capacity for both phenol and cresol. It is the purpose of this paper to determine the coal properties that control the adsorption of o-cresol by coal and to delineate how this irother molecule interacts with the coal surface. It is known that coals of bituminous and lower rank undergo chemical structural changes, especially due to oxidation, during mining activities and storage. For this purpose, coals of different rank were selected for this study. Also, the coal properties were changed by processes such as oxidation and demineralization. The surface of coal is heterogeneous, containing various types of surface groups whose nature and population vary from one type of coal to another [9]. Principal polar sites on a mineral-matter-free coal surface are oxygen-containing functional
Sltrfaces 64 (1992) 7-13
groups [IO]. It is proposed that the surface of coal consists of three kinds of site: strongly hydrophobic, weakly hydrophobic and hydrophilic [l l]. As an approximation, oxidation makes a coal behave as if it were a coal of lower rar,k [12]. A major change in hydrophobicity of coal occurs as content exceeds approximately the oxygen 10% [l l]. Experimental Materials
Three Turkish coals were selected for this study: two lignites (A$kale and Balkaya) and a bituminous coal (Zonguldak). The coals were ground to -40 and - 200 mesh and characterized through chemical analysis. The proximate and ultimate analyses of the coals were carried out using Turkish and ASTM Standards. The Iesults of the analysis are given in iabie i. io see the effect of minerai matter in coal on the adsorption, a fraction of the -200 mesh Balkaya and -40 mesh Zonguldak coals was demineralized. For this, 200 g of coal was agitated with 2 1 of 6 N HCl in a water bath at 60°C for 1 h, after which the solution was decanted. The coal was then washed with distilled water and treated with 800 ml of 48 wt.% HF at 60°C for TABLE I The proximate study
and ultimate analyses of the coals used in this
Analysis
Agkalc
Balkaya
Proximate analysis (wt.% as reccivcd) Moisture 4.46 7.56 Ash 29.64 32.53 Volatile matter 34.28 38.95 Fixed carbon 31.62 20.96 Ultimate analysis (wt.% daf”) Carbon 70.56 Hydrogen 4.28 Sulfur 4.76 Nitrogen 2.20 Oxygen (diff.) 17.90 “Dry, ash-free basis
65.47 4.08 4.72 1.95 23.78
Zonguldak
0.88 II.95 28.60 58.57 .._ . . 88.13
4.89 0.35 0.97 5.66
A. Giirses et al.fColloids
Surfaces &I (1992) 7-13
I h. After this treatment, the leached coal was treated again with 6 N HCl as before, filtered, and reconditioned with distilled water until the pH of the aqueous coal suspension had reached pH 5 for the lignite and pH 6.5 for the bituminous coal, Finally, the demineralized coals were vacuum dried at 50°C [ 13,141. The ash contents of Balkaya and Zonguldak coals treated in this manner were reduced from 35.19% to 3.22% and from 12.06% to 1.64% respectively. In order to investigate the effect of oxidation on the adsorption of o-cresol, fractions of the demineralized Balkaya, and of the fresh Askale and Zonguldak coal samples, were oxidized with hydrogen peroxide and air respectively. In the oxidation with 10 g of the demineralized Balkaya was H202, added to 250 ml of 15 wt.% Hz02 and the oxidation process conducted for 1 h while stirring. At the end of this period, the oxidized coal was recovered by filtration, washed several times with distilled water and then dried under vacuum. However, both the fresh Askale and Zonguldak coals were dry-oxidized under atmospheric conditions in an oven at IOO-200°C for 16 h. The two principal oxygen-containing functional groups, carboxylic and phenolic, were determined in all the coal samples used in this study. Total acidity (carboxylic plus phenolic) was determined (by reaction with barium hydroxide) via the method used by Miller et al. [7]. Carboxylic groups have been determined (by reaction with calcium acetate) according to the method of Blom et al. [!5]. The phenolic grc**p concentration was then calculated by subtracting from the total acidity the value for the carboxylic content. In addition, the area measurements of these samples were carried out through nitrogen gas adsorption (BET analysis). The obtained results are displayed in Table 2. Methods Adsorption experiments were carried out in IO0 ml, glass-stoppered, round-bottom flasks immersed in a thermsotated shaker bath. For this aim, 0.3 g of the coal was mixed with 16 ml of an
9
aqueous so!ution of known concentration of ucresol and shaken vigorously for 2 min by hand to wet the coal. Then, the flask with its contents was shaken by the shaker for the desired time at 21 “C. At the end of the adsorption period, the supernatant was centrifuged twice for 15 min at 3750 rev min-‘. The concentration of o-cresol in the supernatant solution was then determined using a double-beam UV spectrophotometer. It was found that o-cresol had a characteristic peak at 270 nm and the supernatant from the coals did not exhibit any absorbance at this wavelength, and also that the calibration curve was very reproducible and Einear over the concentration range used in this study. This observation is in very good agreement with that of Fuerstenau and Pradip [S]. The amount of o-cresol adsorbed was calculated from the concentrations in solution before and after adsorption. Blanks containing no o-cresol were used for each series of experiments. Results and discussion The amount of o-cresol adsorbed by the -40 mesh fresh Agkale, Balkaya and Zonguldak, and also the - 200 mesh demineralized, and demineralized and oxidized Balkaya coal samples at their natural pHs, is shown as a function of adsorption time in Fig. 1. From this figure, it is seen that equilibrium is achieved in the cases of -40 mesh fresh Askale, Balkaya and Zonguldak coal samples after about 24 h, and also in the case of -200 mesh demineralized and oxidized Balkaya sampIes after 16 h; however, in the case of - 200 mesh demineralized Balkaya, the adsorption continues even after 48 h. In the comparison of the demineralized and oxidized Balkaya with the demineralized Balkaya (Fig. l), both of which are of -200 mesh, the fact that the contact time required to reach saturation on the oxidized coal is shorter can be attributed to the changes in the porosity upon oxidation. In Table 2, it is seen that the oxidation results in a reduction in the nitrogen gas surface areas of the samples.
TABLE
2
Oxygen
functional
groups and surfacc arcas for the cod
satnplcs used in the cxperimcnts
Carborylic
%mple
groups”
Phcnolic
groups” ---
mcc! gNatural (-40 mesh) Fresh &kale Odizcd Oxidized
Agkalc &kAc
Fresh Zonguldak Oxidixd Zonguldnk Oxidized Zonguldak
(I 50 C) (200 C)
(- 200 mesh)
Wet oxidized Balkaya (-700 mesh) Fresh Zonguldak (-40 mesh) “Dry,
E
asI]-free
wt.:‘; 0
0.5X 0.5X
5.57 6.53
I.16 0.30
3.71 !).9h
0.03 0.15 0.65
0. IO 0.48 LOX
15.37 .7 ._74 I .Rc)
0.34 0.49 0.03
8.91
7_.2
10.45
2.1
24.59 5.iX
I.7 2.x
3.0’ 3.65
2.0 I.9 I.8
I .09
6.59
7.6
I .57 0.10
14.9.1
(1.5
3.43
2.4
_._7x 7 6.90
basis.
6
E
a
3
ob-LM
20
50
I hours)
Time cl&t
of time
on
the
adsorption
of o-crcsol
ASkale, Balkaya and Zonguldak coal sampI_s. (0) rrcsh ASkalc: (0) -40 mesh fresh Bulkagz; (A) fresh
wt.?; 0
I I.04
2
Fig. I. The
mcqg-’
0. I x
0.1X (100 C) (ZOO ‘C)
Fresh B:dkay;l
Dcmincralizcd Fresh Balkaya
’
BET Nz (m’g-‘)
Zonguidak:
iii j
-200
mcsil
-40 -40
Jcnlincralizcd
(E) -200 mesh demineralized and oxidized o-crcsol conccnlrations were I85 mg I-‘.
Balkaya.
on
mesh mesh
Balknya: Initial
From work invoiving the ef&t of low-tcmperaturc oxidation of an Australian brown coal on the changes in internal surface area, it was found that oxid*~‘;~~~ significantly decreased the U.. .W ‘r~trnc::.! L.IC CO-, (0°C) area. The decrease was attributed to :-ldsorbed oxygen blocking the micropores [16-J. For a detailed examination of the effect of oxidation and demineralization on the adsorptior, of o-crcsol by the coa! samples which a~: given in Table 2, a series of experiments was conducted with the o-crcsol solutions at various initial concentrations. Cini,, for I6 h. The adsorption isotherms obtained for ASkale, Baikaya and Zonguidak coal samples are presented in Figs 2, 3 and 4 respectively. The data of Fig. 2 (-40 mesh ASkale) clearly indicate that the adsorptive capacity of the samples oxidized both at 100 and 2OO”C, at the same initial concentration in solution, is higher than that of the fresh sample, and that the uptake of o-cresol increases as the temperature of the oxidation process increases. As can be seen in Table 1, the degree of oxidation, which was followed by determination of quantities of carboxyl and phenolic groups, significantly increased with an increase from 100
0
250 Cinit (mglL) ;. 3. Adsorption isotherms of o-crcsol on ASkaIc lignite nplcs of -40 mesh. (Et) fresh: (I) 100°C osidized;( 0) 200°C dizcd.
0
&II
80
$20
Cinit (mgiL
160
200
I
6. 3. Adsorption isotherms of o-crcso) on Baikrrya lignite fresh; (IS) demil~~ralizcd; mpIes of -ZOO mesh. (0) I) deminc~li~cd and oxidized.
I 200°C in oxidation ;:mperature (dry oxidation air). The adsorption isotherms for the fresh, :minera!izcd, and demineralized and then oxized (wet oxidation with H,Oz) Balkaya Iigrlite :mpIes (- 200 mesh) are given in Fig. 3. From this gure, it can be seen that adsorption increases with
Fig. 4. Adsorption isotherms of o-crcsol on Zonguldak bitu nous coal samples of -40 mesh. (U) fresh: (13) demincraliz (@) 150°C oxidized; (0) NO’C oxidized.
oxidation, but it decreases with the removal mineral matter, which is generally more hydrop Iic than the organic material in coal. As indical above, the effect of oxidation on the adsorption the o-cresol for Iignites is similar. Adsorption isotherms of o-crcsol on the fre demineralized, and oxidized at 1.50 and 200°C (( oxidation in air) ZonguIdak bituminous cl samples (-40 mesh) are presented in Fig. 4. shown in this figure, the adsorption first decrea! with oxidation (r50°C) and then increased wit1 further increase in oxidation temperature (200’ of the coal sample. However, there is a substan increase in the content of the carboxylic and pher lit groups with increasing temperature from to 200°C for Zonguldak coal (TabIe 2). A’ the demineralization markedly increased adsorption. The fact that the adsorption of o-cresol for b the lignites (see Figs 2 and 3) increases with increase in oxidation of coal, indicates that oxygen functional groups on the surface of c can play an important part in the adsorpt process. This finding, together with the effect demineraiization, suggests that o-cresol adsorbs
hydrophilic interac!ions from aqueous solution on the surface of the lignite coals used. These results differ from those reported by Fuerstenau and Pradip [SJ for o-cresol, and also differ from the results reported by Miller et al. [7] for dextrin adsorption by coal. In these systems, in which the authors have used higher-rank coals (bituminous and anthracite), adsorption decreases as the extent of oxidation increases, and hence the adsorption phenomenon is thought to involve hydrophobic bonding. In the case of Zonguldak bituminous coa!, the adsorption (Fig. 4) decreased with slighi oxidation (at 15O”C), but it increased by demineraiization in comparison with the fresh coal, in contrast to the lignite coals. This observation is in agreement with the results obtained by Fuerstenau and Pradip [5] and Miller et al. [7], implying hydrophobic bonding with the surface. However, the adsorption increased with additional oxidation (at 200°C) in comparison with the fresh coal. sample, implying hydrophilic bonding. This latter situation agrees in part with the work of Miller et al. [S], who found that the MIBC adsorption density was not particularly sensitive to surface oxidation of coal and suggested that MIBC adsorption by a bituminous coal was not limited to hydrophobic bonding with the surface, but may also include hydrogen bonding. To determine whether the adsorptions follow the Langmuir equation, and to evaluate some parameters of the adsorption, the data in Figs .Y:3 and 4 were applied to the Langmuir equation [i?], which can be written in linear form as
cc x=x,+X&
Surfaces 64 (1992) 7-I 3
A. Giirscs et al./Colloids
I2
1
temperature. K can be re!ated to the adsorption free energy, dG$,, as follows: dG!&, = - RT(ln K + 4.02)
(2)
where R is the gas constant and T the absolute temperature. When adsorption follows the Langmuir equation, a plot of C/X versus C (Eqn (1)) should be a straight line -with slope l/X, and intercept ! !;r’, K. Of the resuits obtained, only the data in Fig. 4 for the Zonguldak coal samples conform to the Langmuir equation (Eqn (I)), Figure 5 illustrates the linear plots of the Langmuir equation for these samples. The values of K and X, were calculated by the least-squares method app!ied to the straight lines of Fig. 5. The value of .4& for each sample was estimated using Eqn (2). Table 3 shows the values of dGzd:, and the adsorption at the pl?teau, together with the corresponding correlation coefficient, r. From Table 3, it can be seen that the highest free energy of adsorption is found for the demineralized coal sample (- 29.3 kJ mol- ‘), which has the highest value of adsorption at the plateau, whereas the lowest free energy of adsorption is
(1)
where C indicates the concentration of the adsorbate in the solution that is in equilibrium with the adsorbent, X represents the amount of adsorbate adsorbed per unit mass of adsorbent, X, is the limiting amount ofadsorbate that can be taken up by unit mass of adsorbent and K is the adsorption constant. Both K and X, are constant for the particular system being studied and for a given
01 0
t 30
I
I
I
60
90
C(mg1L
120
I
150
1
Fig. 5. Application of the Langmuir equation to the adsorption data in Fig. 4. (W) Ircsh; (Cl) demincralizcd; (9) 150°C oxidized; (0) 200°C oxidized.
A. Giirscs
TABLE
er al./Colloids
Swjnces
64 ( 1992)
I3
7- I3
3
The values of the adsorption at the plateau, dG,O,, and r for the adsorption ol o-cresol on Zonguldak bituminous coal f-43 mesh) Sample
Amount adsorbed at the plateau (mg 6-l solid)
dG:Ci, (kJ mol- ‘)
r’
Fresh coal Dcminrrzlized coal Oxidized coai (! 50” C) Oxidized coal (200°C)
5.9 9.0 5.6 6.3
-
0.928i 0.9863 0.9040 0.9703
“Correlation
for the siraight
coefficients
25.7 29.3 22.6 26.8
lines in Fig. 5.
the experimental results for the bituminous coal used in this study confirm that o-cresol adsorbs predcminantly by hydrophobic interactions at the coal surface, but heavily oxidized coal (dry oxidized at 200°C) behaves as if i; were a coal of lower rank in terms of the adsorption of o-cresol. Similar adsorption trends, as in Figs 2, 3 and 4, were found when the amounts of adsorbate adsorbed were also calculated per unit area (based on the N2 surface area) instead of unit mass of adsorbent. References I
found for the coal sampie oxicii~d at 150°C (-22.6 kJ mol-‘), which has the lowest value of adsorption at the plateau. The fact that the demineralized coal sample has the highest free energy of adsorption means that adsorption is more favorable on this coal sample than on the other coal samples [S]. This situation may be attributed to an increase in the hydrophobicity of this sample compared with the other samples as a result of demineralization, indicating that the predominant mechanism of adsorption is through hydrophobic interactions. In the light of these results, it can be suggested that a further oxidation of the fresh coal (Zonguldak) causes the predorninant mechanism of adsorption to switch from hydrophobic to hydrophilic. In addition, the fact that the demineralized coal sample has the highest correlation coefficient (Table 3) can be ascribed to some increase in the homogenization of the coal surface due to the removal of mineral components in coal, since the Langmuir equation assumes that the surface is homogeneous [ 17,181. According to the results given above, in the lower-rank coals (lignites), which have very few hydrophobic sites [l2], the predominant mechanism of adsorption is through hydrophilic interactions between o-cresol molecules and the surface of the coal, as shown by the effect of oxidation and demineralization on adsorption. In contrast,
D.J. Brown, in D.W. Fucrstcnau (Ed.). Froth Flotation -50th Anniversary Volume. AIME. New York, 1962. Chapter 20. 2 F.F. Aplan, in M.C. Fucrstenau (Ed.), Flotation A.M. Gaudin Memorial Volume, Vol. 2, AiiviE. New York, 1976. Chapter 45. 3 R.E. Zimmerman, in J.W. Leonard (Ed.), Coal Prcparntion, Part 3, 4th cdn., AIME. New York, 1979. Chapter IO. 4 F.F. Aplan, in T.D. Wheclock (Ed.), Coal Dcsulfurization: Chemical and Physical Methods, ACS Symp. Ser. 64, American Chemical Society. Washington, DC, 1977. pp. 70-82. D.W. Fucrstcnau and Pradip. Colloids Surfaces. 4 (1982) 229. J.D. Miller, CL. Lin and S.S. Chang, Colloids Surfaces, 7 (1983) 351. J.D. Miller, J.S. Laskowski and S.S. Chang, Colloids Surfaces, 8 (1983) 137. S. Kumar, S.N. Upadhyay and Y.D. Upadhyay. J. Chem. Technol. Biotechnol., 37 (1987) 28 1. Th.F. Tadros, Second European Conference on Coal Liquid Mixtures, Institute of Chemical Engineers, Rugby, UK. September 16-18, 1985, pp. l-17. IO D.W. Fuerstenau, J.M. Roscnbaum and J. Laskowski, Colloids Surfaces, 8 (1983) 153. II J.A. Gutierrez-Rodriguez, R.J. Purcell. Jr., and F.F. Aplan. Colloids Surfaces, I2 (1984) 1. I2 J.A. Gutierrez-Rodriguez and F.F. Aplan, Colloids Surfaces, I2 ( 1984) 27. I3 Y.S. You, Ph.D. Thesis, University of California, Berkeley, 1983. I4 M. Bishop and D.L. Ward, Fuel, 37 (1958) 191. and D.W. van Krevelen. Fuel, 36 i5 L. Blom, L. Edelhausen (1957) 135. I6 O.P. Mahajan. in R.A. Meyers (Ed.), Coal Structure, Academic Press, New York, 1982, p. 66. 17 M.J. Rosen, Surfactants and Interracial Phenomena, Wiley. New York, 1978, pp. 34-39. I8 S. Bayrakceken and A. Giirses, Doga-Tr. J. of Chemistry. I4 (1990) 140.
Colloids ar~J Surjoces, 64 ( ! 992) 7- I3 Elscvicr Science Publishers B.V.. Amsterdam
adsorption
of o-cresol
from. aqueous solution
on coal
Ahmet Giirses”, Samih Bayrakqeken” and M. Sahin Giilaboglub 3Deuartmertt of Chemistry. Atatiirk Universitv, K.K. Eiitim Fakiiltesi. 25240 Erzurwn. Turkev bDe~artrnertt df Chernicai Engitteering, Atatiiik (Received 21 February
199 1; accepted
University,
26 November
Miikendislik Fakiiltesi, 25240 Erzuhn,
Turkey
199 1)
Abstract of o-crcsol, which is commonly used in coal flotation as frother. on two lignites and a bituminous coal The adsorption has been studied. Coal samples used for the adsorption experiments were: fresh; oxidized; demineralized; demineralized then oxidized. The oxidation proccsscs wcrc conducted by dry (in air at 100, I50 and 200°C) and wet (with 15% HZO1) methods. The extent 0: the coal oxidation was quantified by determining the amounts of the two principal oxygcncontaining functional groups, carboxylic and phcnolic, in the coal samples. The UV spectlophotometric technique was used for the analysis of o-cresol in solution. The adsorption for the lignites was found to increase by oxidation, but to dccrcasc by demincralization, implying that n-crcsol is adsorbed by hydrophilic interactions at the coal surface. In contrast, for the bituminous coal, the adsorption was increased by demineralization while it was decreased by slight oxidation, indicating that the adsorption occurs predominantly through hydrophobic interactions bctwecn the brother molecule and the coal surface. Howcvcr, further oxidation of the bituminous coal resulted in an increase in the extent of adsorption 111comparison with the fresh coal sample. Among the coal samples used, the adsorption on the bituminous coal samples conformed to the Langmuir equation, and adsorption free energies for thcsc samples were found to be in the range -22.6 to -29.3 kJ mol-‘. KCJWW~S: Adsorption;
coal; coal demineralization;
coal oxidation;
Introduction
Coal is a heterogeneous mixture c-f the degradation products of pIants and mineral matter. The appearance and properties of each individual coal is determined by the nature of the original vegetation, and the extent of the physical and chemical changes occurring after deposition. Coal is a very important energy resource and the most common limitations on its use result from sulfur and mineral matter impurities. The increase in pollution of the natural environment forces the use of refined coal. This situation, and the increase in amount *Jffine coal during coal production, increases the importance of froth floCorresporrderrce lo: S. Bayrakceken, Dept. of Chemistry. Atatiirk University, K.K. Egitim Fakiiltesi, 25240 Erzurum, Turkey. Ol66-6622/92/SO5.00
0
1992 -
Elsevier
Science
Publishers
o-cresol;
frothcr;
Langmuir
isotherm.
tation as a cleaning process f’or coal [l-3]. Froth flotation is one of the most effective methods of separating fine inorganic mineral matter and pyrite from coal in a relatively fine size rang’-: (up to 0.5 mm). Coal flotation requires the use of various reagents to obtain the desired recovery and grade. There are three general classes of reagent: frothers, collectors or promoters, and modifying agents. The most commonly used frothers are amyl and butyl alcohols, terpinol and cresols (cresylic acids) [3]. Of these frothers, cresols have been very popular in the coal industry, largely because of their proximity and low cost as coke-oven by-products. However, it has been reported that they adsorb in substantial quantities on coal and this action reduces the residual c:>llcentration in solution available for frothing [2]. The frother concentration is an important variable in coal flotation B.V. All rights
reserved.
A. Giirses et al./Colloi&
in terms of the flotation rate in addition to ?he recovery and grade of coal [2,4]. This paper is concerned with the adsorption of o-cresol from its aqueous solution by lignite and bituminous coals. Fuerstenau and Pradip [S) have investigated the adsorption of three frothers, namely methyl isobutyl carbinol (MIBC), a-terpino1 and o-cresol. on a bituminous coa! and an anthracite, and have shown that both coal samples adsorb a considerable amount of the frother. They found that adsorption of o-cresol by the demineralized anthracite reduced as the extent of oxidation of the coal increased. The authors have suggested that adsorption occurs through hydrophobic interactions between the frother molecules and the coal surface. According to the results obtained by Miller et al. [6], the adsorption density of MIBC on a bituminous coal was not particularly sensitive to surface oxidation of the coal. Miller et al. [7] found that the extent of adsorption of dextrin by a bituminous coal decreased with an increased level of oxidation, while it increased by demineralization; according to the authors, the adsorption occurred by hydrophobic bonding. In a study of the adsorption of phenol and o-cresol onto activated carbon and fly ash, Kumar et al. [S] indicated that fly ash, in which the major constituents are alumina and silica, had an appreciable adsorptive capacity for both phenol and cresol. It is the purpose of this paper to determine the coal properties that control the adsorption of o-cresol by coal and to delineate how this irother molecule interacts with the coal surface. It is known that coals of bituminous and lower rank undergo chemical structural changes, especially due to oxidation, during mining activities and storage. For this purpose, coals of different rank were selected for this study. Also, the coal properties were changed by processes such as oxidation and demineralization. The surface of coal is heterogeneous, containing various types of surface groups whose nature and population vary from one type of coal to another [9]. Principal polar sites on a mineral-matter-free coal surface are oxygen-containing functional
Sltrfaces 64 (1992) 7-13
groups [IO]. It is proposed that the surface of coal consists of three kinds of site: strongly hydrophobic, weakly hydrophobic and hydrophilic [l l]. As an approximation, oxidation makes a coal behave as if it were a coal of lower rar,k [12]. A major change in hydrophobicity of coal occurs as content exceeds approximately the oxygen 10% [l l]. Experimental Materials
Three Turkish coals were selected for this study: two lignites (A$kale and Balkaya) and a bituminous coal (Zonguldak). The coals were ground to -40 and - 200 mesh and characterized through chemical analysis. The proximate and ultimate analyses of the coals were carried out using Turkish and ASTM Standards. The Iesults of the analysis are given in iabie i. io see the effect of minerai matter in coal on the adsorption, a fraction of the -200 mesh Balkaya and -40 mesh Zonguldak coals was demineralized. For this, 200 g of coal was agitated with 2 1 of 6 N HCl in a water bath at 60°C for 1 h, after which the solution was decanted. The coal was then washed with distilled water and treated with 800 ml of 48 wt.% HF at 60°C for TABLE I The proximate study
and ultimate analyses of the coals used in this
Analysis
Agkalc
Balkaya
Proximate analysis (wt.% as reccivcd) Moisture 4.46 7.56 Ash 29.64 32.53 Volatile matter 34.28 38.95 Fixed carbon 31.62 20.96 Ultimate analysis (wt.% daf”) Carbon 70.56 Hydrogen 4.28 Sulfur 4.76 Nitrogen 2.20 Oxygen (diff.) 17.90 “Dry, ash-free basis
65.47 4.08 4.72 1.95 23.78
Zonguldak
0.88 II.95 28.60 58.57 .._ . . 88.13
4.89 0.35 0.97 5.66
A. Giirses et al.fColloids
Surfaces &I (1992) 7-13
I h. After this treatment, the leached coal was treated again with 6 N HCl as before, filtered, and reconditioned with distilled water until the pH of the aqueous coal suspension had reached pH 5 for the lignite and pH 6.5 for the bituminous coal, Finally, the demineralized coals were vacuum dried at 50°C [ 13,141. The ash contents of Balkaya and Zonguldak coals treated in this manner were reduced from 35.19% to 3.22% and from 12.06% to 1.64% respectively. In order to investigate the effect of oxidation on the adsorption of o-cresol, fractions of the demineralized Balkaya, and of the fresh Askale and Zonguldak coal samples, were oxidized with hydrogen peroxide and air respectively. In the oxidation with 10 g of the demineralized Balkaya was H202, added to 250 ml of 15 wt.% Hz02 and the oxidation process conducted for 1 h while stirring. At the end of this period, the oxidized coal was recovered by filtration, washed several times with distilled water and then dried under vacuum. However, both the fresh Askale and Zonguldak coals were dry-oxidized under atmospheric conditions in an oven at IOO-200°C for 16 h. The two principal oxygen-containing functional groups, carboxylic and phenolic, were determined in all the coal samples used in this study. Total acidity (carboxylic plus phenolic) was determined (by reaction with barium hydroxide) via the method used by Miller et al. [7]. Carboxylic groups have been determined (by reaction with calcium acetate) according to the method of Blom et al. [!5]. The phenolic grc**p concentration was then calculated by subtracting from the total acidity the value for the carboxylic content. In addition, the area measurements of these samples were carried out through nitrogen gas adsorption (BET analysis). The obtained results are displayed in Table 2. Methods Adsorption experiments were carried out in IO0 ml, glass-stoppered, round-bottom flasks immersed in a thermsotated shaker bath. For this aim, 0.3 g of the coal was mixed with 16 ml of an
9
aqueous so!ution of known concentration of ucresol and shaken vigorously for 2 min by hand to wet the coal. Then, the flask with its contents was shaken by the shaker for the desired time at 21 “C. At the end of the adsorption period, the supernatant was centrifuged twice for 15 min at 3750 rev min-‘. The concentration of o-cresol in the supernatant solution was then determined using a double-beam UV spectrophotometer. It was found that o-cresol had a characteristic peak at 270 nm and the supernatant from the coals did not exhibit any absorbance at this wavelength, and also that the calibration curve was very reproducible and Einear over the concentration range used in this study. This observation is in very good agreement with that of Fuerstenau and Pradip [S]. The amount of o-cresol adsorbed was calculated from the concentrations in solution before and after adsorption. Blanks containing no o-cresol were used for each series of experiments. Results and discussion The amount of o-cresol adsorbed by the -40 mesh fresh Agkale, Balkaya and Zonguldak, and also the - 200 mesh demineralized, and demineralized and oxidized Balkaya coal samples at their natural pHs, is shown as a function of adsorption time in Fig. 1. From this figure, it is seen that equilibrium is achieved in the cases of -40 mesh fresh Askale, Balkaya and Zonguldak coal samples after about 24 h, and also in the case of -200 mesh demineralized and oxidized Balkaya sampIes after 16 h; however, in the case of - 200 mesh demineralized Balkaya, the adsorption continues even after 48 h. In the comparison of the demineralized and oxidized Balkaya with the demineralized Balkaya (Fig. l), both of which are of -200 mesh, the fact that the contact time required to reach saturation on the oxidized coal is shorter can be attributed to the changes in the porosity upon oxidation. In Table 2, it is seen that the oxidation results in a reduction in the nitrogen gas surface areas of the samples.
TABLE
2
Oxygen
functional
groups and surfacc arcas for the cod
satnplcs used in the cxperimcnts
Carborylic
%mple
groups”
Phcnolic
groups” ---
mcc! gNatural (-40 mesh) Fresh &kale Odizcd Oxidized
Agkalc &kAc
Fresh Zonguldak Oxidixd Zonguldnk Oxidized Zonguldak
(I 50 C) (200 C)
(- 200 mesh)
Wet oxidized Balkaya (-700 mesh) Fresh Zonguldak (-40 mesh) “Dry,
E
asI]-free
wt.:‘; 0
0.5X 0.5X
5.57 6.53
I.16 0.30
3.71 !).9h
0.03 0.15 0.65
0. IO 0.48 LOX
15.37 .7 ._74 I .Rc)
0.34 0.49 0.03
8.91
7_.2
10.45
2.1
24.59 5.iX
I.7 2.x
3.0’ 3.65
2.0 I.9 I.8
I .09
6.59
7.6
I .57 0.10
14.9.1
(1.5
3.43
2.4
_._7x 7 6.90
basis.
6
E
a
3
ob-LM
20
50
I hours)
Time cl&t
of time
on
the
adsorption
of o-crcsol
ASkale, Balkaya and Zonguldak coal sampI_s. (0) rrcsh ASkalc: (0) -40 mesh fresh Bulkagz; (A) fresh
wt.?; 0
I I.04
2
Fig. I. The
mcqg-’
0. I x
0.1X (100 C) (ZOO ‘C)
Fresh B:dkay;l
Dcmincralizcd Fresh Balkaya
’
BET Nz (m’g-‘)
Zonguidak:
iii j
-200
mcsil
-40 -40
Jcnlincralizcd
(E) -200 mesh demineralized and oxidized o-crcsol conccnlrations were I85 mg I-‘.
Balkaya.
on
mesh mesh
Balknya: Initial
From work invoiving the ef&t of low-tcmperaturc oxidation of an Australian brown coal on the changes in internal surface area, it was found that oxid*~‘;~~~ significantly decreased the U.. .W ‘r~trnc::.! L.IC CO-, (0°C) area. The decrease was attributed to :-ldsorbed oxygen blocking the micropores [16-J. For a detailed examination of the effect of oxidation and demineralization on the adsorptior, of o-crcsol by the coa! samples which a~: given in Table 2, a series of experiments was conducted with the o-crcsol solutions at various initial concentrations. Cini,, for I6 h. The adsorption isotherms obtained for ASkale, Baikaya and Zonguidak coal samples are presented in Figs 2, 3 and 4 respectively. The data of Fig. 2 (-40 mesh ASkale) clearly indicate that the adsorptive capacity of the samples oxidized both at 100 and 2OO”C, at the same initial concentration in solution, is higher than that of the fresh sample, and that the uptake of o-cresol increases as the temperature of the oxidation process increases. As can be seen in Table 1, the degree of oxidation, which was followed by determination of quantities of carboxyl and phenolic groups, significantly increased with an increase from 100
0
250 Cinit (mglL) ;. 3. Adsorption isotherms of o-crcsol on ASkaIc lignite nplcs of -40 mesh. (Et) fresh: (I) 100°C osidized;( 0) 200°C dizcd.
0
&II
80
$20
Cinit (mgiL
160
200
I
6. 3. Adsorption isotherms of o-crcso) on Baikrrya lignite fresh; (IS) demil~~ralizcd; mpIes of -ZOO mesh. (0) I) deminc~li~cd and oxidized.
I 200°C in oxidation ;:mperature (dry oxidation air). The adsorption isotherms for the fresh, :minera!izcd, and demineralized and then oxized (wet oxidation with H,Oz) Balkaya Iigrlite :mpIes (- 200 mesh) are given in Fig. 3. From this gure, it can be seen that adsorption increases with
Fig. 4. Adsorption isotherms of o-crcsol on Zonguldak bitu nous coal samples of -40 mesh. (U) fresh: (13) demincraliz (@) 150°C oxidized; (0) NO’C oxidized.
oxidation, but it decreases with the removal mineral matter, which is generally more hydrop Iic than the organic material in coal. As indical above, the effect of oxidation on the adsorption the o-cresol for Iignites is similar. Adsorption isotherms of o-crcsol on the fre demineralized, and oxidized at 1.50 and 200°C (( oxidation in air) ZonguIdak bituminous cl samples (-40 mesh) are presented in Fig. 4. shown in this figure, the adsorption first decrea! with oxidation (r50°C) and then increased wit1 further increase in oxidation temperature (200’ of the coal sample. However, there is a substan increase in the content of the carboxylic and pher lit groups with increasing temperature from to 200°C for Zonguldak coal (TabIe 2). A’ the demineralization markedly increased adsorption. The fact that the adsorption of o-cresol for b the lignites (see Figs 2 and 3) increases with increase in oxidation of coal, indicates that oxygen functional groups on the surface of c can play an important part in the adsorpt process. This finding, together with the effect demineraiization, suggests that o-cresol adsorbs
hydrophilic interac!ions from aqueous solution on the surface of the lignite coals used. These results differ from those reported by Fuerstenau and Pradip [SJ for o-cresol, and also differ from the results reported by Miller et al. [7] for dextrin adsorption by coal. In these systems, in which the authors have used higher-rank coals (bituminous and anthracite), adsorption decreases as the extent of oxidation increases, and hence the adsorption phenomenon is thought to involve hydrophobic bonding. In the case of Zonguldak bituminous coa!, the adsorption (Fig. 4) decreased with slighi oxidation (at 15O”C), but it increased by demineraiization in comparison with the fresh coal, in contrast to the lignite coals. This observation is in agreement with the results obtained by Fuerstenau and Pradip [5] and Miller et al. [7], implying hydrophobic bonding with the surface. However, the adsorption increased with additional oxidation (at 200°C) in comparison with the fresh coal. sample, implying hydrophilic bonding. This latter situation agrees in part with the work of Miller et al. [S], who found that the MIBC adsorption density was not particularly sensitive to surface oxidation of coal and suggested that MIBC adsorption by a bituminous coal was not limited to hydrophobic bonding with the surface, but may also include hydrogen bonding. To determine whether the adsorptions follow the Langmuir equation, and to evaluate some parameters of the adsorption, the data in Figs .Y:3 and 4 were applied to the Langmuir equation [i?], which can be written in linear form as
cc x=x,+X&
Surfaces 64 (1992) 7-I 3
A. Giirscs et al./Colloids
I2
1
temperature. K can be re!ated to the adsorption free energy, dG$,, as follows: dG!&, = - RT(ln K + 4.02)
(2)
where R is the gas constant and T the absolute temperature. When adsorption follows the Langmuir equation, a plot of C/X versus C (Eqn (1)) should be a straight line -with slope l/X, and intercept ! !;r’, K. Of the resuits obtained, only the data in Fig. 4 for the Zonguldak coal samples conform to the Langmuir equation (Eqn (I)), Figure 5 illustrates the linear plots of the Langmuir equation for these samples. The values of K and X, were calculated by the least-squares method app!ied to the straight lines of Fig. 5. The value of .4& for each sample was estimated using Eqn (2). Table 3 shows the values of dGzd:, and the adsorption at the pl?teau, together with the corresponding correlation coefficient, r. From Table 3, it can be seen that the highest free energy of adsorption is found for the demineralized coal sample (- 29.3 kJ mol- ‘), which has the highest value of adsorption at the plateau, whereas the lowest free energy of adsorption is
(1)
where C indicates the concentration of the adsorbate in the solution that is in equilibrium with the adsorbent, X represents the amount of adsorbate adsorbed per unit mass of adsorbent, X, is the limiting amount ofadsorbate that can be taken up by unit mass of adsorbent and K is the adsorption constant. Both K and X, are constant for the particular system being studied and for a given
01 0
t 30
I
I
I
60
90
C(mg1L
120
I
150
1
Fig. 5. Application of the Langmuir equation to the adsorption data in Fig. 4. (W) Ircsh; (Cl) demincralizcd; (9) 150°C oxidized; (0) 200°C oxidized.
A. Giirscs
TABLE
er al./Colloids
Swjnces
64 ( 1992)
I3
7- I3
3
The values of the adsorption at the plateau, dG,O,, and r for the adsorption ol o-cresol on Zonguldak bituminous coal f-43 mesh) Sample
Amount adsorbed at the plateau (mg 6-l solid)
dG:Ci, (kJ mol- ‘)
r’
Fresh coal Dcminrrzlized coal Oxidized coai (! 50” C) Oxidized coal (200°C)
5.9 9.0 5.6 6.3
-
0.928i 0.9863 0.9040 0.9703
“Correlation
for the siraight
coefficients
25.7 29.3 22.6 26.8
lines in Fig. 5.
the experimental results for the bituminous coal used in this study confirm that o-cresol adsorbs predcminantly by hydrophobic interactions at the coal surface, but heavily oxidized coal (dry oxidized at 200°C) behaves as if i; were a coal of lower rank in terms of the adsorption of o-cresol. Similar adsorption trends, as in Figs 2, 3 and 4, were found when the amounts of adsorbate adsorbed were also calculated per unit area (based on the N2 surface area) instead of unit mass of adsorbent. References I
found for the coal sampie oxicii~d at 150°C (-22.6 kJ mol-‘), which has the lowest value of adsorption at the plateau. The fact that the demineralized coal sample has the highest free energy of adsorption means that adsorption is more favorable on this coal sample than on the other coal samples [S]. This situation may be attributed to an increase in the hydrophobicity of this sample compared with the other samples as a result of demineralization, indicating that the predominant mechanism of adsorption is through hydrophobic interactions. In the light of these results, it can be suggested that a further oxidation of the fresh coal (Zonguldak) causes the predorninant mechanism of adsorption to switch from hydrophobic to hydrophilic. In addition, the fact that the demineralized coal sample has the highest correlation coefficient (Table 3) can be ascribed to some increase in the homogenization of the coal surface due to the removal of mineral components in coal, since the Langmuir equation assumes that the surface is homogeneous [ 17,181. According to the results given above, in the lower-rank coals (lignites), which have very few hydrophobic sites [l2], the predominant mechanism of adsorption is through hydrophilic interactions between o-cresol molecules and the surface of the coal, as shown by the effect of oxidation and demineralization on adsorption. In contrast,
D.J. Brown, in D.W. Fucrstcnau (Ed.). Froth Flotation -50th Anniversary Volume. AIME. New York, 1962. Chapter 20. 2 F.F. Aplan, in M.C. Fucrstenau (Ed.), Flotation A.M. Gaudin Memorial Volume, Vol. 2, AiiviE. New York, 1976. Chapter 45. 3 R.E. Zimmerman, in J.W. Leonard (Ed.), Coal Prcparntion, Part 3, 4th cdn., AIME. New York, 1979. Chapter IO. 4 F.F. Aplan, in T.D. Wheclock (Ed.), Coal Dcsulfurization: Chemical and Physical Methods, ACS Symp. Ser. 64, American Chemical Society. Washington, DC, 1977. pp. 70-82. D.W. Fucrstcnau and Pradip. Colloids Surfaces. 4 (1982) 229. J.D. Miller, CL. Lin and S.S. Chang, Colloids Surfaces, 7 (1983) 351. J.D. Miller, J.S. Laskowski and S.S. Chang, Colloids Surfaces, 8 (1983) 137. S. Kumar, S.N. Upadhyay and Y.D. Upadhyay. J. Chem. Technol. Biotechnol., 37 (1987) 28 1. Th.F. Tadros, Second European Conference on Coal Liquid Mixtures, Institute of Chemical Engineers, Rugby, UK. September 16-18, 1985, pp. l-17. IO D.W. Fuerstenau, J.M. Roscnbaum and J. Laskowski, Colloids Surfaces, 8 (1983) 153. II J.A. Gutierrez-Rodriguez, R.J. Purcell. Jr., and F.F. Aplan. Colloids Surfaces, I2 (1984) 1. I2 J.A. Gutierrez-Rodriguez and F.F. Aplan, Colloids Surfaces, I2 ( 1984) 27. I3 Y.S. You, Ph.D. Thesis, University of California, Berkeley, 1983. I4 M. Bishop and D.L. Ward, Fuel, 37 (1958) 191. and D.W. van Krevelen. Fuel, 36 i5 L. Blom, L. Edelhausen (1957) 135. I6 O.P. Mahajan. in R.A. Meyers (Ed.), Coal Structure, Academic Press, New York, 1982, p. 66. 17 M.J. Rosen, Surfactants and Interracial Phenomena, Wiley. New York, 1978, pp. 34-39. I8 S. Bayrakceken and A. Giirses, Doga-Tr. J. of Chemistry. I4 (1990) 140.