Langmuir 1992,8, 104-108
104
Surface Reaetivity of Coals toward Water and n-Hexane. An Adsorption Microcalorimetric Study P. F. Rossi,*9t G. Busca,+G. Oliveri,? A. Vettor,$ and G. Milanat Facolth di Ingegneria, Istituto di Chimica, Universith di Genova, Fiera del Mare, Piazzale J.F. Kennedy, Pad. 19.16129, Genouu, Italy, and Eniricerche, Via Maritano 26, 20097 San Donato Milanese, Milano, Italy Received June 28, 1991. I n Final Form: October 9, 1991 Illinois and Polish fine coal samples, previously characterized by conventional chemical analyses, have been separated by densimetric centrifugation. Both sink and float fractions have been analyzed by X-ray photoelectron spectroscopyand by adsorption microcalorimetry in contact with water and n-hexane vapors. The surface reactivity of Illinois coals (refractory to oil agglomeration processes) is higher than Polish ones (very easily agglomerable) toward both water and n-hexane. This different behavior has been attributed predominantly to the nature of the surface inorganicsites, having an acidic character in the case of Illinois coal, and a more pronounced basic character in the case of Polish coal.
Introduction Coal is a very complex and heterogeneous material, composed by aggregates of different components, having both mineral and organic origin, only partially separable from each 0ther.l Coals of different origins differ very much in relation to their elemental and phase composition and behavior under different conditions. On the other hand, coal represents the fossil raw material having by far the greatest availability, although only the organic component in it is, obviously, of industrial interest, both in order to get energy from its combustion and to derive chemical compounds. The efficient separation of the organic coal material from the mineral matter still represents an industrial problem. Several beneficiation processes have been de~ e l o p e d , *among *~ which separations based on surface properties are particularly applied to fine coals. Oil agglomeration processes are based on the tendency of “pure” coal particles to agglomerate selectively in the oil phase when ground coal containing mineral matter is suspended in oil-water mixtures under stirring>* Eniricerche is active in this area,7,8with a process of beneficiation by oil agglomeration to produce beneficiated coal-water mixtures; for this reason, the knowledge of the surface reactivity of different coal samples with respect to water and hydrocarbons is of great technological interest. Heat-flow microcdorirnetry used in connection with an adsorption volumetric line is a powerful research method of chemical surface reactivity of solid materia1s:gJO the + Istituto di
Chimica.
* Eniricerche.
(1)7 h e Science and Technoloev of Coal and Coal Utilization:Coouer. ~
B.R., Ellingson, W. A., Eds.; P l k m Press: New York, 1984.
(2) Fine Coal Processing; Mishra, S. K., Klimpel, R. R., Eds.; Noyes: Park Ridge, NJ, 1987. (3) Wheelock, T. D.; Markusxewski. In ref 1, p 47. (4) Mehrotra, V. P.; Sastry, K. V. S. Min. Eng. (N.Y.)1980,32, 1230. (5) Capes, C. E.; Germain, R. J. InPhysical Cleanirigof Coal: Present and Dewloping Methods; Liu, Y. A,, Ed.; Dekker: New York, 1982; p 293. (6) Swanson, A. R.; B e d e y , C. N.; Nicol, S. K. In Aggtornerotion 77; Sastry, K. V. S., Ed.; Society of Mining Engineers of AIME: New York, 1977; p 939. (7) Pellegrini, L.; Valentini, D.; Vettor, A. 6th Coal Conference, Pittsburgh, 1989. (8) Busca, G.; Milana, G.; Rossi, P. F.; Vettor, A. 1991 International Conference on Coal Science, September 16-20, 1991, Newcastle upon Tyie, pp 921-924. (9) Busca, G.; Kossi, P. F.; Lorenzeili, V.; Benaissa, M.; Travert, J.; Lavalley, J. C. J.Phys. Chem. 1985, 89, 5433.
vapor amount adsorbed step-by-step on the solid under selected pressure and temperature conditionsand the heata evolved during the adsorption process are simultaneously evaluated. This technique has been scarcely applied to the coal surface chemistry in the past.11-13 Following a previous preliminary study on the surface reactivity of Columbian we have undertaken a new microcalorimetric study of water and n-hexane adsorption on samples derived from two different coalswhose behavior in the oil agglomeration process is opposite, arising from Illinois and Poland, respectively. As reported elsewhere,8 Polish coal undergoes much easier agglomeration than the Illinois one. Both float and sink fractions derived by a preliminary separation through densimetric centrifugahave been investigated, in tion, as reported previ~usly,’~ order to differentiate the role of mineral matter and of “clean” organic-rich coal in the adsorption phenomena.
Experimental Section (a) Materials. Coal samples from Poland and Illinois have been investigated. n-Hexane used in calorimetric experiments is RPE from Carlo Erba (Milano, Italy). Polish coals are commercial samples easy to be found, and Illinois coals are furnished by the Department of Materials Science and Mineral Engineering-Berkeley, CA. The different fractions used for surface characterization experiments and the sequential separation procedures are reported in Chart I. (b) Separation Techniques. The densimetric Separation has been performed following the procedure reported in ref 14 (heavy liquid density 1.5 kg/dm3).The beneficiation proceos is described in refs 7 and 8. (c) Characterization Techniques. Ultimate analyses have been carried out by LECO CHN 60 (carbon-hydrogen-nitrogen LECO 60 analyLer)and SC 32 (sulfurC32 malymr). Petrologicd analysis has been performed by optical microscopy and mineralogic analysis by HTA (high-temperature analysis) arid LTAXRD (low-temperatureanalysis-X-ray diffraction). XPS analysis (X-rayphotoelectron spectroscopy)htio been performed with a VG ESCA Lab Mark I1 machine. l’he FT-1R spectra were (10) Hobsi, P. F.; Busca, G.; Loienzelli, V.; Yaur, 0.;Lavalley, J. C. Larigmuir 1987, 3, 52. (11)Kiselev, A. V. Second Interirutiorial Congress on Surface Acciutty, Proceedirigs; Butterwortha: London, 1957; p 168. (12) Melkus, T. A. Ph.D. Thesis, University of Pittsburgh, PA, 19%. (13) Rossi, P. F.; Vettor, A,; Milana, G.; Busca, G. Colloids Surf, 1991, 54, 297-311. (14) Del Piero, G.; Vettor, A. Proc. Cool Science, TOKYO, 1989; p 1002. (15) Rocisi, P. P.;Milana, G.; Vettor, A. Corigrks-AFCAT, Calorimdtrie e t Analyse Thermrgue, Clermont-Ferrand,1990,Proceedings; p 137.
0743-746319212~08-0~04$03.0010 0 1992 American Chemical Society
Surface Reactivity of Coals
Langmuir, Vol. 8, No. 1, 1992 105
C h a r t I. Separation Procedures
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Table I. Ultimate Analyses of Dry Coal Samples (% w/w) Illinois Polish ash 10.0 10.3 carbon 66.8 76.26 hydrogen 4.2 4.74 nitrogen 1.6 1.46 sulfur 4.7 0.66 oxygen (diff) 12.8 6.6 Table 11. Petrographic Analysis: Maceral Distribution of Coal Samples (% v/v) maceral Illinois Polish vitrinite 89.8 602 inertinite 8.0 30.9 exinite 1.5 8.8 recorded by a Nicolet 5ZDX spectrometer. The sink coal samples were pressed into KBr disks. (d) Microcalorimetric Apparatus. A heat-flow differential microcalorimeter, Calvet type, produced by Setaram (Lyon, France) interfaced t o an IBM-AT computer, has been used in connection with a Pyrex glass volumetric line, as reported previ0us1y.l~ About 0.5 g of the different coal samples, previously grounded by means of a rotary hammer mill, has been activated under vacuum at Torr and 130 "C, for 6 h, into the microcalorimetric cell, in order to eliminate the physisorbed water and eventual organic impurities. Successively, measured small vapor doses have been introduced step-by-step in contact with the coal samples into the microcalorimetric cell at 310 K, until saturation is reached. For each dose, microcalorimetric measurements have been performed. The significance and the reproducibility of measurements has been estimated by repeating the measures up to three times. The error on the measure of the evolved heats is estimated to be 3-5 % at most. A similar error is also evaluated for the volumetric measure of the adsorbed amounts. As demonstrated elsewhere, the differential heat measured experimentally in these conditions approximates the differential enthalpy of adsorption AHada,which also is a molar quantity.16
Fbsults (a) Bulk Coal Characterization. The results of characterization measurements on the two parent coals are summarized in Tables I-IV. Several data show that the two materials differ significantly. In particular, even if there is a comparable ash content, we can underline the much higher concentration of iron and lower of magnesium and calcium in the ashes of the Illinois with respect to the Polish material. The coals differ also in their composition in terms of macerals, the smallest physical components of the organic matter in coals distinguishable under the ~~
~~
~
(16) OnPhysicaladsorption;Ross, S., Olivier, J. P., Eds.; Interscience: New York, 1964.
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Figure 1. FT-IR spectra of PS and IS c o d samplatl in KBr disks (K = kaolinite and other silicates, C = calcite a/o dolomite, S = sulfates, H = OH linked with organic part of coal). Table 111. High-Temperature Ash Analyses (HTA) of Parent Coals ( % w/w) oxides
I11inois 47.7 17.4 22.5
Polish 44.9 20.9 9.3
1.0
1.0
3.8 0.9
4.9 3.3 0.6 1.3 6.7
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Table IV. Low-Temperature Ash Analyses (LTA) of Parent Coals (% w/w) phases quartz kaolinite calcite do1omite siderite pirite natrojarosite amorphous materials (diff)
Illinois 24.5 28.0
5.3 15.0 22.2
Polish 10.5 34.8 0.5 14.0 1.8 1.6 36.8
microscope.' It seems relevant to note the higher amount of vitrinite in the Illinois than in the Polish coal. To confirm the XRD data and to have further information about the nature of the amorphous phases present in ashes, we have analyzed the Illinois and Polish sink coal samples (ISand PS, respectively) by infrared spectroscopy (Figure 1). According to the data reported in Table IV, the inorganic phase present in IS and PY samples is responsible for some common absorptions typically due to aluminosilicates, such as kaolinite, which has the typical sharp OH stretching triplet between 3700 and 3600 cm-' as well as the strong bands at 1030 and 1005cm-' (doublet asymmetric Si-0 stretching),the quartet between 800 and 700 cm-' (symmetric Si-0 stretching), and the two strong deformation bands between 600 and 400 cm-l.17 These absorptions could be superimposed to those of other silicates or silicoalminates, including pure silica ( q w t z ) . In the IS sample, we observed the additional absorptions at 1180,1100,and 630 cm-', not present in the PS sample, due to inorganic sulfates (SO stretchings and deformation) such as jarosite KFe3(OH)&O& or natrojarosite NaFes~~
(17) Gadsden, G. A. Infrared Spectra of Minerals und Xelated Inorganic Compounds; Butteiworthu: Lo~iaoii,1975. (18)Painter, P.; Starsinic, M.; Coleman, M. 111 FT-IR Spectroscopy; Ferraro, J. R., Basile, J. S., Eds.; 1985; Vol. 4, p 169. (19) Gravelle P.C. Adu. Catal. 1972,22, 191.
Rossi et al.
106 Langmuir, Vol. 8,No. 1, 1992 Table V. Coal Samples Used for Calorimetric Experiments notation symbol sample ash content, wt % 2.76 PF Polish float 57.90 PS 0 Polish sink 4.94 IF o Illinois float 45.66 IS Illinois sink PBF Polish beneficiated float 2.63 PBS A Polish beneficiated sink 45.66 51.30 PR A Polish refuse Table VI. XPS Analyses of Coal Samples C species,n % surface composition, % sample AI Si C 0 S Fe red. ox. IF 10.1 12.6 43.7 29.2 1.2 3.2 72 28 PF 3.0 2.5 75.4 16.8 0.7 78 22 IS 12.1 19.7 24.6 37.9 1.6 3.9 63 38 PS 13.0 16.3 34.3 33.3 3.0 64 37 Red. = C-C,
CH; ox. = C-0,
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01
20
0
40
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Figure 2. Typical calorimetric peaks of water adsorption (a) and n-hexane adsorption (b) on coals at 310 K.
C=O, COOH.
(OH)&O4)2.17 The strong band at 3300 cm-l (OH stretching) is certainly due to an OH engaged in a hydrogen bond, and this could be due to the oxidized organic phase (phenolic or alcoholic groupsls). On the other hand, the PS sample shows two bands comparatively much stronger than those of the IS sample, at 1430 and 880 cm-' (symmetricstretching and CO deformation),typically due to carbonates, probably alkaline-earth metal carbonates (CaC03 calcite and/or CaMg(CO3)z dolomite17). These data, in agreement with those arising from XRD, show that the ash contained in the Illinois sample should be constituted by inorganic materials (sulfates and aluminosilicates) having an acidic character. In the Polish coal, on the contrary, according to the presence of Ca and Mg carbonates in bigger amounts, the inorganic phase should have a more pronounced basic character. Table V summarizes the bulk ash content in the different fractions obtained by the procedures reported in Chart I, which have been utilized for calorimetric experiments. The efficiency of purification through centrifugation, depending on mineral matter liberation degree, is clearly better in the case of the Polish coal, in spite of the similar ash content with respect to the Illinois coal. This is evidence of the different morphology of the two coals. On the other hand, it is even evident that the beneficiation process is less efficient than densimetric centrifugation, as expected. (b) Surface Coal Characterization. Surface coal composition has been determined by XPS, on the samples previously subjected to separation by centrifugation. Table I1 shows the maceral distribution of coal samples. The main datum arising from the comparison of bulk compositions (chemical analysis, Table V) and surface compositions (XPS data in Table VI) is that the coal surface appears to be enriched in inorganics relative to the bulk. This has already been observed for Columbian ~ 0 a l . l ~ The comparison of the surface and bulk data relative to corresponding Illinois (I) and Polish (P) samples evidences that, probably due to the different chemical nature of the two materials, the inorganic matter exposed on the surface of I samples is, relative to the bulk content, always much more than on P samples. It seems also that, particularly for float samples, the organic matter at the surface is slightly more oxidized on I than on P. (c) Calorimetric Experiments. Water Adsorption. To show the sensitivity of our calorimetric apparatus and the base line stability, typical peaks relative to adsorption of water and n-hexane on coal samples at low coverage are shown in Figure 2. The curves relative to water adsorption on different samples (Figures 3 and 4)clearly show a higher reactivity
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Figure 3. Volumetric isotherms of water on coals. Symbol meanings are reported in Table V.
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Figure 4. Differential heats of adsorption of water on coals. Symbol meanings are reported in Table V.
toward the water vapors of Illinois samples than those of Polish ones. The qualitative phenomena involved in each step can be discussed in relation to the curves reported in Figure 4,where the differential heats of adsorption (the average heats evolved during the adsorption of a single small dose of gas q d = AQ/Ana) are recorded versus the adsorbed amount expressed in number of adsorbed micromoles per gram (na pmol/g). The shape of this curve is similar in all cases as also in the cases of other coal the high ~amp1es.l~ According to our previousdiscu~sion,'~ heats evolved upon water adsorption at very small coverages (140-170 kJ/mol) by each sample can be attributed to the water interaction with the exposed
Langmuir, Vol. 8, No. 1, 1992 107
Surface Reactivity of Coals
, i __1
0 0
IC--
Figure 5. Volumetric isotherms adsorption of n-hexane on coals. Symbol meanings are reported in Table V.
inorganic part of the coal surface. The heats evolved during this step are in fact similar to those measured when water adsorbs on inorganic materials having an acidic nature.2@22 The adsorption heats in all cases rapidly diminish, falling to values (50-120 kJ/mol) that can be attributed predominantly to the water interaction with oxidized organic coal phase, covered by phenolic, carboxylic and, carbonylic groups. At the highest coverages, the evolved heats are much smaller (less than 50-10 kJ/mol) and can be associated to the interaction, much weaker, with reduced graphite-like parts of coal, being similar to those that are evolved when water adsorbs on pure graphite.13 Following this interpretation, the coal-water interaction should be dominated by the interaction of water with the inorganic matter, which can be by far stronger than that with the organic coal particles. The comparison of the curves of the volumetric isotherms (Figure 3) and of the differential adsorption heats (Figure4) shows that the Illinois samples are more reactive than Polish ones toward water. This higher reactivity is not explainable by a quantitative fact (ascribable for example to a higher surface area), but in terms of a qualitative condition as evidenced by the higher heats evolved. This difference could be ascribed to the different amount of inorganic material (see Experimental Section) or to the more reactive nature of the surface. In effect, although the reactivity toward the water in the series IS > IF > P F (IF = Illinois float, P F = Polish float) follows the trend of the decreasing amount of the inorganic material, the PS sample, very rich in inorganic material, behaves differently, being the least reactive toward water. This behavior can be rationalized, if we also take into account the nature of the inorganic material present in that preparation. We have shown above that the inorganics present in the P preparations have likely a predominant basic character, as deduced by the presence of Mg and Ca carbonates. On the contrary, inorganics present in the Illinois materials should have a stronger acidic nature. The comparison of the heats evolved upon water adsorption on metal oxides having different acid/ base characters shows that the evolved heats are stronger on acidic than on basic ionic oxides.22 Even lower heat evolutions can be expected on solids as alkaline-earth carbonates and on covalent materials.22 (20) Rossi, P. F.; Busca, G. Mater. Chem. Phys. 1983, 9, 561. (21) Borello, E.; Della Gatta, G.; Fubini, B.; Morterra, C.; Venturello, G. J . Catal. 1974, 35, 1. (22) Fubini, B.; Bolis, V.; Bailes, M.; Stone, F. S. Solid State Ionics 1989, 32/33, 258.
50
100
203
250
300 iapmol/g
Figure 6. Differential heats of adsorption of n-hexane on coals. In the inset is shown the expanded scale, only Polish coals. Symbol meanings are reported in Table V.
This interpretation is supported by the results obtained for the PBF and PBS (Polish beneficiated float and sink, respectively) samples. These samples have a very similar behavior, respectively, to P F and PS samples. But, the reactivity with water of PBF and PBS, slightly greater than the corresponding samples not arising from previous oil beneficiation (PF and PS), can be inversely related to the inorganic material amount. So, when basic or hydrophobic inorganic materials are involved, their reactivity with water is small, being comparable with that of the oxidized part of coal. Consequently, the reactivity with water should in this case be inversely related to the inorganic content. The behavior of the Polish sample can consequently be explained with the presence in its inorganic matter of small amounts of materials having an acidic nature (silicoaluminates),responsible for the very first adsorption step, corresponding to high heat evolution, and of a predominant presence of hydrophobic inorganic material. The behavior of the Polish sample refused from the beneficiation process (PR), poorly reactive, agrees again roughly with its mineral matter composition. In conclusion, the reactivity (hydrophilic character) of Illinois samples higher than the Polish ones should be mainly ascribed to the acid nature of its inorganic material. The difference between the behaviors of the Illinois float and sink samples, comparatively small with respect to the strong difference in the inorganic material bulk content, shows that, in agreement with the XPS data, the surface is much richer in the (acidic) inorganic material than the bulk. (b) n-Hexane Adsorption. The Illinois samples are by far more reactive than Polish ones also toward n-hexane, especially from the point of view of the adsorption energies. This can be deduced from the experimental differential adsorption heats (Figure 6) and, even more clearly, from the integral adsorption heats (total heat evolved) measured at the same coverage. For instance, the Qint value at the coverage of 100 pmol/g is near 5.0 J/g for IS and near 4.0 J/g for IF while less than 2.0 J/g for all Polish samples. Accordingto our previous in~estigation,'~ the n-hexane interaction with the graphite-like organic part of the coal surface is expected to be weak, only attributable to van der Waals forces. Accordingly, adsorption heats evolved on pure graphite13 are very low (nearly 10 kJ/mol). Consequently, we assign prevalently to this weak interaction the low heats evolved at high hexane coverages (high
108 Langmuir, Vol. 8, No. 1, 1992
equilibrium pressures). On the contrary, the stronger interactions observed at low coverages, with the evolution of heats of the order of 120-60 kJ/mol should involve the inorganic matter surface, particularly if having an acidic nature, as found in the Illinois coals. The strength of exothermic interactions of this type has been confirmed experimentally when hydrocarbons are adsorbed on acidic solids.23~2~The interaction of the hexane with the coal oxidized organic part should evolve intermediate heats (near 50 kJ/mol). In accordance with the previous data, then, the strong exothermic effects measured during the n-hexane adsorption on the IS sample and, less, on the IF sample are ascribed to the large amount of inorganic acid material at the coal surface as well as to oxidized organic matter. The n-hexane interaction with the Polish coals is, except the very f i s t steps, much less exothermic,and this could reflect again the basic or covalent nature of the inorganic material, as well as the lower oxidation state of the organic coal matter. On the other hand, the fairly small differences between PS and P F samples, in spite of the big difference in their organic/inorganic composition balance, could show that the basic inorganic material produces by adsorption with hexane heats similar to the organic part. The comparison of the differential heat curves of the Polish and the Illinois coals shows that only in the latter case a plateau can be seen, corresponding to heats of nearly 40 kJ/mol. Also upon adsorption of hexane on Columbian whose ash composition is similar to the Illinois one, such a plateau was observed. As cited above, these heats can be evolved by interaction of the hydrocarbon with the oxidized organic surface, containing hydroxy, carboxy, and carbonyl groups. Consequently,we can conclude that the organic material is not strongly oxidized in the Polish samples, according to its less reactive maceral composition (Table 11). The behavior of PBF and PBS samples compared to PF and PS shows that the effect of the previous beneficiation process before centrifugation is again poor, although in this case it is "positive", because such a previous process increases the interaction of the float material with n-hexane and minimizes that of the sink one. (23) Kunath, D.; Schulz, D. J. Colloid Interface Sci. 1978, 66, 379. (24) Rossi, P. F. Unpublished results.
Rossi et al.
Discussion The comparison of the data summarized above is evidence first of all that the Illinois coal samples react more than the Polish ones with both water and n-hexane. This behavior can be well related to the lower agglomerability of the Illinois material than the Polish one in the oil agglomeration process, as reported elsewhere.8 This also agrees with the better mineral matter removal obtained by centrifugation of the Polish than of the Illinois coal, as well as with the much higher amount of inorganics present at the surface on the Illinois than Polish coal samples arising from centrifugation, as evidenced by XPS data. Accordingto the analysis data, the behavior of each fraction can be related neither to their bulk inorganic content nor to their maceral distribution. On the contrary, we can conclude that this behavior is related to the nature (acidic or basic, hydrophilic or hydrophobic) of the inorganic material included in coals and largely exposed on their surface. The acid nature of the inorganic material in the Illinois coal should be predominantly responsible for the stronger interaction with both water and the nhexane vapors. This is rationalized, taking into account that the intensities of the polar interactions (including dipole-induced dipole) are greater than those of the apolar interaction^.^^ It seems also reasonable to propose that the stronger acidic nature of the surface of the inorganic particles alsostrengthenstheir interaction with the surface of the organic coal particles, through ionic or coordination bondings with hydroxy, carboxy, and carbonyl groups, as already proposed.13 This would affect strongly the agglomerability. Accordingto this, it seems very reasonable to propose that the key factor inducing easy agglomerability of coals in the oil phase in the presence of water is their low reactivity with water, essentially related to a small content of acidic inorganic materials.
Acknowledgment. This work has been supported by Eniricerche. Registry No. HzO, 7732-18-5; H&(CHz)&Hs, 110-54-3. (25) Ben-Naim. HydrophobicInteractions;Plenum Press:
1980.
New York,
