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The influence of coal rank and mineral matter content on contact angle hysteresis
The influence of coal rank and mineral matter content on contact angle hysteresis Russell J. Crawford,
David
W. Guy and David
E. Mainwaring
Centre for Applied Colloid and BioColloid Science, Department of Applied Chemistry, Swinburne University of Technology, PO Box 218, Hawthorn, 3122, Australia (Received 14 June 1993; revised 76 July 1993)
The degree of surface hydrophobicity of a range of coal types has been previously shown to be dependent on the carbon to oxygen ratio (or coal rank). The surface hydrophobicity of these coals increases during agglomeration due to substantial removal of the mineral matter content of the coals. It has been found that the degree of hysteresis between the advancing and receding contact angles for light gas oil in water against the coal surfaces decreases as the rank of the coal decreases. This is due to the lower rank coals possessing an organic component which is closer in surface free energy to that of the inorganic constituent of the coal compared to that of the higher rank coals (Keywords:
agglomeration;
mineral
matter;
Coal is an important
hysteresis)
resource to many countries, since it not only represents a large part of the total fossil fuel reserves but also often represents a significant export commodity. Coal quality is important to this trade. With increasing quantities of fine coal needing separation and recovery, the efficiency of coal size enlargement and enrichment processes gains added importance. Two such processes, oil agglomeration and fine coal flotation require the spreading of a continuous oil film over the coal surface already wetted by water for oil to be used as a bridging liquid, or collector, respectively. Such processes are highly dependent on the surface characteristics’ of the coals under investigation. Direct measurement of the equilibrium contact angle has been used extensively for the purposes of assessing this surface hydrophobicityzp4. Two other techniques have recently been employed to determine the surface wetting behaviour of coal when in a fine polydisperse particulate form. Tampy et ~1.~used a modified Washburn technique6 to compare the contact angles and wettability of various coals by capillary rise in packed beds of coal, whereas Fuerstenau et al.’ and Ramesh and Somasundaran’ used a film flotation technique to determine, from the fraction of coal particles that sink or float on liquid films of different surface tension, a distribution of surface wetting energies. Recently Guy et al.’ have shown that the ability or inability of a coal surface to form continuous wetting oil films is dependent upon the surface hydrophobicity, which is related to the carbon to oxygen ratio and hence the coal rank. For such films to form, the oil must overcome the thermodynamic quantity, i.e. the minimum spreading pressure. All coal surfaces had appreciable advancing contact angles, hence the equilibrium spreading pressures were found to be significant for all coal ranks examined. Additional information on the constitution of the coal surface can be gained from considering both the advancing and receding contact angles, and their difference or contact angle hysteresis. In this work we 0016-2361/94/05/0742AU 0 1994 Butterworth-Heinemann
742
Fuel 1994
Volume
Ltd
73 Number
5
supplement the data on advancing contact angles and use contact angle hysteresis to probe the surface chemical heterogeneity as the mineral matter is reduced to the theoretical zero limit and relate it to a rank parameter, the carbon to oxygen ratio. THEORY Detailed discussions are availableg-‘2 on the factors which contribute to the differences between advancing and receding contact angles. As illustrated previously’? the contact angle, 8, at equilibrium for any point where two immiscible liquid phases [e.g. oil (0) and water (W)] meet on a solid surface with an adsorbed film of oil of surface pressure, 7c, [e.g. coal (C)l, is given by the Young equation:
70/w cos 8’ + Ye/o= YC” - %
(1)
where the y terms represent the corresponding surface and interfacial tensions, and 8’ the contact angle measured through the oil phase. As rc, is defined as the reduction in ycOdue to the adsorption of oil molecules at equilibrium, Ye/w= Ye - 71,
(2)
and hence the relationship can be written as: 70/w cos 8’ + Ycio= Ye/w
(3)
This situation is illustrated in Figure 1, where at equilibrium, 8’ (and hence yolw cos 0’) represents a unique description of the line of triple phase contact (TPL). Blake and Haynes” noted that subsequent movement of the TPL on a solid surface may be regarded as a dependent thermodynamic variable. If we select as the independent variable the resulting component of the interfacial tension, yojw cos 8’, for a coal surface (simplified by non-inclusion of any spreading pressure component) and plot it as a function of distance moved on the coal surface, it can be seen that without contact
Contact angle hysteresis: R. J. Crawford et al. angle hysteresis a straight line is obtained as the TPL moves from A to B as depicted in Figure 2. As Blake and point out for their analysis of fluid-fluid Haynes” movement in a capillary tube, there is only one value of the independent variable for which the meniscus can exist in a state of mutual equilibrium for any position of the TPL. Hence the contact angle defined above may be called the equilibrium contact angle, @:. Contact angle hysteresis results in the formation of the hysteresis loop A’A”B”B’, also depicted in Figure 2. Here, yolw cos 8’ and the contact angle, 8’, vary between any of the metastable states on the coal surface (bounded by K, the advancing contact angle, and &, the receding contact angle). This hysteresis loop represents the irreversible work unrecovered in the liquid-liquid interface moving and returning to a given point on the coal surface. Neumann12 , in analyses based on free energy changes, points out that although the experimental contact angles f$ and 13: are not the thermodynamic
Figure 1 Diagrammatic representation of the surface and interfacial forces acting on a droplet of oil on a solid coal surface immersed in water, with the contact angle, B’, measured through the oil phase
equilibrium contact angle, they may be inserted in the Young equation. de Gennes has recently13 detailed the major causes of hysteresis as surface roughness, surface heterogeneities and surface adsorption of solids. Hiemenz14 reports that surface chemical heterogeneity usually dominates contact angle hysteresis unless a particularly large surface roughness is involved. The earlier work on the importance of surface roughness’,’ 5 was confined to regular systems of grooves. Subsequently, de Gennes’ 3 reinterpreted the data numerically to provide estimates of the energy barriers involved, and extended this analysis to random surfaces. By considering random weak surface fluctuations to include both random shape and random chemical composition, de Gennes has demonstrated that surface roughness and chemical composition problems coincide to a first order approximation and can be represented by a random function. Random irregularities on a surface of either type having a correlation length of 1 pm were found to influence the displacement of the TPL up to 10 pm. Coal surfaces are chemically heterogeneous due to the contributions from the respective organic and inorganic components. The degree of difference in nature between these components, i.e. their interfacial tensions or, equivalently, their surface free energies, are directly proportional to the hysteresis. Attempts have been made by several workers2v4 to theoretically consider coal surfaces in terms of the relative contributions of the various area fractions of the coal constituents using adaptations of the Cassie equation16,“. Such applications of a simple patchwork model have had success when applied to simple model systems’*, but have had limited success when applied to more complex surfaces such as that of coal. Tampy et ~1.‘~showed that oils of different composition wetted various ranks of coal according to the surface energies of the coal surfaces, and that these were influenced by the presence of polar groups, surface moisture and mineral matter. EXPERIMENTAL
)..-_J
Distance
on coal surface
Figure 2 Schematic representation of the change in yo,w cos 0’ as a function of position of the three phase line of contact (TPL). Points A and B represent contact angles and positions on the coal surface in the absence of hysteresis, whereas points A’, A”, B” and B’ represent contact angles and positions on the coal surface in the presence of hysteresis
Table 1
Ultimate
Coal
analyses
(dry mineral
matter
free wt%) of coal samples
Type
Norwich
Park
Goonyella
Low volatile
bituminous
Medium
volatile
bituminous
Wongawilli
Medium
volatile
bituminous,
Mount
High volatile
Arthur
Collie
Medium
Loy Yang
Brown
bituminous
volatile coal
subbituminous
high mineral
matter
A detailed account of the coals used, agglomeration procedures and descriptions of the reagents used have been reported previously’. Table I gives the elemental composition of the coals used in this study. The liquids used were light gas oil, which had a surface tension of 28.6 m Nm- ’ at 20°C and high purity conductivity water, which had a specific conductivity less than 1x10-6Q-1cm-1andasurfacetensionof72.8mNm-1 at 20°C. Advancing and receding contact angles were measured according to the well known captive drop technique20*‘l on coal samples which had been pressed into plates
used C
H
N
s
89.5
4.8
1.6
0.7
3.4
87.9
5.1
1.8
0.5
4.7
86.4
5.2
1.7
0.6
6.1
83.1
5.3
1.8
0.4
9.4
73.6
4.3
1.2
0.7
20.2
68.4
4.5
0.5
0.4
26.2
Fuel 1994
Volume
73 Number
0
5
743
Contact angle hysteresis: R. J. Crawford
et al.
provide a sample with a smooth flat surface2*. Photographs of the three phase line of contact were taken through a modified microscope with an attachment for mounting a micrometer syringe. The contact angles were measured through the oil phase (determined by direct measurement from photographic images to a precision of + 2”) and were either advancing or receding, depending on whether the oil phase was advancing over, or receding from the coal surface immersed in conductivity water. Advancing and receding contact angles were also measured on the coal samples, which could successfully be agglomerated. These samples were reground, acetone washed in a Soxhlet extractor to remove any oil present and oven dried at 65°C in nitrogen for 4 h to remove any residual acetone. The resulting coal was pressed into discs and contact angles measured. In order to determine the contact angles on the mineral matter free coal, samples of each particular coal type were prepared with varying degrees of mineral matter using the mineral matter liberation pattern of fine coal fractions and a two liquid separation process as described by Guy et al.‘. Their contact angles were then measured. Intrinsic contact angles, characteristic of the mineral matter free coal component, were obtained by plotting contact angle against percentage mineral matter and extrapolating back to give the zero mineral matter limits. To ensure that the liquids used in oil agglomeration and two liquid separations were efficiently removed prior to contact angle measurement, raw coal samples were subjected to identical procedures. The resulting contact angles for raw coal were maintained within the experimental error of the procedure for contact angle measurement. to
RESULTS
AND DISCUSSION
Advancing and receding oil in water contact angles (averaged from at least two trials) were measured on coal samples both before and after reduction of mineral matter by oil agglomeration according to the method described by Guy et al.‘. Loy Yang and Collie coals were not able to be successfully agglomerated. The contact angles measured represent a macroscopic average of the surface properties of the individual surface sites. One measure of the balance between these surface properties is the carbon to oxygen ratio, high carbon contents being associated with primarily hydrophobic surface sites, and the oxygen content being associated with primarily hydrophilic sites such as phenolic and carboxylic functional groups. The advancing and receding contact angles on one coal (Norwich Park) at various mineral matter contents is shown in Figure 3 together with the lines of best fit which provide the respective mineral free limits. From the slopes of the respective lines, Figure 3 shows that the advancing oil contact angle has a greater sensitivity to the surface content of hydrophilic mineral matter, i.e. the degree of hysteresis for any one coal is increased with hydrophilic mineral matter content. Figures 4a-c show that the processes associated with oil agglomeration, i.e. selective wetting and aggregation of the coal and rejection of the mineral matter into the aqueous phase, decreases both the advancing and receding oil contact angles. The hydrophobicity of the coal surface is increased by this process due to the removal of mineral matter from the coal surface constitution. This progressive decrease of the advancing
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and receding oil contact angles continues to the theoretical mineral matter free limit (Figure 4~). It is notable from Figures 4a and b that the contact angle hysteresis becomes more pronounced as the rank of the coal increases. This provides direct evidence that the principal cause of hysteresis is surface chemical heterogeneity, since in the higher rank coals, the difference between the surface free energy of the mineral matter and the organic coal matter is more profound than that in the lower rank coals. Comparison of Figures 4a and b also indicates that reduction of the overall mineral matter in each coal resulted in a corresponding general reduction in the degree of hysteresis, & -9:. This again indicates that contact angle hysteresis is dominated by the surface chemical heterogeneity between the surface mineral matter sites and the organic coal sites. Figure 4c shows the corresponding theoretical advancing and receding intrinsic contact angles at the mineral free limit. It can be seen that at the extrapolated mineral free limit the advancing and receding contact angles tend to converge. This indicates that on the smooth pressed discs, removal of surface heterogeneity induced by mineral matter produces a corresponding reduction in the thermodynamic hysteresis. The role of the surface mineral matter constitution on the free energy and wetting behaviour of coal surfaces has been investigated previously by demineralization using HF1g*23. Such HF treatment may alter the surface hydrophobicity through both organic functionality and inorganic mineral residues, whereas extrapolation back to an equivalent mineral free surface composition from samples with various degrees of physically liberated mineral matter does not chemically alter the surface, although a functional form of the mineral matter dependence must be assumed. The approach of the advancing and receding contact angles (Figure 4c) yields a smooth monotonic decline of contact angle as the rank of the coal, indicated by the atomic carbon to oxygen ratio, is increased. As discussed previously, in the mineral matter free limit contact angle,
I
I
I
I
1 3
60”
8 s
40’
I
I
I
I
I
2
4
6
8
%
I
Mineral matter
Figure 3 Advancing and receding contact angles, o’, measured on Norwich Park coal samples of various mineral matter contents. Extrapolation indicates the zero mineral matter contact angle limit
Contact angle hysteresis: R. J. Crawford
et al.
advancing and receding
L
L
I
I
5
10
15
I
20
I
25
1
5
I
I
10
15
I
20
1
I
25
I
5
I
I
10
15
1
20
I
25
Carbon / oxygen ratio (mineral matter free basis) Figure 4 Advancing and receding contact angles, O’, as a function of carbon to oxygen ratio for various agglomeration, and (c) at the theoretical mineral matter free limit. 0, Loy Yang; V, Collie; v, Mount A, Norwich Park
any hysteresis largely results from the organic coal surface composition itself; and it is instructive to examine the appropriate length scales which influence these contact angles. While all coal surfaces are heterogeneous on a molecular scale, Figure 4c suggests that a smooth composite surface formed from coal particles of average particle size 20pm yields macroscopic contact angles which reflect the bulk surface atomic composition. This is consistent with inducible fluctuations in the TPL of upto10pm l 3. Although the microscopic constituents of coal, the macerals, have characteristic sizes exceeding 3 pm, the composite surfaces produced do not appear to reflect possible differences in wetting behaviour due to variations in elemental composition. Neumann has shown” that althoug h free energy considerations do not provide a lower limit of the lateral dimension of heterogeneities that produce contact angle hysteresis, this may operationally be estimated from the profile of the TPL to be about 0.1 pm. The approach of advancing and receding contact angles shown in Figure 4c produced from smooth composite coal surfaces is consistent with Neumann’s conclusion regarding patchwise heterogeneous surfaces; i.e. that if the TPL is straight and if the average composition of the surface is constant, there are no metastable states and no contact angle hysteresis occurs. de Gennesi3 has also shown that weak perturbations on a surface strictly create no hysteresis and an ideal surface is not needed for the determination of thermodynamic contact angles, but rather surfaces with irregularities below a certain threshold. The disappearance of contact angle hysteresis at the extrapolated mineral free limit on these composite surfaces, which are composed of a homogeneous mixture
coals (a) before agglomeration, (b) after Arthur; 0, Wongawilli; n , Goonyella;
of the heterogeneous organic components (macerals), is consistent with the earlier film flotation studies on individual particles*. Surface hydrophobicity and wetting will alter as the surface composition of the coals changes due to either different macerals at the same rank or individual macerals over a range of ranks. The changing composition of such macerals has been followed elegantly by cross polarization and magic angle spinning (CP/MAS) nuclear magnetic resonance (nmr.) techniques 24 in terms of both aromaticity and oxygen functionality. Potentially, useful studies on wetting, film spreading and contact angle hysteresis may be carried out on planar maceral surfaces to demonstrate the interfacial impact of these changes.
CONCLUSIONS The degree of contact angle hysteresis is a measure of the surface chemical heterogeneity of the coal. Lower rank coals possess an organic component which is closer in surface nature to that of the inorganic constituent of the coal compared to that of the higher rank coals. Hence, as the rank of the coal increases, the contact angle hysteresis also increases. The similarity in slope of the advancing and receding contact angle, @, versus carbon-oxygen ratio for coal before and after agglomeration indicates that for these mineral matter contents, the surface wetting by the advancing oil film (hydrophobicity of the coal surface) is dominated by the oxygen functionality of the organic component of the coal surface rather than the mineral matter content or type. Where the coal has an extremely high mineral matter content (e.g. raw Wongawilli coal has a mineral matter
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Contact angle hysteresis: R. J. Crawford
et al.
content of 24.4 wt%) the contact angle is significantly above the characteristic curve.
ACKNOWLEDGEMENT The authors wish to thank the Coal Corporation of Victoria for their continued support of coal research at Swinburne University of Technology.
10
11 12 13 14 15
(Ed. E. Matijevic), Vol. 2, Wiley-Interscience, New York, 1969, p. 85 Blake, T. D. and Haynes, J. M. in ‘Progress in Surface and Membrane Science’ (Eds J. F. Danielh, M. D. Rosenberg and D. A. Cadenhead), Academic Press, New York, 1970, p. 125 Finch, J. A. and Smith. G. W. Minerals Sci. Enma 1979,11(l). 36 Neumann, A. W. Ad;. Coil. Inc. Sci. 1974, 4: lb5 de Gennes. P. G. Rev. Mod. Phvsics 1985. 57 (3). 827 Hiemenz, P. C. ‘Principles of dolloid and Surface Chemistry’, Marcel Dekker, New York, 1977, p. 231 Mason, S. G. in ‘Wetting, Spreading and Adhesion’ (Ed. J. F. Paddav)., Academic Press. New York. 1978. D. 321 Cassie, A. B. D. Discuss. Faraday Sot. 1948, 3, flCassie, A. B. D. and Baxter, S. Trans. Faraday Sot. 1944,40,456 Crawford, R., Koopal, L. K. and Ralston, J. CoUoids and Surfaces 1987, 27, 57 Tampy, G. K., Prudich, M. E., Savage, R. L. and Williams, R. R. Energy and Fuels 1988, 2, 787 Padday, J. F. (Ed.) in ‘Wetting, Spreading and Adhesion’, Academic Press, London, 1978, p. 127 Good, R. J. Sur$ Colloid Sci. 1979, 11, 1 Labuschagne, B. C. J. Coal Prep. 1986, 3, 1 Fuerstenau, D. W., Yang, G. C. C. and Laskowski, J. Coal Prep. 1987,4, 161 Pugmire, R. J., Zilm, K. W., Grant, D. M., Larter, S. R., Allen, J., Senftle, J. T., Davis, A. and Spackman, W. in ‘New Approaches in Coal Chemistry’ (Eds B. D. Blaustein, B. C. Bockrath and S. Friedman), ACS Symposium Series 169, Washington DC, 1981, p. 23 I,
16 17 18
REFERENCES 1 2 3 4 5 6 7 8 9
746
Guy, D. W., Crawford, R. J. and Mainwaring, D. E. Fuel 1992 71, 935 Keller, D. V. Jr Cobids and Surfaces 1987, 22, 21 Gutierrez-Rodriguez, J. A., Purcell, R. J. and Aplan, F. F. Colloids and Surfaces 1984, 12, 1 Rosenbaum, J. M. and Fuerstenau, D. W. Inc. J. Miner. Process. 1984, 12, 313 Tampy, G. K., Chen, W.-J., Prudich, M. E. and Savage, R. L. Energy and Fuels 1988, 2, 782 Washburn, E. W. Phys. Rev. 1921, 17, 273 Fuerstenau, D. W., Diao, J. and Hanson, J. S. Energy and Fuels 1990,4, 34 Ramesh, R. and Somasundaran, P. Coal Prep. 1991, 9, 121 Johnson, R. E. Jr and Dettre, R. H. ‘Surface and Colloid Science’
Fuel 1994 Volume
73 Number 5
19 20 21 22 23 24
David
W. Guy and David
E. Mainwaring
Centre for Applied Colloid and BioColloid Science, Department of Applied Chemistry, Swinburne University of Technology, PO Box 218, Hawthorn, 3122, Australia (Received 14 June 1993; revised 76 July 1993)
The degree of surface hydrophobicity of a range of coal types has been previously shown to be dependent on the carbon to oxygen ratio (or coal rank). The surface hydrophobicity of these coals increases during agglomeration due to substantial removal of the mineral matter content of the coals. It has been found that the degree of hysteresis between the advancing and receding contact angles for light gas oil in water against the coal surfaces decreases as the rank of the coal decreases. This is due to the lower rank coals possessing an organic component which is closer in surface free energy to that of the inorganic constituent of the coal compared to that of the higher rank coals (Keywords:
agglomeration;
mineral
matter;
Coal is an important
hysteresis)
resource to many countries, since it not only represents a large part of the total fossil fuel reserves but also often represents a significant export commodity. Coal quality is important to this trade. With increasing quantities of fine coal needing separation and recovery, the efficiency of coal size enlargement and enrichment processes gains added importance. Two such processes, oil agglomeration and fine coal flotation require the spreading of a continuous oil film over the coal surface already wetted by water for oil to be used as a bridging liquid, or collector, respectively. Such processes are highly dependent on the surface characteristics’ of the coals under investigation. Direct measurement of the equilibrium contact angle has been used extensively for the purposes of assessing this surface hydrophobicityzp4. Two other techniques have recently been employed to determine the surface wetting behaviour of coal when in a fine polydisperse particulate form. Tampy et ~1.~used a modified Washburn technique6 to compare the contact angles and wettability of various coals by capillary rise in packed beds of coal, whereas Fuerstenau et al.’ and Ramesh and Somasundaran’ used a film flotation technique to determine, from the fraction of coal particles that sink or float on liquid films of different surface tension, a distribution of surface wetting energies. Recently Guy et al.’ have shown that the ability or inability of a coal surface to form continuous wetting oil films is dependent upon the surface hydrophobicity, which is related to the carbon to oxygen ratio and hence the coal rank. For such films to form, the oil must overcome the thermodynamic quantity, i.e. the minimum spreading pressure. All coal surfaces had appreciable advancing contact angles, hence the equilibrium spreading pressures were found to be significant for all coal ranks examined. Additional information on the constitution of the coal surface can be gained from considering both the advancing and receding contact angles, and their difference or contact angle hysteresis. In this work we 0016-2361/94/05/0742AU 0 1994 Butterworth-Heinemann
742
Fuel 1994
Volume
Ltd
73 Number
5
supplement the data on advancing contact angles and use contact angle hysteresis to probe the surface chemical heterogeneity as the mineral matter is reduced to the theoretical zero limit and relate it to a rank parameter, the carbon to oxygen ratio. THEORY Detailed discussions are availableg-‘2 on the factors which contribute to the differences between advancing and receding contact angles. As illustrated previously’? the contact angle, 8, at equilibrium for any point where two immiscible liquid phases [e.g. oil (0) and water (W)] meet on a solid surface with an adsorbed film of oil of surface pressure, 7c, [e.g. coal (C)l, is given by the Young equation:
70/w cos 8’ + Ye/o= YC” - %
(1)
where the y terms represent the corresponding surface and interfacial tensions, and 8’ the contact angle measured through the oil phase. As rc, is defined as the reduction in ycOdue to the adsorption of oil molecules at equilibrium, Ye/w= Ye - 71,
(2)
and hence the relationship can be written as: 70/w cos 8’ + Ycio= Ye/w
(3)
This situation is illustrated in Figure 1, where at equilibrium, 8’ (and hence yolw cos 0’) represents a unique description of the line of triple phase contact (TPL). Blake and Haynes” noted that subsequent movement of the TPL on a solid surface may be regarded as a dependent thermodynamic variable. If we select as the independent variable the resulting component of the interfacial tension, yojw cos 8’, for a coal surface (simplified by non-inclusion of any spreading pressure component) and plot it as a function of distance moved on the coal surface, it can be seen that without contact
Contact angle hysteresis: R. J. Crawford et al. angle hysteresis a straight line is obtained as the TPL moves from A to B as depicted in Figure 2. As Blake and point out for their analysis of fluid-fluid Haynes” movement in a capillary tube, there is only one value of the independent variable for which the meniscus can exist in a state of mutual equilibrium for any position of the TPL. Hence the contact angle defined above may be called the equilibrium contact angle, @:. Contact angle hysteresis results in the formation of the hysteresis loop A’A”B”B’, also depicted in Figure 2. Here, yolw cos 8’ and the contact angle, 8’, vary between any of the metastable states on the coal surface (bounded by K, the advancing contact angle, and &, the receding contact angle). This hysteresis loop represents the irreversible work unrecovered in the liquid-liquid interface moving and returning to a given point on the coal surface. Neumann12 , in analyses based on free energy changes, points out that although the experimental contact angles f$ and 13: are not the thermodynamic
Figure 1 Diagrammatic representation of the surface and interfacial forces acting on a droplet of oil on a solid coal surface immersed in water, with the contact angle, B’, measured through the oil phase
equilibrium contact angle, they may be inserted in the Young equation. de Gennes has recently13 detailed the major causes of hysteresis as surface roughness, surface heterogeneities and surface adsorption of solids. Hiemenz14 reports that surface chemical heterogeneity usually dominates contact angle hysteresis unless a particularly large surface roughness is involved. The earlier work on the importance of surface roughness’,’ 5 was confined to regular systems of grooves. Subsequently, de Gennes’ 3 reinterpreted the data numerically to provide estimates of the energy barriers involved, and extended this analysis to random surfaces. By considering random weak surface fluctuations to include both random shape and random chemical composition, de Gennes has demonstrated that surface roughness and chemical composition problems coincide to a first order approximation and can be represented by a random function. Random irregularities on a surface of either type having a correlation length of 1 pm were found to influence the displacement of the TPL up to 10 pm. Coal surfaces are chemically heterogeneous due to the contributions from the respective organic and inorganic components. The degree of difference in nature between these components, i.e. their interfacial tensions or, equivalently, their surface free energies, are directly proportional to the hysteresis. Attempts have been made by several workers2v4 to theoretically consider coal surfaces in terms of the relative contributions of the various area fractions of the coal constituents using adaptations of the Cassie equation16,“. Such applications of a simple patchwork model have had success when applied to simple model systems’*, but have had limited success when applied to more complex surfaces such as that of coal. Tampy et ~1.‘~showed that oils of different composition wetted various ranks of coal according to the surface energies of the coal surfaces, and that these were influenced by the presence of polar groups, surface moisture and mineral matter. EXPERIMENTAL
)..-_J
Distance
on coal surface
Figure 2 Schematic representation of the change in yo,w cos 0’ as a function of position of the three phase line of contact (TPL). Points A and B represent contact angles and positions on the coal surface in the absence of hysteresis, whereas points A’, A”, B” and B’ represent contact angles and positions on the coal surface in the presence of hysteresis
Table 1
Ultimate
Coal
analyses
(dry mineral
matter
free wt%) of coal samples
Type
Norwich
Park
Goonyella
Low volatile
bituminous
Medium
volatile
bituminous
Wongawilli
Medium
volatile
bituminous,
Mount
High volatile
Arthur
Collie
Medium
Loy Yang
Brown
bituminous
volatile coal
subbituminous
high mineral
matter
A detailed account of the coals used, agglomeration procedures and descriptions of the reagents used have been reported previously’. Table I gives the elemental composition of the coals used in this study. The liquids used were light gas oil, which had a surface tension of 28.6 m Nm- ’ at 20°C and high purity conductivity water, which had a specific conductivity less than 1x10-6Q-1cm-1andasurfacetensionof72.8mNm-1 at 20°C. Advancing and receding contact angles were measured according to the well known captive drop technique20*‘l on coal samples which had been pressed into plates
used C
H
N
s
89.5
4.8
1.6
0.7
3.4
87.9
5.1
1.8
0.5
4.7
86.4
5.2
1.7
0.6
6.1
83.1
5.3
1.8
0.4
9.4
73.6
4.3
1.2
0.7
20.2
68.4
4.5
0.5
0.4
26.2
Fuel 1994
Volume
73 Number
0
5
743
Contact angle hysteresis: R. J. Crawford
et al.
provide a sample with a smooth flat surface2*. Photographs of the three phase line of contact were taken through a modified microscope with an attachment for mounting a micrometer syringe. The contact angles were measured through the oil phase (determined by direct measurement from photographic images to a precision of + 2”) and were either advancing or receding, depending on whether the oil phase was advancing over, or receding from the coal surface immersed in conductivity water. Advancing and receding contact angles were also measured on the coal samples, which could successfully be agglomerated. These samples were reground, acetone washed in a Soxhlet extractor to remove any oil present and oven dried at 65°C in nitrogen for 4 h to remove any residual acetone. The resulting coal was pressed into discs and contact angles measured. In order to determine the contact angles on the mineral matter free coal, samples of each particular coal type were prepared with varying degrees of mineral matter using the mineral matter liberation pattern of fine coal fractions and a two liquid separation process as described by Guy et al.‘. Their contact angles were then measured. Intrinsic contact angles, characteristic of the mineral matter free coal component, were obtained by plotting contact angle against percentage mineral matter and extrapolating back to give the zero mineral matter limits. To ensure that the liquids used in oil agglomeration and two liquid separations were efficiently removed prior to contact angle measurement, raw coal samples were subjected to identical procedures. The resulting contact angles for raw coal were maintained within the experimental error of the procedure for contact angle measurement. to
RESULTS
AND DISCUSSION
Advancing and receding oil in water contact angles (averaged from at least two trials) were measured on coal samples both before and after reduction of mineral matter by oil agglomeration according to the method described by Guy et al.‘. Loy Yang and Collie coals were not able to be successfully agglomerated. The contact angles measured represent a macroscopic average of the surface properties of the individual surface sites. One measure of the balance between these surface properties is the carbon to oxygen ratio, high carbon contents being associated with primarily hydrophobic surface sites, and the oxygen content being associated with primarily hydrophilic sites such as phenolic and carboxylic functional groups. The advancing and receding contact angles on one coal (Norwich Park) at various mineral matter contents is shown in Figure 3 together with the lines of best fit which provide the respective mineral free limits. From the slopes of the respective lines, Figure 3 shows that the advancing oil contact angle has a greater sensitivity to the surface content of hydrophilic mineral matter, i.e. the degree of hysteresis for any one coal is increased with hydrophilic mineral matter content. Figures 4a-c show that the processes associated with oil agglomeration, i.e. selective wetting and aggregation of the coal and rejection of the mineral matter into the aqueous phase, decreases both the advancing and receding oil contact angles. The hydrophobicity of the coal surface is increased by this process due to the removal of mineral matter from the coal surface constitution. This progressive decrease of the advancing
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and receding oil contact angles continues to the theoretical mineral matter free limit (Figure 4~). It is notable from Figures 4a and b that the contact angle hysteresis becomes more pronounced as the rank of the coal increases. This provides direct evidence that the principal cause of hysteresis is surface chemical heterogeneity, since in the higher rank coals, the difference between the surface free energy of the mineral matter and the organic coal matter is more profound than that in the lower rank coals. Comparison of Figures 4a and b also indicates that reduction of the overall mineral matter in each coal resulted in a corresponding general reduction in the degree of hysteresis, & -9:. This again indicates that contact angle hysteresis is dominated by the surface chemical heterogeneity between the surface mineral matter sites and the organic coal sites. Figure 4c shows the corresponding theoretical advancing and receding intrinsic contact angles at the mineral free limit. It can be seen that at the extrapolated mineral free limit the advancing and receding contact angles tend to converge. This indicates that on the smooth pressed discs, removal of surface heterogeneity induced by mineral matter produces a corresponding reduction in the thermodynamic hysteresis. The role of the surface mineral matter constitution on the free energy and wetting behaviour of coal surfaces has been investigated previously by demineralization using HF1g*23. Such HF treatment may alter the surface hydrophobicity through both organic functionality and inorganic mineral residues, whereas extrapolation back to an equivalent mineral free surface composition from samples with various degrees of physically liberated mineral matter does not chemically alter the surface, although a functional form of the mineral matter dependence must be assumed. The approach of the advancing and receding contact angles (Figure 4c) yields a smooth monotonic decline of contact angle as the rank of the coal, indicated by the atomic carbon to oxygen ratio, is increased. As discussed previously, in the mineral matter free limit contact angle,
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Figure 3 Advancing and receding contact angles, o’, measured on Norwich Park coal samples of various mineral matter contents. Extrapolation indicates the zero mineral matter contact angle limit
Contact angle hysteresis: R. J. Crawford
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Carbon / oxygen ratio (mineral matter free basis) Figure 4 Advancing and receding contact angles, O’, as a function of carbon to oxygen ratio for various agglomeration, and (c) at the theoretical mineral matter free limit. 0, Loy Yang; V, Collie; v, Mount A, Norwich Park
any hysteresis largely results from the organic coal surface composition itself; and it is instructive to examine the appropriate length scales which influence these contact angles. While all coal surfaces are heterogeneous on a molecular scale, Figure 4c suggests that a smooth composite surface formed from coal particles of average particle size 20pm yields macroscopic contact angles which reflect the bulk surface atomic composition. This is consistent with inducible fluctuations in the TPL of upto10pm l 3. Although the microscopic constituents of coal, the macerals, have characteristic sizes exceeding 3 pm, the composite surfaces produced do not appear to reflect possible differences in wetting behaviour due to variations in elemental composition. Neumann has shown” that althoug h free energy considerations do not provide a lower limit of the lateral dimension of heterogeneities that produce contact angle hysteresis, this may operationally be estimated from the profile of the TPL to be about 0.1 pm. The approach of advancing and receding contact angles shown in Figure 4c produced from smooth composite coal surfaces is consistent with Neumann’s conclusion regarding patchwise heterogeneous surfaces; i.e. that if the TPL is straight and if the average composition of the surface is constant, there are no metastable states and no contact angle hysteresis occurs. de Gennesi3 has also shown that weak perturbations on a surface strictly create no hysteresis and an ideal surface is not needed for the determination of thermodynamic contact angles, but rather surfaces with irregularities below a certain threshold. The disappearance of contact angle hysteresis at the extrapolated mineral free limit on these composite surfaces, which are composed of a homogeneous mixture
coals (a) before agglomeration, (b) after Arthur; 0, Wongawilli; n , Goonyella;
of the heterogeneous organic components (macerals), is consistent with the earlier film flotation studies on individual particles*. Surface hydrophobicity and wetting will alter as the surface composition of the coals changes due to either different macerals at the same rank or individual macerals over a range of ranks. The changing composition of such macerals has been followed elegantly by cross polarization and magic angle spinning (CP/MAS) nuclear magnetic resonance (nmr.) techniques 24 in terms of both aromaticity and oxygen functionality. Potentially, useful studies on wetting, film spreading and contact angle hysteresis may be carried out on planar maceral surfaces to demonstrate the interfacial impact of these changes.
CONCLUSIONS The degree of contact angle hysteresis is a measure of the surface chemical heterogeneity of the coal. Lower rank coals possess an organic component which is closer in surface nature to that of the inorganic constituent of the coal compared to that of the higher rank coals. Hence, as the rank of the coal increases, the contact angle hysteresis also increases. The similarity in slope of the advancing and receding contact angle, @, versus carbon-oxygen ratio for coal before and after agglomeration indicates that for these mineral matter contents, the surface wetting by the advancing oil film (hydrophobicity of the coal surface) is dominated by the oxygen functionality of the organic component of the coal surface rather than the mineral matter content or type. Where the coal has an extremely high mineral matter content (e.g. raw Wongawilli coal has a mineral matter
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content of 24.4 wt%) the contact angle is significantly above the characteristic curve.
ACKNOWLEDGEMENT The authors wish to thank the Coal Corporation of Victoria for their continued support of coal research at Swinburne University of Technology.
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(Ed. E. Matijevic), Vol. 2, Wiley-Interscience, New York, 1969, p. 85 Blake, T. D. and Haynes, J. M. in ‘Progress in Surface and Membrane Science’ (Eds J. F. Danielh, M. D. Rosenberg and D. A. Cadenhead), Academic Press, New York, 1970, p. 125 Finch, J. A. and Smith. G. W. Minerals Sci. Enma 1979,11(l). 36 Neumann, A. W. Ad;. Coil. Inc. Sci. 1974, 4: lb5 de Gennes. P. G. Rev. Mod. Phvsics 1985. 57 (3). 827 Hiemenz, P. C. ‘Principles of dolloid and Surface Chemistry’, Marcel Dekker, New York, 1977, p. 231 Mason, S. G. in ‘Wetting, Spreading and Adhesion’ (Ed. J. F. Paddav)., Academic Press. New York. 1978. D. 321 Cassie, A. B. D. Discuss. Faraday Sot. 1948, 3, flCassie, A. B. D. and Baxter, S. Trans. Faraday Sot. 1944,40,456 Crawford, R., Koopal, L. K. and Ralston, J. CoUoids and Surfaces 1987, 27, 57 Tampy, G. K., Prudich, M. E., Savage, R. L. and Williams, R. R. Energy and Fuels 1988, 2, 787 Padday, J. F. (Ed.) in ‘Wetting, Spreading and Adhesion’, Academic Press, London, 1978, p. 127 Good, R. J. Sur$ Colloid Sci. 1979, 11, 1 Labuschagne, B. C. J. Coal Prep. 1986, 3, 1 Fuerstenau, D. W., Yang, G. C. C. and Laskowski, J. Coal Prep. 1987,4, 161 Pugmire, R. J., Zilm, K. W., Grant, D. M., Larter, S. R., Allen, J., Senftle, J. T., Davis, A. and Spackman, W. in ‘New Approaches in Coal Chemistry’ (Eds B. D. Blaustein, B. C. Bockrath and S. Friedman), ACS Symposium Series 169, Washington DC, 1981, p. 23 I,
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Guy, D. W., Crawford, R. J. and Mainwaring, D. E. Fuel 1992 71, 935 Keller, D. V. Jr Cobids and Surfaces 1987, 22, 21 Gutierrez-Rodriguez, J. A., Purcell, R. J. and Aplan, F. F. Colloids and Surfaces 1984, 12, 1 Rosenbaum, J. M. and Fuerstenau, D. W. Inc. J. Miner. Process. 1984, 12, 313 Tampy, G. K., Chen, W.-J., Prudich, M. E. and Savage, R. L. Energy and Fuels 1988, 2, 782 Washburn, E. W. Phys. Rev. 1921, 17, 273 Fuerstenau, D. W., Diao, J. and Hanson, J. S. Energy and Fuels 1990,4, 34 Ramesh, R. and Somasundaran, P. Coal Prep. 1991, 9, 121 Johnson, R. E. Jr and Dettre, R. H. ‘Surface and Colloid Science’
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