Coal Preparation, 1991 Vol. 9, pp. 121-130Photocopying permitted by license only<0 1991 Gordon and Breach Science Publishen S.APrinted in the United Kingdom

R. RAMESH end P. SOMASUNDARANHenry Krumb School of Mines. Columbi. University. New Y~ NY 10027

(Receoind J_y 30. 1m; "'filla/form No~r 2. 1m)

..,Undentanding the nature of coal surface heterogeneity is a prerequisite to accurate characterization andefficient control of coal surfaces. A new approach based on film tlotation has led to estimation of surfaceenergy distributions in individual coal partX:les. Changes in the distribution of surface sites due to owatronand polyacrylamide adsorption on coal is studied here using the above approach. New information onpossible conformations of adsorbed polymers is also obtained.

XI}! wordr: Coal surface characterization. oxidation, reagent adsorption. surface heterogeneity

INTRODUCTION

Coal cleaning involves separation of undesirable ash fonning constituents and inor-ganic sulphur fonDS from fine coal particles. Advanced techniques for the preparationof clean coal of required quality exploit the differences in surface properties ofcarbonaceous material and the mineral matter. However, froth flotation and selectiveflocculation, when applied to fine coal, are not very efficient and the reasons are notfully understood. This results partly from the complex physical and chemical natureof the coal surface and to a major extent from the variation in its properties fromsample to sample. Additionally, coal weathers on exposure to ambient atmosphereover time resulting in its oxidation. Oxidation of coal in turn results in markedmodification of surface characteristics that govern beneficiation techniques such asflotation, flocculation and oil agglomeration.

It has been well established that the average structure of the coal "molecule" (basicstructural unit) consists of a variety of organic moieties composed of polynucleararomatic compounds consisting of six and five membered rings connected by alkyl oraryl bridges [I]. The periphery of these rings include carboxyl and phenolic groups.Surface of coal will be heterogeneous depending on the distribution of these moietieson the surface and this heterogeneous nature of its surface is an important propertyaffecting all reactions at the coal interface. It should be pointed out that only minorchanges in surface properties are adequate to affect surface phased phenomena [2-4].In the case of heterogeneous material like coal, knowledge of average properties

'Presented at the 118th annual AIME meeting at Las Vegas, Feb28, 1989.

122 R. RAMESH and P. SOMASUNDARAN

TABLE I

ANALYSIS OF BRUCETON MINE COAL

Ultimate analysisas received

% Moisture% Carbon% Hydrogen% Nitrogen% Chlorine% Sulphur% Ash% Oxygen(by diff.)

1.7980.655.301.740.101.123.297.80

Proximate Analysisas received

% Moisture"leAsh% Volatile

Matter% Fixed Carbon

1.793.29

36.28

58.60

Sulphur Formsas received

% Pyritic% Sulfate% Organic% Total

0.280.00830.831.12

would not provide sufficient information to control interfacial properties. A know-ledge of chemical group distribution on a coal surface and the changes in surface sitedistribution due to various treatment is important in this regard.

MATERIALS AND METHODS

CoalHand picked Pittsburgh bed bituminous coal from the Bruceton mine in Alleghanycounty was used in this study. The proximate and ultimate analysis is given in Table J.All experiments were done with - 35 + 80 mesh size fraction.

PolymerCommercial polyacrylamide polymers were obtained from American Cynamid Com-pany. Cationic Polyacrylamide of 10% and 34% cationicity, anionic polyacrylamideof 30% anionicity and a nonionic polyacrylamide were used for the tests. Themolecular weight of aU the polymers used were approximately four million as indicat-ed by the manufacturer.

123SURFACE SITE DISTRIBUTIONS ON COAL

Film FlotationFilm flotation experiments were conducted, as described by Fuerstenau et at. [5,6], forthe base coal and polymer treated samples. The experimental technique involvessprinkling of coal (typically 0.25 gms.) on water-methanol mixtures of differentcompositions. The resulting float and sink fractions were separated by decantationand dried under ambient conditions, weighed and cummulative partition curvesconstructed. These partition curves provide the cumulative distribution of sites overa range of critical wetting surface tensions.

Polymer treatment was done by conditioning 2 g samples in water for one hour inscintillation vials and then in polyacrylamide solutions for 48 hours. The samples werefiltered and dried under ambient conditions and subjected to film flotation. Oxidationof coal samples was conducted at 700C in an air heated oven for desired periods. Thematerial was subsequently allowed to cool under ambient conditions before expen-mentation.

RESULTS AND DISCUSSIONS

Film FlotationPartition curves obtained from film flotation data show cumulative weight of theparticles as a function of critical wetting surface tension y c. The cumulative partitioncurves can be translated to frequency distribution diagrams by plotting the slope ofthe cumulative curve at all surface tension values in a procedure analogous toconstruction of particle size distribution diagrams. It has been previously establishedby Zisman [7,8] that y c is a surface parameter which depends not only on the natureof the surface molecules but also on their orientation/conformation. Yc is also ameasure of the surface energy and their relationship is given by:

Ys = (J)lyc

where cI» is the interaction parameter and "Is is the surface tension of the solid invaccum [9]. Assuming that the interactions at the interface are of the London type andspreading pressure (x,) equal to zero, cI» could be taken to be unity. Since the filmflotation process is controlled by interfacial forces the weight percent of materialfloated is equivalent to the surface area and the partition curves can be considered togive essentially the distribution of the surface energy sites on the particles.

Surface heterogeneity in a particulate population can be of two classes. In the firstclass, individual members of the population possess homogeneous surface propertiesbut the population as a whole can be heterogeneous. Such particulate populations areheterogeneous mixture of homogeneous particles e.g. a mixture of silica, teflon andnylon particles. In the second class, the individual members of the population canpossess heterogeneous surface. This category falls under the homogeneous mixture ofheterogeneous particles. The partition curve obtained from film flotation is indicativeof gross heterogeneity. It cannot be distinguished from the partition curve if theheterogeneity of the population is of the first class or the second class.

To illustrate the working hypothesis let us consider three homogeneous particleseach having a surface energy values of X, Y and Z respectively and three heteroge-neous particles each having surface energy values of X, Yand Z distributed equally

114 R. RAMESH and P. SOMASUNDARAN

I-a:c..JU-

N

~25 ~ 45 ~ m 75Sl.fiFACE TENS I ~ CF WETT I ~ L I Q.J I D . dJ"'Iea/om

FIGURE la Schematic diagram of film flotation partition curves for a heterogeneous mixture ofhomogeneous particles.

on its surface as shown below.

~.., .'. . '::.:';

':. : ~~~;~ t;J/CLASS I CLASS 2

Both the sets will yield the same data on subjecting them to film flotation viz., thearea occupied by X, Yand Z is 33.33% each. In the case of a heterogeneous mixtureof homogeneous particles, film flotation of various float fractions i.e. particles floatingon solutions of different surface tension, should yield curves similar to those inFigure 1 a; Figure 1 b shows the alternate case, in which the various fractions follow theoriginal curve. 10 Float, 20 Flot and 30 Float correspond to particles floating on a 10volume percent methanol water mixture, 20 volume percent methanol water mixtureand so on. Experiments were conducted to test the nature of the samples used in thepresent tests on the basis of the above hypotheses and results are given in Figure 2.The results presented in Figure 2 indicate that the heterogeneity of coal particlesbelong to the second class discussed above and therefore it is evident that the partitioncurves yield a psuedo map of the surface energy distributions of individual particles.

Various float fractions of the base coal along the partition curve were treated withcationic polyacrylamide and subjected to film flotation. The results given in Figure 3show the distribution of energy sites after polymer treatment also to be representativeof those of a single particle, since the curves follow the trend of those in Figure 1b.

.

SURFACE SITE DISTRIBUTIONS ON COAL 12$

t-~a..Ju..

~

FIGURE I b Schematic diagram of film flotation partition curves for a homogeneous mixture ofheterogeneous particles.

I-~0-Ii£.

FIGURE 2 Cumulative film flotation panition curves ofbasc coal and various float fractions. 10 Boat,20 Boat etc. corresponds to material Boating on 10 percent, 20 percent, methanol water mixture respectively

126 R. RAMESH and P. SOMASUNDARAN

...~0-'It.

...

~ 40 00 ~ 70 8)SURFACE TENSION OF VETTING LIQUID.dynee/c.

FIGURE 3 Cumulative film flotation partition curves of ba~ coal and various float fractions aftertreatment with I ~ ppm of cationic PAM for 24 hours.

Nature of Coal Surface upon Adsorption of PolyacrylamideFigure 4 shows the Yc distribution curves (obtained by plotting the slope of thecumulative curves at incremental surface tension values) of base coal and coal treatedwith nonionic polyacrylamide, cationic polyacrylamide of 10% and 34% cationicityand anionic polyacrylamide of 30% anionicity. An interesting observation is theabsence of bound or hydrated water characterized by Yc of72 dynes/em, on the surfaceof coal treated with cationic polyacrylamide of 34% cationicity and the anionicpolyacrylamide. It appears that when the charge density of the polymer increases thewater retention power is reduced as evidenced by the decrease in the area of sites witha y c of 72 dynes/em. This is plausible since increased charge density would prevent thepolymer from adopting a more compact structure which can hold large amounts ofsolvent water in the interface layer. If the amide groups of the polymer are exposedon the surface, Yc of 4Odynes/cm can be expected [10], the peak at Yc ofyc 39dynes/cmin this study is attributed to the amide group. It is also seen that as the charge densityincreases the area under the peak at y c of 38 dynes/em increases indicating that moreof the surface is covered by amide groups. The trends in the case of 30% anionicpolyacrylamide and 34% cationic polyacrylamide is reversed. Adsorption of cationicpolyacrylamide and anionic polyacrylamide take place by the anchoring of thehydrocarbon chain such that amide groups are pointing away from the surface. It isalso evident that there are sites with Yc ranging from 50 to 68 dynes/em each occupyingan area of 5 to 10%. These are probably representative of C=O groups of thepolymer.

Effect of Oxidation on Surface Site DistributionThe effect of oxidation on the nature of the site distribution curve was examined byheating the coal at 70°C in a convection oven for time intervals ranging from hours

SURFACE SITE DISTRIBUTIONS ON COAL 127

70

~

m~w~~

wu~u.~~cn

...

.«J

:IJ

20

10

020 :I) ~ 00 ~ 70

CR[T[CAl VETTING SURFACE TENSION.dyn88/c8

FIGURE 4 Site distribution curves obtained from film flotation data for base coal and coal treated withl(XX)ppm of various polyacrylamides.

(Figure 5) to days (Figure 6). It is seen that the heating of coal results in a marked shiftin the site distribution curve towards the region of lower critical surface tension, withthe maximum shifting from a Yc of around 44 dynes/cm for the base coal to that of38 dynes/cm for heated coals. There is also an increase in the area under the peakcorresponding to Yc between 37 and 39 dynes/cm, with an increase in heating time. Theabove observations may be explained on the basis of either one or a combination ofthe following three possibilities: a) Loss of surface volatiles, b) Loss of surfacemoisture, c) Change in the structure of adsorbed moisture on coal, after heating.

Effect of Polyacrylamide Adsorption on Low Temperature TreatedCoalChanges in the site distribution curve, upon adsorption of polyacrylamide on coalheated in air for 70°C for 24 hours, are shown in Figure 7. It can be seen clearly, incomparison with Figure 4, that the adsorbed polyacrylamide layer on the oxidizedcoal surface is different from that on the base coal. In the case of nonionic polyacryla-mide, the percentage of sites having a Y.. of 72 dyoes/cm has decreased from 30% onthe base coal to 15% on the oxidized coal. This indicates that the adsorbed layer ofnonionic polyacrylamide on the oxidized coal exhibits a less dense structure than theadsorbed nonionic polyacrylamide layer on the base coal.

Also, the sites with Yt' in the range of 52 to 62 dynes/cm that disappeared on heating,reappears upon adsorption of both nonionic polyacrylamide and anionic polyacryla-mide. In the case of anionic polyacrylamide an increase of 15% in the area corres-ponding to a y, in the range 52 to 62dyoes/cm is noticed. Upon adsorption ofnonionicpolyacrylamide and anionic polyacrylamide the percentage of area with a Y.. from 22to 50 dynes/cm on the oxidized coal surface is identical, in contrast to the site

128 R. RAMES" and P. SOMASUNDARAN

~

50~w~

~40wu~

~:J)~U)

3)

10

20 :m ~ IX) 8) 70 8)CRITICAL VETTING SURFACE TENSION.d~n../cm

FIGURE 5 Evolution of site distribution curve on beating base coal at 70 degrees in air for variousperiods in hours.

wu«u.a:~&n

...

FIGURE 6 Evolution of site distribution curve on heating base coal at 70 degrees in air for variousperiods in dayS.

SURFACE SITE DISTRIBUTIONS ON COAL 129

~w«~

wU~u.«~cn

...

FIGURE 7 Site distribution curves for coal heated in air at 70 degrees in air for two days and subleqUCDt.ly treated with llKKJ ppm of nonionic and anoinic polyacrylamide.

distributions of polyacrylamide on the base coal. More of the hydrocarbon chain partof the polymer molecule is apparently exposed than that of the amide groups asevidenced by the shift in the curve towards lower ')It" values.

CONCLUSIONS

It has been shown for the first time that the observed heterogeneity of an ensembleof coal particles reflects the heterogeneity of each single particle. In essence it has beenpossible to obtain a pseuedo map of the surface of coal and reagent treated coal usingfilm flotation. In the case of base coal it appears that the adsorption of the chargedpolyacrylamide takes place by the anchoring of the polymer hydrocarbon chain suchthat amide groups are pointing away from the surface. As the charge density increasesthe area occupied by amide groups increases from around 30% for the 10% chargedensity polymer to around 50% for the 30% charge density polymer. The adsorbedlayer in the case of nonionic polyacrylamide presents a comparatively heterogeneoussurface indicating a random mode of adsorption. The adsorbed layers of the cationicpolyacrylamide of ]0% cationicity and the nonionic polyacrylamide presents a moredense and compact structure as compared to those of anionic polyacrylamide andpolyacrylamide of higher cationicity.

Heating of coal at 70°C in air decreased the average value of the critical wettingtension indicating the formation of a more hydrophobic surface probably due to theremoval of surface moisture. In the case of adsorbed layers of nonioic and anionicpolymer on oxidized coal, more of the hydrocarbon chain of the polymer is apparent-ly exposed than that of the amide groups. 75% of the area of both polymer layersexhibit similar surface energy distributions. The adsorbed nonionic polyacrylamide

130 R. RAMESH and P. SOMASUNDARAN

layer on heat treated coal exhibits a less dense structure. It is clear that conformationof polymer molecules is an important factor that can determine the surface behaviorof coal.

AcknowledgementsWe thank the U.S. Department of Energy for the support of this research under grantDE-FG-2287-PC-799 19.

References

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Plenum, (1983).10. M. L. Jarvis, R. B. Fox, and W. A. Zisman, in Contact Angle Advances in chemistry series 43,

317-331, (R. F. Gould, ed.), ACS (1964).