Collofdr and Surfitcer, 8 (1983) 137-161 Elrevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

137

DEXTRIN ADSORPTION BY OXIDIZED COAL

J.D. MILLER, J.S. LASKOWSKI end SS. CHANO

Degmfment of Wetolhgy and Metalturgical Engineering, Uniwrrity of Utah, Salt Lake City, UT 841 I2 (U.S.A.)

(Received 13 June 1982;eccepted In final form 7 January 1983)

ABSTRACT

Dextrin can bc used as a depressant to varying degrees of success in the differential flotation of pyrite from coal. The objective of this investigation has been further iden- tification of phycko-chemical factors thal control the adsorption of dentrin by coal. The research program involved oxidation of coal 6amplen (both natural and demineralized), dctcrmination of oxygen Cunctiomd groups (carboxyltc and phenolic), characterization of coal samples in terms of their hydrophobic character and in terms of their electrokinctic behavior, and measurement of dexlrln adsorption densities as related to the oxygen group content.

Experimental results support yreviouscuggcstions that dcxtrin exhibits tengmuir ad- sorption behavlor with hydrophobic bonding. The more hydrophobic demineralized coal was found to exhibit an adsorption density twice the adsorption density of natural coal. In both crises the extent of adsorption deereased with an increasa!d level of oxidation but wns largely indrpendent of pli. Adsorption free energies of about -5.5 kcallmol, indepen- dent of the extent of oxidation, were calculated.

INTRODUCTION

Fine coal cleaning by froth flotation has become a common practice in the U.S. since the fmt coal flotation circuit was installed at a plant in Jm- p&al, Pa. in the 1930s. Now, about 100,000 tpd of coal is cleaned by flota- tion in the U.S. In this process hydrophobic coal particles are usually lloaW away from the hydrophilic ganguo particles which remain in suspension. Flotation, which is very effective in reducing the ash content of coal, is not so efftcient in separating pytitc from coal.

Coal flotation is usually carried out with the USC of reagents such as: Col- lectors - water insoluble nonpolar oils; Frothers - water-soluble surfactants; and Organic Colloids - starch, dextrin, etc., which are known in the field as depressants (Klassen, 1966). It is worth noting that the adsorption of non- polar collectors in this process is very different from the adsorption of water- soluble collectors utilized in the flotation of ores. Our knowledge of the phenomena involved is certainly not adequate in view of the emerging im- portance of coal flotation.

Flotation which can be very simple and effective in the case of metallur-

0166.6622/83/$03.00 o 1983 Elscvier Scfence Publishers B.V.

&al coals becomes very difficult and nonselective for low rank, or oxidized coals. This behavior reflects the effect of the physical chemical properties of the coal surhcc on adsorption processes taking place at the coal/aqueous solution interface.

&uty experiments on the adsorpt.ion of Rotation reagents by coal (Evcson ct al., 1956) showed that. the amount of phenol adsorbed by coal increases remarkably for lower rank coals. The similarity between the heat of wetting ‘and the amount. of phenol adsorbed as a function of coal rank indicated that the extent of phend adsorption was dependent largely upon the specific surface arca. Ilowcver, Evcson et al. (1956) also showed that the rale of phenol adsorption was much higher for low-rank coals than for higher-rank co,aIs. Frangiskos et. al. (1960) found that the adsorption process of rn-cresol on coal consists of a ralaid chcmisorption step followed by stow physical ad- sorption. ‘t’hc aclivation cncrgy of adsorption was found to IX about 18,500 cal/mol and represented, according to the authors, rcasonnblc cst,imatcs of the activation energy of t,hc initial adsorption process. This is far in excess of the activaCon clwrg-y associated with physical adsorption. These findings indicate that- the osygcn fuuctfoual groups on t-he surface of coal can play an important part in adsorption reactions. Other pieces of evidence subdue- tinting such a point, of view come from the work by Coughtin and Ezra (1968). Working with active carbon which was reduced or oxidized, they showed that- while oxidation does not seem to affect. adcorgtion at high l~henol concwltr~tions, it. reduces the adsorption strongly at low conccntm- tions of phenol. ‘I’hcy concluded that chemically bound oxygen strongly in- fluwcc?s phenol adsorption only under the conditions when the n~olccules arc thought to be adsorbed in the prone position on the Imsd plants of grapllite with attractive forces opcrothq over the entire phenol nucleus.

111 this rcslwA, a~ extromcty intcresting paper was l>rcsented by Zubkuva nnct Kuclux (1076) at. the 7th Intenlationd Congress of Surface Active Sub- S~NICCS. ‘I’hcsc nuthors usd in their experinwnts three coats which bad been transformed into f I-forms: I ligh Volatile Dituminous C, High Votat-ite Hiluminous A, and Semianthracitc. In the experiments fresh and oxidized samples of the coals were used, the degree of oxidation was followed by rlctcrminnlion of carboxyl and carbony groulls. For the I IVB-C coat the content of C00ll and CO groups was found to be 3.06 mequivlg and 0.50 nlcquiv/g, rcspcctivcly, for osidited coal, and 2.04 mcquiv/g and 0.48 mcquiv/g, for the fresh sample. For thb other t.wo coals the carboxyl and carbonyl group content. was very similar for both oxidized and fresh samples. A<lsorption of vapors on evacuated samples as welt as adsorption from aqueous soIut.ious w*as st,udicd. While the adsorpbion of aniline (either from gnscous or aqueous phases) increased with an increase in oxidat,ion of coal, osidntion inhibited the adsorption of weak organic acids. The most. interest-

139

ing finding was that adsorption of propargyl alcohol, HC&XHIOH, jkom the gascow phase increased with an increase in oxidat.ion whereas for propargyl alcohol adsorption from the aqueous phase, the extent of adsorption de- creased with an increase in oxidation of the coal. This suggests that propargyl alcohol adsorbed by hydrophobic interactions from aqueous solution,

The effect of oxidation on tho adsorpt.ion of CTAB (cetyltrimethylam- monium bromide) and Aerosol OT (di-2-ethylhexyl sodium sulfosuccinate) by coal was also shown to be quite pronounced (Laskowski and Konieczny, 1970).

‘I’iuo slagc rfxm-sc ftotatiolr

Dcsulfurization is one of the main reasons to clean coal. Pyritic sulfur occurs in coal in the form of finely disseminated marcasite and/or pyrite grains. Traditional pyrite depressants known in the flotat.ion of ores arc not effective to prevent pyrite contamination of the clean coal froth product (Whelan, 1953). Recent studies have demonstrated that. a promising new flotation technique, a two-stage reverse flotation process, can reduce the pyrite content of fine coal to a grcatcr extent than conventional flotation (MiHer and Baker, 1072; Miller, 1973). In this reverse process after convcn- Conal flotation in the first stage, the froth product is processed by float.ing the pyrite particles from dopressed coal in the second st.age.

Organic colloids such as dcxtrin are typically used as coal dupressants. Dcxtrin (C6H,001),I is a water soluble polymer with a molecular weight that ranges from 800 to 79,000. These higher branched polymeric carbohydrates arc composed of dextrose units as shown in Ipig. 1. Even at a dcxtrin conccn- tration of 1.0 mg/l, corresponding to about 10% surface saturation, coal depression occurs (Haung at. al., 1978). I’hc hydrophilic character of the coal under fhcsc circumstances seems to be due to kinetic considerations rather than thermodynamic considerations as revealed by induction time and con- t.act angle measurements prescntcd in Table 1 (Lin, 1982). Surface saturation occurs at 600 mg/l and at such high surface overage attachment dots not occur.

Adsorption density and tl~ermocl~emical measurements suggest that dcxtrin adsorption by coal occurs by hydrophobic bonding (Haung ct al., 1978). The same adsorption phenomenon pwviously had been suggested for the dcxtrin/molyldcnit system (WC and Fuarstcnau, 1974). In fact it seems that dcxtrin adsorption by naturally hydrophoboc solids is independent of the solids’ chemical composition. Coal (hydrocarbon), molybdcniti (sulfide), and talc (silicate) - all naturally hydrophobic minerals - exhibit the same adsorption isotherm as contrasted with adsorption by hydrophilic solids in Fig. 2 (Miller et al., 1982). Of course such coincidence of the isotherms would not be expcctcd as a general rule and may occur because the surface charge appears to be similar as reflected by a zeta potential of 40 mV under these pH conditions. These hydrophobic bonding adsorption reactions have been

140

Glycosydic Hnkoger eonnecttng glucose unlfs

bslrtrln Pig. 1, Starches and dextrins nrc polymers of dextrogc monomrrlc units linked through 1-4 glycasidic joinls (for straight chains) and 1-G joints (for branch chains). In rtarchcs, the? linear components have mokeular weights reaching millions. In dextrin formation them chains arc fragmented and mcomblnr to Form lower molecular weight mol~culcs of highly branched structurcr (liaung cl al., 1978).

TAEtLE 1

Induction lime and contact an& measurement8 for Illinois No. 6 Coal a5 a function of dcxtrin (Am&o 1700) conccntrat ion at pli II.0 (Lln, 1982).

Dcxtrin cone, Induction time (mglU

Conlacl angle (s) (deg.1

0 <3 40 1 25-39 36

10 30-45 33 GO 300 SO 360 I:

GO0 No contact possible aCtcr 20 min.

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0 Coal Pyrite

[

0 Ore Pyrite -

m Quark I

0 20 40 60 60 100 120 140 160 160 ,

Equilibrium Dentrin Concentrrtion (0) , mg/l

Fig. 2. Adsorption of dewtrin by both hydrophablc and hydrophilic solids at 29°C and pH 5-6.5 (Miller ot al., 1982).

shown to have a heat of adsorption of about 4.6 kcal/mol of dextrose monomer and a free energy of adsorption of about --5.O to -5.5 kcal/mol of dextrose monomer (Haung et al,, 1978). In the present study the adsorption of dextrin was studied ha relation to the extent of coal oxidation in order to test t-he hydrophobic bonding hypothesis further.

EXPERIMENTAL

In these experiments both natural and demineralized coal samples were prepared And oxidized under various conditions. The samples were charac- terized with respect to oxyacn function& grouns and electrokinotic behavior. The dextrin ad&ption d&&tics were dot&mined.

Coul

Ah experimental work has hen carried out using a coal sample from Coal Basin Seam, Carbondale County, Colorado. This medium-volatile

the

(volatile matter 24%) bituminous coaJ contained 65% ash and its ultimate analysis is presented in Table 2, Coal was wet ground and a -270 mesh frac- tion was used in the experiments. The sample was split into two parts, one part was demineralized using HF and HCI. For demineralization, 160 g of coal was fiist conditioned with 1000 ml of 40% HF at 50°C for 45 min followed by filtration and washing. Then tie sample was treated with 1000 ml of 10 N HCI at 60°C for an addNional45 min. After this treatment the sample was repeatedly filtered and reconditioned with distilled water for a few days until the pH of the aqueous coal suspension had reached at least pH 4.5. The

142

ElCIllCIlt Wt.% (dmml)

Carbon 90.16

Jlydrogen 5.35 Nilrogcn 2.25 Oxygen 1.50 Sulfur 0.74

sample was vacuum dried at a tcmpcraturc of 40°C. The demineralized coal sample was found to contain less than 0.5% ash.

Samples of both natural and demineralized coal were dry-oxidized under atmospheric conditions in an oven at 160°C and 200°C for 8 h; titration tcch- niqucs were used to dcterminc the content of active oxygen groups. These oxidized samples were then characterized in terms of their oxygen functional groups and their electrokinclb behavior.

‘I’ho two principai oxygen -containing functional mouJ=. carboxylic and phcnolic, were detarmincd by m+ods sst&lished by lhnatowicz (19521. 13Joom et aJ1, (1967), and - ~5 subsequently modified - by Schafer (1970). The sample which had not 3ccn initially demineralized was aJstz lcachcd before the oxygen functional woups determination. “T&al acidity” {car- hoxylic plus phcnolic) was dctcrmincd by reacting a 0.6 g coal sample with a 50 ml sotutian oZ 1,O N barium (0.2 iV Ba(OE)a and 0.8 N F3aCJt) at room tornpcrature for 8 h under nitrogen. ‘The suspcdon was then left overnight and 25 ml of cJt?ar supcmatrlnt was pip&ted oil t.he next day into 26 ml of 0.2 N HCI. hccss of JIG1 was back-titrated with 0.05 NaOH. A difference in cxccss I-ICI h4wccn tllc blank experiment (without coal) and the actual cx- Jlcrimcnt (wit-h coal) gives the amount of barium abstractid by the c~arboxylic and phenolic yroups of the coal, i.e., total acidity.

The number of carbnxyIfc groups was dcwrmincd in a scparatc cxpcrimcnt in wl~ich 0.25 g of coal was reackd with 69 ml of 1.0 N barJum acetate of initial pJ+ 8.25 at room temperature, under nitrogen, for 4 h. Acetic acid released according to the reaction:

-COO 2(-COOl~l) Surf + l~a(CH,CoO)~ = ‘HaSurf+

--COO/ 2CI 1,COOl~

is slowly titratM1 potinMomctrically with 0.05 N Nash in the presence of coal until the initial pH value of 8.26 is restored, The content of carboxylic groups found in this manner, when subtracted from the total acidity, gives the content of phenoIic groups.

143

Elcctrokinetic behavior

The microclectrophoresis apparatus, Zeta-Meter, was used to determine the zeta potential of coal as a function of PH. Both natural and demineralized, fresh and oxidized samples were further characterized by these experiments. Coal was equilibrated with the aqueous solution of adjusted pH for 30 min before measurements were made. Tho points represent an average of 10 readings.

Dcxtrin adsorption by coal

Specific surface area determination for the natural and deminerahzed sam- ples by air permeametry with a Permaran gave 0.37 ml/g and 0.34 m2/gt respectively, This technique of surface arca determination was preferred to the BET, as it does not measure the internal surface area of fine pore struc- tures which probably are not accessible to macromolecular adsorbates. We, however, obtained 0.37 m2/g and 0.34 m2/g for natural and demlncrali-ad samples, respectively,, which seems to reflect the disappearance of very fmc gangc particles in the demineralized sample. Otherwise there is no apparent reason whjt the sample which had been dcmineralixed by leaching with acids, should have a smaller surface area.

The dextrin used in t.his investigation was a methanol precipitated product with an average molecular weight of 1800, manufactured by Matheson, Coleman and Bell, The adsorption density of dcxtrin was measured by a radiochemical method using radiotracer 14C for analysis. The cxtcnt of ad- sorption was determined by measuring the number of disintegrations from the solution of the depressant before and after adsorption with a liquid scintillation counter, model 720, manufactured by Nuclear-Chicago Corpora- tion, Des Plaines, Illinois.

The procedures for t.agging the dcxtrin and measuring the adsorption den- sity am dcscribd below.

Tagging dax trin The dextrin (2 g) was weighed into 20 ml glass vials. Two ml of a 0.06 AI

Na’4CN, 0.16 fbf NaOli solution was carefully spread on top of the dextrin bed. The vials, with polyethylene lined caps loosely screwed on, were placed in a vacuum desiccator and 20” of vacuum was applied for 2 min and then released. Three vacuum cycles were sufficient to expel all the air from the doxtrin sample. The caps were screwed on tightly and the vials stored for 10 days.

By the end of the 1Ot.h day the dextrin had turned to a gelatinous mass. About 8 ml of distilled water was added to thii the bed and then 10 ml of methanol was added to partly precipitate tho dextrin. The vials were placed in ice water to aid the coagulation of the dextrins. The liquors were decanted into’100 ml beakers and more methanol was added to both the ber.kers and

144

vials to complete the precipitation, The precipitates were centrifuged and tin526 wit-h methanol three times. Finally the prwipita~s were dried at room temperature in a vacuum desiccator. A stock solution of 1000 mgll of tagged dextrin was placed in several plastic vials and stored at near freezing tempcra- tures in a refrigerator in order to inhibit bacterial action. The vials were taken out as needed and warmed to the experimental temperature.

Adsorption experiments were carried out in 60 ml, glass4oppered Erlcn- meyer flasks immersed in a temperature controlled orbit water bath shaker, model 3536, manufactured by Lab-line Instruments Inc,, MeIrose, Illinois.

One half gmm of coal was put into a SO ml Erlenmeyer flask with (20 - V,) ml deionized water and shaken vigorously to wet the coal. Vs represents the volume of the tagged dextrin stock solution needed for tho desired initial concentration when diluted to 20 ml. Next, the dextrin stock solution was added to the flask and shaken again for 20 s, The flask was rotated through the bath at 200 r-pm for 1 h.

After adsorption, the susmnsion was Wered t-hrough tightly packed glass wool. This filtration technique produced a clear fxltratc and made ccntrifuga- tion unncccssary. One ml of the filtrate was mixed with 10 ml Aquasol in a

t 1 I 100 200 300

E.quilibrium ConccnttatiOn. mgr I I

1 I I

C?c*trin, reagent grade t M-W. l80Ot Lower Freeport Coal

l

l 0

m- m

Fig. 3. &mparison of cxpcrimental techniques for the dctcrmination of dextrin adsorp Lion isotherms (Milter, 1982).

145

scintillation vial. The vial was mounted in the scintiUation counter and counted for a chosen length of time (10 min typically).

The radiotracer technique was found to be a quite satisfactory analytical technique. The dextrln adsorption isotherm for a Lower Freeport Coal was determined by both the radiotracer technique and by a wet chemical spectro- photometric technique and the results are compared in Fig, 3 (Miller, 1982). Notic& the initial Langmuir type behavior followed by apparent multilayer formation at high dextrin concentration. In flotation systems the adsorption characteristics below 100 mg/l are of particular interest.

RESULTS AND DISCUSSiON

Oxygen functionat groups

The amount of carboxylic and phenolic groups determined by the pro- cedures outlined previously are presented in Table 3. As seen, oxidation of the natural (6.5% ash) and the demineralized (0.6% ash) samples of coal yielded almost the same amount of oxygen groups. Dry-oxidation in air at 160°C for 8 h raised only slightly the conknt of ozqgen &roups while the oxidation at 200°C resulted in appreciable oxidation of the coal surface.

TABLE 3

Effect of oxidation condittonr on the formation of oxygen functional groups for natural and deminctalized ramplcs

Sample Carboxylic groups Phenollc groups

moVg %O *01/g RO

Nalural Frcrrh 3.43 x 10-s 0.01 4.73 x lo-* 0.008 Oxidized (150-C) 2.84 x 10-1 0.09 6.05 x 10-s cI.;=P O*ldimd (200°C) 4.31 x 1o’a 1.38 9,76 x 1o’e 1.56

DeminomlCzcd Fresh 3.43 x 10-L 0.01 4.73 x 10-c 0.008 Oxidized (16OV) 2.46 x lo-‘ 0.008 6.24 x lo’* 0.1 Oxidized (2OO’C) 4,34 x lo-’ 1.39 I.05 x 10” 1.68

Efcctrokinstic behavior

The results of electrokinetic measurements are given in Figs. 4 and 6. In both cases oxidation shifts the i.e.p. of coal toward more acidic pl1 values. As shown in Table 3, oxidation increases the content of acidic oxygen groups (carboxylic and phenolic) and it is then not surprising that the position of the i.e.p.C on the pH scale are shifted to the left for samples which had been

6 I I I I I I

5 ~- COAL BASIN HATURAL

0 Fresh t

0 150°C Oxldited

A 200% Oxidized

-5 I I I 1 1 I I 0 2 4 6 8 10 12 14

PH Pig. aI. Elcctrokinctic behavior of the natural coal sample Cram the aal Uasin Scam at diffwanf tawls uf oxidaliun.

6~

5 ‘-

4-

I I I I I I

COAL BASIN tEMINERAtlZE0

0 Fresh 0 150°C Oxidized

n 200% Oxidized

-1 -

-2 -

-3 ‘-

-4 -

-5 0

1 2

I I I I 6 8 IO 12 14

PH Pig. 5. Electrokinctic behavior of the dcmincralizcd coal sample from the Coal Basin Seam at different lcvcls of oxidation.

147

oxidized. The electrokinetic measurements have been conducted without any supporting electrolyte as we have ‘observed that the i.e.p’s were further shifted in the presence of v&ous “indifferent” electrolytes. It is to be noted that deminerahzation hti not caused any significant changes in the electrokinetic behavior of t-he coal, These results are a further characterization of the coal surface and supplement the previous results on oxygen group functionality. Wen and Sun (1977) found a similar shift in i.e.p.*s with oxidized macerals and suggested t-hat the shift was due to differences in oxygen-containing func- tional groups.

Adsorption isotherms

Adsorption of doxtrin hy the natural and demineralized coal samples is shown in Figs. 6 and 7, respectively. In both cases adsorption decreases with increased oxidation. This supports strongly the hypothesis which has recently been put forward that adsorption of dextrin on naturally hydrophobic solids occurs via hydrophobic interact.ions as discussed in conjunction with the results presented in Fig. 2. The isotherms presented in Figs. 6 and 7 follow the Langmuir equat.ion,

(W 1 W) 1’=I‘,K+I’

where K = oxp (-AQ”/HT) from which the free energy of adsorption was determined to be from -5.2 to -S,8 kcal/mol dextrose monome,: which ices we11 with the --5.O to -5.6 keal/mol of dextrose monomer dobrmined p&iousIy (Haung et al., 1978). _

I I I I I I

COAL DASIH NATURAL 20% 0

0 Fresh 0 160% Oxidized A 200% Oxldixed

10 20 30 40 50 60 70

Equitibrium Dertrin Concenlration (0) , mg/t

Fig. 6. Adsorption of dcxtrin by the natural coal sample from the Coal Basin Scam at pJJ 7.2-7.4 and different levels of oxidation.

I I I I I I I I

COAL BASIN DEMlNEAALlZED

20% pH = 3.9 - a3

h-O-

0 Fresh 0 rso% Oxidized A 200% Oxidized

I I I 10 2D 30 40 50 60 70 80

Equllibrlum Oerrtrin Concentration (0) , mg/l

Fig. 7. Adsorption uf r!ch;lrin lay lhc rlrmincratizcd currl sample from lhc Coal fkin Sea~n at 1~11 X9-4.3 anal tliffcrcnt Icvcls of uxidalion.

F lydrophobic bonding usually arises from t-be tendency of nonpok g:rouW of organic molecules to adhcrc to one another in a polar aqueous environ- ment (Ncmcthy et d., 19012; The free energy change corresponds to the re- movrd of n nonpolar group from its aqueous environment and the resulting Vnn dor Waal’s lmnding lmtw*ccn the nonpolar group and the hydrophobic surface. In fact, the attractIon of nonpolm groups (such as hydrocarbon chains) for each other is small (Parsegian and Ninham, 1971) and plays only a minor rdr! in the mlsorptian process, The major contribution to the ener- gctics of adsorption probably arises from the change in the nature of the hydrogen bonds of the structured water molcculcs near the nonpolar surface (Tanford, 1973). ‘I’hc total free energy change for the removal of a nonpolar CI Iz unit from the aqueous phase into a nonpolar environment has been cstimatcd to bc GO0 t,o 800 caX/mol of GHt group {Hoynolds et nl,, 1974). The magnitude of the cxl.lorimental adsotptton free energy, -5.2 to -5.8 kc&no1 of dr?xt.rosc? monomer for the dcxtrin-coal system, suggests that the adsorpt,ion involves hydrophobic bonding between the nonpolar groups of the dcxt.rosc monomeric unit and t-he co& surface, with the polar greups directed away from the surface producing a hydrophilic surface state. The hydrophobic bonding of a dcxtroSc unit to the coal surface could involve as many as six Cl1 groups which would correspond to an adsorption free energy of 4.8 kcal/mol of dextrose monomer, similar to the experimental adsorption free onorgies.

The parking area for the dextrose monomer with the distorted ring parallel to the surface was estimated from a molecular model to be 50 A2, which

would correspond to a maximum loading at saturation of 3.3 X lo-” moles of dextrose monomer/cm’. Experimentally the apparent maximum surface saturation can be found to be about 1 X low9 moles dextrose monomer/cm* in the case of the natural sample at pH 7.2 to 7.4 and 1.7 X 10S9 moles dextrose monomer/cm* in the case of the demineralized sample at pH 3.9 to 4.3. These calculations suggest that the dextrose monomer is oriented in a different position at t.he surface or, in fact, some monomers (- 2 out of 3) do not bond at the surface.

Further detailed examination of the adsorption isotherms shows t.hat adsorption by the demineralized samples (Fig. 7) is approximately twice the adsorption density of the corresponding natural samples (Fig. 6). This reflects the increased hydrophobicity of the demineralized samples due to the M- moval of hydrophilic mineral matter components.

It is also to be noted that the pH of the suspension in the dcxtrin adsorp- tion tests was in the range of pH 7.2-7.4 in the case of natural samples and pW 3.9-4.3 for the demineralized sampies. The difference in pH values occurs because the demineralized sample consists of the coal in its protonatcd form. As seen from Figs. 3 and 4, the natural coal sample nt pli 7.2-7.4 is ncgativc- ly charged while tho demineralized snmpla at pH 3.9-4.3 is at its i.e.p. This circumstance may also contribute to the higher adsorption density of dcxtrin on t.he demincralizcd coal. More recent work shows that the pli effect on dcxtrin adsorption is not particularly significant and the increased adsorption is due to dcmincralization. Further, the recent findings on brother adsorption by coal havr? been explained by a hydrophobic bonding mechanism and, interestingly, the adsorption densities show a definite independence 011 sys- tcm pH (Fucrstcnau et al., 1982).

The inrlepcndcncc of dextrin adsorption by coal on pH suggests that the hydrophobic character of the coal should also be independent of 111~. Contact angle measurements (equilibrium mcasurc of hydrophobicity) support this inference as shown in Table 4. fiowevcr, induction time measurements (kinetic measure of hydrophobicity) arc dependent on pH and as a result tic flotation rate would be expected to reveal 8 pH dcpcndcncy. Indeed such results have been report-cd (Rosenbaum ct al., 1982).

The experimental rc5ults generally confirm the notion that dextrin adsorbs by hydrophobic interactions at the coal surface. RCMOV~ of hydrophilic mineral matter sites from the coal surface by deminerahzation increases the level of doxtrin adsorption as was expected, whereas an increase in the acidic oxygen functional groups of the coal by controlled oxidation decreases the level of dextrin adsorption. The increase in the number of polar oxygen sites, as quantified in terms of carboxyl and phenol content, causes a reduction in the hydrophobic character of the coal and a decrease in the extent of dextrin adsorption.

150

TABLE 4

The hydrophobic characler of Coal Basin Coal as described by Induction time measure- ments and contact angle measurements for w!ccled pH vatues

System pH Induction time (ms) Contact angle (deg.)

4.3 (i.e.p.) 76 40 5.6 100 40 7.2 116 40

ACKNOWLEDGMENTS

The authors thank Mr. Gonlon Yang for the electrokinetic measurementi, Mr. Joven Calara and Mr. Jin-sheng Hu assisti in the adsorption density measurements. Dr. M, hlisra’s assistan- also contributed to the completion of this work. The fmanclal support provided by the Demment of Energy under Contract No, AC22-7SET11419 is gratifully recognized.

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