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Dextrin adsorption by oxidized coal
Collofdr and Surfitcer, 8 (1983) 137-161 Elrevier Science Publishers B.V., Amsterdam
DEXTRIN ADSORPTION J.D. MILLER,
(Received
137
BY OXIDIZED COAL
J.S. LASKOWSKI
of Wetolhgy City, UT 841 I2 (U.S.A.)
Degmfment
- Printed in The Netherlands
end SS. CHANO
and Metalturgical
13 June 1982;eccepted
Engineering, Uniwrrity of Utah, Salt Lake
In final form 7 January 1983)
ABSTRACT Dextrin can bc used as a depressant to varying degrees of success in the differential flotation of pyritefromcoal. The objective of this investigation has been further identification 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 adsorption 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 dent of the extent of oxidation, were calculated.
of about -5.5
kcallmol,
indepen-
INTRODUCTION
Fine coal cleaning by froth flotation has become a common practice in U.S. since the fmt coal flotation circuit was installed at a plant in Jmp&al, Pa. in the 1930s. Now, about 100,000 tpd of coal is cleaned by flotation in the U.S. In this process hydrophobic coal particles are usually lloaW away from the hydrophilic ganguo particles which remain in suspension. the
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: Collectors - 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 nonpolar collectors in this process is very different from the adsorption of watersoluble collectors utilized in the flotation of ores. Our knowledge of the phenomena involved is certainly not adequate in view of the emerging importance of coal flotation. Flotation which can be very simple and effective in the case of metallur0166.6622/83/$03.00
o 1983 Elscvier
Scfence Publishers B.V.
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
&al
solution
interface.
of Rotation reagents by coal (Evcson 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 &uty
experiments
ct al., 1956)
showed
on the adsorpt.ion
that. the amount
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 adsorption. ‘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 subduetinting 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 conccntmtions of phenol. ‘I’hcy concluded that chemically bound oxygen strongly influwcc?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. 111this rcslwA, a~ extromcty intcresting paper was l>rcsented by Zubkuva nnct Kuclux (1076) at. the 7th Intenlationd Congress of Surface Active SubS~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
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 decreased 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 (cetyltrimethylammonium 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 convcnConal 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 conccntration 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 cont.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, (mglU
Induction (s)
0 1 10 GO SO GO0
<3 25-39 30-45 300 360 No contact possible aCtcr 20 min.
time
Conlacl (deg.1 40 36 33 I:
angle
141
0 Coal Pyrite
0 Ore Pyrite [
0
20
40
60
60
Equilibrium Dentrin
100
120
Concentrrtion
Fig. 2. Adsorption of dewtrin by both hydrophablc 5-6.5 (Miller ot al., 1982).
(0)
140
-
m Quark
I 160
160
,
, mg/l
and hydrophilic solids at 29°C and pH
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-hehydrophobic bonding hypothesis further.
shown
EXPERIMENTAL
In these experiments both natural and demineralized coal samples were prepared And oxidized under various conditions. The samples were characterized 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 the Coal Basin Seam, Carbondale County, Colorado. This medium-volatile (volatile matter 24%) bituminous coaJcontained 65% ash and its ultimate analysis is presented in Table 2, Coal was wet ground and a -270 mesh fraction 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.%
Carbon
90.16
Jlydrogen
Nilrogcn Oxygen Sulfur
(dmml)
5.35 2.25 1.50 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 tcchniqucs 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 phcnolic, were detarmincd by m+ods sst&lished
mouJ=. carboxylic and 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” {carhoxylic 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?arsupcmatrlntwaspip&ted oil t.he next day into 26 ml of 0.2 N HCI. hccss of JIG1was back-titrated with 0.05 NaOH. A difference in cxccss I-ICI h4wccn tllc blank experiment (without coal) and the actual cxJlcrimcnt (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: 2(-COOl~l) Surf + l~a(CH,CoO)~ =
-COO --COO/
‘HaSurf+
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 microclectrophoresisapparatus, 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 measurementswere made. Tho points represent an average of 10 readings.
Dcxtrin adsorption
by coal
Specific surface area determination for the natural and deminerahzed samples 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 structures which probably are not accessibleto macromolecular adsorbates. We, however, obtained 0.37 m2/g and 0.34 m2/gfor 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 investigationwas 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 14Cfor analysis. The cxtcnt of adsorption was determined by measuring the number of disintegrationsfrom the solution of the depressant before and after adsorption with a liquid scintillation counter, model 720, manufactured by Nuclear-Chicago Corporation, Des Plaines, Illinois.
The procedures for t.aggingthe 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.kersand
144
vials to complete the precipitation, The precipitateswere centrifuged and tin526wit-hmethanol 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 tempcratures 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 Erlcnmeyer 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-pmfor 1 h. After adsorption, the susmnsion was Wered t-hroughtightly packed glass wool. This filtration technique produced a clear fxltratc and made ccntrifugation unncccssary. One ml of the filtrate was mixed with 10 ml Aquasol in a 1
I
I
C?c*trin, reagent Lower Freeport
grade
t M-W.l80Ot
Coal
l l
t
1
I
E.quilibrium
Fig. 3. &mparison of cxpcrimental Lion isotherms (Milter, 1982).
300
ConccnttatiOn.
techniques
m
I
200
100
0
m-
mgr I
for the dctcrmination
of dextrin adsorp
145
scintillation vial. The vial was mounted in the scintiUation counter and counted for a chosen length of time (10 min typically). The radiotracertechnique 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 spectrophotometric 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 characteristicsbelow 100 mg/l are of particular interest. AND
RESULTS
Oxygen
DISCUSSiON
functionat groups
The amount of carboxylic and phenolic groups determined by the procedures 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 and deminctalized ramplcs Carboxylic
Sample
on the formation groups
of oxygen
Phenollc
functional
groups
moVg
%O
*01/g
RO
(150-C) (200°C)
3.43 x 10-s 2.84 x 10-1 4.31 x 1o’a
0.01 0.09 1.38
4.73 x lo-* 6.05 x 10-s 9,76 x 1o’e
0.008 cI.;=P 1.56
DeminomlCzcd Fresh Oxidized (16OV) Oxidized (2OO’C)
3.43 x 10-L 2.46 x lo-‘ 4,34 x lo-’
0.01 0.008 1.39
4.73 x 10-c 6.24 x lo’* I.05 x 10”
0.008 0.1 1.68
Nalural Frcrrh Oxidized O*ldimd
Efcctrokinstic
groups for natural
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 surprisingthat 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 COAL
5 ~-
I
I
BASIN
HATURAL
t
-5
0
I
I
I
2
4
6
Pig. aI. Elcctrokinctic
diffwanf
behavior
tawls uf oxidaliun.
6~
I
5 ‘-
Fresh
0
150°C
Oxldited
A
200%
Oxidized
1
I
10
12
coal sample
I
I
I BASIN
I
0
1 8
PH of the natural
I COAL
I
Cram the aal
I
14
Uasin Scam at
I
tEMINERAtlZE0
4-
-1
0
Fresh
0
150°C
Oxidized
n
200%
Oxidized
-
-2 -3 ‘-4
-
-5 0
1 2
Pig. 5. Electrokinctic
at different
I 6
I 8 PH
I IO
I 12
14
behavior of the dcmincralizcd coal sample from the Coal Basin Seam lcvcls of oxidation.
147
oxidized. The electrokinetic measurementshave been conducted without any supporting electrolyte as we have ‘observedthat 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-hecoal, These results are a further characterizationof 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-hatthe shift was due to differences in oxygen-containing functional 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 decreaseswith 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.ionsas discussed in conjunction with the results presented in Fig. 2. The isotherms presented in Figs. 6 and 7 follow the Langmuir equat.ion, W)
(W 1 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 we11with the --5.O to -5.6 keal/mol of dextrose monomer dobrmined p&iousIy (Haung et al., 1978). _ I
I
I
COAL DASIH
I
I
I
NATURAL
20%
0
Fresh 160% 200%
0 0 A
10
20
30
Equitibrium Dertrin
40
50
Concenlration
Oxidized Oxldixed 60
(0)
70
, mg/t
Fig. 6. Adsorption of dcxtrin by the natural coal sample from the Coal Basin Scam at pJJ and different levels of oxidation.
7.2-7.4
I
I
I
BASIN
I
I
I
I
COAL
I
DEMlNEAALlZED
20%
pH= 3.9
- a3
h-O-
0
10
2D
30
Equllibrlum Fig. 7. Adsorption
uf r!ch;lrin
at 1~11 X9-4.3 analtliffcrcnt
Oerrtrin
40
rso%
Oxidized
A
200%
Oxidized
50
Concentration
lay lhc rlrmincratizcd Icvcls of uxidalion.
Fresh
0
currl
I
I
I
60
70
80
(0)
, mg/l
sample
from
lhc Coal
fkin
Sea~n
g:rouW Flydrophobic bonding usually arises from t-be tendency of nonpok of organic molecules to adhcrc to one another in a polar aqueous environment (Ncmcthy et d., 19012; The free energy change corresponds to the removrd of n nonpolar group from its aqueous environment and the resulting Vnn dor Waal’s lmnding lmtw*ccnthe 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 energctics 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 Mmoval of hydrophilic mineral matter components. It is also to be noted that the pH of the suspension in the dcxtrin adsorption 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 ncgativcly 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 011systcm 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 ments and contact angle measurementsfor w!ccled pH vatues System
pH
4.3 (i.e.p.) 5.6 7.2
Induction 76 100 116
time (ms)
Contact
time measure-
angle (deg.)
40 40 40
ACKNOWLEDGMENTS
The authors thank Mr. Gonlon Yang for the electrokinetic measurementi, Mr. Joven Calara and Mr. Jin-shengHu 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. RKYERENCES
Rloom, L., L. Edclhauscn and D.W. \*an Krevelen, 1957.Fuel, 38: 135. Coughlin, 1t.W. and F-S. Ezra, 1968. Role of surface acidity tn the adsorption of organic wltutants on the surface 0f cartmn, Environ. Sci. Tcchn01., 2: 291. Evcson, G.P., S.a. Ward and F. Worthington, 1958. Flotation of lowrank coal, part 1. Ttac adsurpllon of phenol from aqueous xolutbns by coal ot various rank, J. Inst. Fuel, 29: 540. Frangiskor, N.Z., C.C. Harris and A. Jowelt, 1960, The adsorption of frothing agents on coals, 3rd Intcm. Congress on SuFfaCC Actfwz Substances, Koln, 1980, Congress Proceedings, V01. 4, pp. 404-415. Fucrstcnau, D-W.. 197 1. In terfaciat process tn mineral water nystcms. Pure Appl. Chem., 24: 135. vuerstcnau, D.W. and Fradip, 1982. Adsorpllon of Bothers at -at/water intorfaces, C0lluidc Burraces, 4: 220. ftaung, II.I#., J.V. Calaro, D.L. Bauer an& tD, Miller, 1978. Admrplkn rcactionl In the depression of real by organic colloids, In: Rcmnt Developments in Separation Science, CRC Press, Florida, vol. 4, pp. 115-133. Ihnat0wicr, A., 1952. Studies of owygcn groups in bituminous roads, Praca Gi0wncg0 lnstylutu Gorntctwa (Proc. Centml ReMarch Mining Institute), No. 126, Katowi(in Polish). Klasscn, V.I., 19GG. Coal Flotation, Wyd. Slark, Katowice, (Polish text). Lasko~ski, J. and 6. Koniccrny, 1970. An attempt to the determination of ah oxidation of coal by adsorptlon of the surf’actants, Arch. Oom., 16: 29. Lin, CL., 1982. Characterization of pyrite in reverse Rotation product&, M.S. Thesis, University of Utah, Salt Lake City, Utah. Miller, J-D., 1982. Radiotracer measurements of the ads0rptlon of organic collo1dr hy coat, Proceedings of the August 1979 Rindgc Coal Cleaning Conference, NSF, DOE and Engineering Foundation, p. 30. Miller, J.D., S.S. Chang and C.L. Lin, 1982. Coadsotption phenomena in the flotation of pyrite from coal by re\~rsc flotation, to be submitted f0or publication.
161 Miller, K.J. and A.F. Baker, 1972. Flotation
of pyrilc from coal, Technical Progress Rcport - 51, DOE, Pittsburgh Energy Research Center, February. Miller, K.J., Flotation of pyrite from coal: pilot plant rtudy, U.S. Bureau of Mines Report of Investigations, 7822, (1973). G, and H. Scheragc, 1962, The structure of water and hydrophobic bonding in proteins, 111.The lhermodynamic properttes of hydrophobic bonding in proteins, J.
Nemethy,
Phys, Chcm,,
66; 1773.
Parsegfan, D.A. and B.W. Ninham, 1971. J. colloid Interface Rosenbaum, J.M., D.W. Fucrstenau and J. Laska’a+ski, 1983. groups on the flotrttfon of coat, Coltoids Surfaces, 8: 153. Reynolds, J.A., D-B. Bitgert and C. Tanford, 1974. EmpirlcaI hydrophobic free energy and aqueous cavity surface area,
71(S):
Schafer,
197. Schafer,
2926.
H.N.S.,
1970a. Carboxyl
H.N.S., 1970b.
groups and ion exchange
Determination
of the total acidity
Sci.. 37(2): 332. Effect of surface functional correlation hctwccn Proc. Natl. Acad. Sci. U.S.A.,
in low rank coals, Fuel, 49:
of low rank coats, Fuel, 49:
271. Tanford, C.T., 1973. The Hydrophobic Effect: Formation of Micelle and Biological Mcmbranw, John Wiley & Sons, New York, p. 3. Whelan, P.F., 1954. l%ely diwminatcd sulfur compounds in British coals, J. inst. Fuel, 27: 455. Wcn, W.W. and S.C. Sun. 1977. An electrokint:tic study on tho amine fiotation of oxidized
coal, SMEIAIME
Trans.,
262:
174.
Wie, J.M. and D.W. Fucrstenau, 1974. The effect of dcxtrfn on surface properties and the flotatran of malybdcnite, fnt. J. Miner. Process+, 1: 17. Xuhkova, J.N. and R.V. Kucher, 1970. Molecular interactions at the coat/equcour solution of surface active agents intcrfacc, in: FL 8chukin (Ed.), 7th Int. Congress on Surface Active Subttanceq Moscow, Congress Proceedings, Vol. 2, pp. 866-652 (Russian text).
DEXTRIN ADSORPTION J.D. MILLER,
(Received
137
BY OXIDIZED COAL
J.S. LASKOWSKI
of Wetolhgy City, UT 841 I2 (U.S.A.)
Degmfment
- Printed in The Netherlands
end SS. CHANO
and Metalturgical
13 June 1982;eccepted
Engineering, Uniwrrity of Utah, Salt Lake
In final form 7 January 1983)
ABSTRACT Dextrin can bc used as a depressant to varying degrees of success in the differential flotation of pyritefromcoal. The objective of this investigation has been further identification 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 adsorption 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 dent of the extent of oxidation, were calculated.
of about -5.5
kcallmol,
indepen-
INTRODUCTION
Fine coal cleaning by froth flotation has become a common practice in U.S. since the fmt coal flotation circuit was installed at a plant in Jmp&al, Pa. in the 1930s. Now, about 100,000 tpd of coal is cleaned by flotation in the U.S. In this process hydrophobic coal particles are usually lloaW away from the hydrophilic ganguo particles which remain in suspension. the
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: Collectors - 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 nonpolar collectors in this process is very different from the adsorption of watersoluble collectors utilized in the flotation of ores. Our knowledge of the phenomena involved is certainly not adequate in view of the emerging importance of coal flotation. Flotation which can be very simple and effective in the case of metallur0166.6622/83/$03.00
o 1983 Elscvier
Scfence Publishers B.V.
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
&al
solution
interface.
of Rotation reagents by coal (Evcson 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 &uty
experiments
ct al., 1956)
showed
on the adsorpt.ion
that. the amount
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 adsorption. ‘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 subduetinting 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 conccntmtions of phenol. ‘I’hcy concluded that chemically bound oxygen strongly influwcc?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. 111this rcslwA, a~ extromcty intcresting paper was l>rcsented by Zubkuva nnct Kuclux (1076) at. the 7th Intenlationd Congress of Surface Active SubS~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
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 decreased 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 (cetyltrimethylammonium 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 convcnConal 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 conccntration 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 cont.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, (mglU
Induction (s)
0 1 10 GO SO GO0
<3 25-39 30-45 300 360 No contact possible aCtcr 20 min.
time
Conlacl (deg.1 40 36 33 I:
angle
141
0 Coal Pyrite
0 Ore Pyrite [
0
20
40
60
60
Equilibrium Dentrin
100
120
Concentrrtion
Fig. 2. Adsorption of dewtrin by both hydrophablc 5-6.5 (Miller ot al., 1982).
(0)
140
-
m Quark
I 160
160
,
, mg/l
and hydrophilic solids at 29°C and pH
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-hehydrophobic bonding hypothesis further.
shown
EXPERIMENTAL
In these experiments both natural and demineralized coal samples were prepared And oxidized under various conditions. The samples were characterized 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 the Coal Basin Seam, Carbondale County, Colorado. This medium-volatile (volatile matter 24%) bituminous coaJcontained 65% ash and its ultimate analysis is presented in Table 2, Coal was wet ground and a -270 mesh fraction 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.%
Carbon
90.16
Jlydrogen
Nilrogcn Oxygen Sulfur
(dmml)
5.35 2.25 1.50 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 tcchniqucs 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 phcnolic, were detarmincd by m+ods sst&lished
mouJ=. carboxylic and 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” {carhoxylic 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?arsupcmatrlntwaspip&ted oil t.he next day into 26 ml of 0.2 N HCI. hccss of JIG1was back-titrated with 0.05 NaOH. A difference in cxccss I-ICI h4wccn tllc blank experiment (without coal) and the actual cxJlcrimcnt (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: 2(-COOl~l) Surf + l~a(CH,CoO)~ =
-COO --COO/
‘HaSurf+
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 microclectrophoresisapparatus, 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 measurementswere made. Tho points represent an average of 10 readings.
Dcxtrin adsorption
by coal
Specific surface area determination for the natural and deminerahzed samples 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 structures which probably are not accessibleto macromolecular adsorbates. We, however, obtained 0.37 m2/g and 0.34 m2/gfor 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 investigationwas 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 14Cfor analysis. The cxtcnt of adsorption was determined by measuring the number of disintegrationsfrom the solution of the depressant before and after adsorption with a liquid scintillation counter, model 720, manufactured by Nuclear-Chicago Corporation, Des Plaines, Illinois.
The procedures for t.aggingthe 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.kersand
144
vials to complete the precipitation, The precipitateswere centrifuged and tin526wit-hmethanol 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 tempcratures 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 Erlcnmeyer 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-pmfor 1 h. After adsorption, the susmnsion was Wered t-hroughtightly packed glass wool. This filtration technique produced a clear fxltratc and made ccntrifugation unncccssary. One ml of the filtrate was mixed with 10 ml Aquasol in a 1
I
I
C?c*trin, reagent Lower Freeport
grade
t M-W.l80Ot
Coal
l l
t
1
I
E.quilibrium
Fig. 3. &mparison of cxpcrimental Lion isotherms (Milter, 1982).
300
ConccnttatiOn.
techniques
m
I
200
100
0
m-
mgr I
for the dctcrmination
of dextrin adsorp
145
scintillation vial. The vial was mounted in the scintiUation counter and counted for a chosen length of time (10 min typically). The radiotracertechnique 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 spectrophotometric 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 characteristicsbelow 100 mg/l are of particular interest. AND
RESULTS
Oxygen
DISCUSSiON
functionat groups
The amount of carboxylic and phenolic groups determined by the procedures 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 and deminctalized ramplcs Carboxylic
Sample
on the formation groups
of oxygen
Phenollc
functional
groups
moVg
%O
*01/g
RO
(150-C) (200°C)
3.43 x 10-s 2.84 x 10-1 4.31 x 1o’a
0.01 0.09 1.38
4.73 x lo-* 6.05 x 10-s 9,76 x 1o’e
0.008 cI.;=P 1.56
DeminomlCzcd Fresh Oxidized (16OV) Oxidized (2OO’C)
3.43 x 10-L 2.46 x lo-‘ 4,34 x lo-’
0.01 0.008 1.39
4.73 x 10-c 6.24 x lo’* I.05 x 10”
0.008 0.1 1.68
Nalural Frcrrh Oxidized O*ldimd
Efcctrokinstic
groups for natural
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 surprisingthat 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 COAL
5 ~-
I
I
BASIN
HATURAL
t
-5
0
I
I
I
2
4
6
Pig. aI. Elcctrokinctic
diffwanf
behavior
tawls uf oxidaliun.
6~
I
5 ‘-
Fresh
0
150°C
Oxldited
A
200%
Oxidized
1
I
10
12
coal sample
I
I
I BASIN
I
0
1 8
PH of the natural
I COAL
I
Cram the aal
I
14
Uasin Scam at
I
tEMINERAtlZE0
4-
-1
0
Fresh
0
150°C
Oxidized
n
200%
Oxidized
-
-2 -3 ‘-4
-
-5 0
1 2
Pig. 5. Electrokinctic
at different
I 6
I 8 PH
I IO
I 12
14
behavior of the dcmincralizcd coal sample from the Coal Basin Seam lcvcls of oxidation.
147
oxidized. The electrokinetic measurementshave been conducted without any supporting electrolyte as we have ‘observedthat 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-hecoal, These results are a further characterizationof 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-hatthe shift was due to differences in oxygen-containing functional 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 decreaseswith 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.ionsas discussed in conjunction with the results presented in Fig. 2. The isotherms presented in Figs. 6 and 7 follow the Langmuir equat.ion, W)
(W 1 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 we11with the --5.O to -5.6 keal/mol of dextrose monomer dobrmined p&iousIy (Haung et al., 1978). _ I
I
I
COAL DASIH
I
I
I
NATURAL
20%
0
Fresh 160% 200%
0 0 A
10
20
30
Equitibrium Dertrin
40
50
Concenlration
Oxidized Oxldixed 60
(0)
70
, mg/t
Fig. 6. Adsorption of dcxtrin by the natural coal sample from the Coal Basin Scam at pJJ and different levels of oxidation.
7.2-7.4
I
I
I
BASIN
I
I
I
I
COAL
I
DEMlNEAALlZED
20%
pH= 3.9
- a3
h-O-
0
10
2D
30
Equllibrlum Fig. 7. Adsorption
uf r!ch;lrin
at 1~11 X9-4.3 analtliffcrcnt
Oerrtrin
40
rso%
Oxidized
A
200%
Oxidized
50
Concentration
lay lhc rlrmincratizcd Icvcls of uxidalion.
Fresh
0
currl
I
I
I
60
70
80
(0)
, mg/l
sample
from
lhc Coal
fkin
Sea~n
g:rouW Flydrophobic bonding usually arises from t-be tendency of nonpok of organic molecules to adhcrc to one another in a polar aqueous environment (Ncmcthy et d., 19012; The free energy change corresponds to the removrd of n nonpolar group from its aqueous environment and the resulting Vnn dor Waal’s lmnding lmtw*ccnthe 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 energctics 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 Mmoval of hydrophilic mineral matter components. It is also to be noted that the pH of the suspension in the dcxtrin adsorption 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 ncgativcly 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 011systcm 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 ments and contact angle measurementsfor w!ccled pH vatues System
pH
4.3 (i.e.p.) 5.6 7.2
Induction 76 100 116
time (ms)
Contact
time measure-
angle (deg.)
40 40 40
ACKNOWLEDGMENTS
The authors thank Mr. Gonlon Yang for the electrokinetic measurementi, Mr. Joven Calara and Mr. Jin-shengHu 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. RKYERENCES
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