Langmuir 1993,9, 1330-1333
1330
Adsorption of Dextrin onto Graphite S.Subrammiant and J. S . Laskowski' Department of Mining and Mineral Process Engineering, The University of British Columbia, Vancouver, British Columbia, Canada Received September 8,1992. In Final Form: February 3,1993
The interaction of dextrin with a hydrophobic solid, such as graphite, hae been investigated through adsorption, electrokinetic, and flotation experiments. The results revealed that further purification of the graphite by leaching increased its hydrophobicity,but significantlydecreased dextrin adsorption;this suggests that the interactions between the metallic sites on the graphite surface and dextrin are mainly responsible for adsorption. The electrokineticbehaviors of the original and the leached graphites in the presence of dextrin are very different,consequent to the pronounced differencesin the dexttin adsorption. In accordance with the adsorption results, dextrin was found to strongly depress the flotation of the original graphite but did not affect the flotation of the leached sample, which was more hydrophobic. and metal hydroxidesresult from the formation of chemical Introduction complexes, with the maximum adsorption density coinDextrins, which are low-molecular-weightpolysacchaciding with the pHiepof the metal hydroxides.12 Further, rides have been used in the mineral processing industry their results highlighted the importance of metallic sites for over half a century, chiefly as depressanta of inherently for the adsorption to be effective. It was found that while hydrophobicsolids. Followingthe early patents by Brown' dextrin did not adsorb onto either hydrophilic quartz or and Booth213for the application of dextrins as depressanta methylated hydrophobicquartz, it adsorbed stronglyonto for molybdeniteand carbonaceousmaterials, respectively, both these sampleswhen activatedwith lead.13 Thus,these Klassen4investigated the depressive action of dextrin on findings unequivocallyindicate that hydrophobicbonding coal flotation. Dextrin has been used as a depressant for per se is not the primary mechanism governing dextrin coal in a two-stage reverse flotation process for the adsorption. Since it is known that hydroxides may appear separation of pyrite from ~ o a l Fundamental .~~~ studies on on the surface of sulfides under certain physicochemical the effect of dextrin on the surface properties of molybconditions, it became logical to examine these findings denite have been carried out by Wie and Fuerstenau.' with respect to typical polymetallicsulfides. In conformity Haung et al.* and Im and Aplang have reported that with the above mechanism,it was establishedthat selective dextrins are excellent depressants for hydrophobic coals. separation of galena from chalcopyrite,l4and chalcocite Dextrin has also been used in the depression of talc.l0 from heazlewoodite,16could be achieved. These laboratory Recently, Grano et ale" have used an organic depressant tests were further scaled up to pilot plant and full scale containing dextrin to reduce the impact of the natural plant testa on typical copper-lead bulk concentrates at flotability of graphite-coated iron sulfide minerals assoBoliden, Sweden, and yielded encouraging results that ciated with the copper concentrate. In all these applihighlighted the importance of closer control of pH and cations, high adsorption densities of dextrin on different dextrin dosage.16 hydrophobic minerals were observed and consequently In the present investigation,the validity of the chemical led to the "hydrophobic bonding" theory being proposed interaction mechanismpostulated for the sulfideshas been as the adsorption mechanism for dextrins onto mineral further examined using an inherently hydrophobic solid, surfaces. namely, graphite. Extensive basic investigations to elucidate the interaction mechanism between dextrin and various mineral Experimental Section surfaceshave been carried out by Liu and L a s k o w ~ k i . ~ ~ - ~ ~ They established that the interactions between dextrin Materials. High-quality graphite grom Sri Lanka was procured from Ward's Natural Science Establishment, New York.
* To whom correspondence should be addressed.
On sabbatical leave from the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. (1)Brown, E. H. U.S.Patent 2,070,076,1937. (2)Booth, R. B. U.S.Patent 2,145,206,1939. (3) Booth, R. B. U.S.Patent 2,211,686,1940. (4)Klassen, V. 1. Coal Flotation; Gosgortiekhizdat: Moscow, 1963. (5)Miller, K. J.; Baker, A. F. USBM Technical Progress Report No. +
51;1972. (6)Miller, K. J. USBM RI-7822;1973. (7)Wie, J. M.; Fuerstenau, D. W. Int. J.Miner. Process. 1974,1,17. (8)Haung, H.H.; Calara, J. V.; Bauer, D. L.; Miller, J. D. In Recent Deuelopnents inSeparationScience;Li,N.N.,Ed.;CRCPress: Chicago, 1978;Vol. 4, pp 115-133. (9)Im, C. J.; Aplan, F. F. In Proceedings of the First Australian Coal Preparation Conference,Newcastle,New South Wales; Swanson, A. R., E .: -d-. , 1981. - - - -. (10)Makarinsky, F. M. Can. Min. J. 1975,96,26. (11)Grano, S. R.; Griffin, L. F.; Johnson, N. W.; Smart, R. St. C.; Ralston, J. In Proceedings of the Fourth Mill Operators' Conference, Burnie, Tasmania; 1991.(12)Liu, Q.;Laskowski, J. S. J.Colloid Interface Sci. 1989,130,101. (13)Liu, Q.;Laskowski, J. S. Int. J. Miner. Process. 1989,26,297. (14)Liu, Q.;Laskowski, J. S. Int. J.Miner. Process. 1989,27,147.
The graphite lumps were dry ground in an agate mortar grinder and sieved to produce a coarse sample for the Hallimond tube flotation experiments (-120+ 104pm) and a finer sample for the adsorption and electrokinetic studies (-38pm). Testa were also conducted on "leached" graphite samples. These were prepared by agitating ca. 10 g of the sized graphite sample in 100 mL of 1 M HCl solution at 45 "C for 18 h in each leaching cycle. The sampleswere subjectedto 6-8 leaching cycles before being fiitered. Then they were washed thoroughly with distilled water until no trace of chloride ions was detected and the conductivity of the residual wash water approached that of the distilled water. The leached graphite was then dried at 50 "C. A typical analysis of the leach liquor is given in Table I. Further, the chemical analyses of the original and the leached graphite samples are compared in Table 11. An examination of both the graphites under a scanning electron microscope (ETEC, Autoscan) gave no indi~~
(15) Nyamekye, G. A.;
Laskowski, J. S. In Proceedings of the Copper91 International Symposium; Dobby, G. s.,Argyropoulos, s.A,, Rao, s. R., Ed.; Pergamon Press: Toronto, 1991;Vol. 11, pp 231-243. (16)Bolin, N. J.; Laskowski, J. S. Int. J.Miner. Process. 1991,33,235.
0743-7463/93/2409-1330$04.00/00 1993 American Chemical Society
Langmuir, Vol. 9, No. 5, 1993 1331
Adsorption of Dextrin onto Graphite Table I. TyDical Analyses of the Leach Liauor Samples
1.5
AI
~~
leaching cycle no. 1 2 3
4 5 6 7 8
Fe 42.7 18.0 7.4 2.6 1.7 0.6 0.6 0.5
amt of trace elems (ppm) Ca Mg 33.2 12.9 4.7 1.2 0.5 0.2 0.2 0.2
13.6 5.4 2.2 0.4 0.2 0.1 0.1 0.1
Al 10.1
5.9 2.6 1.2 0.5 0.2 0.0 0.0
Table 11. Chemical Analyses of Graphite Samples amt of trace elems ( % ) type of graphite Fe Ca Mg A1 0.008 0.100 0.004 synthetic 0.210 0.020 0.056 0.026 original 0.130 0.002 nd' 0.004 leached 0.070 a
1.5
nd = not detectable.
cation of alterations of the mineralogical structure introduced by the leaching process. The Brunauer-Emmett-Teller (BET) specific surface areas measured using a "Quantasorb" (Quantachrome Corp.) apparatus and nitrogen as the adsorbate were 8.42 and 9.04 m2/gfor the original and leached -3&rtm graphite samples, respectively. In a few tests, synthetic graphite from BDH Limited, having a BET nitrogen specific surface area of 5.08 m2/gwas also used. The trace elements present in this graphite are also given in Table 11. Graphon, a graphitized carbon black, was supplied by Cabot Corp., and ita BET nitrogen specificsurface area was 91.39 m2/g. Poly(tetrafluoroethy1ene) (PTFE) was obtained from Aldrich Co., and its BET nitrogen specific area was 2.02 m2/g. The dextrin used wasa J.T. Baker product (G193-7) purchased from Johns ScientificInc. Its molecularweight has been reported7 to be 7900. Potassium chloride was used to control the ionic strength, and HCI and KOH were utilized to adjust the pH of various solutions. All reagents were of analytical grade. Methods. Adsorption isotherms were determined by contacting 0.5 g of graphite with 25 mL of dextrin solution of desired concentration in closed vials, which were agitated at a constant rate (300 rpm) in a temperature-controlled (25 "C) orbit shaker (Lab-line Instruments Inc.). Kinetic studies showed that equilibration times of 1h were more than adequate for the adsorption to attain equilibrium. After adsorption, the supernatant solution was separated from the solids by centrifuging at 8000 rpm for 3 min in an IEC Model H T centrifuge, for the determination of pH and the residual concentration of dextrin. The analytical procedure used to determine the dextrin concentration was basically that of Dubois et al." Electrophoretic mobilities were measured with a microelectrophoresis apparatus manufactured by Zeta Meter Inc. For each measurement about 10 mg of graphite was conditioned with the appropriate solution for 1h under a constant ionic strength (10-3 M KCI). About 10-15 particles were tracked, and the average mobilities were determined. Flotation experiments were carried out in a modified Hallimond tube,l8 using 1.5 g of graphite in about 150 mL of solution volume. The graphite particles were conditioned for 10 min and then floated at 100mL of nitrogedmin for 5 min. The concentrate and the tailings were collected, filtered, dried, and weighed to determine the yield of the products.
Results and Discussion Adsorption of Dextrin on Graphite. The adsorption
isotherms of dextrin on various graphite samples are portrayed in Figure 1. It is apparent that the adsorption density of dextrin onto the original Sri Lankan graphite (17) Dubois, M.; Gilles, K. A.; Hamilton, J. K.; Rebers, P. A.; Smith, F. Anal. Chem. 1956,28,360. (18)Fuerstenau,D. W.;Metzger, P. H.; Seele,G. D.Eng.Min. J. 1957, 93.
Equilibrium Concentration,mg/l
. z N
E
-
Synthetic
0 0
Synthetic (Leached) Leached
B-I
,._I_!__
uriginal
0'
1' 45 0.1
4
E
pH=4.5-7.5 Temper~ture=25~ I
-0
' " ' I
1
2
5
10
I
I
20
SQ
,
, , , I
im
I
am
Equilibrium Concentration,mg/l Figure 1. Adsorption isotherms of dextrin onto various graphite samples.
is higher than that on the leached sample (Figure 1A).A similar trend is observed with respect to the synthetic graphite. Taking into consideration the presence of the trace elements in the different graphite samples, as summarized in Table 11, it becomes clear that the adsorption density is dependent on the amount of the metallic impurities present. The high affinity (Langmuirian) isotherms can plausibly be attributed to the chemicalinteractionbetween dextrin and the metallic sites, acting as adsorption centers. The removal of the adsorption sites consequentto the leaching operation lowers the adsorption density of dextrin onto the leached samples. These findings are in good agreement with the results of the adsorption of carboxymethyl cellulose on graphite reported by Solari et al.19 In order to confirm the role played by the metallic adsorption centers in dextrin adsorption, a few testa were also conducted on Graphon, a graphitized carbon black, which is considered to be chemically unreactive, possessing reproducible and practically uniform, nonpolar surfaces and thereby serving as a model adsorbent.20 As anticipated, the results for Graphon (Figure 1A)indicate a substantial reduction in dextrin adsorption, due to the absence of the metallicsites. These findingswere further corroborated bythe adsorption tests on poly(tetrafluoroethy1ene) (PTFE),a synthetic hydrophobicmaterial, which did not show any adsorption of dextrin. The adsorption isotherms replotted with concentration on a log scale (Figure 1B) show without any doubt the ~
~~
(19)Solari,J. A.; de Araujo,A. C.; Laskowski, J. S . Coal Prep. (Gordon & Breach) 1986, 3, 15. (20) Kitchener, J. A. J. Photogr. Sci. 1965, 13, 297.
Subramanian and Laekoweki
1332 Langmuir, Vol. 9,No. 5, 1993 ,-
Leached Graphite
0.7
2
18 3
5
i
80-
-
0.6 0.5
Original
;r;
w
-
s J!
0.4 -
40-
K
Dextrin: 100 ppm
0.3
so-
Temp: 25OC
0.2 0
2
wO
4
6
2o
!
Qraphon 10
8
12
t
14
PH
Figure 2. Adsorption densities of dextrin onto various graphite samples as a function of pH. loo,
I
./-.-.
Leached Graphite
Original Qraphiie
-~
8
0
85t
low-affinity adsorptiQn of dextrin onto Graphon and corroborate the postulated adsorption mechanism via the metallic adsorption centers. These isotherms confirm lower adsorption of dextrin onto the leached samples, with the adsorbing dextrin macromolecules satisfying the available adsorption centers at very low saturation concentrations. The adsorption in this case seem to be localized to the high-energy adsorption centers only. The curves for the original graphites suggest the presence of some high-energy sites as well as some lower-energy sites. The effect of pH on dextrin adsorption onto different graphites is shown in Figure 2. It is evident that the adsorption density is independentof pH for allthe samples studied. A similar behavior with respect to pH was observed for dextrin adsorption onto molybdenite, also a naturally floatable The observed differences in the magnitude of dextrin adsorption onto the various graphites in the pH range investigated are in conformity with the resulte of the adsorption isotherms depicted in Figure 1. It is pertinent to recall that Liu and La&owski12 found an excellent correlation between the pH of optimum adsorption and the isoelectric pointa (iep's) of the respective metal hydroxides, using the same dextrin. Since traces of various metallic impurities are ratill present even in the leached graphite (Table 11),the adsorption optima can be expected at different pH ranges and their overlapping may thereby lead to the apparent lack of pH dependence on the adsorption density. Floatability of Graphite. The floatabiIities of the original and the leached graphites are compared in Figure 3. As can be observed, the leaching process has enhanced the hydrophobicity of the original sample in the pH range
studied. It is also evident that the flotation recovery-pH relationship for both the graphites is similar and shows very little variation over the pH range investigated. It is significant that a similar trend was observed in the case of molybdenites7 Additionally, contact angle measurementa on graphite:' molybdenite: and paraffin waxz2 showed a close parallelism to the floatabilities, and both were independent of pH. As seen from Figure 4, in the case of the leached hydrophobic graphite, the flotation is initially slightly affected at low dextrin concentrations, but it remains almost constant when the dextrin concentration is further increased. The effect of the dextrin concentration on the flotation of original graphite is much more pronounced, and the flotation depression intensifies with the dextrin concentration in the whole tested range. It is obviousthat the surface of the original graphite becomes much more hydrophilic as a result of dextrin adsorption. This is probably due to the higher dextrin adsorption onto the original graphite, and also ita less hydrophobic nature as compared with the leached sample (Figure 3). Thus, there is a close correlation between the adsorption results and the flotation behavior for both the graphite samples studied. Figures 3 and 4 also indicate that while the leached graphite is more hydrophobic, ita surface properties are less affected by dextrin. These findings reiterate that it is not the hydrophobic interactions but the interactions with the metallic adsorption centers on the solid surface that are primarily responsible for dextrin adsorption. ElectrokineticStudies. The {potentialof the original graphite as a function of pH in the absence and presence of different concentrationsof dextrin is portrayed in Figure 5. The measurements have been conducted in the presence of le3 M KC1as an indifferent electrolyte. In the absence of dextrin, the t potential is negative in the entire pH range investigated, with the electronegative character increasing with the increase of pH. A similar trend has been observed by Parreira and SchulmanzZfor parrafh wax and Arbiter et al.21 for other inherently hydrophobic solids. In the presence of dextrin, the {potential becomes less negative at all the pH values studied. It is apparent that the electrokineticbehavior is more or lessindependent of the dextrin concentration for the original graphite (Figure 5). For the leached graphite (Figure 61, the { potential value is reduced in proportion to the dextrin concentration, presumably without any shift in the iep ~
(21) Arbiter, N.; Fujii, Y.; Hansen, B.; Raja, A. AIChE Symposium Series No. 160; AIChE: New York, 1975; Vol. 71, p 176. (22) Parreira, H. C.; Schulman,J. H. Adu. Chem. Ser. 1961,33, 160.
Adsorption of Dextrin onto Graphite
.-
2
3
4
5
t3
7
a
Langmuir, Vol. 9, No. 5, 1993 1333
s i 0 1 1 1 2
PH
Figure 5. Effect of pH and dextrin concentration on the 3 potential of the original graphite.
-201 I"
1(r3t.4 KCI
-30
-90 1 2
Nil
.
3
4
9nnnmi
5
6
7
a
s i o i i 1 2
PH Figure 6. Effect of pH and dextrin concentration on the potential of leached graphite.
disappears from the adsorption isotherm. The {potential curves reported in this paper indicate a strong interaction of dextrin macromoleculeswith the original graphite and conformational rearrangements with the loading, leading to the increased extension of the looping chains for the leached sample. While the flotation is not affected by these conformational changes at the isolated adsorption sites, it is clearly correlated with the adsorption density. Of special interest to this study are our electrokinetic measurements made with heazlewoodite (Ni&z) and chalcocite ( C U Z S ) .A~ ~high affinity of dextrin toward heazlewoodite and a low affinity toward chalcocite observed in the adsorption experiments were paralleled by quite a different electrokinetic behavior. For example, while in the case of copper hydroxide and chalcocite increasing the concentrations of dextrin produced different { potential-pH curves, all intersecting at the iep and resembling the effect of an indifferent electrolyte, in the case of heazlewoodite and nickel hydroxide a strong interaction between dextrin macromolecules and nickel hydroxide was manifested in quite a different {potentialpH relationship. It is worthy of mention that recent experiments with the use of ATR-FTIR spectroscopy confirmed strong chemical interaction between heazlewoodite and dextrin and lack of such an interaction between dextrin and chalcocite. Note Added in Response to the Reviewers' Comments. As one of the reviewers remarked, the findings reported in this paper seemed not in keeping with some of the earlier publications28on the adsorption of dextrin by oxidizedcoal. The results discussed in the quoted paper indicated that the dextrin adsorption density increased substantially when the bituminous coal (which contained 6.5 % ash) was purified by leaching to less than 0.5 % ash, and decreased when the coal was oxidized. The purified coal used in the experiments described earlier28 contained 0.5% ash. In comparison with the graphite and synthetic Graphon samples utilized in this project, this was an "impure" sample that still contained a lot of various adsorption centers, and it is, therefore, difficult to compare these results.
value, resemblingthe influence of an indifferent electrolyte. Since the polymer appears merely to reduce the {potential in absolute magnitude under all the pH conditions studied, the primary effect of the large macromolecules seems to be to shift the slipping plane further away from the interface. These results suggest that some rearrangement of the macromolecules adsorbed at the adsorption sites takes place with increasing extension of the looping chains when the dextrin concentration is further raised. These observations are in good agreement with the results of the effect of adsorption of large nonionic molecules on the { potential, reported by many researchers.23-25 As it was shown in our previous publications on the ~ u b j e c t , l lalthough ~~ the metallic centers are mostly responsible for dextrin adsorption, the hydrophobicity of the matrix on which these adsorption centers reside further enhances the adsorption through some sort of synergistic effects. The adsorption of dextrin onto the original graphite (Figure 1) seems to be taking place on two different (the higher- and lower-energy)adsorption sites. It seems reasonable to assume that the lower-energy adsorption sites can be ascribed to the mentioned synergistic effects. For the reduced number of the adsorption centers on the leached samples, the synergistic effect
(1)The purification of graphite by leaching reduces the content of metallic impurities and increases graphite hydrophobicity. (2) The dextrin adsorption density was found to be correlated with the number of metallic adsorption centers on the surface of the tested graphite samples. (3) Electrokinetic experiments hinted that dextrin macromolecules adsorb strongly at the surface of the original graphite, and become more fully extended with the loading at the leached graphite; the latter drives the shear plane further away from the interface. (4) In accordance with the adsorption and electrokinetic measurements,dextrin was found to depress strongly the floatability of the original graphite and to only marginally affect the floatability of the leached sample. ( 5 ) The findings of this study confirm the previously postulated chemical in nature interaction mechanism between dextrin and solid surfaces.
(23) Brooks, D. E. J. Colloid Interface Sci. 1973,43, 687. (24) Lyklema, J. Pure Appl. Chem. 1976,46, 149. (25) Garvey, M. J.;Tadros, Th. F.;Vincent,B. J. Colloid Interface Sci. 1976, 55, 440. (26) Laekowski, J. 5.; Liu, Q.;Bolin, N. J. Int. J . Miner. Process. 1991, 33, 223.
(27) Nyamekye, G. A.; Laskowski,J. S.Adsorption and Electrokinetic Studies on the Dextrin-Sulphide Mineral Interactions. J. Colloid Interface Sci., in press. (28) Miller, J. D.; Laskowski, J. S.;Chang, S. S. Colloids Surf. 1983, 8, 137.
Conclusions
