Z« Reprinted from ENERGY a FUEI3, It93, 1.Copyright @ 1993 by the AmericaD Chemical Society and reprinted by permiaiOD of the copyright owner

Role of Electrical Double Layer Forces and Hydrophobicityin Coal Flotation in NaCI Solutions

Chin Li and P. Somasundaran'

Langmuir Center for Colloids and Interfaces, Henry Krumb School of Mines, ColumbiaUniversity, New York, New York 10027

Received December 8, 1992. Revised Manuacript Received December 10, 1992

It is well-known that coal floatability can be enhanced by the addition of inorganic salts. Althoughconsiderable effort has been made in the past to investigate the role of these salta, their functionin the coal flotation system is not yet clear. In this study, the floatability of a bituminous coal inNaClsolutions were evaluated using a modified Hallimond tube to delineate the role of the electrt8taticinteraction between bubbles and particles and the coal hydrophobicity. Zeta potential measurementsshowed that bubbles are negatively charged in the entire pH range tested, while coal particles arenegatively charged only above pH 6.0. Also, both sodium and chloride ions were coDflrmed to actas indifferent ions for both coal particles and bubbles. Floatability of coal measured as a functionof pH exhibited a maximum and minimum at low salt concentrations which is shown to be determinedmainly by the coal hydrophobicity as measured by a new film levitation technique. At high saltconcentrations (above 0.1 moll dm3), the floatabllitywithout such maximum or minimum is controlledby the electrostatic interaction between the bubble and the particle.

onstrated that the flotation maximum dos not necessarilyOCCUI' at the isoelectric potential of the coal}.,15 None ofthe above studies, however, bas attempted to isolate theelecaoetatic interaction effect from the hydrophobicityeffect. This is likely due to a lack of information on thecharge of bubbles which is essential in estimating the effectof electrostatic interaction in flotation.

A systematic approach to understand the role of boththe electrostatic interaction between bubbles and particlesand the hydrophobicity of the coal particles by isolatingthe two effects is used in this study, The effect of the saltaddition on the bubble charge and its contribution to theelectrostatic interaction is also discussed.

Experimental Section

Materials. PittabW'gh seam pgh bituminous coal. obtainedfrom Bruceton mines in Alleghany county, P A. was uaed in thisatudy. Large chunks of as-mined coal samples ~re broken intosmall pieces using a hammer and subsequently crushed in aQuaker mill in open atmosphere. The crushed coal was then dryground in a closed mill with ~ramic balls for 5 miD. The groundproduct was sieved into different size fractions which were storedin plastic bags under an argon atmosphere to minimize ambientoxidation. Floatability and hydrophobicity experiments wereconducted usinc the 35 x 80 mesh size fraction. The sampleused for zeta potential determination was prepared by crindingthe 35 x 80 mesh coal sample down to mintD 200 mesh. Prozimateand ultimate analyses of the coal sample are Ihown in Table I.

ACS ~rtified grade sodium chloride (NaCI) and pH modifyingreagents, hydroch1oricacid (HCl) and sodium hydroxide (NaOH) ,were purchased from Fisher ScientifIC Inc. Triply distilled waterwas used in all the experiments.

Flotation Experiment. One gram of 35 X 80 mesh coal samplewas conditioned for 5 min in 100 J of sodium chloride solutionat the desired salt concentration in a l5O-cmJ beaker. The pH

(11) Jaycock. M. J.; Ottewill, R. H. TroM. IMM 1963, 12, .97-506.(12) ChaDder, S.; ~u, D. W. Trona. AlliE 1m. 252. 62-69.(13) WeD, W. W.; S\IIl. S. C. Trona. AIAlE 1m, 8, 174-1-(1~ CeIik. M. S.; &-~~~PaD, P. CoilcMd. Surf.l_l,121-u.o(15) J~, R. R.; Stzettoo, J. L. Fue11t69, 48, 317-320.

C 1993 American Chemical Society

Introduction

Froth flotation is becoming increasingly recognized 88a potential process for the removal of pyritic sulfur andash impurities to produce ultraclean coal. It bas beenreported that the floatability of naturally hydrophobiccoal can he increased aignificantly by the addition ofinorganic electrolytea.l~ Several attempts made in thepast to discern the role of the salts in the flotation processhave led to a considerable degree of controversy on thistopic. Kl~n and MakrouaoVl and Kitchener and hisco-workers5.6 stated that the addition of salt destabilizesthe hydrated layers surrounding the particles and henceiDcre88e8 the floatability of coal Mamm and Nicodemo. 7on the other hand, pro~ the addition of electrolytesto cause an increase in the surface potential of bubbleswhich results in reduced bubble coaleac:ence and in turnto improve the flotation rate. Compression of the doublelayer .by the added electrolyte, which can subsequentlycause thinning and rupture of the wetting r11m betweenbubbles and particles, bas also been propoeed 88 a reasonfor the salt effects on flotation. 8,9 This argument bas beenfurther supported by the results of several other studiesshowing flotation recovery to reach a maximum at min-imum zeta potentia1.10-13 This view, on the other hand,has been criticized by some other ~chers who dem-

(l) ~ v. L; M~, V. A. An I rIt7OduetioII to the Theoryof FIot4tion; TnDIIated by J. Leja aDd G. W. PoliDC; Butterwortb8:LODdOD. 1963; pp 338-342-

(2) ~ J. CollUry G&IG1diGn 1M$, 211, :.1-366.(3) YOGa, R. H. MiniIJI COIIIr. J. 118%, ~, 7~.(4) YOGa, R. H.; Sabey, .1. B. In Interfacial PfIeIIOmeno in Coal

Technology; Botaria, G. D., Glazman, Y. M., Eda.; Marcel Dekker: NewYork, 1-' pp 87-114-

(5) Blake, T. D.; ~, J. A. J. CMm. S«~ FGlUday TraIU. 1Im.~, 1435-1442.

(6) ~ A. D.; Kikhener, .1. A. J, CoUoid Int-rtace Sci. ltG', ~,391--

(7) M8mM:ci. G.; N~. L C1IeJIL EIIB. sa. IM7, 22. 1257-~(8) Brown. D. J. In Froth FfotGtion 60th ~ Volume;

Fuentenau, D. W.. Ed.; AIME: ~ York. 1962; pp 518-538.(9) Lukowski. J.; Iskra. J. TrIIM. IAiM 1977, 79, C6-clo.(10) FU8nt-.u, D. W. TrvIII. AlAiE 1157, Q,I36S-l.-I.

0887-0624/93/2607-0244$04.00/0

Coal Flotation in NaCI Solutio", Energy & he,., Vol. 7, No.2, 1993 245

Table I. ProDIBaae, illtiJDaae. aDd Forms of SulfurAaaly... for the AllerbaDy Coal

>E

jC.'0G-o..N

U1timateAD8ly8il;As~moisture 1.79carbon *».65hydrogen 5.00nitrogen 1.74chloriDe 0.10auJfur 1.12BIb 3.290XYJeD (by diff) 7 ~

ProsimateADaJY8i8:As~volatile matter 36.29medcarboa 58.60calorie value 14CXX) ~/Ib

Sulfur Forma: As Receivedpyritic 0.28sulfate 0.0083OrIaDic 0.33

pH

Figure 1. Zeta potential of bubbles as a function of pH in N.Clsolutions.

40

>E

:2c.'0G-o..N

20

0

-20

-40

-eo

1 4 . 10 12

Ficare %. Zeta potential of coal - a function of pH in NaClsolutions.

of the slurry was then adjusted to the deaired value and theslurry conditioned for five more minutes. The slurry was thentransferred to a mOOified H aIlimolxi tube and the flotation carriedout for 10 min using nitrocen at a flow rate of 10 em'/min. Floatand .ink fractions were filtered, dried. and weighed.

Zeta PoteDtJal Measan8e8t. Coal slurries uaed in the.tapotential measurements were prepared using a procedure 8imi]arto that used in flotation experiments except that a 0.1 wt % coe.lslurry W88 uIed here. After conditioning, approximately 25 em'of the slurry was transferred into the sample cell of a Lazer ZeeMeter for the zeta potential meaaurement.

Zeta potential of bubb188 wa measured in a letup rl~~for monitoring the electrokinetic behavior of gas bubbles; adetailed deecrlption of the experimental letup and procedurehas been prwented earlier. II

Bubble St. DetermiDatiOD. The bottom portion of theH~~ tube W88 uaed fA) generate bubbles which were pumpedinto a specially constructed cell 8imi]ar in design to the samplecell of the Lazer Zee Meter. The procedure for obtainingstatiooary bubbles was aiJnilar to that uaed in zeta potentialmeasurements. The bubble size distribution was determinedusmc an Omnicon Model 3000 image analyzer.

Hydrophobicity EvalaatioD. Hydrophobicity of the coe.lsamples was evaluated usmc a f1lm levitation technique developedspecifically for the present purpoee with a modified 35OoCIn3Buchner funnel provided with a ~ cJaas frit at the bottom(~ II.In in pore size). A detailed description of the experi.mental ~tup and procedure is given eJsewbere.11 A 0.25-1 sampleof 35 x ~ mesh CX)8} wa tint conditioned in a beaker followiDcthe same procedure as in the flotation experiments. Theconditioned slurry W8 then tl'aDaferred into the funoel fromwhich the Uce8 eolution was pumped out through the pores ofthe Class frit usmc a peristaltic pump. The pump W88 reversedas lOOn as air bubbJes were IeeD breaking ~ the g1aas frit.The hydrophobic particles, carried by the air-water interface,were separated from the hydrophilic ones which remained on theg1asa frit. These two fractions were then filtered, dried, and~iIhed. Hydrophobicity index, numerically equivalent to weigbtpercent float in tbeee testa, is presented on a scale of 0-100.

zeta potential (Figure 1). This suggests that Na+ and CI-ions do not specifically adsorb at the gas-liquid interface.

Effect of Salt Concentration on the Charge of Coal.Zeta potential of coal was determined as a function of pHin triply distilled water and in NaCI solutions of threedifferent concentrations (1 X 10-1, 1 X 10-2, and 1 X 1~moVdm3). Zeta potential-pH curves for coal shown inFigure 2 yield an isoelectric point of 6.0 *' 0.1. Also,increase in NaCI concentration caused only a reduction inthe magnitude of the zeta potential with no effect on theisoelectric point. This indicates that neither sodium norchloride ions specifically adsorb on the coal surface.

Effect of Salt Concentration on Coal Flotation.Coal flotation was conducted in triply distilled water aswell as in NaCI electrolyte solutions. It is found thatchanges in solution pH and salt concentration have a verycomplex effect on the flotation behavior of coal (Figures3 and 4). The floatability of coal can be seen to decreasewith increase in salt concentration at low salt concentra-tions and increase at high concentrations.

Figure 3 shows that in triply distilled water and 1~moVdm3 NaCI solution around 75% of the coal can beftoeted at and below 4.5 (i.e., the natUl'al pH of the coalslurry after 10 min conditioning). The floatability of coalthen drops precipitously with increase in solution pH andreaches a minimum of -15 % around pH 6. Above thispH, coal floatability increases and peaks at - pH 9 beforeit drops again. An increase in NaCl concentration to 0.01moV dm3 results in a significant decrease in coel floatability,

Results and Discussion

Effect of Salt Concentration on the Charge ofBubble. According to the earlier paper,16 bubbles in theNaClsolution are negatively charged above -pH 1.5 (theisoelectric point of bubble), and an increase in saltconcentration causes a decrease in the magnitude of the

(16)Li, Co; ~ P. J. Colloids Surf. 1"1,146,215-218.(17) Li, C.; ~ P. ,ubmitted for publication in CoUoida

Surf.

Z46 EMrIY &. hell. Vol. 7. No.2. I., LiandSomoaundarcln

100

i

I~~~-~:~~...~:,~

:"~t

to

f~tI .0 ...

. 1810_1 f'.0 1810_1a ~10

m' . . . . .2 4 . . 10 12

pH

Figure s. EffectofN.C1 ooneentration on coeJ floatability <N.CICOD~tr.tiOn: 0-0.1 mol/dID').

100

~

10"I~0

k:. 40

20

I' 8 10

pH

Ficw'e 5. Effect of N.CI concentration on the hydrophobicityof coal as determined by the film levitation technique.

the re&poD8e8 exhibiting a minimum value at -pH 6. Thehydrophobicity index of coal is -95 below pH 4.5 andreaches a minimum of 86 at around pH 6 while thefloatability of coal exhibits a much sharper decrease (from75 to 15 wt %). Above this pH. ooel hydrophobicitygradually increases with increasing pH and reaches 94 atpH 9.5, whereas the flotation recovery increases from 15to 60%. It is thus clear that at low salt con~trationacoal flotation is controlled mainly by the hydrophobicityof coal. It is aJso clear that the hydrophobicity of ooeldecreaSM .ignifi~nt1y 88 the 8altco~tzation is increasedfrom 1 X 10""6 to 0.1 molldm3, except in the pH range of&-9 where the hydrophobicity of ooel remains almost thesame. This decrease in coal hydrophobicity is very likelyresponsible for the observed sharp decrease in the coalfloatability. This result again sugg_ts that at low saltconcentrations the hydrophobicity of the coal is thecontrolling factor for the flotation pr0CM8.

Further increase in salt concentration to 0.5 moll dm3does not have any significant effect on the coal hydro-phobicity while the flotation recovery is noticed to increaseat least by a factor of 2. This means that above 0.1 mo1ldm3 NaCI the observed upward shift of the % Rec-pHcurves cannot be attn"buted to any increue in the coalhydrophobicity. Also, it is seen that no simple correlationcan be found between hydrophobicity and the flotationbehavior in 0.1 and 0.5 molldm3 NaCl solutions. Thehydrophobicity index remains constant at a value of 88below pH 6. Above this pH, the hydrophobicity indexstarts increasing and reaches a value of 93 at -pH 9.5before it decreases again, whereas the floatability testsshow a continuous decrease in flotation with increasingsolution pH. These results suggest that above 0.1 molldm3 NaCI the coal hydrophobicity is no longer controllingthe flotation procesa.

Bole of Electrostatic Interaction on Coal Flotation.As mentioned earlier, ooel particles are positively chargedbelow pH 6 and negatively charged above this pH whilebubbles are negatively charged in the complete pH rangeof 2-12. It is then clear that below pH 6 an attractiveforce will exist between bubbles and particles since theyare oppositely charged. This attractive force decreaseswith increase in pH and vanishes at pH 6 where coalparticles are neutral. Above pH 6, the force betweenbubbles and coal particles becomes repuJsive and increasesin magnitude with increase in solution pH. The effect ofelectrostatic interaction can be demonstrated more clearlyin tenDS of an -attraction index.. Acx:ording to Healy and

. pHFicure 4. EffectofNaCl OODceDtratiOD OD CXI8l n~tability (NaClooD~DtratiOD: 0.1-0.5 mol;dml).

although the % Rec-pH curve is sti11somewbat IimiJar inshape. The floatability of coal reaches a minimum at&round pH 6, increuea and peaks at pH 7 (instead of pH9), and then decrealea. Further increaae in salt concen-U'ation to 0.1 molldm3 causes not only an additionaldecrease in the coal floatability but also the disappearanceof the similarity in the % Rec-pH curve.

Above 0.1 molldms NaCl, any inClM&e in salt concen-U'ation caU8e& an increase of the coal floatability and thusan upward shift of the entire % Rec-pH curve (Figure 4).This observation is in agreement with the earlier fiDdiDgsof 8everal other re8earcherB that the floatability of naturallyhydrophobic materials increuea significantly upon theaddition of inorganic electrolytea.l-4 Also, it is Been that,at a salt concentration of 0.1 moll dm3 and above, the coalfioatability is highest at the low pH end and decreasessteadily with increase in pH.

Role of Hydrophobicity on Coal Flotation. Hydro-phobicity of fine particles is generally evaluated in termsof the induction time for the a~chment of a bubble tothe particle. However, bubble charge can also playa crucialrole in this type of measurement and give a distortedpicture of the effect of hydrophobicity. The experimentsCOM ucted in this study, as discU88ed earlier, were thereforedesigned to eliminate the electrostatic effects.

Coal hydropoobicity in triply distilled waw and in NaCIsolutions of various salt concentrations is plotted in Figure5 as a function of pH. It is Been that in triply distilledwawand l0-6molldms NaClaolution the hydrophobicityand the notation recovery show IimiJar trends, with both

Cool Flotation in NaCl Solutionl Energy &: hell. VoL 7. No.2. 1993 247

Table II. F.tiaated Values of Zeta PoteDtial (iD mV) for BubblM aDd Particle8 iD Various N.Cl Solution.

pH(NaClJ. DM)i/dm3 I 4 5 6 " 8 . 10

-140

-170

0

-11

-21

-24-6

-19-8

-a-10-27-u

-21-15

-.-18~-22

100NaCI c.-. W

-$ -I. 1810_2 . 281O_ID 1w10_1 ~ JalO_1. 1.10 ~ 5010

/.

3,80 r

r-~"V 10."00

i::

M 40 '" , " ,~ .~

0.5 M bubble -e -8 -12pertide I 5 3

0.3 M bubble -7 -10 -14particle 11 6 4

0.2 M bubble -8 -12 -17particle 13 7 4

co-workers,l8.l9 the double layer interaction between twospherical particles is a function of the product of theirStem potentials .1 and~. If the bubbles are consideredto be spherical particles, and the zeta potential ..umedto be the same 88 the Stern potential, the double layerinteraction between the bubbles and the coal particleswill be a function of the product of their zeta potentials,l"1l"2. Therefore. the attraction index is defined here 88the product of the zeta potential of the bubble and thatof the particle with a negative sign. Negative term isintroduced to show attraction as a ~ve value andrepul.ion as negative.

Due to the limitations of the Lazer Zee Meter. zetapotential values for bubbles and coal particles cannot beobtained experimentally in NaClaolutions &bow 0.1 molldm3 and have to be estimated. For a charged surfa~ ofpotential fA), the potential tp at a distance % from the surfaceis

0-6000J -2000 0 2000

Attroction Index

Ficw'e 6. Flotation reoovwy as a fUI¥:tion of the attraction iDda.

-400c

tp = /Po exp(-a) (I)

where K is the reciprocal of the electrical double layerthickness and can be expre88ed 88 follows

- [~~ ]1/2

K fiT (2)

where e is the elementary electric charge, Rj ia the numberof ions per unit volume of type i. Zi is the valency of ioni, E is the permittivity, k is the BolhYnAnn constant, andT is the absolute temperature.

If .x in eq 1 is the distance of the shear plane from theparticle surface, tp can be replaced by t. and eq 1 thenbecomes

t ~ to exp(-a) (3)

Since for the unknown zeta potential, tl - so exp(-«lz)and for the known experimental value t2 = so exp(-«2X),thus

attraction index until a minimum is reached at 0 attractionindex. The floatability starts increasing once the attractionindex becomes negative. This indicates that the electro-static interaction is not control1ing the flotation processat least when both the bubbles and the part.icles arenegatively charged. This result further supports thesuggestion made in the previous section that at low saltconcentration the flotation process is controlled mainlyby the coal hydrophobicity. It is to be noted that all theflotation recovery data at 0.1 mol/dm3 NaCleolution andthose with positive attractive indices at 1 Q-6 and 10-2 mol/dm3 solutions can be represented by a single curve, whereasabove 0.1 mol/dm3, the results obtained at differentconcentrations are widely separated from each other. Ineach case, the floatability of coal decreases steadily withdecreasing attraction index suggesting that the coe.lflotation under these conditions is controlled mainly bythe electrostatic interaction between the bubbles and theparticles. Thus, at high salt concentrations (0.1 mol/dm3and higher) the el~tatic interaction between bubblesand particles becomes the major factor control1ing theflotation pr0ce88.

It can be concluded at this point that, below 0.1 molldm3 NaCl, the flotation is mainly controlled by thehydrophobicity of the coal, whereas above this saltconcentration it is controlled by the electrostatic inter-action. This is poesibly due to the long-range nature ofthe electrical double layer forces at low salt concentrationsin comparison to the short- range hydrophobic interactions.Kinetic energies of the bubbles and the part.icles aresufficient to overcome the energy barrier resulting fromthe electrostatic interaction enabling them to approacheach other within a distance at which the hydrophobicinteraction can take place and cause the three-phasecontact. As the salt concentration is increased, theeffective dmtance for the double layer interaction alsodecreases due to the compression of the double layers.The electrostatic interaction becomes significant andcontrolling once the effective distance of the electrostatic

rl = r2 exp[(~ - «J%] (4)

For the present calculations, r2 was assumed to be thatobtained in the 0.1 moVdm3 solution and % to be 3. 72 A~for both the bubblM and ~ particles (aaaumiDg thelocation of the shear plane is the same 88 the Stern plane),K.I were estimated to be 1.47 x 10', 1.80 x 10', and 2.32 x10' m-1 for 0.2, 0.3, and 0.5 mol/dm3 NaCI solutions,respectively, and K.2 1.04 X 1oe m-l. The results obtainedare tabulated in Table n.

In Figure 6, the floatability of coal is plotted 88 a functionof the attraction index. -rItz, for various NaClsolutions.In 10""6 and 10-2 mol/dm3 NaClaolutions, it is found thatthe floatability of coal first decreases with decreasing

(18) Hog. R.; Healy, T. w.; FuerIt-.u. D. W. TruIU. FaIadGy Soc.I"', 62, 1638-1651.

(19) W_,G. R.;H88ly, T. W. Trana.FGlUdCJySoc.It70,66,4~99.(00) DevaDathm, M. A. V.; Tilak, B. V. K. S. R. A. Chem. Rev. 1165,

65, 635-e84.

248 Energy &; hell, Vol. 7, No.2, 1993 Li and Somasuradaran

Table III. MeaD Babble Di.--r iD NaCI Solatioas atDifferent pH- -

KharJamOVl has shown that DO significant change in bubblesize could be observed for NaCI solutions below 0.25 N.

mean bubble diameter (18m)

(NaCl].mol/dm3

pH63 4 10 11

Conclusions

ADalysi8 of the effect of salt addition on the zeta potentialof bubbles and ooal particles showed the sodium chlorideto behave as an indifferent electrolyte in both cases. Theisoelectric point of coal was pH 6.0 and that of the bubble,obtained by extrapolation, was pH 1.5. The floatabilitybehavior of ooel, obtained using a mOOified Hallimondtube, was found to be complex with respect to variationsin both the pH and the salt con~tration. At low saltconcentrations (below 0.1 mol/dm3) flotation of coaldecreases with increaae in salt exhibiting a minimum ataround pH 6 and a m.Yimum between pH 7 and 9. Above0.1 moJ/dm3, however, flotation exhibits a marked increasein flotation with salt concentration without any suchmaximum or minimum. Hydrophobicity of ooal particlesis measured by a new hydrophobic levitation techniqueand it is shown that the floatability of coal dependspredominantly on the hydrophobicity of coal at low saltconcentrations. Above 0.1 mol/dm3, on the other hand,the flotation is governed mainly by electrostatic interac-tions represented here as -attraction indexw.

~-~

1&-1 - 110 109 110 1101&-2 - 102 106 103 -1&-5 102 110 104 107 109

interaction between bubbles and particles is comparableto that of the hydrophobic interaction.

Above 0.1 moi/dm3, any increase in salt concentrationwill result in a significant increase in coal floatability whichis not explained by the changes in either the hydrophobicityor the electrostatic interaction. The reason for this is notyet clear; one ~ble explanation is that an increase insalt concentration can destabilize the hydration layeraround coal particles and thus facilitate the drainage ofthe water film between the bubble and the particle In. Icing

the coal particles more floatable. It may be sugestedthat the observed changes in the coal flotation behaviorbe attributed to the variations in the bubble size. Resultsof the bubble size measurement given in Table ill clearlyshows that there is no significant change in bubble size atleast below 0.1 moi/dm3 NaCI in a pH range of 3-11 whilethe floatability of coal does change markedly under theseconditions. Lukowski and Iskra' have also suggested thatthe frothing effect in the salt flotation of naturallyhydrophobic materials can be neglected. Moreover,

Acmowledcment. The authors acknowledge the fl-nancia1 support of NSF(INT-87-O(303) and the New YorkMining and Mineral Resources Research lD8titute.