ii! - columbia universityps24/pdfs/solution chemistry of surfactants and...solmon chemistry of...

1,7

lil j lII! (SOLmON CHEMISTRY OF SUR.FACTANTS AND THE ROLE OF IT IN ADSORPTION

AND FROTH n.OTAnON IN MINERAL-WATER SYSTn6

P. Somasundaran and K.P. Anantbapad=anabhan

Henry Krumb School of MinesColumbia UniversityNew York, NY 10027

IIAdsorption of surfactants on minerals and froth

flotation can be expected to be determined in additionto' the aqueous chemistry of minerals and the interac-tions between the surfactant and the dissolved inorganicspecies also by the solution chemistry of surfactants.The role of solution chemistry becomes ~ortant par-ticularly due to the dependence of the type and con-centration of surfactant species on solution conditionssuch as pH. In particular, hydrolyzable surfactants$c~h as fatty aciGs and ~nes ca~ undergo variousassociative interactions to form ionomolecular complexessuch as acid-soap in the appropriate pH range. Equi-librium species distribution diagrams that have beencomputed recently from literature data and estimates ofenergy of interactions between molecules for two cypicalsurfactants, namely, oleic acid and amine, are reviewedin this paper, and the role of various cC':lPlexes indetermining interfacial processes such as flotation andsurface tension lowering is considered. Formation ofcertain ioDODOlecular complexes is found to coincidewith observed conditions of maximum surface activity asmeasured by surface tension, surface tension decay rate

and flotation.

777

I

,

b

P. SOMASUNDARAN AND K. P.ANANTHAPADMANABHAN778

1~'"!B.ODUCTION

Adsorption of surfactants on the mineral surface from an aque-ous _dium and the resultant hydrophobicity of it are strongly in-fluenced by the surface chemical and electrokinetic properties ofthe solid as well as by the solution chemistry of surfactants. Theflotation of mineral particles caused by the attachment of air bUb-bles to particles and their sUbsequent levitation, on the otherband, is determined by the degree of bycrophobiciry of the tWo in-terfacial regions (solid/liquid and liquid/gas), which in turn areaffected by, among other things, the adsorption densit:y of the sur-factant as well as the orientation of the adsorbed surfactant DM>le-cules. The overall process of flotation is governed by a large num-ber of such factors, but ~st importantly. it can be expected to beinfluenced by the solution cbe:istry of ~he surfactants. Neverthe-less, the role of surfactant solution chemistry in flotation has re-ceived only limited at~ention in the past. It is the aim of thispaper to discuss the nature of che:ical equilibria of certain commonsurfactants in solution and ev~uate their influence on flotation.Particularly the type of cQC?lexes ~hat can result from the associa-tion of various species under different conditions and their poten-tial role in flotation will be considered here. It is however real-ized that a complete quantitative treatt:ent incorporating all thepossible interactions &mOng surfactant and mineral species willhave to await the availabiliry of relevant thermodynamic da~a for

such interactions.

CHEMICAL EQUILIBnA IN SURFACTANT SOLUTIONS

The behavior of surfactant species in solution is determinedessentially by the properties of their polar heads and hydrophobictails and the resultant tendency of the solvent to accommodate themunder various conditions. For e%amPle, a weakly acidic surfactantIH will exist as ions (R-) at high pH values, neutral molecules atlow pH values and may fo~ an ionomolecular complex. (RKR-) in theintermediate pH range.l-ll As the surfactant concentration isincreased, micellisation or precipitation will occur in solution,depending upon the Kraft point of the system. In addition to this,surfactant species can also unlergo associative interactions to formother aglre!ates such as the R22- dimer even in sUb-micellar solu-tions,5- ,I -18 depending upon the nature of hydrophilic and hy-drophobic parts of the surfactant species. As the surface activi-ties of aggregate species as R~2- or R2S- will be different fromthose of the monomers, R- or BE, one can expect flotation of miner-als using them to be dependent upon the solution conditions, suchas pH and surfactant concentration.

In this paper, the above centioned associative interactionsbetween surfactant species ~re discussecand the possible role of

~779FROTH FLOTATION IN MINERAL-WATER SYSTEMS

resultant complexes in flotation is examined in detail. Also therole of Gdcellisation, hemi-micellisation--a process analogous toaicellisation at the solid/liquid interface--and precipitation arediscussed with the help of examples from the literature wheneverpossible.

Associative Interactions in Surfactant Solutiona

Surfactants have been in general considered in the past to ex-ist as monomers up to a particular concentration and above that toaggregate to form micelles or to precipitate. A nUDber of proper-ties of surfactant solutions such as conductiVity,12 colar vol~,l~osmotic coefficient,17,18 hydrolysis,2,~ etc. have, however, beenreported to exhibit deviations from their expected ideal behaviorbelow the critical micelle concentration (CHC). For example, spe-cific conductivity of certain surfactant solutions below CHC hasbeen found to undergo positive deviations from the behavior expect-ed on the basis of the Debye-Huckel-Onsager relationship. Such de-viation could be accounted for by considering the forDation of di-mer. and other multimers which will have a higher electrophoreticmobility owing to their lower hYdrod~udc resistance as comparedto an -equivalent number of monomers. 2 There have been attempts to

account for the deviations in other properties also to such aglre-gation in aqueous phase and to estimate the theraod~c constantsfor association complexes from the data obtained for such propertiesas a function of concentration.S-8,l~,lS However, SoDe of the abovedeviations can also arise from possible artifacts that can existunder the expert.ental condi tions used.

Associati~o of surfactant. is maio1y due to the '~ydropbobicboodiO'" (i.e. cha1o-cha1o interaction) betw~o their Don-polarparts. .13 It is vide1y accepted that the hydrocarbon molecules eo-hance the vater structure around it.19.20 Under such conditiODS.association of monomer. can be expected to decrease the number ofvater molecules in such an ordered state.21.22 The resultant increaseio entropy vould favor the formation of .ultimers. Following thisconcept. NeDethy and Scheraga21 had formulated a statistical tber-modynaudc procedure for the estimation of hydrophobic free energyinvolved io the association of proteins. In subsequent vorks.Scheraga and co-vorkers extended their investigation to the studyof the contributions of hydrophobic bonding energy to the dimeri-zation of fatty acids.23 In another approach, Bangs' assumed thatthe changes in interfacial energy accompanying the changes 10 io-terfacia1 area due to dimerization to be entirely due to the bond-ins betWeen the assocIating specIes. The change in interfacialarea during dimerization vas estimated using molecular models andthe correspond1og change in interfacial energy vas obtained byassum1og the surface energy of a molecule to be the same as that atoil/vater interface.' It might ~e 1oteresting to note that the

)

P. SOMASUNDARAN AND K. P. ANANTHAPADMANABHAN780

estimates of hydrophobic energy for d1aer1~tion by Scheraga andco-workers23 and Bangs7 were in good agreement with each other.

In addition to hydrophobic interactioDS, that of the polar orionic heads with various solvent and solute 8pecies will a180 in-fluence the for88tion of complexes. For example, the formation ofa doubly charged di.er (R22-) will be bindered by charge/charge re-pulsion. 12 Foraation of an acid-soap dimer (R2H-) berween an ion

and a 8Olecule, on the other band, is acre probable, because of theabsence of charge/charge repulsion and the p08sibility of polar/ionic head interaction through hydrogen bonding, if the ionic andpolar head. are placed at the same end of the dimer.6,7,lO,ll

It must be noted here that the formation of higher aultimerssuch as tri8ers or tetramers also can cause deviations in solutionproperties. It is, however. difficult i6 distinguish the effectso~ higher ~t~ers from dimers at this stage. On the other hand,higher multimers cannot be expected to exist in significant amounts,because of the accentuated repulsion betWeen the ionic heads whichwould now" have to be placed closer to one another compared to thatfor the case of a doUbly charged di8er. It is therefore assumed forfurther analysis that higher multimers such .s trimers or tetramersare not present 10 the system.

Based upon the above considerations. the associative interac-tions that are likely to occur in rwo major surfactant systems. i.e.potassium oleate and dodecylamine solutions are discussed below.

Oleate solutions. Formation of a 1: 1 complex betWeen potassiumolea~e and ~leic aciJ has been considered as eaTlv &5 ~n 1927.2~Subsequent studies by ~ain,l Ekwall,l Mukerjee,6 and others25-27have provided evidence for such associations in oleic acid systems.On the basis of data available at present for this system the fol-lowing chemical equilibria c~ be WTitten for oleate solutions.

RCO<II(1) ~ aCOOB(aq) (1)

,+(2)RCOOH -::: RCOO- + B

2I.COO- ::: (I.COO) 22- (3)

acoo- + I.COOH :: (B.COO) 2B-

+Na + (RCOO) 2U- :: (RCOO) 2UNa

(4)

pKSAS --9.358 (5)

The value of pI of oleic acid shown above is that obtained byJung8 by extTapolati!n of the pi values of ShOTt chain fatty acids.

a

Analysis of the literature values showed that the dimerizationcons~t (K ) for oleate ions varied over 3 to 4 orders of magnitude.

D

781FROTH FLOTATION IN MINERAL-WATER SYSTEMS

The value chosen here is again that obtained by Jung8 ass\Cing alinear relationship between the logarithm of solubility aDd the hy-arocarbon chain length.

It is poss~ble to est~~te the equilibrium constant for theionomolecular complex (acid-soap). since the extra stability of t~eacid-soap over the dimer will be essentially due to the absence ofcharge/charge repulsion and possible hydrogen bonding between theionic and polar heads. as shown below.

6Go - - 6Go + AGo + 6~o- (6)

R22 C-C Elec. -x:E

(-) (+) (+)

0- 6GC-C

(-)

+ toC: + toC:E

(-) (+)

(7)

- lI-G.-~ - lI-CO-B Elec.

(-) (+)

lI-C~ B-2- Exp (- )

RT

(8)

KAD (9)

where ~G~-O ~G~lec., ~~E and ~G§ represent the free energy changedue to the chain-chain interaction, charge/charge repulsion, kineticenergy changes and the hydrogen bonding respectively. By esti=atingthe ionic repulsion energy and the hydrogen bonding energy, one canobtain K~, provided the formatioD1constant for R22- is known. Muk-erj ~e6 est~ted on this basis the value of KAD to be thirty timesthat of KD' by assuming a 4Ao separatioc betWeen ~he ionic beads ina dimer and 1000 cal/mole for the hydrogen bonding. The value of~ shown ~~ equation (4) was obtained in our analysis b~ assUQinga 13.8 Ao separation betWeen ionic heads. The latter value of dis-tance of separation is obtained- from molecularly scaled models.'

It m~~ be pointed out here that the evaluation of charge!Charge repulsion involves the use of a value for the dielectric con-stant ~! the medium surrounding the charges which is expected to belower than that of pure water. However. in our treatmentll as wellas the one earlier by Mukerjee.6 a dielectric constant of 78 forwater has been used.

Using equations (1-5). and the mass balance equation:

(10)CT - CRCOOH + CRCOO- + 2C(RCOO)2Z- + 2C(RCOO)ZR-

where CT is the total surfactant concentration, it is possible todetermine the activities of the individual species and their distri-

~-\

782 P. SOMASUNOARAN AND K. P. ANANTHAPADMANABHAN

-3\

K-OLEATE (RCOOt<) 3. 'O'" M-4

RCOO-

-:I

-If)1&1U -..1&1A.If)

1&1

%-7.-

...0

>-.- -85:

~U4- -90

0

..J

-10

(RCOO)

RCOOH

-11J H-

Z

.bu~ion, as a func~ion of solu~ion condi~ions such as pH below theprecipi~ation li8it of ~he acid-soap salt. A typical oleate speice84istribu~ion diagram a~ a total oleate concentra~ion of 3 x IO-~i. shown in Figure 1 as a fUDC~ion of pH. It i. to be noted frO;this diagram that (I) a maximum in the activity of acid-soap(RCOOH.RCOO-) occurs at pH 7.8, (b) precipitation of neutral oleicacid (RCOOH) occurs at pH 7.8, (c) ac~ivities of the ionic 8Onomerand the ionic dimer increase up to pH 7.8 and remain practicallyconstant above it.

The significance of these observations to flotation 1. dis-cussed in a later section.

Dodecvlamine 80lutions. Hydrolyzable cationic surfactantssuch as dodecylamine can also underso associative interactions si-milar to that discu8Sed for the oleate s~stem. The possible chemi-~al eauilibria in this system ace:

FROTH FLOTATION IN MINERAL-WATER SYSTEMS 783

(11)RNB~ ~ RNH2 + B+

2RNB~ ::: (RNB3) 22+

pK' - 1063a .

PKi» - -2.08 (12)

pKlD - -3.12 (13)+ .. ( ) +

llNB 3 + 1lNB 2... B.NH2 . B.NH 3

(15)CT. CI.NB2 + CRNH1 + 2C(RNH3} 22+ + 2C(RNH2.RNH1>

Species distribution diagrams have been determined as a func-tion of pH for two different concentrations of dodecylamine hydro-chloride (see Figures 2 and 3). The total concentrations of aminehave been chosen in such a way that one (5 x lO-5:!1> is above the

0

-1

DODECYlAMINE HYDROCHLORIDE (RNHz.HCI) 5 a IO-IM

-2

-3

-4

-51

-6

- ,

-",~uWA..,WZI-

...0

>-I->I-U~

C0-J

RNH+~RHHZ

++(RNH)Z

RNH+)

.'RNH)-8

+RNHz"RNH)

139 10 11 124 5 6 1 8pH

Figure 2. Dodecylamine species distribution diagra~ as a functionof pH. Total amine ~ 5 x 10-511.

RNH2(1) : RNB2(aq) pK' 1 - 4.69 (14)so

The dimerization constant for dodecylamine hydroch1oride usedhere is assumed to be the same as the one obtained by Mukerjee fordodecylamine thiosulfate.l? The KkD constant for the amine-aminiumcomplex formation was estimated in the same manner as that describedfor the oleate system. The mass balance equation for this systemcan be written as:


DODECYLAMINE HYDROCHLORIDE (RNHzoHCI) 10~M

-31

-4

-5

-6

I-71 I

!i

-U)1&1U1&1A-

U)

1&1

X

...

""

0

>-I-

~...

~

00

-J

...RNH~ RNH2

, +RtiH,(RNH ) ++

32-8

-9

+RNH,)

-10 12 135 7 8 9 10 it4 .pH

Figure 3. Dodecylamine s~ecies distribution diagram as a functionof pH. Total amine - 10- ~ (below solubility limit).

precipitation limit of the neutral dodecylamine and the other (10-~is below it. It is to be noted that the pH of precipitation oftencoincides with the pH of maximum complex formation and this makesit difficult to distinguish between the roles of these two phenomena.By conducting tests at the above two concentrations, the effects ofprecipitation can be distinguished from those of complex formation.A comparison of Figures 2 and 3 reveals that, apart from precipita-tion, the pH of maximum amine-aminium complex has shifted fro:higher pH (10.9) to lower pH (10.4) values with increase in the totaldodecylamine concentration. The implications of these observationsare discussed in a later section.

SURFACE ACTIVITIES OF OLEATE AND DODECYLAMINE SOLUTIONS

Based on their charge and molecular size. the surface activi-ties of various oleate and dodecylamine species in solution can beexpected to be different. For example. ionomolecular complexes.RCOOH.RCOO- and RNH2.RNH1 will have a higher surface activity com-pared to their corresponding ionic monomers. Similarly. charge con-


siderations viII again make ionomolecular complexes Dare surfaceactive than the corresponding dimers. On the other hand, a directcomparison of the surface activities of dimer and the ionic monomeris difficult because of the opposing effects of their molecularsize and charge. At low surface coverages, it is possible that thedimers can adsorb at the air/liquid interface with an inverted 'Utshaped configuration, thereby allowing the ionic heads to be insolution and the hydrocarbon chains in air, provided che chain issufficienCly long Co bend around. At high surface coverages, on theother hand, it is reasonable to expect the adsorption of dimer wichits ionic heads at che opposite ends to be less probable than theadsorption of individual ionic monomers. On che contrary, adsorp-cion of such dimers at the solid/liquid interface can be feasible,as ic would orient hydrophilic ends towards che mineral as well as

the bulk. solution.

Surface properties of oleate" and dodecylamine solutions ob-tained from surface tension results are shown in Figures 4 and 59.28as a function of pH. Comparison of Figure 4 with Figure 1 and Fig-

Surface tension of potassium oleate solution as a func-Total oleate - 3 x 10-5~.

Figure 4.t.ion of pH.

1~---!

786

,HF~gure 5. Surface tension of dodecylamine solution as a functionofpB. Total amine c 4 x lO-4H.

ure 5 vith Figure 2, clearly shows that the minimum in surface ten-sion in both cases occurs in the pH region vhere the maximum in ion-omolecular complex activity and the precipitation edge of the neu-tral mole~uie also occur in solution. These observations are, thus,in accordance vith the expected surface activities of different spe-cies considered above.

In the light of the above discussion, the flotation dependenceof minerals using oleic acid and dodecylamine is discussed below.

FLOTAnON USING OLEATE

Typical results obtained for the dependence of flotation ofhematite using oleate is shown in Figure 6.9 The sharp maximum inflotation at pH 8 is seen to correspond to that where the surfacetension for the same total concentration of oleate is minimum. Also,pH of maximum flotation coincides with the pH of maximu= acid-soapforcation (see Figure 7). These observati~s indicate a possiblecorrelation betWeen flotation and the activities of these speciesin solution. In addition, changes in concentration of other species,and in properties of the mineral such as the interfacial potential,can also cause accountable differences in.the flotation behavior.

..,.


4 5 6 7 . ,FLOTATION pH

figure 8. Bematite flotation as a functio~ of DB.7.5 x 10-6 M(I). 1.5 x 10-~(8) and 3 x 10-SM(0).

Total oleate -

I t is to be noted that the above flotation 8echanisms based onthe solution chemistry of oleate can also explain the dependence ofpH of aaxi8um flotation of hematite on the concentration of totaloleate species in solution (see Figure 8). Tbe pH of maximum acid-soap formation and the onset of oleic acid precipitation has beencalculated at various oleate concentrations and is given in Fiaure 9.It can be clearly seen that this parameter also follows the sametrend with oleate concentrations as the pH of caximum flotationgiving further support to the proposed .ech.ni~ based on oleatesolution chea1stry. Tbe strong dependence of flotation of gineralson the oleate chemistry can also be seen from the results of Polkinand Najfonow29 (see Figure 10) where the max1m\:::lln flotation for anumber of minerals of significantly different surface chemical andelectrochemical properties occurs in the pH ra~ge of 7 to 8. Again.the possible role of acid-soap which forms in ~ximum acoubts inthis region needs to be noted.

790 P. SOMASUNDARAN AND K.. P. ANANTHAPAOMANABHAN

-~u-.0.J

-5.5

i.o ~5 8.0

~i;\;re 9.

formation.

. .pH. -pH . pH OF MAXIMUM RZH FORMATION

Total olp~te concentratinn vs. p9 of maximum acid-soap

FLOTATION USING DOD£CYL\.'iI~

Flotation using amines usually exhibits a caximum around pH10-11. Typical results obtained for the flotation of quartz using10-5 and 5 x 10-5 mole/l dodecylamine solutions using a Hallimondcell are liven in Figures 11 and 12. The?H of maximum flotationat 5 x 10 5 mole/l is found to correspond ~o the pH of formation of

maximum amounts of aminium dimer and amine-ar.1nium complex whichalso coincides with the pH of onset of amine precipitation. At10-5 mole/I, however, there is no precipitation of dodecylamiDe andin this case the pH of maximum flotation corresponds simply to thatof the pH of the ioDomolecular complex for--ation. In this CLSe, itis thus possible to distinguish be~'een the role of the collectorcomplexes and that of the neutral molecule precipitation. Theseobservations fully support the proposed mec~ani~ based on the forca-tion of complexes between collectOr species.

"'- -FROTH FLOTATION IN MINERAL-WATER SYSTEMS 791

Fig~r~ 10. Flotation recovery of minerals as a function of pRoCollector oleic acid (1000 gms/ton). (1) Columbite (2) Zircon(3) Tantalite (4) Ilmenite (5) Rutile (6) Garnet (7) Tourmaline(8) Albite (9) Perovakite (from Reference 29).

ROLE OF DIMERS IN nounoN

The discussion above indicates that the role of associatiC3c~plexes in governing the flotation properties of minerals usinghydroly&able surfactants such as oleic acid and dodecylamine can beim?°rtant. In the case of strongly ionizable surfactants such assulfonates and sulfates. appreciable a80unts of ionO8Olecular com-plexes can be present only at very low pH values «2). On the otherhand. depending upon the hydrocarbon chain interaction and the ionicrepulsion involved in the formation of multtmers. dimers and othermultimers can be present under 8Ost pH conditions.

As mentioned earlier, adsorption of dimers at the 8olid/l~quidinterface can also lead to an energetically compatible situation.

792 P. SOMASUNDARAN AND K. P. ANANTHAPADMANABHAN

,HFisure 11. HallimDDd cell flotation of quartz as a function of pH.Dodecylaainebydrocbloride - 5 x 10-SM.

From the point of view of flotation, adsorption of doublycharaed di8ers with the polar heads at the opposite ends at the8olid/liquid interface may not be suitable, as this would render a"hydrophilic surface" to solution and thus would affect the flota-~ion deleteriously. In fact, such effects could possible contribute~o the decrease in mineral flo~ation often observed at higher sur-factant concentrations. In the past, this was attributed to-a.num-ber of factors such as the adsorption of a second layer of surfac-~ant with a reverse orientation, decrease in bUbble size and the sat-uration of the bUbble/solution and the solid/liquid interfaces with

8urfactants, creating incompatibility with the hydrophobic surfaceof the aiDeral.

Fieure 12. Hallimond cell flotation of quartz as a function of pH.

Dodecvlaminehydrochloride - la-SM.

I


HEMI-~CELLlZAT10N AND MICELLlZATION

Bemi-micellization is a phenomenon that ~s analogous to the ~-celIe formation, occurring at ~he solid/liquid interface as a resultof the lateral interactions between the adsorbed ~lecules. A sche-matiC diagram of hemi-micelle formation is give~ in Figure 13. Freeenergy involved in the removal of hydrocarbon chains from an aqueousenvironment upon hemi-micelle formation can be expected to contributetowards increased adsorption of the surfactant at the solid/liquidinterface. Such interactions at the solid/liquid interface lead tomarked changes in interfacial parameters such as adsorption density,zeta potential, flotation, etc. A typical exa~le is given in Fig-ure 14 wh,re the results for such interfacial parameters obtainedfor dodecylsulfonate/alumina systems are given as a function of sul-fonate concentration. 3D Flotation data for quartz using alkyl am=c-nium acetate (see Figure 15) also provide additional support forsuch behavior. 31 In fact, this flotation dependence can be made useof conveniently to o~tain the free energy involved in hemi-micelli-

zation.

In a system such as quartz/alkylamine where only electrostaticand lateral interaction forces are operative, the Stern-Graham

equation can be written as:32

(-W' + n+')yo _?-r 'P !" e 1 (16)'i - ~ "At'.~ kT J

Where r i is the adsorption density, y~ is the electrostatic inter-action tietWeen the cineral and the surfactant molecule, ~' is theenergy involved in the removal of a single 'CE2' group from aqueoussolution to the ~eci-micelle 2nd tn' is thenu5ber of 'CH2' groupsin the surfactant molecule, k is the Boltzmann constant and T is

A~SOLUTION

SOLUTION<:>

<:>

~0

E>

e

Figure 13.


Figure 14. Adsorpt;ion density of dodecylsulfonate, electrophoreticsobility and settling rate of alucina-sodiuo dodecylsulfonate sys-tem as a function of the coucentr2.tionof sodium dodecylsulfonate.(B.eproduced by pe~ssion of t;be t"uive:-sity of California.)


the absolute temperature.

At the concentration ~, When hemi-micelles begin to form,

, (V' - n~')~ r EXP: e ]. (17)-

2r kTor

'W'

-!.._£.t:kT kT

(18)r'lnC-_- - - 10(-)+

-HK 2r

Thus,a plot of log CBM va. n should be a straight line with aslope (~) which can be used to evaluate.'. Fuerstenau et a132obtaine~ a concentration parameter by extrapolating the linearrising portion of flotation recovery vs. log concentration curveand assuming it to be proportional to~. This parameter vas thenplotted as a function of the number of carbon atoms and 4>' the ener-gy involved in transferring -CH2- groups from aqueo~s phase to hemi-micelles vas computed (see Figure 16). The va1u~ of 4>' so obtainedis 1 kT per -CB2- group ~'hich is quite comparable to the energygain due to mdce~isation in bulk solution (see Figure 17).

Figure 16. The effect of chain length on the concentration para-meter of quartz ~-ith alkylammonium acetates31.


Micelle formation also has been known to influence the adsorp-tion of surfactants at the solid/liquid interface. Micelles them-selves are believed to be non-surface active. However, exclusionof micelles from the interfacial region either due to the solvationcharacteristics of the solid and the surfactant34,35 or due to thecharge/charge repClsion of the micelle from the solid surface36 canconceivably yield manm& in ads6rption isotherms. Adsorption frommicellar solutions has been discussed elsewnere.34,35

PB.ECIPITA'IION

Interaction of surfactantswith the counter ions in the system,leading to precipitation is not unca=mon in flotation systems. In

fact, on the basis of an extensive study of the solubility productsof a number of cationic soaps of coucon fatty acids, du Rietz37,38has suggested the point of precipitation to be a requirement for in-cipient flotation. Fuerstenau and Miller,39 using a homologous ser-ies of aklyl sulfonates have also recently obtained a direct corre-lation between calcite flotation and the solubil~ty product of cal-cium salt. On the other hand, excessive precipitation (probably dueto the supersaturation of the corres?onding counter ions in the bulksolution) in the bulk has been found also to adversely affect theflotation.~O For example, addition of calcium nitrate to a calcite!'potassium oleate system around pH 10 decreases the calcite flotation

GAS£OUS STATE

~~

Figure 17. SChematic diagram for free energy transfer of -CH2-groups from aqueous solution to various environments.33

~


significantly; this has been attributed to the depletion of surfac-

tant owing to precipitation.~O

It is often considered that while precipitates nucleated onthe solid surface and thus 'physically adsorbed' on the surface cancause flotation, the bulk precipitates themselves may not act ascollectors.ltl The adsorption density (or, more strictly, the appar-ent adsorption density) obtained under precipitation conditions canbe quite high, if the nucleation of the precipitates can occur atthe solid surfaces. The adsorption results obtained for calcite!oleate system shown in Figure 18 illustrate this point as the apfar-ent sharp increase in slope of the adsorption isotherm above 10- Hof oleate can be seen to correspond to the precipitation limit of

calcium oleate in the bulk.lta

CONCLUDING REMARKS

Flotation is a complex phenomenon governed by the propertiesof the three interfaces, namely, solid/liquid, liquid/gas and solidI

100 . . J..' 11 t ~-

.'10.0.'0E..ci...

: I0..

0c

...I-C 1.0...5 0

0

10,8 10'$ .0-4EOUtLI8RIUM CONCEHTRATIOfi OF OLEATE. _'e"

Figure 18. Adsorption isotherm of potassium oleate on calcite at

a natural pH of 9.6.40

10

798 P. SOMASUNDARAN AND K. P. ANANTHAPADMANABHAN

gas. Attachment of air bUbbles to particles results from the favor-able energy balance created by the adsorption of surfactants at vari-ous interfaces. Adsorption in turn is controlled by, among otherthings, the solution chemistry of surfactants including their asso-ciative interactions such as micellization premicellar aggregationand precipitation. Adsorption studies at the solid/liquid and liquid/air interfaces, coupled with the study of other interfacial proper-ties, such as electrokinetic potential as a function of relevantvariables such as surfactant concentration and pH, have yielded use-ful information on mechanisms of adsorption and therefore the depen-dence of flotation on such solution conditions. Thus, the resultsobtained for surface tension and flotation in systems containinghydrolyzable surfactants such as oleic acid and dodecylamine as afunction of pH, along with the data estimated for activities of var-ious surfactant species, have thrown considerable light on the im-portance of the role of solution chemistry. Premicellar association,hemimicellar association at solid/liquid interfaces, precipitation,and even micellisation have a role to play in determining surfactantadsorption on minerals and flotation. A complete quantitative under-standing of the surfactant mineral interaction will, however, bepossible only when the thermodynamic data for all relevant reactionsbetWeen various surfactant species and dissolved mineral species be-come available.

AC~~EDGMENT

Support of the National Science Foundation (ENG-76-80l39) 1s

gratefully acknowledged.

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