FLOTATION MECHANISM BASEDON IONOMOLECULAB COMPLEXES

R. D. Kulkarni - P. Somasunda,.n

K. P. AMnthapadmanabhan

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1'he collector adsorption at the solid/liquid (S/LI interface inrelation to flotation has received considerable attention in the past.11lis has ~tly been expl aiDed in ~ of the swface chemical andelectrokinetic properties of the mineral with a little emphasis on thechemical nature of the surfactant 1-3. For example, the high flotawnof hematite obtainedaround the neutral pH region in hematiteoleatesystem has been attributed to the chemisorp"tion of oleate at the neutralsurface-OH sites which are in high concentrations under theseconditions 1,3. The high concentration of OH neutral sites on ~~.surfa~ has in turD been attributed to the occurrence of pzc of hematitearound the neutral pH region 1,3. On the contrary, the adsorption ofoleate on hematite does not show any maximum around the neutral pHrange 4. Also it can be shown that the concentration of neutralohydroxyl sites does not change so drastically to account for a sharppeak in flotation under the above conditions 15. Therefore, the abovetheory cannot adequately explain the notation and adsorption behavior-'of hematite-oleate system. On the other hand, in exp1ainins the aboveresults, the above theory has not taken into account the state of oleicacid and its effect on adsorption and flotation of hematite.

The present study is an attempt to explain the flotation behavior ofhematite-~eate system by taking into account the solution chmistry ofoleic acid. .

A fatty acid such as oleic acid is known to undergo varioo shydrolysis reactions to form different products depending upon thesolution conditions such -as pH, ionic strength and temperature. Theseproducts of hydrolysis further interact to form complexes in solution.Complexes such as those -l'ormed between the ions and neutral"molecules of the collector a.re termed a§QoDOJDoleouJ,ar complexes 16.The different hydrolyzed products and the complexes with varyingdegrees of surface activity will act as collectors depending upon theconditions such as pH.

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Reprinted from XIIth International MineralProcessing Corigress~ IRON, Vol. II, SaoPaulo. Brazil. 1977.

The ionomolecul.. CtaJIlPIex. because of their higher chain length orlower solubility are ~.ed to be DX)re surface active compared totheir parent species and hence one would expect a higher flotationrecovery of the mineral when they are present in high concentrations,provided the other co.-litioDS such as the surface characteristics of themineral are also favorable for flotation. It is thus important to considerthe various aspects cI ~r solutioo chemistry and their effect onflotation, while studyiD£ the froth flotation mechanisms.

The existence and the role of ionomolecular complexes indetermining the flota1ioD JKoperties of quartz-amine ~tem have beendiscussed recently /6. Awni- exist in the form of neutral molecules athigh pH values and in the form of ions at low pH values. In theintermediate range. the mas and the neutral DX)lecules interact to form1 : 1 ionomolecular .:::~.--=:':"'-:A in the system. The high fl<X.ation of quartzar~nd pH 10.0 has been attributed to the formation of ionolOOlecularcomplex in high concea1nlioDs in that pH region.

In the present stIMIy we have eYAmined the role of oleate-oleicacid soap complex in ~ining the pH dependence of hematiteflQtation. The sharp iri..a~ in hematite flotation around the neutralpH range has been correlated with the sharp increase in theconcentration of 01eate-oJeic acid soap complex in the solution.Meas:urement of syriamic and equilibrium adsorption studies at theliquid/gas (L/G) interf~ have provided an indirect evidence for theexistence of a highly 5m'face active species in neutral pH region. Anexcellent colT8lation has been obtained between flotation recovery anddynamic surface te~ .

ESPERIMENTAL

MaterialsMassive red uiDDeSCM;a hematite obtained from Ward's Natural

Scien~ E.~tahljsh~t was used for basic studies while - 65 meshtacQnite ore .obtained &om upper Michigan Peninsula was used forDenver ~ll flotation te5ts.. The Minnesota hematite was found to be94% pure with quartZ - the main impurity whereas the taconite wasof 40% Fe with mainly quartz and magnetite in the non-hematiticportion. 100 X 150 mesh ~tite was used fir Hallimond cell flotationtests.

Oleic acid purchased from Applied Science Laboratories. in 1 gramsealed ~\iles had a specification- orC!'~'1ia:ritY and was potfurther purified. It was rC'i&erated till use. A stock solutioo of 7.5 X10- 4 mole/liter potassium oleate was' prepared by dissolvingappropriate amounts of oleic acid in deaerated water cootaining

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enough potassium hydroxide to yield a pH of 11.2 after saponification.The stock solution was refrigerated under 8 nitrogen atmosphe~.

Potassium nitrate. potassium hydroxide and nitric acid used wereFi£her certified grade. Triple distilled water. distilled in a pyrex stiJIand collected and stored in teflon bottles. was used for making allsolutions. .

Flotation Experiments

Modified Hallimond cell and 1.25 liter Denver cell were used forall flotation experiments. Two special features of the present Halliomndcell set-up were automatic control of flotation time and stirringintensity and time /8. The details are describel elsewhere 8.

Hallimond test procedure consisted of desliming 0.8 gram of thesample till free of visible fines and _then transfening it.to a cylinder towhich the desired collector solution is added. It was then conditionedby stirring for ten minutes in a constant temperature bath maintainedat 259 C. After conditioning, the pH of the pulp supernatant ismeasured and the pulp was transferred to the cell. Flotation \,'asconducted for ten seconds at a nitrogem flowrate of 20 ml/minute.

For Denver cell tests, 300 gram sample of the ore was deslinwdtwice with 1800 ml. of distilled water and then reagentized at 60%solid cont:!!nt at desired temperature for ten minutes. Pulp was thentransferred into the Denver cell and floated for thirty 'econds at 20%solid content.

Surface Tension

A dynamic surface tension measuring technique was developedutilizing the Wilhelmy plate method and 'a microbalance 9. The cellcontaining the test solution is jacketed for circulation ot water at thedesired temperature. A hole was provided on the side of the cell at ale'i.el above the solution in order to remove surface layers using acapillary connected to an aspirator and thereby to create a nascentsurfac3. In order to test the nature of the contact of t.~e solutionsurface with the sensor, the balance was mounted on a camera screwring enabling smooth raising or lowering of the seJlsQr. The ()U~ut ofthe microbalance is fed to the y-channel of - X"..;.~.y '~or«'8r witll theX- axis for recording aging time.

For measurements of surface tension decay as a function of time.a thin surface layer is removed by suction and the surface tensionrecording is begun at the end of suction. The recording directly yieldssurface tension versus time plots. The suction was inifonnly applied for

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30 sewnds in all cases even though the surface tension values for purewater were obtained in lea than ten .conds. Repeated recordingsfollowed by suctions o~ed for most test solutions showed therepr<xlucibility to be satisfactory.

RESULTS

Plotatt..The effect of pH on the Hallimond cell flotation of natural hematite

is shown in Fig. 1. A strong dependence of flotation (Xl pH is evidentfrom this figure. Maizmum flotation is obtained around the neutral pHrange and this is in agreement with the results repoI1ed in theliterature. In the past, this maximum around the neutral pH range hasbeen attributed to the presence of pzc in this pH region. It waspro~ that the oleic acid pre~rably adsorm on the neutral surfacesites which are present in high concentration near the pzc. Flotationrecovery ~. pH curves for various concentrations of oleate is shown inFig. 2. I~ can be ~n from this figure that the increase in oleateconcentration has increased the flotation recovery of hematite. At thesame time the pH of mammum tlotatim response has shifted to higherpH values with the increase in oleate concentratien. This is clear ftomFig. ~ where the pH of ma~um flotation response is plotted againstthe concentration of oleate. If surface characteristics of hematite aloneis responsible for flotation behavior, such a change in the pH ofmaz]ml,m flotation w(Rl}d not have occunoed.

The Denver cell flotation results for taconite as a function of'pH isshown in Figure 4. The results show a maximum in flotation recoveryin the neutral pH range whjch is in agreement with the Hallimond cellflotation of hematite.

Surf~ TeDSMIn

Typical dynamic surface tension decay cmves for 3xlo- 5 mole/loleate solutions at various pH levels are shown in Figure 5. It is seeDfrom these figures that. (a) the initial surface tension values for all thEcurves correspond to the surface tension of the pure water, bl in alJcases equilibrium Surface tension values are attained in a finite timeoand (cl the surface tension decay rate as well as the total surCac itension lowering is stnmg1y iDf1uenced by pH.

In general the surface tension decay curves exhibit three distinc;'regions: Region I in which there is a negligible surface tensiordecrease, Region ll, characterized by a linear decrease in surfacttension with time and Region ill characterized by an apparentl\

83

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exponential surface teD8GI decay. While all the th fee regions exist a1low pH values, the ~ .. higher pH values seem to exhibit only th6latter regions.

The slope (dy/dt) of the linear regiml of the surface tension decaycurves (ie., Region m was found to be characteristic of eactl curveand was the1'8fore ~ to t:haracterize the dynamic surface tensionproperty.

The effect of pH cm the total surface tension lowering oould beseen more clearly in F"JIIIre 6, whe1'8 surface pressure is plotted asfunction of solutioo pH. 'nle plot in this figure shows a do~ shapewith maximum surface pressure in the neutral pH range. Thism~yJmum obtained iD the MUtral range can be aUributed to thefonnation of the acid SO8p. which wxist in this pH ~ange and is moresurface active than other forms of oleate.

ms:O$SiONThe results of this inftStigation has clearly defined two main

features of hematite-*tate flotation systeIn. These are:( 1) Hematite flotatiml ~nse is very sensitive to pH especially in

the neutral pH range aDd the maximum floatal:ility is observed clC:8e topH 8.? with 3xl0 -5 u.-J1iter total oJeate concentration.

(2) For a given bematite sample the pH. of maximum flotationresponse (ph*) is dc--; t on the oleate concentration and folmwsthe followq relation:where Ct is oleate .~--=tration in JIK)le/Jiter and A and B arearihitrary constants.

Both these featur8 m this flotation system cannot be adequately.explained on the ~ m tbe exixting theories. For example. if the stateand the extent of oIe8Ie ~tion. and hence the flotation response, isdependent only on the I.matite solution interfacial JrqJel1ies then theph of m~~~ ~~ response should be independent m oleateconcentration which is contrary to the finding of the presentinvestigation.

In the following ..m-::;;~sioo -we will make an auem~ to explaintheses results based - tt. oleate s~~n cbemi$tIy and the pl'8~of iom-molecular co~~~~ in this system. Before attempting such atask it is first ~t to briefly describe the oleate solutionchemistry.

Oleic acid is a weakIJ acidic insoluble cmn~d in the aqueoussolutmn, which fonm hiIhIJ lOIuble S8lts with monovalent alkali metalions such as sodium - potassium. These soluble salts. UDderappropiate mlutim1 ~tiom can undergo series of hydrolysis

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NacLions yieldinl oompiex oleate species / I 0.11. 'M1e-. espedel havebeen identified to be 01.8te ions (R .). oleic acid (RH). acid-soap (R2H.).8cidics08p salt (R2HNa) and oleate dimer (R2..1. The rel8tive~n of t.he88s species in the aqueous ~lutims is stl'OnalY andionic streJlltb.

1118 chemical equilibrium. beteween tIieses 0le8te speci88 can bere..-ented in the foUowinl manner 10:

These equations clearly indicate that at a given total oleateooncentration in the solution. the d-=re.. in ph increases the oleic8cid concentration and decreases the ol.8te ion concentr8tion. theoel8te complex species being stable mostly in the neutial ph renge. Thebeh8vior of this system is rather complex. especially becau. of theva:ryng solubilities of these species in water. For example at roomtemper8ture while oleic acid exbimt equi~um solubility of only2.S1xl0 -8 D¥)le/Uter. the oleate ions are highly soluble /10/. Besides.the selt o( acid SO8p in its undissociated form is expected to exbibithleut solubility. In general. true oleate solutions in water are obtainedonly in the basic ph range. The neutral and acidic ph solutions areturbid containing fiDe stable dispersion of oIe8te species.

'MJe concentr8tion of oleate species in aqueous solution can bedetermined using equations /6-10/. The rearrangement of theseeqU8tions along with the substitution of the activity of the species bytheir conespooding D¥)lar concentration in the solution OM obtainsfollowing equations:mo18r ooncentr8tion of total oleate species. oleate ions. oleic acid.acid-~p salt and 01e8te dimer respectively.

11M last equatioo is obtained on the basis of mass balance and isvalid only for tnle 01e8te solutions. Under these condictiom. since theooncetration of RH and R2HNa are very low as compared to other.ate species. o~ can oeglect their contributioo to the ~ oleateooncentration. l11us the mass balance equ.tion becomes:

FIg. (71 represents a ph8se di8gram describing the concentrationsoC various o.leate in solution as a function of ph 8t 2 Soc. This diagrambaa '-en contnlced 00 the basis of various equations described aMve8t 3xl0 -5 mole/litel" total oleate. and 8xl0 -5 mole/Uter sodiumnitrate concentration. It is observed from the diagram that: (alacid-~p is ~nt in significant ooncentrations in the MUtral pbran... with i.. Ift.~mum at Pi 7.8. Its ooncentra~n rapdlv ~8188as the ~ drifts from the value of 7.8 (see Figure 81. (bl Acid-soappreciptation does not occur at any ph conditions. Howev... suchprecipitation would occur in the neutral ph if the total oleateoonC8ltration is raised above 10 .4 mcHe/Uter. (cl The ph of oleic acidpreciptation. aM the ph of Iftaximum add-map formation are identical.

87

Id) At any given ph the concentration of R2-- is comparativelylower than that of R-. However, its concentration is expected toincrease rapidly with increase in the total added oleate concentration.

Ie) At ph 7.8 'oleic acid precipitates and thus below this ph asecond phase appears. which will be present as dis~rsion. The amountof this dispersed phase will increase with the deacrese in solution ph.

SURFACE ACTnnTY OF OLEATE SPE~

It should be noted that among the various forms of oleatediscussed above, acid soap complex essentially represents a surfactantwith a chain 1ength of 36 carbon as compared to 18 carbon chainlength of o~ei.c acid or oleate ions. Thus, on the basjs of higher effectivechain length together with lower charge desity of the polar head group(due to partial neutralization by h.c ions,) one would "expecte acid-soapto be much more sui-face active compared to other forms of oleate.

The surface activity of these oleate species can be. examinedeffectively by monitoring the dynamic as well as static surface tensionbehavior of oleate solutions under dffferent ph conditions. Fig. (6)depicts the equilibrious surface tension of 3xl0 -5 mole/liter oleatesolution as a function of 'bulk solution ph. Here maximum surfacepressure, and hence maximum surface acitivity of this solution isobserved in the ph range of 8.5 - 9.5. It should be noted that the abovestated ph refers to bulk ph. The surface ph, i.e, the ph close to thesolution/gas interface is expected to be lower due to the solution/gasinterface is expected to be lower due to the anionic nature of theadsorbed oleate layer. Using Gouy's theory of electrical double layer, itcan be shown that, under these condictions the surface ph can, in fact,be )ower by as much as 1.5 to "2.0 ph units at bulk ph of 9.5. Thus,with this consideration, the maximum surface activity correlatesremarkably weel with the maximum concentration of acid soap

.jrompare Figs 6 and 8). A similar correlation is presented in fig. (9)wh~re rate of surface tension decay dy/dt i.e. dynamic surface tensionis plotted along with the concentration of acid-soap as a function ofbuIll. ph. It is again seen that the maximum rate of surface tensionIt:lwering is obtained in the ph range where acid-soap is a predominantspecies

The lower surface activity. and slower surface tension decay rates,as see!' in Figs (5,6) at acidic ph valucs.,bave been attributed to thepre;;enre of oleic acid which exist as precipitate Under thesecCJnditions. Also the lower surface activity, but relatively higherdynamic surface tension behavior at higher ph values has been

89

90

attributAtd to the higher solubility which leads to lower surface activityand highter molecular mobility of the R2 and R- oleate ~peci. whichare predominant under these condictions.

On the basis of above discussion one can expect the oleate to bemost effective as flotation aid when p-esent as acid-soap complex.~vided other conditions such as properties of solid/liquid interfaceare favorable.

The actual role of acid-soap in the hematite-oleate flotationsystem can be evaluated by determining quantitatively theconcentrations of tb8Ie species in ~lutiOD under various experimentalcondutions and by correlating them with the flotation respoDM underidentical conditions.

ACID SOAP COMPLEX: IN RELATION TO HEMATITE FwrATlON

'nle Hallimond cell flotation response of hematite mineral ispresented in figUJoe (10) along with the concentration of acid map as afunction of bulk solution ph. at a ~ oleate concentration of 3xlO -5mole/Uter. An excellent correlation is evident from this figure. It isseen that ph of maximum floatability corresPonds to the ph ofmaximum acid soa~ concentration in the soJution. Also the shery dropin hematite floatability at hig!1err aDd lower }:il value is associatedwith rapid decrease in the acid soap concentration. Again it soouJd berecalled that the slight difference in the }:il of maximum floatabilityand acid-soap concentration is due to the difference in the value of thehematite/soJution surface ph and the buJk solution ph. Unlike the caseof solution/gas interface. however. the hematite/solution inr.erface.being onJy weakly negative in charge. will show onJy marginallowering of surface ph in the ph range of 7-9.

Table (1) Jists th~ flotation resP>DSe of hematite-oleate system attwo different ph levels and different oleate admrption densities. It is tobe noted that at ph 8.0 where acid-soap complex predominates. theflotation respmse is much more seMitive to adsorption density ascompared to ph 4.8 where adsorption is expected to take place m~tlyin the form of oleic acid. This suggests that flotation reSJX>nse ofhematite is DOt jUst related to the adsorption density but also on thenature of oleate species being adsorbed. These results are described indetail elsewhere i 12/. The implicaticms beiRg. perna.- orientation ofthe ~late molecules at the surface is of importance in obtaining goodfiotatkm resJX)nse. FCB' a given adsorption density acid-soap impartsmuch more floatabiity to the mineral as compared to oleic acid.

91

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CONCLUSIONS

Two main features of the hematite-oleate system evident from thepresent study are (1) a sharp peak in flotation around the neutral. phrange and (2) the shift. in the ph corresponding to the m~~umflotation with the increase in the concentration of oleate. These resultshave been explained here" by considering the role of ionoumlecularcomplex loleate-oleic acid soap) in flotation. Independentthermodynamic calculations have shown that the ph region where ~cidsoap complex is present in maximum concentration corresponds to theregion of high flotation. The higb flotation in the presence ofiondmolecular complex has been attributed to the high surface activityof this species and this bas been confirmed by surface tension aIKisurface tension decay studies. Most importantly. the present study hasshown that the solution chemistry and the chemical nature of thesurfactant play an important part in determining the flotationcharacteristcs of hematite-oleate system. In the light of the aOOve facts.a systematic study of the solution chemistry of other mineral-collectorsystems is warranted.

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Rl'EREHCES

1 Pope,' M.I." and Sutton, 'D.I., cThe Cori'elation Between FrothFlotat,ion Response and Collector Ad.-ption from Aqu«)us Solution-.Powder Tech.. 7,271. 1973./2/ Fuerstenau, D. W., cCorrelation of Contact Angles, AdsorptionDensity, Zeta Potentials and Flotation Rate.. Trans. AllIE, 2~, 1365,1957. '

/3/ Howe. T.M. and Pope, MM.I.. The Quantitative Determination ofFlotation Agents Adsorbed on Mineral Powders Using DTA, PowderTech.. 4, 338. 1970/71./4/ Kulkarni. R.D., cf1otation Properties of Hematite-Oleate System andTheir Dependence on the Interfacial Adsorption.. Ph. D. Thesis,ColumlXa Univ., 1975./5/ Somasundaran. P.. .The Role of lonomolecualr Complexes inFlotation», International JnL of Mini. Proc.. 3,35, 1976./6/ Finch J.A. and Smith C.W.. Dynamic Surface Tension of AlkalineDodecylamine Solutions». J.CoU, Int!. Sci.. 45 (1). 1973./7/ Somasundaran, P. and Moudgil. B.M.,J. CoU Inti. Sci., 45. 591.1973. ./8/ Somtiundaran, P.. Danitz. M, and Mysels, K. J.. cA New Apparatusfor Measurements of Dynamic Interfacial Properties,. J.Co1l- Intf. Sci.

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48,410, 1974.191 Jung R.F., M. Sc. Thesis. Univ. of Melbourne. Australia, 1975.1101 Goddard E. D.. Goldwass~~, S., Golikeri, G., and Kung, H.C. in«Molecular Association in Biological and Related SystemsJ, p.67, 1968,ed. Goddard, E. D. Pub. Am. Chem. Soc., W~~.bL~gton, D.C., 1968.1111 Kulkarni, R.D., and .SOmasunda,te, P., Oleate AdSOtption atHematite/Soln. Interface and Its Role in Flotation, Presented in l04thAnnual AIME Meeting, N.Y., 1975.

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