P. Somasundaran and J. T. Kunjappu

Abstract - Spectrosc9pic techniques are becomingincreasingly popular for ch4mcterizing the interior ofad30rbed layer;! of .surfactant.! a1ld polymer3 at solid-liquid interface a3 well a3 in the identirwation of com.pound.ton mineml.surfaces. Thi3 review encompassesthe application of /luore.!cence and ESE .spectroscopictechniques to reveal the micro.!c9pic environment ofthe adsorbed layer.! of .surfactant.! a1ld polymer.! onminemb with a .special reference to the adsorption ofanionic .surfactant.! (e.g., .!odium dodecY' sulfate) a1ldpolvelectrolytes (e.g., pol~crylic acid) at po3itivelychcit'ged mmeml (e.g., alumina) - ~ter interface.!.Finally, the Utility of XPS a1ld auger techniques toidentify the .surface elemental compo3ition of differentmineml .!pecies has been related to their flotationefrlCiency.

Introduction

Ever since Langmuir laid the foundation of adsorp-tion phenomenon of gases on solids (Langmuir, 1918),a great number of theories and experimental method-ologies have revolu~on1zed the domain of surface

P. Somasundaran, member SMe, and J.T. Kunjappu are withLangmuir Center for Colloids and Interfaces, School ofEngineering and Applied Science, Columbia University, NewYork, NY. M&MP paper 88.621, Special Topics in MineralProcessing Seminar, Pune, India, Dec. 30, 1987. Jan. 1, 1988.Manuscript April 1988. Discussion of this paper must besubmitted. in duplicate. prior to July 31, 1988.

science, expanding the scope of Langmuir's discoveryto the solid/llquid adsorption process, which plays ama)orrole in industrial processes. For example,manyprocesses such as flotation (Somasundaran, 1975)derive their efficacy from the modification of surfaceproperties of solids owing to the adsorption of surfac-tants or polymers.

A molecular understanding of the structure of theadsorbed layer at the solid/liquid interface is essentialfor adsorption to the resultant processes. Such anunderstan(ltng is considered necessary for improvingthese processes by manipulating the adsorbed layersfor optimum configurational characteristics. How-ever, unW recently, methods of surface characteriza-tion were Urtllted to measurement of macroscopicproperties like adsorption density, zeta potential,flotation efficiency, etc. (SomasundaI'an et al., 1964;Hough and Rendall, 1983). Such s:tudies, while helpfulfor developing insight into the adsorption mechanisms,could not provide any direct information on the micro-scopic characteriStics ot the adsorbed species. Withthe advent of spectroscopic and other allied tech-niques, the interior of the adSOrbed layer has becomeaccessible. These techniques answered many of thequestions, on atomic and molecular levels, concerningthe bonding of adsorbates onto sunace sites of solidparticles.

A variety of specb"oscopictechniques are currentlypracticed in the area of surface science. To cite a few,the elemental nature of the surface could be character-ized by X-ray photoele~tron spectroscopy (Roberts,1981) and auger spectroScopy (CZandema, 1975), andits chemical state could be Inferred from valence-level

II MAY 1M8 MINERALS AND METALLUAGICA'- PAOC£SSINO

In addition to T. ferrooxidan.s and T. thiooxidan.s.many other bacterlahave been reported to playa rolein the leaching process although their activtties are notsignificant from the point of metal solubilization.However, moderately thermophilic thlobacilli (Brierlyand Lockwood. 1977) and sulfur.oxidizing Sulfolobusspecies (Marsh et al., 1983) have been successfullyused in the leaching process.

The thermophilic thiobacilli can be isolated byenrichment in the conventional media described pr-e-VI0U.;lY. UtI:: 111(;Uuai.1UI1 l~illpe!""l.u.t: (J~ijl6 ~v - "-' <' :;

above.

The Swfolobw species can be enriched on a sulfur.containing ba.sal salts medium supplemented with0.01% yeast extract. The medium pH is acidic (pH 2.0)and the incubation is at 10oC for several days.

Using the methods described, potentially usefuliron. andsulfur.oxidlzing bacteria were isolated fromdifferent ecosystems including ores, wastes, soils:acid.mine waters, etc., collected from various coppermining areas in India (Table 1). The major or~msisolated by enrichment techniques belonged to T.f~n.s and T. th~n.s species. In addition, afew thermophilic species like Swfolobw species andthermophilic thiobacilli were also isolated. Startingfrom flask-level experiments, the process of copperleaching was systematically scaled up to a 3OO.kg levelin PVC columns (Fig. 1).

"1

tJ~::~

Table 1 - Microorganisms Isolated from Indian Copper Mines

No. of Iso"I..1172851

Cuhut8rl/iQbaci/lus f...rooxidansTlliobaciffuS thiooxidansThermophilic Thiollaci/lussP.Sulfo/abus sp.To'" 151 Fig. 1 - Bat1ery of PVC columns for copper leaching.

Preservation of cultures used incopper leaching

Table 2 - Activity check on selected cultures preserved bydifferent methods and using different suspending

media/carriers after storage for 12 months. Activity isexpressed in percent loss in iron oxidation rate.

L ~lIlZ8t1onSucro.. s..c-

Liquid N2SIatq. Mlzlrtg WIth In.rt C.m.rs

(Glyc«Ol1 CNlcopyril. Li9nll.0,.

Bacteria

T. 'erroOM/danS

MCM 8.8MCM 8.185MCM 8.191MCM 8-231MCM 8-175

28.428.231.718.320.2

2.82c5.125.1Z5.125.12

SI.851.8«1.851.848.2

70.375.858.773.8.

70.3

57.8

54.2

51.8

64.1

In laboratories around Ule world, cultures of T.feTT~M and T. thiooxidaM have been routinelymaintained by serial transfers in Ulelr respective~dia every two monUlS or so. 'I1lis meUlod has twolimitations for long-term culture preservation. First,monthly transfer of cultures into fresh media is alaborious and time-consuming task, and. second. it isnot possible to maintain Ule genetic uniformity of Ulesecultures since thiobacilii presumably have mobilegenetic elements (D.H. Holmes. personalcommunica-tion).

Lyophilization or storage in liquid nitrogen areeffective meUlodS for preserving many types of micro-organisms. However, thiobacilli cannot be storedeffectively by these conventional methpds. Hence, wehave developed a novel method for thelr storage inwhich T. ferrooxidaM andT. th~M cultures aremixed with sterile chalcopyrite ore and stored at goC(Gupta and Agate, 1986). ThismeUlod has resulted inretention of the viability of these bacteria for pro-longed periodS (Table 2). In addition, we observed thatsuch preserved cultures retained thelr copper extrac-tion abUlty after one year's storage (Table 3) when theleaching experlrnentswere carried out on an expandedscale in PVC columns (Gupta and Agate. 1986a).

. Not possible to revive a,ter storage for 4 months.

Table 3 - Leaching of Copper from Rakha Chalcopyrite OreUsing T. ferrooxidans MCM 8-231 in PVC Columns

(75 kg Ore)

% Copper Eatf8CtedAIt., ~ 0.,.

28.620.24.313.1

SetFresh culture InoculumPreserved culture I~utumUnstefUlzed (posltlvel Ole control 'Chemically sterilized (negative) or. controt

MAY 1- 67MINERALS AND METALLUftGICAL PN3CESS-.G

photoemission spectra and molecular vibrationalspectra. Stmnarly, the local order or geometry couldalso be probed with vibrational spectroscoPY, whilethe long. range order could be investigated by diffrac-tion methods. Many of these techniques needed ultra.high vacuum and, hence, failed to yield in situ informa-tion on the adsorbed layer. But, some of the vibrationalspectroscopic methOds, e.g.,ffi (Hayden, 1987), ~(Takenaka, 1979), etc., have been found to be partic-ularly useful for elucidating the structure of theadsorbed layer under eouiltbriurn conditions. I,urnl.nescence and eleclron spin resonance spectroscopictechniques with proven capabilities in the biologicalsciences (Weber,lm; Ohnishi and McConnell. 1965)also find interesting appllcations in understanding themicrostructure of adsorbedsurfactants and polymerson solids.

chains (this phenomenon is referred to as hemi-micellization (SOmasundaran and Fuerstenau, 1966)].

. Region m is marked by a decrease In the slopethat Is ascribed to the Increasmg electrostatic hin-drance to the surfactant association process followinginterfacial charge reversal (see Fig. Ib).

. The plateau adsorption In region IV corresponds tothe maximum surface coverage as determined bymicelle formation In bulk or monolayer coverage,whichever is attained at the lowest surfactant concen.tration; further increase in surfactAnt concentrationdoes nota!t~1' the adsorption density.

The surfactant aggregation process on the surface ofa solid as described above has been generally acceptedas a working hypothesis to explain the general patternof the adsorption iSOtherm, though some modificationof this treatment has appeared in literature (Harwellet aI., 1985).

The exact natUre of these aggregates, referred to ashem1m1celles, could not be ascertained until spectro-scopic teclmiques were applied to Inquire Into thedetalls of the adsorbed layers. The succeeding sectionsdel1neate the utU1ty of spectroscopic techniques toinvestigate the structure ansi. In some cases, evolutionof adsorbed lavers. .

~

Scope of the review

This review addresses the application of certainspectroscopic techniques for in situ characterizationof adsorbed layers at solid/Uquid biterfaces. Mostof the examples, viz., application of fiuorescence,electron spin resonance (ESR), and X-ray photo. -electron spectroscopies (XPS) are selected fromauthors' publications. Adsorption of two clasSes ofmaterials - surfactants and polymers - is detailed to .exemplify the validity of these spectroscopic methods. 10-Sodium dodecyl. sulfate (SDS) and polyacrylic acid(P AA) are selected as the adsorbates and alumina(Linde-Union Carbide, 0.3 Ilrrl, 15 m2/g) as the adsor- .bent.

While the fluorescence technique demonstrates theestimation of micropolarlty, microviscosity, andaggregation number of the adsorbed layers, based on ~ 1_"the fluorescence intensity and lifetime of. pyrene and ~dinaphthyl propane probes, the ESR studies estimatethe variation of microviscosities in the adsorbed layeras reported by 5-, 12-, and 16-doxylstearic acid spinlabels. The conformational changes of P91yacrylicacid in the adsorbed state With change in pH aremonitored by making use of the excimer-formbigability of a pyrene-labeled polyacrylic acid. Finally,the importance ofXPS (ESCA) in mineral engj,neerlngprocesses has been brought forth with reference to thesurface characterization of some carbonate, phos-phate, and sulfide minerals.

~mICMC

SDS/ALUMINAO.lM HoCI, pH 6.5

J[~fC4

1&1

~ 1af2..JJ(/)

..J>-u 10"'"1&1c00

Y A- I C SOS ONLY

~ SOS WITHPYRENE

~

1a"4I"IIIII" "l""" ."., ""'1'"I 10~5 10-4 10~3 to-2

RESIDUAL DODECYL SULFATE, mo"s/literAdsorption of sodium dodecyl sulfateon alumina.

Fig. 1 a - Adsorption i.sotherm of sodium dodecyl sulfate(50S) on alumina at pH 6.5 in 10-1 kmol/m3 NaCI.

]I ,I

~Ol--.,

>e 60 I.;j 40::! 20...

~ 0 I- O~m 0-20

4.-40 IV:c(I- .601&1N.ao 1""""""1""" i..,.",II,.,.,...1

0 ~-5 10-4 10.3 10"2RESIDUAL DODECYL SULFATE. ~Ies/llt.r

Fig. 1 b - Zeta potential of:alumlna as a function of equilibriumconcentration of SOS (designation of regions based on shapeof Isotherm In Fig. 1a).

As most ot the spectroscopic applications cited inthis review make use ot the adsorbed layer of sodiumdodecyl sulfate on alumina as a working model, it isexpedient to be tamiliar with the features ot theadsorption isotherm ot SDS on alumina. Fig. 1a repre-sents a typical isotherm ot SDS on alumina at pH 8.5and a salt concentration ot 10-1 kmol/m3. This iso-therm is characterized by tour regions, which areattributed to the different stages ot adsorption. Resultsof zeta potential studies corresponding to these regionsare depicted in Fig. lb. Mechanistically, these regionsmay be viewed as tollows.

. Region I with a slope of unity under constant ionicstrength conditions is indicative of the existence otelectrostatic interaction between the anionic surfac-tant and the positively charged mineral.

. The conspicuous mcrease m adsorption m region nmarks the onset ot surfactant association at the surfacethrough lateral interaction between hydrocarbon

MINERA1.S AI«> METAlWAGICAL PNx:ESSING MAY 1- 69

'j 10-10. IZ0t=

Fluorescence spectroscopic studies change occurs In a region U1at is well below the ~ICand approximately coincides with the transition In theadsorption Isothenn from region I to ll. At the plateauregion, the II/II value coincides with the maximum1./11 value for SDS solutions (Fig. 2a), Indicating thecompletion of aggregation on the surface. It may beobserved U1atthe II/II values of pyrene are relativelyconstant (-1) throughout most of region n and aboveand, hence, seem to be independent of surface cover-age. Thus, fluorescence studies provide a means tomeasure the micropolarity of the adsorbed layers.

'i'he lact that pyrene ill small coIlcenlrations doesnot perturb hemimlcelle fonnation can be seen fromthe adSorption Isotherms In Fig. 1a.

1.5

~~T'

-......If)...

0 .,. ., "",1 . 0" 10 ."to 0' 0 .,..,1 i , ..." ..1

0 10-5 .10-4 10-3 10-2OOOECYL SULFATE CONCENTRATION.~les/lite'

Fig. 2a - 13'1, fluorescence parameter of pyrene in sodium~odecyl sulfate (50S) solutions in 10-1 kmolfm3 NaCI (13 =383 nm,l, = 374 nm).

Subsequent to absorption of light by a molecule, a ~of the light may be emitted radiatively from its firstsinglet excited state. nus phenomenon, called fluores-cence e~sion, bears a wealth of Infonnation regard-ing the environment of the Ught.ab8()rbing species.Parameters oftundamentalimportance in fluorescenceemission are (1) emission maximum (wavelength ofmaximum emission intensity), (2) quantum yield offluorescence (emission efficiency as measured by theintensity of fluorescence), and (3) fluorescence'life-time (time taken by the excited state to decay to lie ofits initial value, e being the base of natural logarithm).They are quantitatively related to various eXtJerimen.tal parameters by the following expressions.

-(cl +I, = 10(1-10 ) f

where I, is the fluorescence intensity, 10 the intensityof the incident light, £ the molar absorptivity, c themolar concentration of the solute, Z the path length ofthe sample, and +, the quantum Yield of fluorescence.

-tIT1 I[t] = 11[0] exp

where 1/(0] is the initial fluorescence intensity attime t = 0, l/[t] the fluorescence intensity at a giventime t, and T the average lifetime of the excitedsinglet state.

The fluorescence measurements are generallycarried out by a steady-state fluorescence spectra.fluorometer and lifetime of fluorescence by a timeresolved fluorescence lifetime instrument (Lakowicz,1983 ) .

The dependence of fluorescence intensity and life-time on the physicochemical environment of thefluorescing molecule has been well documented(Thomas, 1984). Such data have been applied tomicellar photophysics and photochemistry to study theproperties of micelles and also to understand micellarcatalysis (Turro et al., 1985). Application of the fore-going technique to the adsorbed layer of surfactants onsolids can provide infonnation on the interior of theadsorbed surfactant layers on solidS at the solid/liquidinterface under in situ conditions. We now proceed todescribe the application of the foregoing solutionphase methodology to probe the micropolarity. micro-fluidity, and aggregation number of adsorbed surfac.tant,layers.

1.5PYRENE IN ADSORBED LAYER 50S/ALUMINA

.1M NoCl, pH 6.5

SQS MIC~LLEI.IM NoCI) A., ... """ .,.

WATER IO.IM NoCI)"'.'."""" '.., , ,

-...~-.5

",...,!) MicTOpOlarity 0 ., . . ... ...1 1. ,.,1 . . ,. '..',

0 10-5 10-4 10-3 10-2

RESIDUAL DODECYL SULFATE. moles/lile,

Fig. 2b - 13111 fluorescence parameter of pyrene in 50S-alumina slurries.

Microviscoaity

To obtain the microfluldity changes, the excimer-forming ability of l,3-dinaphthyl propane (DNP) isuWized. The intramolecular excimer formation ofDNP is a sensitive function of the microviscoslty of itsneighborhood. This property, expressed as the ratio ofthe yields of monomer and excimer {1",/le}' is deter.mined in solution and in the adsorbed layer (Somasun,.daranetal., 1986) forvarlous regions of the adsorption

Here, advantage is taken of the fact that the poly-aromatic hydrocarbon. pyrene, possesses a highlystructured fluorescence spectrum whose vibrationallines are susceptible to intensity fluctuations broughton by the polarity changes of the medium. A properlyresolved fluorescence spectrum of pyrene in fluids hasfive vibrational fine structures in the region from 370to 400 nm. The intensities of first (11) and third (13)vibrational bands are found to be particularly depen-dent on the polarity of the environment. Thus, 13111value for pyrene changes from -0.6ln wa~r to a valueofgrea~r than unity in hydrocarbon media.

131 11 values for pyrene were de~rmlned for aluminalSDS/water systems for various regions of the adsorp-tion lsothenn. The data are shown in Fig. 2, which ismarked by an abrupt change in the local polarity of theprobe from an aqueous environment to a relativelynonpolar micelle-type environment. This abrupt

MINERALS AHa METALLURGICAL PROCESSING70 MAV 1-

isotherm. These values are compared with the (1",1 Ie)values of DNP in mlxtures.ofethariol and glycerol ofpredetermined viscosity. The results are shown inFigs. 3a. b. and c. The extent of excimer formation inthe adso~tion sample (1mlle -4:) is significanUylowerthan that in the micellar solution (1 ml Ie = 1.7).

Based on the Imll~values of DNP calibrated inethanol-glycerol mixtures. a microviscosltyv8.lue of90 to 120 Cp Is obtained for the adsorbed layer (inde-pendent of Surface coverage) com~red to a value of8 Cp for micelles. The constancy of microViscosity asreported by DNP is Indicative of the e)dstence of acondensed surfactant assembly on the Surface thatholds the probe In a densely packed state.

Aggregation number)-

tooihz'"tooZ

'">~<..J'"~

320 340 360 380 400 420 440 460 480 500

WAVELENGTH,nm

Fig. 3a - Dinaphthyl propane (DNP) fluorescence spectra in

(A) SDS micellar solution and (8) SDS-alumina slurry; ({SDS]I(DNP]) refers to ratio of micellized or adsorbed SDS to addedDNP.

The dynamics of fluorescence emission of pyrenehas been previously studied in homogeneous andmicellar solutions using a time-resolved fluorescencespectrometer (Demas, 1983). While the decay kineticsof monomer and excimer emission may be deriveddirectly for a homogeneous solution (continuousmedium), statistical methods are to be applied toarrive at similar kinetics m aggregated micellarensembles (fragmented medium). This stems from aneed to recogJ:1ize the possibllity of random multipleoccupancy of the probe in the aggregates (Infelta,1979), which affects the probabllity of excimer forma-tion within the aggregate..If the micellar system isviewed as groups of individual micelles with probeoccupancies of 0, 1, 2, 3, etc. P., the probabllitv ofmicelles with n probes, P", may be related to ii, theaverage number of probes per micelle by Poissonstatistics through the relation

P" = ii"exp(-ii)/n!6MONOMER/EXCIMER RATIO OF DNP INADSORBED LAYER .1M NaCl, pH 6.5 This model yields the following relation for th~ time

dependence of monomer emission.5

. - _120 -~ - -- -~~__~ 6- ~.,. {-~ i1m[t1 = 1m[0I.exp(-ko t + n (exp (-ke t) -1»)

where ko is the reciprocal lifetime of excited pyrenein the absence of excimer formation, ke is the intra-miceUar encounter frequency of pyrene in excited andground states, and 1m [01. and 1m [tl. represent theintensity of monomer emission at zero time and time t.

Knowledge of ii leads to the aggregation number Nfor the adsorbed layer as given by the e~re5Sion

.: 3

...e

2 B P(SDS MICELLE)c . ,.. c,- ~--c+-~~"'"-.

0 j . 0 . "",1 . '0" ,..1 0 . 0 'I"'" '0' ",..I '" '" .,.

10-f4 10-f3 10.12 10-11 10-10 10-9

DODECYL SULFATE ADSORPTION. moles/cm2--Fig. 3b - Monomer to exclmer rat.io (Im/l.) of dinaphthyl

propane (DNP) in SDS.aluminaslurries as a function of. SDSadsorption density (A.m = 340 nm, A.e = 420 nm).

u-E-

n = [P]/[Agg] = [P] N /( [5] - [5eql)

where fP] is the total pyrene concentration, [AggIis the cbncentration of aggregates, and ([8] - [5eq])

is the concentration of adsorbed surfactant.A kinetic analysis based on this relation was carried

out from the decay profiles of pyrene in the adsorbedlayer (Chandar et aI., 1987) for different regions of theA40,/SDSadsorption isotherm. Figure 4a representstypical decay curves for monomer (curves A and B)at two pyrene levels and excimer (curve C) corre.sponding to curve B. The fluorescence spectra of thesetwo cases are represented in Fig. 4b.

The aggregation number was calculated from n usingthe foregoing equation. The SDS aggregation numbersthus obtained are marked on the adsorption isothermin Fig. 5. The aggregates in region n appear to berelatively of uniform size (121 to 128). But in region Ill.there is a marked growth In Ute aggregate size (166 to356). These results are of special significance withrespect to Ute evolution and structure of Ute adsorbedlayer. Region II and above seem to be characterizedby surfactant aggregates of limited size. Here, thesurface is still not fuijy occupied and enough positivesites are available (Fig. Ib). Further adsorption occursmainly by increasing the number of aggregates as

00 20 40 60 80 100 120 140 160 180 200

VISCOSITY,cPFig. 3c - Monomer to excimer ratio (Im/le) of dinaphthylpropane (DNP) to ethanol-glycerol mixtures (viscosities ofsolvent measured by caplilary flow method).

MAY 1~ 11MINERALS AND M£T ALLUAGICAL f"AOCESSING

6

5

4

3

2

revealed by a near constant aggregation number. Thetransition b'Om region n to m corresponds to theisOelectric point ot the mineral, and adsorption inregion m Ia likely to occur tlu'Ough the gr()wth ofexiating aggregates rather than the formation of newones. 'nt1s Ia possible by the hydrophobic Interactionbetween the hydrocarbon tails of the already adsorbedsurfactant molecule and the unadsOrbed ones. Such asituation can be expected to result In a reverse orienta-tion of the surfactant molecules as illustrated In Fig. 6,where the whole process of adsorption has beenpull..'Q.j"ed sc'i"'lllatically.

The fact that the aggregation number In region IIessent18.1ly remainathe same suggests that adsorptionin U1js region occurs by increas~ the number ofaggregates on the positive sites of the particle. Whenthe positive charge on the mineral is neutralized. theenergetics favors the growth of existing aggregatesrather than the formation of new aggregates. Thus. inregion III. the size of aggregates increases significant-ly with adsorption density (N >166)..

'~~.. -t~~.'/2

~

TIME nsec

Fig.4a - Pyrene monomer and excimer decay profiles in SOSmicellar solution, {SOS] = 8.2 x 10-2 kmollm3; {NaCl] = 1 x10-1 kmollm3, CMC = 1.5 x 10-3 kmollm3; pyrene .levels areindicated as the ratio of micellized SOS to added pyrene(emis$ion monitored at 383 nm for monomer and 480 nm forexcimer); (A) monomer emission for SOSIPV = 2160, (B)monomer emission for SOSIPY = 108, (C) excimer emissionfor SOSIPV = 108.

Fig. 5 - Surfactant aggregation numbers determined atvarious adsorption densities (average numbers at each adsorp-tion density shown along isotherm).

~NO AGGREGATION

.

amIgUIlUUBER 0'AGGRE4ATtS_REAS"-20-130 WO\ ECUI.£SPtR AGGREe4TE

._~. .. ~ ...Fig. 4b - Pyrene emission spectra In SOS micellar solutionsunder varying levels of pyrene (A and B correspond to pyrenelevels indicated in Fig. 4a).

In summary. these studies on the adsorbed layer ofSDS on alumina establlsh the earlier concepts ofhem1mlcel11zation. Surfactant aggregation occursabove a critical concentration referred to as hemi-micellar concentration, which is marked by a sharpIncrease In the adsorption isotherm. eventually lead.ing to the formation of highly organized and finite sizeassemblies even at relatively low surface coverages.

~~ZE 0' AGGREGAT£S-CREASES~MO~~ES~ &OGREGAT!

.~ ...;

Fig. 6 -Schematic representation of the correlation of surfacecharge and the growth of aggregates for various regions of theadsorption Isotherm depicted In Fig. 5.

72 MAY ,- MINERALS AND METALLUAGCA&. PAOCESSING

Polymer conformation in the adsorbed state positively charged alumina was rapid enough so thatthe adsorbed polymer adopted a conformation thatreflected its state in solution prior to adsorption. Tht.SIs represented in Fig. 8. which shows simUar mono-mer/exc1mer ratio curves as a function of pH for thesolution and the adsorbed cases. Further. it wasobserved that the polymer adsorption at high pH Is anirreversible process with respect to the conformation.as the efficiency of excimer formation remainedunaffected by the change of pH to acidic values. At thesame time, the polymer adsorbed at low pH could be:;::":;lched 1:C :;o::,,~ e}::,'~' ~y 1!1Creasing the pH 'hes...processes are represented mechanistically in Fig. 9.

The adsorption of polymeric materials onto solidsurface Is quite dUferent from the adsorption of smallmolecules (Morawetz, 1975). This dUference stemsfrom the existence of polymers in different sizes andconformations. In addition, macromolecules possessmultifunctional groups, each having the potential toadsorb on a surface.

Of the polymeric materials finding a variety ofapplications, polyelectrolytes stand out because of...,l~.i partlCll,JaUup, i," lilany ~lu:;::gical proce;:,::;,,:;;(Llpatov and Sergeeva, 197j) involving globularproteins and other biopolymers. Polyelectrolytes, e.g.,polyacryUc acid, also have importance in understand-Ing and controlling the processes of colloidal stabUiza-tion and flocculation, which have immediate applica-tion in energy-related areas like enhanced oil recoveryand in other fields such as paint formulation andsolid-liquid separation processes.

PolyacryUc acid can exist in dUferent confonnationsdepending on the solvent, pH, and ionic strengthconditions (Arora and Turro, 1987). Such a flexibilitywould influence the adsorption of macromoleculesfrom solution onto solids and, in turn, would affect thedispersion behavior of suspensions. We have used apyrene-labeled fluorescent polyacryUc acid to inves-tigate the adsorbed polymer conformations at thesolid-liquid interface of alumina in water (Chandaretal., 1987a) (1.5 mole % of pyrene-beartng moiety;molecular wt jO,OOO; degree of polymerization -510).

The rationale behind the implementation of thisteclu1ique is the observation that the extent of excimerformation, which depends on the interaction of anexcited state pyrene of polymer pendant group withanother pyrene group in the ground state, has a directbearing with the polymer conformation. This may beunderstood with reference to Fig. 7a, which showsthat, at low pH, there is a better probability for intra-molecular excimer formation between pyrene groupsresulting from a favorable coiled conformation.Similarly, a low probability for the excimer formationat high pH may be understood as a consequence ofrepulsion between highly ionized carboxylate groupsin the polymer and the subsequent stretching of thepolymer chain. This difference Is discernible in theirfluorescence spectra (Fig.7b).

350 ~oo 450 500Wavelength (ntn)

550 coo

Fig. 7b - Fluorescence emission spectra of ads~d polymerat two pH values.

6

,. 4! 3

2.

1

Fig. 8 - Monomer to excimer ratio as a function of pH foraqueous solution of polymer and adsorbed polymer on alumina.

Electron spin resonance studies (ESR)

ESR studies as applied to miceliar systems rely onthe sensitivity of a free radical probe to respond to itsmicroenvironment. Molecular species with a freeelectron possess intrinsic angular momentum {spins}.which in an external magnetic field undergoes theZeeman splitting. For a sYstem with S = 1/2.c twoZeeman energy levels are possible whose energy gap{~E} is given by the equation:

low pH HIgh pH

Fig. 7a- Schematic representation of the correlation of theextent of excimer formation and intrastrand coiling of pyrenelabeled polyacrylic acid.

E = 11." = gBHo

where 9 Is a proportionality constant, B is the Bohrmagneton (nab1ral unit for the magnetic moment ofthe electron), Ho is the applied magnetic field, and" is the frequency of electromagnetic radiationcorresponding to AR, whieh Muses resonance absorp-tion. The position of an absorption line is generallyspecified by the 9 value.

'l11e foregoing methodology was applied to theadsorbed layer of pyrene-labeled polyacrylic acid(PP AA) to investigate the polymer conformations inthe adsorbed layers on alumina. It was observed thatadsorption of PP AA from aqueous solution onto

MAY 1- 73MINERAlS AI«> METALWAGICAL ~SSeIG

~) "'9" oH

0 ~-'""'-( - - ~--«) '0. pH

,~ ~-A

. + . +coiled/solutIon

. . . ...pa"dtd/saJ"fla...

chemically, or radiolytically by tra.~ping the radicalU1ro~h chemical reactions With a spin. trap (a mole-cule like t.butyl n1troxide) and converting the radicalinto a stable free radical to be examined by ESRtechnique.

In the study under review, stable free radicaln1troxide spin labels were chosen for use of electronspin resonance spectroscopy. They were the threeisomeric doxylStearic acids bearing the doxyl groupsat positions 5. 12, and 16 of the stearic acid chain. Theirmolecular structures may be represented as follows:

A-H~~~~-~ coo 16-doxyisteartc acid (16-D)

B E

§~~.: coo H12.doxylstearlc acid (12-D)

~---'"~~0 Ho.COOH~

5.doxylsteartc acid (5.D >.i

:~~.~t

,.10. pH

~~/-+ + + +

..ponded/odIOtbtd

N~hpHC - --~-~ -~-

+ + + +

partialtr .aponded/adsorbed

Fig. 9 - Schematic representation of the adsorption processof pyrene.labeled polyacrylic acid on aluminac (A) At low pH,the polymer is coiled in solution, which leads to (B), adsorptionIn the, coiled form. (C) Subsequent raising of the pH causessome expansion of the polymer. (0) Polymer at high pH .insolution is extended and b.inds (E) strongly to the surface inthis conformation. (F) Subsequent lowering of the pH does notallow tor sufficient intrastrand interactions for colling to occur,

The magnetic moment of the free electron is suscep.tible to secondary magnetic moments of neighboringnuclei. Thus, the Zeeman splitting will be superim-posed by the effect called hyperfine splitting, whichbrings further splitting of the absorption signal. Theseparation between the hyperfine line is an indicatorof the nuclear neighborhood about the free electron.The hyperfine splitting pattern depends on the spins

~and actual number of the neighboring nucleI withspins. If the electron is in the field of a proton (8 = 1/2),then the ESR spectrum would Yield two lines of equalintensity. Similar interaction by a n~cleus with 8 = 1,as in nitrogen, would produce a triplet of equalintensity.

The line shape of ESR signals is subject to variousrelaxation processes (Wertz and Bolton, 1986) occur-ring within the spin system (spin.lattice and spin-spinrelaxations) as well as anisotropic effects (due to thedifferently oriented paramagnetic centers being actedupon by an external magnetic field). These effectsresult in broadening of the spectral lines.

Three types of ESR studies may be applIed to themicellar systems (Ranby, 1977). They are spin-probing, spin-labelIng, and spin-trapping techniques.In the spin-probing technique, a molecule with a spinis extemally added to the system, whereas in the spin-labeling technique, a spin.bearing moiety throughcovalent bonding forms a part of the molecule. Thespin-trapping technique is mainly applied in the iden-tification of radicals produced thermally, photo-

'So

These spin labels (in micromole levels) were co-adsorbed Individually on alumina along with the ma~adsorbate, m., sodium dodecyl sulfate.. and the mainregions of the adsorption Isotherm were studied.

Infonnation on micropolarity and microviscosity ofthe environment was obtained by measuring thehyperfinesplitting constant A.vand rotational correIa.tion time Tc. For this, the ESR spectra of 5-D, 12-D.and 16-D were recorded In water, ethanol, and ethanol-glycerQI mixtures. Ethanol-glycerol miXtures servedas calibration standards to monitOr the viscositychanges within the matrices of Interest, viz.. micellesand hemimicelles. Fi~re 10 represents the ESRspectra of 16-D In different ethanol-glycerol mixtures.The three-tine isOtropic specb'Um of 16-D in pureethanol (rJ = 1 Cp) considerably broadens as theglycerol content of the ethanol-glycerol miXture isincreased, imparting anisotropic features to thesystem due to the decreasing mobllity of the spin.label.The rotational correlation time for the three spinlabels are depicted in Fig. 11 for diUerent solventviscosities. It may be seen that although the Tc valuesIncrease with increasing solvent viscosity. their mag-nitude is ~ferent for different probes. This is attrib-uted to the diUerenceln the entropy of internal rotationof the probes.

Before applying the methodology to AlzOJ/SDSadsorbed layers, it. is beneficial to examine the ESRspectral changes observed InSDS solution (Fig. 12).The nitrogen hyperfine splitting constantS (AN) androtational correlation times In these cases are listed InTable 1. It is obvious that. all the probes experience awaterlike atmosphere as could be seen from A.vvalues, comparable to the value of 16.0G In pure water.Microviscosities could be evaluated by comparing theESR spectra In micelles and in ethanol-glycerol m'1x-tures. The fact that the spectral features of theseprobes matched well with the ESR spectra of theseprobes In an ethanol;;glycerol mixture (40:60 v/v; rJ =40 Cp) shows that the nitroXide groups of the probesfeel the same environment as indicated earlier by thepolarity parameter AN'

Figure 13 gives a comparative picture of the ESRspectra of 16-D in Al2OJ/SDS hemimicelles, SDSmicelles, and various eti)anol;;glycerol mixtures withrespect to their estimated rotational correlation timeTc. Thehem1micellar case reported a nitrogen hyper-~

74 MAY 1988 MINERALS AND MET Au.uAGICAL PRcOCESSING

fine splitting value of 15.0 G, indicating a less P9larenvironment in hemimicelles (AN in water = 16.0 G,AN in ethanol = 14.6 G. and AN in SDS micelles =15.6 G). 'l11ree different microviscosities were obtainedfor the probes in hemimicelles, indicating that thenitroJdde group in each ~ felt a different viscositywithin the hemlmlcelle. Figure 14 depicts the ESRspectra of the three probes in hemlml~elles and thecorresponding ESR spectra simulated in ethanol.glycerol mixtures of appropriate viscosities. Theest\m"t,,~ ""1"",, ,..fvl~,.n~ttle~ WE're 12h 1An ~T1rl >~MCp for 16-, 12-, and 5-D, respectively.

.t'T- 16-0~~"""--COOH

""X' , ,~

v,.o-COOtt"

~

- -"--~ ~

-;...!1OQ~ ETHANOL

"1 81cP? ~12-D

'"""~ coo H

67% ETHANOL

"1a8cP5""0

" ~ ~~~ COO H

(A)WATER

(8)SDS H::CELLE33~ ETHANOL

~ .65cP Fig. 12 - ESR spectra 0'5-, 12-, and 16~doxylstearlc acid inSOS solutions below (A) and above CMC (B).

20% ETHANOL7] -165 cP

Table 1 - Nitrogen Hyperfine Splitting Constant andAotationar Correlation Times of 5-. 12-. and 16-Doxylstearic

Acid .in SOS Solutions Below and Above CMC

~~/

(50S) - 1 . 10-4M<CMC

(SOSI - 5.2- 10--M>CMCI~% ETHA~L

"1 . - 300 cP 7V r~-~ Probe

16-012,0s.o

AN- gauss

16.0158158

T~.

1.5.c

1.1.'2.0.'

AN. gauss

156

15.4

15.4

TC,lec4x 10-10

18x10-10

15 x 10-10Fig. 10 - ESR spectra of 16-doxylstearic acid (16-0) in

ethanol-glycerol mixtures...

MAY 1988 75MINERALS AND METALLUAG1CAL ~SSING

-10-1010-1010-10

nltroxide spin probes 15 demonstrated to be a facilemethod for probing microenvtronments Within hemi-micelles.

.'I:; ~;

..

(A) (8)

Fig. 14 - ESR spectra of 5-. 12-. and 16.doxylstearic acid InSOS adsorbed layer and equivalent spectra obtained in ethanol-glycerol mixtures of indicated composition.

The foregoing observations may be e~lained byass~ a model for Ute adsorption ot Ute probe inwhich Ute ~oxylate group is bound to Ute aluminasurface. Such a model would necessitate attributinggreater mobility for Ute nltroxide moiety near UteSDS/H2O interface (as in Ute 16-D case) and lessmobility for l2.D- and 5-0 cases. Figure 15 is aschematic representation of such a model.

( /

J

++ + + +1 + +1 + + + .. + + +1 + +t+ + +

(4) (b\

Fig. 15 - Schematic representation of hemimicelle structureshowing flexibility of chain segments as reported by nitroxldeof doxylstearic acid In 16th position (a) and 5th position (b).

X-ray photoelectron spectroscopy (~) orelectron spectroscopy for chemical analysis(ESCA). and auger. electron spectroscopy (AES)

XPS (ESCA) and AES are two complementa:ryspectroscopic techniques proven to b:e effective ineSt<l.ullSlung surface elemental composition. ESCAcan even distinguish the electronic structure, Uteoxidation state, and bonding of atoms of the element(Seigbahn. 1967). Characterization of surfaces by thesetechniques has found beneficial use in mineral engi-neering processes involving the complex phenomenaof flotation. selective fiocculation, etc., as well as insemiconductor technology to characterize the dopants.

Briefly, XPS takes advantage of the fact that mono-energetic X-rays [Mg(Ka), Al(Ka); >1000 eV), onimpinging a surface, dislodge the inner electrons ofsurface elements producing photoelectrons. Theseelectrons are sorted out in an electric field accordingto their kinetic energies. The kinetic energy of thephotoelectrons depends on the binding energy of theshell electrons, which is discreet and easy to associat'ewith an element. XPS can view a surface region of upto 100"\ in depth. The peak position and the peakintensity provide qualitative and semiquantitativeinformation about the identity of the element.

In AES, high energy electrons form the irradiationsource, and the emitted secondary electrons (augerelectrons) are analyzed as in XPS. But in AES, thechemical shift effect (the change in the peak positionof an element due to change in chemical environment)is not as pronounced as in XPS.Another advantage ofXPS over AES is that the high energy electron beamused in AES can cause more surface damage thanX-rays used in XPS. However, both the methods..being high vacuum techniques, can give informationonly under ex-situ conditions.

We have applied these techniques for characterizingsome mineral surfaces. Information gathered fromthese, coupled with that from morphological studieswith scanning electron microscopy (SEM), has beenhelpful in revealing surface chemical composition(Roussev and Somasundaran, 1986; Somasundaranand Moudgil, 1979; Kulkarni and Somasundaran, 1976).For Instance, grinding of litharge with sulfur canchange the morphology of the particles as shown bySEM. 'n1iS is attributed to the formation of galena onthe litharge surface (Roussev and Somasundaran,1986). Direct proof for the latter hypothesis is obtainedfrom XPS, which can identify sulfur on the surface inthe sulfide form.

Application of XPS in a surface science laboratorymay commence with the examination of solids forpurity prior to experimentation. This may be illus-trated with the calcite-apatite system. Since thebinding of one mineral on the other can be expected tohave detrimental effects on the selective flotation ofthese minerals, they were studied individually aftertreatment in the supernatant of the other. Figure 16shows the XPS spectra of pure calcite and apatite. Theabsence of any extraneous peaks is an indication of thepurity of these minerals.

The effect of calcite supernatant on apatite at pH 11may be understood with reference to Fig. 11, where itsXPS spectrum is com~d with the XPS spectra ofindividual minerals In water under identical condi.tions. The former spectrum contains the features of

ThIs work is the first reported indication of varia.tions in microviscosity within a heminilcelle as esti.mated by any known technique (Watennan etal..l986;O1andaret aI., 1987b). Thus, ESR spectroscopy with

78 MAY ,- MINERALS N«J METALWRGICAL ~

the latter. suggesting the fonnation of a layer of Cacoson the surface of the apatite. But the Cacos layer maybe less than 100..\ since features of apatite also are

obvious in Ute XPS spectrom and since XPS cannot seeUlrough more than the 100 A thick layer of the surface.This observation is reinforced by the fact that thephosphorous peak in apatite is reduced in intensity asa result of its surface coverage by CacoI, This Isrepresented in Fig. 18, The effect of carbonate speciesalone in the supernatant of the foregoing systemmakes aninterest1ng case study. EarUer experiments(Amankonah, 1~) based on electrokinetics, dissolu-tion behavior, and equilibrium pH of the systemindicated that the effect of carbonate addit1onto theaque""~"'-"""""" asto . O' l ."".t ,;.., "".,J-' t... " urf. '~"J..~ ~~I:"'-"~L~.. Yt L. . .~. - -'- ~J:'-- ~ ~ "'--

into calcite under high pH conditions. XPS reaultsshown at dltferent KaCOI concentrations reveal thatcarbon 19 peak of apatite, due to KaCOI addition, Istransformed into carbonate as if It were in a calciteenvironment. This is shown in Fig. 19.

133.5 PIp

-'.t&.0dz>"..-U)Zt&J..-Z.-I

BINDING ENERGY. eV

Fig. 16 ~ Reference spectra of pure calcite and apatite(untreated samples).

140 135 130 125BIND1NG ENERGY. eV

120

Fig. 18 - Effect of calcite supernatant on apatite phosphorus2p peak intensity.

-Ut1&100~~U1&1-J1&1

~~wc2~Z

>-~

U)Z1&1~ZH

~-~ ~

CALCITE IN WATER

Fig. 19 - Effect of K2CO3 addition on the intensity and shapeof apatite carbon 15 (CO3--) peak.

295 The problem of selectivity in mixed mineral systemsdue to the presence of dissolved mineral species maybe further seen by e~g the examples (Acar.1985) of certain sulfide minerals: chalcopyrite. pent-1andite. m11lerlte. pyrrhotite. and coveUlte.

290 280 2752.85BINDING ENERGY.. eV

Fig. 17 - Illustration of the surface conversion of apatite(conditioned in calcite supernatant) Into calcite.

MINERALS AND METAWIRGICAI. PROCESSING MAY 1988 77

XPS spectrum of chalcopyrite conditi()ned In pent-landite supernatant and rinsed with water showed thatnickel Ions can be found on the surface of chalcopyrite.'l111s Is shown In Figs. m and 21, which show XPSspectra of pure chalcopyrite and chalcopyrite condi-tioned In pentlandite supernatant. On the other hand,spectrom of pentlandite conditioned In chalcopyrite$Upernatant and rinsed with water also showed thatcopper Is present on the pentlandlte surface (FIgs. 22and 23).

~z0~t-U&4J;..J&4J

~0

~wm

1:=>z

.

j

BINDING .eV

Fig. 22 - XPS spectrum of pentlandite.

(Fe.Ni)gSe f chalco sup

10066 806 6 6066

ENERGY.eV

AnZ0Ct:.-UI.IJ-JI.IJ

LI..0

Q:I.IJm~::>Z

BINDING

Fig. 20 - XPS spectrumot chalcopyrite.

CuFfS: Ipent. sup\U~.

,,

I/)z0a:..-

(jLAJ-J~

I.:-0

~~aJ

~:JZ

,j

.,

~10066 806.6

ENERGY

6066

,eVBINDI NG

Fig. 23 - XPS spectrum of pentlandite conditioned inchalcopyrite supematant.9J66 8316

Treatment of millerite with FeClz solution showsthat ferrous ions are electrostatically adsorbed ontothe millerite surface. These are depicted in Figs. 24and 25. Similarly. millerite conditioned with CuClzsolution gives evidence for the adsorption of copper onmillerite. Other minerals also showed comparablebehavior: covellite treated with FeCiz showed iron incovellite spectrum; covellite conditioned with NiClz

1006.6 8066 6066

BINDING ENERGY. eV

Fig. 21 - XPS spectrum of chalcopyrite conditioned inpentlandite supernatant.

MINERAlS AND METALLURGICAL p..ocESSlNG78 MAY 1-

p ve ev1dence..for nickel in covellitespectrum; pyrrho-tite spectra after conditioning wiUt O1Cla and NIClzsolutions and rinsing gave evidence. respectively. forcopper and iron on Ute surface of pyrrhotite.

'nlese observations have direct be&ring on the floc-culation behavior ot minerals. This ~y be elaboratedfor the case of millerite. Flocculation ot millerite withpolyethylene oxide is enhanced by CuClz. On the basisot the foregoing discussion, one can understand thisbehavior as due to the formation ot the partly hydro-phobic covellite on the surface of millerite.

In conclusion, XPS studies along with other measure-ments show that dissolved mineral species can interactwith the mineral surface, causing surface conversionsand thereby drastic alteration of their behavior inprocessing systems. Moreover, such sluUles can aSsl£tin controlling and regulating the reagents used inmineral processing. .tn

Z0Ct:"'"'UI.&J.oJI.&J

11-0

Q:I.&Jm

~~z

Acknowledgments

The authors are thankful to the National ScienceFoundation and the US Department of Ene~ for theirfinancial assistance. JTK thar1ks BARC, India:. forgranting extraordinary leave of absence.

References

Fig. 24 - XPS spectrum of millerite.

NiS,F.

InZ0Q:t-V\JJ-JW

lI..0

a:1IJm1:-:JZ

9=56

1006.6 8d6.6 606.6

ENERGY.eV81 NDING

Acar. s...1W5. DESc Thesis, CoI~ia Un~tY. N- YOtIc.AmankOMll, J.O~ 1W5. PlIO The$iS, Columbia Uni..,.ily,N- YOtIc.Arora. I(.S~ and Tuno. N.J~ 1.7, Joumal of ~m.f ScieIIc.. (8). Vol. 25, p~ 2~.Chand8f, P.. Som8StIndaran, P.,..,d Tuno, N.J~ I~, Journal of Colloid andInt.,,- Sci_, Vol. 1fl,p. 31.Chandar, P.. SOInaSundaran, P~ Tuno. KJ.. and Waletman, K.C.. 1.7a, l.angmutf,Vol 3, Po 298.Chandar, P~ Somasundaran. P., Waterman, K.C., and Tuno. N.J., 1987b.Joumalof PllY$icaJ Chemi$Iry. VOl. 91. p. 150.Czanderna, A.W.. 1975, Melhod$ 01 Surlace Analysl$, Elsevier, Am$terdam.Demas, J.N~ 1~, Exci,.d Sta,. LIfetime M"$Ute_ts. Academic Pr8$S.N.wY OtIc.H_ell, JH.. Hoskins, J.C.. SclMCter, A.S., and Wade, W.H.. 1985. Langmuir.Vol1.p.251.Hayden, 8.E~ 1.7. Vibrational sp.ctroscopy 01 Molecules on Surfac.s. J. TYates. and T.~ Madey, eds., Plenum Pres.. New York.Hough. D.B., and Rendall. H.M.,1~,Adsorprton from SolutIon at tile Solid.LlquidInterlac-. G.D. Parfitt and C.H. Rochester. eds.. AcademiC Press. New Vorl\.

Inlelta, P., 1979, Chemic8t Physics Lett.rs. Vol 61. p. 86.Kulkarni. R.D.. and Somasundaran. P.. 1976. Powder Technology. Vol t4. P 279lakowicz. JA., 1983. Princip/e$ of Fluore$c.nc. Spectroscopy. Plenum. New York.Langmuir. I.. 1916. Journal of Amencan C"~icaISocie/y. Vol. 40. p. 1361.Lipatov. YS.. and Sergeeva, L.M. 1974. Adsorption orPolym.rs. John Wiley &Sons. New York.Mor_etz. H., 1975. M.cromolecu/es in Solution. 2nd ed., John Wiley. New YOrk.Ohnishi. S., and McConnetl. H.. 1965. Jou"..1 of Am.riCan Ch.mical SOCI./y.VOl 87. 11.2293.Ranby, B~ 1977. ESR Spectro.coPY in PoIym.r R4searcll, J.F.Aabek, ed- SilringerV.rlag, Berlin.Rob8f1a. W.M., '~1. Chemistry in Britain. Vol. 17, II. 510.ROussev. fl. and Sornasundalan. P., 1986, Particulat. Science .nd Technology.Vo!..4, II. 305.Seigbahn. K.. 1967. ESCA. Almquist and Wiksells.Ullllsaia, Sweden.Soma$undaran. P~ Healy. T .W.. and Fuerstenau. D.W. 1~. Journal 01 PllysicalChemi$try. Vol 68. II. J562.Somasundaran. P, and Fuer$tenau. O.W.. 1966. Jour~I 01 Physical CIIemlslry.VOl. 70. II. 90.Somasundarall. P.. 1975. "Interfacial Chemi$try 01 Particulate Flolatiott;.Advanusin Interfacial Phenomena 01 Par1iculate/SolutionlGas systems, P. Somasundaranand R.B. Grieves, eds., AlChE, SympoSium Series 71, PIl1.15.Somasundatan, P., and Moudgil, B.M., 1979, Surface Contamination, Vol I,K.L Mltla!. ed., Plenum. New York.Somasundaran, P., Turro. N.J.. and Chand.,. P.. ,_, CollOids and Surtacft,Vo!..20. p. 145.Takenaka. T...1979, Advances In Colloid Intertac. Science. Vol. 11, p 291.Thomas. J.K., 1984. "The Chemistry 01 Excitation at Interfaces. ~ American

Chemical Society Monograph.Turro, N.J., Cox, G.S. and PacZkOwski. M.A., 1985. .'PtIotoctlernlslry in Micelles;'Topics in ~nt Ch.mistry. Vol. 129.Waterman. K.C.. Tuno. N.J~ Chand." P.. and Soma$undaran. P.. 1988. Journal 01P/lySi~1 ChemirffY. Vol. ~ pp. 6829.Weber, G., 1972. Annual R.view o7'Biophyslcs .nd 8;o.nginftring. Vol. 1, p. 553.Wertz. J.H., and Bolton. J.A.. 1986, Electron Spin Resonance - ElementaryTheOly and Pf3Ctical Application., Chapman and Hall, N- YOtIc.

Fig. 25 - XPS spectrum of millerite conditioned In 10-2kmol/m3 FeCI2 . 4H2O.

MAY'~ 79MNERALS AND METALlUAGK:AL PftOC£SS-.G

Pradip

Ab8tract - Chelating agents, beooU.!e of their""""rrl ~7""'ificity, CI:t'1 fi""'"fil»1 ll.'t Rffective m.inp"(l~processing reagents. A brief review of the workcarried out in thia area ia presented. Some oj theaalient feature.! of mineral flotation and flocculationwith chelating-type reagenu, such 0,8 bulk v.!. surfacechelation, the role of pH, mineral .!olubility, .!Olutionchemiatry of reagent.!, and temperature, are iU1.&.9tratedwith appropriate e%ample.!. The problem.! a.!.!ociatedwith the commercialization of chelating reagent3 forore beneficiation and the re.!earch e/l0rt3 required inthi.9 area are also di.9C1.&.93ed.

Introduction

A number of successful attempts have been reportednn developIng tA1Inr-rnRI1~ rp~f!~nts With ('hplAt!n~funcuonal groups for specific applications. In UU'Spaper, the published work on the application ofchelating reagents In mineral processing has beenreviewed, with special emphasis on identifying certaincommon trends observed In such systems. The ulti-mate objective of researchers In this area sWlremalnsto be the development of a comprehensive theory ~tcan help practicing engineers to a priori select theappropriate reagent combinations for the parlicularseparation problem at hand.

Chelating agents and chelation

A chelate-forming reagent'" must hav~ at least twoatoms that can be coordinated by the metal at tltesame time. Such atoms are usually oxygen, nitrogen.sulfur, and phosphorus. The "coordinating" speciesproviding these donor atoms is known as the . 'ligand. "When more than one atom of a single ligand moleculeor ion Interacts with the metal ion.. it may be presumedto bend itself pincerlike around the central atom toform a complex ring structure Called a .'chelate"(from the Greek for claw). The ligands arce refelTed toas bidentate, tridentate, tetradentate, pentadentate,or hexadentate according to their respective attach-ments In two, three, four, five, or six positions In thesphere of coordination around a positively chargedmetal ion. An example of (1: 2) chelation of a bidentateS-S type ligand of the diethyl dithlo carbamate reagentwith nickel is shown below. Here, all the four Ni-Sbonds are equivalent, and, therefore, the chelate ringhas a flat, square-shaped configuration (Zolotov, 1970).

'

i.~'

," ; -"..."

:?,"

Chelatirtg agents can be classlfled on the basis ofdonor atoms involved (0-0, N-O, N-N, S-S, N-N) orring size (4-, 5-, or 6-membered) or charge on thecomplex (anionic, cationic, or neutral) or the numberof bonds to the metal for every chelating molecule(Somasundaran and Nagaraj, 1984).

To form chelate com~ds, the reagent must haveat least one active group With a mobile hydrogen atom(acid group) that is substituted by the metal duringcompound formations. Examples of such groups are-OH. -COOH, -gOsH, -AsOsHa, -POsHa. -NH,- NHa, - NOH, and - SH. The other groups may beacidic or. more often, basic, usually gI'9Upings like=0, -OH, -0-, -N=, -NHa, =NOH, =S, -S-.The locations of these groups in the reagent moleculeshould allow formation of at least one closed ring(Zolotov,1970).

Froth flotation is one of the most widely usedseparation processes for beneficiating low-grade ores.The key element responsible for the success of aflotation separation flowsheet is the choice of thereagent combination (collector-activator-depressant)used for achieving the desired selectivity. With steadydepletion of rich and medium-grade ores, the exploita-tion of more complex and disseminated ore reserveshas become essential. This calls for iImovative meansof concentrating desired minerals at relatively finersizes and/or separating them from a number ofassociated minerals havingsimllar surface properties.Flotation separation in such complex ore systems ispossible only with highly selective collectors.

A promising technology for achieving mineralseparation at a subsleve size range of particles hasrecently been developed. The process, known as selec-tive flocculation, is based on the preferential adsorp.-tion of a polymeric flocculant on Ute desired mineralsurface causing its aggregation while leaving Uteremainder in suspension. The success of a selectiveflocculation scheme largely depends on a judiciouschoice of a flocculant-dispersa.nt combination. Exceptfor the successful use of starches in beneficiatinglow-~de taconItes at Cleveland Cliffs.. Ute workcaoied out Utus far with commonly available reagentshas not shown encouraging results (Yu and Attia, 1987).Efforts are underway to design, synUtesize, and testnew or modified polymers having desired functionalgroups for specific applications.

The basic surface chemical principles underlyingboth these processes, namely flotation and selectiveflocculation, are very similar. The selectivity ofseparation in eiUter case is primarily controlled by UteselecUve adsorption of reagents at Ute mineral/waterinterface. It is essentially a function of Ute affinity ofUte reagent funcUonal groups for the adsorption siteson the mineral surface. ChelaUng-type functionalgroups, because of Uteir high specificity towardcertain metal ions, Utus appear to be ideal constituentsof mineral processing reagents.

Selectivity of chemical groups in chelationPradlp is with Tata Research Development and Design Centre,Pune, India. M&MP paper 88.624, Special Topics In MineralProcessing Seminar, Pune, India, Dec. 30, 1987. Jan. 1, 1988.Manuscript April 1988. Discussion of this paper must besubmltted,ln duplicate, prior to July 31,1988.

There have been a number of monographs publishedon the stability of chelate compounds and the factorsgoverning the specificity of certain chemical groupsfor cations (Martell and Calvin, 1952: BaUar and

MINERALS ANa METALWAGK:AL PROCESSING., MAY 1-