Behavior of Colloidal Suspensions of Zinc Carbonate in the Presence ofCopolymers Designed for Selective Flocculation

S. QUARESIMA,*"K. SIVADASAN,t., A. MARABINL*M. BARBARO,* AND P. SOMASUNDARANt,2

*CNR, IslitUIO Trallamenlo Minerali, Via Bologf!Qla 7, 1-00138 Rome, Italy; and tLangmuir Center for Colloidsand Interfaces, Henry Krumb School of Mines, Columbia University, New York, New York 10017

Received April 26.1990; accepted November 28. 1990

The flocculation / dispersion behavior of smithsonite fIDes in the presence of copolymers tailor-madefor selective flocculation is investigated. Three copolymers of ethylene glycol methacrylate ethylenedioxybenzoate (EGMEDB) with acrylic acid (BE-39, EGMEDB 20%; BE-44, EGMEDB 10%) andmethacrylic acid (BE-48, EGMEDB 10%) are used. BE-44 and BE-48 showed entirely opposite effectson the colloidal stability of smithsonite suspensions though their chain length and copolymer compositionswere identical. At high pH values an increase in the dispersion power of the copolymer is observed andthis is attributed to electrosteric stabilization of the fines by the polyelectrolytes. The measured zetapotential of the colloidal particles fails to explain the different effects of copolymers on smithsonitesuspensions. fluorescence spectroscopic investigations showed these copolymers to have a hypercoiledconformational state at low pH values due to intramolecular association of the hydrophobic groups onthe polymer chain. At pH 7 and above, the acrylic acid copolymer, BE-44, unfolds and exists as anextended chain. However, an increase in the catecholic group (EGMEDB ) content on the polymer chain,as in the case of BE-39, increases the association of the hydrophobic groups and its conformationalproperties are similar to those of methacrylic acid copolymer. The copolymers whicll flocculated smith-sonite suspensions showed the presence of hydrophobic rnicrodomains in the adsorbed layers of copol-ymers. The difference in the behavior of colloidal suspensions in the presence of structurally identicalcopolymers, BE-44 and BE-48, is attributed to such differences in the conformational states of the co-polymers. @ 1991 Academic Prcss. Inc.

INTRODUCfION

The economic as well as the environmentalproblems associated with the recovery ofmin-eral fines provide the impetus for devising ef-ficient processes for the treatment of fine par-ticles. Selective flocculation is a promisingtechnique (1-3) for the beneficiation of ul-trafine minerals. Selectivity is achievedthrough interactions between mineral surfacespecies and specific functional groups of thereagents. The choice of proper selective floc-culants is usually made on an empirical basisfrom commercial polymers of natural or syn-thetic origin. In most cases the selectivity in-

I On extraordinary leave of absence from the Chemistry

Section, Oil and Natural Gas Commission, Bombay, India.2 To whom correspondence should be addressed.

159

0021-9797/91 $3.00CopyriIIIt@ 1991 by Al:adtmic PI-. Iec.All richls of ~ in any form -*.~...~-~ s v~ 144, No. I.lu~ 1991

dicated from a combination of tests with pureminerals is not confirmed by tests with mineralmixtures or raw ore (4). The traditionalmechanism of molecular bridging is insuffi-cient to explain this flocculation behavior andit is necessary to take into account the inter-actions of the dIssolved mineral species withpolymer and the resultant conformation ofpolymer molecules at the interface and its ef-fect on selectivity.

There have been very few theoretical inves-tigations of problems related to the morphol-ogy of polymeric flocculants. Recently, a sim-plified theoretical model of the structure oftailor-made polymeric flocculants has beenproposed (5 ). This is based on structural pa-rameters that can be optimized for each min-eral and correspond to three different chemical

160 QUARESIMA ET AL

functions that must be present on the poly-mers. The three parameters of interest are:

( I) a functional group that can selectivelyinteract with the mineral particle,

(2) pH-sensitive hydrophilic groups whichcontrol solubility as well as the overall con-formation of the polymer chains, and

(3) a random distribution of functionalgroups which assures uniform composition ofthe polymer chain.

In the present work, the flocculation / dis-persion of smithsonite fines in the presence ofcopolymers tailor-made for selective floccu-lation is investigated. Based on the character-ization of the adsorbed polymer layers and theelectrokinetic properties of the particles, anattempt is made to explain the behavior ofsmithsonite suspensions in the presence ofstructurally similar copolymers. A fluorescentprobe which reports the micropolarity of itsenvironment is used to obtain information onthe conformational states of the copolymersin the bulk as well as the adsorbed layer.

EXPERIMENTAL

TABLE I

HomopolymeR and CopolymeR Used in This Study..1

CojK)Iymercomposition

(~ of EGMEDB)Decree of

poIymeriDIiOD~~.;;

AA/IO (EGMEDB/AA) 10.10 2100AA/20 (EGMEDB/AA) 20.20 7100MAllO (EGMEDB/MA) 9.93 2200PAA (homopolymer) - 2100PMA (homopolymer) - 1800

Pure smithsonite mineral from the Lampingdeposit, China. was wet-ground in a porcelainmill and the minus 15-Itm fraction was usedfor all experiments. Copolymers and homo-polymers were prepared by Professor V. Ber-tini and co-workers at the Organic and Mac-romolecular Chemistry Laboratory of theUniversity of Calabria, Italy. The random co-polymers were synthesized from homogeneousmixtures of monomers using azobisisobu-tyronitrile as the initiator. The procedure isdescribed in detail elsewhere (5). The catecho-lic-functional-group-containing monomer isethylene glycol methacrylate ethylene dioxy-benzoate (EGMEDB) and the comonomeris either acrylic acid (AA) or methacrylicacid (MA).

The copolymers after fractionation werecharacterized by IR and NMR spectroscopy.MAllO (MA/EGMEDB 10%) is a copolymercontaining about 90% methacrylic acid groups

Journal o{Co/loid and llfler{«e Science. Vol 144. No 1. June 1991

while AA/20 (AA/EGMEDB 20%) and AA/10 (AA/EGMEDB 10%) are copolymers withabout 80 and 90% acrylic acid groups, respec-tively, on the polymer chains. The exact c0-polymer compositions and degree of poly-merizations are listed in Table I.

The sedimentation tests carried out weresimilar to those reported earlier ( 6). The floc..culating power (F) or dispersing power (D) ofeach copolymer was calculated with respect topure smithsonite in the absence of the polymerunder identical experimental conditions.

The zeta potential was measured using aRank Bros. Mark II apparatus with a flat celland platinum electrodes.

Micropolarity tests were done with pyrene-containing (ca. 2 X 10-7 mol/liter) polymersolutions and polymer / smithsonite suspen-sions using the steady-state fluorescence emis-sion method (7). Test samples were preparedby adding aqueous saturated solutions of py-rene to polymer stock solutions or smithsonitesuspensions. One-half gram of smithsonite wasequilibrated for 24 h with 10 ml copolymer /pyrene solution. The concentration of the c0-polymer was adjusted to 100 ppm. The sus-pension was centrifuged and the supernatantremoved. The steady-state fluorescence emis-sion spectra of pyrene were recorded on a SpexFluorolog spectrofluorometer. Spectra forslurry samples were measured as emissionsfrom the front face of the cell at an angle of22.5° from the incident light. The two mono-mer emission bands of interest in the fluores-cence emission spectra were at 373 nm (I,)

161SUSPENSIONS OF ZINC CARBONATE

m;-48

and 384 nm (/3)' Ratios of their intensities,/3/ / I, were calculated from the spectra.

Triple-distiUed water or high purity waterfrom a Millipore water system, Milli Q, wasused for aU the experiments. Polymer solutionsand smithsonite suspensions were prepared in10-3 MNaCI solutions.

01£-..

~

RESULTS AND DISCUSSIONS

Sedimentation results obtained for tests withsmithsonite suspensions containing 50 ppm .

0 20 81 100 I--copolymers or homopolymers at pH 7 and 9 Co.. ~".L" ~y~. """are presented in Table II. Low pH solutions FIG. 1. The uta potential ofsmitbsonite as a functioncaused dissolution of the zinc carbonate and of roncentmtion oftbe copolymer.have therefore been avoided. As can be seenfrom the table, both acrylic acid and meth-acrylic acid homopolymers have a weak dis-persing effect on smithsonite. The copolymershad the same catecholic functional groups(EGMEDB) randomly distributed on thepolymer chains. The copolymers BE-44(acrylic acid copolymer) and BE-48 (meth-acrylic acid copolymer) were of comparabledegrees of polymerization and also had thesame copolymer compositions (EGMEDB10%). Most interestingly, their effects on thecolloidal stability of smithsonite suspensionsare entirely opposite those on each other. Thecopolymer with 10% acrylic acid has a dis-persing effect while the copolymer with 10%methacrylic acid has a flocculating effect. AtpH 9 the polymers in general increase the dis-persing power or decrease the flocculatingpower.

~ 60 '"

TABLE II

flocculation/Dispersion Behavior of SmithsoniteSuspensions in the Presence of 50 ppm Homopolymer orCopolymer

The choice of the specific functional groupthat interacts with the mineral particle wasmade on the basis of published data (8,9) onthe thermodynamic stability of the complexesbetween various known ligands and metallicions of interest. The efficiency of catecholicgroups as selectively interacting functionalgroups has been tested and found satisfactoryfor various minerals such as chalcocite, rutile,quartz, and calcite (5). In the case of smith-sonite also, the specific interaction of cate-cholic groups with mineral particles is evidentfrom the observed effect on the dispersing andflocculating powers of the copolymers in com-parison with those of the homo polymers. Theflocculating/ dispersing effect of homopoly-mers on the mineral is not very significant asthese polymers contain only the functionalgroups which interact mainly with the solvent.On the other hand, the copolymers behave asgood flocculants or dispersants due to the spe-cific interactions of the catecholic functionalgroups with the mineral particles.

Zeta-potential values obtained for smith-sonite in the presence of BEM and BE-48 areplotted in Fig. 1 as a function of polymer con-centration at pH 7. Even at a very low con-centration of 10 ppm, adsorption of the C0,-polymers on the positively charged mineralsurface results in its charge reversal and sug-gests chemisorption of the copolymer mole-cules through forces that are nonelectrostatic

Journal ofCol/oid aJId /lIlt'lj"Dl:rSdmce. Vol 144, No I, June 1991

Rocxulalinc (F) or dispenins (D)

power

Name of Ute poly-. pH 7.0 pti9.0

PAAPMAAA/IOAA/20MAllO

D8.SOD36.0D63.6F 55.3F 81.8

D II.SO

D 100

F 51.78

162 QUARESIMA ET ~

in nature. Because of the dissociation of car-boxylic acid groups of acrylic and methacrylicacids, the particles covered with the polymermolecules are negatively charged at neutraland alkaline pH conditions.

Figure 2 shows the effect of pH on the zetapotential of pure smithsonite and smithsonitetreated with copolymers. Pure smithsonite hasa positive zeta potential (+40 mY) at pH 7and is an isoelectric point of pH 8.6. At pH7, smithsonite treated with BEM is highlynegative (-32 m V), whereas smithsonitetreated with BE-48 is less negative ( -16 m V).It has been reported ( 10) that flocculation byinterparticle bridging is possible up to a zetapotential of 15 to 20 m V and hence, the floc-culation behavior of a smithsonite suspensionwith BE-48 is expected. As the pH of the sus-pension is increased, both copolymers furtherincrease the negative ~tentials of the particlesand the zeta potentials become identical at pH9 ( - 35 m V). However, no drastic change inthe flocculation / dispersion response of col-loidal suspensions is observed, suggesting thatthe electrokinetic properties alone cannot ex-plain this behavior. At pH 9, the copolymer-treated colloidal suspensions become moredispersing. This is attributed to electrostericstabilization ( II ) which is a combination ofelectrostatic and steric stabilization as the co-

"" ---0 ~ aJ'(L YIER

6 ~-48

0 ~-44-'~~w

~a:!ii

~~

-00

:;;,:,;~,~,.-'..,::~:.;i"1t: 12r*i

FIG. 2. The zeta potential ofsmithsonite in the presenceand the absence of copolymers as a function of pH (co-polymer concentration, 200 ppm).

...tf'~-'l~Scw-. Val 144, No. I,June 1991

polymers used are basically polyelectrolytes.The dissociation of carboxylic acid groups in-creases the electrostatic repulsions between theparticles covered with polymers and also en-hances the steric stabilization due to the in-creased hydrophilicity of carboxyl groups athigher pH.

Since the electrokinetic properties of theparticles failed to explain the conflicting fea-tures of the colloidal suspensions in the pres-ence of two structurally identical copolymers,it became clear that the conformational char-acteristics of the copolymers at the interfacemust play an important role in these systems.i 0 determine this role, we obtained a descrip-tion of conformation of these copolymers atthe interface as well as in the bulk.

Synthetic polyelectrolytes possessing pen-dant hydrophobic groups often show a markedpH-induced conformational transition. Thisbehavior in bulk solutions has been extensivelystudied and confirmed for poly( methacrylicacid) and its copolymers (12, 13). The poly-mer molecule exists in a compact form at lowpH and in a more expanded conformation athigh pH. In aqueous solutions, the hydropho-bic interactions among the methyl groups ofthe polymer lead to the intramolecular ass0-ciation of methyl groups and the resulting hy-drophobic microdomains stabilize the com-pact ~lymer structure. As the pH is increasedcoulombic repulsions between the ionized acidgroups ov~rcome the forces which stabilize thecompact form of the macromolecule and thesystem undeIEoes a conformational transition.Though poly(acrylic acid) does not show asimilar behavior, some of its copolymers hav-ing pendant hydrophobic groups do exhibitpH-dependent conformational changes ( 12).The stability of hydrophobic microdomainsand the pH at which the conformational tran-sition occurs in such systems are functions oftheir copolymer compositions.

Information on the conformational statesof the copolymers !s obtained by monitoringincorporation of a fluorescent probe into theirhydrophobic microdomains. Pyrene is a probe

163SUSPENSIONS OF ZINC CARBONATE

TABLE UI

The Ratio, lJil" of PyIene in the Adsorbe4 Layers ofCopolymers at the Interface (Copolymer Concentnttion,l00ppm)

1.11, ofpyRneiD dIC~pCIIymer layer

C-"ymer pH7 pH'

ANIOMAllOAN20

0.600.740.70

0,580.71

that prefers to be in hydropbobic environ-ments and its spectral characteristics are de-pendent on its microenvironment. As the py-rene molecule experiences a change in its en-vironment, the relative intensities of the thirdand first vibrational bands of the monomeremission are affected. The ratio, 13/1., is ameasure of the effective polarity of the me-dium 3round the pyrene molecule.

The pyrene fluorescence parameter, 13/1.,is plotted in Fig. 3 as a function of pH forsolutions (200 ppm) of various copolymers.The value for pyrene in pure water is 0.56 andis unaffected by changes in pH. At lower pH(pH < 7), all three copolymers have high 13/I. values, indicating the presence of hydro-phobic microdomains on the chains. The c0-polymer molecules exist in a hypercoiled con-formational state due to the intramolecularassociation of the hydrophobic groups. As thepH is increased, the 13/ I. value decreases andthis decrease is a consequence of the unfoldingof the copolymer chain, which reduces thenumber of hydrophobic sites. BE-44, whichhas comparatively lower 13/ II values, showsno hydrophobicity at pH 7 and above (the 13/I. values are the same as those in water). Onthe other hand, BE-48 is stroQgly hydrophobicat pH 7 and shows the presence of hydropho-bic guest sites for pyrene even at pH 9. Onlyat pH II the 13/ I. value corresponds to thatofpyrene in water. BE-39, the acrylic acid co-

polymer with a higher molar ratio of ca,techolicgroups on the chain (EGMEDB 20%), alsoshows conformational characteristics similarto those of BE48. Thus, the methacrylic acidcopolymer and the acrylic acid copolymer withan increased nuD)ber of catecholic groups onthe chain produce hydrophobic microdomainsunder the experimental conditions of sedi-mentation tests.

13111 values obtained for pyrene in the ad-sorbed layers of copolymers at the smithson-itelwater interface at pH 7 and 9 are presentedin Table III. BE-39 and BE48 show high 131I, values, indicating the presence of hydro-phobic domains in the adsorbed layers. In thecase ofBE-44, the values are very close to thoseof pyrene in pure water. Moreover, the ob-served monomer emission intensities werevery weak, suggesting very low pyrene con-centration in the adsorbed layers.

The results show two different conforma-tional states for BE44 and BE48 at thesmithsonite 1 water interface. BE44 is ad-sorbed at the interface in an extended chainconformation while BE48 retains its hyper-coiled conformational state at the interface.The hydrophobic microdomains at the inter-face are proposed to enhance interparticularinteractions which result in smithsonite floc-culation.

a~-~~

6 ~-:»

O~-~

.sLATER

C.5. 6 8 10 12~

FIG. 3. The characteristic nuor~nce parameter, IJ/1'0 of pyrene in the presence of copolymers in solution(200 ppm).

CONCLUS[ONS

Properties of colloidal suspensions of finesmithsonite in the presence of synthetic c0-polymers are examined. Sedimentation testsconfirmed the presence of selective inter-

Journal (){Colioid and Inlerjaa. Science, Yol. 144, No I, June 1991

164 QUARESIMA ET AL

actions of ethylene glycol methacrylate ethyl- REFERENCESene dioxybenzoate (EGMEDB) groups withsmithsonite mineral. Importantly, the meth- I. Sresty, G. C., and Somasundaran, P., Int. J. Miner.

I. .d I ed fl u1 Process. 6, 303 (1980).acry lC aCl copo ymer act as a occ ant 2 A . Y A I d Ki h J A . "Dr.., A;~ft.hil h I. .d f . tba, . .., an tc ener, . ., In . """"""'-'wet e acry lC aCl copolymer 0 the same IlthlntemationalCo~onMineral~ng,copolymer composition was a dispersant. Cagliari, 1975" (M. Carta, Ed), p. 1233.When the catecholic group (EGMEDB) con- 3. Baudet, G., Morio, M., Rinaudo, M., and Nematol-tent in the acrylic acid copolymer was in- Iahi, H., Rev. Ind. Min. MetalI. 78( 1),19 (1978).creased it also behaved as a flocculant. The 4. Sha~ng, Y., and Attia, Y. A.: in "Aoccu.~tion infl I . d '. . . BIotechnology and Separation Systems (Y. A.

occu anon/ lSperslon behavior of slnlth- Attia, Ed.), p. 601. Elsevier, Amsterdam, 1987.

sonite suspensions in the presence of these co- 5. Bertini, V., Marabini, A., De Munno, A., Pocci, M.,polymers cannot be explained on the basis of and Barbaro, M., in "Aocculation in Biotechnologyelectrokinetic properties alone. Fluorescence and Separation Systems" (Y. A. Attia, Ed.), p. 247.

b. h ed th f h dr h b. Elsevier, Amsterdam, 1987.pr? 109 s 0':'" e p~esence 0 y Op 0 lC 6. Bertini,V.,Pocci,M.,Marabini,A.,Barbaro,M.,andmlcrodommns at the Interface when the co- De Munno, A., Part. Sci. Techno/. 4, 203 (1986).polymers behaved as flocculants. The inter- 7. Kalyanasundaram, K., and Thomas, J. K., J. Am.particular association through the hydropho- Chern. Soc. 99, 2039 ( 1977).bic microdomains of the adsorbed polymer 8. Sillen, L G., and Martel\, A. E., Spec. PubI. Chern.I . ed be h r f1 Soc. 2S, 865 (1970).aye~ IS sugg~ to. t e r~n lor the oc- 9. Marabini, A., Barbaro, M., and Ciriachi, M., Trans.culanon of smrthsornte suspenSions. Inst. Min. MetaiI. Sect. C92, (1983).

CKNO 10. Tilton, R. C., Murphy, J., and Dixon, J. K., WaterA WLEDGMENTS Res. 6, 155 (1972).

This work was carried out under a U.S.-Italian coop- II. Napper, D. H., "Polymeric Stabilization of Colloidalerative researeh program supported by the U.S. National Dispe~ons:' Academic Press, New York, 1983.Science Foundation (NSF-INT-84-15103) and CNR of 12. Bekturov, E. A., and Bakauova, Z. Kh., "SyntheticItaly. P.S. and K.S. also acknowledge the financial support Water-Soluble Polymers in Solution, " Hiithig &

from the Minerals & Primary Materials Processing Pro- Wepf, New York, 1986.gram of the NSF(MSM-86-17183) and NALCOchemical 13. Olea, A. F., and Thomas, J. K., Macromolecules 22,company. K.S. thanks ONGC, India, for granting an ex- 1165 (1989).traordinary leave of absence. 14. Thomas, J. K., Chern. Rev. SO, 283 (1980).

,J.

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