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PROCEEDINGS OFTHE 7TH INTERNATIONAL MINERAL PROCESSING
SYMPOSIUMiSTANBUL/TURKEY /15-17 SEPTEMBER 1998
Edited by
Suna Atak, Giiven anal & Mehmet Sabri
-
Innovations in MineralandCoa/Processing, Atak. ()nal&
ge/ic(eds)~ 1998 BaJmIma. ftJtt8Idam.ISBN9058090132
Aocculation and selective flocculation - An overview
P. Somasundaran & Kalyan K. DasNSF 11Uc.'R c.'enter for
Advanced Studies in Novel Surfactant\'. Henry Krumb School of
Mine.\'. Columbia
Uni\'ersity. N. r. U.SA
ABSTRACT Flocculation/stabilization has recently become an
important step in processing offine particles inmineral
beneficiation. In today's world of high technology, the demand for
processing of fine particulates isever- increasing with more
stringent quality requirements on the finished products
Flocculation/stabilization ofcolloidal systems find widespread
applications in addition to minerals processing, in as diverse
areas asmaterials processing, food and beverage industry, health
care and personal care industries A number of factorscontrol and
govern the polymer induced flocculation/stabili7.8tion processes
This paper discusses some of theissues involved in flocculation and
presents some of the recent advances made in the field of
mineralsprocessing
1 INTRODUCTION
Treatment of the fine particulates for the recovery ofmineral
values as well as for the colloidal processingof ceramic
suspensions are of importance in fineparticle technology. This is
due to the dwindlingmineral resources worldwide as well as
ever-increasing need for manufacturing of specialitymaterials for
high technology applications Asignificant proportion of the world's
mineral valuesare lost as slimes in the mineral processing
industries(Sresty and Somasundaran, 1978; Gebhardt andFuerstenau,
1986). To optimally recover finevaluable particles, flocculation
and selectiveflocculation induced by natural and syntheticpolymers
have received increased attention in the lastseveral years
Considerable progress in bothfundamental understanding and
practical applicationshas been made due to the development of
newgroups of tailor-made polymers, more comprehensiveunderstanding
of the mechanisms involved, and bettercontrol of the operating
parameters (Somasundaranet al 1996) For several decades,
inorganicelectrolytes, natural polymers and synthetic highmolecular
weight polymers have been used ascoagulants or flocculants for
clarification of effluents,acid mine drainage and treatment of
paper, textile,
sugar and other industrial wastewater processingapplications
Research has shown that interactionsbetween the functional groups
on particulatesurface(s) and those on polymer are responsible
forthe adsorption and subsequent flocculation and canbe
appropriately controlled by tailoring polymerswith specific
functional groups suitable for theparticular application The
effectiveness of a polymerfor a given flocculation system depends
on severalparameters e.g. the polymer dosage, amountadsorbed on the
solid surface(s), molecular weight ofthe polymer, particle size,
shape, porosity, ionicstrength of the system, the electrostatic
interactions,electrokinetic properties and more importantly,
theconformation of the polymer at the solid-liquidinterface.
Polymeric reagents influence significantlythe flocculation/
stabilization behavior of particulatesuspensions The following
recent examples illustratethe diverse areas of research involving
study ofparticle-polymer interactions
The stabilizing action of sodium polyacrytate onthe colloidal
dispersion of calcite has been studiedusing viscosity, ion
concentration and electrophoreticmobility studies The stabilizing
action was evidentfrom the sharp fall in viscosity observed at low
levelsof polymer addition and the invariance of this lowviscosity
throughout the remainder of the
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concentration region (Rogan et al 1994) Besidesapplication in
materials processing industries,polymeric flocculation also finds
widespreadapplication in paper and pulp making industry Inone
interesting study involving flocculation of woodpolymers
(unbleached thermo-mechanical pulpsuspensions) with linear cationic
polyacrylamides ofdifferent charge densities (but same
molecularweight), formation of the polyelectrolyte complexeswas
observed and its role in the destabilization of thecolloidal
particles was delineated. The properties ofthe polyelectrolyte
complexes was found to dependon the charge density of the cationic
polyelectrolyte( Nylund et aI, 1995 ). In another
interestingapplication involving DNA, the flocculation ofpositively
charged latex micro spheres and negativelycharged single-stranded
DNA of varying chainlength from 3-mers up to 1400 mers
wasinvestigated The coagulation kinetics at the optimumpolymer dose
was found to be diffilsion limited Theauthors have proposed a
mechanism which dependsupon the relative spacing of charged groups
on thepolymer backbone and particle surface, the size andinherent
flexibility of the polymer in question and thenature of the initial
polymer-particle interactions(Walker and Grant, 1996)
Attempts have also been made in the recent pastto develop
predictive mathematical models forflocculation and selective
flocculation processes(Moudgil and Behl, 1992; Behl et at, 1993;
Hogg,1994) . Many mechanisms involving particle-particleand
particle-polymer interactions have beenproposed. In addition to
chemical factors,hydrodynamical conditions in
flocculation/dispersionare found to have significant effect, which
isespecially important for practical applications, wheremechanical
agitation is used to ensure proper mixingof added reagents, to aid
dispersion prior toselective flocculation and then to promote
flocformation and growth ( Hogg, 1992).
adsorption of polymer, or by using activators thatinduce
adsorption of polymers on desired minerals
(Somasundaran, 1978)Adsorption of polymers on solid surfaces
is
dependent on polymer properties such as molecularweight and
configuration, distribution of functionalgroups, solid properties
such as surface charge,ox:idation state and solution properties
such as ionicstrength, temperature and the solvent power for
thepolymer It is this dependence of adsorption that caneffectively
be used for obtaining selective adsorption
(Somasundaran, 1979) Aggregation betweenparticles can take place
if the interparticle repulsiveforces do not exceed the
corresponding attractiveforces (Figure I). Therefore, aggregation
can beachieved if the solution properties are adjusted suchthat the
electrostatic repulsive force is minimized.Dispersion can be
affected by maximizing theelectrostatic repulsive forces
The various forces responsible for the adsorptionof the polymers
on the mineral surfaces resultprimarily from three types of bonding
namely,electrostatic, hydrogen and covalent bonding (Srestyet al.
1978). The solution chemistry conditions, therheology of the solid
suspension and the polymerproperties dictate the nature of
interactions Oftenseveral mechanisms may be operative at the
sametime in controlling the adsorption/flocculationbehavior of the
system.
Adsorption of the polymers on the particle Surfaceis a complex
phenomenon which can be considered
+AEP\A.SlOH
9tERGY
0181-~MftP8rt1cl"-
ATTRACTIONetERGY2 THEORETICAL ASPECTS
2 I Adsorption of polymers a,1d .'ilIrface coverage
Polymer molecules can be made to selectively adsorbon particles
by (i) adjusting the chemical compositionof the suspending medium,
(ii) introducing intopolymer. active functional groups that will
formcomplexes or salts with the metal atoms on thesurfaces of the
desired minerals and (iii) usingdepressants such as sodium silicate
that would adsorbon the undesired mineral surface thereby
preventing
Figure I Sum of repulsive and attractive interactionsbetween
particles as a function of distance ofseparation between them under
various conditions
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polymer but remains well dispersed with the anionicpolymer
Polymers can also flocculate similarlycharged particles provided
the magnitude of theinterfacial potential is not very high Under
suchconditions, the electrostatic repulsive forces are notstrong
enough to prevent the particles from comingwithin the polymer
bridging range of each other.
to be an irreversible process particularly in dilutesuspensions
that are often encountered in flocculationsystems Generally, the
residual concentration ofpolymers in the system is only a fraction
of the initialconcentration thereby suggesting a strong
adsorptionaffinity of the polymers on the particle surfaces(Linke
and Booth, 1960) Some of the issues relatedto the bonding
characteristics in the flocculationsystem are briefly addressed
here H):dro~en bonding
Surface oxygen species on the particles may formhydrogen bonds
with the nonionic polymers therebyassisting the flocculation
process Such adsorptionmechanism is possible when the electrical
doublelayer repulsions are not very strong.
Electrostatic bonding
This is a particularly probable mechanism when thecharge density
of the polymer is relatively high andopposite in sign to the
mineral particle The surfacepotential of the solid particulates
gets neutralized bythe adsorption of the oppositely charged
polymerspecies even at a very low concentration of thereagent and
this can even lead to charge reversalEffective flocculation by
interparticle bridging canoccur even when the zeta potential is
non-zero(either positive or negative) and the
electrostaticrepulsive forces are not very high The role
ofelectrostatic forces was illustrated in the case offlocculation
of negatively charged synthetic silica(Biosil-A) with cationic,
Nalcolite-610 andstabilization with anionic polymer Separan
AP-30,Figure 2 (Sresty et ai, 1978 ) It may be noticedfrom this
figure that the silica particles undergoexcellent flocculation in
the presence of the cationic
Covalent bonding
The most widespread mechanism of flocculation maybe the result
of strong covalent bonding between themetal ions present on the
particulate surface(s) andthe active groups of the interacting
polymer (s)Flocculation of kaolin by polyacrylamides wasreported as
a result of the formation of salt-typecompounds by reaction between
the polymer and theCa 2- ions present in kaolin (Michaels and
Morelos.
1955)Sufficient coverage of the particulate surface
facilitates polymer bridging leading to a progressivebuild-up of
the fine particles into three-dimensionalnetworks which are
reported to be stabilized uponcomplete coverage (LaMer and Smelie,
1962) Theseauthors predicted existence of an optimum
polymeradsorption layer and reported a value of 05 surfacecoverage
by the polymer molecules for maximumflocculation. Subsequently, the
concept of stericstabilization due to complete coverage of
theparticles was suggested [t was reasoned that thestability could
be disturbed if the Gibbs free energydue to interpenetration of the
polymer chain isnegative (Napper. 1970) The interactions leading
tothe change in entropy and enthalpy of the free energycomponent
due to the interpenetration of the polymerchains depend on several
factors. namely, theinteractions among the polymer, solvent and
theinorganic electrolyte species (Napper. 1970; Napperand Netschey.
1971) In such cases flocculation canproceed if the increase in
entropy due to the releaseof S(uvent molecules outweighs the loss
of entropydue to interpenetration of the polymeric chains TheGibbs
free energy change is considered in such casesto be determined
essentially by the change in entropydue to the release of the
solvent molecules and due tothe decrease in randomness of the
polymer chain and
Figure 2 Percentage of synthetic silica settled as afunction of
concentration of Nalcolyte -610 andSeparan AP- 30, reagentizing
time 30 seconds;settling time: 45 seconds (Sresty, Raja
andSomasundaran, 1978 )
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by the enthalpy of dissolvation of the polymer chain.Therefore,
for flocculation to occur , the increase inentropy due to the
release of solvent moleculesshould outweigh the loss of entropy due
to theinterpenetration of polymer chains and increase inenthalpy of
dissolvation
three interactions, VT determines if a collision willbe aborted
or be successful collision leading toadhesion. DL VO ( Derjaguin
and Landau, 1941;Verwey and Overbeek, 1948) and HHF (Hogg,Healy and
Fuerst~ 1966) theories considerthe London van der Waals attractive
energy andelectrical double layer energy and yield anexpression for
the total energy, VT, which showsthat it will be attractive at
large and smalldistances In the case of polymeric flocculation,the
interaction between polymer layers adsorbedbetween two particles
and also between such alayer and a bare particle playa predominant
role.Of particular interest in the case of polymericflocculation is
the nature and magnitude of V s,which can be positive or negative
depending onthe solvent power of the medium for the exposedportion
of the adsorbed layer Interpenetration ofthe branches of the
polymer adsorbed on differentparticles will be favorable if the
solvent power isnegligible, thus assisting flocculation
i\~ = i\Ht - T i\ s.
L\H( arises from polymer dissolvation and L\S(increases from
released solvent molecules anddecreases from interpenetrating
polymersFlocculation under complete surface coverage ispossible
only if the free energy of interpenetration ofadsorbed layers, ~ is
negative. It has been foundthat partial coverage allows the free
loops of thepolymers adsorbed on one particle to adsorb on thebare
region of another particle and thus produceinterparticle bridging
and hence cause flocculation
2.2 Physico-chemica/ collditilJl/S
3 IMPORTANT FACTORS IN FLOCCULATIONThe physico- chemical
conditions responsible forselective aggregation in solution are (a)
collisionsand forces governing collision rate (b) adhesionduring
collisions; repulsive and attractive forcescontrolling the
probability of adhesion (c) effect ofdissolved mineral ions and
other ions in water on thesurface properties of individual
minerals
3. I PreJreatmenl
Collisions:
Collisions generally result from thermal motions thatare
determined by particle size, temperature,viscosity etc. Controlled
agitation by stirringenhances both the rates of collision and
adhesionFlocculation assisted by such external forces areknown as
orthokinetic flocculation Collision ratesbetween particles of
different sizes can be calculated(Levich, 1962)
Adhesion
Flocculation can be enhanced or retarded by externaladdition of
ions which subsequent to adsorption onthe mineral surfaces either
activate or depressflocculation Selective flocculation of hematite
-quartz using anionic polyacrylamide has beenreported to be
effective after the addition ofCalgonR and sodium fluoride (Read,
1972) Treatmentprior to the flocculation may also consist
ofcomminution and liberation of constituentcomponents, mechanical
and chemical dispersion,and any other process to remove possible
coatingmatters. In the flocculation system, dispersants
playasignificant role in obtaining full liberation particularlyfor
removing interparticle adhesion However, thecomplexity of
polymer-polymer interactions can havea detrimental effect on the
successive flocculation
Many investigators have found that methods usedsuccessfully for
the separation of synthetic mixturesof pure mineral components
often fail when real oresamples are used The primary reason for the
loss inselectivity has been attributed to the coating andcementing
matters on one or more solid surfaces. Ithas been shown that
removal of surface coating andorganic cementing matters can
significantly enhanceselective separation of francolite from
phosphateslimes Pretreatment used to remove the coatinginclude
solubilization using acid or cornplexing agentand oxidation by
chemicals like hydrogen peroxide
The important interactions that decide whether or notparticles
will collide and adhere are thefollowingi) London Van der Waals
attractive forces, V"
ii)Double layer forces that can be attractive orrepulsive in
nature, V.1 and
iii)Steric forces that arise from the overlap ofadsorbed layers,
V s These forces can also beattractive or repulsive in nature
depending onwhether the adsorbed layers are in contact withthe
solvent or not. The combination of the above
84
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and ion exchange with excess salt (Anderson andSomasundaran,
1993) One way to prevent 'slimecoating' and 'hetero-coagulation' is
by the use ofdispersants to prevent particle-particle adhesion
Theobjective criterion in the selection of dispersants isthe
minimum interference by these reagents on thesubsequent adsorption
of the tlocculant Bothinorganic (e.g sodium hexa-metaphosphate)
andorganic ( eg low molecular weight polymers likeP AA, PEO and
PVP) reagents are known to beeffective dispersants for mineral
systems
32 Conditioning time
The conditioning time of the mineraI with thepolymer determines
the extent of surface coveragesignificantly Prolonged conditioning
may result in re-dispersion possibly due to the saturation coverage
ofthe particle surface by the polymer molecules(Michaels, 1954;
Kitchener, 1972) The role ofpolymer conformation is important in
inter-particlebridging mechanism. Extended chains of thepolymers
have been found to be more effective inbridging The coiling of
polymer molecules ismaximum at their respective iso-electric points
andany excess charge resulting from the solutionconditions may lead
to an extended conformationthus facilitating inter-particle
bridging
in some others ( Sato and Yoshida, 1967). Themobility of the
polymer molecules and solvent powerof the suspending medium depend
on the temperatureand affect significantly the adsorption behavior
of thepolymers on the solid surfaces It is generallybelieved that
when the suspending medium is a poorsolvent for the polymer, the
polymer adsorption rateis greater The conformation of the polymer
isaffected by the nature of the solvent and, hence, to agreat
extent determines the chances of collision ofthe polymer molecules
with the particulate materialsParticle morphology and surface
chemical propertiesof the solid also contribute to the kinetics and
extentof adsorption (Kraus and Dugone,1955; Frisch et aI.1959;
Linke and Booth, 1960; Stromberg and Kline,1961)
Polymer adsorption and flocculation involvesseveral steps eg
transport of polymer molecules tothe suspended particles followed
by initial attachmentof the polymer chain, which then rearranges to
givean equilibrium configuration (Akers, 1972). Collisionof the
particles with the adsorbed polymers result information of flocs
which may subsequently break-upAdsorption and flocculation steps
have beenvisualized (Gregory, 1988) as either due to
diffiIsion(rapid adsorption) or induced by the motion of thefluid
in the system (slow adsorption). The effect ofcharge density and
molecular mass on theflocculation kinetics of silica suspensions
has beeninvestigated and it was found that the rate of
polymer(copolymers of acrylarnide and dimethylarninoethylacrylate)
adsorption could be slow compared to theparticle collision rate at
low charge densities and lowpolymer molecular weights (Lee and
Gregory, 1991)
Differences in the rate of adsorption or in the rateof other
rate controlling steps could result fromchanges in the process
conditions eg thereagentizing times in hematite-quartz system
usingSeparan AP -30 Maximum flocculation for hematitewas observed
with about 60 seconds reagentizing atneutral pH whereas for quartz
the correspondingtime was found to be - 600 seconds (Sresty
etal,1978 ) Using a simple technique which involvedimmersion of a
glass plate in cationic polymersolutions for short periods followed
by deposition ofsilica spheres on the adsorbed polymers to
decoratethem for monitoring, , it was shown that adsorptioncould be
complete in less than one minute In the firstfew seconds, there was
very little adsorptionfollowed by a .. catastrophic" sudden
increase inadsorption, FigureJ (Somasundaran and Sivakumar1988;
Onoda and Somasundaran, 1987)
33 Kilteti(,'s
Rate of uptake of polymer by the solid substrate isinfluenced by
molecular weight, structure andconfiguration of the polymer
molecule, polymerconcentration, agitation, temperature, ionic
strengthand pH Rate of adsorption increases with increase
inagitation and polymer concentration ( Stromberg andKline, 1961)
The kinetics of the adsorption of thepolymer molecules on the
particulate surfaces isgoverned by the rate of diffiJsion of these
moleculesto the surfaces It is also important to note that
thekinetics and the extent of adsorption are dependenton the porous
nature of the particles With porousparticles, the extent of
adsorption may also varydepending on the solution properties of the
systemand hence profoundly affect the adsorption behaviorThe
solution chemistry conditions as well as thepolymer properties
including the molecular weightand conformation of the polymer, its
dosage and thetemperature are some of the important parameterswhich
govern the diffiJsion and adsorption of thepolymer on the surface
However, an increase inmolecular weight of the polymer can result
in anincrease in adsorotion in ceratin cases and a decrease
85
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~ici..
~c~
, f 101Ol'.u ~.cf fll(. N:
-,..
Figure J Adsorption of polyacrylamide ( PAM) onglass as a
function of time for different PAMconcentrations (Somasundaran and
Sivakurnar, 1988)
quartz. In a similar fashion, selective flocculationbetween
galena and calcite failed in the absence ofother additives since
the zeta potentials of theseminerals are not sufficiently high at
natural pH toproduce required dispersion of fines prior to
selectiveflocculation (Yarar and Kitchener, 1970)
The selectivity of starch in its interaction with ironoxide
depends to a large extent on the functionalgroups present in the
starch molecule (Iwasaki et al.I %9) Selective flocculation of a
hematite-quartzmixture using causticized com starch was carried
outin our laboratory. It was found that the recovery ofiron oxide
increased sharply with flocculantconcentration The perfonnance of
the selectiveflocculation of hernatite-quartz mixture is shown
inFigure 4 where separation index is plotted as afunction of the
starch concentration.Separation index takes into account both
therecovery and grade of the mineral of interest and isdefined as "
[ % of valuable mineral recovered in the
concentrate + % of gangue rejected in tailings 100 ]II ()()" A
value of unity denotes complete separationwhile a value of zero
implies total failure of selectiveseparation. The results show that
a satisfactoryseparation index of 0 7 could be obtained after a
one-
4 SELECTIVITY
;~
~c:=c
A serious problem often faced by the practicingengineers in
devising efficient selective flocculationschemes is the lack of
selectivity in the adsorptionof polymers on the desired mineral
surfaces due tothe non- specificity of the commercially
availablepolymeric reagents This has necessitated research inthe
realm of selective flocculation by incorporatinginto the polymer
chain specific functional groupsthat can complex or chelate with
the solid surfacesand thereby cause selective
adsorption/flocculation.
The success of separation by selective flocculationdepends on
the liberation of the particulate mattersfrom each other.
preferential adsorption of theflocculants on the particles of
interest. bridging ofthe flocs by the adsorbed polymer molecules
andeffective separation of the flocculated mass from thesuspension
without re-dispersion of the flocs Thepreferential adsorption of a
polymeric reagent in acomplex system often depends on the
interfacialpotential on the particle surface, fractional coverageof
the particle surface by the flocculant, the chemicalnature of the
flocculant and the solids present in thesystem
In a study involving separation of galena fromother minerals, it
was found that galena flocculationwith anionic polyacrylamide could
be enhanced bythe addition of lead nitrate and retarded by
sodiumsulfide It was also found that galena dried in contactwith
air did not undergo selective flocculation fromquartz and this was
attributed to zeta potential ofgalena being zero in a medium
containing oxidizedlead salts and hence undergoing coagulation
with
Figure 4 Separation index achieved from theselective
flocculation of hematite-quartz mixture as afunction of
concentration of starch, reagentizing time30 sec, settling time; 45
sec
~
-
i~~~-...,c "':"5 .
80
I 5
~~c
~o#
ic40
~ ~I~ ,u ,oJ ,C ,%u .0
#..cw~t\c 40 ~.n + NWTLMOtTE
3 . ~---,.,..O. 0 O_~EO&65- P«O+ttr4_/.JOPG
202 ~ ~ ~ ~ I~ j4'ZOpH
Figure 5 Recovery and assay of: (a) chalcopyrite and (b)
pentlandite
stage cleaning of the flocculated product (Sresty
andSoma.'IlJndaran, 1980)
Complexing agents have also been used toenhance the selectivity
of flocculation processDiphenylguanidine (DPG), a complexing agent,
hasbeen extensively used in the flotation separation ofINCa matte,
which consists of CuzS and Ni3Sz dueto its unusual selectivity for
copper minerals (Tipmanet al., 1976) The effect of addition of DPG
in thepresence and absence of polyethylene oxide (PEO)on
chalcopyrite and pentlandite flocculation indicatedthe possibility
of selectivity The results of selectiveflocculation tests conducted
with synthetic mixturesof chalcopyrite and pentlandite is shown in
Figures5a and 5bThe recovery and assay of chalcopyrite is found
toincrease from about 700/0 to 900/0 and from 500/0 toabout 600/0
respectively, in the presence of DPG andPEG together throughout the
pH range studiedHowever, the recovery and assay of pentlandite
wasfound to reduce compared to the results obtainedwhen only PEG
was present (Acar andSomasundaran, 1989).
Selective flocculation of hematite from itsmixtures with quartz,
calcite etc has been achievedby many investigators ( Usoni et al,
1968; Yarar andKitchener, 1970 Read, 1971) Subsequent to
thesuccessful use of many chelating type collectors inthe
flotation-separation. applications have beenattempted by the
incorporation of some of the activegroups into the polymeric chain
(Iwasaki et al,1969; Attia and Kitchener, 1975; Clauss et aI
1976)
Starch and related polysaccharides have beenwidely applied in
the mineral processing industry as
depressants, flocculants and selective flocculants(Weissonbom et
aI. 1994) Also modified starchesare regularly used in other
industries ego in papermanufacturing where it is used as an
additive in theparticulate suspension When the molecular weight
ofthe starch is lowered, converted starch is obtained.Modification
can also be carried out by substitutingdifferent functional groups
into the polymer chainNumerous mechanisms for the adsorption of
starchand related poly-saccharides onto particles have
beenproposed. Hydrogen bonding and hydrophobicinteractions between
polysaccharides and paniclesurfaces are mainly responsible for the
adsorptionHowever, more recent evidences have shown thatchemical
interactions may playa significant role(Laskowski et aI., 1991 ).
Using spectroscopictechniques, it has been shown that many
derivativesof starch such as dextrin, fructose, glucose andsucrose
have complexing abilities with metal ions
Polymers and surfactants have also been used incombination with
each other for flocculation!stabilization. The stability of
suspensions in thepresence of polymers and surfactants are
governedby many factors besides the electrostatic interactionsIn
the case of stahilizatiOfI, considerations have beengiven to the
role of the adsorbed polymer andsurfactant layer on particle
surfaces which canproduce repulsive steric interactions (Sato and
Ruch,1980 ) For j1occulatiOfI of suspensions, bridging byadsorbed
surfactants and polymers is a widelyaccepted mechanism in addition
to chargeneutralization In the case of flocculation, severalfactors
affect the performance eg flocculant dosage,
87
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'-"'--"-""-"-"'-"~
c_~
-
molecular weight, charge density, as well as mode ofaddition
5 RECENT ADVANCES
In a study on the selective flocculation of ultra-fineiron ore
Weissenborn et aL, 1995 carried out an in -depth investigation of
the mechanism of theadsorption of starch and its components
namelyamylopectin and amylose onto hematite surfaceUsing OiffiJse
Reflectance Infrared FourierTransform (DRIFT) Spectroscopy, these
authorsdetermined the nature of the adsorbed species on thehematite
surface and also the extent of adsorption Itwas found that wheat
starch and amylopectin werestrongly adsorbed on the surface
ostensibly by theformation of a surface complex rather
thanadsorption by hydrogen bonding. Using the dataobtained from
adsorption and thermogravimetricresults together with supportive
evidence frominfrared and zeta potential results, the
authorsproposed preferential adsorption of starch ontohematite due
to the higher concentration of metaladsorption sites They suggested
that flocculationoccurs by classical bridging mechanism
Jarnstrom et al., 1995 have studied flocculation ofkaolin
suspensions induced by modified starchesThey measured the
fluctuations in the turbidity inflowing kaolin suspensions using a
fibre-opticalsensor It was found that a cationically modifiedstarch
was an excellent tlocculant. The effect of ionicstrength was also
studied and it was observed thatthe flocculation response was
satisfactory if the ionicstrength was maintained between I and 100
roM.However, it was interesting to note that theflocculation
response was poor at I mM NaCIconcentration when the kaolin
suspension was pre-treated with Na-polyacrylate. But the
flocculationwas satisfactory in presence of Na-polyacrylate athigh
temperatures and when the salt and starchconcentrations were high.
The authors attributed theflocculation of kaolin by the
cationically modified andconverted starch to a bridging
mechanism.
One interesting study on the adsorption andflocculation of
negatively charged polystyrene latexparticles using cationic
modified polyacrylarnides wasconducted by Eriksson et al,1993. In
this work.conditions were chosen such that the adsorbedpolymers
approached equilibrium conditions beforethe occurrence of
appreciable flocculation Polymerswith very low charge density was
found to formloops and tails at the latex surface and
theyflocculated the latex particles by bridging But
polymers with higher charge density adsorbed in aflat
configuration and flocculation occurred bycharge neutralization
These authors suggested thatflocculation could also result from the
long rangeattraction between the positive and negative
chargecenters on the particles They found that at maximumadsorption
of highly charged polymers the latexparticles acquire a high
positive charge. This has beenargued as the mean distance between
positivecharges on the polymer being smaller than thedistance
between the charges on the latex, such thation binding of the
polymer to the ionized latexsurface groups can not take place
without thesimultaneous attachment of an excessive positivecharge
to the latex surface.
Conformation and orientation of the adsorbedmolecules also playa
major role in polymericflocculation (Tjipangangjara and
Somasundaran,1991). However, the exact role of the conformationis
stiD not well understood mainly due to the complexnature of the
adsorption of the polyelectrolytes andthe lack of reliable
techniques to monitor theconformation in situ. Fluorescence and
Electron SpinResonance spectroscopy techniques are powerfultools to
probe the micro structure, orientation andconformation of adsorbed
surfactant and polymerlayers ( Chander and Somasundaran, 1987;
Chanderet aI 1988, Somasundaran and Yu, 1994) Thedetails regarding
the application of these techniquesare beyond the scope of this
paper and can be foundin the above references
Enhanced flocculation obtained by maneuveringpH shift could also
be obtained by using acombination of polymers, one of which can
adsorbstrongly and another that can bridge A combinationof two or
more polymers produces synergism in theflocculation or
stabilization of suspensionsFlocculation of alumina suspensions
with sequentialaddition of polystyrene sulfonate and
cationicpolyacrylamide was found to be superior to thatobtained by
a single polymer The cationic polymerdid not produce any
flocculation by itself while theanionic polystyrene sulfonate had a
minor effect.However, excellent flocculation was obtained withthe
dual polymer even when the charge on aluminaparticles were
significant, Figure 6 (Yu andSomasundaran. 1993)From adsorption
studies. it was found that cationicpolyacrylamide when used alone,
does not adsorb onthe alumina surface due to electrostatic
repulsionHowever, both polystyrene sulfonate and poly-acrylamide
co-adsorbed completely on alumina whenthey were used in combination
with each other,Figure 7( Yu and Somasundaran, 1993).
88
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R8IiduaI PSS COftC. ~
Figure7 Adsorption isotbenn of individual polymersand their
combination on the alumina surface; fJ ,premixture of polystyrene
sulfonate and cationicpolyacrylamide; V , 12 million mol wt.
polystyrenesulfonate; 0 , 4600 mol. wt. polystyrene sulfonate; 0,
cationic polyacrylamide (Yu and Somasundaran,1993)
Figure 6. Comparison of flocculation response ofalumina with
double tlocculants to that with singletlocculants; A. , polystyrene
sulfonate followed bycationic polyacrylamide, 0 , cationic
polyacrylamidefollowed by polystyrene sulfonate; V, premixture
ofpolystyrene sulfonate and cationic polyacrylamide; 0,4600 mol wt.
polystyrene sulfonate alone, O,cationicpolyacrylamide alone
arising due to interactions of the various processparameters,
the industrial success has been ratherlimited The benchmark Tilden
Iron Ore mine'ssuccess in selective flocculation is the first
industrialapplication of this technique Considering that anumber of
parameters govern adsorption/flocculationof the fine particulates,
it is necessary to make anintegrated effort to correlate the
influence of theprocess parameters as the development of
futureselective flocculation technology wiD essentially bean
evolutionary process
This co-adsorption is attributed to the interactionof complexes
between the two at the solid/liquidinterface. The superior
flocculation in the case ofdual polymers was explained on the basis
ofelectrostatic attraction of the anionic polystyrenesulfonate on
positively charged alumina surface. thepre-adsorbed polymer
molecules acting as anionic"anchors " for the adsorption of the
long chain
cationic polymer and cause good interparticlebridging and hence
excellent flocculation
ACKNOWLEDGMENTS6 CONCLUSIONS
The authors acknowledge the financial supports ofNational
Science Foundation (rNT-9415417) andIUCR Center for Advanced
Studies in NovelSurfactants (NSF/EEC-9804618)
Flocculation /stabilization of colloidal suspensions isof great
technological importance Based on ourknowledge of colloid and
surface chemistry,significant achievements have been made in
obtainingdesired end results In the case of mineral
processing,selective flocculation is a promising techniqueAttempts
have been made to exploit this technologyto achieve selective
separation both in the laboratoryscale as well as at a industrial
level However,because of the enormous complexity of the system
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