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Vol. 10 Special Issue Trans. Nonferrous Met. Soc. China Jun.
2000
Article ffi: lOO3-6326(2000)Sl-OOO8-04
Selective flocculation of fines
P. Somasundaran, V. RunkanaLangmuir Center for Colloids and
Interfaces,
ColumbiaUniwrsity, New York. NYlOO27. USA
Abstract: A number of factors that affect selective fkJCcu1ation
of fines have been identified and the effect of ~e of pa-rameters
on p~ behavior has been explained with the help of a few case
studies. Experiments with francolite-montmo-rillonite and
francolite-palygorskite mixtures indicated that francolite recovery
depends on pH and the type of dispersant.The results showed that
removal of multivalent ionic species on clay mineral surfaces seems
to enhance flocculation. andseparation efficiency increases as Caz+
ions are removed from the surface. When chalcopyrite and quartz are
present to-gether. it is however necessary to clean the fIocs
obtained to remove entrapped quartz.Key 'WordS: flocculation; clay
minerals; p~phate minerals; dispersant Document code: A
INTRODUCTION1 1.1 Basic principles and mechanisms of
separationThe basic principles of selective flocculation are
actually similar to those of the conventional selectiveflotation
process. First, the individual mineral parti-cles need to be
liberated and dispersed, then a reagentis required that can
selectively adsorb onto one or sev-eral of the mineral surfaces.
So, the two main criteria(or selective flocculation are dispersion
of the fines andselective polymer/reagent adsorption.
When particles are in suspension, they collide ei-ther due to
the Brownian motion or due to externalforces induced by agitation,
magnetic field etc. Theprobability of aggregation during such
collisions is de-termined by the nature of interactions between
theparticles. Attractive interactions arise due to London-Vander
Waals type of forces whereas repulsive inter-actions arise
generally due to the electrostatic forces.There is always a net
total energy of interaction and itis necessary that the following
conditions are met sothat selective aggregation can take place:
1) All types of particles should carry the samecharge so that
there will be no heterocoagulation be-tween them. Repulsive energy
should be larger thanthe energy of attraction between different
particles.
2) Charge on the particles to be aggregatedshould be such that
repulsive energy between themshould be less than that of the
attractive energy.
Floc formation takes place by either of the fol-lowing three
mechanisms:
1) Reduction of electrostatic repulsion between
particles;2) Formation of polymer bridges between parti-
As the earth's resources are being consumedsteadily and as
demand for mineral products continuesto increase, the need to
utilize -low-grade ores moreefficiently has become more severe.
This indeed re-sults in the generation of a large quantity of fine
parti-cles during mining and processing of such ores. Largeamounts
of fine mineral values are discarded currentlyas techniques to
recover ultra-fine minerals are not ad-equate. Conventional
processing techniques such asflotation are not efficient to process
fines due to theintimate mixing of values and gangue materials.
Thisis especially true in the case of fines in the sub-micronrange.
Also, these fines cannot be disposed off easilyas it results in
environmental hazards and in some cas-es, waste of useful land.
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Thus, there is a great need to develop processesto utilize the
fine ores and selective flocculation hasshown some promise and
potential in this regard. Theobjective of the present communication
is to elucidatethe basic principles and mechanisms of selective
floc-culation, identify the critical process parameters, pre-sent
results obtained for materials of industrial impor-tance and
suggest directions for future research.
2 SELECflVE FLOCCULATION
This process involves selectively flocculating thedesirable!
undesirable mineral from a mixture of min-erals. Though this
appears simple, it is quite complexin nature. Selective
flocculation has to always dealwith a binary or even a
multicomponent mixture ofmaterials. Also, long chain polymers are
normallyused as flocculants. So, it is necessary to understandnot
only the interactions between particles, polymersand the solvent
but also the interactions between dif-ferent particles in the pulp.
The surface chemistry ofdifferent materials in the presence of each
other be-comes an important factor in this situation.
cles; \3) ion-exchange reactions between polymers and
particle surfaces. .Selectivity in flocculation can be achieved
by any
one or combination of the following treatments:1) Altering the
surface potential of different
minerals;2) Incorporating specific functional groups into
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Vol. 10 Special Issue Selective flocculation of fines 9
with the dissolved species.Experimental results obtained from
the studies
on selective flocculation of synthetic mixtures of chal-copyrite
and pentlandite using PEa and DPG are pre-sented in Fig.!. It can
be observed from Fig.! thatflocculation of the mixture is enhanced
by the pres-ence of DPG and PEa together. particularly underacidic
pH conditions.
the polymer chain;3) Controlling flocculation! polymer-particle
in-
teraction time;4) Selective polymer adsorption;5) Coating
impurities or specific ions on solid
surfaces.
2.2 Important process parametersSome of the important factors
that affect selec-
tive flocculation are1) size distribution of colloidal
particles;2) shape and surface heterogeneity of particles;3)
concentration, molecular weight distribution
and charge density of the polymer;4) functional groups present
on the polymer
chain;5) suspension pH,' temperature, density ~d vis.
cosity;6) dissolved ionic species in solution;7) speed of agi
tation and type of impeller.
3 CASE STUDIFS
We now present a few examples of selective floc-culation in
which the effect of some of the factorsmentioned above is discussed
in detail.
3.1 Selective flocculation or sulfidesAs indicated earlier,
selective flocculation of a
mixture of minerals is significantly different from thatof a
single mineral. One of the reasons for this is thedissolution,
precipitation! readsorption of mineralspecies from one mineral onto
another. When mineralfines are suspended in solution, species such
as Cu2 + ,N? +, F e2 + and 52 - present on the surface of a
min-eral can dissolve and then precipitate! readsorbon an-other
mineral surface. I t has been reported thatspecifically adsorbing
ions can change and/or even re-verse the zeta potential of a
mineral and thereby causeheterocoaguiation as well as enhanced
polymer floccu-lation[1-3]. Acar and SomasundaraJ4] studied
sepa-ration of chalcopyrite and pentlandite from a mixtureof these
minerals using polymers such as polyacry-lamide (PAM) and
polyethylene oxide (PEO). Floc-culation was observed both in the
presence and ab-sence of the polymer. Though the presence of
PEaenhanced the efficiency of overall flocculation, theseparation
efficiency was not significant.
Electron spectroscopy for chemical analysis (ES-CA ) and
electrophoretic mobility measurementsshowed that nickel ions
present on the chalcopyritesurface and copper ions present on the
pentlanditesurface cause nonselective adsorption of polymer.
Theamount of dissolved species was significant especiallyunder the
acidic pH conditions. In order to overcomethis problem
diphenylguanidine (DPG) was intro-duced into the system so that it
can form complexes
3. 2 Separation of phosphate minerals from clayminerals
Florida phosphatic clay is disposed off in enor-mous quantities
though it is rich in phosphate miner-als and causes serious
environmental hazards. Thewaste is in the form of fine slimes and
because of thisthe clay minerals such as montmorillonite and
paly-gorskite are intricately mixed with the ph~phatemineral,
francolite. Traditional separation processessuch as flotation are
extremely inefficient because theslimes are in the micron and
sub-micron size range.Another problem is the presence of organic
matter andcementing materials such as oxides and hydroxides ofAI
and Fe. Andersen and Somasundaran[S] studiedthis separation problem
by preparing synthetic mix-tures of clay and ph~phate minerals and
using thenaturally occurring slimes. After initial screening
ex-periments. polyacrylic acid with an average molecularmass of 3
million was found to be best suited for poly-mer flocculation.
Sodium silicate and sodiumtripolyph~phate were used as dispersants.
Electroki-netic measurements indicated that all the pure miner-als
as well as the natural slimes are negatively chargedin the entire
pH range (4-11) studied.
Experiments with francolite-montmorillonite(Fig.2) and
francolite-palygorskite (Fig.3) mixturesindicated that francolite
recovery depends on pH andthe type of dispersant. For both the
mixtures, recov-ery decreased at high pH, where it is likely that
elec-trostatic repulsion is inhibiting polymer adsorptionand
flocculation. The behavior of the two systemswith respect to
francolite recovery at lower pH values
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Jun. 200010
was quite different. In the cue of montmorillonite,gel formation
was observed after polymer addition,resulting in a decrease in the
recovery. In the case ofpalygorskite, recovery increased with
decreasing pH.The floc grade in both systems is hardly affected
bypH. The difference between the two systems can beexplained by
examining the differences in .the crystalstructure of the two clay
minerals. Due to the edgeface charge characteristics, the
plate-shaped montmo-rillonite particles can form a card-house
structure insuspension. This prevents the clay particles as well
asthe francolite particles from settling which results inlow
recovery of francolite. The needle-shaped paly-gorskite crystals
cannot form such an expanded struc-ture in the suspension. So these
particles flocculateand setde together with francolite. For both
the sys-tems, maximum separation occurred in the pH rangeof 9-9.5
with S(x{ium silicate as the dispersant.
Trans. Nonferrous Met. Soc. China. - -~
0
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II~ 40
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Fig.4 Effect of the presence of exchangeablecalcium and sodium
on separation of francolite-montmorillonite mixtures by poiyacrylic
acid
3:
Fig.1 Separation results for francolite-montmorillonite mixtures
using polyacrylic acid
9Or 1:1 Palll~:F~~.A -- ",.04 x 10-iPAA 0-101lll~.
.2-. dids oA-O. 5 l1li STPP/..# 80
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Fig.! Effect of the presence of exchangeablecalcium and sodium
on separation of francolite-
palygorskite mixtures by polyacrylic acidFig.) Separation
results for francolite-
palygorskite mixtures using polyacrylic acid Selective
flocculation of natural phosphatic slimeswas attempted next.
Sedimentation experiments indi-cated that the as-received natural
slimes flocculatedand settled at pH 7 within one-half hour. But.
selec-tive flocculation experiments. under the optimumconditions
obtained for synthetic mixtures. did notresult in any specific
separation of francolite. The
The separation achieved in the above experi-ments was far from
the maximum that can beachieved theoretically. Pure francolite
contains 32 %P2o,. The feed had 16% P2o, while the best thatcould
be achieved was 28% in the presence of mont-
0.6 jto
-J A" o-101D1 $/1
I . A'~ .A-o..51D1 s:w/IIO'.0 7.5 8.0 8.5 9.0 9.5 10.0
pH
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Vol. 10 Special Issue Selective flocculation of fines 'n-
main reason for this appeared to be the presence of ce-menting
materials such as organic matter and inorgan-ic (mainly AI and Fe)
hydroxide precipitates whichinhibit dispersion of particles. AI and
Fe were foundto be 10 times more abundant in natural slimes thanin
pure clay minerals. The organic and inorganic Q18t-ter causes
aggregation of particles. In addition, theAI and Fe compounds might
be causing non-selectivepolymer adsorption also.
I3 . 3 Beneficiation of mineral slimes
The type of functional group on a polymer chaincan also modify
the behavior of flocculation. Srestyand Somasundaran[6] studied
selective flocculation ofchalcopyrite-quartz system in the presence
of hydrox-ypropylcellulose xanthate. Hydroxypropylcellulosewas
modified to incorporate the xanthate functionalgroup, as it is
known that xanthates adsorb selective-ly on heavy minerals such as
chalcopyrite, galena andsphalerite. Fig. 6 shows the variation of
percent solidssettled as a function of xanthate concentration. It
canbe observed that chalcopyrite flocculates and settlesvery well
whereas quartz stays suspended in solution.When the two minerals
were present together, it washowever n~ to clean the flocs obtained
to re-move entrapped quartz.
can be overcome by introducing a complexing agentinto the
system.
A number of ,fa~tors that affect selective floccula-tion of
fines have been identified and the effect ofsome of theSe
parameters on process behavior has beenexplained in this paper with
the help of a few casestudies.
It has been found[7] that polymer: conformationat the
solid-liquid interface has a significant impact onflocculation. It
is necessary to understand how theconformation changes depending on
the mineral sur-face heterogeneity and take advantage of that for
se-lective adsorption. Also, there is a need to developnew
processing techniques to remove undesirable ions(for example, Ca2+
ions in the case of Florida phos-phatic slimes) present on the
mineral surfaces in mul-ticomponent mixtures of minerals.
Hydrodynamics will playa major role in the caseof large scale
industrial separation operations. Howev-er, very little research
has been done in this direc-tion. It needs to be seen as to how
existing knowl-edge on mixing of fluids can be extended for
particu-late suspensions.
Even though a number of mathematical modelshave been proposed in
the literature, very few takepolymer adsorption into acCbunt. It is
important toincorporate knowledge on all aspects of flocculation
inorder to arrive at meaningful conclusions.
ACKNOWLEDGMENTSThe authors acknowledge the National Science
Foundation for its financial support (NSF/EEC-9804618).~
~~~~~
REFERENCES
3000 100 200
Xanthate COlIC. (Dry dids basis)/10-'
Fig. 6 Percentage of chalcopyri te fines and quartzfines settled
as a function of concentration of
hydroxypropylcellulose xanthate(Reagentizing time, 30 s;
settling time. 45 s)
4 CONCLUSIONS
Selective flocculation has been found to be a fea-sible
technique for the separation of sulfides, ox.idesand phosphatic
minerals. However, dissolution ofmineral species has a significant
impact on the selec-tivity of polymer adsorption. This problem
however,
[1] Critchley J K and Jewit S R. The effect of Cu' + ions on
zeta potential of quart~ (J]. Trans Inst Min Metall (SectC:
Miner Process Extr Metall) , 1979,88: C57-59.
[2] DrzYmala J and Fuerstenau D W. Selective flocculation
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acidsystem. 1. Activation and deactivation of quartz (J]. IntJ
Miner Process, 1981, 8: 265-277.
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