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. -

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

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

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

80 8#

1 I .~'.4 J

60.0

II~ 40

~c.. 20

1".8#

1

1:llobJt.:F~4 x 10-' PM2~ 8OIids 0 66 0 I a

6 a 0 A .. IC

6 1t-°'-

I)

55.2

0

~

145II. I

" J-- ~

. i Lo_~! """""'~"'I"""'b

1:1~.:F~. ~r~s9.3-9.S :I ~~ 10~.'.PAA :I 2 ~ IOIi.dI . : ..

0 20 40 ~ .. 100'

Ca bdiII8 011 da,./ ~

IO.~0

---~ -~,~-

-c 111.2

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

'toI 70

"1 60

I&. so

0.8M

6 1t-°'-

04 OJ~

--:l '0

A_~

. ~~40' _~~_~-A. '-!

8.S 9.0 9.S 10.0 10.SpH

0.2

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

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 ofhematite in the hematite-quartz-ferric ion-poiyacrylic acidsystem. 1. Activation and deactivation of quartz (J]. IntJ Miner Process, 1981, 8: 265-277.

[3] Critchley J K and Straker P. Flotation of nickel sulphideand zeta potentials in a nickel (2)-xanthate system [J].Trans Inst Min Metall (Sect C: Miner P~ Extr Met-all), 1981, 80: 44-45.

[ 4 ] Acar S and Somasundaran P. Flocculation of sulfides andthe role of a OOffiplexing agent in it (J]. Int J Miner Pro-cess, 1989.. 27: 111-123.

[5] Andersen B and Somasundaran P. Mechanisms determin-ing separation of phO5phatic clay waste by selective floccu-lation (J]. Miner and Metall Process. 1993: 200-205.

[ 6 ] Sresty G C and Somasundaran P. Selective flocculation ofsynthetic mineral mixtures using modified polymers [J].

Int J Miner Process, 1980, 6: 303-320.[7] Yu X and Somasundaran P. Role of 'polymer conforma-

tion in int~rticle-bridging dominated flocculation [J].J Colloid Interface Sci, 1996, 177: 283-287.