Reprinted from "Beneficiation of Phosphates. Advances in REsearch and Practi.. ~,

Ge" , Patrick Zhang et al. ed. p.141-154, 1999

CHAPTER 1.2

* ,P. Somasundaran and Lei Zhang

ABSTRACT

The beneficiation of phosphate is complicated dueto its sparingly soluble nature. The surface proper-ties of phosphate and associated minerals aredetermined, in addition to the ionic compositionof the solution, by the dissolution of the latticeions of the minerals and subsequent complex-ation, hydrolysis, adsorption, and precipitation ofthe dissolved species. These, in turn, are influ-enced by the pretreatment of the mineral. Thesecomplex interactions determine the interfacialproperties of phosphate and associated mineralsand thereby the efficiency of beneficiation pro-cesses such as flotation and flocculation. Under-standing of the chemistry involved in such systemsis helpful to develop the optimum beneficiationschemes. It is shown that desired selectivity can beachieved by controlling the chemistry of the sys-tems. More important, the need to conduct devel-opment tests under plant conditions is clearlydemonstrated.

efficient for the benefication of phosphate ores withsilicate gangues, but those with carbonaceousgangues are not as easily separated by flotationtechniques. The poor selectivity in the latter casehas been attributed to the similarities in the surfacechemical properties of the constituent minerals.The surface properties of phosphate are affectednot only by phosphate's own solution chemistry butalso by the dissolved species from other salt-typeminerals present such as calcite and dolomite. Fur-thermore, these properties are influenced by thewater chemistry of the reagents involved.

In this paper, the surface chemistries of phos-phate and associated minerals is discussed fol-lowed by their influence on the reagentsadsorption and phosphate beneficiation by flota-tion and flocculation.

SURFACE CHEMISTRY OFPHOSPHATE ANDASSOCIATED MINERALS

Adsorption of surfactants and polymers on solids isdependent on, among other factors, the interfacialproperties of the solids, which in turn are governedto a larger extent by their surface chemistries.Solid particles, when contacted with water, willundergo dissolution and develop a surface charge.The extent of dissolution is dependent on the typeand concentration of the chemicals in the solution.

INTRODUCTION

Phosphate is one of the most important mineralsprocessed by techniques such as flotation and floc-culation/dispersion, which rely heavily on differ-ences in the surface chemical properties of theminerals in the system. For example, flotation is

.1.4.1.

Beneficiation of Phosphates: Advances in Research and Practice1.42

FIGURE 3. Effect of Ca, PO4' and F on the zeta potential of natural apatite 85 a function of pH

calcium makes it more positively charged. Fluo-ride is found to make the mineral more positivelycharged under acidic conditions and more nega-tively charged under alkaline conditions. It canbe concluded that solution pH and concentra-tions of calcium, phosphate, and fluoride have aprofound influence on the electrochemical prop-erties of apatite.l,2

The dissolved mineral species can undergo furtherreactions such as hydrolysis, complexation,adsorption, and precipitation. The complex equi-libria involving all such reactions govern the inter-facial properties of the minerals as well as theirflotation and flocculation behavior.

Electrochemical Properties

The zeta potential of a natural apatite(CalO(PO~6(F, OH)v sample in water and vari-ous uni- and multivalent salt solutions is illus-trated in Figure 1 as a function of pH. Clearly,hydrogen and hydroxyl are (surface) potentialdetermining ions. KNO3 does not shift the iso-electric point, so r or NO3 have no potentialdetermining role in the case of apatite. Theeffects ofCa(NO~2- KF, KH2PO4 on the isoelec-tric point of apatite can thus be attributed toCa2+, r, and PO4 ions and their complexes.Under all pH conditions, phosphate additionmakes the apatite more negatively charged while

Effects of PretreatmentIt is to be noted that pretreatment such as clean-ing and storing can have marked effect on theelectrochemical properties of these minerals.3.4The isoelectric point of apatite has an initial valueof 4.5, which shifts to about 5.5 upon equilibrium.When the mineral was tteated with a concen-trated fluoride solution (1 moVIiter) for a pro-longed time (8 hours), the zeta potential ofapatite became more positive (or less negative)throughout the pH range studied5 (Figure 2).

+50

+40

+30>E +20

~-.J~ +10

Z 0

~

~ -10~

-20I&JN -30

-40

-50

FIGURE 2 Effect of prolonged fluoride contact on the electrochemical properties of natural apatite (Zeta potential In2 x .1.0-3 M KN03 solution of freshly cleaned apatite, cleaned and aged apatite, and F-treated [using .1.M KF following

cleaning and aging] apatite.)

Surface conversion due to the reaction of thedissolved species with the mineral surface can bepredicted using stability diagrams for heteroge-neous mineral systems. 7 ,8 This is illustrated in Fig-

ure 3 for the calcite-apatite-dolomite systemunder open conditions (open to atmosphericCO2). The activity of Ca2+ in equilibrium with vari-ous solid phases shows that the singular point forcalcite and apatite is 9.3. Above this pH apatite isless stable than calcite and hence conversion ofapatite to that of calcite can be expected in the cal-cite-apatite system. Similarly, apatite is more sta-ble than calcite below pH 9.3. For calcite-dolomiteand apatite-dolomite systems the singular pointsoccur at pH 8.2 and 8.8, respectively. It is also tobe noted that Ca2+ in equilibrium with calcite-dolomite-apatite in an open system could be sig-nificantly different from that in a closed system.

The surface conversions in a calcite-apatite sys-tem have been confirmed experimentally. Electro-kinetic data obtained for a calcite-apatite systemin water and in the supernatant of each other areshown in Figure 4. When apatite is contacted with

Surface Conversion

During the processing of carbonaceous phosphateores, apatite, calcite, and dolomite will dissolve inwater followed by pH-dependent hydrolysis andcomplexation of the dissolved species. These dis-solved species can have a marked effect on theirinterfacial properties. From theoretical consider-ations, depending on the solution conditions, theapatite surface can be converted to calcite andvice versa through surface reactions or bulk pre-cipitation of the more stable phase. The equilib-rium governing the conversion of apatite to calcitecan be written as6:

CalO(PO~6(OH)2 (5) + lOCO~-

= lOCaCO3 (5) + 6PO~- + 20H-

It can be seen from this equation that, depend-ing on the pH of the solution, apatite can be con-verted to calcite if the total carbonate in solutionexceeds a certain value. In fact, the amount of dis-solved carbonate from atmospheric CO2 doesexceed that required to convert apatite to calciteunder high pH conditions.

144 Beneficiation of Phosphates: Advances In Research and Practice-- -

~E...'0E..

.

.0u~0)-t-

:Et-UC"0-'

10 126 .pH

-- -' - -AGURE 3 Stability relations In calclte-apatite-dolomlte system open to atmospheric carbon dioxide at 25°C ([Mg2+)= 5.0 X 1.0-4 M)

the calcite supernatant its zeta potential is seen toshift to that of calcite and vice versa, suggestingsurface conversion of a~atite to calcite and calciteto apatite, respectively.

The zeta potential data obtained in mixed super-natants of calcite and apatite also show the effect ofdissolved mineral species. If supernatants of calciteand apatite are combined as a 1:1 mixtt1re, the twominerals have almost identical surface charge char-acteristics in the basic pH range (Figure 5).

The surface conversion of apatite and calcite isfurther supported by ESCA measurements. Theresults in Figure 6 show that when apatite is con-ditioned in the supernatant of calcite at pH -12,its surface exhibits spectroscopic pro~rties char-acteristic of both calcite and apatite. This behav-ior is attributed to the precipitation of calcite onapatite. Note the implication of such conversionson the efficiency of processes such as flotation andflocculation, which depend on the differences inthe surface properties of component minerals inthe system.

Surface Chemistry of Phosphatic Slimes

In the beneficiation of Florida phosphoric claywaste by selective flocculation to recover francolitefrom clay minerals (montmorillonite), the surfacechemistry of phosphate as well as associated min.erals was found to play an important role. Poly-acrylic acid (PAA) was used as a flocculant forfrancolite since there is specific interactionsbetween the carboxylate group on PAA and thecalcium sites on francolite. While high separationefficiency could be obtained with artificial mix-tures of francolite and monnnorillonite using poly-acrylic acid as a flocculant, the efficiency fornatural phosphate slimes was very low. This isattributed to the fact that polyacrylic acid interactsalso with montmorillonite when there are calciumspecies on its exchangeable sites. In the case ofnatural phosphate slimes, dissolved calcium spe-cies from francolite can adsorb on the exchange-able sites of the clay surfaces and activatepolyacrylic acid adsorption on the clay, which inturn leads to a decrease in separation efficiency. to

>e

J4

...ZW...

f

4...WN

5 Eo 7 e 9 10 Jt 12 13

pH

FIGURE 4 Effect of supernatants on the zeta potential and isoelectric point of calcite and apatite

SURFACE CHEMISTRY OF PHOSPHATEMINERAL PROCESSING

Adsorption of reagents at the solid-liquid inter-face is greatly affected by the surface charge char-acteristics of the minerals. In addition to reagentadsorption, interactions among dissolved mineralspecies and various reagent species can also beexpected. Chemical equilibria in aqueous solu-tions containing both the minerals and thereagents can be expected to be much more com-plex than in either of the individual systems(reagents-'solution equilibrium and minerals-solution equilibrium). All of these interactions canaffect the surfactant adsorption and subsequentbeneficiation processes.

Furthermore, with aging of the slime, there is anadditional drop in the separation efficiency. Ther-modynamic calculations of the stability of miner-als in phosphate slimes suggest that a mixture offluorapatite with common clay minerals is not sta-ble under conditions prevailing in the slimes stud-ied. It was found that millisite, a calcium-aluminum phosphate, is the stable phosphatephase. Formation of this compound at the clay-water interface leads to loss of sefaration effi-ciency by selective flocculation.!

From the above discussion, it can be concludedthat the surface properties of phosphate are a func-tion of various salt concentrations, pH, species fromthe dissolution of the minerals, as well as pretreat-ment. These factors will playa major role in deter-mining the adsorption of reagents on them andselectivity in subsequent flotation or flocculation.

1.46 Beneficiation of Phosphates: Advances in Research and Practice

->E..Jct~%\oJ.-00-

ctI-\&IN

AGURE 5 Illustration of the similarity in zeta potentials of calcite and apatite in mixed supernatants

HOI(l) ~ H01(aq) PKsoI ~ 7.6H01(aq) K ~ + 01- PKa - 4.952 01- ~ (OI)~- P~ - -3.7HOI + 01- = (OlhH- P~ = -5.25

The species distribution of oleic acid as a functionof pH based on the abOve equilibria at a given con-centration is shown in Figure 7. It can be seenfrom this figure that:

1. pH of the precipitation of oleic acid at thegiven concentration is 7.45.

2. Activities of oleic monomer and dimer remainalmost constant abOve the precipitation pHand decrease sharply below it.

3. The activity of the acid-soap (OlhH- exhibitsa maximum in the neutral pH range.

It is important to note that the surface activi-ties of various surfactant species can be markedlydifferent from those of each other. It has beenestimated that the surface activity of the acid-soap (OI)2H- is five orders of magnitude higherthan that of the neutral molecule (HOI) and

Oleic Acid Solution Chemistry

Long-chain fatty acids such as oleic acid areamong the commonly used reagents for flotationof phosphates. Flotation using fatty acids is mark-edly affected by solution properties such as pH, asweakly acidic fatty acids can undergo special asso-ciative interactions that influence their adsorptionand flotation properties.12 For example, oleic acidspecies will undergo dissociation to form ions (01)at high pH values and exist as neutral molecules(Hal) at low pH value. In the intermediate region,the ionic and the neutral molecular species canassociate to form ion-molecule complexes((OI)2H). As the surfactant concentration isincreased, micellization or precipitation of the sur-factant can occur in the solution. In addition, sur-factant species can associate to form otheraggregates, such as the dimer (Ol~-), in premicel-lar solutions. Also, long-chain fatty acids such asoleic acid have very limited solubility, which is asensitive function of pH.13-15

The solution equilibria of oleic acid (Hal) areexpressed as follows:

FIGURE 6 ESCA spectra of C(1.s) peak of apatite conditioned in calcite supematant at pH -12

about seven orders of magnitude higher thanthat of the oleate monomer 01-.16

Adsorption and Precipitation ofOleate in Mixed Minerai Systems

As a result of the low solubility of oleic acid, theadsorption of oleate on solid, in some cases, is infact due to precipitation of the surfactant in theinterfacial region.I7 If, at any stage, activity ofCa2+ in solution is greater than that in equilibriumwith Ca2+-oleate, Ca2+-oleate can be expected toprecipitate. Adsorption isothenns of oleic acid on

both francolite and dolomite has been observed tobe a two-region linear isotherm with a change ofslope at about 10-4 kmoVm3 18 (Figure 8). Simul-taneous an~ysis of the dissolved mineral speciesin the supernatants of the samples used in theadsorption experiments (Figures 9a and 9b)shows a sharp decrease in the concentrations ofboth Mg and Ca species when oleate concentra-tion exceeds 1.0 x 10-5 kmoVm3 in the case offrancolite and 3.0 x 10-5 kmoVm3 in the case ofdolomite. This suggests that bulk precipitation ofcalcium and magnesium species can also occur

Beneficiation of Phosphates: Advances in Research and Practice.1.48

FIGURE 7 Oleate species distribution diagram as a function of pH (Total oleate concentration = 3 x 1.0-5M.)

RGURE 8 Adsorption Isotherms of C3.4 labeled oleic acid on francolite and dolomite

of particles by surfactant precipitate does not nec-essarily make the particles hydrophobic and float-able. For example. we did not find calcium oleateto act as a collector when we added it to mineralsuspensions in several cases.19

under such conditions. Bulk precipitation cancause depletion of valuable reagents and retardtheir full utilization for processing. Bulk precipita-tion can also coat the mineral particle and changeits colloidal behavior. It is to be noted that coating

j

FIGURE 9 Dissolved Ca and Mg levels from: (a) francolite and (b) dolomite suspensions as a function of oleateconcentration

species of calcite and apatite on fatty acid flota-tion of both minerals ~as been studied using min-eral supernatant solutions containing variousdissolved species,. and typical flotation results aregiven in Figure 10. Both supernatants of calciteand apatite are found to depress the calcite flota-tion by oleic acid in the tested pH range, with apa-tite supernatant exhibiting a larger depressioneffect. Similar results have also been obtained forapatite flotation. The supernatants of calcite andapatite depressed the apatite flotation under alltested pH conditions.7

Studies on the dissolved species responsible forthe observed effect revealed that for calcite flota-tion, the depression role of apatite supernatantresults from the combined effects of calcium andphosphate species in solution. The depression roleof calcite supernatant is mostly that of the calciumand possibly some carbonate. The depression dueto calcium is caused by the depletion of reagentowing to the precipitation of calcium oleate. In thecase of apatite flotation, the depression was due tophosphate and carbonate species in solution. Weproposed the competition between oleate andphosphate/carbonate for the surface calcium siteto be the main reason for the depression of apatiteflotation. Calcium-oleate precipitation, in thiscase, does not occur to a significant extent due tothe low concentration of oleic acid used in flota-tion. It must be noted from the above discussionthat water chemistry will playa crucial role in theflotation of apatite-calcite systems by affectingthe surface chemistry of the solids.

Major chemical equilibria for the precipitationof Ca and Mg species by oleate can be given asfollows:

Ca2+ + 201- = Ca(01)2 KcaCOl)2 = 3.81 X 10-13Mi+ + 201- = Mg(OI)2 KMgCOl)2 = 1.58 X 10-11

The onset of the precipitation of Ca(OI)2 andMg(OI)2 is calculated from the solubility productsgiven above and marked in Figure 9. The calcu-lated oleate concentrations at the onset of precipi-tation are in good agreement with experimentalobservations.

It is postulated that, in the case of oleate adsorp-tion on dolomite and francolite, different mecha-nisms govern the adsorption process. In the lowconcentration range «10-4 kmoVm3) , the adsorp-tion of oleate on both minerals occurs mainly dueto chemical bonding on surfaces without any pre-cipitation. At an intermediate concentration ofabout 10-4 kmoVm3, the solubility limit of Ca andMg oleate can be reached in the interfacial regionbut not in the bulk solution, suggesting surfaceprecipitation of oleate on both minerals. In thehigh concentration range (>5 x 10-4 kmoVm3) ,oleate depletion may be dominated in the case ofboth minerals by its precipitation with Ca and Mgin the bulk solution.

From the above discussion, it is clear that a flo-tation separation scheme designed on the basis ofthe surface properties of single minerals is notlikely to perform satisfactorily due to variousinteractions among the dissolved minerals species,surfactants, and the particles, leading to signifi-cant surface alterations. The effect of dissolved

Beneficiation of Phosphates: Advances In Research and Practice1.50

pH

5 8 7 . ..- . 8 .pH

d

pH

c

FIGURE 3.0 a. E~t of apatite supernatant on calcite flotation; b. Effect of calcite supernatant on calcite flotation;c. Effect of calcite supernatant on apatite flotation; d. Effect of apatite supernatant on apatite flotation

TABlE 1. Comparison of laboratory and plant conditions

pHWaterSolid, %Time, min

9.1 to 9.5

Distilled and plant water

10 (2 g sample)

120 (except for kinetics)

Lightnin Labmaster L1UOS. four-bladedcruciformpropeller operating at 350 rpm

9.1 to 9.5

Plant water

72 (1.000 g sample)

3 (except for kinetics)

surfactants and particularly differences in kineticsof adsorption on different minerals during condi-tioning can be the single overriding factor control-ling the efficiency of an industrial flotationoperation. Kinetics of surfactant adsorption on sol-ids can indeed be e~ted to be affected by theirsolution chemistry. Anionic conditioning is a unitoperation that precedes rougher flotation and skinflotation of phosphates in Florida flotation plants.The effect of solution chemistry on the oleic acid

Adsorption Kinetics Under Laboratoryand Plant Conditions

A major factor that usually does not receive muchattention in fundamental investigation of the flota-tion process is that flotation is a dynamic process.Another important factor that is invariably ignoredin basic research is the possible differencesbetWeen the performances of an operation in thelaboratory and the plant. Adsorption kinetics of

:L

Role of Surface Chemistry of Phosphate in Its Beneficiation 2.51.

1ft1~

AGURE 1.1 Kinetics of oleic acid adsorption on francolite in distilled water (D.W.) and plant water (P.W.) underplant conditions

adsorption on francolite during anionic condition-ing has been recently studied in detail.2o In orderto identify the effect of process variables on theadsorption, the experiment was carried out underboth laboratory and plant conditions. The labora-tory and plant conditions are compared in Table 1.

The kinetics of oleic acid adsorption on fran-colite under both laboratory and plant conditions,using distilled water and plant water, is shown inFigure 11. The adsorption density and kinetics areclearly quite different depending on the condi-tions and the water. Under laboratory conditions,the adsorption in the plant water is significantlylower than that in the distilled water. This wasproposed to be due to reagent loss resulting fromoleic precipitation by the dissolved species in plantwater. In contrast, under the plant conditions,adsorption behavior of oleic acid in plant waterand distilled water is similar and adsorption den-sities are lower than those under laboratory condi.tions. The lower adsorption density is due to theintense agitation under plant conditions that canremove some of the bound oleate. The similaradsorption behavior under plant conditions forboth the plant water and the distilled water is dueto the high solids loading in the process. The highsolid/liquid ratio under plant condition reduces

the amount of dissolved species and thus reducesthe reagent loss due to precipitation.

The adsorption isotherms of oleic acid on fran-colite under laboratory and plant conditions arecompared in Figure 12. Adsorption is markedlyhigher under the laboratory conditions than underthe plant conditions. On the other hand. under theplant conditions the adsorption is similar in thedistilled water and the plant water. This suggeststhat the effect of dissolved species is reducedunder plant conditions.

Use of Surface Chemistry PrInciplesto Improve Phosphate flotation

From the above discussion, it can be seen that thesurface chemistry of phosphates and associatedminerals plays a vital role in controlling the bene-ficiation processes. Understanding the chemistryinvolved in these systems offers opportunities tomanipulate such processes by optimizing the con-tributing factors such as alteration of the surfaceproperties; complexation of ions, which causesprecipitation of the surfactant; prevention orenhancement of collector adsorption; and changesin the adsorption kinetics to achieve the desiredselectivity in flotation and flocculation.

In the anionic flotation of phosphate, Ca2+ affectsthe grade of phosphate by activating the quartz

FIGURE 12 Adsorption Isotherms of oleic acid adsorption on francolite in distilled water (D.W.) and plant water(P.W.) under laboratory and plant conditions

Based on the oleic acid solution chemistry, atwo-stage conditioning process for the flotation ofdolomite from apatite was proposed.23 The mixedminerals were first conditioned at pH 10 witholeic acid collector. The system was then recondi-tioned below pH 4.5 where dolomite was floated.The selectivity of dolomite from apatite is attrib-uted to two factors in this process: (1) highadsorption of oleate on dolomite during the firststage at pH 10, which is maintained after recondi-tioning at lower pH, and (2) oleate to oleic acidtransformation upon reconditioning reducing itsefficiency and this reduction being more severefor apatite than for dolomite. In the high pH rangeoleate adsorbs on apatite and calcite through spe-cific interactions, while at low pH when oleic acidis the major species, the adsorption is throughweaker physical interaction. Thus, oleic acid is apoor collector compared to oleate.

Modification of collector adsorption on mineralscan be used to control their flotation response. Inone stUdy, alizarin red 5, a dye that stains calcite,was tested as a modifying agent in a calcite-apatite system due to its preferential adsorptionon these minerals. Even ,though alizarin red 5adsorbs more on apatite than on calcite, itdepresses the flotation of apatite using oleate ascollector more than that of calcite (Figure 13). Inthe absence of alizarin red S, both calcite and

through formation of calcium-bearing precipitatesat high pH. This detrimental effect can be pre-vented by adding sodium silicate, which can futer-act with Ca2+ and form calcium silicate. Sfucecalcium silicate and quartz are negatively charged,detachment of calcium silicate from quartz canoccur and thus quartz flotation can be depressed.21

It has been found that in carbonate/phosphatesystems, with fatty acid as the collector, apatite isdepressed fu the acid media (pH 5.5-6.0) whilecarbonate is floated. l1le depression of phosphateat this pH is possibly due to the adsorption (or for-mation) of aqueous CaHPO4 on its surface, pre-venting surfactant ions from approaching thesurface of the phosphate particles. Free Ca2+ fusolution can affect the formation of aqueousCaHPO4' From thermodynamic considerations itcan be predicted that the selective flotation of car-bonates from phosphates fu acid media can beenhanced by minimizing free Ca2+ fu solution andby fucreasing HPO~- in the system. This can bedone by: (1) decreasfug free Ca2+ concentration futhe system to low values by addfug suitable chem-ical reagents such as sulfuric acid or chelatingagents such as oxalic acid, and (2) adding solublephosphate salts to enhance the depression of thephosphate mfuerals. Results from experimentswith natural phosphate ores are in agreementwith the theoretical predictions.22

Role of Surface Chemistry of Phosphate In Its Beneficiation 153

FIGURE 1.3 Rotation of calcite and apatite from their mixture (.1.:1.) at pH 1.0.5 as a function of alizarin red 5concentration

apatite float with oleate at pH 10.5. When alizarinred S concentration increases to 5 x 10-6 moVm3,the flotation of calcite is affected very little with arecovery of about 90%, while apatite flotation isdepressed to 5-100/0. Calcite flotation is affectedonly at higher concentrations of alizarin red S.Alizarin red S or its derivatives can hence be apromising reagent for the beneficiation of phos-phate with carbonaceous gangues.24

ions. The dissolved mineral species can furtherinteract with mineral solids leading to surfaceconversion of the minerals. Surfactant can exist indifferent forms in solution depending on the solu-tion pH and the surfactant concentration. The dis-solved mineral species can interact with surfactantspecies leading to surface and bulk precipitation.All of these processes can significantly affect theadsorption of reagents on minerals. A full under-standing of various interactions in a surfactant-solid-solution system is essential for developingefficient separation schemes. Indeed, desiredselectivity can be achieved by the use of appropri-ate additives to control dissolved species, modifysurface chemistry of the solid, and optimizereagent adsorption and particularly the kineticsinvolved during the conditioning.

SUMMARY

The surface chemistry of phosphates plays a pri-mary role in determining reagent adsorption onthe phosphates and hence their separation by pro-cesses such as flotation and selective flocculation.The complex interactions among solid surface spe-cies, dissolved species, and reagents species alsohave marked effects on the reagent adsorption aswell as flotation/flocculation. Minerals canundergo dissolution, with the extent of dissolutiondepending on solution conditions such as pH,ionic strength, and concentration of constituent

ACKNOWLEDGMENTS

The authors acknowledge the financial support ofthe National Science Foundation (CfS-9622781and EEC-9804618).

3.54 Beneficiation of Phosphates: Advances in Research and Practice

REFERENCES

1. Somasundaran, P., Agar, G.E., "Further StreamingPotential StUdies on Apatite in Inc- :ganicElectrolytes," Trans AIME, 252, J )72, p. 348.

2. Somasundaran, P., Wang, Y.H.C., "SurfaceChemical Characteristics and AdsorptionProperties of Apatite," in Adsorption on and SurfaceChemistry of Hydroxyapatite, D.N. Misra, ed.,Plenum Press: New York, 1984, p. 129.

3. Somasundaran, P., "Pretreatment of MineralSurface and its Effects on their Properties," in CleanSurfaces: Their Preparation and CharacterizationforInterfacial Studies, G. Goldfinger, ed., MarcelDekker: New York, 1970, p. 285.

4. Somasundaran, P., 1968, "Zeta-Potential of Apatitein Aqueous Solutions and Its Change duringEquilibration," Journal of CoUoids and InterfaceScience, 27, p. 659.

5. Phillips, S., Kulkarni, R.D., and Somasundaran, P.,"Effect of Pretreatment with Fluoride Solutions onApatite Electrochemical Properties," J. Dental Res.,54, 1975, p. 180.

6. Arnankonah, J.D., Somasundaran, P., andAnanthapadmanabhan, K.P., 1985, "Effects ofDissolved Mineral Species on the Dissolution!Precipitation Characteristics of Calcite andApatite," CoUoids & Surfaces, 15, p. 295.

7. Ananthapadmanabhan, K.P., Somasundaran, P.,1984, "The Role of Dissolved Mineral Species inCalcite-Apatite Flotation," Minerals & MetallurgicalProcessing, 1, p. 36.

8. Somasundaran, P., Amankonah, J.D., andAnanthapadmanabhan, K.P., 1985,"Mineral-Solution Equilibria in Sparingly SolubleMineral Systems," CoUoim & Surfaces, 15, p. 309.

9. Arnankonah, J.D., Somasundaran,P., andAnanthapadmanabhan, K.P., 1985, "Effects ofDissolved Mineral Species on the ElectrokineticBehavior of Calcite and Apatite," CoUoids &Surfaces, 15, p. 335.

10. Andersen, J.B., Somasundaran, P., 1993,"Mechanism Determining Separation of PhosphaticClay Waste by Selective Flocculation," Minerals &Metallurgical Processing, 10, p. 200.

11. Andersen, J.B., Somasundaran, P., 1993, "The Roleof Changing Surface Mineralogy on the Separationof Phosphatic Clay Waste," International Journal ofMineral Processing, 38, p. 189.

12. Ananthapadmanabhan, K.P., Somasundaran, P.,"Oleate Chemistry and Hematite Flotation," inInterfadal Phenomena in Mineral Processing,B. Yarar and D.J. Spottiswood, ed., EngineeringFoundation, New York, p. 207.

13. Kulkarni, R.D., Ananthapadmanabhan, KP., andSomasundaran, P., "Flotation Mechanism Based onlonomolecular Complexes," in Proc. XIlth Int. Min.Proc. Congress, Round Table-Iron Ore, Sao Paulo,Brazil, 1977, p. 80.

14. Somasundaran, P., Ananthapadmanabhan, KP.,and Ivanov, I.B., "Dimerization of Oleate inAqueous Solutions," J. Coli. Interf Sci, 99, 1984,

p.128.15. Ananthapadmanabhan, K.P., Somasundaran, P.,

"Acid-Soap Formation in Aqueous OleateSolutions," J. CoIL Interf. Sd., 122,1988, p. 104.

16. Ananthapadmanabhan, KP., "AssociativeInteractions in Surfactant Solutions and Their Rolein Flotation," D. E. Sci. Thesis, ColumbiaUniversity,1980.

17. Ananthapadmanabhan, K.P., Somasundaran, P.,1985, "Surface Precipitation of In organics andSurfactants and its Role in Adsorption andFlotation," Colloids & Surfaces, 13, p. 151.

18. Somasundaran, P., Xiao, L., and Wang, D.,"Solution Chemistry of Sparingly, SolubleMinerals," Minerals & Metallurgical Processing,8(3),1991, pp. 115-121.

19. Somasundaran, P., Wang, Y.H.C., unpublished results.20. Maltesh, C., Somasundaran, P., and Gruber, GA.,

1996, "Fundamentals of Oleic Acid Adsorption onPhosphate Flotation Feed During AnionicConditioning, " Minerals & Metallurgical Processing,

13, p.157.21. Dho, H., Iwasaki, I., 1990, "Role of Sodium Silicate

in Phosphate Flotation, " Minerals & Metallurgical

Processing, 7, p. 215.22. Elgillani, D.A., Abouzeid, A.-Z.M., 1993,

"Floatation of Carbonates from Phosphate Ores inAcidic Media," International Journal of MineralProcessing, 38, p. 235.

23. Moudgil, B.M., Chanchani, R., 1985, "SelectiveFlotation of Dolomite From Francolite Using Two-Stage Conditioning," Minerals & MetallurgicalProcessing, 2(1), p. 19-25.

24. Fu, E., Somasundaran, P., 1986, "Alizarin Red S asa Flotation Modifying Agent in Calcite-ApatiteSystems," International Journal of MineralProcessing, 18, p. 287.

!1.'"

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