CoUoids and Surfaces, 15 (1985) 335-353Elsevier Science Publishers B. V., Amsterdam - Printed in The Netherlands

EFFECfS OF DISSOLVED MINERAL SPECIES ON THE ELECfRO-KINETIC BERA VIOR OF CALCITE AND APATITE

J. OFORI AMANKONAH and P. SOMASUNDARAN

School of Engineering and Applied Science, Columbia University, New York, NY 10027(U.S.A.)(Received 7. May 1984; accepted in final form 22 March 1985)

ABSTRACT

The interfacial behavior of salt-type minerals such as calcite and apatite is governedto a large extent by their electrochemical properties which in turn are governed by pHand concentration of dissolved mineral species. In mixed mineral systems, the interfacialbehavior of various minerals is often quite different from what might be expected onthe basis of their individual properties. Predictions for flotation separation based onsingle mineral tests often fail in these systems mainly owing to the interactions betweendissolved mineral species.

The effect of dissolved mineral species of the calcite-apatite system is investigatedin this study using the electrokinetic technique. The electrochemical, dissolution andspectroscopic properties of each mineral were markedly altered by the supernatants ofthe other mineral. Most interestingly, under the test conditions, the isoelectric pointof each mineral in the supernatant of the other was observed to be similar to the super-natant mineral source. It is clear that, under these conditions, a flotation separationscheme designed on the basis of the surface properties of the single minerals is not likelyto perform satisfactorily. Tests in inorganic solutions have shown that surface reactionand/or bulk precipitation could be responsible for the observed shifts. Surface chemicalalterations predicted from theoretical considerations using species distribution diagramsare correlated with the experimental results.

INTRODUCfION

Separation of phosphates from carboni tic gangues by flotation has beena major problem in the phosphatic industry for years. Direct applicationof various processes described in the literature to phosphate ores has oftenfailed to reduce the carbonate content of the final product to an acceptablelevel [1, 2]. Furthermore, a previous investigation [3] has shown predictionsof selective flotation on the basis of single mineral tests to fail even forsynthetic mineral mixtures.

The difficulties encountered in the separation of calcite-dolomite typeimpurities from phosphates have been attributed to the similarities in thesurface chemical, electrokinetic and dissolution properties of these minerals

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as a result of which both the carbonates and phosphates respond similarlyto anionic and cationic collectors [4, 5].

The dissolution characteristics of the minerals can be expected to playa major role in determining the nature of the interactions taking place inthe bulk solution or in the interfacial region and hence the efficiency ofsuch interfacial processes as flotation and flocculation. In salt-type mineralsystems where the solubility is markedly higher than in most other systemssuch as oxides and silicates [6], this role is expected to be even more sig-nificant.

Effects of dissolved species in various mineral systems have been reportedby several investigators. Healy [7] observed drastic changes in the point ofzero c~e of magnetite when stored in glass containers. The observedchanges were attributed to the silicate species released from the glass con-tainer. In a similar manner, the dissolved species can cause significant activa-tion or depression of flotation under certain solution conditions. These ef-fecw have been correlated with both the specific adsorption of hydroxycomplexes of the cations as well as the stronger tendency of multivalentions over monovalents to compete with the surfactant ions for the adsorp-tion sites [8-14]. Fuerstenau and Miller [15] have obtained good correla-tion between the pH range in which quartz flotation, using anionic sur-factants, is activated by cations and the pH range in which the first hydroxycomplex of the cation is formed. This illustrates the role of the specificadsorption of these complexes. The high adsorption affinity of partiallyhydrolyzed ions on charged surfaces has been recognized by Wolstenhammeand Schulman [16]. Dissolved ferric and other metallic species have beenreported to activate the flotation of silica using fatty acid as the collector[11, 12, 14, 15]. This suggests that the dissolved species from hematitecan affect silica flotation in hematite-silica systems. It should prove usefulto study the effect of hematite supernatant itself on silica flotation. In aseparate study by Iwasaki and Soto [17], the effect of calcium and mag-nesium species released into the solution by the impurities associated withhematite have been shown to have a significant effect on the selectiveflocculation of hematite from quartz. This study clearly shows that thewater in equilibrium with the ore can contain sufficient amoun~ of calciumand magnesium ions to significantly alter the surface properties of silica.As has been shown by several investigators [18-21], surface precipitationof a different form of the mineral than its original one from a solution thatis supersaturated is also possible. This fact has been considered in detail byParks [22] for alumina systems and by Brown and co-workers [19,20,23]for phosphates.

Due to the complex nature of salt-type mineral systems, much controver-sy exists in the literature on the role of polyvalent ions. Polkin [24] con- ,siders the polyvalent ions to be essential for the formation on hydrophobicmultilayers on calcite. Sun et al [25] and Eigeles [26] consider such ions tobe harmful due to precipitation as metal soaps in the pulp.

337

Surface or bulk precipitation of a more stable phase is perhaps one ofthe major effects due to dissolved species. For example, in the presenceof 10-4 kmol m-3 calcium, MacKenzie and Mishra [27] observed that theapatite surface closely resembled that of the calcite. This could be due tothe precipitation of calcium carbonate on the surface of the apatite. Theimplication of this observation is that a separation scheme based on theinterfacial properties of the individual minerals is not likely to performsatisfactorily for beneficiation of ores. The observation by Le Bell andLindstrom [28], and Calara and Miller [29] on the electrokinetic behaviorof fluorite after various treatments must be noted. These authors observeddrastic changes in the point of zero charge of fluorite when it was con-ditioned in carbonate solutions.

The effects of dissolved mineral species of calcite and apatite on theflotation of these minerals has been shown recently in our laboratory tobe drastic under certain solution conditions [6]. Flotation of calcite andapatite in mineral supernatants was drastically depressed due to the pre-sence of dissolved species.

The electrokinetic behavior of calcite and apatite in water and in mineralsupernatants is discussed in this work. Experiments have also been con-ducted in various electrolytes containing mineral constituent ions in orderto identify their role when present in mineral supernatants. The observedchanges in the electrokinetic behavior have been correlated with thermo-dynamic predictions.

EXPERIMENTAL MATERIALS AND METHODS

Materials

CalciteThe calcite of lO-micrometer size was prepared during an earlier investi-

gation by Somasundaran and Wang [4, 30] and was used in this study.

ApatiteSynthetic hydroxyapatite was used in this study. It was prepared by

the precipitation technique described by Moreno et al. [20] and modifiedby Somasundaran and Wang [4, 30]. The method involves mixing appro-priate amounts of K2HPO4 in a KOH solution and Ca(NO3h in water andboiling the reaction products under appropriate solution conditions toavoid the formation of octa-calcium phosphate as a precursor. To avoidthe possible precipitation of CaC~ in the system or the formation of car-bonate species in the crystal lattice of the apatite, the experimental set-upwas constantly flushed with purified and dried nitrogen gas. Settled crystalswere washed and then freeze-dried and characterized using electron spectro-scopy for chemical analysis (ESCA).

The stoichiometry of the samples was determined by measuring concen-trations of dissolved species in the mineral supernatants. In these tests,

338

the conditioning of the minerals was done in a nitrogen glove box to mini-mize the interference by atmospheric carbon dioxide. The molar ratios ofcalcium/phosphorus (Ca/P) and calcium/carbonate (Ca/Ct) were 1.59 and0.93 for apatite and calcite, respectively, and their calculated thermo-dynamic solubility products were 10-115 and 10-8)45. ESCA was used todetermine the surface chemical composition of the freshly prepared (un-treated) mineral surfaces. Comparison of the ESCA spectra with standardsprovided by the Physical Electronics Division of Perkin-Elmer [31] showedthe samples to be pure calcite and apatite. Surface areas of the calcite andapatite samples as determined by the BET technique using Quantasorbwere found to be 0.71 and 30.3 m2 g-l, respectively.

lnorganicsAll inorganics such as KN~, KOH, K2C~, ~PO4, Ca(N~h, HN~ etc.

were of certified ACS grade purchased from Fisher Scientific Company.

Methods

Reactions at the mineraholution interface were studied in this work bymeasuring the zeta potential as a function of pH under various conditions.The zeta-potential values were measured by the electrophoretic techniqueusing the Lazer Zee Meter and the Zeta-Meter manufactured by Pen Ken Inc.and Zeta-Meter Corp., respectively. The equilibration of the mineral for zeta-potential measurements was done by stirring for 30 min 0.1 g of the mineralwith 100 ml of triply distilled water containing the required amount of KOHor HNO3. Thirty minutes conditioning time was selected on the basis ofkinetic studies. Depending on the initial pH, significant changes in pH andzeta potential were observed only within the first 10-20 min of conditioning.Furthermore, the data in the literature indicate that conditioning times forflotation tests do not often exceed 10 min [6]. It is to be noted, however,that several hours of conditioning are required for complete equilibrium [32,34]. The equilibration as well as electrokinetic measurements were conductedin the presence of atmospheric carbon dioxide.

Zeta-potential measurements were conducted in mineral supernatantsas well as in water. The supernatant was prepared by stirring the mineralwith water at the desired pH for 30 min followed by centrifugation. Thesupernatant obtained was checked under an optical microscope to ensurethe absence of solid particles. The pH was then readjusted and the solutionsused for conditioning of the mineral prior to zeta-potential measurements.To check for possible effects of variations in ionic strength, experimentswere conducted also in 2 X 10-3 kmol m-J KN~.

Additional measurements were made in mixed supernatants preparedby combining 50 ml each of the supernatants of calcite and apatite pre-pared at about the same pH.

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RESULTS AND DISCUSSION

Results obtained for the zeta potential of apatite and calcite in water,and in 2 X 10-3 klnol m-3 KN~ solutions are given in Figs l(a) and l(b),respectively. It can be seen that the isoelectric points of calcite and apatitein both water and KNO3 solutions are about 10.5 and 7.4, respectively.This is in agreement with the results reported in the literature [27, 32, 33].

The effect of the supernatant of calcite on the zeta potential of apatiteis shown in Figs 2(a) and 2(b). Examination of the results shows that theapatite surface is more positively charged in calcite supernatant than inwater over the entire pH range investigated, and that the isoelectric pointhas shifted from 7.4 to about 11, towards that of calcite. Similarly, thezeta potential of calcite in apatite supernatant was measured as a functionof pH. The results given in Fig. 3 show the zeta potential of calcite to bereduced drastically by the supernatant of apatite, and the isoelectric pointto shift from 10.5 to 6.5. The decrease in the magnitude of the zeta-poten-tial values in KN~ solut.ions from those in water alone could be attributedto the compre~ion of the double layer under high ionic strength conditions[34].

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In order to identify clearly the effects of supernatants, the results inFigs 2(b) and 3(b) (experiments under controlled ionic strenth conditions)are replotted in Fig. 4. The results show calcite and apatite to interchangeapproximately their isoelectric points in the supernatants of each other.

The observation of Ananthapadmanabhan and Somasund~, reportedin Ref. [6], for the effect of the supernatants of calcite and apat.ite ontheir flotation behavior must be noted here. These effects are obvi~,,-due to the dissolved mineral species present in the supernatants. It )s-iD-teresting to note that the flotation of these minerals was found t9---be af-fected also by their own supernatants. However, the zeta potehtial of amineral is not expected to be affected to any appreciable extent by its ownsupernatant if similar conditions are maintained since the equilibriumconcentration of the potential determining ions on the surface shouldbe identical whether treated in water or in their own supernatants. Figure5 showing the results of the zeta potential of calcite and apatite in theirown supernatants indicates that, indeed, the zeta potentials are only slightlyaffected.

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The results of the zeta-potential measurements in mixed supernatants,presented in Fig. 6, further show the effects of dissolved species; if calciteand apatite are present as a 1:1 mixture (weight basis), the two mineralshave identical surface-charge characteristics in the basic pH region.

Evidently, these effects are due to some or all of the dissolved mineralspecies. In order to identify the species actually responsible, zeta-potentialmeasurements of calcite and apatite were conducted in inorganic solutionscontaining each species. The results obtained in the presence of Ca(N~h,K2C~ and K2HPO4 are given in Figs 7 to 9. Data given in Fig. 7 for thezeta potential of apatite in Ca(N~h solutions of several concentrationsshow that the surface of the mineral is more positively charged in Ca(N~hsolutions at all pH and levels of concentration tested. This is in agreementwith the results in the literature [4, 30, 34] .

The effect of K2C~ on the zeta potential of apatite (Fig. 8) is foundto be minimal in the concentration range studied in accord with the factthat the carbonate will be present mostly in the HCO; form in the pHrange where apatite is negatively charged and, therefore, cannot be expected

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The effect of K3PO4 addition shown in Fig. 9 indicates that the surfaceof apatite has become more negatively charged in the pH range examinedin agreement wiUl past results [27, 35]. This decrease is mainly due to therole of phosphate as potential determining ions for apatite.

Figures 10 and 11 present results for the zeta potential of calcite inthe above inorganic solutions. As can be seen from Fig. 10, the effect ofCa(NO3)1 addition is to make the surface of calcite more positively charged.In the concentration range tested, only positive potentials were recordedover the entire pH range.

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Similar to the effects on apatite, K3PO4 addition reduces the zeta-potentialvalues of calcite (Fig. 11). It must be noted that zeta-potential values ob-tained for calcite in 10-3 kmol m-3 K3PO4 closely agree with those obtainedfor apatite in water. As shown later, even less than 10-9 kmol m-3 totalphosphate can cause precipitation of apatite from calcite supernatants underneutral pH conditions. The observed changes in zeta potential can there-fore be attributed to the precipitation of apatite on the calcite surface.

Analysis of the zeta-potential results presented above show the observedshifts in the isoelectric point of calcite and apatite in the supernatants ofeach other to be the result of many complex surface processes. The mecha-nism of surface charge generation for these minerals has been examinedin detail during an earlier investigation [4, 5, 34, 35]. The electrokineticbehavior of apatite and calcite in various inorganic solutions has also beendetermined [32, 35]. These properties, however, have not been studiedfor heterogeneous mineral systems. In a recent investigation [36] we haveshown from theoretical considerations that, depending on the solutionconditions, apatite surface can be converted to calcite, and vice versathrough surface reaction or bulk precipitation of the more stable phase.

349

The observed changes in the electrokinetic properties of calcite and apatitecan therefore be examined on the basis of the mineral-solution chemicalequilibria involving dissolved mineral species.

From studies of solubility isotherms for apatite and calcite at 2SO C[19,23,37], the singular point for these minerals is identified to be 9.3.Above this pH calcite is more stable than apatite. The implication is that,if apatite is brought in contact with alkaline solutions (pH> 9.3) previouslyequilibrated with calcite, calcite can precipitate on the surface of apatite.From similar considerations, the conversion of calcite to apatite is also pos-sible below the singular point. We have shown theoretically [36] that, ifcalcite supernatants are prepared above pH 9.3, the total carbonate presentin these solutions can exceed the amount required for the conversion ofapatite to calcite as illustrated in Fig. 12(a). As shown in Fig. 12(b), thetotal phosphate in equilibrium with apatite below the singular point canalso convert calcite surface to apatite under appropriate conditions. Asmentioned earlier, much lower phosphate concentrations are required inthis case. Our investigation on the solubility behavior of these mineralsbased on supernatant analysis under various solution conditions [38] has

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also shown that, indeed, the precipitation of these minerals is quite pos-sible.

The surface conversion of calcite and apatite is further illustrated bythe results of recent spectroscopic investigations in our laboratory usingESCA [39]. The minerals were conditioned under solution conditionssimilar to those used for zeta-potential studies. The data given in Fig. 13show that, when apatite is conditioned in the supernatant of calcite atpH ,..., 12, its surface exhibits spectroscopic properties characteristic ofboth calcite and apatite. This behavior is attributed to the precipitationfo calcite on apatite.

On the basis of the electrokinetic, spectroscopic, flotation, dissolutionand precipitation behavior of these minerals in heterogeneous systems,as well as the analysis of the mineral-solution chemical equilibria govern-ing them, conditions for selective flotation have been discussed in detailin a recent publication [40].

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SUMMARY AND CONCLUSIONS

Dissolved mineral species playa major role in the interfacial behaviorof sparingly soluble minerals such as calcite and apatite. Electrokineticstudies were conducted on the calcite--apatite heterogeneous system todevelop an understanding of the manner in which dissolved species affectthis behavior. The isoelectric points of calcite and apatite in both waterand KNO3 solutions are 10.5 and 7.4, respectively. When the mineralsare conditioned in the supernatants of each other, they approximately

352

interchange their points of zero charge; when apatite is conditioned in thesupernatants of calcite, its isoelectric point shifts from 10.5 to about 6.5(8.0 in the presence of 2 X 10-3 kmol m-3 KN~) towards that of apatite.Similarly, the isoelectric point of apatite is shifted from 7.4 to about 11.0towards that of calcite. If calcite and apatite are present as a 1: 1 mixture(weight basis), the two minerals have identical surface-charge characteristicsin the basic pH range. These effects are discussed on the basis of mineral-solution equilibria controlling these systems. Thermodynamic considerationsindicate that the apatite surface can be converted to calcite in the super-natant of the latter and vice cersa. Based on the results of the electrokinetic,spectroscopic, flotation, dissolution and precipitation studies, the observedshifts in isoelectric points are attributed to the precipitation of one mineralon the surface of the other.

ACKNOWLEDGEMENTS

Support of the National Science Foundation (CPE-83-04059) and FloridaInstitute of Phosphate Research is greatly acknowledged. We wish to thankK.P. Ananthapadmanabhan and Kenneth Wong for helpful discussions.

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29 J. V. Calara and J.D. Miller, The Electrokinetic Behavior of Fluorite in the Absenceof Surface Carbonation, Colloids Surfaces, submitted.

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33 R.K. Mishra, S. Chander and D. W. Fuerstenau, Effect of Ionic Surfactants on theElectrophoretic Mobility of Hydroxyapatite, Colloids Surfaces, 1 (1980) 105.

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35 P. Somasundaran and G.E. Agar, Further Streaming Potential Studies of Hydroxy-apatite in Inorganic Solutions, Trans. AIME, 252 (1972) 348-352.

36 P. Somasundaran, J.O. Amankonah and K.P. Ananthapadmanabhan, Mineral-Solution Equilibria in Sparingly Soluble Mineral Systems, Colloids Surfaces, 15 (1985)309-333.

37 S.K. Mishra, The Electrokinetics of Apatite and Calcite in Inorganic ElectrolyteEnvironment, Int. J. Miner. Process., 5 (1978) 69-83.

38 J.O. Amankonah, P. Somasundaran and K.P. Ananthapadmanabhan, Effect of Dis-solved Mineral Species on the Dissolution/Precipitation Characteristics of Calcite andApatite, Colloids Surfaces, 15 (1985) 295-307.

39 P. Somasundaran, J.O. Amankonah and K.P. Ananthapadmanabhan, SpectroscopicAnalysis of Calcite-Apatite Interactions Using ESCA, unpublished results.

40 P- Somasundaran, J.O. Amankonah and K.P. Ananthapadmanabhan, Calcite-ApatiteInteractions and Their Effects on Selective Flotation Using Oleate. Accepted forpresentation at the XV Int. Miner. Proc. Congress, Cannes, France, June 1985.