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

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1 CoUoids and Surfaces, 15 (1985) Elsevier 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 127 (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 governed to a large extent by their electrochemical properties which in turn are governed by ph and concentration of dissolved mineral species. In mixed mineral systems, the interfacial behavior of various minerals is often quite different from what might be expected on the basis of their individual properties. Predictions for flotation separation based on single mineral tests often fail in these systems mainly owing to the interactions between dissolved mineral species. The effect of dissolved mineral species of the calcite-apatite system is investigated in this study using the electrokinetic technique. The electrochemical, dissolution and spectroscopic properties of each mineral were markedly altered by the supernatants of the other mineral. Most interestingly, under the test conditions, the isoelectric point of each mineral in the supernatant of the other was observed to be similar to the supernatant mineral source. It is clear that, under these conditions, a flotation separation scheme designed on the basis of the surface properties of the single minerals is not likely to perform satisfactorily. Tests in inorganic solutions have shown that surface reaction and/or bulk precipitation could be responsible for the observed shifts. Surface chemical alterations predicted from theoretical considerations using species distribution diagrams are correlated with the experimental results. INTRODUCfION Separation of phosphates from carboni tic gangues by flotation has been a major problem in the phosphatic industry for years. Direct application of various processes described in the literature to phosphate ores has often failed to reduce the carbonate content of the final product to an acceptable level [1, 2]. Furthermore, a previous investigation [3] has shown predictions of selective flotation on the basis of single mineral tests to fail even for synthetic mineral mixtures. The difficulties encountered in the separation of calcite-dolomite type impurities from phosphates have been attributed to the similarities in the surface chemical, electrokinetic and dissolution properties of these minerals 1985 Elsevier Science Publishers B. V.

2 336 as a result of which both the carbonates and phosphates respond similarly to anionic and cationic collectors [4, 5]. The dissolution characteristics of the minerals can be expected to play a major role in determining the nature of the interactions taking place in the bulk solution or in the interfacial region and hence the efficiency of such interfacial processes as flotation and flocculation. In salt-type mineral systems where the solubility is markedly higher than in most other systems such as oxides and silicates [6], this role is expected to be even more significant. Effects of dissolved species in various mineral systems have been reported by several investigators. Healy [7] observed drastic changes in the point of zero ce of magnetite when stored in glass containers. The observed changes were attributed to the silicate species released from the glass container. In a similar manner, the dissolved species can cause significant activation or depression of flotation under certain solution conditions. These effecw have been correlated with both the specific adsorption of hydroxy complexes of the cations as well as the stronger tendency of multivalent ions over monovalents to compete with the surfactant ions for the adsorption sites [8-14]. Fuerstenau and Miller [15] have obtained good correlation between the ph range in which quartz flotation, using anionic surfactants, is activated by cations and the ph range in which the first hydroxy complex of the cation is formed. This illustrates the role of the specific adsorption of these complexes. The high adsorption affinity of partially hydrolyzed ions on charged surfaces has been recognized by Wolstenhamme and Schulman [16]. Dissolved ferric and other metallic species have been reported to activate the flotation of silica using fatty acid as the collector [11, 12, 14, 15]. This suggests that the dissolved species from hematite can affect silica flotation in hematite-silica systems. It should prove useful to study the effect of hematite supernatant itself on silica flotation. In a separate study by Iwasaki and Soto [17], the effect of calcium and magnesium species released into the solution by the impurities associated with hematite have been shown to have a significant effect on the selective flocculation of hematite from quartz. This study clearly shows that the water in equilibrium with the ore can contain sufficient amoun of calcium and magnesium ions to significantly alter the surface properties of silica. As has been shown by several investigators [18-21], surface precipitation of a different form of the mineral than its original one from a solution that is supersaturated is also possible. This fact has been considered in detail by Parks [22] for alumina systems and by Brown and co-workers [19,2,23] for phosphates. Due to the complex nature of salt-type mineral systems, much controversy exists in the literature on the role of polyvalent ions. Polkin [24] con-, siders the polyvalent ions to be essential for the formation on hydrophobic multilayers on calcite. Sun et al [25] and Eigeles [26] consider such ions to be harmful due to precipitation as metal soaps in the pulp.

3 Surface or bulk precipitation of a more stable phase is perhaps one of the major effects due to dissolved species. For example, in the presence of 1-4 kmol m-3 calcium, MacKenzie and Mishra [27] observed that the apatite surface closely resembled that of the calcite. This could be due to the precipitation of calcium carbonate on the surface of the apatite. The implication of this observation is that a separation scheme based on the interfacial properties of the individual minerals is not likely to perform satisfactorily for beneficiation of ores. The observation by Le Bell and Lindstrom [28], and Calara and Miller [29] on the electrokinetic behavior of fluorite after various treatments must be noted. These authors observed drastic changes in the point of zero charge of fluorite when it was conditioned in carbonate solutions. The effects of dissolved mineral species of calcite and apatite on the flotation of these minerals has been shown recently in our laboratory to be drastic under certain solution conditions [6]. Flotation of calcite and apatite in mineral supernatants was drastically depressed due to the presence of dissolved species. The electrokinetic behavior of calcite and apatite in water and in mineral supernatants is discussed in this work. Experiments have also been conducted in various electrolytes containing mineral constituent ions in order to identify their role when present in mineral supernatants. The observed changes in the electrokinetic behavior have been correlated with thermodynamic predictions. EXPERIMENTAL MATERIALS AND METHODS Materials Calcite The calcite of lo-micrometer size was prepared during an earlier investigation by Somasundaran and Wang [4, 3] and was used in this study. Apatite Synthetic hydroxyapatite was used in this study. It was prepared by the precipitation technique described by Moreno et al. [2] and modified by Somasundaran and Wang [4, 3]. The method involves mixing appropriate amounts of K2HPO4 in a KOH solution and Ca(NO3h in water and boiling the reaction products under appropriate solution conditions to avoid the formation of octa-calcium phosphate as a precursor. To avoid the possible precipitation of CaC in the system or the formation of carbonate species in the crystal lattice of the apatite, the experimental set-up was constantly flushed with purified and dried nitrogen gas. Settled crystals were washed and then freeze-dried and characterized using electron spectroscopy for chemical analysis (ESCA). The stoichiometry of the samples was determined by measuring concentrations of dissolved species in the mineral supernatants. In these tests, 337

4 338 the conditioning of the minerals was done in a nitrogen glove box to minimize the interference by atmospheric carbon dioxide. The molar ratios of calcium/phosphorus (Ca/P) and calcium/carbonate (Ca/Ct) were 1.59 and.93 for apatite and calcite, respectively, and their calculated thermodynamic solubility products were and 1-8)45. ESCA was used to determine the surface chemical composition of the freshly prepared (untreated) mineral surfaces. Comparison of the ESCA spectra with standards provided by the Physical Electronics Division of Perkin-Elmer [31] showed the samples to be pure calcite and apatite. Surface areas of the calcite and apatite samples as determined by the BET technique using Quantasorb were found to be.71 and 3.3 m2 g-l, respectively. lnorganics All inorganics such as KN, KOH, K2C, PO4, Ca(Nh, HN etc. were of certified ACS grade purchased from Fisher Scientific Company. Methods Reactions at the mineraholution interface were studied in this work by measuring the zeta potential as a function of ph under various conditions. The zeta-potential values were measured by the electrophoretic technique using the Lazer Zee Meter and the Zeta-Meter manufactured by Pen Ken Inc. and Zeta-Meter Corp., respectively. The equilibration of the mineral for zetapotential measurements was done by stirring for 3 min.1 g of the mineral with 1 ml of triply distilled water containing the required amount of KOH or HNO3. Thirty minutes conditioning time was selected on the basis of kinetic studies. Depending on the initial ph, significant changes in ph and zeta potential were observed only within the first 1-2 min of conditioning. Furthermore, the data in the literature indicate that conditioning times for flotation tests do not often exceed 1 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 conducted in the presence of atmospheric carbon dioxide. Zeta-potential measurements were conducted in mineral supernatants as well as in water. The supernatant was prepared by stirring the mineral with water at the desired ph for 3 min followed by centrifugation. The supernatant obtained was checked under an optical microscope to ensure the absence of solid particles. The ph was then readjusted and the solutions used for conditioning of the mineral prior to zeta-potential measurements. To check for possible effects of variations in ionic strength, experiments were conducted also in 2 X 1-3 kmol m-j KN. Additional measurements were made in mixed supernatants prepared by combining 5 ml each of the supernatants of calcite and apatite prepared at about the same ph.

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8 342 RESULTS AND DISCUSSION Results obtained for the zeta potential of apatite and calcite in water, and in 2 X 1-3 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 apatite in both water and KNO3 solutions are about 1.5 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 apatite is shown in Figs 2(a) and 2(b). Examination of the results shows that the apatite surface is more positively charged in calcite supernatant than in water over the entire ph range investigated, and that the isoelectric point has shifted from 7.4 to about 11, towards that of calcite. Similarly, the zeta potential of calcite in apatite supernatant was measured as a function of ph. The results given in Fig. 3 show the zeta potential of calcite to be reduced drastically by the supernatant of apatite, and the isoelectric point to shift from 1.5 to 6.5. The decrease in the magnitude of the zeta-potential values in KN solut.ions from those in water alone could be attributed to the compreion of the double layer under high ionic strength conditions [34]. > E -J Z 1&1 4-1&1 N Fig. 4. mration ph or the effects or 8upernatants on the IEP of calcite and apatite.

9 343 :J: Q...; c,., a GI Co...s 3 i. -- i i '41 Co GI 'u g.s GI '(3 g :3 C QI i. N ad rz

10 344 In order to identify clearly the effects of supernatants, the results in Figs 2(b) and 3(b) (experiments under controlled ionic strenth conditions) are replotted in Fig. 4. The results show calcite and apatite to interchange approximately their isoelectric points in the supernatants of each other. The observation of Ananthapadmanabhan and Somasund, reported in Ref. [6], for the effect of the supernatants of calcite and apat.ite on their flotation behavior must be noted here. These effects are obvi,,- due to the dissolved mineral species present in the supernatants. It )s-idteresting to note that the flotation of these minerals was found t9---be affected also by their own supernatants. However, the zeta potehtial of a mineral is not expected to be affected to any appreciable extent by its own supernatant if similar conditions are maintained since the equilibrium concentration of the potential determining ions on the surface should be identical whether treated in water or in their own supernatants. Figure 5 showing the results of the zeta potential of calcite and apatite in their own supernatants indicates that, indeed, the zeta potentials are only slightly affected.

11 The results of the zeta-potential measurements in mixed supernatants, presented in Fig. 6, further show the effects of dissolved species; if calcite and apatite are present as a 1:1 mixture (weight basis), the two minerals have identical surface-charge characteristics in the basic ph region. Evidently, these effects are due to some or all of the dissolved mineral species. In order to identify the species actually responsible, zeta-potential measurements of calcite and apatite were conducted in inorganic solutions containing each species. The results obtained in the presence of Ca(Nh, K2C and K2HPO4 are given in Figs 7 to 9. Data given in Fig. 7 for the zeta potential of apatite in Ca(Nh solutions of several concentrations show that the surface of the mineral is more positively charged in Ca(Nh solutions at all ph and levels of concentration tested. This is in agreement with the results in the literature [4, 3, 34]. The effect of K2C on the zeta potential of apatite (Fig. 8) is found to be minimal in the concentration range studied in accord with the fact that the carbonate will be present mostly in the HCO; form in the ph range where apatite is negatively charged and, therefore, cannot be expected M5 6 4 > E -J..- Z t&j 2 to Q. <r..- t&j N ph Fig. 7. Effect of Ca(NO,h addition on the zeta potential of apatite.

12 346 1l Ị :i... c.8 8- law) lvij.n3j.od VJ.3Z i os c c "D :a 'Q. o' ] rz1.!i. 1. :j j 8..s = CI Ạ.. Q Q :.; ' '.! rz1.'w

13 to produce any specific effects. On the other hand, high concentrations of K1 can be expected to cause the precipitation of CaC on the surface of apatite to produce more positive zeta-potential values. However, relatively higher concentrations of carbonate are required for the calcite precipitation. While the concentrations of K1 u in the experiments (1-3 and 1-1 kmol m-3 K1C) would be insufficient for this process, calcite supernatants could contain enough carbonate to effect the precipitation. The possibility of surface conversion processes from a thermodynamic point of view is discussed Jater in this section. The effect of K3PO4 addition shown in Fig. 9 indicates that the surface of apatite has become more negatively charged in the ph range examined in agreement wiul past results [27, 35]. This decrease is mainly due to the role of phosphate as potential determining ions for apatite. Figures 1 and 11 present results for the zeta potential of calcite in the above inorganic solutions. As can be seen from Fig. 1, the effect of Ca(NO3)1 addition is to make the surface of calcite more positively charged. In the concentration range tested, only positive potentials were recorded over the entire ph range. 6[ CALCITE kmovm3 Co ()2 4 " ", --'\.-- \ WATER '- \ '\ \\ \ \ -4oL -l. I,.L I I.L II ph Fig. 1. Effect of Ca(NO.>' addition on the zeta potential of calcite.

14 348 '> _E- -' ct.- Z '".- ct.- '" N ph Fig. 11. Effect of K,PO. addition on the zeta potential of calcite. Similar to the effects on apatite, K3PO4 addition reduces the zeta-potential values of calcite (Fig. 11). It must be noted that zeta-potential values obtained for calcite in 1-3 kmol m-3 K3PO4 closely agree with those obtained for apatite in water. As shown later, even less than 1-9 kmol m-3 total phosphate can cause precipitation of apatite from calcite supernatants under neutral ph conditions. The observed changes in zeta potential can therefore be attributed to the precipitation of apatite on the calcite surface. Analysis of the zeta-potential results presented above show the observed shifts in the isoelectric point of calcite and apatite in the supernatants of each other to be the result of many complex surface processes. The mechanism of surface charge generation for these minerals has been examined in detail during an earlier investigation [4, 5, 34, 35]. The electrokinetic behavior of apatite and calcite in various inorganic solutions has also been determined [32, 35]. These properties, however, have not been studied for heterogeneous mineral systems. In a recent investigation [36] we have shown from theoretical considerations that, depending on the solution conditions, apatite surface can be converted to calcite, and vice versa through surface reaction or bulk precipitation of the more stable phase.

15 The observed changes in the electrokinetic properties of calcite and apatite can therefore be examined on the basis of the mineral-solution chemical equilibria 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) previously equilibrated with calcite, calcite can precipitate on the surface of apatite. From similar considerations, the conversion of calcite to apatite is also possible below the singular point. We have shown theoretically [36] that, if calcite supernatants are prepared above ph 9.3, the total carbonate present in these solutions can exceed the amount required for the conversion of apatite to calcite as illustrated in Fig. 12(a). As shown in Fig. 12(b), the total phosphate in equilibrium with apatite below the singular point can also convert calcite surface to apatite under appropriate conditions. As mentioned earlier, much lower phosphate concentrations are required in this case. Our investigation on the solubility behavior of these minerals based on supernatant analysis under various solution conditions [38] has 4 I Q 349 If) E" " E -!oj I- oct Z m oct U -J oct I- I-... -J (OPEN) - HAP CALCITE /, / tot/calcite- (OPEN)., ""'" (CLOSED) ph Fig. 12(a). Illustration of the conversion of apatite to calcite.

16 E " E ok -- I&J -2 '4 x Q.. (/) X. Q.. -J C 6 t- Co? -J ph Fig. 12 (b). illustration of the conversion of calcite to apatite [36]. also shown that, indeed, the precipitation of these minerals is quite possible. The surface conversion of calcite and apatite is further illustrated by the results of recent spectroscopic investigations in our laboratory using ESCA [39]. The minerals were conditioned under solution conditions similar to those used for zeta-potential studies. The data given in Fig. 13 show that, when apatite is conditioned in the supernatant of calcite at ph,..., 12, its surface exhibits spectroscopic properties characteristic of both calcite and apatite. This behavior is attributed to the precipitation fo calcite on apatite. On the basis of the electrokinetic, spectroscopic, flotation, dissolution and precipitation behavior of these minerals in heterogeneous systems, as well as the analysis of the mineral-solution chemical equilibria governing them, conditions for selective flotation have been discussed in detail in a recent publication [4].

17 351 - cn I&J a: I- <.J I&J -J I&J 11- a: I&J m z >- I- cn Z I&J I- Z H CALCITE IN WATER '--"'-"'\ 1 APATITE IN CALCITE SUP NO ING ENERGY, ev Fig. 13. ESCA spectra of C(1s) peak of apatite conditioned in calcite supernatant at ph- 12 [39). SUMMARY AND CONCLUSIONS Dissolved mineral species playa major role in the interfacial behavior of sparingly soluble minerals such as calcite and apatite. Electrokinetic studies were conducted on the calcite--apatite heterogeneous system to develop an understanding of the manner in which dissolved species affect this behavior. The isoelectric points of calcite and apatite in both water and KNO3 solutions are 1.5 and 7.4, respectively. When the minerals are conditioned in the supernatants of each other, they approximately

18 352 interchange their points of zero charge; when apatite is conditioned in the supernatants of calcite, its isoelectric point shifts from 1.5 to about 6.5 (8. in the presence of 2 X 1-3 kmol m-3 KN) towards that of apatite. Similarly, the isoelectric point of apatite is shifted from 7.4 to about 11. towards that of calcite. If calcite and apatite are present as a 1: 1 mixture (weight basis), the two minerals have identical surface-charge characteristics in the basic ph range. These effects are discussed on the basis of mineralsolution equilibria controlling these systems. Thermodynamic considerations indicate that the apatite surface can be converted to calcite in the supernatant of the latter and vice cersa. Based on the results of the electrokinetic, spectroscopic, flotation, dissolution and precipitation studies, the observed shifts in isoelectric points are attributed to the precipitation of one mineral on the surface of the other. ACKNOWLEDGEMENTS Support of the National Science Foundation (CPE ) and Florida Institute of Phosphate Research is greatly acknowledged. We wish to thank K.P. Ananthapadmanabhan and Kenneth Wong for helpful discussions. REFERENCES 1 E. Sorensen, J. Colloid Interface Sci., 45 (1973) J.E. Lawver, R.L. Wiegel, R.E. Snow and C.L. Huang, Benefication of Dolomitic Florida Phosphate Reserves, XIVth Int. Miner. Proc. Congress, Session IV, Flotation, Paper IV (2), CIMM, Ontario, Canada, P. Somasundaran, On the Problem of Separation of Calcite From Calcareous Apatite, XIth Int. Miner. Proc. Congress, Instituto di arte mineraria e preparazione dei minerali, Cagliari, Vol. 2, 1975, pp P. Somasundaran and Y.W. Wang, Surface Chemical and Adsorption Properties of Apatite, in D.N. Mishra (Ed.), Adsorption and Surface Chemistry of Hydroxyapatite Plenum, New York, NY, 1984, pp H.S. Hanna and P. Somasandaran, Flotation of Salt-Type Minerals, in M.C. Fuerstenau (Ed.), Flotation, A.M. Gaudin Memorial Vol. 1, AIME, New York, NY, 1976, pp K.P. Anathapadmanabhan and P. Somasundaran, The Role of Dissolved Mineral Species in Calcite-Apatite Flotation, Minerals and Metallurgical Processing, SME AIME, 1 (1984) T_W. Healy, personal communications, M.C- Fuerstenau and D.A. Eigillani, Trans. AIME, 235 (1966) F.F. Aplan and D. W. Fuerstenau, Principles of Non-metallic Flotation, in D. W. Fuerstenau (Ed.), Froth Flotation, 5th Ann. Vol., AIME, New York, NY, 1962, p R.W. Smith and S. Akhtar, Cationic Flotation of Oxides and Silicates, in D.W. Fuerstenau (Ed.), Flotation, A.M. Gaudin Memorial Vol. I, AIME, New York, NY, 1976, p J.B. Clemmer, B.H. Clemmons, C. Rampacek, M.F. Williams and R.H. Stacey, U.S. Bur. Mines Rep. Invest., 3799, A_M. Gaudin and A. Rizo-patron, Trans. AIME, 153 (1942) 462.

19 S.R.B. Cooke and M. Digre, Trans. AIME, 184 (1949) H.J. Modi and D.W. Fuerstenau, Trans. AIME, 217 (196) M.C. Fuerstenau and J.D. Miller, Trans. AIME, 238 (1967) G.W. Wolstenholme and J.H. Schulman, Trans. Faraday Soc., 46 (195) I. Iwasaki and H. Soto, Fundamentals of Phosphate Flotation, Prog. Rep., (1981) W. Stumm and J.J. Morgan, Aquatic Chemistry, 2nd edn, Wiley-Interscience, New York, NY, W.E. Brown and L.C. Chow, Chemical Properties of Bone Mineral, Annu. Rev. Mater. Sci., 6 (1976) E.C. Moreno, T.M. Gregory and W.E. Brown, Preparation and the Solubility of Hydroxyapatite, J. Res. Nat. Bur. Stand Sect. A, 72 (1968) F. Morel, Principles of Aquatic Chemistry, Wiley-Interscience, New York, NY, G.A. Parks, Am. Mineral., 57 (1972) W.E. Brown, Physicochemistry of Apatite Dissolution, Physicochimie et Crista1lographie des Apatites d'interet Biologique: Colloques Internationaux C.N.R.S., No. 23, 1973, pp S. Polcn, Interaction Between Reagents and Minerals during Flotation of SnO." Fe.O], and CaCO], Izv. Vyssh. Uchebn. Zaved. Chern Metal., 1 (1958) S.C. Sun, R.E. Snow and W.I. Purcell, Characteristics of Florida Leached Zone Phosphate Ore With Fatty Acids, Trans. AIME, 28 (1957) M.A. Eigeles, Selective Flotation of Non-8ulfide Minerals, Progress in Mineral Dressing, Trans. 4th Int. Mineral Dressing Congress, Stockholm, Almqvist and Wiksell, 1958, pp J.M. W. MacKenzie and S.K. Mishra, Zeta-Potential Studies on Apatite, Paper presented at the 4th National Convention, Australian Chemical Institute, Canberra, Australia, 197, p J.C. Le Bell and L. Lindstrom, Electrophoretic Characterization of Some Calcium Minerals, Finn. Chem. Lett., (1982) J. V. Calara and J.D. Miller, The Electrokinetic Behavior of Fluorite in the Absence of Surface Carbonation, Colloids Surfaces, submitted. 3 Y.H. Wang, Zeta-Potential Studies on Hydroxyapatite, M.S. Thesis, Columbia University, New York, NY, Handbook of Photoelectron Spectroscopy for Chemical Analysis, Physical Electronics Division, Perkin-Elmer, Minnesota, U.S.A., P. Somasundaran and G.E. Agar, Zero Point of Charge of Calcite, J. Colloid Interface Sci., 24 (1967) R.K. Mishra, S. Chander and D. W. Fuerstenau, Effect of Ionic Surfactants on the Electrophoretic Mobility of Hydroxyapatite, Colloids Surfaces, 1 (198) P. Somasundaran, Zeta Potential of Apatite in Aqueous Solutions and Its Change During Equilibration, J. Colloid Interface Sci., 27 (1968) P. Somasundaran and G.E. Agar, Further Streaming Potential Studies of Hydroxyapatite in Inorganic Solutions, Trans. AIME, 252 (1972) P. Somasundaran, J.O. Amankonah and K.P. Ananthapadmanabhan, Mineral- Solution Equilibria in Sparingly Soluble Mineral Systems, Colloids Surfaces, 15 (1985) S.K. Mishra, The Electrokinetics of Apatite and Calcite in Inorganic Electrolyte Environment, Int. J. Miner. Process., 5 (1978) J.O. Amankonah, P. Somasundaran and K.P. Ananthapadmanabhan, Effect of Dissolved Mineral Species on the Dissolution/Precipitation Characteristics of Calcite and Apatite, Colloids Surfaces, 15 (1985) P. Somasundaran, J.O. Amankonah and K.P. Ananthapadmanabhan, Spectroscopic Analysis of Calcite-Apatite Interactions Using ESCA, unpublished results. 4 P- Somasundaran, J.O. Amankonah and K.P. Ananthapadmanabhan, Calcite-Apatite Interactions and Their Effects on Selective Flotation Using Oleate. Accepted for presentation at the XV Int. Miner. Proc. Congress, Cannes, France, June 1985.