EFFECfS OF DISSOLVED MINERAL SPECIES ON THE ELECfRO

CoUoids and Surfaces, 15 (1985) 335-353

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 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 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 super-

natant 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

0166-6622/85/$03.30

@ 1985 Elsevier Science Publishers B. V.

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 sig-

nificant.

Effects of dissolved species in various mineral systems have been reported

by several investigators. Healy [7] observed drastic changes in the point of

zero c~e of magnetite when stored in glass containers. The observed

changes 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 hydroxy

complexes of the cations as well as the stronger tendency of multivalent

ions 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 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 mag-

nesium 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 [19,20,23]

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 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.

337

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 10-4 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 con-

ditioned 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 pre-

sence 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 con-

ducted 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 thermo-

dynamic predictions.

EXPERIMENTAL MATERIALS AND METHODS

Materials

Calcite

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

gation by Somasundaran and Wang [4, 30] 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. [20] and modified

by Somasundaran and Wang [4, 30]. The method involves mixing appro-

priate 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 car-

bonate 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 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 of

calcium/phosphorus (Ca/P) and calcium/carbonate (Ca/Ct) were 1.59 and

0.93 for apatite and calcite, respectively, and their calculated thermo-

dynamic solubility products were 10-115 and 10-8)45. ESCA was used to

determine the surface chemical composition of the freshly prepared (un-

treated) 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 0.71 and 30.3 m2 g-l, respectively.

lnorganics

All 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 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 zeta-

potential measurements was done by stirring for 30 min 0.1 g of the mineral

with 100 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 10-20 min of conditioning.

Furthermore, the data in the literature indicate that conditioning times for

flotation 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 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 30 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 10-3 kmol m-J KN~.

Additional measurements were made in mixed supernatants prepared

by 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 apatite

in 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 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 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 attributed

to the compre~ion of the double layer under high ionic strength conditions

[34].

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Fig. 4. m~ration or the effects or 8upernatants on the IEP of calcite and apatite.

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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-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 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.

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(N~h,

K2C~ and K2HPO4 are given in Figs 7 to 9. Data given in Fig. 7 for the

zeta potential of apatite in Ca(N~h solutions of several concentrations

show that the surface of the mineral is more positively charged in Ca(N~h

solutions at all pH and levels of concentration tested. This is in agreement

with the results in the literature [4, 30, 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

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346

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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 (10-3 and 10-1

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 10 and 11 present results for the zeta potential of calcite in

the above inorganic solutions. As can be seen from Fig. 10, 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.

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Fig. 10. Effect of Ca(NO.>' addition on the zeta potential of calcite.

348

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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 ob-

tained for calcite in 10-3 kmol m-3 K3PO4 closely agree with those obtained

for apatite in water. As shown later, even less than 10-9 kmol m-3 total

phosphate can cause precipitation of apatite from calcite supernatants under

neutral pH conditions. The observed changes in zeta potential can there-

fore 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 mecha-

nism 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.

349

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 pos-

sible 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

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Fig. 12 (b). illustration of the conversion of calcite to apatite [36].

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

sible.

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 govern-

ing them, conditions for selective flotation have been discussed in detail

in a recent publication [40].

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CALCITE IN WATER

APATITE IN CALCITE SUP.

295 290 285 280 275

81 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 10.5 and 7.4, respectively. When the minerals

are conditioned in the supernatants of each other, they approximately

352

interchange their points of zero charge; when apatite is conditioned in the

supernatants 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 11.0

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 mineral-

solution equilibria controlling these systems. Thermodynamic considerations

indicate 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 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-83-04059) and Florida

Institute of Phosphate Research is greatly acknowledged. We wish to thank

K.P. Ananthapadmanabhan and Kenneth Wong for helpful discussions.

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