'lWELFTH INTERNATIONAL MINERAL PROCESSING CONGRESS

SAC PAULO, BRAZIL, 1977

FLOTATION MECHANISM BASED ON IONOMOLECULAR COMPLEXES

P. Somasundaran, K. P. Ananthapadmanabhanand R. D. Kulkarni.

Henry Krumb School of MinesColumbia Uni versi ty

New York, N. Y. 10027, u. S. A.

*Present address:Union Carbide CorporationTechnical CenterTarrytown, N. Y. 10591, U. S. A.

APRIL 1977

ABSTRACT

F~A!IQN MECHANISM BASED ON

IONOMOLECUIAR SURFACTANT COMPLEXES

P. Somasundaran, K. P. Ananthapadmanabhanand R. D. Kulkarni

Studies of flotation, surface tension and surface

tension decay rate as a function of pH along with solution

chemistry of collector solutions suggest the formation of

ionomolecular surfactant complexes to be a factor governing

froth flotation of hematite using oleate and quartz using

amine. Formation of the complexes coincides in these systems

wi th maximum surface acti vi ty as measured by surface tension

In this paper, the pH dependenceadhesion tension tests.

of hematite flotation using oleate is discussed in terms of

role of ionomolecular complexes

FLOTATION MECHANISM BASED ON

IONOMOLECULAR COMPLEXES

P. Somasundaran, K. P. Ananthapadmanabhanand R. D. Kulkarni

The collector adsorption at the solid/liquid (S/L)

interface in relation to flotation has received considerable

This has mostly been explained inattention in the past.

terms of the surface chemical and electrokinetic properties of

the mineral with a little emphasis on the chemical nature of

the surfactant{I~3). For example, the high flotation of

hematite obtained around the neutral pH region in hematite-

oleate system has been attributed to the chemisorption.of

oleate at the neutral s~~face-Og sites which are in high con-

centrations under these conditions(1,3) The high concentration

of 08 neutral sites on the surface has in turn been attributed

to the occurrence of pzc of hematite around the neutral. pH

region(l,3). On the contrary, the adsorption of oleate on

hematite does not show any maximum around the neutral pH range(4)

Also it can be shown that the concentration of neutral hydroxyl

sites does not change so drastically to account for a sharp

peak in flotation under the above conditions(5). Therefore,

above theory cannot adequately explain the flotation and

On the otheradsorption behavior of hematite-oleate system.

hand, in explaining the above results, the above theory has

not taken into account the state of oleic acid and its effect

on adsorption and flotation of hematite.

3

In the intermediate range, the ions andat low pH values.

the neutral molecules interact to form 1:1 ionomolecular

complex in the system. The high flotation of quartz around

pH 10.0 has been attributed to the formation of ionomo1ecu1ar

complex in high concentrations in that pH region.

In the present study we have examined the role of

oleate-oleic acid soap complex in determining the pH dependence

The sharp increase in hematite flotationof hematite flotation.

around the neutral pH range has been correlated with the

sharp increase in the concentration of oleate-oleic acid soap

complex in the solution. Measurements of dynamic and equilibrium

adsorption studies at the liquid/gas (L/G) interface have

provided an indirect evidence for the existence of a highly

An excellentsurface active species in neutral pH region.

correlation has been obtained between the flotation recovery

and surface tension and between flotation recovery and dynamic

surface tension

EXPERIMENTAL

Materials

Massive red Minnesota hematite obtained from Ward's

Natural Science Establishment was used for basic studies while

-65 mesh taconite ore obtained from upper Michigan Peninsula

was used for Denver cell flotation .test~. The Minnesota hematite

was found to be 94% pure with quartz as the main impurity

whereas the taconite was of 40% Fe with mainly quartz and

magnetite in the non-hematitic portion. 100 X 150 mesh

hematite was used for Hallimond cell flotation tests.

5

For Denver cell tests, 300 gram sample of the ore was

des limed twice with 1800 mI. of distilled water and then

reagentized at 60% solid content at desired temperature for

ten minutes. Pulp was then transferred into the Denver cell

floated for thirty seconds at 20% solid content.

Surface Tension

A dynamic surface tension measuring technique was

developed utilizing the Wilhelmy plate method and a microbalance (9)

The cell containing the test solution is jacketed for circula-

of water at the desired temperature. A hole was provided

on the side of the cell at a level above the solution in order

to remove surface layers using a capillary connected to an

aspirator and thereby to create a nascent surface. In order to

test the nature of the contact of the solution surface with the

sensor, the balance was mounted on a camera screw ring enabling

The output of thesmooth raising or lowering of the sensor.

microbalance is fed to the Y-channel of an X-Y recorder with

the X-axis for recording aging time.

For measurements of surface tension decay as a function

time, a thin surface layer is removed by suction and the

surface tension recording is begun at the end of suction. The

r.ecording directly yields surface tension versus tilOO plots.

The suction was unifo~y applied for 30 seconds in all cases

even though the surface tension values for pure water were

6

Repeated recordingsobtained in less than ten seconds.

followed by suction,s obtained for most test solutions showed

the reproducibility to pe satisfactory.

RES ULTS

~a~The effect of pH on the Hallimond cell flotation of

natural hematite is shown in Figure 1. A strong dependence

of flotation on pH is evident from this figure. Maximum

flotation is obtained around the neutral pH r~ge and this is

Inin agreement with the results reported in the literature.

the past, this maximum around the neutral pH range has been

attributed to the presence of pzc in this pH region. It was

proposed that the oleic acid preferably adsorbs on the neutral

surface sites which are present in high concentration near the

Flotation recovery vs. pH curves for various concentrationspzc.

It can be seen from this figureof oleate is shown in Figure 2.

that the increase in oleate concentration has increased the

At the s ame ti~ the pH offlotation recovery of hematite.

maximum flotation response has shifted to higher pH values

This is clear iranwith the increase in oleate concentration.

Figure 3 where the pH of maximum flotation response is plotted

against the concentration of oleate. If surface characteristics

of hematite alone is responsible for flotation behavior, such a

change in the pH of maximum flotation would not have occurred.

7

Th~ Denv~r cell flotation results for taconite as a

function of pH is shown in Figure 4. The results show a

maximum in flotation recovery in the neutral pH range which is

in agreement with the Hallimond cell flotation of hematite.

Surface Tension

Typical dynamic surface tension decay curves for

3xlO-S mole/l oleate solutions at various pH levels are sh~

in Figure 5. (a) the initialIt is seen from these figures that,

surface tension values for all the curves correspond to the

surface tension of the pure water, b} in all cases equilibrium

surface tension values are attained in a finite ti~, and (c)

the surface tension decay rate as well as the total surface

tension lowering is strongly influenced by pH.

In general the surface tension decay curves exhibit

three distinct regions: Region I in which there is a negligible

surface tension decrease, Region II, characterized by a linear

decrease in surface tension with time and Reqion III characterized

by an apparently exponential surface tension decay. While all

the plots for higherthe three regions exist at low pH values,

pH values seem to exhibit only the latter regions

The slope (dy/dt) of the linear region of the surface

tension decay curves (i.e., Region II) was found to be charac-

teristic of each curve and was therefare used to characterize

the dynamic surface tension property.

The effect of pH on the total surface tension lowering

could be seen more clearly in Figure 6, where surface pressure

8

'l11e plot in thisis plotted as a function of solution pH.

figure shows a dome shape with maximum surface pressure in the

This maximum obtained in the neutral rangeneutral pH range.

can be attributed to the formation of the acid soap, which exist

in this pH range and is more surface active than other forms

of oleate.

9

DISCUSSION

The results of this investigation has clearly defined

Thesetwo main features of hematite-oleate flotation system.

are:

(1 Hematite flotation response is very sensitive to pH

especially in the neutral pH range and the maximum floatability

is observed close to pH 8.0 with 3 X 10-5 mole/liter total

oleate concentration.

For a given hematite sample the pH of maximum flotation(2)

response (pH.) is dependent on the oleate concentration and

follows the follC7l1ing relation:

log (CT) = A + B X pH*

where CT is oleate concentration in mole/liter and A and B

arbitrary constants.

Both these features of this flotation system cannot be

adequately explained on the basis of the existing theories. For

example, if the state and the extent of oleate adsorption, and

hence the flotation response, is dependent only on the hematite-

solution interfacial properties then the pH of maximum flotation

response should be independent of oleate concentration which

is contrary to the finding of the present investigation

In the following discussion we will make an attempt

to explain these results based on the oleate solution chemistry

and the presence of iono-molecular complexes in this system.

Before attempting such a task it is first important to briefly

describe the oleate solution chemistry.

10

Oleic acid is a weakly acidic insoluble compound in

aqueous solution, which forms highly soluble salts

with monovalent alkali ~tal ions such as sodium or potassium.

under appropriate solution pH conditionsThese soluble salts,

undergo series of hydrolysis reactions yielding complex

oleate species (lO,ll) - These species have been identified to be

oleate ions (R-), oleic acid (RH), acid-soap (~H' , acid-

soap salt (~HNa) and oleate di~r (R;-).

proportion of these species in the aqueous solutions is

The relative

strongly dependent on the total oleate concentration, pH,

temperature and ionic strength.

The chemical equilibrium between these oleate species

can be represented in the fo1lowing manner(lO) .

Kl7.6 (1RH(solution)RH (L) pKl

=

K2 ,+~ R- + H- 4.95 (2)RH(soln. pK2

=

-3)-4.7~H pK3~ + R' =KJ .-

-4.0K4 --

~R'-- ~-~- 2 4)-2R pK4=

++ Na -9.35 (5)KS

~ ~HNa (precip- PKS~ itate)*~H

=

*The equilibria shown here are for Na-oleate. It is assumedthat these values may not change appreciably for K-oleate.The thermodynamic data is not available for K-oleate.

11

so that:

IRH]/[RH(C)]K1

~

fR-] [H+j

IRH]K2

=

/ [RH] [R-][~HKJ=

[~-]

~]2K4

~

total oleate concentration in the solution, the decrease in pH

increases the oleic acid concentration and decreases the oleate

ion concentration, the oleate complex species being stable

mostly in the neutral pH range. The behavior of this system

is rather complex, especially because of the varying solubilities

of these species in water. For example at room temperature

while oleic acid exhibit equilibrium solubility of only

2.51 X 10-8 mole/liter, the oleate ions are highly solub1e(10)

Besides, the salt of acid soap in its undissociated form is

expected to exhibit least solubility. In general, true oleate

solutions in water are obtained only in the basic pH range.

The neutral and acidic pH solutions are turbid containing fine

stable dispersion of oleate species.

The concentration of oleate species in aqueous solution

The rearrangement ofcan be determined using equations (6-10).

these equations along with the substitution of the activity

(11)

14

maximum surface activity of this solution is observed in the

pH range of 8.5 - 9.5. It should be noted that the above

stated pH refers to bulk pH. the pHThe surface pH, i.e.

close to the solution/gas interface is expected to be

lower due to the anionic nature of the adsorbed oleate layer.

Using Gouy's theory of electrical double layer, it can be

shown that, under these conditions the surface pH can, in fact,

be lower by as much as 1.5 to 2.0 pH units at bulk pH of

Thus, with this consideration, the maximum surface activity

correlates remarkably well with the maximum concentration of

acid soap (compare Figures 6 ~d 8). A similar correlation

is presented in Figure (9) where rate of surface tension decay

R i. e. dynamic surface tension is plotted along with the

concentration of acid-soap as a function of bulk pH. It is again

seen that the maximum rate of surface tension lowering is

obtained in the pH range where acid-soap is a predominan t species

The lower surface activity, and slower surface tension

decay rates, as seen in Figures (5,6) "at acidic pH values, have

been attributed to the presence of oleic acid which exist as

precipitate under these conditions. Also the lower surface

activity, but relatively higher dynamic surface tension behavior

at higher pH values has been attributed to the higher solubility

which leads to lower surface activity and higher molecular

mobility of the R;- and R-

under these conditions.

oleate species which are predominant

On the basis of above discussion one can expect

oleate to be most effective as a flotation aid when present as

IS

acid-soap complex, p;rovided othe;r conditions such as properties

of sol~q/l~quid interface are favorable.

The actual role of acid-soap in the hematite-oleate

~lotation system can be evaluated by determining quantitatively

concentratiQ~s of these species in solution under various

experimental conditions and by correlating them witn the

flotation response under identical conditions.

Acid Soap Complex In Relation to Hematite Flotati~

The Hallimond cell flotation response of hematite mineral

(~O) along with the concentration of acidis presented in Figure

soap as a function of bulk solution pH, at a total oleate

concentration of 3 X 10-5 mole/liter. An excellent correlation

It is seen that pH of maximumis evident from this figure.

floatability corresponds to the pH of maximum acid soap con-

Also the sharp drop in hematitecen trati on in the solution.

floatability at higher and lower pH value may be associated with

Again it shouldrapid decrease in the acid soap concentration.

be recalled that the slight difference in the pH of maximum.~:

floatability and acid-soap concentration is .due to the difference

in the value of the hematite/solution surface pH and the

unlike the case of solution/gas interface,.

bulk solutJ.on,pH.

the hematite/solution interface, being only weaklyhowever,

negative in charge, will show only marginal lC7l1ering of surface

pH in the pH range of 7-9.

lists the flotation response of hematite-oleateTable (1

system at two different'pH levels and different oleate adsQ~t}on

It is to be noted that at pH 9.0 where acid-soapdensities.

17

CONCLUSIONS

Two main features of the hematite-oleate system

a sharp peak inevident from the present study are (1

flotation around the neutral pH range and (2) the shift in

the pH corresponding to the maximum f iota tion wi th the

These results haveincrease in the concentration of oleate.

been explained here by considering the role of ionomolecular

complex (oleate-oleic acid soap) in flotation. Independent

thermodynamic calculations have shown that the pH region

where acid soap complex is present in maximum concentrations

The hiQh flotationcorresponds to the region of high flo~ation.

in the presence of ionomolecular complex has been attributed

to the hiqh surface activity of this species and this has

been confirmed by surface tension and surface tension decay

Most importantly, the present study has sho~ thatstudies.

the solution chemistry and the chemical nature of the surfactant

play an important part in determining the flotation character-

In the light of the aboveistics of hematite-oleate system.

facts, a systematic study of the solution chemistry of other

mineral-collector systems is warranted.

.ACKN~ DGMENTS

for the support of this research.Foundation (BNG-76-80139 1

18

REFERENCES

1. Pope, M. I., and Sutton, D. r., "The Correlation BetweenFroth Flotation Response and Collector Adsorption fromAqueous Solution," Powder Tech., 7,271, 1973.

2. Fuerstenau, D. W., "Correlation of Contact Angles,Adsorption Density, ~eta Potentials and Flotation Rate,"Trans. AIME, 208,1365,1957.

3. Howe, T. M. and Pope, M. I., The Quantitative Determinationof Flotation Agents Adsorbed on Mineral Powders Using DTA,Powder Tech., 4, 338, 1970/71.

4. Kulkarni, R. D. and Somasundaran, P., -Kinetics of OleateAdsorption at the L/A Interface and Its Role in HematiteFlotation,- in Adv. in Interfac. Phenomena on Particulate/Solution Gas S stems, APP1J,.catlop to FlotatIoQ ~s~ar~I\~:;'P. Somasundaran and R. B. Grieves, eds., AIChE Symp. SeriesNo. 150, 124, 1975.

5. deBruyn, P. L. and Agar, G. E., "Surface Chemistry ofFlotation," in Froth Flotation, 50th Ann. Vol., D. W.Fuerstenau, ed., Pub. AIME, N. Y., 1962, p. 91.

6. Somasundaran, P., "The Role of Ionomolecular Complexesin Flotation," International Jnl. of MinI. proc., 3, 35, 1976.

7. Pinch, J. A. and Smith, G. W., Dynamic Surface Tension ofAlkaline Dodecylamine Solutions," J. ColI. Intf. Sci.,45 (I), 1973.

8. Somasundaran, P. and Moudgi1, B. M., J. ColI. Intf. Sci.,45,591, 1973.

9. Somasundaran, P., Danitz, M. and Mysels, K. J., "A NewApparatus for Measurements of pynamic Interfacial Properties,J. ColI. Intf. Sci., 48, 410, 1974.

Jung, R. F., M. Sc.1975.

Thesis, Univ. of Melbourne, Australia,

11. Goddard, E. D., Goldwasser, S., Golikeri, G., and Kung,H. c. in "Molecular Association in Biological and RelatedSystems," p. 67, 1968, ed. Goddard, E. D., Pub. Am. Chern.Soc., Washington, D. C., 1968.

Kulkarni, R. D., .and SomasWldaran, P., Oleate Adsorptionat Hematite/Soln. Interface and Its Role in Flotation,Presented in lO4th Annual AIME Meeting, N.Y., 1975.

TABLE 1

Adsorption 10

Density X 10

Mo1e/cm2 , Floated

3.00.228.0

27.00.78.0

86.08.0 1.6

1.00.81

2.36 9.040.03.06

TABLE 2

pH* = pH of maximum

acid-soap complexforma tionCT = Total Oleate in Solution

-53.5 X 10 M 7.78

1.5 X 10-5 M 7.63

7.5 x 10-6 M 7.37

Figure 2

100HEMATITEKN03 2 X 10-3 M

TEMPERATURE 25°C. K-OLEATE 7.5 x 10-6Me K - OLEATE 1.5 x 10-sM

0 K-OLEATE 3x 10-5 M80 f-

60a&IJ

~0-JLA..

oe 40

20

0ld94 5 6 7 8

FLOTATION pH

~ Figure 5(\J

4

§C\J

~~ ~

".buX fcACONI~E~ ~ OLEAtE 1 ~ x 10-~M.o

KNO3 -1 M 0:. <.D

URE 25°C II:I:Co

La.

Hc],La. -

00t()r-

'"

60

-u~-w~~

>-0:W

~UW0:

40

~0

20I{)(\J--..

:I:Q.

00L!)

(\J

a>"

:J:Q.

<'1

07 8 9 104 5 6 <,

FLOTATION

0C5CD

ff)

0Z L&J~ .-

(Jas/sau.(p) NOISN31 3:)~.::I~nS

-Eu.n!~"0-U

J0:::>~U

J~U

JU~0:::>U

')

40302010'4 T

25°C

0N

KN

O3 J6

pH ,,-8

Figure

6I

0

,

-1

Fiqure 8

3200

2880 TOTAL OLEATE ='- -TEMPERATURE 25° C=

~ 2560r-

--0r-

-X 2240w..JQ..~0 1920(,)

Q..«0~ 16000G«~0 1280z0-~

~ 960..-z~(,)

~ 640(,)

-

-~

-

-

-

-'-

-

320 -

106 7 -8 94 5pH

5 6 7

pH

8 9

Figure 9

10

Figure 10

~,..

0-M)(W-IQ..~0UQ..40(/).a

~u.0z0-

~a:..-zwU20U

4 5 6 7 8 9 10

pH