by P. Somasundaran

An experimental and theoretical treatment of the depressing effect of inorganicelectrolytes on the amine flotation of quartz is presented. Experimental data ob-tained for the amine flotation of quartz as a function of pH with potassium nitrateas depressant was used to test the models for the effectiveness of inorganic ions asdepressants. The validity of commonly used assumptions are examined. The surfacetension data obtained for dodecylammonium acetate solutions as a function of KNO.concentration indicated that the collector adsorption on bubble cannot be neglectedin the treatments, since this adsorption changes significantly with the addition 01the inorganic electrolyte. It also indicated the possibility of significant changes inthe specific adsorption potential of the collector occurring with the addition of in-organic electrolytes.

history of the mineral including the nature of the pre-treatments that it has received. In water quartz surfaceis negatively charged and as a result the positive ionsin solution are preferentially attracted towards theinterface. If the solution contains cationic surfactantssuch as dodecylammonium acetate, they will be electro-statically adsorbed at the interface causing a decrease inthe hydrophilic properties of the particles and thus ren-dering them more amenable to flotation. These surfac-tant ions however will have to compete for adsorptionsites with other cations such as sodium and calcium inthe bulk water. Adsorption density, r.', of ions of type iin the stern plane 6 is given by the Stern-Graham equa-tion:

Inorganic electrolytes are often used as depressantsand activators in the froth flotation of minerals. Eventhough some research has been done on the froth flota-tion of minerals in the presence of inorganic salts, amodel capable of satisfactorily explaining or predictingdepression by the above salts is not yet available. Anunderstanding of the actual mechanisms would behelpful in solving mineral flotation problems.

Some of the noteworthy attempts to explain the ac-tion of inorganic electrolytes in froth flotation includethose of Onoda and Fuerstenau,' Hopstock and Agar,"and Clarke and Cooke.. Onoda and Fuerstenau were thefirst to use flotation and zeta potential data to deducea model with the help of which they calculated theratio of solution concentration of sodium ions to that ofbarium ions for equivalent flotation. The calculatedratios were, however, approximately twice that of theexperimental ratios. This author' modified the modeland obtained a significant improvement in agreementbetween the experimental and calculated values; how-ever this discussion appeared in print without the actualresults submitted to support the modified model. Themodified model will be briefly discussed later along witha comparison of the calculated and experimental ratios.Hopstock and Agar used zeta potential and flotation datafor the calculation of the specific adsorption of alkalineearth ions and Clark and Cooke correlated the adsorp-tion of divalent cations to the flotation of activatedquartz with fatty acids. In the present paper the validityof the various assumptions that have been made in thepast is examined with the help of new data-for ex-ample, attention is paid to the effects of added inorganicelectrolytes on the adsorption at the liquid/gas interface,this adsorption indeed being an important parametergoverning flotation..

-W,'r,' = 2r, c, exp

\ RT ;

where r, is the radius of the counterion i, c, the bulk

concentration, and W,' the work to be done to bring the

ion from bulk to plane 3. W,' for electrostatic adsorption

is equal to z, F ~. where z, is the valency of the counter-

ion, F the Faraday constant, and ~. potential at plane 3.

If there is any specific adsorption, then W,' contain also

a specific adsorption energy term, tj>,. Eq. 1 then becomes

, z,F~'+tj>,'

(1)

= 2,.. c. exp ( (2)

TheoryMinerals such as quartz in aqueous solutions possess

a surface charge, the sign and magnitude of which isdependent upon the concentration of potential-deter-mining ions. In the case of quartz, the surface charge isknown to be negative above approxjmately pH 2 andpositive below it. The exact pH value for the point of zerocharge is dependent among other things on the previous

P. SOMASUNDARAN, Member AIME, is Associate Professor of Min-eraI Engineering, Henry Krumb School of Mines, Columbia University,New York. TP 728237. Manus<ript, June 27, 1972. Discussion of thispaper, submitted in duplicate prior to June 15, 1974, will appear inSME Transactions, 1974, and in AIME Transactions, 1974, Vol. 256.

.. RT ,The major assumptions involved in using this equationin the past for developing the model for flotation depres-sion by inorganic electrolytes are:

1) .p' is equal to the zeta potential.2) .<--potential is constant under conditions of equiva-

lent flotation.3) Adsorption of divalent ions under constant .<--po-

tential conditions is half of that of the adsorption ofmonovalent ions.

4) r. for ions such as sodium shall be the radius of ahydrated sodium ion and for ions such as barium shallbe that of the unhydrated ions.

5) Adsorption density of the collector ions at thesolid/liquid interface is constant for a given amount offlotation.

6) The addition of electrolytes has no effect on thespecific adsorption potential of the collector ions, i.e.,association energy of the hydrocarbon chains is inde-pendent of the ionic strength of the solution.

The foregoing assumptions will now be examined fortheir validity.

The assumption that .p' is equal to .<--potential is safeunder conditions of low adsorption in the Stern plane.

64 - MARCH 1974 Society of Mining Engineers, AIME TRANSACTIONS - VOl. 255

I11

,..-,,\:(,,~.Cfl

"'f;:-, .. I

: ~--Drill holes forpillar softening

fig. 12-Application ofpillar softening at in-tersections.

fA)

~"'.

~

Fig. 1O-Plan of field instrumentation-plan view.(1) Gage plates for Whittemore and Vernier anglegages. (2) Gage plates for Whittemore gage. (3)Suspended roof bolt for micrometer gage.

\

En.,y

Fig. 11-Application of pillar softening at single and multipleentriet-sectian and plan views.

SummaryThis pillar softening technique for alleviating the cut-

ter-roof caving problem has been shown to be analyti-cally sound and effective. This technique may also beuseful for reducing tensile roof failure and coal ribsloughing. Underground practice of this method shouldbe simple, practical, effective, and inexpensive. It is rec-ommended that this method be tested underground, withinstrumentation for monitoring.

AcknowledgmentsThe authors wish to express their sincere apprecia-

tion to Anthony J. Barry for his valuable suggestionsand discussions during the preparation of this report,and to Verne E. Hooker, Fred D. Wright, and C. S. Wangfor their review and suggestions.

References'Gordon, W., and Adler, L., "Analyzing Development Roof Falls,"

Coal Age, Mar. 1971, pp. 103-111.0 Blake, W., "Destressing Test at the Galena MIne, Wallace, Idaho,"

Trans. SME/AIME, Vol. 252. 1972. pp. 294-299.. Roux, A.J., Leeman, E.R., and Denkhaus, H.G., "Destressing: AMeans Of Ameliorating Rockburst Conditions, Part I. The Conceptionof Destressing and the Results Obtslned from Its Application."lou71l4!, South African Institute of Mining" Metallurgy, Oct, 1957,pp, 101-127.

'Talman, W.G.. and Schroeder. J.L., Jr.. "Control of MountainBumps in the Pocahontas No.4 Seam," Transactiom AIME, Vol. 211,1958, pp. 888-891.

. Zienkiewicz, O.C., and Cheang, Y.K., The Finite Element Methodin Structural and Continuum Mechanic., McGraw-Hill, New York,1967.

. Wang, F.-D., et al., "Structural Analysis of Mine Opening andEntry System at Choctaw MIne." Laboratory Report, Ground ControlTechnology Group. Kerr-McKee Corp., Apr. 1971.

1 Heteni, M., Beams on Elastic Foundation, University of MichiganPress, Ann Arbor, 1964, pp. 68, 94-. Adler, L.. "Rib Control of Bedded Roof Stresses," Proceedingo,4th Symposium on Rock Mechanics, The Pennsylvania State Uni-versity, University Park, Pa.. 1961, pp. 203-209.

. Babcock. C.O., "Plates with Rows of Holes Considered as Aniso-tropIc Soft-Inclusion Models of Three-Dimensional Room-and-PlllarMIning Systems, Report of Investigation 7436, U.s. Bureau of Mines,1970.

JOGeyer, R.W., and Myung, J.L: "The 3-D VelocIty Lge: A Toolfor in-SItu Determination of the Elastic Model of Rocks," DynamicRock Mechanics, G.B. Clark, ed.. AIME, New York, 1971, PP. '71-107.

U Panek, L.A., "Determination of the Modulus of RIgidity of Rockby Expanding a Cylindrical Pressure Cell in a DrlI1hole," Proceed-ing., 6th Symposium on Rock MechanIcs, University of MissourI-Rolla, 1964, pp. 427-449.

It is suggested that the following steps for the applica-tion of the pillar softening technique be considerednecessary:

1) Determine the physical properties (modulus ofelasticity, Poisson's ratio, and density) of coal, immedi-ate roof, and fioor rock layers by using a sonic loggingtechnique,'" in-situ static test with ti.le cylindrical pr~':'sure cell developed by U.S. Bureau of Mines,l1 or lab-oratory static core sample test.

2) Perform finite element structural analyses of thespecific mine structure, with rock properti~found fromStep 1 to determine the relation between reduction ofstress concentration versus reduction of pillar stiffness,.and the location of the softening. Calculate the optimumsize and spacing of drill holes, and design the drillingpattern for field test.

3) Verify the effectiveness of the pillar softeningtechnique through field tests.

TRANSACTIONS YOLo 155 Society of Mining Engincers, AIM' MARCH 1974 II

Preseatwe.lated

CN./cB.

Prevlou..caleulated

c"./cs.CN.. Mole

per LExperimental

cNa/caaCR_. Hole

per LFloated, % rHo. mv ra.. my

-SO-35-22-10

90'105030

1 X 10-38 X 10-31.5 X 10-&5 X 10-1

84331810

50402010

2 x 10-31.5 X 10-48 X 10-45 X 104

-83

-49-40-32

110623823

r-potential is not obtained under constant flotation con-ditions. The ratio r"./ra. under constant flotation condi-tions should be less than two. In the absence of any ex-perimental data this ratio has however been assumedto be two for obtaining the values given in Table 1. Pos-sibly, the effect of the difference between value 2 andthe actual value for rN./raa is counterbalanced by theeffect of the difference between the value used forTN./Ta. (assuming adsorbed sodium ion is hydrated andadsorbed barium ion is dehydrated) and its actual value.

Assumption 4 concerns the question of the thickness ofthe adsorbed layer that is to be used for calculating theadsorption density in terms of moles per sq cm fromadsorption density expressed in terms of moles per cc.Here when we consider the fact that these equations foradsorption densities of inorganic ions are derived inorder to estimate the competition that they will offer tothe collector ions for adsorption by reducing the re-sponsible electrostatic potential, it appears that what oneneeds to calculate is not r,', the adsorption in the Stemplane, but r," given by

At high adsorption densities, however, the large numberof counter ions between the 6-plane and the shear planewill make r-potential significantly lower than the 'it' andtherefore the foregoing assumption will not be valid.Changes in viscosity at the interface and resultant pos-sibility of a shift in the shear plane make the calcula-tions very complex. The problem of 'it' not being equalto r is, however, immaterial for the present purpose.This is due to the fact that it is uncertain whether it isthe adsorption of the collector ions in the Stern planethat should be calculated. Roy and Fuerstenau. havesuggested that the adsorption of the surfactant takesplace possibly on several layers of adsorbed watermolecules. Barium on the other hand could possibly goclose to the solid surface. This particular point will betaken into account while developing the new model.

The validity of assumption No.2 that the ,('-potentialis constant under conditions of equivalent flotation isfound to be poor on examining the data of Onoda andFuerstenau (see Table 1). The zeta potential value inthe presence of barium ions is lower than in the presenceof sodium ions at conditions of equivalent flotation. Onecan then rewrite the equation used for calculatingC"./Ca. ratio for equivalent flotation in the followingmanner:

rN. r". CN. ( .".. + 2 F ,('.. - F ,('N.

Ic'i" -.' '4, (5)r." = 6.' c. exp

(S)~;: --expI'a. ra. Ca. ' RT ,

I'N. and I'a. are, respectively, the adsorption densities ofsodium and barium conuter ions under conditions ofequivalent flotation and constant concentratiofi of thecollector. CN. and Ca. are corresponding bulk concentra-tions of sodium and barium ions and r... and ra. corre-sponding r-potentials. r". and Ta. are the radii of thehydrated sodium' and the unhydrated barium ion, re-spectively, and 40.. is the specific adsorption energy ofone mole of barium ions. The adsorption of hydratedsodium ion on oxide minerals is nonspecific, since itspresence is not known to change the point of zero chargeof these minerals in water. Using the same treatmentas of Onoda and Fuerstenau' we obtain from Eq 3:

r... TN. CN. ( F (2 ra. - rH.) ,exp 3-

RT ;

where b' is the effective distance of approach of the col-lector ions to the solid surface. b' will be different from3 if, as mentioned earlier, the adsorbed collector ions areseparated from the surface by several layers of watermolecules. Instead of 2 rN. and 2 r.., we will use b' as thethickness of adsorbed layers in the present treatment.This is because it is 3' that would determine adsorptiondensity of collector ions and "'" in turn would be af-fected by all the sodium ions and barium ions that areadsorbed between the solid surface and 3' plane.

The next factor to be considered is the criterion forflotation in terms of the collector adsorption. It has beenassumed in the past that equal flotation will be obtainedat equal collector adsorption at the solid/liquid inter-face. Important role of the collector adsorbed at theliquid/air interface has been neglected. It is not yetestablished whether the criterion for equal flotation isthe adsorption at liquid/gas interface, solid/liquid inter-face, solid/gas interface, or the sum total of the ad-sorptions at the liquid/gas and the solid/liquid inter-faces. We have established earlier that the amount ofcollector adsorption that is possible at the solid/gasinterface for the quartz-dodecylamine solution-gas sys-tem that we studied is approximately equal to the ad-sorption at the liquid/gas interface and that these twoadsorptions are of a considerably higher magnitudethan the adsorption at solid/liquid interface.s For thisreason it was considered essential that collector adsorp-tion at all interfaces should be taken into account whilestudying the flotation phenomenon. At this point it isinteresting to note that while addition of inorganicelectrolytes will decrease the adsorption of collector ionsat solid/liquid interface, it can be expected to increasethe adsorption at the liquid/gas interface. Surface ten-

(f)- --rBA TBa CB.

Eq. 4 can now be used to recalculate the relative effec-tiveness of sodium and barium salts in depressing amineflotation of quartz. The new values for the ratio CHa!CBaare given in Table 1. It can be seen that, whereas theearlier calculated values were twice the experimentalvalues, the new values are in fair agreement with theexperimental values.

Even though the agreement is now fair, the other as-sumptions need to be checked for their validity, par-ticularly since we do not have at present more data toverify whether a model based on Eq. 3 will be univer-sally satisfactory.

Assumption 3 that rHa is twice rBa under constant r-potential conditions is now unnecessary, since a constant

RT

TRANSACTIONS - YOlo 2SS MARCH 1974 - 65Society of Mining Engineers, AIME

less modifications that are currently possible should bemade. Eq. 5 is the basis in the present study. The ad-sorption of collector ions at the solid/liquid interfaceat a given flotation recovery in the presence of so-dium chloride is thereby given:

...' ~, ( FIJI"D... + .D,N. .

J. D,N. = U CO,N. exp - (10)RT

Adsorption of sodium ions under the same conditions isgiven by F .,," .., D,Na

(1.1)r"Na,D = a'CNo.D exp

(12)

sion experiments were done in the present work to testthe possibility of increase in collector adsorption at theliquid/gas interface due to the addition of inorganicelectrolytes.

If collector adsorption of the liquid/gas interface isfound to be affected by the addition of electrolytes, aquestion would arise as to whether the specific adsorp-tion potential of the collector ion and hence the hemi-micelle concentration could not be expected to changewith the addition of electrolyte. Indeed the specificadsorption potential responsible for hemimicelle asso-ciation should be expected to increase with an increasein electrolyte concentration; this is supported by thefact that critical micelle concentration of surfactants de-crease with increase in ionic strength. The critical hemi-micelle adsorption density at the solid/liquid interface,at which the hemimicelle association begins, thereforecould be expected to be lowered by the addition of anelectrolyte. However this critical adsorption densityis achieved by the electrostatic adsorption of collectorions and this would suffer under competition from theadded inorganic ions. Again, there is no data availablein the literaure to determine the nature of the totaleffect of addition of electrolytes on hemimicelle concen-tration. The only statement that could be made at pres-ent is that the specific adsorption potential of collectorions could not be considered to remain constant whenthe concentration of the inorganic electrolyte is altered

Eq. 2 with tiI' = r could in fact be treated for constantflotation conditions to show that two of the assumptions,5 and 6, could not simultaneously remain valid. Towardsthis purpose, we rewrite this equation for the case ofadsorption of dodecylammonium ions on quartz from asodium chloride solution . FrD.R. + ~D,R.

rO,M. = 2 TnCO,M. exp - (6)RT

The first subscript stands for the ion under considerationfor adsorption and the second subscript stands for theother ion that is present. D here stands for dodeclyam-monium ion and Na for sodium ion. Adsorption ofdodecylammonium ions from a barium chloride solutionis similarly

('1)rD,.. = 2 1'DCD,.. exp -

(8)= 2 rDCp... exp -I

when CO.No = CO.Bo, it follows from Eq. 8 that

RT ;

It might be noted that it is the total adsorption ofsodium ions between solid surface and 6' plane that iscalculated here since as mentioned earlier all the sodiumthat is adsorbed in that region will affect the adsorp-tion of collector ions at plane 6', the plane of closest ap-proach of collector ions to the surface under flotationconditions. The ratio of the adsorption of sodium ions tododecylammonium ions for a given flotation is obtainedby dividing Eq. 11 by Eq. 10.

1"'"..0 C"..o( -400... '

-=-exp -1"'0.". CO.". RT

Eq. 12 predicts the ratio of adsorbed sodium ions toadsorbed dodecylammonium ions to be independent ofthe r potential of the mineral and hence the solution pH.

Experimental Methods and MaterialsBrazilian quartz crystals were crushed and sized to

separate 28/65 mesh fraction for the flotation studies.The sized quartz was cleaned using warm dilute nitricacid, washed with triple distilled water until free ofnitrate and then stored in it at pH 2.8 until it wasused. A pH closer to the point of zero charge of themineral was chosen for storing the mineral for reasonsdiscussed elsewhere (7). Dodecylammonium acetate wasprepared from high purity dodecylamine. Sodiumchloride used was of reagent grade.

For flotation tests, 0.5 to 0.6 g of the mineral was con-ditioned in the desired solution for 10 min and thentransferred into a modified Hallimond tube for flotation.After measuring the pH, flotation was conducted for 10sec at a flowrate of 0.6 cc per sec. Reproducibility of theflotation time and stirring rate was achieved in theseexperiments using a timer. One of the cams of the timerwas connected to a solenoid valve placed between thegas reservoir and the Hallimond tube so that the gasflow would occur for 10 sec. The second cam was con-nected to a magnetic stirrer so that the stirrer will beturned on 5 sec before the beginning of the gas flowand will be turned off 2 sec after the end of the gas flow.This arrangement permitted the setting of the stirringrate to be left undisturbed between experiments andthus prevented any nonreproducibility that would occurif the stirrer was manually operated for each test.

Surface tension of the solutions were determined byWilhelmy plate method using a platinum foil as sensOr.A Beckman microbalance was used for measuring theforce exerted on the sensor. Wettability of the solutionswith the sensor was checked during every test bysmooth lowering and raising of the solution containerwith the help of a platform mounted on a camerafocusing ring. Samples for the determination of surfacetension were taken from flotation solutions after con-ditioning and before flotation.

exp- =exp-

(9)

Since according to the data of Onoda and FuerstenaurO,M. is not at all equal to rO,B.. ~o.". cannot be equalto ~O,B.. Hence both rO,B. = rO,N.. and ~O,B. = ~D,H' to-gether cannot remain valid for constant flotation con-ditions.

We can reexamine the model on the basis of the fore-going discussion on the previous assumptions. It must beremembered that uncertainties regarding ~' and changesin specific adsorption potential make it rather difficultat this point to obtain a very rigorous model; neverthe-

Results and DiscussionTo select a collector concentration that is satisfactory

for the present experiments, flotation tests were first

TRANSACTIONS - vat.. 2SS66 - MARCH 1974 Society of Mining Enginee.., AIME

done at natural pH (-5.8) in the absence of externalelectrolytes and as a function of dodecylauunoniumacetate concentration. Results obtained are shown inFig. 1. A major purpose of the work was to study thedepressing effect of external electrolytes at differentpH values. Therefore, 1.5 X 10-- mole per 1, a concen-tration that gave a fairly high recovery was selected forthe tests.

The results obtained at different pH values (4.6, 5.8,and 6.4) for the effect of potassium nitrate on the flo-tation of quartz are shown in Fig. 2. It can be seen thatpotassium nitrate is effective in depressing flotationeven at as Iowa concentration as 10-- mole per 1. Cal-cium nitrate, as expected, is effective at even muchlower concentrations. The importance of this becomesevident when one considers that the natural watersused in flotation plants can indeed have much highersalt concentrations than these. The increase in depressionobtained in the present work with increasing salt con-centration is fairly regular except in the case of testsdone at pH 6.4. In this case the results obtained fromfour tests done in salt concentration range from 10-1 to3 X 10-1 mole per 1 were higher than what would havebeen obtained had the flotation decreased regularlywith salt concentration along the broken line in Fig. 2.A similar irregular behavior was obtained also whencalcium nitrate was used as the external electrolyte(see Fig. 3). The nature of the froth obtained was alsodifferent in these cases. Frothing was voluminous in

10-. 10-0 10-. 10-0

DOOECYLAMMONIUM ACETATE CalCENTRATION. mol. II

Fig. I-Flotation recovery of quartz as a function ofdodecylammonium acetate concentration at natural pH(-5.8), no external electrolyte added.

...'"..ro (pH..F.) C".,o (pH..F.)

From Eqs, 14 and 15 we obtain

r"N.,O (pH.,FJ r"..,D (pH..F.)

r"N.,O (pH"F.) ",'Ha... (pH..FJ

CHa.D (pHl,FJ C..,D (pH..F.)(16)=~i!";~"...C,~ '. .

C...,O (pH.,F.) CN..O (pH..F.)

If it is assumed that the change in adsorption densityof collector at the solid/liquid interface for changein flotation from one level to another is independent ofsolution pH, it can be stated that:

rt'N.,O (pHt,F.) rt'.a,D (pH..F.) (17)-,," ., .

"~"

rI'".,n (pH.,F.) 1"""..0 (pH..F.)

This expression simply means that to reduce the ad-sorption density of collector by a given factor at any

TRANSACTIONS - VOL 155 Society of Mining Engineers, ACME MARCH 1974 - '7

sponsible for the specific adsorption potential, "'0, it alsowill certainly increase with an increase in ionicstrength. Assumption in previous models regarding theconstancy of the specific adsorption energy is hence aweakness of the models that has to be rectified. Further-more, since adsorption at liquid/gas interface is an im-portant parameter determining flotation recovery," it isevident from the indicated increase in adsorption at thesolution/gas interface that one cannot assume constantcollector adsorption density at the solid/solution inter-face for constant flotation recovery if the ionic strengthof the solution is varying. In fact, the collector adsorp-tion at the solid/solution interface for constant flotationshould be expected to decrease as the ionic strength ofthe solution is increased. At present there is however nodata available in literature for the collector adsorptiondensity at the solid/solution interface for constantflotation as a function of concentration of inorganicelectrolytes. There is also a need for quantitative infor-mation on the variation of specific adsorption potentialof surfactant species as a function of ionic strength be-fore a more rigorous model for the cationic depressionof oxide flotation could be developed.

Table 2-CNa,D(fJ/CNa,D(f,! Ratios Obtained for pH 4.6 and 5.9

C..,o(I'u/C..,u(I'.)1', . Ie, F. - 18 Fl = 10. F. = SO

4.85.9

4.13.0

4.32.8

I'.'8.2

pH value, the adsorption density of the competing in-organic ion also has to be increased by a constant factor.Substitution of Eq. 17 into 16 yields:

C".,n (pH.,FJ C".,n (pH.,F)(18)~ !_,~ !~'C"~c~,~~., "~-

C..a,o (pHl,F.) C"a,O (pH..F.)

This ratio can now be calculated at different pH valuesfrom the results obtained for flotation as a function ofthe concentration of potassium nitrate. It can be seenfrom Fig. 2 that the results at only two pH valuesare useful for this purpose. The range of flotation inwhich the salt concentration is of significant influenceat botJ1 pH 4.6 and 5.8 is 0 to 30%. The ratio C"a,o(FJ/CNa, 0 (F.) is calculated for different F. and F. values andis presented in Table 2.

Even though errors of the order of 5% in flotationrecovery could give rise to a significant change in theratios, they do show a tendency to decrease with in-crease in solution pH. This becomes more evident if thetrend of the data obtained at pH 6.4 is also examined.

Possible reason for the foregoing tendency becomesevident when surface tension data for dodecylammo-nium acetate solution is examined as a function of con-centration of an inorganic electrolyte. Results of thesurface tension experiments are given in Fig. 4. Surfacetension decreased from a value of 70.5 dynes per cm inthe absence of KNO. to 32.6 dynes per cm at a KNo. con-centration of 1 mole perl. This decrease, being the resultof an increase in the adsorption of the collector ions atthe solution/air interface, suggests that the free energychange for transfer of surfactant species from bulksolution to solution air interface increases with increasein ionic strength. Part of this increase in transfer ofenergy will be due to the compression of the electricaldouble layer at the solution/air interface, but a signifi-cant part of it will be due to an increase in the energyof transfer of -CH.- groups from an aqueous elec-trolyte." Since it is the same transfer energy that is re-

Summary and ConclusionsThe depressing effect of potassium nitrate on the

amine flotation of quartz was examined as a function ofsolution pH. The data was used to test theoreticalmodels developed for the effectiveness of inorganic ionsas flotation depressants. Even though the agreement ofthe predicted data with the experimental data was with-in the possible experimental error, the weaknesses ofcertain common assumptions were evident.

Experiments conducted to determine the surface ten-sion of collector solutions as a function of the concen-tration of potassium nitrate indicated that the adsorp-tion of the collector ions on the surface of the bubbleswould increase when the salt concentration of the solu-tion is increased. Since the collector adsorbed at theliquid/gas interface plays an important role in flotation,it is necessary to take this also into account along withadsorption at the solid/solution interface while study-ing the role of inorganic electrolytes as depressants forflotation. The surface tension data also indicated that thespecific adsorption energy of the collector at the solidisolution interface could not remain constant when thesolution concentration of inorganic electrolytes is in-creased. It is noted that quantitative infonnation onthese effects is necessary for developing a more rigorousmodel for the depression of flotation of oxides using in-organic electrolytes.

-1"~ f'

~

1.5)(10"'_/1 DOA

-pH-5880

~

~60~0;z....-... 40u~~

~

,

~

References'0noda. G. Y.. and Fuerstenau. D. W.. 7th lntentatlonal Mineral

Processing COftgTess. Vol. I. Gordon and Breach. New York. I...p.301.. HoPstock. D.M.. and Agar. G.E.. Trans. SME/AIME. Vol. 241,1968. p. 466.

. Clark, S. W., and Cooke, S.R.B., Trans. SME/ AIME, VoL 241,

1968, p. 334. Somasundaran. P.. 7th lnte.-national Mineral Processing Congress,

Discussions Digest, N. Arbiter. W. Giftord, and D. Welsh, eds.. Gor-don and Breach. New York, 1969, p. 39.

. Somasundaran. P.. Trans. SME/AIME. Vol 241. 1968. P. 105."Roy. P., and Fuerstenau. D.W.. Journal of Colloid Interface Sci-

ence, Vol 26. 1968, p. 102.7 Somasundaran, P., "Pretreatment of Mineral Surfaces and Its

Effect on Their Properties." Clean Surfaces: Their Preparation andCharacterization for Interfacial Studies. G. Goldfinger, ed., MarcelDekker, New York. 1970. P. 285.

8Fuerstenau. D.W., Metzger, P.H., and Seele, G.D.. Engineering &Mining Journal. Vol. 158. 1957. p. 93.

. Li, H.C., and de Bruyn. P .L., Surface Science. Vol. 5, 1966. p. 203.to Gaudin, A.M.. and Chane. C.S.. rrans. AIME. Vol. 193, 1952.

p.I93. .U Un, I.J.. and Somasundaran, P., Journal of Colloid Interface SCI-

ence. Vol. 37. 1971, p. 731.

~

OL-,.!-- ! i I - I]0 10-4 10-3 10' 10-' ,

KNO) CONCENTRATION, ..oJ./ I

Fig. 4-Surface tension of dodecylammonium acetate so-lutions at pH ,..,5.8 as a function of KNo. concentration.

Society of Mining Engineers. AIME TRANSACTIONS - VOL. 25568 - MARCH 1974

~