FATTY ACIDS AS FLOTATION COLLECTORS FOR CALCITE

Flotation experiments using stearic, oleic, linoleic, linolenic, and ricinoleic acids and naturally occuring products rich in these acids as collectors for calcite are described. The results confirm the validity of the rule of Hukki and Vartiainen relating the collecting power and unsaturation of the C18 acids. Reasons for this relationship are discussed and a close relationship between mineral depression and critical micelle concentration i s reported.

by M. H. Buckenham and J. M. W. Mackenzie

I t has long been recognized that the unsaturated fatty acids a r e in general superior flotation collec-

tors to their saturated homologues. Whereas earlier workers concentrated on saturated and monoethe- noid acids, recent investigations have centered on oleic, linoleic, linolenic, and the substituted mono- ethenoid acid: ricinoleic.

From studies of the collecting properties of pal- mitic, oleic, linoleic, and linolenic acids on ilmenite, rutile, hematite, and magnetite, Hukki and Vartiai- nenl concluded that the collecting power of fatty acids increased with unsaturation of the hydrocarbon chain.

Sun, Snow, and Purcel12 investigated the collecting properties of unsaturated fatty acids a s collectors for phosphate ores and concluded that the collecting power increased with increasing unsaturation of the hydrocarbon chain up to two double bonds and that further unsaturation decreased the collecting power. More recently Sun3 studied the collecting power of the C18 fatty acids stearic, oleic, linoleic, and lino- lenic on 37 minerals. The results of these experi- ments with a few exceptions showed that the col- lecting power of the acids increased in the order stearic, oleic, linoleic, and linolenic . The relative collecting powers of linoleic and linolenic acids were in many cases altered by the cleanliness of the mineral surface prior to flotation. Sun concluded that linolenic acid may become oxidized by atmos- pheric oxygen during flotation, a reaction which would reduce its effectiveness as a collector. This deduction is supported by Gaudin and Cole4 who con- cluded that oleic and linoleic acids do not undergo appreciable oxidation of the double bonds during flo- tation but that the linolenic acid double bonds a r e measurably affected.

M. H. BUCKENHAM, formerly Senior Lecturer in Mineral Engineering, Faculty of Technology, University of Otago, Dunedin, New Zealand, is now at Columbia University, New York; and J. M. W. MAC KENZ IE i s Research Officer, Bmken Hil l Pty. Ltd., Central Research Laboratories, Shortland, Newcastle, Australia. TN 608232. Manuscript, Oct. 6, 1960. Discussion of this DaDer. submitted in ~ U D -

licate prior to July 1, 1962, will' abpebr in AlME ran;., 1962, vol . 223.

The most complete investigation on the influence of hydrocarbon chain unsaturation on the collecting properties of fatty acids is that of Kivalo and Leh- m ~ s v a a r a . ~ These workers using linolenic acid of a higher purity than Hukki and Vartiainen showed that this acid was superior to oleic and linoleic acids as a collector for magnetite. They also found that ri- cinoleic acid was a more effective collector for this mineral than the other acids investigated, an obser- vation particularly apparent a t high collector con- centrations. Seeking to explain the superiority of the unsaturated acids these authors considered the ef- fects of critical micelle concentration, surface ac- tivity, hydrolysis, and cross sectional a rea of the hydrocarbon chain in relation to unsaturation. Using what, according to the results of Caviere a r e erro- neous surface tensions of the soap solutions of these acids, they concluded that the surface activity de- creases and the critical micelle concentration in- creases with increasing unsaturation. In view of the data of Cavier, whose results show that surface ac- tivity of these soaps increases with unsaturation, the deductions of Kivalo and Lehmusvaara a r e open to criticism. The degree of hydrolysis of the soap solu- tions a s measured by Kivalo and Lehmusvaara de- creases as the unsaturation of the fatty acid in- creases, a result which helps to explain the superior- ity of unsaturated acids a s collectors.

In view of the superiority of unsaturated acids as collectors for many minerals, attention has been directed towards the utilization of fatty acid raw materials such as tall oils and linseed oils which contain considerable quantities of these acids. This paper describes test work to determine the effect of unsaturation of the hydrocarbon chain on the collect- ing properties of fatty acids on calcite, and includes comparable test work using natural products rich in unsaturated acids.

TEST WORK

Reagents: The pure fatty acids used were oleic, linoleic, and linolenic. Of these, oleic was supplied by British Drug Houses Ltd. and the others were produced by the Hormel Inst. When not in use these

oleate o linokate

a 20 -

P

0. I 10 CONCENTRATION GMS- LITRE

Fig. 1-Hallimond tube results showing the effect of concentrations of collectors-oleate, linoleate, linolenate, ricinoleate, and stearate-on calcite re- covery.

acids were stored under nitrogen in a refrigerator to prevent oxidation of the double bonds. The ricino- leic acid was a sodium sal t supplied by the Distilla- tion Products Div. of the Eastman Kodak Co., an analysis of which is included in Table I together with analyses of s tear ic acid and locally available fatty acid products which were also evaluated a s collectors for calcite. Sodium sal ts of the pure acids were pre- pared by neutralization with aqueous sodium hydrox- ide and sodium sal ts of the crude products by sapon- ification with ethylinic sodium hydroxide.*

*Sodium hydroxide dissolved in ethyl alcohol. The alcohol was sub- sequently removed by distillation at reduced pressure.

Calcite: The calcite used in the investigation was marble from Takaka, Nelson, New Zealand. The usual precautions were taken during and after prep- aration to ensure that contamination of the mineral surfaces was avoided. The calcite was periodically subjected to Hallimond tube testing in distilled water to confirm surface cleanliness. Flotation Tests: Flotation tests on -65 + 100 mesh Tyler calcite were carried out using a Hallimond tube, conditioning and flotation times being 40 and 1.5 min, respectively. Tests were conducted a t the natural pH to avoid the influence of added OH and hydrogen ions on hydrolysis and micelle formation. Critical Micelle Concentration: The technique of dye solubilization was used to determine critical micelle concentrations. This method does not yield a sharply defined concentration but rather a range of concen- tration a s required by the mass law. The dye used was an adsorption indicator di-iodo ( r ) dimethyl (r)

Table I. A n a l y s i s of I m p u r e Fa t ty A c i d Coll*ctors

Linseed Castor Linseed Recovery Ricinoleic Stearic

Tall Oil* Oil Oil Oil Acid Acid

Palmitic 11.87 2.63 4.43 4.84 2.08 3.54 Stearic 9. % - 1.21 1.21 93.15 Oleic 34.45 9.48 15.52 13.30 11.11 0.88 Linoleic 6.51 8.45 14.52 18.55 4.17 - Linolenic - - 64.10 55.65 - - Ricinoleic - 78.90 - - 82.64 -

*The tall oil also contained 8.42 pct lauric and 6.89 pct myristic acids.

tall oil soap o recowry cil s w p + linseed cil soap I

" !

CONCENTRATION GMS- LITRE - Fig. 2-Hallimond tube results showing the effect of concentration of collectors-tall oil soap, recov- ery oil soap, linseed oil soap, and castor oil soap- on calcite kecovery.

fluorescein, a small excess of which was added to the soap solution and allowed to stand for 40 min. The solution was then centrifuged and the resulting color tested for optical density using a Spekker pho- tometer with green filters. Hydrolysis: Standard concentrations of soap solu- tions were prepared and the pH immediately meas- ured by a Cambridge pH meter.

RESULTS AND DISCUSSION

The results of flotation tests using stearic, oleic, linoleic, linolenic, and ricinoleic acid soaps (Fig. 1) show, with the exception of s tear ic acid, a linear in- crease in calcite recovery for low collector concen- trations followed by a region of optimum recovery after which a t high collector concentrations recovery rapidly falls to zero.

Calcite recovery with the crude acids (Fig. 2) in general follows that of the fatty acids of which they a r e composed. An exception however is the castor oil which despite having 78.9 pct ricinoleic acid ceases to follow the recovery for this acid above 0.4 g per 1. The recovery curves for the linseed oils show high recoveries at low concentrations, in agree-

0 4 - z E Q 3 -

w a . II: w 2 - I-

8 k : C L .

* oleatc 0 linoleate

l~nolenate - -< -. -o + r~cinoleate

-. :1,7j-)i\ 1 : -a-.

. - 1 1

0.1 1.0 CONCENTRATION GMS-LITRE

Fig. 3-Test results showing the intensity of dye solubilization against concentration of collectors- oleate, linoleate, linolenate, ricinoleate, and tall oil soap.

CONCENTRATION GMS-LITRE Fig. 4-Hydrolysis curves showing the effect of concentration of collectors-oleate, linoleate, lino- lenate, and ricinoleate-on pH.

ment with their high linoleic and linolenic acid con- tents. Compared with the raw linseed oil, high acid recovery oil* is a superior collector a t low concen-

*Impure byproduct recovered during refining of raw linseed oil. -

trati& but depression a t high concentrations oc- curs at lower values than for the linseed oil. These differences in collecting properties can be recon- ciled with the linoleic acid contents of these reagents, (18.55 and 14.52 pct, respectively). The curves for the linseed oils and tall oil show a distinct irregu- larity in form in the plateau region, a trend which is also evident to a lesser degree in the castor oil curve. The reasons for this irregularity, not evi- dent in the curves for the pure acids, is not under- stood but it may be due to the fact that the impure collectors contain mixtures of fatty acids and there- fore ions of different types may be adsorbed onto the calcite surface. If this is s o the curves would represent the cumulative effect of different collector entities.

It is apparent from Fig. 1 that the collecting prop- er t ies of s tear ic , oleic, linoleic, and linolenic acids follow the original rule of Hukki and Vartiainen. The curves show, however, that the order of superiority of the acids as collectors for calcite depends to some extent on the concentration and pH at which the comparison is made. This may explain some of the anomalies which have been reported when C18 fatty acids have been compared a t one concentration only.

From Fig. 3 the approximate collector concentra- tions a t which micelles begin to form in appreciable quantities a re :

Linoleate 0.15 gpl Linolenate 0.20 gpl Oleate 0.30 gpl Ricinoleate 0.45 gpl Tall Oil Soap 0.50 gpl

The value for the oleate (0.001 mol per 1) com- pares reasonably well with that of Flockhart and Graham7 of 0.0015 mol per 1. The results reported above a r e in contrast to the trend proposed by Kivalo and Lehmusvaara whose results, however, were de- rived from surface tension measurements, which in view of the work of Cavier may be incorrect. Our re-

fa) f b) (c) Fig. 5-Models showing possible orientations of carboxylate collector ions at the mineral surface. a) Oleate, low concentration. b) Linoleate, low con- centration. c) Linoleate, high concentration.

sults a r e in better agreement with the order which may be inferred from rather incomplete osmotic co- efficient data of McBain. '

The pH measurements graphed in Fig. 4 show that degree of hydrolysis decreases in the order oleate, linoleate, linolenate, a result confirming the trend found by previous worker^,^, although the actual pH values a r e somewhat different in magnitude.

The improved collecting properties of the C18 fatty acids with increasing unsaturation may be as- sociated with 1) the oxidizability of the double bond, 2) the degree of hydrolysis of the fatty acid soap solutions, 3) the orientation and cross sectional a rea of the hydrocarbon chains, and 4) the critical micelle concentrations of the fatty acid soap solutions. Oxidation of Double Bonds: Gaudin and Cole con- cluded that the ordered increase in the collecting powers of the C18 fatty acids was not due to oxidiza- bility of the double bonds. The double bonds of lin- oleic acid may however undergo some oxidation a s a result of flotation cell aeration and oxidation may be the reason for linolenic acid appearing inferior

(a) (b)

Fig. 6-Models showing possible orientations of carboxylate collector ions at the mineral surface. a) Linolenate, low concentration. b) Linolenate, high concentration.

(a) (b, Fig. 7-Models showing possible orientations of carboxylate collector ions at the mineral surface. a) Ricinoleate, low concentration. b) Ricinoleate, high concentration.

to linoleic or a t least for the anomalous results which have been reported. Degree of Hydrolysis: Hydrolysis of soap solutions, whether to form acid soaps o r the f ree acid, may be expected to affect the flotation properties of a soap solution. Soaps such a s sodium linoleate which hy- drolyze to a lesser degree than sodium oleate may be expected to be superior collectors. pH varied considerably in the Hallimond tube tests but did not enter the range where OH ions could be expected to cause marked depression. Addition of hydrogen o r OH ions to control the pH would have masked the hydrolysis effects and altered the physicochemical conditions existing in the soap solutions, in particu- lar the critical micelle concentration. At the low pH values encountered below concentrations of 0.1 gpl, reduction of the collector ion concentration in accordance with the Henderson equation may have had an effect on calcite recovery. Orientation and Cross Sectional Area of Hydrocar- bon Chains: Studies of monomolecular films of fatty acids on water with particular reference to force- a r ea curves by Schneider, Holman, and Burrm have yielded information regarding the apparent c ross sectional a r ea of the fatty acid molecules. These experiments showed that the hydrocarbon chains in - crease in cross sectional a r ea with increase in un- saturation.

The results of Schnieder, Holman, and Burr a r e :

Acid

Stearic Oleic Elaidic Linoleic Linoelaidic Linolenic Elaidolinolenic Ricinoleic Ricinelaidic

Apparent Crosz Sectional Area A'

According to Langmuir's original postulate, the double bonds in the fatty acids have an affinity for water, a conclusion which is in agreement with the surface tension data of Cavier. However, mere so-

lution tendencies of unsaturated centers of the hy- drocarbon chain cannot alone be responsible for this increase in apparent molecular c ross sectional a r ea as some unsaturated fatty acids such a s ricinelaidic form condensed monolayers.

Possible adsorption configurations for the fatty acids a r e shown in Fig. 5, 6, and 7. It i s suggested that at low concentrations the carboxylate ions of linoleic and linolenic acids tend to be oriented paral- lel to the mineral surface, and at higher concentra- tions the orientation changes until the ions a r e al- most perpendicular to the surface. Under these con- ditions the hydrocarbon chains would cover an a rea determined by the space requirements of the chain rather than by the chain orientation. The tendency of the hydrocarbon chains to orient themselves more or less parallel to the mineral surface would in- crease with increasing unsaturation. This effect is not noted with s tearic acid, which forms a compact mono!ayer with a limiting c ross sectional a r ea of 24.4 A2. While this effect may help to explain the superiority of the doubly and triply unsaturated acids a t low concentrations the behavior of ricinoleic acid is anomalous. The hydroxyl group is water avid a s is suggested by the high apparent c ross sectional a rea of 109.4 A2. However, the flotation results do not indicate that ricinoleic acid is adsorbed parallel to the calcite surface but suggest that it is adsorbed in much the same manner as oleic acid, the structure of which it closely resembles. It is possible that in the case of ricinoleic acid, hydrogen bonding between the OH groups will cause the hydrocarbon chains to orient themselves in a manner similar to that of oleic acid and in a more compact form than that of the more unsaturated acids.

Orientation of the hydrocarbon chain a t the min- e ra l surface is not the only factor determining the surface coverage of the mineral by collector ions. The unsaturated fatty acid cis-isomers will by their very nature have increasing cross sectional a rea with increasing unsaturation. This increase in space requirement, a s well a s the increase in thermal agi- tation of fatty acid molecules with an increase in unsaturation, explains the expanded type of mono- layer formed a t the mineral surface by the cis- isomers of the unsaturated fatty acids. Critical Micelle Concentration: Collector ions when adsorbed on a mineral surface have their hydrocar- bon chains oriented towards the solution. When the ions at the surface reach a certain concentration corresponding to the critical micelle concentration of the bulk solution they may form what Gaudin and Fuerstenaul1 term "hemi-micelles." These may be visualized a s being half a McBain lamellar micelle with the mineral surface taking the place of the wa- t e r phase. The formation of complete micelles at the mineral surface, a phenomenon which may be expected to occur a t higher concentrations than a r e necessary for the formation of hemi-micelles, re- sults in the collector coated mineral surface becom- ing hydrophyllic. From the curves in Figs. 1 and 2 it is suggested that through the plateau region hemi- micelle formation becomes almost complete and that the abrupt decrease in recovery with increasing col-

lector concentration is due to the formation of com- plete micelles a t the interface. If the above hypoth- esis i s correct, there should be a correlation be- tween the critical micelle concentration of the bulk solution and the concentration a t which depression commences.

The corresponding values a r e : Critical Micelle Concentration Concentration, a t which Depression

G P ~ Commences, Gpl

Linoleate Linolenate Oleate Ricinoleate Tal l oil soap

Considering the approximations made in deriving these results and the broad zone over which micelle formation occurs there is a close correlation be- tween the two concentrations. The reason for the concentration a t which depression commences being approximately four times the critical micelle con- centration is not understood, but it may be due to the fact that the micelles which form initially a r e capable of adsorbing a dye but a r e not sufficiently dominant to inhibit flotation.

CONCLUSIONS

Aqueous solutions of the alkali metal soaps of the fatty acids constitute complex systems the proper- ties of which are difficult to investigate and relate to flotation performance. These solutions a r e par- ticularly complex in the case of the saponified crude fatty acids, but their performance may be roughly interpreted on the basis of their fatty acid content.

Sufficient test work has been done on the calcite- fatty acid system to further confirm the validity of the Hukki and Vartiainen rule relating collecting power and unsaturation of fatty acids. The improved flotation properties associated with unsaturation a r e connected with the structures of the fatty acids and a r e dependent upon the hydrolysis, critical micelle concentration, and surface activity of the soap solu- tions a s well a s the cross sectional a rea and orien- tation of the adsorbed fatty acid ions.

In the test work described there is a close corre- lation between depression of mineral due to exces- sive collector and the critical micelle concentration of the soap solutions a s determined by dye solubili- zation. This suggests that depression is due to the formation of a double layer of collector ions corre- sponding to a lamellar micelle a t the mineral solu- tion interface.

ACKNOWLEDGMENTS

The authors acknowledge the assistance given them by the staff of the Geology and Chemistry Depts. of the University of Otago, and F. B. Shorland of the Fats Research Laboratory, Dept. of Scientific and Industrial Research, Wellington. J. M. W. Macken- zie acknowledges the financial assistance provided by the James Park Scholarship. This paper is con- densed from a thesis presented to the University of New Zealand a s partial fulfillment of the require- ments for the degree of Master of Engineering.

REFERENCES 'R. T. Hukki and 0. Vartiainen: An Investigation of the Collecting

Effec ts of Fat ty Acids i n Ta l l Oil on Oxide Minerals, particularly on Ilmenite, AIME Trans., 1953, 196, p. 818.

'S. C. Sun, R. E. Snow. and V. I. Purcell: Flotation Characterist ics of a Florida Leached-zone Phosphate Ore with Fa t t v Acids. AIME Trans., 1957, 208, p. 70.

'S. C. Sun: Single Mineral Flotation with Linolenic, Linoleic. Oleic and Stearic Acids, presented a t AlME Annual Meeting, February 1959.

'A. M. Gaudin and R. E. Cole: Double-Bond Reactivity of Oleic Acid during Flotation, AlME Trans. , 1953, 196, p. 418.

'R. Kivalo and E. Lehmusvaara: An Investigation into the Collecting Properties of some of the Components of Ta l l Bil , Transactions of Inter- national Mineral Dressing Congress, Stockholm, 1957, p. 577.

'R. Cavier: Valeur de l a Tension Suferficielle d e Solutions d e diffbrents Savons de Sodium, Comptus Rendes , Acad. Science (Paris), 1941, 212, p. 1146.

'B. D. Flockhart and H. Graham: Studv of the Dilute Aqueous Solu- t ions of Sodium Oleate, Journal of Col l . Science, 1953, 8 , p. 105.

'J. W. McBain: Col lo id Science, D. C. Heath and Co., 1959, p. 252. 'J. Powney and D. Jordan: The Hydrolysis of Soaps a s Determined

from Gla s s Electrode pH Measurements, Transac t ions , Faraday Soc., 1938, 34, p. 363.

'V. L. Schneider, R. T. Holman, and G. 0. Burr: A Monolayer Studv of the Isotherm of Unsaturated and Oxy Fa t t y Acids, Journal of Phys . Coll . Chem., 1949, 53, p. 1016.

"A. M. Gaudin and D. W. Fuerstenau: Quartz Flotation with Cationic Collectors, AIME Trans. , 1955. 202, p. 958.