P. Somasundaran, K. P. Ananthapadmanabhan

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'lwelfth INTERNATIONAL MINERAL PROCESSING CONGRESS SAC PAULO, BRAZIL, 1977 FLOTATION MECHANISM BASED ON IONOMOLECULAR COMPLEXES P. Somasundaran, K. P. Ananthapadmanabhan and R. D.

1 'lwelfth INTERNATIONAL MINERAL PROCESSING CONGRESS SAC PAULO, BRAZIL, 1977 FLOTATION MECHANISM BASED ON IONOMOLECULAR COMPLEXES P. Somasundaran, K. P. Ananthapadmanabhan and R. D. Kulkarni. Henry Krumb School of Mines Columbia Uni versi ty New York, N. Y , u. S. A. *Present address: Union Carbide Corporation Technical Center Tarrytown, N. Y , U. S. A. APRIL 1977

2 ABSTRACT FA!IQN MECHANISM BASED ON IONOMOLECUIAR SURFACTANT COMPLEXES P. Somasundaran, K. P. Ananthapadmanabhan and 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 adhesion tension tests. In this paper, the ph dependence of hematite flotation using oleate is discussed in terms of role of ionomolecular complexes

3 FLOTATION MECHANISM BASED ON IONOMOLECULAR COMPLEXES P. Somasundaran, K. P. Ananthapadmanabhan and R. D. Kulkarni The collector adsorption at the solid/liquid (S/L) interface in relation to flotation has received considerable attention in the past. This has mostly been explained in terms of the surface chemical and electrokinetic properties of the mineral with a little emphasis on the chemical nature of the surfactant{i3). For example, the high flotation of hematite obtained around the neutral ph region in hematiteoleate system has been attributed to the chemisorption.of oleate at the neutral sface-og sites which are in high concentrations 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 adsorption behavior of hematite-oleate system. On the other 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.

5 3 at low ph values. In the intermediate range, the ions and 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 of hematite flotation. The sharp increase in 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 surface active species in neutral ph region. An excellent 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.

7 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 circulaof 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 smooth raising or lowering of the sensor. The output of the 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 unifoy applied for 30 seconds in all cases even though the surface tension values for pure water were

8 6 obtained in less than ten seconds. Repeated recordings followed by suction,s obtained for most test solutions showed the reproducibility to pe satisfactory. a RES ULTS 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 rge and this is in agreement with the results reported in the literature. In 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 pzc. Flotation recovery vs. ph curves for various concentrations of oleate is shown in Figure 2. It can be seen from this figure that the increase in oleate concentration has increased the flotation recovery of hematite. At the s ame ti the ph of maximum flotation response has shifted to higher ph values with the increase in oleate concentration. This is clear iran 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.

9 7 Th Denvr 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. It is seen from these figures that, (a) the initial 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 three regions exist at low ph values, the plots for higher 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 characteristic 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

10 8 is plotted as a function of solution ph. 'l11e plot in this figure shows a dome shape with maximum surface pressure in the neutral ph range. This maximum obtained in the neutral 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.

11 9 DISCUSSION The results of this investigation has clearly defined two main features of hematite-oleate flotation system. are: These (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 (2) concentration. For a given hematite sample the ph of maximum flotation 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 hematitesolution 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.

12 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. These soluble salts, under appropriate solution ph conditions 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', acidsoap salt (HNa) and oleate dir (R;-). The relative proportion of these species in the aqueous solutions is 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). RH (L) Kl RH(solution) pkl = 7.6 (1 RH(soln. K2,+ R- + H pk (2) = KJ - + R'. = ) H pk3-2r - K 4 -- '-- R = ) pk4 *H + KS + Na HNa (precip- PKS = (5) itate) *The equilibria shown here are for Na-oleate. It is assumed that these values may not change appreciably for K-oleate. The thermodynamic data is not available for K-oleate.

13 11 so that: K 1 IRH]/[RH(C)] K2 = fr-] IRH] [H+j KJ = [H / [RH] [R-] K4 [-] ]2 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 can be determined using equations (6-10). The rearrangement of these equations along with the substitution of the activity (11)

16 14 maximum surface activity of this solution is observed in the ph range of It should be noted that the above stated ph refers to bulk ph. The surface ph, i.e. the ph 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- oleate species which are predominant under these conditions. On the basis of above discussion one can expect oleate to be most effective as a flotation aid when present as

17 IS acid-soap complex, p;rovided othe;r conditions such as properties of solq/lquid interface are favorable. The actual role of acid-soap in the hematite-oleate lotation system can be evaluated by determining quantitatively concentratiqs 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 is presented in Figure (O) along with the concentration of acid soap as a function of bulk solution ph, at a total oleate concentration of 3 X 10-5 mole/liter. An excellent correlation is evident from this figure. It is seen that ph of maximum floatability corresponds to the ph of maximum acid soap con- cen trati on in the solution. Also the sharp drop in hematite floatability at higher and lower ph value may be associated with rapid decrease in the acid soap concentration. Again it should 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 bulk however,. solutj.on,ph. unlike the case of solution/gas interface, the hematite/solution interface, being only weakly negative in charge, will show only marginal lc7l1ering of surface ph in the ph range of 7-9. Table (1 lists the flotation response of hematite-oleate system at two different'ph levels and different oleate adsqt}on densities. It is to be noted that at ph 9.0 where acid-soap

19 17 CONCLUSIONS Two main features of the hematite-oleate system evident from the present study are (1 a sharp peak in flotation around the neutral ph range and (2) the shift in the ph corresponding to the maximum f iota tion wi th the increase in the concentration of oleate. These results have 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 corresponds to the region of high floation. The hiqh flotation 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 studies. Most importantly, the present study has sho that the solution chemistry and the chemical nature of the surfactant play an important part in determining the flotation character- istics of hematite-oleate system. In the light of the above facts, a systematic study of the solution chemistry of other mineral-collector systems is warranted..ackn DGMENTS Foundation (BNG for the support of this research.

18 REFERENCES 1. Pope, M. I., and Sutton, D. r., "The Correlation Between Froth Flotation Response and Collector Adsorption from Aqueous Solution," Powder Tech., 7,271, 1973. 2. Fuerstenau, D. W.

20 18 REFERENCES 1. Pope, M. I., and Sutton, D. r., "The Correlation Between Froth Flotation Response and Collector Adsorption from Aqueous Solution," Powder Tech., 7,271, Fuerstenau, D. W., "Correlation of Contact Angles, Adsorption Density, eta Potentials and Flotation Rate," Trans. AIME, 208,1365, Howe, T. M. and Pope, M. I., The Quantitative Determination of Flotation Agents Adsorbed on Mineral Powders Using DTA, Powder Tech., 4, 338, 1970/ Kulkarni, R. D. and Somasundaran, P., -Kinetics of Oleate Adsorption at the L/A Interface and Its Role in Hematite Flotation,- in Adv. in Interfac. Phenomena on Particulate/ Solution Gas S stems, APP1J,.catlop to FlotatIoQ sari\:;' P. Somasundaran and R. B. Grieves, eds., AIChE Symp. Series No. 150, 124, debruyn, P. L. and Agar, G. E., "Surface Chemistry of Flotation," in Froth Flotation, 50th Ann. Vol., D. W. Fuerstenau, ed., Pub. AIME, N. Y., 1962, p Somasundaran, P., "The Role of Ionomolecular Complexes in Flotation," International Jnl. of MinI. proc., 3, 35, Pinch, J. A. and Smith, G. W., Dynamic Surface Tension of Alkaline Dodecylamine Solutions," J. ColI. Intf. Sci., 45 (I), Somasundaran, P. and Moudgi1, B. M., J. ColI. Intf. Sci., 45,591, Somasundaran, P., Danitz, M. and Mysels, K. J., "A New Apparatus for Measurements of pynamic Interfacial Properties, J. ColI. Intf. Sci., 48, 410, Jung, R. F., M. Sc Thesis, Univ. of Melbourne, Australia, 11. Goddard, E. D., Goldwasser, S., Golikeri, G., and Kung, H. c. in "Molecular Association in Biological and Related Systems," p. 67, 1968, ed. Goddard, E. D., Pub. Am. Chern. Soc., Washington, D. C., Kulkarni, R. D.,.and SomasWldaran, P., Oleate Adsorption at 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 8.0 0.

21 TABLE 1 Adsorption 10 Density X 10 Mo1e/cm2, Floated

22 TABLE 2 CT = Total Oleate in Solution ph* = ph of maximum acid-soap complex forma tion X 10 M X 10-5 M x 10-6 M 7.37

25 Figure f- HEMATITE KN03 2 X 10-3 M TEMPERATURE 25 C. K-OLEATE 7.5 x 10-6M e K - OLEATE 1.5 x 10-sM 0 K-OLEATE 3x 10-5 M a &IJ J LA.. oe ld FLOTATION ph

$Figure 5 (\J 4 >- 0: W U W 0: 0 ". La.$ $ff) URE 25 C II - 0 :I: Co Z L&J.- C\J 0 0 t() r- ū - w 20 0 I{) (\J -.$

27 Figure 5 (\J 4 >- 0: W U W 0: 0 ". La. H c], '" bu X fcaconie OLEAtE 1 x 10-M.o KNO3-1 M 0:. <.D La. ff) URE 25 C II - 0 :I: Co Z L&J.- C\J 0 0 t() r- ū - w 20 0 I{) (\J -.. :I: Q. 4 5, 6 7 < FLOTATION (\J a> " :J: Q. <'1 0 0 L!) C5 CD (Jas/sau.(p) NOISN31 3:).::InS 0

29 T J 0 N KNO3,, Figure 6 I, 25 C 40 Ēu.n! "0 - UJ 0: ::> UJ UJ U 0: ::> U') ' ph

31 Fiqure 8 r TOTAL OLEATE = = r-- X 2240 ẉ.j Q (,) Q.. « G « z z (,) 640 (,) ' '- TEMPERATURE 25 C ph 10

32 Figure ph

33 Figure 10, M )( W -I Q.. 0 U Q (/). a u. 0 z 0 - a:..- z wu 2 0 U ph

R. D. Kulkarni and P. Somasundaran

THE IN R. D. Kulkarni and P. Somasundaran Hen?)' Krumb School of Mines Columbia University New York, New York 127 VOL. 71, No 15 975 PAGES 124-13 KINETICS OF OLEATE ADSORPTION AT THE LIQUID/AIR INTERFACE