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Langmuir 1994,10, 510-513
Designing and Selecting a Collector for Optimum Field Performance on the Basis of Pressure-Area Isotherms B. P. Singh Regional Research Laboratory (Council of Scientific & Industrial Research), Bhubaneswar 751 013,India Received August 7,1992. I n Final Form: August 18,1 9 9 P This investigation has been aimed at developing a method for characterizing and predicting the performance of collectors based on pressure-area isotherms. This procedure can be used both to choose the proper collector and to develop effective new compounds by determining the effect of changes in interfacial properties. The surfactant activity of the collector is dependent on the bulk phase behavior of the chemical when dispersed in a three-phase system. This behavior can be monitored by determining the collector surface pressure isotherms for adsorption at the interface. Further, collector adsorption is dictated by bulk physical properties of the collector. These properties are described and discussed. This paper presents a technique with which a modified Langmuir film balance may be used to study the effectivenessof a collector for maximum floatability in terms of grade and recovery. Experimental data are presented for sulfideore. Collectorschosen are those which are already tested for flotation performance. Strong correlationswere obtained between a collector performance and film pressure. A careful selection of solvent can significantly improve the surface activity of the collector. It is proposed that the correct solventis one which allows the collectorto dissolve totally without irreversibleaggregation. The consequences of this selection of a solvent are improved surface activity and collector performance.
Introduction The design and application of collectorshave historically involved evaluation of numerous products to arrive at a product that gives acceptable performance. Because collectors are surfactants, this work was initiated in an attempt to apply surface chemical principles to select suitable collectors. The success of any flotation scheme largely depends on a judicious choice of a collector. The ultimate objective of researchers in this area still remains to develop a comprehensive theory which can help practicing engineers to select appropriate reagents for the particular separation problem at hand. There has been an attempt' to correlate collector effectiveness with some of the physical properties governing flotability of minerals. A method of calculation has been developed for choosing the collecting reagent theoretically by using the selective collecting power for a given mineral' and conditional stability constants which can be calculated from published values of equilibrium constants for each experimental c o n d i t i ~ n .However, ~~~ our understanding in this area is still limited. Consequently collector selection has been traditionally based on a trial and error method with hundreds of chemicals in the field. The classical theories of flotation have considered the elementary act of flotation to be the process of attachment of mineral particles to the liquid/gas interface. The conditions of equilibrium are based on a comparison of the magnitudes of the initial and final free energies for attachment of a bubble to a particle in an isolated system. Later theories have considered the stability of thin films of liquid, that must be present at the bubble approaches the mineral surface on collisions, to establish the thermodynamic criteria of flotation. These later theories are based on the concept of disjoining pressure suggested by
* Abstract published in Advance ACS Abstracts, January 1,1994.
(1) Marabini, A. M.; Barbaro, M.; Ciriachi, M. Trans.-Znst. Min. Metall., Sect. C 1983,29, 20. (2) Sillen, L. G.; martal, A. E.Spec. Publ.-Chem. SOC.1964, 17. (3) Perrin, D. D. Organic ligands; Pergamon: Oxford, 1979.
Derjaguin. For flotation to occur, the film must thin and rupture; that is, the disjoining must be negative. Several investigator^^^^ have analyzed the properties of the thin films to explain the stability and kinetics of flotation. This paper describes the results of our initial investigations in determining the necessary conditions for predicting the performance of new collectors on the basis of surface film pressure-area isotherms. Our goal is to develop a property-performance relationship for different types of collectors. The important interfacial properties governing the efficacyof collectors are interfacial tension and surface film pressure. In particular, we have found that to be effective, a collector must lower the surface film pressure and interfacial tension gradient; its ability to do so depends on the rate at which surfactants attain equilibrium or orient themselves at interfaces and is an important factor in determining their effectiveness. This paper will also discuss the surface active behavior of a selected collector and identify how its efficacy can be enhanced by changing its bulk physicochemical behavior. The enhancement is demonstrated at the molecular level using interfacial adsorption isotherm behavior.
Experimental Section Materials. Collector. The collectors used in this studywere commercially available material supplied by chemical dealers. The collectors are designated simply as A and B throughout this text. The former (A) comprises LIX65N (with 99.9% purity), i.e. substituted 2-hydroxybenzophenone, and was supplied by the Henkel Corporation, Germany; the latter (B) was sodium isopropyl xanthate procured from the M/SAceto chemical private limited, Calcutta. Both of the collectors were purified in the laboratory.s The collectors were selected as suitable after evaluating ita performance in the laboratorybatch scale flotation testa and identifying the optimum collector doses and other conditions. Flotation recovery and grade by these two collectors are shown in Figure 1. (4) Johansaon, G.; Pugh, R. J. Znt. J . Miner. Process. 1992,34, 1. ( 5 ) Nikopv, A. D.; Wasan, D. T.J. Colloid Interface Sci. 1989,133(1). (6)Vogel, A. I. A tezt book of practical organic chemistry, 3rd ed.; Longman: London, 1956.
0743-7463/94/2410-0510$04.50/00 1994 American Chemical Society
Predicting the Performance of Collectors
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CoI\ector do sa2 e (mt/ I J Figure 1. Flotation of copper ore as a function of reagent concentration.
Concen+rati o n cwa 11) Figure 2. Adsorption of LIX65N and NaIPX on chalcopyrite.
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Methods All the experimentswere carried out in a modified Langmuir type film balance fabricated in the laboratory. The f i i balance used in this study has already been reported by Singh and coworker.'~~The complete assembly of the film balance can be divdied into two parts: the trough with a movable barrier and a Du-Nouy tensiometer for measuring interfacial tension. The movable barrier is yoked to two threaded guide bars and can travel along the lenth of the trough. The correspondingreading is taken on a graduated scale fixed parallel to the bottom of the trough. Two teflon foils are used to prevent any leakage of liquid from the sides of the moving barrier. The interfacial tension of the system was measured by the ring method and was finally converted into film pressure, according to the following relationship (fiim pressure = initial IFT - next compressed IFT). Thecorrespondingarea of the f i i was convertedinto normalized area (the area at a given time divided by the initial area between the barrier and float). Calibration with stearic acid dissolved in benzene was carried out to know the nature of pressure-area isotherm for known systems. Measurement of Film Pressure. If a monolayer is compressed in a Langmuir balance and the surface pressure plotted as a function of the area, one frequently obtains curves which show that the monolayer undergoesphase changes in a manner somewhat analogous to a three-dimensionalsystem. Whereas simple three-dimensionalsystemsexhibitthree-phasestates (gas, liquid, and solid),surfacefilms may exhibit up to six-phasestates. These are summarizedby Harkinseas(a)gaseous or vapor expand, (b) liquid expanded, (c) intermediate (liquid or high compressibility), (d) liquid condensed stat (low compressibility), (e) superliquid (low compressibility),and (f) solid. The two-dimensional isothermal surface compressibility ( K ) of a film is defined by Davies and Rideal'O as K
= l/A(aA/aII),
which canbe calculatedfromthe plot. The surfacecompressional modulus K - ~ is the reciprocal of the compressibility. The compressional modulus is more convenientto use thanR because of its magnitude. The unit of r1is dyn/cm.
Results and Discussions Adsorption of Collectors at Chalcopyrite. The adsorptions of LIX65N and sodium isopropyl xanthate ~~
~
(7) S i g h , B. P. Ph.D. Thesis, ISM,Dhanbad, 1991. (8) Singh, B.P.; Pandey, B. P. Indian J. Technol. 1991,29,443. (9) Harkins, W.D. The physical chemistry of surfaces film; Reinhold Publishing Company: New York, 1962. (10) Davies, J. T.;Rideal, E. K. Interfacial Phenomenu; Academic Prees: London, 1961.
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Normalised Area
Figure 3. Film pressure characteristicsof stearicacid on distilled water. on chalcopyrite (purity 99.9 % ) are given in Figure 2. A hand-picked high-purity natural mineral sample in the form of lumps was crushed and ground in an agate mortar to the desire size. Semiquantitative spectroscopic analysis of the minerals showed the presence of the following impurities Cu, 0.02 % ;Si02,0.05% ;Fe, 0.02 7% ;and A1203, 0.01% ;apart from chalcopyrite (99.9%). The surface area of the sample determined by the BET method was found to be 1.72 m2 gl. Standardization of the Equipment. A film balance experiment with stearic acid dissolved in benzene was carried out for several reasons. The equipment had to be tested with a material of known film-forming characteristics in order to evaluate the proper functioning of the film balance. A solution of 0.05 % of stearic acid in benzene was prepared and 250 mL was placed in contact with the water phase. The contact time prior to compression was 30 min. The ambient temperature was 28 "C. The pH of the distilled water was 6.9 and the interface was compressed at a speed of 1.2 cmlmin. Figure 3 shows the pressurearea isotherm for stearic acid at a hydrocarbonlwater interface. The compressibility value (4)of 0.0120 in the normalized area region between 0.79 indicates that the film exists in the intermediate state. Harkinsg found compressibility values for stearic acid monolayer on aqueous substrates in the range of 0.0063 to 0.0098. The corresponding pressure range for liquid condensed state is indicated by Harkins is between 20 and 24 dynlcm. The
Singh
512 Langmuir, Vol. 10, No. 2, 1994
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Figure 4. Compressional modulus vs normalized area.
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Normalixed Area Figure 5. Effect of collector on fib properties.
transition from on liquid condensed state to a solid state occurs for a stearic acid monolayer at a film pressure of about 27 dyn. The film behaves very much the same as analogous films at the water-air interface. Physical Characteristics of Interfacial Films. Several distinct regions in the curve reflecting changes in the physical state of the film could be identified in the curve (Figure 3). The region from point A to B resembles that of an expanded film, that from B to C a much more condensed but still highly compressible film, that from C to D a region of collapse on microscopic scale, and point E is a region of completecollapseand multilayer formation. The compressional modulus ( K - ~ ) vs normalized area (A) is plotted in Figure 4. The compressional modulus is more convenient to use than compressibility ( K ) because of its magnitude. Graphs of K-1vs A was found useful to infer the physical characteristics of the film from pronounced changes in the slope of the curve. Effect of Surface Active Agent (Collector)on Film Properties. Figure 5 shows the pressure-area isotherms with collectors A and B. The most effective, LIX65N (A), and the least effective sodium isopropyl xanthate (B), collectors were chosenllJ2 and were used at concentration levels. of 0.05 and 0.02 g/kg, respectively. All other parameters were maintained similar to that of batch scale flotation experiments with collector A & B. From the (11)Das, B.;Sahoo, R. K.; Praead, A. R. Proceedimgsof the National Seminar on Research and Development in Mineral Preparation, 14-15 August, 1992,NML, Jamehedpur, India. Erunetal, (C) 1992. (12)Das, B. Internal Report, RRL, Bhubaneewar, 1990.
0
01 0.2 0 3 0.4 0.5 Q6
0.7
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Normalised Area Figure 6; Film properties in different solvent with collector LIX65N.
comparison of pressurearea isotherms of collector A & B, it is evident that in the case of the effective collector, the film pressure was found to be minimum, and maximum in the case of less effective one (B). The collapse pressure was also found to be minimum in the case effective collector and vice versa. It has been shown that a low concentration of effective commercial surface active agent eliminatad rigid film. A similar result waa reported by Singh and co-worker7**with demulsifier surfactants. The more effective demulsifier surfactant causes greater reduction in film pressure than the less effective one. And hence the effectiveness of the collectors is apparently related to the film pressure. This indicates that the film no longer remains as rigid as in the absence of collectors. Further, it can be said that all the above presaure-area isotherms show adsorption of polar compoundsat the interface,which eliminate the rigidity of the film and hence facilitate the particle bubble attachment for effective flotation. Although there is some variation in physical characteristics of these films, it can be said that the pressure-area isotherms for the above experiments exhibit a rather similar behavior. Surface Activity of the Collector in Different Solvent. It has been demonstratedI3that the interfacial behavior of surfactants can be significantly affected by the solvent phase in which they are diluted. The solvents were selected for this study were benzene, xylene, and acetone. It has been shown by studying the interfacial tension adsorption isotherm that the collector has an enhanced surface activity when diluted in the solvent acetone. In other solvents the interfacial activity is only partialy enhanced. The enhanced surface activity is due to ready solvation of collector in acetone giving almost complete molecular solubility. However, the same collector is more (irreversibly)aggregated in the other solvents thereby having a reduced surface activity. Figure 6 shows the pressurearea isotherms with collector A the most effective one. The film,pressureobservedwithA in acetone is significantly reduced compared to the other solvents. This clearly indicates that the film no longer remains 88 rigid as in the absence of collector in solvent acetone. The collapse pressure of the film is also reduced significantly. Collapse pressure is the point where film is completely broken, i.e. particle-bubble attachment is facilitated. It has been shown that the collector which is able to reduce surface film pressure (in a suitable solvent) significantly was more effective than the collector which is not able to (13)Graham, D. E. In surface active agents; SCI Publication, 1979; p 127.
Predicting the Performance of Collectors reduce film pressure significantly. The more effective collector causes greater reduction in film pressure than the less effective one. The consequences of this selection of a solvent are improved surface activity and collector efficiency. Hence, the performance of a particular collector would be predicted on the basis of pressurearea isotherms and their solvent properties.
Conclusions The results obtained demonstrate the usefulness of the surface film pressure measurements in discussing the designing and selection of collectors for optimum field performance. This approach gives additional data supporting the disjoining pressure mechanism of flotation. It became apparent to us during the course of our investigations that the more effective collector causes greater reduction in film pressure as compared to the less
Langmuir, Vol. 10, No. 2, 1994 513 effective one, which is desirable for a good collector. Thus the study of physical characteristics of collectors prior to flotation can be useful in flotation and selection of a suitable collector for flotation of minerals. It is not feasible to make broad generalizations regarding selection of collectors via a particular technique, as there is a wide variability in collector physicochemical properties.
Acknowledgment. The author thanks Professor H. S. Ray, Director, Regional Research Laboratory, Bhubaneswar, for his kind interest, encouragement, suggestion, and permission to publish the paper. The author wishes to thank Professor B. P. Pandey, Department of Petroleum Engineering, Indian School of Mines, Dhanbad, for his help and discussion about this research. The author duly acknowledges B. Das for his help.
