27 Use of MacroemulsionsinMineral Beneficiation BRIJ M. MOUDGIL
Macro- and Microemulsions Downloaded from pubs.acs.org by YORK UNIV on 12/03/18. For personal use only.
Center for ResearchinMining and Mineral Resources, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611
Macroemulsions of water insoluble reagents can enhance the coating efficiency because of large surface area of the droplets coming in contact with solid particles. However, stability of oil droplets by fine particles is encountered in these operations, and causes problems in recycling of the water immiscible reagents. Applications of emulsifying the reagents in achieving desired solid-solid separation by electronic ore sorting, emulsion flotation, liquid-liquid extraction, and froth flotation are discussed. Role of surface charge of the particles and interfacial tension in spreading of fluids on solids is presented.
Macroemulsions are extensively used in coating of different substrates. In such cases, generally the coating material is insoluble or sparingly soluble in the bulk solvent, thereby, limiting the area of contact between the coating material and the substrate. Emulsification of the insoluble fluid using a surface active agent helps in overcoming this limitation. Pesticides, paints, and road surfacing by bitumen or tar are some of the examples in which emulsification of the coating medium is necessary to obtain an effective coating of the substrate. It should be noted that in a l l these cases, a nonselective coating is obtained. However, selectivity of the coating is essential in applications of oil in water macroemulsions in solid/solid separation operations encountered in mineral processing. A discussion of this aspect of macroemulsions in coating of mineral particles is presented in this paper. 0097-6156/85/0272-0437$06.00/0 © 1985 American Chemical Society
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MACRO- AND
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Background Reagent coating process involves c o l l i s i o n between oil droplets and the p a r t i c l e , and spreading of the f l u i d over the s o l i d substrate. C o l l i s i o n between droplets and substrates can be enhanced by agitation or imparting other mechanical forces. Spreading of the f l u i d during the time it is in contact with the substrate is influenced also by surface chemical forces such as e l e c t r o s t a t i c charge and s t e r i c i n t e r a c t i o n s . In both these cases adsorption of surfactant molecules at various interfaces can have a major influence on the spreading of droplets on s o l i d substrates. Adsorption of surfactant molecules w i l l modify the e l e c t r o k i n e t i c properties of the oil droplets and s o l i d surface, thereby r e s u l t i n g in more f r u i t f u l c o l l i s i o n s . On the other hand, surfactant adsorption at the l i q u i d - l i q u i d interface can reduce i n t e r f a c i a l tension which favors spreading of the f l u i d over the s o l i d surface. In c o r r e l a t i n g electrophoretic mobility of the bitumen droplets with the rate of spreading of the droplets on quartz p a r t i c l e s , Lane and O t t e w i l l (J) determined that spreading occured only below the HTAB (hexatrimethylammonium bromide) concentration of O.5 mM at which the amount of surfactant adsorbed on quartz was equivalent to a v e r t i c a l l y oriented monolayer. They suggested that at the lower concentrations, the surfactant molecules pre- f e r e n t i a l l y adsorb on the quartz surface, thus, depleting the bitumen-water interface and d e s t a b i l i z i n g the emulsion. E f f e c t of surfactant concent r a t i o n on emulsion droplet coating is shown schematically in Figure 1. In achieving s o l i d - s o l i d separation in mineral beneficiation, selective coatings of water insoluble or sparingly soluble reagents are required in electronic ore sorting, froth f l o t a t i o n , and emulsion f l o t a t i o n operations. Use of macroemulsion in achieving the selective coating for these applications is discussed below. Electronic Ore Sorting Electronic sorting of minerals based on the surface chemical differences involves achieving a selective coating of a coloring or fluorescent dye on the desired mineral component. The f l u o rescent dye may or may not be water soluble. In the case of water soluble dyes, it is e s s e n t i a l to make only the desired mineral component water wetted leaving the other p a r t i c l e s water unwetted or vice versa. In such cases applications of macroemulsion technology in achieving the desired coating are l i m i t e d . On the other hand, when the dyes are water insoluble, an e f f e c t i v e coating of the dye may only be achieved by macromulsion technology which uses an intermediatory reagent compat i b l e with the dye and the s o l i d substrate. For example, in separation of limestone from quartz to obtain a coating of f l u o ranthene on limestone p a r t i c l e s , the intermediatory chemical is f a t t y acid (oleic acid) which adsorbs on limestone pieces but
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Macroemulsions and Mineral Beneficiation
Solid
Low
Cone.
Medium Cone.
High Cone. F i g u r e 1 . E f f e c t of s u r f a c t a n t c o n c e n t r a t i o n on s p r e a d i n g of an oil d r o p l e t on a s o l i d s u r f a c e .
439
MACRO- AND MICROEMULSIONS
440
not on quartz p a r t i c l e s . I t should be noted that o l e i c acid is sparingly soluble in water and, therefore to achieve an e f f i c i e n t coating, the dye is f i r s t dissolved in an oil and/or o l e i c acid, which is agitated with water to form a oil in water macroemulsion. After a suitable coating of the fluoranthene on limestone is achieved, the coated and uncoated p a r t i c l e s are separated using the Oxylore sorting machine. (2) Results of such a separation are presented in Table 1. Table I. Separation of Limestone from Quartz by Fatty Acid Coating Feed: Coating Emulsion:
Limestone and Quartz Mixture Fluoranthene Dye, O i l and Fatty Acid Mixture 1,3-10.0 cm
P a r t i c l e Size:
Limestone Content (%)
Sample
Limestone
Recovery (%)
Feed
47.5
100
Concentrate
94.5
93.7
however, i f only the quartz p a r t i c l e s are to be made f l u o r e scent, the dye is dissolved in a beta amine which has limited s o l u b i l i t y in water.(2) The amine is then added to water, and an emulsion is made by agitating the mixture. Results of limestone separation from quartz are presented in Table I I . Table I I .
S i l i c a Content of Limestone after Separation by Electronic Ore Sorting
Feed: Coating Emulsion: P a r t i c l e Size:
Sample
Feed Concentrate
Limestone and Quartz Mixture Fluoranthene Dye, and Armeen L-9 (Beta Amine - Armak Chemicals) 1.3 - 10.0 cm
Silica Content (%) 25.1 1.1
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In another example of separating coal from slate, fluoranthene is dissolved in decylalcohol which is then dispersed into water. Results of separation of fluorescent dye coated coal p a r t i c l e s from uncoated slate pieces using the Oxylore sorting machine are presented in Table I I I . (3) Table I I I .
Separation of Coal From Slate by Decyl Alcohol Coating
Feed: Coating Reagent: P a r t i c l e Size:
Coal (87%) and Slate (13%) Mixture Fluoranthene in Decyl Alcohol 10.0 cm
Sample
Ash (%)
Pyritic Sulfur (%)
Thermal Valve (BTU/lb)
Coal Recovery (%)
BTU Recovery (%)
Feed
17.3
O.2
11339
100
100
Concentrate
5.0
O.1
13384
96
98
Emulsion Flotation In emulsion f l o t a t i o n technique, a neutral hydrocarbon oil, together with a surface active agent function as c o l l e c t o r s for s p e c i f i c minerals. In this process as shown in Figure 2, hydrophobic mineral p a r t i c l e s and oil droplets form an aggregate which is then floated out of the pulp by a i r bubbles. The mixture of oil and surfactant is either emulsified (O/W) p r i o r to addition to the pulp, or in the pulp i t s e l f . The e f f e c t of neutral oil in emulsion f l o t a t i o n is not yet completely known. According to Fahrenwald (4) and Karjalahti (5) the neutral oil coats the hydrophobic p a r t i c l e s which then form an agglomerate. L i v s h i t s and Kuzkin (6^) on the other hand have reported that hydrophobicity of the p a r t i c l e s is unaltered in the presence of neutral oil, and that the added oil reduces the induction time of mineral p a r t i c l e s in contact with the a i r bubbles. Lapidot and Mellgren Ç7) concluded that addition of the neutral oil improved the dispersion of the c o l l e c t o r and also increased the hydrophobic nature of the p a r t i c l e s . I t has been reported that the f l o t a t i o n a c t i v i t y of nonpolar o i l s depends on their v i s c o s i t y and d i s p e r s i t y . (8) It is to be expected that there is an optimum droplet size of emulsions which is a function of p a r t i c l e size and mineral chemistry. This aspect of emulsion f l o t a t i o n , however, has not been systematically studied. Emulsion f l o t a t i o n has been employed in the treatment of ores containing, iron, manganese, molybdenum and titanium oxide. (9-11) A major role of emulsion of f a t t y acids in the f l o t a t i o n of Florida phosphate rock also cannot be
MACRO- AND MICROEMULSIONS
Emulsion
Flotation
L i q u i d - L i q u i d E x t r a c t i o n and
Flotation
Froth F l o t a t i o n F i g u r e 2. Schematic o f e m u l s i o n f l o t a t i o n , l i q u i d - l i q u i d e x t r a c t i o n and f l o t a t i o n , and f r o t h f l o t a t i o n .
MOUDGIL
Macroemulsions and Mineral Beneficiation
ruled out. In this case, probably ionization of the fatty acid provides the required surfactant molecule for emulsification of the f u e l oil which is used as a c o - c o l l e c t o r . McCarrol (9) reported that recovery of manganese ore increased from 70 to 84% when an emulsion of oil, soap and a surfactant (Ornite-S) was used as the c o l l e c t o r as compared to a mixture of only soap and f u e l oil. Burkin and Bramley (10) observed that less than 1% coal floated in 3 minutes when f u e l oil was used as the c o l l e c t o r . On the other hand, 100% of the coal floated when f u e l oil was emulsified using a nonionic surfactant, Lissapol NDB. I t should be noted that in both cases the droplet size of the f u e l oil was maintained at 7 microns. The increased e f f i c i e n c y of coal f l o t a t i o n in the presence of oppositely charged surfactant was attributed to lowering of the zeta potential, which indicated adsorption of surfactant on coal p a r t i c l e s , and thereby increased tendency of the fuel oil to spread on surfactant coated p a r t i c l e s . These investigators suggested that i f the k i n e t i c energy of approach between p a r t i c l e s and droplets is less than the energy of repulsion, c o l l i s i o n between them does not lead to the spreading of oil on c o a l . I f the energy barrier is smaller than the k i n e t i c energy to overcome it, c o l l i s i o n between coal p a r t i c l e and droplet occurs but does not y i e l d a hydrophobic coal p a r t i c l e . Flotation of coal occurs only when the energy b a r r i e r is much smaller than the k i n e t i c energy available, and a surface active agent is present in the system. Liquid-Liquid Extraction This process involves extraction of fine p a r t i c l e s from an aqueous phase into an oil phase. The effectiveness of this technique, as shown in Figure 2, is based on the s t a b i l i t y of emulsion droplets with s o l i d p a r t i c l e s . If a p a r t i c l e is p a r t i a l l y wetted by two immiscible liquids the p a r t i c l e w i l l concentrate at the l i q u i d - l i q u i d i n t e r f a c e . The thermodynamic c r i t e r i a f o r d i s t r i b u t i o n of solids at the interface of two immiscible liquids is the lowering in the i n t e r f a c i a l free energy of the system when p a r t i c l e s come in contact with two immiscible l i q u i d s . (12) I f Y , Y and are the i n t e r f a c i a l tensions of solid-water, water-oil and s o l i d - o i l interfaces respectively, and i f y > y + y then the s o l i d p a r t i c l e s are p r e f e r e n t i a l l y dispersed within the water phase. However, i f γ > γ + γ , the s o l i d is dispersed within the oil phase. On the other hand, i f γ . > γ + γ , or i f none of * ' 'wo 'so 'sw' the three i n t e r f a c i a l tensions is greater than the sum of the other two, the solids in such case w i l l be distributed at the oil-water i n t e r f a c e . The surface charge of the s o l i d p a r t i c l e s has been reported to play an important role in the recovery process. Maximum recovery is achieved when the p a r t i c l e s exhibit net zero surface charge.(13-15) L a i and Fuerstenau (16) have reported that u l t r a f i n e (O.1 μπι) alumina p a r t i c l e s can be extracted from gw
W Q
g o
W Q
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Λ
g w
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444
M A C R O - A N D
M I C R O E M U L S I O N S
aqueous suspension into iso-octane ( o i l phase) through the adsorption of sulfonates. These investigators reported that sulfonate molecules control the hydrophobic!ty of the alumina p a r t i c l e s and also act as emulsifying agent, thus, forming a large oil-water i n t e r f a c i a l area. Maximum amount of alumina was extracted into the oil phase when the contact angle exceeded 90°, and the e l e c t r o k i n e t i c p o t e n t i a l was zero. Under these conditions, the p a r t i c l e s have a net zero charge and, therefore, can transfer into the oil phase without experiencing repulsion due to e l e c t r o s t a t i c forces. One of the major l i m i t a t i o n s of the l i q u i d - l i q u i d extraction and f l o t a t i o n process is the breaking of the stable emulsions to recover the separated s o l i d s , and to recycle the oil phase. Some of the methods examined to break the emulsions include f i l t r a t i o n , modifying the phase volume r a t i o , centrifugation and sedimentation. An e f f i c i e n t and economic solution to this problem is yet to be developed. Froth F l o t a t i o n In this process, as shown in Figure 2, hydrophobic p a r t i c l e s attach to the a i r bubbles and r i s e to the top of the c e l l where they are removed by skimming. Separation by froth f l o t a t i o n is based on s e l e c t i v e hydrophobicity of the p a r t i c l e s . The surfactant molecules which s e l e c t i v e l y adsorb on the p a r t i c l e s are mostly water soluble. In cases where the reagents are water insoluble, an e f f i c i e n t coating is achieved by emulsifying the reagent. Brown and co-workers (17) have reported a marked increase in the f l o t a t i o n of complex low-grade Michigan phosphate ore when the c o l l e c t o r (fatty a c i d - f u e l oil mixture) was emulsified using oil soluble petroleum sulfonate. The potential of applying the emulsification technology in froth f l o t a t i o n has not been investigated to any extent. Summary I t is evident from the above discussion that an e f f i c i e n t coating of a water insoluble or sparingly soluble reagent can be achieved through e m u l s i f i c a t i o n . Adsorption of the emulsifying agent (surface active agent) on the s o l i d substrate reduces the surface charge and oil-water i n t e r f a c i a l tension, which leads to the spreading of reagent on the s o l i d substrate. Reagent coating on the desired substrate can be achieved by s e l e c t i n g an emulsifying agent which w i l l adsorb also on the s p e c i f i c s o l i d water interface, e.g. by charge a t t r a c t i o n or by chemical bonding. The enhanced effectiveness of emulsion coatings in case of water insoluble reagents can be attributed to increased area of contact between the substrate and the reagent droplets.
27.
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Macroemulsions and Mineral Beneficiation
Acknowledgments The author wishes to acknowledge the College of Engineering-EIES (COE Funds) for p a r t i a l f i n a n c i a l support of this work.
Literature Cited 1. Lane, A.R.; Ottewill, R.H. in "Theory and Practice of Emulsion Technology," Smith, A.L. Ed.; Academic: New York, 1976; p. 157. 2. Moudgil, B.M. US Patent 4 208 272, 1980. 3. Moudgil, B.M.; Messenger, D.F.; US Patent 4 208 273, 1980. 4. Fahrenwald, A.W. Mining Congress J. 1957 43, 72-74. 5. Karjalahti, K. Trans. Inst Mining & Met (London), 1972, 81, C219-C226. 6. Livshits, A.K.; Kuzkin, A.S. Tsvetn. Metal, 1964, 37, 7677. 7. Lapidot M.; Mellgren, O. Trans. Inst Mining & Met. (London), 1968, 77, C149-C165. 8. Glembotskii, V.A.; Dmitrieva, G.M.; Sorokin, M.M. "Non-polar Flotation Agents,"; Israel Program for Scientific Translations, Jerusalem, 1970, p. 45. 9. McCarroll, S.J. Mining Engineering, 1954 (March), 289-293. 10. Burkin, A.R.; Bramley, J.V. J. Appl. Chem. 1961, 11, 300309. 11. Hoover, R.M.; Molhotra, D. In "Flotation-Gaudin Memorial Volume," Fuerstenau, M.C., Ed.; AIME: New York, 1976, Vol. I, p. 485. 12. Green, E.W.; Duke, J.B. Trans. SME-AIME 1962, 223, 389393. 13. Shergold, H.L.; Mellgren, O. Trans. SME-AIME, 1970, 247, 149-159. 14. McKenzie, J.M.W. Trans SME-AIME, 1969, 244, 393-400. 15. McKenzie, J.M.W. Trans SME-AIME, 1970, 247, 202-247. 16. Lai, R.W.M.; Fuerstenau, D.W. Trans. SME-AIME, 1968, 241, 549-556. 17. "Characterization and Beneficiation of Phosphate-Bearing Rocks from Northern Michigan," U.S. Bureau of Mines, RI 8562, 1981. RECEIVED
December 18, 1984
