THE PHYSICAL CHEMISTRY OF FLOTATION. I V A CRITICISMOF OSTWALD’S THEORY OF FLOTATION IAN WILLIAM WARK A N D ALWYN BIRCHMORE COX
Department of Chemistry, University of Melbourne, Melbourne, Australia Received March 1, 1988
Wo. Ostwald (1)has advanced a theory of the action of soluble collectors, an essential part of which is that only a ring of the collector is necessary for flotation, the location of the ring being the air-water-mineral line of contact. The process by which this ring is assumed to be held is styled by him “adlineation.” This theory is in opposition to that adopted by US (2) , namely, that for soluble collectors a substantially complete unimolecular film is adsorbed by the surface of the mineral, and that for oleaginous collectors a ’ continuous thin film spreads over the surface. The only evidence advanced by Ostwald in direct opposition to this view is a statement based on an observation of Kellermann (3) that the amount of added reagents is insufficient to form such a unimolecular film. This evidence has been questioned by Siedler, Moeller, and Reddehase (4),who cite work by Gaudin, Glover, Hansen, and Orr (5) which demonstrates that the amount of collectors used in practice is of the order required to form a unimolecular film over the surface of the mineral floated. Ostwald, in reply, questions the assumptions upon which Gaudin’s and Siedler’s calculations are based, but does not offer any further substantiation for Kellermann’s views, and, in fact, admits the impossibility of determining accurately the number of molecules adsorbed per unit area of the mineral surface, except by a complete adsorption analysis. In our opinion, it has not been proved that in any single case the number of effective collector molecules (certain frothers are also collectors) is insufficient to form a unimolecular film, but, apart from this, there is so much evidence against Ostwald’s “adlineation” theory that even if, as seems unlikely, it were proved that Kellermann’s view is correct, some explanation other than Ostwald’s must be sought. Though finality has not been reached in deciding between the views of Kellermann and Gaudin, it is certain that in practice quantities of reagents must be added enormously in excess of those required to give an “adlineation” ring, and as it is impossible with the available technique to determine what fraction of these reagents is consumed by the minerals floated, any direct proof or disproof along these lines is impossible. 815
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Indirect methods must therefore be adopted in determining whether the theory possesses the essential desiderata of a useful working theory. A theory need not be true to be useful, but to be useful a new theory should be simpler and more comprehensive than previous theories, and it should be in accord with known facts. Being an extreme development of the unimolecular film theory, Ostwald’s theory cannot claim greater simplicity than it, and, not being applicable for insoluble collectors, it is not as comprehensive as the theory of complete filming. More important, the “adlineation” theory is not in accord with several experimental facts and accredited principles of surface physics. Considering the turbulent conditions of the flotation pulp it is doubtful whether a ring of collector of the type suggested by Ostwald could lead to a sufficiently stable contact between air and mineral. Only if the ring acted as a perfectly mobile barrier could the air-mineral aggregates withstand the sudden stresses imposed upon them. Ostwald, in his second paper, attempts to show how the “adlineation” ring possesses this necessary mobility. Great mobility, however, could arise only from one of two causes. Firstly, the line of collector might be so loosely bound to the surface that free motion could occur. Actually xanthate films, far from being loosely held, are held by the surface with great tenacity. For example, chalcopyrite retains a xanthate coating after being washed for several minutes in running water, as is shown by the contact angle remaining the same as in a xanthate solution. Such tenacious adsorption is of considerable import.ance, since the xanthate film is the link which holds the mineral to the bubble. Secondly, great mobility might be due to the ring of the collector being securely bound to certain atoms of the crystal lattice which themselves possessed great mobility in the surface. Though this type of mobility is assumed for liquid films, we know of no evidence suggesting sufficient mobility of the atoms of a solid. Indeed Adam ( 6 ) states: “As a general rule, spreading on solids only occurs through the vapour . . . .; if the liquid is non-volatile, no spreading occurs, a t least in any reasonable time.” Adam’s reference was to solids in contact with the atmosphere, but the generalization may apparently be extended to include contact with a liquid phase, in which case spreading only occurs through solution. Our experience would support such an extension, for the methods of experiment adopted in this laboratory would not have been possible had adsorbed films possessed appreciable mobility. Only one side of our test specimens of mineral is protected from contamination, and it is known that a t least portions of the other sides are contaminated by grease and/or air, but experiment has shown that the area of contamination does not spread to the test face. Perhaps the most convincing argument against great mobility is the phenomenon known as hysteresis. Several observers (7) have noticed
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that the air-mineral-water boundary is not perfectly mobile. The imperfect mobility is responsible for a difference between the angles of contact when water is replacing air and when air is replacing water at the mineral surface. The difference between these two angles has been called the hysteresis; it is due to friction. The existence of a definite contact angle a t the line of triple contact between air, water, and mineral has been accepted by Ostwald. It is universally accepted that if T,,, T,,, and T,, signify the surface energies a t the air-solid, water-air, and solid-water interfaces, respectively, the magnitude of the angle of contact 8 is determined by the relationship,
DeWitt, Roper, and Makens (8) have shown that the magnitude of T,, is practically unaltered by common collectors and frothers a t the concentrations customarily employed. If, as Ostwald suggests, there is but a ring of collector at the line of triple contact, the air-solid and solid-water interfaces on either side of this line cannot be influenced by the collector, and the magnitudes of their interfacial energies would be identical with the corresponding values in pure water. Thus, if the “adlineation” theory were correct, the angle of contact would be independent both of the addition of a collector and of its nature. This is contrary to experience. At clean surfaces of many of the common minerals the angle of contact in pure water is zero, and if a collector be added the resultant finite angle varies with the nature of the essential non-polar radical of the collector (2). Ostwald repeatedly stresses a generalization of Mayer that only unwettable particles float. We will return to a more exact statement of this principle later, but far from agreeing better, as Ostwald suggests, with his own theory than with other theories, Mayer’s generalization is quite incompatible with Ostwald’s theory, for it is only after the surfaces of the sulfide minerals have been changed by the collector that they become “unwettable.” Furthermore, experiment shows that the whole of a collector-conditioned mineral surface is actually different from a surface which has had no contact with collector; the change is not confined to a single line on the surface or to any small fraction of the surface. For if the specimen be washed and placed in distilled water a bubble of air will effect contact with any part of the surface even though the surface be “swept” several times by air bubbles. It is well to consider what is meant by the term “unwettable.” Ostwald cites Edser (7) as stating that in no known case does the angle of contact a t the air-water-solid boundary reach 180”. This implies that when contact with air is possible, equilibrium can be established with a portion
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of the solid surface in contact with air and a portion with water, the airwater interface being inclined a t an angle to the solid suface. The extent of the displacement of the water from the surface depends upon the volume of the air, the angle of contact, and the size and shape of the surface of the particle. The term “unwettable” is, therefore, misleading, and we suggest in place of Mayer’s generalization the statement, “NOparticle will float at an air-water interface unless both air and water are in contact with its surface, that is, unless there is a finite contact angle a t a line of triple contact between air, solid, and water.” At surfaces of substances such as diphenyl and paraffin wax, water is readily displaced by air, the equilibrium contact angles being approximately 84”and 105”,respectively, for the above two substances. Bubbles of air will readily attach themselves to such solids, and particles of diphenyl (sp. gr. = 1.16) may be floated readily to the surface of water. There is, moreover, no visible difference between the type of contact of an air bubble with diphenyl and with a mineral surface which has been in contact with a soluble collector, nor in the type of flotation. Since the surface of diphenyl must consist exclusively of hydrocarbon groups, there is no possibility of the presence of a mobile triphilicl ring a t the line of triple contact. Ostwald’s theory, therefore, cannot have universal application in accounting for the mechanism of flotation. Indeed, Ostwald does not use the “adlineation” theory for oleaginous collectors. For these he makes use of a theory of “laminar” flotation. This theory was propounded in very similar terms by Christmann (9). The essential feature of the theory is that superimposed on the collector film at the mineral surface there is a second film composed of frother molecules and that the dual film is air-avid and induces flotation. Ostwald states that there must be an adherence between the collector and frother films, and Christmann that an orientated hydrocarbon film of collector is soluble in the hydrocarbon film of the frother. The orientation of the frother molecules necessary for these conditions to be fulfilled, however, would be such that the assumed dual film could not be air-avid. The collector is bound to the mineral surface by its active group, the non-polar group being orientated away from the mineral. The frother molecule, if it be soluble in this film as Christmann suggests, or if it adhere as Ostwald ’
1 Ostwald considers that soluble collectors must be of a triphilic nature, i.e., contain three groups of essentially different type, whose function is t o stabilize the air-water-mineral contact, each group of the collector making contact with only one phase. Since the hydrocarbon groups in the surface of diphenyl can make contact with both air and water such an hypothesis is not essential to account for flotation. Moreover, i t would require abandonment of the usual conceptions of atomic attractions to ascribe a triphilic nature to such soluble collectors as the quaternary ammonium salts and mercaptides.
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suggests, must therefore be orientated with its non-polar group towards, and its polar or water-avid group away from the mineral. Therefore, the dually filmed particle could not float. Actually the orientation of the frother molecule a t the air-water interface is such that its non-polar group is directed towards the air phase and the polar group towards the water phase; such orientation would oppose any adherence or mutual solubility of the collector and frother films. It is our view that the essential function of the frother is to stabilize the large air-water interface necessary for froth flotation, which stabilization is undoubtedly achieved because of adsorption (either positive or negative) a t this interface. During the process of formation of the air-mineral aggregate the frother is squeezed from the surface of the bubble, and air then makes direct contact with the air-avid collector film. This collector film on the mineral surface may be an orientated adsorbed unimolecular layer, as in the case of soluble collectors, or a more discrete film of an insoluble collector as in the case of oils. The difference between soluble and insoluble collectors does not therefore lead to a difference in the mechanism of formation and stabilization of air-mineral aggregates, but to a difference in the means by which they render a mineral surface airavid. It is thus unnecessary to postulate a number of complex theories of flotation to cover the use of different types of collectors. REFERENCES (1) OSTWALD, Wo.: Kolloid-Z. 68, 179 (1932);60, 324 (1932).
Cox AND WARK:J. Phys. Chem. 37,797 (1933). KELLERMANN: Kolloid-Z. 47, 273 (1929). SIEDLER, MOELLER,AND REDDEHASE: Kolloid-Z. 60, 318 (1932). GAUDIN,GLOVER,HANSEN,AND ORR: Flotation Fundamentals, Part I. University of Utah, 1929. (6) ADAM:The Physics and Chemistry of Surfaces, p. 213. Clarendon Press, Oxford (1930). (7) SULMAN: Trans. Am. Inst. Mining Met. Engrs. 29, 44 (1919-20). EDSER:Brit. Assoc. Fourth Report on Colloidal Chemistry 4, 263 (1922). LANGMUIR: Trans. Faraday SOC.16,62 (1920). ADAMAND JESSOP: J. Chem. SOC.127,1865 (1925). WARKAND Cox: Am. Inst. Mining Met. Engrs., Tech. Pub. No. 461. (8) DEWITT,ROPER,AND MAKENS:J. Am. Chem. SOC.64, 444, 455 (1932). (9) CHRISTMANN: Am. Inst. Mining Met. Engrs., discussion following Tech. Pub. No. 461. (2) (3) (4) (5)
