THE PHYSICAL CHEMISTRY OF FLOTATION. I1 THE NATUREOF
THE
ADSORPTION OF
ALWYN BIRCHMORE COX
AND
THE
SOLUBLECOLLECTORS
IAN WILLIAM WARK
Department of Chemistry, Universitu of Melbourne, Melbourne, Australia Received September 17, 1998
It having been demonstrated that the angle of contact plays a very important part in determining the tenacity of attachment between a conditioned mineral surface and air, it becomes important to determine what factors determine the angle of contact. The magnitude of the angle of contact is fixed by the interfacial tensions of the three phases meeting a t the line of triple contact mineral-air-water. Consequently, any factor having an effect on any one of the three surface tensions (or surface energies) will influence the angle of contact. Every surface change may be expected to influence the surface energy, and thus no factor which influences the surface is without influenceon flotation. Confusion has arisen in the literature owing to a lack of appreciation of this fact. Thus several writers, in emphasizing the importance of some one factor, have put forward their views as a complete theory of flotation. Even the writers of textbooks refer to the “contact angle theory of flotation,” the “adsorption theory of flotation,” etc. Though no one factor is responsible for flotation, no factor is more important in flotation by soluble collectors than adsorption, and this is particularly true of adsorption at the solid surface. It has been shown (1) that air does not even partially replace water from clean surfaces of many common sulfide or gangue minerals. Metals and oxidized minerals behave similarly. It is only after the surfaces have acquired a coating of some “collector” that any displacement of water by air is possible at the surface. I n the early application of flotation, the collectors used were oils and the surfaces were filmed by oil. Latterly, oils have been largely displaced as collectors by soluble non-oleaginous compounds, of which potassium ethyl xanthate is the most used. These, only, are considered in the present discussion. Under certain conditions unimolecular films are adsorbed by the minerals which then display a definite air avidity. The orientation of the adsorbed molecule is all-important. If there be no polar group in the surface, contact with air (and displacement of water) is possible; if there be a polar group in the surface, contact with air is impossible. The lead, zinc, and copper salts of mandelic acid float very 797
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ALWTN BIRCHMORE COX AND IAN WILLIAM WARK
readily, though they are heavier than water. Mandelic acid contains two active groups; it is evident that both are orientated inwards in these salts, which are coordination compounds, and that the non-polar phenyl group is orientated outwards. The parent acid and the sodium salt are not collectors, however, for the sulfide minerals, proving that if they are adsorbed, there is an active group outermost. Other acids might be cited whose heavy metal salts float readily but which, themselves, do not act as collectors for the sulfide minerals. On the other hand, it is probably true that an acid, or its soluble salts, would not cause floatability if the heavy metal salts were not themselves readily floatable. There are three aspects of the adsorption of soluble collectors which need examination. 1. Under what conditions does adsorption of the collector occur? 2. At what rate does adsorption occur? 3. If adsorption occurs, how is the angle of contact related to the constitution of the collector? The first question is the most difficult to answer. Modern selective flotation is based upon the adsorption of a collector by one mineral and not by another, and an explanation must be sought for the differences between minerals in this respect. It is avoiding the issue to say that certain minerals attract the CS.S- group, though this is the customary method of accounting for adsorption. Incidentally, a valid explanation for the adsorption of xanthates by minerals would mark an important step forward in the theory of adsorption. Of the theories which have been advanced, none has received more attention than that which postulates that adsorption is due to the formation of an insoluble compound on the mineral surface, and attention should be drawn to certain exceptions, lest it be taken for granted that the theory is established. Of the other theories, that which seeks to explain adsorption in terms of surface charges is also worthy of more consideration, but in the absence of definite experimental verification we have perforce to confine our attention to the former, the so-called “chemical” theory. The chemical theory has been sponsored by Taggart and Gaudin and their collaborators. Gaudin, Haynes, and Haas (2) state that it has appeared more and more as their experimental work has progressed that the action of reagents on the flotation of sphalerite is strictly chemical. Taggart, Taylor, and Knoll (3) advance the generalization, “Simple chemical reaction underlies the functioning of the flotation reagents which control mineral collection, when these reagents are soluble in and act from solution in the water of the pulp.” The proviso excludes collectors which are oils. It was not claimed that this generalization was substantiated exhaustively, and we therefore propose to discuss its usefulness as a working hypothesis. A truly chemical theory implies that adsorption can occur only when the
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solution is saturated with respect to the adsorbed compound. As early as 1915, Paneth and Horowitz (4) pointed out that the amount of adsorption of certain radioactive elements is dependent on the degree of insolubility of their salts. This conclusion was confirmed by Fajans and Horowitz (5). Evidence has been adduced by Taggart and Gaudin and their collaborators to show that there is a close connection between adsorption of xanthate a t a mineral surface and the solubility of the xanthate of the metal in the mineral. Before the chemical theory can be accepted, it must be shown that this connection amounts to identity. In other words, it is necessary to show that the forces controlling adsorption are of the primary valence type. The adsorbed film is surprisingly stable. Once adsorption has occurred, contact with air continues to be possible though the surface be washed with several changes of water. The alkali metal xanthate of the solution from which adsorption has occurred is extremely soluble in water; the heavy metal salts are of extremely low solubility. The heavy metal xanthates, moreover, flocculate readily about an air bubble and then float readily. There is thus some presumptive but not conclusive evidence that the adsorption is due to an insoluble film of the heavy metal salt. Taggart (3) has stated, “We are rather inclined to think that in every case the solubility of the substance as a surface coating is somewhat less than that of the same substance independently put into solution, but I do not know.” The solubility term used in describing adsorption may thus be different from that of the solubility of the same compound in bulk. It is of significance that the statement quoted arose in a discussion following the presentation by Taggart and his associates of a paper in which an attempt was implicitly made to explain “adsorption” in terms of primary chemical reactions. Does it not amount to an admission that the explanation was incomplete, i.e., that secondary valences must be considered? Gaudin, Haynes, and Haas (2) evidently doubt the universal vali&y’ of the theory, for they state that though sphalerite can be floated by certain amines, no insoluble sulfides or zinc salts are formed by them. Apparently the substituted hydrazines may be classed with the amines. Thus, though there is undoubtedly much evidence of a qualitative nature in favor of this theory of adsorption, there are many cases which apparently are exceptions. Upon examination some of the exceptional cases may later be explained on the grounds of oiling, but until such exceptional cases have been classified no sweeping generalization is justified. Many cases have been cited in the earlier paper (1)which seem to conform to the chemical theory. Firstly, concerning the mode of action of activators, e.g., copper sulfate for sphalerite, the information available suggests that this followsthe order of solubility of the sulfides. The chemical theory, if applicable here, would state that lead nitrate is effective in
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ALWYN BIRCHMORE COX AND IAN WILLIAM WARK
activating sphalerite because lead sulfide is less soluble in the solution than zinc sulfide and so on, but experimental proof of this principle, which alone could decide whether the action were purely chemical, has not been advanced. Secondly, concerning the mode of action of the xanthate, it has generally been found that contact with air becomes possible a t about the concentrations of metal and xanthate ions a t which a precipitate of the metal xanthate forms. This is illustrated by the use of zinc sulfate to activate sphalerite. This type of evidence, which could be added to from the works of other writers, constitutes the case for the theory. Let us consider the case against it. Firstly, objections based on experimental evidence will be considered; secondly, certain theoretical objects will be discussed. 1. Some work on calcium heptoate has been cited by Gaudin and Hansen (6), which a t first sight seems to support the theory. They deduce, “When the solution is saturated (with calcium heptoate) the superficial film of calcium soap (heptoate) is complete. At a concentration less than saturated, the superficial film is incomplete.” A truly chemical theory implies no adsorption until the solution is saturated, and therefore the data may be used as an argument against the theory. (2) It has not been shown decisively that contract first occurs under exactly the same conditions as precipitation. It would be difficult to prove this with the xanthates, because of their low solubility, but Gaudin’s case of flotation of calcite by heptoic acid appears to be suitable for the test. Unfortunately, as pointed out above, this case does not indicate exact coincidence of these two conditions. Taggart and his collaborators have evidently noticed some similar flaws in the theory. (3) Some unpublished work dealing with the effect of cyanide and alkali on adsorption of xanthates has shown that in many cases adsorption occurs under conditions where no heavy metal xanthate precipitation has occurred and that in others, despite the precipitation of xanthates, no adsorption of xanthate has occurred. On theoretical grounds two difficulties arise. Firstly, it follows from the constancy of the angle for ethyl xanthate for different minerals (l),that the number of alkyl groups in the surface is independent of the mineral. The size of this group, rather than any dimensions of the crystal lattice, evidently determines the extent of adsorption. Therefore, every metallic and/or sulfide group of the surface may not be involved in anchoring the adsorption complex. Though (if this solubility or chemical theory is to hold) each xanthate ion takes the place of a sulfide ion in the surface, not every sulfide ion can be so replaced. The fate of those not replaced is uncertain. Do they pass into solution or is the final condition a mixed xanthate/sulfide surface with the xanthate portions shielding the sulfide? The solubility theory deals only with the possibility of and not with the extent of adsorption.
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Secondly, the xanthates are stated by Weinig and Palmer (7) to be more soluble than the corresponding sulfides. The replacement of the sulfides of the surface by xanthates therefore needs further explanation. Taggart, Taylor, and Ince (8) and Taggart (9) have assumed that sulfides change on the surface to more soluble thiosulfates, sulfates, carbonates, hydroxides, etc. The surface of galena can under certain conditions be changed to sulfate. Evidence has been adduced which shows that sulfate may be detected i n solution after flotation of galena by xanthate. This evidence does not show, however, whether this sulfate is first formed a t the surface and then replaced by xanthate, or whether it is formed by oxidation of sulfide ions which dissolve in the solution or which enter it as the result of metathesis with the xanthate. It would seem impossible to devise experiments to distinguish between these possibilities. It is perhaps significant that anglesite (natural lead sulfate) is more difficult to float than galena. Accurate solubility determinations of xanthates and sulfides might prove it unnecessary, however, to assume the formation of any oxidized compounds on the surface. Even though the xanthate were more soluble than the sulfide, it is conceivable that the relatively high xanthate ion concentration might displace the equilibrium PbS
+ 2X-
= PbXa
+ S--
sufficiently to the right to allow the formation of lead xanthate on the surface. The adsorption of xanthates by the noble metals (see following paper) can hardly be explained by metathesis. Excluding the doubtful possibility of surface oxidation, it must be concluded that xanthate ion or xanthic acid is directly adsorbed. Such a conclusion is a t variance with the solubility or chemical theory of adsorption. The balance of the evidence appears to be against the chemical theory, and for the present it is suggested that in its place there should be substituted the generalization, “The ability of a mineral to adsorb a soluble chemical flotation collector containing sulfur is closely related to the solubility of the salt formed by the collector and the metal of the mineral.’’ Concerning the second question, the time required after the mineral i s placed in a xanthate solution before contact with air can be effected, was discussed very briefly in the paper previously mentioned (1). This discussion was concerned mainly with the development of the full equilibrium angle. Flotation, however, is possible, provided that an angle is possiblenot necessarily the equilibrium angle,-and it is therefore desirable to consider the speed with which sufficient adsorption occurs to enable any contact to occur. Using any of the common sulfide minerals except galena, if contact is going to take place it is usually possible as soon as the bubble can b e
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ALWYN BIRCHMORE COX AND IAN WILLIAM WARK
brought into contact with its surface after its immersion in the xanthate solution (25 mg. per liter). Even where activation is necessary, e.g., where sphalerite must be activated by copper sulfate, response is equally rapid. With lower xanthate concentrations adsorption may be a little slower, but generally, if contact is going to be possible, there is some indication of it within a minute or two. Other collectors (see the following paper) are adsorbed just as quickly. Using galena, a second reaction apparently is possible which may interfere with the standard method used by us of detecting an adsorbed xanthate film. The nature of this reaction is at present unknown. It is prevented or removed by violent agitation of the test specimen with fine particles of galena or sand, such as would normally occur in a flotation machine. It is removed also by wiping the surface with a pad of linen, a process more easily carried out than agitation in a miniature flotation machine. It is now customary to wipe the surfaces of all minerals with linen prior to making the final measurements of contact angle. Generally, this is without effect on the angle, but occasionally, as in this case, or where a fine precipitate has settled on the surface, it is essential. Wiping the surface in this way does not remove the xanthate film. It is surmised that, where it is effective, the pad of linen removes an irregular film of some precipitate which has formed or settled on the surface and which mechanically prevents contact between the bubble and the adsorbed film on the surface. With these precautions a chalcopyrite surface responds f u l l y to 1 mg. per liter of potassium ethyl xanthate within five minutes and a galena surface within ten minutes. Comparatively little attention has been paid in this laboratory to measurements of the rates of adsorption of xanthates by mineral surfaces. Arising out of earlier work the conviction has grown that an ideal flotation process must be based upon conditions where only one of the minerals to be separated is responsive to air, the other being completely non-responsive. Processes based upon differences in the rate of adsorption of xanthate have not appeared attractive. The third question-the relationship between the angle of contact and the constitution of the collector-has been discussed in the paper already referred to (1). It is considered in greater detail in the next paper. We wish to thank Mr. H. Hey for his constructive criticism in the preparation of this paper. REFERENCES (1) WARKAND Cox: Am. Inst. Mining Met. Engrs., Tech. Pub. 461. (2) GAUDIN,HAYNES,A N D HAAS:Flotation Fundamentals, Part 4. University of
Utah, 1930. (3) TAGGART, TAYLOR, AND KNOLL: Am. Inst. Mining Met. Engrs., Milling Methods, p. 247 (1930).
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(4) PANETH AND HOROWITZ: Z. physik. Chem. 89, 513 (1919). (5) FAJANS AND HOROWITZ: Z. physik. Chem. 97, 478 (1921). (6) GAUDINAND HANSEN:Flotation Fundamentals, Part 1. University of Utah, 1928. (7) WEINIQ AND PALMER: Quart. Colo. School Mines 24, No. 4 (1929). The Trend of Flotation. (8) TAGQART, TAYLOR, AND INCH: Am. Inst. Mining Met. Engrs., Milling Methods, p. 285 (1930). (9) TAGGART: J. Phys. Chem. 36, 130 (1932).
