T H E PHYSICAL CHEMISTRY OF FLOTATION.
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THEPROCESS OF ACTIVATION ELSIE EVELYN WARK
AND
IAN WILLIAM WARK
Department of Chemistry, University oj Melbourne, Melbourne, Australia Received April $4, 1988
Potassium ethyl xanthate, which is not normally able to induce the flotation of sphalerite (ZnS), becomes effective in the presence of a low concentration of copper sulfate. This action of a copper salt, known technically as “activation,” is used for the flotation of sphalerite following the flotation of galena by xanthate alone. It is generally assumed that the sphalerite becomes coated with a thin film of copper sulfide, which can adsorb the xanthate. In support of this interpretation, it is urged that many copperbearing minerals do adsorb xanthate from very dilute solutions. It has been demonstrated by several methods that sphalerite acquires a coating of a copper-bearing film when it is immersed in a copper sulfate solution. This coating is not removed by a water washing, but is removed by treatment with a dilute solution of sodium cyanide (12). Three molecules of cyanide per atom of copper are sufficient to prevent the activation of the sphalerite by copper sulfate, presumably because a soluble cupricyanide is formed which greatly reduces the copper-ion concentration. The minimum concentration of copper ions required to activate sphalerite is of the order 10-28 (12). Many lead-zinc ores contain sufficient soluble copper to activate the sphalerite and thus to interfere with its separation from galena. Addition of sodium cyanide during grinding and conditioning of the ore usually prevents flotation of much of the sphalerite with galena, but some operators think that cyanide does not completely prevent activation. If this be true, the activation must be due to some other ion, not removable from the sphalerite surface by cyanide. Lead salts would cause activation of this type but sodium carbonate, if present, would tend to precipitate them and thus to prevent activation. The results cited later may therefore be of more than academic interest. Berl and Schmidt (l),using a spectroscopic method to determine the amount of heavy metal ions removed by adsorption, have demonstrated that galena and sphalerite both adsorb copper and that sphalerite also adsorbs lead from solutions of their soluble salts. Ravitz and Wall (8), who used an iodometric method to measure the amount of copper removed 799 THE JOURNAL OP PHYBICAL OREXISTRY,
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ELSIE EVELYN WARK AND IAN WILLIAM WARK
from a copper sulfate solution by sphalerite, claim that the adsorption is almost complete within a minute; that for particles of 50 microns and over, which is the size preferred in practice, the amount of copper sulfate required for maximum recovery in flotation is approximately equivalent to the amount that would be required for the formation of a unimolecular film, and that considerably greater amounts of copper up to a fixed maximum value can be adsorbed from concentrated solutions. They suggest that the maximum amount adsorbed is just sufficient to coat with a unimolecular film the surfaces of all the unit crystal blocks, whose size is set a t 0.37 micron, penetration into the crystal lattice being assumed. In some instances activation of a mineral can be effected by adding a salt of the metal of the mineral. Thus, when using methyl xanthate as collector, chalcopyrite can be activated by copper sulfate. Similarly, the addition of zinc sulfate helps in the flotation of sphalerite by amyl xanthate. This type of activation is connected with the mechanism of the adsorption of the xanthate, and has led us to the conception of an “adsorption solubility product.” Another type of activation, exemplified by the action of sodium sulfide on anglesite and cerussite, which has already been considered (14),will not be discussed here. It has generally been assumed that activation of sphalerite by copper sulfate is due to the formation of a surface coating of cupric sulfide, formed, in accordance with the solubilities, by the action ZnS
+ Cu++ = GUS + Zn++
If one accepts the evidence of Ravitz and Wall that a unimolecular film suffices for flotation’, one would not expect such a film to possess the surface properties of massive covellite (CuS). Experiment does, in fact, indicate that the film has not the properties of massive covellite, a t least with regard to depression by sodium cyanide or caustic soda. Figure 1, constructed from earlier papers (12, 13), shows for covellite and for preactivated sphalerite the relationships between the concentrations of cyanide and the pH value necessary to prevent air-mineral contact in the presence of 25 mg. per liter of potassium ethyl xanthate. (By preactivation is meant immersion of the sphalerite specimen in a copper sulfate solution before it is placed in the xanthate solution.) Contact for either mineral is possible below or to the left but not above or to the right of its curve. If copper sulfate is present as well as xanthate, a considerably higher concentration of cyanide is necessary to prevent contact. With 150 mg. 1 One could not a t the same time accept the suggestion of Taggart, del Giudice, and Ziehl (9) that this film must become oxidized before i t can adsorb xanthate, for if it did we should have simply a unimolecular adsorbed film of copper sulfate on zinc sulfide.
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of CuS04.5Hz0 and 25 mg. of the xanthate per liter, the corresponding curve for sphalerite, also taken from an earlier paper (13), is shown in figure 2. A similar curve has been determined by Mr. A. B. Cox for stibnite which, like sphalerite, requires activation before it will respond to a neutral solution of ethyl xanthate. It will be seen that the curves for the two minerals are very similar. The corresponding curve for covellite in the presence of copper sulfate was not determined completely, but it lies very much higher than these two curves. It is apparent, therefore, that
pll VALUL
FIQ. 1. Relationship between pH value and concentration of sodium cyanide necessary to prevent contact a t surfaces of covellite and activated sphalerite. Potassium ethyl xanthate = 25 mg. per liter. No added copper sulfate.
when copper sulfate activates a sulfide mineral, the coating produced is not identical with covellite. Nor is it probable that the coatings for different minerals are held equally firmly: the non-identity of the activation curves for galena and sphalerite when using sodium diethyl dithiophosphate as a collector suggests that they are not (13). For reasons which will be cited elsewhere we do not agree with the contention of Taggart, del Giudice, and Ziehl (9) that the possibility of adsorption of a collector is governed entirely by the solubilities of the metallic
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ELSIE EVELYN WARK AND IAN WILLIAM WARK
salts of that collector. However, as these authors state, there is little evidence to show whether adsorption of an activator is governed entirely by solubility considerations. We set out, therefore, to determine which metallic salts are effective as activators for sphalerite. If solubilities alone are responsible, immersion of 8 sphalerite specimen in a solution of a salt of any metal whose sulfide is less soluble than sphalerite may cause activation, but immersion in a solution of a salt of a metal whose sulfide is more soluble than sphalerite should not. I n testing this view we proposed to use ethyl xanthate to indicate whether adsorption of the metal ions 140
120
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FIQ. 2. Relationship between pH value and concentration of sodium cyanide necessary to prevent contact a t surfaces of sphalerite and stibnite. Specimens preactivated in a solution of copper sulfate. CuSOa.5H20 = 150 mg. per liter; potassium ethyl xanthate = 25 mg. per liter.
had occurred, the procedure being to immerse the pretreated sphalerite in a xanthate solution and to observe whether a bubble of air could effect contact with it. An assumption was made here, namely, that if the surface is filmed by the metal ions, it will respond to ethyl xanthate. When the tests were begun this seemed probable enough, for the heavy metal sulfide minerals that had been tested had all responded to ethyl xanthate. However, during the progress of the work it was found that stibnite (Sb2SJ does not respond to neutral ethyl xanthate solutions, but only to slightly acid solutions of ethyl xanthate. This raised doubts concerning the validity of our initial assumption. Consequently, if the treated sphalerite
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specimen failed to respond to a neutral ethyl xanthate solution it was tested also in an ethyl xanthate solution at a pH value of between 4 and 5. More potent collectors could not be used, since they cause contact at a sphalerite surface in the absence of activators (13). Despite these precautions, the absence of a response to ethyl xanthate, though it suggests that adsorption of the metal ions has not taken place, does not prove it. On the other hand, if a response has been obtained, the conclusion is definite that activation has taken place. Kolthoff (4)cites three values between 5 X and 8 X for the solubility product of a-zinc sulfide, and the single value for 0-zinc sulfide. Let us consider activation by the salt of a bivalent metal (M). If the process is dependent only upon solubilities, it follows that the filming process, ZnS + M++ = Zn++ MS
+
occurs if on the addition of the salt of M,
(M++>
(MS) i.e.,
O’(Zns)’
,
Solubility product of MS Solubility product of ZnS
The concentration of zinc ions must be very small: we should therefore expect filming by any bivalent metal, the solubility product of whose sulfide is equal to or lower than the solubility product of zinc sulfide. For filming to occur under this mechanism it would be necessary that the term:
(9 (ii) (iii) Solubility product of (MzSa) 1’3 for tervalent metal salts
(M++)2‘3 Unfortunately, the solubility products of the metallic sulfides are not known with any degree of certainty. Table 1 sets out recorded values from various sources. Because ,of the decomposition of ferric sulfide it is doubtful whether the figures for it are significant. There is an enormous discrepancy between the figures cited in Landolt-Bornstein’s tables for the solubilities and solubility products of heavy metal sulfides. Thus the corresponds to a solusolubility of mercuric sulfide, given as 1.2 X the recorded value is For lead bility product of the order sulfide the corresponding figures are 10-e and 10-2g. Though greater care
ELSIE EVELYN WARK AND IAN WILLIAM WARK
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PHYSICAL CHEMISTRY OF FLOTATION.
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is usually taken in the determination of solubility products, the values recorded for them are of doubtful value, for the assumptions upon which the determinations are based cannot be justified. The greatest weight must be attached to Kolthoff’s figures, which were obtained from a critical study of the work of earlier writers. Even if it were proved that solubilities alone governed activation, it is evident that the solubilities of some of the sulfides are not known with sufficient accuracy for one to be able to predict whether zinc sulfide should be activated by solutions of the corresponding heavy metal salts. The results obtained in this investigation do not, therefore, enable one to decide whether activation of sphalerite should be attributed to simple double decomposition or to “exchange adsorption” (2). There is no doubt that salts of the metals that form the least soluble sulfides activate sphalerite, nor that salts of the metals that form the most soluble sulfides do not activate it. It has not been settled, however, whether activation is governed entirely by solubility considerations for salts of metals whose sulfides are of the same order of solubility as zinc sulfide. EXPERIMENTAL
Method I : The sphalerite specimen was polished in the usual manner (ll),placed in a 10 mg. per liter solution of the heavy metal salt, and after thirty minutes, 25 mg. per liter of potassium ethyl xanthate was added. After a second period of thirty minutes the specimen was tested with a captive bubble of air to ascertain whether it had acquired a xanthate film. If contact was possible between the bubble and the surface the angle of contact was measured at intervals up to two hours. The recorded values weremeasured two hours after the xanthate addition; usually a steady value was reached much sooner than this. Method 11: An alternative method of activation was tried for each metal, namely, to stand the specimen for thirty minutes in a 1g. per liter solution of the salt, then after rinsing in water, to transfer to a 1g. per liter xanthate solution. This method possesses the advantage that there is no precipitate of heavy metal xanthate in the solution to hinder contact with the surface. Method 111: For reasons already stated, if neither of these procedures proved that activation had occurred, a third was tried, namely to test the pretreated specimen in an acidified 200 mg. per liter ethyl xanthate solution a t a pH value of between 4 and 5. Control tests showed that in the absence of activators contact with sphalerite was impossible under these conditions: in solutions containing 500 mg. of potassium ethyl xanthate per liter, weak and irregular contact is obtained if the pH value is reduced to 4 by addition of hydrochloric acid, and with very high xanthate concen-
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ELSIE EVELYK WARK AND IAN WILLIAM WARK
trations and acidities an angle of contact only two or three degrees less than 60" is obtained. This contact is not due t o products of the decomposition of the xanthate in acid solutions. Nor is it due to the iron that is almost invariably contained as an impurity in the sphalerite, for resin blende, marmatite (a solid solution of iron sulfide in zinc sulfide), and cleiophane (an iron-free blende) all behave similarly. Curves obtained prcviously (13) suggest that sphalerite might respond, without activation, to high ethyl xanthate concentrations in acidified solutions. Method IV: The contact tests were supplemented by direct flotation tests in stoppered cylinders. The procedure was similar to that adopted in methods I, 11, and 111, except that a suspension of sphalerite in water was used for the attempted activation; after additions of 20 mg. per liter of terpineol as frother, of xanthate as collector, and of acid, if there was no response in neutral solutions an attempt was made to float the mineral by shaking the stoppered tube vigorously. PURITY O F CHEMICALS
Since even 1mg. of copper sulfate per liter is an activator for sphalerite, it is essential that the compounds tested should be free from any substantial amounts of copper or other heavy metal salts. The purest specimens obtainable were recrystallized before use, and the xanthates were purified as described previously (11). Antimony trichloride was redistilled. Titanium trichloride was crystallized from the commercial 15 per cent solution by adding alcohol and ether, washing with ether, and then recrystallizing from alcohol by addition of ether. The water used was distilled from glass apparatus and was copper-free. TECHNIQUE O F POLISHING
At one time during the investigation it became so difficult to obtain clean polished specimens that, had we not had several years' experience to convince us that clean specimens of sphalerite were not air-avid, we should have believed that they were. As some other investigators have apparently experienced similar difficulties, a description of the methods adopted to overcome them may not be out of place. Since one of us experienced greater difficulty than the other, and since trouble was encountered particularly in hot weather, it was suspected that the natural grease of the hands was responsible. Linen gloves lessened but did not completely eliminate the trouble, but surgical rubber gloves, when properly treated, did remove it. The gloves must be kept in such a condition that water readily wets them; this can be done by washing them with wet talc powder. Subsequently it was found that rubbing the hands with talc powder may suffice to remove the natural grease, and it is then possible to dispense with gloves.
PHYSICAL CHEMISTRY OF FLOTATION.
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808
ELSIE EVELYN WARK AND IAN WILLIAM WARK EXPERIMENTAL RESULTS
The results of the investigation are summarized in table 2. Except for titanium the results from direct flotation tests are in agreement with those from contact tests. The contact induced by some of the salts was not of the same order as that induced by copper sulfate, for example. When using copper sulfate as activator, the xanthate caused a rapid and complete response to an air bubble, but when using a titanium salt the reaction was slow and irregular; only on rare occasions was the characteristic angle (60") obtained, and then only over a portion of the surface. In direct flotation tests, the presence of copper sulfate results in the formation of a highly mineralized froth that is stable for days and up to 75 per cent of the sphalerite can be floated. On the other hand, titanium salts, though they cause more sphalerite particles to reach the surface than would do so in their absence, do not result in the formation of a permanent mineralized froth. With thallium nitrate as activator, it seems a t first that a stable froth will form, but although much of the mineral is carried to the surface by the bubbles, the froth does not persist, and most of the mineral falls. With cobalt sulfate the flotation is still less permanent. Owing to hydrolysis, difficulties arose in testing salts of bismuth, tin, and antimony. Activation was attempted both in solutions acidified to prevent hydrolysis and in neutral suspensions containing the hydroxide. With bismuth, sufficient of the salt remains in solution at pH = 7 to activate sphalerite, and ethyl xanthate then induces the customary contact angle and leads to excellent flotation. With antimony trichloride, however, only in slightly acid solutions is there sufficient antimony for activation; the xanthate solution also is effective only in acid solutions. Neither in acid solution nor in neutral solution was stannous chloridean activator for sphalerite. Titanium trichloride also hydrolyzes, and it was the partly decomposed solution (pH = 3.5) that was in part effective as an activator. Arsenic trioxide dissolves very slowly in water; hydrochloric acid hastens the solution process and the excess acid can be neutralized before testing. Sphalerite reduces chloroauric acid; a precipitate of metallic gold is formed on the surface of the sphalerite when using gold chloride. It is doubtful, therefore, if the activation should be attributed to the formation of a film of gold sulfide. Cobalt and nickel are generally believed tq form fairly insoluble sulfides. The failure of hydrogen sulfide to precipitate the metals from acidified solutions of their salts is difficult to understand, for the sulfides themselves do not dissolve in dilute acid. Middleton and Ward (7) have shown, however, that the mechanism of the precipitation is complex, and that precipitation of the true sulfides does not usually occur. Attempts to activate sphalerite by a solution of nickel sulfate, made alkaline by ammonia and stabilized by ammonium chloride, were not successful. It is surprising that neither for stannous nor for stannic salts could con-
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ditions be found that led to activation of sphalerite. We found, however, that stannic sulfide is not precipitated from a 1 g. per liter solution of the double chloride by sodium sulfide, despite the reported low solubility of stannic sulfide, 0.0002 g. per liter. Some results for silver and mercury, cited in an earlier paper (ll),seemed to indicate that if the heavy metal salt was in stoichiometric excess of the xanthate, activation was not obtained. This indication was not substantiated, for on varying the concentrations over a wider range, it was found that contact was sometimes possible with the metal salt in excess. Precipitates of the heavy metal xanthates are responsible for the difficulties, and if the surface is freed from them (by wiping with a clean linen pad) contact is possible whatever the relationship between the metal and xanthate additions. In all the tests now recorded the surfaces tested were treated in this manner to free them from precipitates, visible or invisible, that would hinder contact. SUMMARY
1. It has been found that salts of the metals platinum, gold, bismuth, mercury, silver, copper, cadmium, lead, cerium, antimony, and arsenic “activate” sphalerite in a manner such that it responds to ethyl xanthate and floats readily. Thallium and cobalt induce a somewhat weaker response and titanium, though it does have a weak influence on the response of the mineral to an air bubble, is not a sufficiently powerful activator to cause flotation. 2. In general, the metals that are effective as activators form relatively insoluble sulfides and those that are not effective form relatively soluble sulfides. 3. It is not possible to decide whether solubility considerations alone govern the activation process. Thallium, whose sulfide is reputed to be more soluble than that of zinc, does activate sphalerite, and tin salts, which give less soluble sulfides, do not activate sphalerite. However, the reported solubility products are not to be relied upon. One of us (E. E. W.) wishes to express her thanks to the University of Melbourne for a scholarship that has rendered the work possible. The otber author wishes to acknowledge the help of the companies by which he is employed, vis., Broken Hill South Pty. Ltd., North Broken Hill Ltd., Mt. Lye11 Mining & Railway Co., Zinc Corporation Ltd., Electrolytic Zinc Co. of Australasia Ltd., and the Burma Corporation Ltd. Our thanks are due also to Messrs. H. Hey and A. B. Cox for valuable help and advice. REFERENCES (1)
BERLAND SCHMIDT: Kolloid-Z. 66, 264 (1933).
(2) FREUNDLICH: (Hatfield’s translation) Colloid and Capillary Chemistry, p. 204. London (1926).
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ELSIE EVELYN "ARK
AND IAN WILLIAM WABK
(3) Handbook of Chemistry and Physics, p. 845. Chemical Rubber Publishing Company, Cleveland (1934). (4) KOLTHOFF:J. Phys. Chem. 36, 2716 (1931). Physikalisch-chemische Tabellen, Vol. 11, 5th edition, (5) LANDOLT-BORNSTEIN: p. 1180. Springer, Berlin (1923). (6) LATrniER AND HILDEBRAND. Reference Book of Inorganic Chemistry, p. 389. The Macmillan Co., New York (1929). (7) MIDDLETON AND WARD:J. Chem. SOC.1936, 1459. (8) RAVITZAND WALL:J. Phys. Chem. 38,13 (1934). DEL GIUDICE, AND ZIEHL: Trans. Am. Inst. Mining Met. Engrs. (9) TAQGAXT, 112, 344 (1934). (10) TREADWELL-HALL: Analytical Chemistry, Vol. 1, p. 458. London (1932). (11) WARKAND Cox: Trans. Am. Inst. Mining Met. Engrs. 112, 189 (1934). (12) WARKAND Cox: Trans. Am. Inst. Mining Met. Engrs. 112, 245 (1934). (13) WARKA N D Cox: Trans. Am. Inst. Mining Met. Engrs. 112, 267 (1934). (14) WARKAND Cox: Am. Inst. Mining Met. Engrs., Tech. Pub. 659 (1936).
