T H E PHYSICAL CHEMISTRY OF FLOTATION. V
FLOTATION OF GRAPHITE AND SULFUR BY COLLECTORS OF THE XANTHATE TYPEAND ITS BEARINGON THE THEORY OF ADSORPTION IAN WILLIAM WARK
AND
ALWYN BIRCHMORE COX
Department of Chemistry, U,niversity of Melbourne, Melbourne, Australia Received August 20, 1934
Gaudin (3) states that graphitmeand sulfur are intensely water-repellent and that to float them it is often sufficient to add a “frother” such as pine oil. Though other oils are sometimes added, Gaudin does not attribute their beneficial effect to their functioning as collectors. Nor does he attribute any collecting power to the frothers, but assumes that the natural floatability of these two minerals is sufficient to account for their flotation. Xanthates, it is claimed, exercise no collecting function toward sulfur and graphite, and their addition would be merely wasteful. We have found, however, that concentrations of frothers considerably in excess of those required to produce a good froth are necessary to induce good flotation of graphite. With certain other frothers in neutral solutions, no substantial flotation is obtained whatever their concentration, and it has been concluded that graphite, like most other minerals, requires a collector for satisfactory flotation. In those cases where a reagent, usually regarded purely as a frother, is alone sufficient to induce good flotation, it has been shown that the reagent is itself a collector for the graphite. Contrary to the generally accepted view, it has been found that collectors of the xanthate type (xanthates, dithiocarbamates, dithiophosphates, etc.) are excellent collectors for both graphite and sulfur. Sulfur occupies a different position from graphite since it usually floats readily without any collector. Under some conditions, however, the floatability is considerably enhanced by collectors of the xanthate type, and it has been shown, moreover, that they are abstracted from solution by the sulfur.1 The response of graphite and sulfur to collectors of the xanthate type is of considerable theoretical significance. It does not seem possible to connect the response of a mineral with the possession of any particular type of lattice structure by the mineral. Sulfur and cleiophane, cerussite and I n an earlier paper (6) we made the statement that sulfur is not influenced by xanthates. This incorrect view was due to our not having experimented with sufficiently high concentrations of ethyl xanthate. 551
552
IAN WILLIAM WARK AND ALWYN BIRCHMORE COX
anglesite, graphite, galena and chalcopyrite, copper and gold represent among them several different types of crystal lattice, but they all adsorb xanthates from solution, and, what is more significant, all lead to the same maximum angle of contact for each xanthate a t a line of triple contact air-solid-xanthate solution. The last-mentioned fact indicates that, following the adsorption of the xanthate, the external or effective surface is the same in all cases, and that it consists entirely of the non-polar groups of the orientated adsorbed xanthate molecules, I t would be premature to conclude that the nature of the forces binding the xanthate molecules to these vastly different surfaces is the same for all of them. Nevertheless there emerges from these observations sufficient evidence to prove that certain of the proposed theories of adsorption cannot be of general application. Taggart ( 5 ) claims that chemical (soluble) collectors function by forming a substantially insoluble orientated water-repellent film on the surface of the mineral to be floated. He has endeavored to show that this film is formed by double decomposition between the soluble collector and the surface of the mineral. In cases where the solubility of the mineral is so low as to render such action improbable, as, for example, galena, it is assumed that a previous oxidation has already changed the mineral surface into some compound of suitable solubility. Though it has been shown that the surface of galena may become oxidized and that the oxidized film may react with ethyl xanthate by double decomposition, it has not, in our opinion, been shown that unchanged galena cannot adsorb ethyl xanthate. Indeed, the procedure for floating anglesite in practice, namely, to add sodium sulfide as well as xanthate, points to the conclusion that lead sulfide itself adsorbs ethyl xanthate more readily than does lead sulfate. This may be demonstrated by air-mineral contact tests, as will be shown in a future publication. At the surfaces of sulfur and graphite there can be no chemical reaction with xanthate of the type postulated by Taggart. The double decomposition theory cannot therefore be of general application, even if it be true in certain cases. The constancy of the contact angle, whatever the adsorbing solid, suggests, however, that the forces causing adsorption of xanthates are similar for all solids, and that any theory of the action of xanthates should explain their action on graphite as well as on galena. While it is contended that the simple chemical theory of adsorption should logically be abandoned, certain observations that lent support to it should not be forgotten, though the reason for them is obscure. The readiness with which a sulfide mineral adsorbs a collector of the xanthate type increases as the solubility of the compound formed between the base metal of the mineral and the collector decreases. It should be pointed out however, that since the solubilities of the heavy metal xanthates run
PHYSICAL CHEMISTRY OF, FLOTATION.
V
553
strictly parallel with the solubilities of their sulfides, this generalization might be expressed in an alternative form, namely, that the readiness with which a sulfide mineral adsorbs a collector of the xanthate type increases as the solubility of the mineral decreases. It is dissatisfying to reject one theory of adsorption without substituting for it another theory that is in agreement with experimental facts. It is therefore proposed to consider the nature of the forces a t the surfaces of solids with a view to deciding whether sufficient is known concerning them to provide an explanation for the adsorption of collectors. It is generally agreed that surrounding each of the atoms or ions of a solid there exist strong forces which are responsible for the cohesion that opposes cleavage or grinding, and which lead to high melting and vaporization temperatures. A t the surface these forces are available for attracting other ions, atoms, or groups of atoms, but until recently very little was known of their magnitude. De Boer and Custers (2) have, however, made an attempt t o calculate the energy of adsorption of phenol on sodium chloride, dividing the binding forces into two types, the van der Waals forces and the electrostatic forces. It was shown by them that molecules with a high dipole moment are preeminently adsorbed electrostatically by the ions of the crystal lattice, whereas the van der Waals forces are operative for the nonpolar portion of the adsorbed molecules. The soluble flotation reagents known as collectors possess either a high dipole moment or a high degree of dissociation. Most of the adsorption of common collectors on solids possessing an ionic lattice structure must therefore be attributed to electrostatic forces emanating from the surface ions, acting either on the dipoles or the dissociated ions of the collector. For solids of the graphite type, in which the electrons are shared equally between the atoms of the crystal lattice, different considerations must apply, and in this case it is difficult to reconcile the interior structure with an apparently undiminished ability to adsorb the xanthate type of collector. A t the surface of such solids there must be a different arrangement of the electrons and a development of more polar characteristics.2 The structure of graphite is such that the surface consists of a hexagonal pattern of carbon atoms with alternate carbon atoms having available one electron for sharing with any atom or group also capable of supplying an unshared electron. Before it can adsorb xanthate or during the adsorption, a rearrangement of electrons is necessary. Of the carbon atoms carrying an unshared electron every second one would be able (if spatial
* Adams and Jessup (1) have shown by means of contact angle measurements on original crystalline surfaces and on surfaces produced by scraping, that the surface forces differ according to the method of producing the surface. Surface rearrangements are therefore sometimes necessary for thermodynamic equilibrium, but the rate of rearrangement may be infinitesimally small.
554
IAN WILLIAM WARK.AND ALWYN BIRCHMORE COX
considerations permitted) to adsorb a xanthate ion, while the other similar atoms would accept the electron available from these atoms on which adsorption occurs. It has been found that the adsorption of ethyl xanthate by graphite does not change the pH value of the solution; it follows that xanthic acid3 alone is not adsorbed, for if it were, the solution would develop alkalinity. It has been shown, too, that the xanthate adsorbed on graphite may be approximately determined by an iodine titration; it cannot therefore be present as dixanthogen. Since xanthic acid is a strong acid, it is probable that undissociated potassium xanthate is not adsorbed. We are therefore led independently to the conclusion that the xanthate ion is adsorbed. Graphite can supply no ion to replace the xanthate ions adsorbed, and unless a positive ion (potassium ion) is also adsorbed, the graphite must become strongly negatively charged following the adsorption. There probably exists a surrounding layer of potassium ions. It is well to consider the difference between this conception of the adsorption of xanthate ions and Taggart’s conception of chemical action, since the external appearance of the solid with its adsorption film is identical according to the two theories. Nothing is known of the crystal structure of lead xanthate, but it must certainly be very different from that of lead sulfide. The interior forces of the lead xanthate lattice must differ from those of the lead sulfide lattice, and the surface forces also must differ. These differences are manifested in the different solubilities in water and alcohol, and in the great ease with which air displaces water a t the surface of the former. Consequently, the forces available to bind a xanthate ion t o the surface of lead sulfide (by adsorption) must differ from those available to bind the same ion to the surface of lead xanthate (by crystal growth). The concentration of xanthate ions in equilibrium with two such different surfaces must therefore differ. Hence, in an earlier paper (4)Taggart was led to express tentatively the view that “the solubility of the substance as a surface coating is somewhat less than that of the same substance independently put into solution.” This statement must be interpreted as meaning that lead sulfide binds a xanthate ion to its surface more firmly than does lead xanthate. We have found it useful to retain the conception of an “adsorption solubility product” of lead xanthate at such a surface. Provided that it is not confused with the true solubility product of lead xanthate, there can be no confusion due to its use. It is merely implied that the concentration of the lead ion in solution influences the adsorption of xanthate ions a t a lead sulfide surface in a manner qualitatively similar to but differing quantitatively from the manner in which it is known to influence the deposition of sulfide ions a t the surface. a
Barsky (7) has suggested that xanthic acid may be adsorbed by galena.
PHYSICAL CHEMISTRY O F FLOTATION. V
555
Taggart’s suggestion that there must first be formed a film of sulfate (or other sulfoxide) before adsorption can occur, leads to an alternative picture, with an intermediate layer of lead sulfate between the lead sulfide and the xanthate film, (If this intermediate film be not retained in the final model, the supposed conversion to sulfate loses its significance. For if a xanthate ion can ultimately be attached to the sulfide lattice it is clearly unnecessary t o postulate a sulfate layer a t any stage of the process.) Here again the forces binding the xanthate ion to the surface differ from those binding it to a lead xanthate lattice, and the surface cannot be identical with a surface of pure lead xanthate. EXPERIMENTAL METHODS
A . Contact angle measurements These have been carried out by the methods described in the third paper of this series (8). The sulfur specimens were cut from a solid obtained by allowing fused flowers of sulfur to cool. The usual methods of polishing are satisfactory for sulfur, but not for graphite. None of the methods tried gave a polish as good as can be obtained on specimens of the sulfide minerals or metals, and the use of linen as a final polishing medium was useless. Magnesium oxide as abrasive on a plane lead or aluminum plate proved moderately satisfactory.
B. Flotation tests Grinding was accomplished under water with a porcelain pestle and mortar, and the pulp was deslimed by shaking with water in the test tube and pouring off the mineral that did not settle within fifteen seconds. The flotation tests were carried out in rubber stoppered test tubes (size 6 in. x 1 in). About 2 g. of ground material with 30 cc. of water were introduced and the reagents added. The tube was then shaken vigorously. If a frother is present an excellent froth can be produced in this way, and if the mineral is in a condition susceptible to flotation from 50 per cent t o 100 per cent of it will float in a well-mineralized froth that does not collapse on standing. Even when the mineral is not in a condition susceptible t o flotation, a few particles usually float in an incomplete film on the surface. Unless the particles in the surface showed crowding this small tendency to float was ignored. Though the method is incapable of giving results that could be applied without modification in practice, it is a more rapid and more easily controlled medium for studying the effects of reagents than an ordinary flotation machine. The major differences between the test tube and the flotation machine lie in the method of aeration, and in the pulp density.
556
IAN WILLIAM WARK AND ALWYN BIRCHMORE COX
C. Consumption of xanthate The xanthate was estimated before and after the abstraction tests by titration against standard iodine. The precautions necessary because of the low concentrations of xanthate and the consequent use of N/1000 iodine will be described elsewhere. EXPERIMENTAL RESULTS
Graphite Flotation in the absence of reagents. Ground graphite will float in an incomplete film if it is fed carefully on to the surface of water. If, however, it be once submerged, only a small proportion of the particles will float again. Washing with ether, to remove any adherent oil, has little effect on the flotation of graphite, nor does heating to a red heat, with or without subsequent quenching in water. It does not seem to matter whether the graphite is ground dry or under water. Efect of the recognized frothers. Under the conditions of these test tube tests, 10 mg. of terpineol (one of the major constituents of pine oil) per liter gives a fairly stable froth in water, but between 25 and 50 mg. of terpineol per liter is required to produce satisfactory flotation of graphite. With isoamyl alcohol 30 mg. per liter gives a fairly stable froth in water, but between 200 and 500 mg. per liter is required to produce a good flotation of graphite. Neither of these frothers induces satisfactory contact in contact angle tests, though solutions of high concentration lead to slight and irregular contact. Acetone, though a frother, is not, in neutral solution, a collector for graphite. One gram of acetone per liter gives a froth of low stability, and with 200 g. per liter the froth is still not very stable. Whatever its concentration, however, acetone is without influence on the floatability of graphite. If a suitable collector is present as well (e.g., 5 mg. of potassium amyl xanthate per liter), the acetone exerts the usual function of a frother in facilitating the formation of a stable mineralized froth; about 10 g. per liter is necessary for this purpose. Ethyl alcohol and cyclohexanol are frothers that behave in a manner similar to acetone. In alkaline solutions all of th’ese frothers give stable froths a t lower concentration and all of them lead to flotation of graphite. Whereas in the absence of a collector 2 g. of acetone per liter gives appreciable flotation a t pH = 12, and 10 g. per liter gives good flotation, in the presence of 10 mg. per liter of potassium amyl xanthate 250 to 500 mg. per liter of acetone gives good flotation. In general it has been found that the addition of amyl xanthate permits of the flotation (especially of the larger particles) with considerably lower concentrations of “frother” than would otherwise be possible, and this is true whether alkaline or neutral solutions are used.
PHYSICAL CHEMISTRY OF FLOTATION.
V
557
Efect of collectors of the xanthate type. Ethyl xanthate, if a t sufficiently high concentration, is a good collector for graphite in test tube flotation tests. In these tests isoamyl alcohol (30 mg. per liter) was used as the frother. About 25 mg. of ethyl xanthate per liter definitely gives some flotation, but higher concentrations are required to give a permanent mineralized froth. Contact angles varied from approximately 30" for a concentration of 5 mg. per liter to 60" for concentrations greater than 200 mg. per liter. Copper sulfate did not improve either contact or flotation a t the lower concentrations of xanthate; it is not an activating agent for graphite. A concentration of 5 mg. of potassium amyl xanthate per liter gives good flotation of graphite; again 30 mg. of isoamyl alcohol per liter was used as frother. Contact angles varied from 57" for a concentration of 5 mg. per liter of potassium amyl xanthate to 80" for a concentration of 500 mg. per liter. Cyanide (1 g. of sodium cyanide per liter) is without influence on flotation or on contact, but after a short conditioning period strong alkali (pH = 13) prevents flotation, probably because it slowly saponifies the xanthate. If a solution containing 5 mg. of potassium amyl xanthate per liter is kept a t pH = 13 for an hour before use, it fails to induce flotation of graphite. Of the dithiocarbamates the butyl compound is more effective than the ethyl. Potassium dibutyl dithiocarbamate (10 mg. per liter), with isoamyl alcohol (30 mg. per liter) as frother, induces excellent flotation of graphite. Solutions containing 5, 50, and 500 mg. per liter induced contact angles of 58", 65", and 74", respectively. Sodium diethyl dithiophosphate must be a t a high concentration to cause flotation. As the concentration is increased, flotation first becomes possible at about 500 mg. per liter, but 10 g. per liter gives excellent flotation. The latter concentration leads to a contact angle of 60". Thus, graphite generally responds to collectors of the xanthate type (containing the CS2 group) in the same order as the sulfide minerals, which are least susceptible to the dithiophosphates, most susceptible to the dithiocarbamates, and of intermediate susceptibility to the xanthates. Like those on sulfide minerals, the collector films on graphite are very stable. Amyl xanthate cannot be recovered from the surface by washing with hot water or alcohol. Consumption of xanthate. Graphite rapidly abstracts ethyl xanthate and amyl xanthate from their solutions. Within ten minutes 2 g. of dry ground graphite completely removed the amyl xanthate from 20 cc. of a 20 mg. per liter solution. Within half an hour 2 g. removed 69 per cent of the amyl xanthate from 40 cc. of a 500 mg. per liter solution, and 50 per cent from a second similar solution. The pH value remained unchanged
558
IAN WILLIAM WARK AND ALWYN BIRCHMORE COX
at 6.8; had xanthic acid been abstracted the p H value would have changed to between 10 and 11. Similar results were obtained for ethyl xanthate. It is impossible to recover the adsorbed xanthate by digestion of the graphite with hot water or hot alcohol. That the xanthate has been adsorbed unchanged on the surface of graphite is, however, suggested by the results of a titration of a suspension of the conditioned graphite against N/1000 iodine. The reaction is a slow one, several hours' contact with the iodine solution being necessary, and.an accurate titration is impossible. However, a t least 80 per cent of the adsorbed ethyl xanthate can be evaluated in this way. In a control test, graphite that had not been treated with xanthate consumed less iodine than corresponds to 10 per cent of this amount. Sulfur Sulfur differs from graphite in that it is inherently much more readily floatable.
Sulfur that has been ground very coarsely under water does
I
TABLE 1 Abstyaction o f amyl xanthate bg two grams of sulfur
I
NO.
Initial ?iq.
1 2 3 4
.
I
I
CONCEN'PRATION OF XANTHATE
per liter
312 156 94 31
1
Final mg. per lite?
268 108 50 0
I O D I N E EQUIVALENT CJP XAN?HATE ABSTRACTED
cc.
3.4 3.7 3.4 2.5
XANTadTE ABSTRACTED
mg
.
0.70 0.77 0.70 >0.5
not float completely on shaking it up with water or dilute solutions of the frothers already mentioned, but if it be allowed to slide slowly on t o the surface a heavy film flotation can usually be obtained. However, by using coarse sulfur and low concentrations of frothers, it can be demonstrated that the xanthate type of collector, if present in sufficiently high concentration, enhances the floatability of sulfur. The adsorption of the collector by sulfur may also be studied by measuring the contact angle a t its surface in the presence of a solution of the collector. Thus 500 mg. of potassium di-n-butyl dithiocarbamate per liter led to an angle of contact of 78". Amyl xanthate did not, however, induce the customary angle of contact. We cannot confirm a statement by Christmann (7) that whereas sulfur is not wetted by water, pine oil causes it to be readily wetted by water. We find that the floatability of sulfur is considerably increased by the addition of pine oil. If sufficient pine oil is added to give large undissolved globules, the sulfur, being wetted by pine oil in preference to water, goes
PHYSICAL CHEMISTRY OF FLOTATION.
V
559
into the pine oil phase, or into the interface. The combined sulfurpine oil aggregate is readily wetted by air and floats readily, unless sufficient sulfur is present to make its density greater than that of water. Christmann’s conclusion that the polar group of the pine oil is orientated outwards a t the surface of sulfur is therefore unjustified. The adsorption of collectors by finely divided sulfur has been determined by measuring the abstraction of xanthate from solution. Table 1 demonstrates that the amount of amyl xanthate removed is independent of the initial concentration of the xanthate, and that the whole of the xanthate may be removed from sufficiently dilute solutions. Two grams of flowers of sulphur and 16 cc. of xanthate solution were used for each test, three hours’ contact being allowed between sulfur and solution. The sulfur was then washed until the washings were free from xanthate and the unconsumed xanthate determined by titration against N/1000 iodine. In order that the sulfur should be wetted by the xanthate solution, it was first wetted by 1 cc. of acetone. I n a control test without sulfur, but with acetone, there was no appreciable loss of xanthate. This work was carried out for the following companies: Broken Hill South Ltd., North Broken Hill Ltd., Zinc Corporation Ltd., Electrolytic Zinc Company of Australasia Ltd., Mount Lye11 Mining and Railway Co. Ltd., and the Burma Corporation Ltd. The authors with to.express their thanks to Mr. H. Hey, under whose general direction they have worked, and to Professor E. J. Hartung, who, has generously provided laboratory accommodation in the Chemistry Department of the University of Melbourne. REFERENCES (1) ADAMSAND JESSUP:J. Chem. SOC.127, 1863 (1925).
DEBOERAND CUSTERS:2. physik. Chem. 26B,225 (1934). GAUDIN:Flotation, Chapter XIII. McGraw-Bill Book Co., New York (1932). TAGQART: Am. Inst. Mining Met. Engrs., Milling Methods, p. 247 (1930). TAQGART: J. Phys. Chem. 36,130 (1932). WARKAND Cox: Am. Inst. Mining Met. Engrs., Tech. Pub. 461. WARKAND Cox: Am. Inst. Mining Met. Engrs., Tech. Pub. 461, discussion of paper. (8) WARKAND WARK:J. Phys. Chem. 37,804 (1933). (2) (3) (4) (5) (6) (7)
