THE PHYSICAL CHEMISTRY OF FLOTATION. VI THEADSORPTIONOF AMINESBY SULFIDEMINERALS ELSIE EVELYN WARK AND IAN WILLIAM WARK Department of Chemistry, University of Melbourne, Melbourne, Australia
Received March 15, 1955
Wark and Wark (4)have used the measurement of contact angle a t the line of triple contact air-solid-water as a means of studying adsorption by the solid of the class o l compounds known in flotation as “collectors.” It was shown that, for a group of soluble collectors containing the -SH group or its alkali metal salts, the angle of contact is dependent only upon the specific non-polar group of the adsorbed collector. Though the collectors most commonly used today for the flotation of the sulfide minerals are almost all of this type, several other types are of considerable theoretical interest, namely: (1) soluble salts of the fatty acids, in which, as in the xanthate type, the non-polar group is in the negative ion; (2) soluble salts of the amines, in which the non-polar group is in the positive ion; (3) sparingly soluble oils. The first of these types is being investigated elsewhere by the contact angle method. Work on the second type is described in this paper. Work on the third type is to be undertaken shortly . Amines have been chosen to facilitate a comparison between (a) different members of the series of primary amines from the methyl t o the hexyl derivatives, (b) different members of the series of quaternary amines from the methyl to the amyl derivatives, (c) the ethyl derivatives of the primary, secondary, tertiary, and quaternary series, and (d) the aliphatic and the aromatic amines. EXPERIMENTAL METHOD
Preparation of amines The primary, secondary, and tertiary amines were recrystallized as the hydrochlorides, usually from alcohol, by addition of ether. The quaternary ammonium salts were recrystallized from water.
Determination of contact angle The experimental procedure of the preceding papers was followed, but in the present work higher concentrations of the collector were generally 1021 THE JOURNAL OF PHYSICAL CHEMISTRY, VOL. XXXIX, NO.
7
1022
ELSIE EVELYN WARK AND IAN WILLIAM WARK
necessary to induce contact. Since limited amounts of the amines were available, the response of the minerals could not be tested in solutions of greater concentration than 1g. per liter. I n every test, the reaction of the mineral to the amine was observed over a period of at least half an hour, and the bubble was pressed against the mineral for approximately fifteen seconds to see whether true contact was established. The pH value of the solution was found to influence the adsorption of amines. Consequently, if a mineral failed to respond to the natural solution of the amine hydrochloride, its behavior was observed also in acid (pH = 1) and alkaline (pH = 12) solutions. Even with this procedure, adsorption of isoamylamine by sphalerite was not det,ected. Gaudin, Haynes, and Haas (1) have shown that flotation in this case occurs only within the pH range 9 to 11. It is possible, therefore, that in a few cases contact angles are reported as “nil” where tests at, other pH values would .have shown contact to be possible. RESULTS
No matter what the concentration of an amine may be, a characteristic contact angle is never exceeded. Near the minimum effective or “threshold” concentration of amine, or near the critical p H value-at which contact ceases to be possible-there is a range of solution compositions over which the maximum contact angle is not attained. However, over a wide range of concentrations and of p H values, the contact angle is constant within an experimental error of rt2O. The determined maximum contact angles are recorded in table 1. Over a wider range of pH values than other minerals, bornite and chalcocite do not give the usual maximum angles for the aliphatic amines. On the other hand, both these minerals adsorb sulfur-bearing collectors more readily than does chalcopyrite, and the incomplete response t o amines may be due to fouling of the surface by cuprous chloride or iodide. Certainly both minerals float readily with hexylamine under solution conditions that lead to angles of contact of only 40”to 50°, the amine itself serving as frother. With triethylamine, however, for which the measured contact angles were still lower, only a very rapidly collapsing mineralized froth could be obtained, even in the presence of an added frother. When placed in amine-free distilled water, a chalcopyrite surface that has previously responded to hexylamine or to a quaternary ammonium salt retains for some time the ability to attract an air bubble. I t is evident, t,herefore, that the amine changes the surface of the mineral. The change, attributed to adsorption of the amine, is most permanent for those amines that are most readily adsorbed. Methylamines. Trimethylamine hydrochloride and tetramethylammonium iodide were tested. Within the pH range considered, none of the
PHYSICAL CHEMISTRY O F FLOTATION.
1023
VI
minerals with which the investigation was concerned would have responded to a 1 g. per liter solution of the primary or secondary methylamines. None of the minerals responds to a concentration of 1 g. per liter of trimethylamine hydrochloride a t pH values of 7 or 12. To a solution of 1 g. per liter a t a pH value of 1.0, only chalcopyrite and bornite responded, and then but weakly; in normal hydrochloric acid, however, the response TABLE 1 Maximum values of contact angle for various minerals and amines AXIMUM VALUES OF CONTACT A N G L E S FOR VARIOUS M I N E R A L S
-
d m AMINE
O
Y
N
s
a
16
8
. I
u
$9 Ev m
12 m
_ _ .
Trimethylamine hydrochloride . . Monoethylamine hydrochloride. . . Diethylamine hydrochloride. . . . . . Triethylamine hydrochloride, . . . Isoamylamine hydrochloride.. . . . .
61* 58* 61* 61 63
Nil Nil Nil Nil Nil
59* Nil Nil 63 60
Isohexylamine hydrochloride . . . . .
63
61
56
.. .. ... . .
56
Nil
57
62 (55)
62
61 61 60 61 61 62 59
58*’ 62 61 61 60 59 57
.
Aniline hydrochloride . . .
a-Naphthylamine hydrochloride.
.
.
Piperidine hydrochloride . . . . . . . , Tribenzylamine hydrochloride$. . . Tetramethylammonium iodide.. . . Tetraethylammonium iodide.. . . . Tetrapropylammonium iodide. . . . Tetrabutylammonium iodide.. . . . . Tetraamylammonium iodide.. . . . .
.
Nil Nil Nil Nil Nil 60 60
3
s.
V
.L
‘G
1
ii 4 (49)* (42)* (44)* (52)* (52) (47)’ 64t 59
0
g
6
d
G
%&
8 - -
Nil Nil Nil Slight Slight
Nil Nil Nil Nil Nil
Nil Nil Nil Nil Nil
{
63
Nil
Nil
Nil
Nil
Nil
Nil Nil Nil Nil Nil Nil Nil
Nil 60 Nil
;;;(
57
{ ~ $ ~59, 56 62 61 60 56 61 60
03
Slight (56 1 (50) 61 (52 1 61 ,60
54) *
521 62 62
* A t pH = 1.0. t At pH = 12.0. 1Maximum pH value 3.5.
of chalcopyrite to the amine was complete. Sphalerite that had been previously activatedl responded in acid solution in a manner similar to chalcopyrite. Sphalerite that has been immersed in a dilute copper sulfate solution becomes coated by a copper-bearing film, probably of cupric sulfide. When in this condition, it responds more readily to most collectors, and is regarded as being “activated.”
1024
ELSIE EVELYN WARK AND IAN WILLIAM WARK
The copper minerals respond to 1 g. per liter of tetramethylammonium iodide, but neither galena, sphalerite, nor pyrite responds within the p H range 1 to 12. Chalcopyrite, however, responds to so low a concentration as 25 mg. per liter, and the response of bornite, chalcocite, and sphalerite activated by copper sulfate, to neutral 1 g. per liter solutions is rapid. The contact angle for chalcocite (see table 1) was influenced by the formation of a heavy precipitate, presumably of cuprous iodide. I n neutral solution, the angle of contact for bornite was 61", but contact was slower and less complete (= 50") a t p H = 12. Ethylamines. The mono-, di-, tri-, and tetra-substituted ammonium salts indicate a slight increase in the readiness with which adsorption occurs as the number of non-polar groups in the adsorbed molecule increases. Solutions of a constant concentration (1 g. per liter) were used for all four amines. The differences between them would have been slightly greater had equivalent concentrations been taken. Monoethylamine is a slightly less potent collector than trimethylamine. None of the minerals responds to 1 g. per liter of ethylamine hydrochloride in neutral or alkaline solutions (pH = 12) and only chalcopyrite responds completely in an acid solution of p H value = 1. Bornite gives an angle of approximately 40" at pH = 1, the response being irregular over the surface. Diethylamine hydrochloride was adsorbed only by those minerals that adsorbed the primary amine. Again, none of the minerals responds to 1 g. per liter in neutral solution or a t pH = 12, and only chalcopyrite and bornite respond in acid solution. The response of bornite is again incomplete. With triethylamine the response of the minerals is slightly more general. Pyrite, sphalerite, and galena, however, fail to respond to 1 g. per liter of the hydrochloride over the whole pH range 1 to 12. Activated sphalerite does respond, particularly in acid solution. Chalcocite responds incompletely, the angle of contact being only 38" a t pH = 1, and it does not respond in alkaline solution. Though bornite responds over the whole pH range, the contact angle is low, a maximum angle of 51" being obtained a t p H = 1. Chaleopyrite responds rapidly to the neutral solution. The quaternary ammonium salt is more effective than triethylamine as a collector. The copper minerals and activated sphalerite respond rapidly to a 1 g. per liter solution of tetraethylammonium iodide in neutral solution; indeed, much lower concentrations than 1 g. per liter may be used. Galena responds in a strongly acid solution (pH = l ) , but the full contact angle is not attained. Neither pyrite nor sphalerite responds to a 1 g. per liter solution within the pH range 1 to 12. Isoamylamine. Though more effective as a collector than monoethylamine, isoamylamine is no more effective than triethylamine. Galena and pyrite do not respond to a 1 g. per liter solution of isoamylamine hydro-
1025
PHYSICAL CHEMISTRY OF FLOTATION. VI
chloride a t pH values 1, 6, and 12. However, sphalerite, which does not respond a t pH values of 6 and 12, does respond at 10. The copper minerals and activated sphalerite respond to a 1g. per liter solution a t the natural pH value (3.8). Chalcopyrite responds to very low eoncentrations in acid solution (see figure 2). E5
f 300
Ed
ID0 Y
250
90
8, s
3x1
rn
E
W
=
50
f gz
Y
z
F
E
$ z
Y
‘*O
L
+o 30
100
4
5
20
50
Y
10
z
0
“
4
5
8
7
pH VALUE
8
9
10
41
I2
IJ
pH VALUC
FIG.1 FIG,2 FIG. 1. RELATIONSHIP BETWEEN THE pH VALUE AND THE CONCENTRATION OF WNAPHTHYLAMINE HYDROCHLORIDE NECESSARY TO INDUCE CONTACT BETWEEN AN AIR BUBBLEAND A CHALCOPYRITE SURFACE 0, contact possible; X contact impossible. Contact possible only to left of curve. FIG.2. RELATIONSHIP BETWEEN THE pH VALUEAND THE CONCENTRATION OF AMINE HYDROCHLORIDE NECESSARYTO INDUCE CONTACT BETWEEN A N AIR BUBBLE AND A SURFACE OF CHALCOPYRITE p
io00
=
edo
3
g
$
900
700
600
0
500
2 w w
p 2
200
2
lm
x
-
300
0
4
5
6
7
8
9
10
11
11
13
pU VALUE
FIQ. 3. RELATIONSHIP BETWEEN THE pH VALUEAND THE CONCENTRATION OF IBOHEXYLAMINE NECESSARY TO INDUCE CONTACT BETWEEN AN AIR BUBBLEAND SURFACESOF SPHALERITE AND PYRITE Contact possible only above curves
Isohezylumine. Isohexylamine is a still more effective collector than the corresponding amylamine. It is a good collector for sphalerite, the optimum pH value being about 10 (see figure 3). Even pyrite responds to
1026
ELSIE EVELYN WARK AND IAN WILLIAM WARK
sufficiently concentrated solutions of the hydrochloride over a limited p H range. Galena alone fails to respond a t the p H values tested. Aniline. As a collector, aniline resembles isoamylamine. Sphalerite, galena, and pyrite do not respond to a 1 g. per liter solution of aniline hydrochloride within the p H range tested, but the copper minerals and activated sphalerite do respond to a solution a t the natural p H value of 3.5. a-Naphthylamine. a-Naphthylamine is a little more effective than aniline: sphalerite responds slowly and incompletely to 1 g. per liter of the hydrochloride in acid solutions. However, neither galena nor pyrite responds to it. Accordingly, sphalerite can be floated away from galena by a 1 g. per liter solution of the amine hydrochloride at a pH value of 3.5. The amine itself serves as a frother. Piperidine. Piperidine and a-naphthylamine are of approximately equal value as collectors, the former being slightly the more active. Tribenzylamine. The solubility of tribenzylamine hydrochloride in water is less than 200 mg. per liter; a saturated solution was therefore employed. Since alkalis precipitate the amine from this solution, no tests could be made in alkaline solution. Only the natural solution (pH = 3.5) and a solution of pH value = 1 were tested. This amine is the strongest collector of the aromatic derivatives tested. Even with this low concentration, only sphalerite and pyrite failed to respond to it in acid solutions. Tetrapropylammonium iodide. Of the minerals tested, only pyrite and sphalerite do not respond to 1 g. per liter of this reagent within the p H range 1 to 2. The response of chalcocite is masked by the formation of a precipitate of cuprous iodide, and though galena responds to a neutral 1 g. per liter solution, its response is not fully developed. The copper minerals respond in the natural solution (pH = 6.6) to considerably less than this concentration of the amine; indeed, chalcopyrite responds fully to 25 mg. per liter. Tetrabutylammonium iodide. With the increase in the size of the nonpolar portions of the amine, adsorption becomes still more general, and sphalerite responds to this collector without the need for activation. Pyrite, however, remains uninfluenced by 1 g. per liter of the reagent over the pH range 1 to 12. Concentrations as low as 25 mg. per liter are effective for the copper minerals and activated sphalerite. Tetraamylammonium iodide. Of the minerals tested, pyrite alone does not respond to a 1 g. per liter solution of tetraamylammonium iodide within the pH range 1 to 12. The other minerals respond to considerably lower concentrations. INFLUENCE OF
PH
VALUE
In general, the amines are more readily adsorbed from acid than fromalkaline solutions. However, in two cases a t least-namely, for sphalerite
PHYSICAL CHEMISTRY OF FLOTATION.
VI
1027
and pyrite with hexylamine-adsorption ceases to be possible in acid solutions, and in fact takes place more readily from alkaline solutions. I n these cases, there is an optimum p H value between 9 and 11for contact between air and mineral; Gaudin, Haynes, and Haas (1) give a n optimum p H value of about 10 for flotation of sphalerite by isoamylamine. The copper minerals exhibit no such optimum p H value within the range tested (3.4 to 13). With them there is, for each p H value, a concentration of collector above which contact is possible between mineral and air and below which contact is impossible. This concentration is dependent both on the mineral and on the amine. Curves have therefore been determined to show the influence of pH value on the threshold concentration of the amine for a number of minerals and amines. The method of determining the curves is best illustrated by figure 1, which shows for chalcocite the relationship between p H value and concentration of a-naphthylamine hydrochloride. The response of chalcocite to a number of solutions of differing concentration and p H value was determined. The compositions of solutions in which contact with an air bubble was possible appear on the diagram as circles; those of solutions in which contact was not possible appear as crosses. Where the response over the surface was irregular, or where the contact angle fell short of 60°, the solution composition appears on the diagram as a crossed circle. A smoothed line is then drawn between the two sets of points; obviously it separates solutions in which contact is possible from those in which contact is not possible. Fuller details of the method are given elsewhere (2). In general, the curves for amines cannot be determined with the same precision as those for xanthate, the range of incomplete contact being bigger. In figure 2 are shown the curves for chalcopyrite with two different amines, and in figure 3, the curves for sphalerite and pyrite with isohexylamine. DISCUSSJON OF RESULTS
The maximum angles of contact cited in table 1 are approximately constant for all the amines tested; the mean value is 60” and the variation therefrom is usually within the experimental error. This angle is independent both of the nature of the mineral and of the concentration of the collector, provided that the latter exceeds a “threshold concentration” characteristic of the mineral, the amine, and the p H value of the solution. The threshold concentration is lowest for the copper minerals, and then follow, in order of their susceptibility to the quaternary ammonium salts, galena, sphalerite, and pyrite. Sphalerite preactivated by copper sulfate closely resembles chalcopyrite in its response to amines; this is in accord with its response to the sulfur-bearing collectors. Though galena responds more readily than sphalerite to the quaternary amines, sphalerite responds more readily to hexylamine. Using 50 mg.
1028
ELSIE EVELYN WARK AND IAN WILLIAM WARK
per liter of hexylamine at a pH value of 10.5,sphalerite can be floated away from galena. The amine serves both as collector' and frother. With collectors of the xanthate type, galena can be floated away from sphalerite; in this case a frother is necessary. It follows that the function of the collector in selective flotation is not merely to enhance differences in the inherent floatability of the minerals to be separated, as has been suggested by some writers, but actually to alter the surface of the desired mineral so that it can be floated. Of the amines tested, the quaternary ammonium salts are the most readily adsorbed. Further, the higher the homolog the lower is the concentration necessary for the mineral to respond. For example, considering the ethylamines. the primary compound induces a marked response only in one mineral, chalcopyrite, and then only in strongly acid solutions; the secondary compound is little if any more active; the tertiary compound is, however, very much more active, though it does not influence pyrite or galena, and only slowly influences sphalerite in acid solutions; while the quaternary compound induces a response from all the minerals except pyrite and sphalerite. Similarly, tetraamylammonium iodide is very much more active than primary amylamine. Among the primary amines the higher members of the series are far more active than the methyl and ethyl compounds. Three of the four aromatic amines considered are of about the same order of activity as isoamylamine, but tribenzylamine is more active. An analysis of the influence ol acids and alkalis suggests that in general the amine may be adsorbed in the form of an ion, e.g. (N(Et)s)+, (N(Et)3H)+, (NEt2H2)+,(NEtH3)+, etc. The greater the hydrogen-ion concentration the greater will be the percentage of the amine present in the ionic form (and the less in the form of the amine itself or of the hydroxide) and consequently a smaller addition of amine would produce the threshold concentration of the amine ion necessary for adsorption. The loss of floatability with decrease of pH value for sphalerite and pyrite in the presence of hexylamine cannot be explained in this manner. There, up to a certain point, an increase in alkalinity permits of a smaller addition of amine; thereafter (figure 3), the curves are of the normal form. No explanation is offered for the increase in concentration shown on the left of these two curves. Gaudin, Haynes, and Haas have shown (1)that the relationship between flotation and pH value may be very complex. The approximate constancy of the angle of contact for all the amines tested indicates that the forces responsible for the adsorption of the amines differ from those responsible for the adsorption of the soluble collectors of the xanthate type. This is perhaps due to the fact that the non-polar groups are in the positive ion in amines and in the negative ion in xanthates.
,
PHYSICAL CHEMISTRY OF FLOTATION.
VI
1029
If, as has been suggested (3), ionic forces bind the xanthate ion to the surface, the forces of adsorption must be of a different nature in the two cases. Using sphalerite and hexylamine, it has been possible, for the first time, to determine experimentally how closely contact angle tests parallel actual flotation tests carried out in identical solutions. Concentrations of this amine that are sufficient to cause contact between air and mineral are also sufficient to produce frothing. Moreover, the amount of the aminc nbstractcd by the mineral is low and the concentration of the amine in solution can therefore be calculated approximately from the amount added. By means of flotation tests carried out in test tubes (3) a curve can be constructed showing the relationship between the p H value and the amount of amine necessary to induce flotation. Such a curve approximately coiiicides with thc corresponding contact curve of figure 3, but there is not absolute agreement between them. I n comparing flotation tests with thc contact curve, it was found that, keeping the amine concentration constant at 1 g. per liter, flotation of sphalerite is impossible a t pII values below 5; that at B pH value of between 5.5 and G an ephemeral mineralized froth forms; that when the pH value is increased to 8, flotation becomes stable; but that a t a p H value of 13 only the very fine material floats. Keeping the pH value constant and gradually increasing the concentration of the amine it was found that: (a) with the p H value a t 9.5, no flotation occurs with 15 mg. per liter, only an ephemeral mineralized froth forms a t 25 mg. per liter, and a permanent mineralized froth forms a t 50 mg. per liter; (b) with the p H value a t 6.6, no flotation occurs a t 100 mg. per liter or 200 mg. per liter, slight flotation occurred at 250 mg. per liter, and good flotation occured a t 300 mg. per liter; (c) with the pH value a t 13, good flotation occurred a t 2 g. per liter. Except in the last-mentioned caw Ihese results are in agreement with the contact curve. A similar comparison has been made for chalcopyrite and a-naphthylamine (compare figure 2). With a concentration of 500 mg. per liter in strongly acid solutions no froth is produced, but the mineral gives good film flotation. As alkali is added the solution acquires the capacity to form a froth, and between pH values of 6 and 9 a stable mineralized froth is produced. Beyond pH = 10, although the frothing power increases, flotation falls away, until a t pH = 12 no permanent mineralized froth can be formed. At lower concentrations an atlditionnl frother is necessary, :ind thc comparison loses significance. SUMMARY
1. Table 1 shows that the maximum contact angle is independent of the particular amine choseii, being within a few degrees of 60" for all amines. 2. The amine induces a more or less permanent effect on the mineral surface, and it is concluded that adsorption of the amine is responsible for this.
1030
ELSIE EVELYN WARK AND IAN WILLIAM N A R K
3. Of the minerals tested, the copper minerals respond most readily, and pyrite least readily, to amines; activated sphalerite closely resembles chalcopyrite in its response. 4. Using hexylamine, sphalerite can be floated away from galena. 5 . Of the amines tested, the quaternary ammonium salts are most readily adsorbed. 6. The higher the homolog, the lower is the concentration necessary for the mineral to respond, i.e., the ethylamines are more effective than the methylamines, the propylamines than the ethylamines, etc. 7. The primary aniines are the least active, the secondary amines are more active, and the tertiary amines are still more active. 8. Three cyclic amines-aniline, a-naphthylamine, and piperidine--sw of about the same order of activity as isoamylnmine, but tribenzylaminc is much more active. 9. Contact tests closely parallel actual flotation tests carried out i l l identical solutions. 10. Figures 1 , 2 , and 3 show how the contact induced a t certain mineral surfaces by certain amines is prevented by addition of alkali or acid. We wish to record our thanks to Professor E. J. Hartung for having made available a laboratory for this work, and to Messrs. H. Hey and A. B. Cox for help during its progress. The cost of the work was borne by the University of Melbourne and 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 Company Ltd., and the Burma Corporation Ltd. REFERENCES (1) G,AUDIN,HAYNES, AND HAAS:Flotation Fundamentals, Part 4. University of Utah, 1930. (2) WARKAND Cox: Am. Inst. Mining Met. Engrs., Tech. Pub. 495 (1933). (3) WARKAND Cox: The Physical Chemistry of Flotation. V. J. Phys. Chem. 39, 551 (1935). (4) WARKAND WARK: J. Phys. Chem. 37, 805 (1933).
