T H E PHYSICAL CHEMISTRY OF FLOTATION. VI1
TRIMETHYLCETYLAMMONIUM BROMIDEAS
A
FLOTATION AGENT
IAN WILLIAM WARK
Department of Chemistry, University of Melbourne, Melbourne, Australia Received January SO, 1086
Solutions of trimethylcetylammonium bromide possess, to an unusual extent, certain surface properties that influence the flotation of minerals. Films thin enough to exhibit beautiful interference colors may persist for weeks, and solutions can be used for such experiments with bubbles as are described by Boys (1). The compound is one of the best frothing agents known, even a 10 mg. per liter solution giving a very stable froth. The stability of the bubbles is not due to the formation of a thick semirigid film of the type produced by saponin, for, in dilute solutions, the substance does not seem to be present in colloidal form. From aqueous solutions the salt is adsorbed by a wide variety of minerals, and since it possesses the polar-nonpolar structure characteristic of collectors, it is almost a universal flotation reagent of the collector class. The angle of contact, determined as described in previous contributions (11, 9), is 60" for chalcopyrite and antimonite. This maximum angle has not been obtained for galena and pyrite, however, nor for the oxidized and silicate minerals. For these minerals the angle is usually within a few degrees of 50". It is of interest that silver bromide is not precipitated from solutions of the compound immediately upon the addition of silver nitrate, but upon boiling a precipitate slowly forms. For the work to be described the compound was purified by recrystallizing twice from alcohol and washing with ether. TRIMETHYLCETYLAMMONIUM BROMIDE AS A COLLECTOR
Dilute solutions of trimethylcetylammonium bromide induce flotation of chalcopyrite, pyrite, sphalerite, galena, and other sulfide minerals. Several silicate minerals also respond to it, quartz, for example, responding to acid, neutral, and alkaline solutions, and rhodonite to neutral solutions. Tinstone responds to alkaline solutions, but calcite does not float in acid, neutral, or alkaline solutions. Captive bubble tests with polished mineral specimens confirm these observations. The compound modifies the surface of glass in such a way that an angle of contact of about 40"is obtained; 661
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IAN WILLIAM WARK
it is therefore impossible to keep glassware “clean” when in contact with dilute solutions. Solutions of the compound possess a property that is unusual among collectors, a property moreover that is general for all the minerals floated by it. As the concentration is increased from 0 to about 50 mg. per liter, the floatability increases steadily. Further increases in concentration decrease the floatability, until somewhere below 500 mg. per liter, not a single particle floats. Meanwhile the frothing power of the solutions has steadily increased. Parallel results are obtained when polished mineral specimens are tested by a captive bubble of air, contact ceasing to be possible at a concentration of about 100 mg. per liter. The mechanism of the process preventing contact was suggested by tests with galena in a 50 mg. per liter solution. A newly formed bubble effected contact with the center of the galena surface, but a bubble thirty seconds old did not. Khatever the age of the bubble, contact could be effected with the sharp edges of the specimen, and when once established, the contact could be retained as the bubble was moved across the plane surface,-even to the central area with which contact could not be established with an aged bubble. The galena specimen, when transferred to pure water exhibited, for a time, an air avidity; this indicates that a film of the collector had been adsorbed. It is presumed that the galena was in a suitable condition for contact with air, but the surface of the bubble, as it aged, passed into a condition that hindered contact; nevertheless, the sharp edges of the galena could still rupture the bubble surface. I n slightly more concentrated solutions, contact with the center could be established only by starting at an edge, while in a 500 mg. per liter solution contact at)the edges could not be established. These results were not unexpected. It has already been suggested (11) that the difficulty of rupturing an orientated adsorbed film of frother at the surface of a bubble would hinder its contact with a collector-conditioned mineral surface. But it is only when the film is particularly stable, as here, that its presence can be demonstrated by the bubble test. Support for this interpretation is given by a test in which a chalcopyrite specimen, previously conditioned by amyl xanthate, was placed in a 500 mg. per liter solution of trimethylcetylammonium bromide. Contact with air could not be effected, though the specimen, when replaced in water, still carried an air-avid film with the characteristic amyl xanthate contact angle. Reagents of the type of trimethylcetylammonium bromide-that is, substituted ammonium salts and amines containing a large nonpolar group-may be of value for the flotation of silicate and other oxygenbearing minerals. They possess the advantage over soap solutions that they do not form insoluble salts with such cations ae calcium, copper, and
TRIMETHYLCETYLAMMONIUM BROMIDE AS A FLOTATION AGENT
663
iron. Very low concentrations of the collector should suffice, since there would be no waste due to precipitation by cations derived from the ore or present in the water used. DEPRESSANTS
Because a reagent is a depressant for a mineral when using a collector of the xanthate type, it does not follow that it will be a depressant also when using a nitrogenous collector. Thus 1g. of Na2Sa9HaO per liter, which is much more than sufficient to prevent contact with the sulfide minerals in the presence of xanthates, does not prevent contact in the presence of 50 mg. per liter of trimethylcetylammonium bromide. Neither does it influence contact with or flotabion of quartz. Similarly, a small concentration of sodium cyanide, though an excellent deactivator and depressant for
TABLE1 Conditioned chalcopyrite i n solutions of trimethylcetylamnzonium bromide CONCENTRATION
CONTACT ANQLE
mg. per liter
Nil
(54)
10 25 50
60 59 60
INDUCTION PERIOD O X PLANB SURFACE
Extreme vsluea
Mean value
8ffiond8
seconds
0.6 and 2.4 0.6 and 8.4 2.8 and 6.2 22 and 48
1.1 1.9 4.2 31
>m >600
DIRECT FLOTATION TWTS
Slight film flotation Excellent froth flotation Excellent froth flotation Good froth flotation Fair froth flotation No flotation
* For this determination contact had first to be established with an edge. sphalerite and pyrite respectively in xanthate flotation, is not a depressant with this reagent. The reagent causes flotation of so many minerals that its application in practice will depend upon the discovery of suitable depressants, which must be differential in their action: Depressants OF some minerals have been found. Thus, galena is depressed by alkalis; in a solution containing 10 mg. per liter of the compound and 20 mg. per liter of lime, a partial separation of sphalerite from galena is possible. When the separation was attempted in a test tube, a little of the fine galena floated with the sphalerite. Copper sulfate and sodium cyanide when used together-but not singly-prevent the flotation of tinstone, sphalerite, and pyrite, but it has not been ascertained whether this action is differential between sphalerite and pyrite. Presumably the cupricyanide ion is the depressant. Acids and alkalis both prevent the flotation bf rhodonite. In acid solutions quartz can be floated away from tinstone.
664
IAN WILLIAM WARK INDUCTION PERIOD FOR CONTACT BETWEEN AIR AND MINERAL
Some tests now to be described have a bearing on the explanation of the “induction period” for the establishment of contact. Sven-Nilsson (8) has observed that true contact (spreading) does not usually take place until some time after a bubble has come into apparent contact with a suitably conditioned mineral surface. The time that elapses before spreading commences, he terms the induction period. It increases with the size of the bubble, but the manner of its dependence upon other variables has not yet been established. Using a stopwatch, the induction period has been determined for a chalcopyrite specimen in a series of solutions of trimethylcetylammonium bromide of different concentrations (see table 1). To ensure that the surface condition was as nearly as possible identical in all tests, the newly polished specimen was conditioned in a 500 mg. per liter solution and rinsed in water before it was placed in the TABLE 2 Dependence of angle of contact at a submerged parafin surface on concentration of trimethylcetylammonium bromide CONCENTRATION OF AMINE
ANGLE OF CONTACT
INDUCTION PERIOD
mg. per liter
Nil 10 25 50 100 250
m
107, 106, 109” 100 94.5
Too small to be measured by a stopwatch
92 80, 79 66
Up to 2 seconds Very irregular
Nil
test solution. Evidently the induction period is dependent upon the concentration of the compound. This is presumably because at high concentrations the adsorbed film is difficult to rupture. CONTACT AT A PARAFFIN SURFACE
The condition of the surface of the bubble cannot account completely for the influence, recorded in table 2, of the concentration of trimethylcetylammonium bromide on the angle of contact at the surface of paraffin wax. The wax was purified first by heating with a strong sulfuric acid solution and then with a strong caustic soda solution. The test surface was formed on a glass base by allowing molten wax to solidify in contact with the air; the hysteresis effect was almost absent because of the smoothness of this surface. Since the angle of contact changed slowly with time even in distilled water, the recorded angles were measured immediately after immersion of the wax in the solution.
TRIMETHYLCETYLAMMONIUM BROMIDE AS A FLOTATION AGENT
665
Variations in the surface tension of the solution, no matter how large, could not account for a reduction in the angle to below go”, since in the Young equation for the cosine of the contact angle, the surface tension occurs only as the sole term of the denominator and therefore changes in it could not alter the sign of the cosine of the angle of contact, as happens when the angle passes through 90” (10). The results prove that the surface energy at the paraffi-water interface has changed in such a way as to render the paraffin more water-avid or less air-avid. Such a change may be a chemiadsorption of the amine, with the paraffin wax attracting the alkyl groups of the substituted ammonium salt and the polar group orientated outwards, or it may be a kind of Gibbs’ adsorption at this interface. If a film is formed, it can have no very strong binding forces, for on placing the paraffin in pure water the usual contact angle (107.5’) was obtained. SURFACE TENSION MEASUREMENTS
Perhaps the best method of studying adsorption is to attempt to apply the Gibbs’ equation to measurements of the change in surface tension of a solution with concentration. Most of the methods of measurement of surface tension are inapplicable if, as here, the solution forms a finite contact angle with glass. The “maximum bubble pressure’’ method, which is stated to be applicable when the contact angle is finite, was therefore adopted (7). It was found, however, that the finite contact angles that dilute solutions of trimethylcetylammonium bromide form with glass, though they may not directly influence the maximum bubble pressure, do prevent the apparatus from functioning smoothly. Consequently results could not be obtained for solutions of concentration lower than 50 mg. per liter. There is an unexplained lack of agreement between the results of those who have measured surface tensions of aqueous solutions of surface-active compounds. The work of Harkins and Brown (4) suggests that insufficient attention has been paid to the time necessary for the establishment of the equilibrium or static value of surface tension. Schmidt and Steyer (6) claim that equilibrium is not immediately established even in pure water. The surface tensions of solutions of trimethylcetylammonium bromide, measured by the bubble pressure method, vary with the time interval between the formation of successive bubbles in the manner shown in figure 1. For each strength of solution the measured surface tension approaches a steady minimum value as the rate of bubble formation is decreased. This steady value is presumably the static value corresponding to the attainment of an adsorption equilibrium. The dynamic value, i.e., the value corresponding to a surface concentration equal to the bulk
666
IAh’ WILLIAM WARK
concentration, was not measured, but-whatever the concentration-it is evidently very much closer to, and may not differ greatly from, the value M)
75 70
8
0’
5 2
5s
y w z
e B
45
? m w
3 as 2, 0
P
4
E
a
M
I2
14
M
18
20
22
24
01
28
30
TIME OF FORMATION OF BUBBLE (SECOWDSI
FIG.1. Relationship between the value of the surface tension of solutions of trimethylcetylammonium bromide of various concentrations and the “age” of their surfaces. Temperature, 25 =k 1°C. M)
75
i m B 05
8.. 6 I
5’
2
gu,
P
43
w
Yu, P 3 34 a0 0
400
loo0
xx)o
3ooo
CONCENTRATION OF TRI-METHYL CETYL AMMONIUM BROMIDE
4000
5000
MG. PER L I T E R .
FIG.2. Relationship between concentration of solutions of trimethylcetylammonium bromide and their static surface tensions. Temperature, 25 1°C.
for pure water. The indicated slow attainment of equilibrium is due to the slow diffusion to the surface of such a large molecule as trimethylcetylammonium bromide. It is evident, moreover, that equilibrium is
TRIMETHYLCETYLAMMONIUM BROMIDE AS A FLOTATION AGENT
667
most rapidly established in the most concentrated solutions. This is because of the greater concentration of solute molecules that are available close to the surface for the formation of the adsorbed layer. Figure 2, constructed from figure 1, shows the relationship between the concentration of trimethylcetylammonium bromide and the static surface tension. Up to a concentration of 1 g. per liter, the higher the concentration the lower is the static surface tension and, presumably, the more of the dissolved collector is adsorbed at the surface, but already in a 500 mg. per liter solution the surface must consist largely of orientated solute molecules. It is apparent that if the adsorbed layer around a bubble is a hindrance to contact with a suitably conditioned solid surface, contact will be most readily established in dilute solutions. FROTHING
In figure 2 a condition has been reached at which an increase in concentration no longer lowers the surface tension. H. M. Cassel (2) in discussing a similar case, states that at ~t point of zero slope in the surface tension-concentration curve for sodium palmitate, the Gibbs’ theorem indicates that no adsorption at all takes place. The application of the Gibbs’ theorem is inadmissible, however, when complete, or almost complete orientated films of polar-nonpolar compounds are present. Langmuir (5) states that such films at the surface of aqueous solutions may possess the properties of liquids or solids. Corresponding to the large difference between the static and dynamic values of the surface tension, even the stronger solutions froth strongly (compare Foulk (3)). This certainly indicates that there is an adsorption film at the interface, despite the horizontal nature of the concentrationsurface tension cuive beyond 1 g. per liter. The writer gratefully acknowledges his indebtedness to the companies for which the work was carried out, namely, Broken Hill South Ltd., North Broken Hill Ltd., Zinc Corporation Ltd., Electrolytic Zinc Co. of Australasia Ltd., Mt. Lye11 Mining & Railway Co. Ltd., and the Burma Corporation Ltd.; to Mr. H. Hey, under whose general direction the work was carried out; and to Professor E. J. Hartung for providing laboratory accommodation. REFERENCES
(1) BOYS:Soap Bubbles. London (1924). (2) C ~ S S E LJ.: Am. Chem. SOC.67,2009 (1935). (3) FOULK:Kolloid-2. 80, 115 (1932). (4) HARKINS AND BROWN: J. Am. Chem. SOC.41,499 (1919). (5) LANGMUIR: Gen. Elec. Rev. 38,402 (1935). (6) SCHMIDT AND STEYER: Ann. Physik 79,442 (1926). (7) SUQDEN: The Parachor and Valenoy. London (1930).
668
IAN WILLIAM WARK
(8) SVEN-KILSSON: Proc. Royal Swedish Inst. Eng. Research, 10. 133 (193;); Kolloid-Z. 69, 230 (1934). (9) WARK,E. E.,AND WARK,I. W , : J . Phys. Chem. 37, 797 (1933). (10) W A R KI. ~ W.: J . Phys. Chem. 37, 623 (1933), see equation on p. 624. (11) WARK,I. W., AND Cox, A . €3.: Tians. Am. Inst. lliiiing X e t . Engrs. 112, 180 (1934).
