Critical surface tension of wetting and of floatability of molybdenite and sulfur

Critical Surface Tension of Wetting and of Floatability of Molybdenite and Sulfur S. K E L E B E K * Department of Mining and Metallurgical Engineering, McGill University, 3450 University Street, Montreal Quebec H3A 2.47, Canada

Received October 28, 1986; accepted April 2, 1987 Molybdenite and sulfur samples have been used for adhesion tension determinations in aqueous methanol solutions. The critical surface tension of wetting, ~c, of the crystal faces of molybdenite and sulfur was found to be 29 and 27 mN/m, respectively. Both "re and/3, the slope of the adhesion tension line, increased as the polar nature of the sample increased, Polar characteristics of various specimens were estimated from the octane contact angle measurements. These were related to the fl values of respective samples. The adhesion tension behavior of aqueous solutions of Syntex(a commercial surfactant consisting of sulfonated monoglycerides of coconut oil) on molybdenite was also studied and compared to that of methanol. A good correlation has been observed between wettability and floatability of the samples. The flotation test data have been employed to illustrate the effect of grinding on the critical surface tension distributions of molybdenite. © 1988AcademicPress,Inc. INTRODUCTION Molybdenite and sulfur belong to a group o f minerals which exhibits inherent hydrophobicity. These two are well-known natural floaters. In the mineral industry, the separation o f molybdenite from other ore constituents is based on exploiting the natural floatability o f molybdenite. In the emulsion flotation (1), the natural floatability o f m o l y b d e n i t e is enhanced by using hydrocarbons in order to achieve a desirable recovery level in the process. The natural floatability o f sulfur is important f r o m the standpoint o f not only the recovery o f sulfur but also its effect on the flotation o f c o m m o n sulfides such as chalcopyrite and galena. F o r m a t i o n o f free sulfur on sulfide minerals due to oxidation, which results in so-called "collectorless flotation" behavior, has been well d o c u m e n t e d in the recent literature (2-4). * Present address: Energy,Mines and ResourcesCanada, CANMET/Coal Research Laboratories, P.O. Bag. 1280, Devon, Alberta, Canada T0C 1E0. 0021-9797/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

Some f u n d a m e n t a l surface properties o f inherently h y d r o p h o b i c minerals including m o lybdenite and sulfur were investigated (5, 6). These investigations mainly dealt with the effect o f p H on their contact angle, electrokinetic, and f o t a t i o n behavior. The authors discussed specifically the nature o f the correlation a m o n g these surface properties. However, no work seems to have been reported on the effect o f surface tension on the wettability and floatability o f these minerals until recently. Recent research efforts on this area resulted in a series o f publications (7-15). Molybdenite and sulfur have some interesting structural features. The cleavage planes o f molybdenite contain a layer o f sulfur atoms. It is interesting to see to what extent these c o m m o n characteristics are reflected by their critical surface tension o f wetting. In the present work, the wettabilities o f various specimens prepared f r o m these two minerals are compared. Polar characteristics o f each specimen are estimated from the measurements o f the octane contact angles. The data are then used to illustrate the relationship

504 Journal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

CRITICAL SURFACE TENSION OF MOLYBDENITE AND SULFUR

between the surface polarity and the slope of the adhesion tension line for each specimen. The correlation between the results of wettability and floatability experiments is discussed in relation to sample preparation. The flotation data are used to evaluate the effect of grinding on the critical surface tension distribution of molybdenite.

Structural Considerations Molybdenite has a hexagonal layer type structure consisting of two sheets of sulfur atoms between which is sandwiched a single sheet of molybdenum atoms (16). There are strong covalent bonds within the S-Mo-S layers, but only weak van der Waals forces between them to yield relatively large S-S interlayer distances. These bonds are easily ruptured, giving rise to characteristic cleavage along the [0001] plane. Orthorhombic sulfur consists of the S ring molecules (17), which are puckered; that is, all the atoms are not coplanar. The intermolecular distance (i.e., ringto-ring) is about 1.5 times the interatomic distance in the rings. Despite similarities sulfur does not have a layer structure as readily cleavable as that of molybdenite. Some relevant differences of chemical nature may also be noted. An electronegativity difference of 0.7 for the bond Mo-S (18) corresponds to a partial ionic character of 12.9% according to the empirical formula of Hannay and Smyth (19). On the other hand, the bonds of the elemental sulfur do not have any ionic character unless they are broken and subsequently oxidized. Generally the surface oxidation of minerals is unavoidable. In the case of molybdenite and sulfur, oxidation produces the M o - O and S-O bonds having 37.3 and 19.5% ionic character, respectively. With oxidation, the electrostatic fields on such surfaces become strong enough to attract the water dipoles. The molybdenite edges (produced by the rupture of the S-Mo-S bond) are known to be completely hydrophilic (6). This is presumably due to creation of the M o - O and SO bonds which have a relatively large percentage of ionic character.

505

EXPERIMENTAL MATERIALS AND PROCEDURES

Methanol was the principal reagent used to control the bubble surface tension in the wettability and floatability tests. A surfactant commercially known as Syntex (courtesy of Climax Molybdenum Co., CO) was included in the experiments to determine its wetting behavior on molybdenite. Syntex, a sodium salt of a mixture of sulfonated monoglycerides of coconut oil derivation, is used in the molybdenite flotation at Climax (1). Octane (3qv = 21.8 mN/m; bp range, 124126°C) was used for contact angle measurements to characterize the polarity of the samples in terms of the polar component of their surface free energy, 3"~. The water (3qv = 72.5 mN/m) used in these tests was doubly distilled. The surface tension values were verified by measurements. The octane and water phases had been saturated with each other before the contact angles of octane droplets on specimens were measured under water. All glassware used in the experimental work was cleaned with freshly prepared chromic acid solution and later with distilled water employing an ultrasonic bath. Molybdenite (Ontario, Canada) and sulfur (A.D. Mackay Inc.) samples used in the experiments assayed over 99.4%. Each mineral was ground and screened to obtain the 180 × 106 mesh fraction. One gram powder of this size class was used in flotation tests. A specialpurpose molybdenite sample was prepared for flotation testing. This involved cutting previously cleaved molybdenite layers in the form of squares and rectangles with sides ranging from 0.5 to 3.0 m m (Fig. 1). This sample was used to check the extent of correlation between the wetting and the flotation behavior of molybdenite faces containing no (or the least possible number of) hydrophilic sites. Flotation tests were conducted using an all-glass microcell with an 18-cm-long cylindrical body. The captive bubble method was employed to measure so-called equilibrium contact angles. The bubbles and octane droplets were of 1.5 +__0.2 m m in diameter. The contact angles Journal of Colloid and Interface Science, Vol. 124,No. 2, August1988

506

s. KELEBEK

FIG. 1. Nonground molybdenite sample used in the flotation experiments (grid dimensions, 1 X lmm).

were measured on the crystal faces as well as on disk samples prepared from molybdenite and sulfur. The molybdenite face sample was obtained by cleaving the sample along the [0001 ] plane, while the sulfur crystal sample was prepared by recrystallization from carbon disulfide. The crystal specimens are shown in Fig. 2. The disk samples were obtained by compacting the powder samples of each mineral that had been prepared for flotation tests. As opposed to the homogeneous and strongly hydrophobic character of crystal faces, the disk specimens were heterogeneous with a number ofhydrophilic and hydrophobic sites. The ratio of these two types of sites determines the degree of wetting of the samples. Aqueous Journal of Colloid and Interface Science,

Vol. 124,No. 2, August1988

methanol solutions of various surface tensions were employed as the wetting and flotation media. All experiments were carried out at a room temperature of 23 + 2°C. The surface tensions and related properties of the methanol-water system have recently been reported (20). More details regarding the sample preparation and procedures may be found elsewhere (11, 13). RESULTS AND DISCUSSION

Representation of the Wettability Data The contact angle (0)-surface tension ('rlv) data are usually represented in two different ways. One way is to plot cos 0 against 3qv.

CRITICAL SURFACE TENSION OF M O L Y B D E N I T E AND SULFUR

507

FIG. 2. Crystal samples of molybdenite(left) and sulfur (right) with their numerical 3'~ values indicated (in mJ/m2).

When the data are produced using pure liquid droplets, there is generally a linear relationship between cos 0 and 3'iv (7, 15, 21). Bernett and Zisman (22) found that the use of droplets of aqueous surfactant solutions produced the same % values for polyethylene and Teflon as those obtained using pure liquids, but the cos 0 vs % relationship was no longer linear. An alternative way of representing the wettability data is through the adhesion tension, 3q~ cos 0 vs 3% In Fig. 3, these two ways of representation are compared using the results obtained for sulfur samples in methanol solutions. It is evident from this comparison that the representation in terms of adhesion tension is more preferable. This way, as in the case of the Zisman method, the wettability of a hydrophobic surface may be conveniently characterized by two parameters, Yc and a slope value. The linear behavior can be mathematically expressed as 71vCOS0 =/3~{lv+ (1 -/3)%.

[ 1]

The slope of the adhesion tension line (fl) has been related to the adsorption densities of solute at the three interfaces by combining Young's equation with Gibbs' equation (23)

r=

d(TlvCOS 0) _ I'sv - Psi d3qv rl~ '

[2]

where Psv, F~l, and rlv are the relative adsorption densities of the solute at the three interfaces. The simplest stated explanation for the linearity of adhesion tension line (i.e., constant fl value) is the assumed proportionality of the three adsorptions over the concentration range studied. The linear wetting behavior as expressed empirically by Eq. [1] has been predicted mathematically. Johnson and Dettre (24) obtained the relation 2% - q/iv cos 0 = - -

[3]

"Ylv

More recent derivations of Eq. [3] have appeared in the literature (25, 26). This equation represents the case where fl = - 1 , i.e., equal adsorption of solute at the solid/liquid and liquid/vapor interfaces, I'sl = I'lv and I'sv = 0, i.e., negligible adsorption at the solid/vapor interface. It is clear from Eq. [2] that when Fsv = 0 the slope fl is equal to -I'sdFlv. Pyter et al. (27) have shown how the linear wetting Journal of Colloid and Interface Science, Vol. 124,No. 2, August 1988

508

S. K E L E B E K

Cos 8 = 1

1.0

Sulphur ~ Crystal Face ooisc

0.8

~. ,%.

I "..I .... ' \ : ]"".-~"-... ~ _ _ ~

.... %

0.6

I

"':.

"o'"...

~D ().4

.......

0.2

i

I 3'~

I I%

i

I

I

_

I "-

o

¢..)

7 I !?.::: ~

~"--"20

./ I i~

I~

¢"

I

-~ 0 m

'~ -I0

~

i ~ ~,,,.,.~

~{clf

~

\ ~0

0

'Tcmf

i r I I r 20 50 40 50 GO Surfece Tension, ~lv (mN/m)

I 70

FIG. 3. Zisman'swettabilitydiagram (top) and adhesiontension and floatabilitydiagram (bottom) for sulfur.

behavior is affected by the change of the adsorption density ratios within a wide range of limits. For certain surfactant-solid systems (e.g., perfiuorooctaic acid-paraffin) they have also presented experimental evidence for complex (nonlinear) adhesion tension variation. For the aqueous methanol solution-hydrophobic mineral systems the variation of adhesion tension with surface tension has been found to be consistently linear (11, 14, 15). The linear behavior can also be seen from the analysis of the experimental data reported for aqueous butanol solutions and hydrophobic minerals such as talc and stibnite (28). Journal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

Adhesion Tensions and Wetting Parameters. "yc, fl From the adhesion tension diagrams shown in Figs. 3 and 4 it may be noted that yc values established in methanol solutions are about 27, 29 mN/m for the crystal faces and 33, 42 mN/m for the disk samples of sulfur and molybdenite, respectively. These values are consistent with their structural features noted previously. In the case of Syntex solutions 7c for molybdenite is about 31 mN/m. The slope values are -0.48, -0.43 for the crystal faces and +0.05, +0.34 for the disk samples of sulfur

CRITICAL SURFACE TENSION OF MOLYBDENITE AND SULFUR I

I

[

I

Molybdenile 70 --

~ Face/Methanol o Disc/Methanol

e Face/Syntex

60

~" 5 0 Z

E 4O

u

50

g

,:

!' I

o)/ e,

~ -I0

cose=o

i

,%

-20

l0

20

Surface

I

I

I

I

I

30

40

5Q

60

70

Tension, ~lv (rnN/m)

F~G, 4. Adhesion tension diagram for molybdenite in aqueous solutions of methanol and Syntex.

and molybdenite, respectively. The adhesion tension variation of the molybdenite-Syntex system is described with two straight lines having slopes of - 1 . 4 and about 0. Negative slopes are characteristic of a significant nonpolarity. Examination of the literature data (22, 28-30) shows that for various aqueous surfactant solutions the slope fl is close to - 1 for strongly hydrophobic solids such as paraffin and Teflon. It has been indicated (30) that for such nonpolar solids £~1 = Psv + fly (consider Eq. [2]) and also that equilibrium surface pressure, 7re, is zero, which means that I's~ = 0. A zero value of ~re for water on Teflon has been recently demonstrated (31). The mineral samples are naturally less nonpolar. Therefore, the fl values of their wettability lines are less negative and in the case of disk samples they are actually positive. According to Eq. [2] a positive fl value means r~v > Psb Therefore the more positive the fl value is, the greater the adsorption density at

509

the solid/vapor interface, which may be regarded as a consequence of the greater amount of polar character of the solid surface. Other aspects of aqueous methanol solution-hydrophobic mineral systems are discussed elsewhere (20). In the case of the molybdenite-Syntex system the negative slope value (-1.4), which holds for the range 72.5 > £1v > 60 m N / m may be interpreted as £s~ being greater than both I'sv and Pxv. Syntex has a rather long hydrocarbon chain, i.e., R = C12H23 (1). In a low concentration region, its molecules are likely to lie prone to the surface through hydrophobic bonding between the hydrocarbon group and the crystal face of molybdenite. In more concentrated solutions (3% < 60 mN/m), additional molecules are adsorbed at the three interfaces. The slope assumes a value of nearly zero which suggests that now Psv = Fs~. At ~flv = 31 m N / m (-~7c), the surfaces presumably become saturated with polar groups so that no contact can be established between the bubble and the molybdenite surface. A finite value of Fsv considered here requires the transfer of solute to the solid/gas interface by several mechanisms which have been suggested by Blake (26). However, if rsv is negligibly small due to any reason (e.g., nonvolatility of Syntex) a slope of zero may also be interpreted as P~l Fly. Polar Characteristics

Virtually all hydrophobic solid surfaces possess some polar sites due to surface oxidation or the presence of impurities. For example, on Teflon 6, about 1 in 200 surface sites is polar and can accept highly polar adsorbates such as water (32). The % value of polyethylene has been found to increase with an increase in the surface concentration of polar sites due to various pretreatments (33, 34). Polar characteristics may alternatively be evaluated by recently developed semiempirical approaches. The method suggested by Hamilton (35) involves the use of octane contact angles measured on hydrophobic solids in disJournal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

510

S. KELEBEK

tilled water. This approach makes use of Fowkes' equation expressing the components of the solid-liquid interfacial free energy (36) with an additional term, lsw, introduced by Tamai et aL (37) to account for nondispersion forces (e.g., polar interactions). This equation when combined with Young's equation for the solid-octane-water contact angle (0ow) leads to the expression cos 0ow-

(Vw - Vo - Isw)

,

[4]

~Yow

where 3"w and 3'0 are measurable surface free energies (or surface tensions) of water and octane phases and "Yawis the interracial free energy between the two phases. Hamilton (35) inferred that 0ow (measured through the octane phase) would be 50 ° for solids interacting only through dispersion forces and larger for solids which m a y interact also by polar sites. Bagnall and Green (38) modified this conclusion by introducing new values of surface tensions. The term Isw in Eq. [4] has been replaced by a geometric mean in which 3"~ and 3"°ware the polar components of the surface free energy of the solid and water, respectively. The resulting equation is c o s 0ow =

(3"w- 3"0 - 2 ~ )

[5]

anal solutions in Table I. It is notable from the table that for all specimens studied 3,~p regularly increases as fl increases. However, this does not seem to be the case for 3"c; for some specimens it decreases in spite of an increase in 7~p and ft. The relationship between j5 and 7~ is represented by a curve as shown in Fig. 5. However, when [3 is plotted as a function of the cubic root of %0 a linear relationship becomes apparent, fl = 0.55(3,~P)~/3 - 1.06.

There is some scatter of data points. Regression analysis has indicated a correlation coefficient of 0.993. Increasing the order of root to 4 has not improved linearity of the data points. Recently the extent of polarity for a number ofhydrophobic minerals including the samples considered here has been alternatively characterized using the fractional area occupied by hydrophilic sites, Xhl. The Xhl values as estimated by employing Cassie's equation have then been related to fl and that following expression has been proposed (14), fl = 2Xhl- 1.

3,sp = 12.70(1 - cos 0ow)2.

fl = 0.56(7D ~/3 - 1.

[6]

According to this equation, complete spreading of an octane droplet on a solid surface in water (i.e., 0ow = 0) means that the 3"~ value of the solid is zero. In contrast, a solid surface in water, which cannot establish any contact with octane droplets (0ow = 180°), is represented by a m a x i m u m 3"sp value of 50.8 m J / m 2. For the crystal faces of sulfur and molybdenite (Fig. 2) the 3"~ values have been estimated to be 1.2 and 3.1 m J / m 2, respectively. The 3'~ values of various specimens are compared with their wetting parameters in methJournal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

[8]

The following version of Eq. [7] conforms to the conditions fl = - 1 , 3"~ = 0 m j/m2; and fl = 1, Vsp = 50.8 m J / m 2,

3"ow

Using the numerical values of 3'w, 3'0,~3"ow, and 3,v~and rearranging Eq. [5] for 3"~Pleads to the reduced form

[7]

[9]

TABLE I Comparison of the Polar Components, 3"~,of Inherently Hydrophobic Minerals with Their Wetting Parameters Mineral sample

/5'

(mN/m)

(m J/m2)

Sulfur ( c f ) a Molybdenite (cf) a

-0.48 -0.43 0.05 0.34 0.50 0.71 1.00 1.00

27 29 33 42 35.5 39.5 >--72.5 ~>72.5

1.2 3.1 6.7 15.0 21.2 37.0 50.8 50.8

Sulfur disk Molybdenite disk Talc disk Stibnite Molybdenite edge Talc edge

Note. Slope/3 and critical surface tension of wetting, 3'~, obtained using aqueous methanol solutions. a cf, crystal face.

511

CRITICAL SURFACE TENSION OF MOLYBDENITE AND SULFUR Polar 10

Component,

'](P s 40

20

50

IO O.B

cO_ 0 6 .5 -.i

0.4

~

O~

O0

~ -0.2 .c

L,__--0.4 o

n

/

0

o. -0.6

2

-0.8

-Lol

I

I

I

I

I

I

I

05

[0

1.5

2.0

2.5

5,0

3.5

FIG. 5. The relationship between the slope of adhesion tension line (obtained using aqueous methanol solutions) and the polar component of the surface free energy obtained for various hydrophobic minerals.

Fig. 6 it may be noted that for both molybdenite samples, the lower limits of surface tension of floatability, yell, are practically the same (i.e., 28-29 mN/m). This surface tension value correlates well with the 3'c obtained for the molybdenite cleavage plane (29 raN/m). One of the main reasons for this correlation is attributable to the fact that methanol solutions establish their apparent surface tension equilibrium rapidly. With long-chain surfactant solutions a lack of correlation is likely because their surface tension equilibrium may not be reached within the time scale of particle capture during flotation. In this case, high recoveries may still be possible even at a surface tension corresponding to 3'c determined by the captive bubble contact angle measurements. Recently reported results (13) involving the use of decyl xanthate solutions appear to support this possibility, which needs to be further investigated. The correlation between flotation and wetting of sulfur is also notable (Fig. 3). The value of'~clf is essentially the same as the 3'c value obtained with a crystal surface (27

Elimination of ;3 from Eqs. [8] and [9] leads to a simple relation between ;3 and xh~, ~

=

50.8x~1.

.....

mo

~

~

-

[10]

It is of interest to find out the extent of applicability of this empirical relation for other materials. An improvement of this kind of expression is likely. In the present work, its formulation is based on only eight data points.

Correlation between Flotation and Wetting The floatabilities of sulfur and molybdenite samples are shown as a function of surface tension in Figs. 3 and 6, respectively. Over a certain surface tension range the recoveries drastically decrease from 100% to nearly zero. This flotation behavior has also been observed for a number of other hydrophobic solids (10, 15, 39). The floatable/nonfloatable transitions are directly related to the nonwettable/wettable transitions of the solids, which may be characterized by critical surface tension. From

8O

:~ 60 0 ¢o L

0

0

0

4O

.9

o 2O /

50

9 Ground sample

f

t

t

l

40

50

60

70

Surface Tension (rnN/m)

FIG.6. Floatabilityofmolybdenitein aqueousmethanol solutions of varyingsurfacetensions. Journal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

512

s. KELEBEK

mN/m). It has fortuitously been found that a disk sample prepared by compacting a powdered sulfur sample produced an adhesion tension line which could be related to the upper limit of surface tension of floatability, %r~f, by assuming a small contact angle for flotation, 0of (11). A % value of 31.5 m N / m reported recently (10) for a sulfur sample (preparation procedure not specified) falls within the two "rc values reported in the present work for crystal and disk specimens (Table I). At this point, it is interesting to mention an early reference to the flotation behavior of some inherently hydrophobic minerals. While examining the flotation of flowers of sulfur at the distilled water surface Edser (40) observed the difficulty in sinking the particles of sulfur. "On adding a solution of sodium oleate, in successive small quantities, the flotation of the sulphur was observed to diminish progressively. • . ." None of the sulfur floated when the solution surface tension was 28 mN/m. This surface tension is in good agreement with %if and % (crystal surface) reported here. The results of similar experiments carried out recently using well-defined beads with hydrophobic surface coatings also showed good correlation with % for flat surfaces of the same materials (12).

expected to show variations, for example, in deformity, surface roughness, shape and chemical composition. The overall heterogeneous character of the ground flotation feed may be evaluated in terms of critical surface tension distribution. The percentage recovery versus surface tension curves given in Fig. 6 essentially represent the cumulative distributions. The percentages on the ordinate axis show in effect the critical surface tensions in the distribution that are equal to or less than the value indicated on the abscissa. Alternatively these data may be expressed in histogram form as shown in Fig. 7. It may be noted from these histograms that only a small fraction of particles (<5%) have surface tensions that correlate well with % obtained using the cleavage plane of molybdenite. These particles are probably the ones whose surfaces have been least affected by grinding. The centers of distributions (medians) for the cut and ground samples are 30.5 and 46.5 mN/m, respectively. The former value is close to % of the cleavage plane while the latter resembles to some extent % of the disk specimen prepared from the ground sample (Table I). As mentioned previously, grinding produces

-]

Critical Surface Tension Distributions 60

The critical surface tension range of floatability characterized by its lower and upper limits reflects various degrees of wetting of particles in flotation. This critical range is much narrower for the molybdenite sample prepared by cutting its layers (i.e., nonground sample) than for the sample powdered using a pulverizer (Fig. 6). Evidently this difference is brought about by a direct consequence of grinding. The effect of communition on various surface properties of minerals has recently been reviewed (41, 42). Due to repetitive impacts prior to actual size reduction the lattice of minerals is subjected to alterations both physically and chemically. Therefore, the molybdenite particles produced by grinding are Journal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

5o w ~ ,o ~, Q. ~ 3o ~" ~ zo la_

I

4

~o 28

32

30

40 ~=f (mN/rnJ

50

60

FIG.7. Frequencyhistogramillustratingthe criticalsurface tension distributions for the nonground (left) and ground (fight)samplesof molybdenite.

CRITICAL SURFACE TENSION OF MOLYBDENITE AND SULFUR

particles which show differences both physically and chemically. These differences are likely to cause contact angle hysteresis. As discussed by Marmur et al. (12) this phenomenon may also be a contributing factor for the occurrence of a wide range of surface tension floatabilities in the case of ground sample. In a future work it may be interesting to compare the extent of correlation between floatability and wettability when the contact angle data are obtained using advancing and receding angles. SUMMARY AND CONCLUSIONS

The wettability of molybdenite and sulfur in aqueous methanol solutions has been evaluated using the adhesion tension diagrams. These diagrams indicate the critical surface tension of wetting values, % of 27 and 29 mN/ m, and slope values, fl o f - 0 . 4 8 and -0.43, for the crystal faces of sulfur and molybdenite, respectively. Polar characteristics of various samples have been evaluated in terms of the so-called polar component of their surface free energy, ysp. The 7~p values found for the crystal faces of sulfur and molybdenite are 1.2 and 3.1 mJ/m 2, respectively. The greater hydrophobic nature of sulfur is reflected by its smaller %,/3, and %vvalues. This is consistent with the structural features of the two minerals. The following empirical expressions have been found between fl and 7 p, and between %Pand Xhl

fl = 0.56(Ts°) 1/3 -- 1 ~ sp = 5 0 . 8 ( X h l ) 3.

The wettability of the molybdenite face sample in Syntex solutions has also been studied. The Syntex system is characterized by two slopes, -1.4 and ~0, and has a Tc of ~31 mN/m. The flotation behavior of the samples in aqueous methanol solutions has been discussed in reference to Zisman's critical surface tension of wetting. The lower limit of the critical surface tension range of floatability, %~f, has been found to correlate well with yc values of the crystal faces. The flotation test data for

513

two samples of molybdenite have been employed to evaluate the critical surface tension distribution of particles. In contrast to a ground sample, the sample prepared by cutting the molybdenite layers has shown a very narrow distribution due to its homogeneity. The effect of grinding on the wetting properties of molybdenite has been illustrated using the frequency histograms constructed. ACKNOWLEDGMENTS The author acknowledges and thanks the NSERC (Canada) and TDCI (Turkey) for financial assistance and The Climax Co. (CO) for supplying the Syntex sample. REFERENCES 1. Hoover, R. M., and Malhotra, in "Notation: A, M. Gaudin Memorial Volume" (M. C. Fuerstenau, Ed.), Vol. 2, pp. 485-505. AIME, New York, 1976. 2. Trahar W. J., Int. J. Miner. Process. 11, 57 (1983). 3. Luttrell, G. H., and Yoon, R. H., Int. Z Miner. Process. 13, 271 (1984). 4. Walker, G. W., Waiters, C. P., and Richardson, P. E., Int. J. Miner. Process. 18, 119 (1986). 5. Arbiter, N., Fujii, Y., Hansen, B., and Raja, A., in "Advances in Interfacial Phenomena of Particulate/Solution/Gas Systems: Application to Notation Research" (P. Somasundaran and R. B. Grieves, Eds.), Vol. 71, pp. 176-182. AIChE Symposium Series, No. 150, 1975. 6, Chander, S., Wie, J. M., and Fuerstenau, D. W,, in "Advances in Interfacial Phenomena of Particulate/Solution/Gas Systems: Application to Flotation Research" (P. Somasundaran and R. B. Grieves, Eds.), Vol. 71, pp. 183-188. AIChE Symposium Series, No. 150, 71/150, 1975. 7. Parekh, B. K., and Aplan, F. F., in "Recent Developments in Separation Science" (N. N. Li, Ed.), Vol, IV, pp. 107-113. CRC Press, West Palm Beach, FL, 1978. 8. Hornsby, D. T., and Leja, J., Colloids Surf. 1, 425 (1980). 9. Hornsby, D. T., and Leja, J,, Colloids Surf 7, 339

(1983). 10. Yarar, B., and Kaoma, J., Colloids Surf. 11, 429 (1984). 11. Kelebek, S., and Smith, G. W., Int. J. Miner. Process. 14, 275 (1985). 12. Marmur, A., Chen, W., and Zografi, G., J. Colloid Interface Sci. 113, 114 (1986). 13. Kelebek, S., Finch, J. A., Yrrtik, S., and Smith, G. W., Colloids Surf. 20, 89 (1986). 14. Kelebek, S., Inst. Min. Metall. Trans. 96, C103 (1987). Journal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

514

S. KELEBEK

15. Kelebek, S., Smith, G. W., Finch, J. A., and Y6riik, S., Sep. Sci. Technol. 22, 1527 (1987). 16. Dickenson, R. G., and Pauling, L., Y. Amer. Chem. Soc. 48, 1466 (1923). 17. Abrahams, S. C.,Acta Crystallogr. 8, 661 (1955). 18. Pauling, L. "The Nature of the Chemical Bond and the Structure of Molecules and Crystals," 3rd ed. Cornell Univ. Press, Ithaca, NY, 1960. 19. Wells, A. F., "Structural Inorganic Chemistry," 3rd ed. Oxford Univ. Press (Clarendon), Oxford, 1962. 20. Kelebek, S., Colloids Surf. 28, 219 (1987). 21. Zisman, W. A., in "Contact Angle, Wettability and Adhesion," pp. 1-51. Adv. Chem. Ser. 43, Amer. Chem. Soc., Washington, DC, 1964. 22. Bernett, M. K., and Zismaia, W. D., J. Phys. Chem. 63, 1241 (1959). 23. Lucassen-Reynders, A. H., J. Phys. Chem. 67, 969 (1963). 24. Johnson, R. E., Jr., and Dettre, R. H., in "Surface and Colloid Science" (E. Matijevic, Ed.), Vol. 2, pp. 85-153. Wiley-Interscience, New York, 1969. 25. Owens, N. F., Richmond, P., Gregory, D., Mingins, J., and Chan, D., in "Wetting, Spreading and Adhesion" (J. F. Padday, Ed,), pp. 127-145. Academic Press, New York/London, 1978. 26. Blake, T. D., in "Surfactants" (Th.F. Tadros, Ed.), pp. 221-275. Academic Press, London/New York, 1984. 27. Pyter, R. A., Zografi, G., and Mukerjee, P., J. Colloid Interface Sci. 89, 144 (1982).

Journal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

28. Fowkes, F. M., and Harkins, W. D., J. Amer. Chem. Soc. 62, 3377 (1940). 29. Wada, M., Sci. Rep. Res. Inst. Tohoku Univ. Ser. A 2, 102 (1950). 30. Bargeman, D., and Van Voorst Vader, F., J. Colloid Interface Sci. 42, 467 (1973). 31. Fowkes, F. M., MacCarthy, D. C., and Mostafa, M. A., J. Colloid Interface Sci. 78, 200 (1980). 32. Zettlemoyer, A. C., J. Colloid Interface Sci. 28, 343 (1968). 33, Allan, A. J. G., J. Polym. Sci. 38, 297 (1959). 34. Baszkin, A., and Ter-Minassian-Saraga, L., J. Colloid Interface Sci. 43, 190 (1973). 35. Hamilton, W. C., J. Colloid Interface Sci. 40, 219 (1972). 36. Fowkes, F. M., Ind. Eng. Chem. 56, 12, 40 (1964). 37. Tamai, Y., Makuuchi, K., and Suzuki, M., J. Phys. Chem. 71, 4176 (1967). 38. Bagnall, R. D., and Green, G. F., J. Colloid Interface Sci. 68, 387 (1979). 39. Hornsby, D. T., and Leja, J., CoalPrep. 1, I (1984). 40. Edser, E., in "Colloid Chemistry and Its General and Industrial Applications," Report IV, pp. 263-326. Department of Scientific and Industrial Research, London, 1922. 41. Laskowski, J., in "Surface and Colloid Science" (E. Matijevic, Ed.), Vol. 12, pp. 315-357. Plenum, New York, 1982. 42. Leja, J., "Surface Chemistry of Froth Flotation," pp. 352-356. Plenum, New York/London, 1982.