S O M E C O N S I D E R A T I O N S ON T H E INTERACTIONS B E T W E E N PARTICLES IN T H E P R E S E N C E OF A WALL-FLOCCULATION 1 A N D A D H E S I O N AS INFLUENCED BY THE CUMULATIVE NET WEIGHT OF T H E PARTICLES 2

Max Bender 3

Chemistry Department, New York University, New York, New York

Received June 14, 1954

~kBSTRACT

Experiments with aqueous zinc sulfide suspensions illustrate that the height of the suspension can be an important factor influencing the degree of adhesion of the par- ticles which have settled to the glass container bottom. In alkaline medium where zeta potential measurements and shallow cell microscopic observations show rela- tively strong repulsion between individual particles and glass and between particles themselves, there being no flocculation, it was observed that much adhesion of par- ticles to the container bottom developed after settling, if the suspension was rela- tively tall. On the other hand, in the acid medium, where the zeta potential measure- ments and shallow cell observations show attraction between glass and individual particles but still slight repulsion between particles themselves and therefore no flocculation normally, the effect of increased suspension height was to decrease the adhesion at the glass bottom while bringing about flocculation.

INTRODUCTION

For a suspension, originally nonflocculated, when the to ta l number of part icles sett l ing into the ne ighborhood of the container b o t t o m is no greater t han t h a t corresponding to a ]ayer one part icle high on the bo t tom, it is expected t h a t the degree of adhesion to the b o t t o m will be direct ly dependent on the balance of electrostat ic and nonelectrostat ic repulsion- a t t r ac t ion forces between the particles and the glass surface and also indi- v idual part icle characterist ics, such as Brownian movement , weight, size, and shape (3, 7, reference in footnote 2).

Bu t if the to ta l number of particles corresponds to a heavier sediment,

1 Flocculation (coagulation) is defined in all this work as the grouping together of the majority of the "individual" particles of a suspension, which are mostly mi- croscopic (1-10 p) and submicroscopic (< 1 tL) in size, to become macroscopic (> 100 ~) in size.

2 The data cited are from Bender, h~. : "Brownian Movement--Electrical Charge- 7- Flocculation," Ph.D. Dissertation, New York University (1949). See also reference (2).

3 American Cyanamid Company, Bound Brook, N. J.

359

3 6 0 MAX BENDER

whether due to increased height of the suspension or greater particle coneen- tration~ it may become necessary to reckon with still another factor in order to understand the adhesion. This is the cumulative net weight of the particles one on top of another, and it can bring about not only adhesion results quite different and opposite to those expected but also flocculation.

This is important to consider in connection with suspensions in general. Also it should be useful in ore flotation research, where one may be studying the relationship between the degree of particle adherence at the bottom of a container, the zeta potential, and the degree of flocculation, in various electrolyte media and interpreting this in terms of the tendency of the particles to "float." Papers by Bankoff (1), Kellogg (6), Taggart et al. (8), and Sun (7) are interesting in this connection.

The following experiments with sphalerite particles in alkaline and in acidic media are illustrative of the importance of the "cumulative weight" factor.

EXPERIMENTAL

Sphalerite suspension was prepared by first dry-grinding the mineral in a "Diamonite" mortar and pestle, then pasting with a small amount of distilled water and letting this down with more water to 0.025 % solids by weight. This "master" suspension was then diluted 50:50, respectively, with O.O02M Na2COz and O.O02M tt2S04 to give the preparations now under consideration. Most of the particles were between 0.1 and 0.5 ~ in radius. Neither suspension was flocculated when first prepared, the degree of dispersion appearing to be the same as judged by comparative settling rates and examination with the microscope.

Five different suspension heights were considered for each of the two preparations. The lowest was Jess than 35 ~ and consisted of the "shMlow cell" which is prepared by placing a microscope cover glass over a drop of the suspension on a glass slide. Here the extent of the adhesion was deduced on the basis of microscopically observing whether the particles were in Brownian movement, since adhering particles do not exhibit this activity (2). The next lowest height was about 1 mm. and corresponded to droplets of the suspensions positioned on glass slides with no cover glasses. The other heights, namely 2.5, 4.0 and 5.0 cm., respectively, were the suspension heights in 50 cc. beakers and also 10 cc. vials, 6 cm. high. Degree of ad- hesion for all experiments outside the shallow cells was judged visually after drawing off the preparation, lightly rinsing the surface, and drying over a flame. All suspensions were well shaken before any settling-ad- herence studies were made. Note that prior to use, the glassware was first washed with soap and water and rinsed, then treated with chromic acid solution, followed by thorough rinsing with distilled water and finally alcohol.

I N T E R A C T I O N S B E T W E E N P A R T I C L E S 361

TABLE I Adhesion of Sphalerite to Glass in O.O01M Na2CO, and in O.O01M H2S04

as a Function of Suspension Height

Expt. No.

1

2 ~

3

4 5

Height of suspension

<35~

About 1 ram.

2.5 cm. (beaker)

4.0 em. (vial) 5.0 era. (vial)

Period of settling

1-2 min.

20 min.

3 hrs.

Several hours 20 min. 2 weeks

Degree of adhesion

In 0.001M Na~CO,

Practically none

Some deposit

Much

Much Much Very dense ad-

hering com- pact cake

In 0.001M H2SO4

Practically all the particles

More deposit than in Na~CO3

Quite small visually

Visually little or none Visually little or none Granular loose cake

coming readily off the bottom

Droplets taken from preparations of No. 5 after shaking after the 20-minute settling. No. 2 was repeated by first placing single drops of 0.002M Na2COa and of 0.002M H~SO~ on the slides and to these solutions adding one drop of the 0.025% "master" suspension. Now the adhesion was about the same from the acid as from the alkaline media.

The experiments which were made are summarized in Table I. These da ta illustrate very well tha t the degree of adhesion can vary extremely, depending on the height of the suspension. The periods of settling (prior to examination of the container bot toms for adhesion) were of sufficient durat ion to allow most all the partie]es of the suspensions to settle fairly close to each other at the bot tom. With further passage of t ime the degree of adhesion changes but little except for the taller alkaline suspensions where the cake becomes more firmly packed and adhering. In Exper iment No. 5, the extreme of two weeks caused the alkaline cake to become so dense tha t vigorous shaking was required to resuspend it. However, the original degree of dispersion was obtained, although, as described below, this was not quite so with the acid sediment.

Flocculation was not evident a t all for the alkaline suspensions. The acid ones, however, al though not flocculated when first prepared, became so when a sufficient number of the particles (in the case of the taller suspen- sions) settled into close proximity in the vicinity of the bo t tom of the container. This could be seen for instance in Exper iment 5, when the vials were carefully inverted after standing for about 20 minutes. The acid sediment was granular and flocculated, its coarseness relative to the alkaline sediment (which went readily back into suspension in the form of a cloud of fine particles) being very obvious.

Nevertheless, whatever flocculation was observed was not completely of

362 ~Ax BENDER

TABLE II Electric Charge of Sphalerite Particles and Glass Surface in Alkali and Acid

0.001M Na~COa 0.001M H2S04

Sphalerite

c.v .~

--2.11 --27.5 --1.12 --14.6

C.V. ~

--2.95 --0.38

Glass

-38.3 --4.9

a Cataphoretic velocity in microns/sec./volt/cm. b Zeta potential in millivolts. Calculated from C.V. by the Helmholtz-Smolu-

chowski equation.

permanent nature because the floccules could be broken up again into indi- vidual particles by shaking. This was true for acid suspension settling for 20 minutes as well as for suspension settling for two weeks. Actually, the original degree of dispersion was not quite obtained on shaking, this being more true in the situation of the two weeks settling. For instance, although most all the floccules were broken up on shaking, it could be seen visually and by microscope that these individual particles so obtained were on the whole somewhat coarser than originally.

DIscussIost

An explanation of how the height of the suspension can have such pro- nounced effect on the adhesion in either the alkaline or the acid medium and also on the flocculation in acid medium, may be sought with the aid of theoretical considerations such as have been made by Hamaker (5) and Verwey and Overbeek (9) applied to the electrokiuetics in the present sys- tems. Table II gives these data for the particles and the glass surface as obtained with a Northrup-Kunitz cataphoresis cell (2).

The appreciable repulsion between particles and glass in the alkaline medium is the reason for there being no adhesion of particles in the alkaline shallow cell. In the acid shallow cell, however, with the glass ahnost at the isoelectric point, 4 there is adhesion.

Flocculation was absent in the alkaline medium owing to the high mutual repulsion between the particles. In the acid medium with the particle charge being less negative and the repulsion between particles accordingly less, there was flocculation but only in the taller suspensions after they had been allowed to settle. These floccules could be broken up by vigorous shaking, although not quite to the original degree of dispersion, indicating some strong attraction by virtue of the particles' penetrating into a mini- mum energy field.

This difference in flocculation in the acid medium depending only on the

4 With greater acidity the charge of the glass becomes positive. For instance, at 0.002M H2S04 the zeta potential is -{-16.6 millivolts.

INTERACTIONS BETWEEN PARTICLES 363

suspension height must be explained in terms of the greater cumulative net weight of particle on particle in the taller suspensions. With the in- creased force due to gravity, the particles can penetrate the comparatively low repulsion barriers of each other, the breakthrough being so rapid that they attach at random, that is, at first sight so to speak, thus giving the flocculated loose structure observed.

Meanwhile the adhesion to the bottom (or the cohesion with other itoccules) becomes practically nil, because any area of contact of the volu- minous and irregular shaped floccules with the glass (or other floccules) is so small compared to the over-all floccule surface and weight that they do not hang on in the face of any degree of disturbance such as inverting the container or rinsing (see Ehrenberg (4); yon Buz~gh (3), p. 166-7; Hamaker (5) ; Verwey and Overbeek (9), p. 15, 17; and Sun (7)).

In the tall alkaline suspensions despite the higher zeta potentials in the system there is also penetration of the repulsion barriers owing to the cumulative net weight of the particles. But now, with the stronger re- pulsion between particles and particles and glass bottom, this action is quite slow compared to tha t in the acid medium. The particles, being in Brownian movement, move about and slide over each other and over the glass bottom as they are forced closer with settling. There is time to squirm and fit into crevices before the attraction forces prevail so that a compact sedimentation cake in close contact and strongly adhering to the con- tainer bottom eventually results, 5 I t is interesting that the alkaline sedi- mentation cake, unlike the acid floccules, does revert to the original degree of dispersion on shaking, thus showing that the repulsion barriers have not yet been too strongly overstepped. Continued settling as in Experiment 5 (two weeks) brings the particles more into the field of minimum energy, as is evident from the more compact cake more difficult to redisperse. Evidently, centrifuging would probably bring the particles so close to each other tha t there would no longer be reversibility of dispersion (see t tamaker (5)).

I t is thus seen that the height of a suspension may have much to do with the adhesion of particles at the bottom and also any flocculation. Explanation is given. These possibilities should be carefully considered in connection with adhesion and flocculation studies in general.

]:~EFERENCES 1. BANKOFF, S. G., Am. Inst. Mining Met. Engrs., Tech. Publ. No. 1391 (1942). 2. BENDER, M., AND ~[OUQUIN, H., J. Phys. Chem. 56, 272 (1952).

5 With a still higher repulsion barrier, the formation of this compact adhering sedimentation cake is at least delayed. This was shown by adding one drop of Tergitol penetrant surface active agent (Carbide and Carbon Chemical Corporation) per 25 cc. suspension and allowing to sediment at 2.5 cm. height. No adhesion occurred. In the acid medium with the same concentration of agent and the same height there was a slight deposit, but this was very small compared to the original acid suspension.

3 6 4 MAX BENDER

3. BUZ~Gtt, A. YON, "Colloid Systems." Technical Press, Ltd., London, 1937. 4. EHRENBERG, P., "Bodenkolloide," p. 83ff. Dresden, 1918. 5. ttAMAKER, It. C., Rec. tray. chim. 55, 1015 (1936). 6. KELLOOG, H. H., Am. Inst. Mining Met. Engrs., Tech. Publ. No. 1841 (1945). 7. SUN, S., Am. Inst. Mining Met. Engrs., Tech. Publ. No. 1580 (1943). 8. TAGGART, A. :F., TAYLOR, T. C., AND KNOLL, A. F., Trans. Am. Inst. Mining Met.

Engrs. 87, 217 (1930); or Am. Inst. Mining Met. Engrs., Tech. Publ. No. 312 (1930).

9, VERWEY, E. J. W., AND OVERBEEK, J. WE[. G., "Theory of the Stability of Lyopho- bie Colloids." Elsevier, Amsterdam, 1948.