Electroflotation of colloids without surfactants

Electroflotation of Colloids without Surfactants C. MANOHAR, V. K. KELKAR, AND J. V. Y A K H M I Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400085, India Received June 2, 1981; accepted J a n u a r y 4, 1982 It is shown that the electroflotation of colloidal Fe(OH)3 does not require any surfactant and that the flotation efficiency is independent of the surface charge of the colloid. It is further shown, using powdered activated carbon, that coagulation is the sufficient condition for flotation provided that there are a large number of small bubbles. A formula for flotation efficiency based on the coagulation theory is derived and it is shown that this model qualitatively describes all the observed aspects of flotation.

hydrophobic, and thus forcing the colloid to the surface of the rising bubble (4). However, in practice both mechanisms operate. In this paper it is shown that one can float the colloids without surfactants provided one uses very small bubbles and simple coagulants like alum, etc. The mechanism for flotation also operates in an entirely different way. For instance, the efficiency of flotation appears to be independent of the surface charge (as shown later). This immediately implies that this way of floating the colloids is not useful in mineral beneficiation where the surface charge plays an important role in selectivity. In effluent treatment, selectivity is not needed; one has to remove all the suspended particles to the specified limits irrespective of their surface charge. It is this flexibility which has been taken advantage of in the present context. From this point of view the use of surfactants in flotation for effluent treatment is an undesired hangover from its previous use in mineral flotation. In fact the possibility of eliminating the surfactant has been indicated in experiments on the use of coagulants like alum along with the surfactants. In these experiments it was observed that the use of alum increased both the range and efficiency of flotation (5). Looking back on these results now, in view

INTRODUCTION

Recently, the technique of flotation has emerged as a unit process for effluent treatment (1). The adsorbing colloid flotation, in particular, has been shown to be a powerful technique for the removal of traces of toxic metals from effluents, on both laboratory and pilot plant scale (2). The technique of adsorbing colloid flotation involves the preparation of a suitable colloid, like Fe(OH)3, AI(OH3), HgS, etc., in the effluent in order to adsorb the toxic metal ions on the surface and then effect the solid-liquid separation by flotation after adding a suitably chosen surfactant. One of the major hindrances for the scaleup of this technique is the high cost of the surfactants. It has been argued that the use of a surfactant with a proper charge, i.e., opposite to the surface charge on the colloid, is essential to effect such a flotation. There are two theoretical models which have been suggested to describe the process of flotation. According to the first model (3), the surfactant is first adsorbed at the gaswater interface charging electrically the bubble surface with a charge opposite to that of the colloid, so that the colloid is attracted to the rising bubble. The second model assumes that the surfactant is first adsorbed on the colloid surface, making the surface 54 0021-9797/82/090054-07502.00/0 Copyright © 1982by AcademicPress, Inc. All rights of reproductionin any form reserved.

Journal of Colloidand InterfaceScience, Vol. 89, No. 1, September 1982

ELECTROFLOTATION

of present work, it appears the surfactants possibly served the purpose of reducing the bubble size. In the present paper, small bubbles are generated by electrolysis and perhaps similar results will be obtained by dissolved air flotation. The present work is reported in three stages. First it is shown that the colloidal ferric hydroxide can be floated without surfactants over a wide pH range including the point at which surface charge changes sign (PZC). This gives a clue that the bubblecolloid interaction is not electrostatic in nature. In the second stage, it is shown, by choosing the system of powdered activated carbon (PAC), that flotation can be effected without surfactants provided the suspension of PAC is coagulated with alum or ferric hydroxide. These experiments suggest that the coagulation is a necessary step for flotation. The other condition, it is argued, is that there should be sufficient density of bubbles to effect flotation. In the third stage, the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory (6) is used to derive curves for flotation efficiency and the effect of parameters like concentration of coagulants and strength of van der Waals interaction are discussed. In the last part of the paper an attempt has been made to comment on how best to exploit the mechanism for colloid flotation and several examples to support this conjecture are indicated. EXPERIMENTAL

Conventionally for flotation using surfactants, one uses a sintered disk of fine porosity at the bottom of a glass column and, a suitable gas, at a controlled flow rate, is forced through the sintered disk into the column from a gas cylinder. Such systems are described in detail in a number of references ( 1). However, in the present work a flotation cell that uses electrolysis for the generation of fine gas bubbles has been employed. The cell is a glass jar of about 1.5-liter capacity, with a height of 30 cm, in which two parallel

OF COLLOIDS

55

graphite rods of 5-mm diameter are fitted horizontally to serve as electrodes. An outlet for taking samples is located about 1 cm above the electrodes. A dc voltage supply is connected to the electrodes and the amount of gas is controlled by adjusting this voltage. A very fine stream of bubbles emerges from the electrodes and rises slowly upward. Although, mostly in Russian literature, there has been mention of using electrolysis for generation of gas, no scientific study of such a process for studying the mechanisms of flotation seems to have been reported (7). In particular the point that surfactant is not necessary has not been stressed. Initially a number of colloids, e.g., HgS, CuS, and Fe(OH)3 were prepared and flotation experiments were attempted with the electrolysis cell and the sintered disk cell without adding any surfactant in both the cases. It was observed that all these colloids could be floated in the electrolysis cell but not in the sintered disk cell. Thereafter, it was decided to concentrate on just one colloid, namely, Fe(OH)3, and study in detail its flotation in the electrolysis cell as a function of pH, with a view to study the underlying mechanism. AR grade ferric nitrate and distilled water were used for the preparation of Fe(OH)3 colloid. The cell was operated at 20 volts and the current, as expected, varied with the pH of the solution. The pH was adjusted to the desired value by adding sodium hydroxide or hydrochloric acid. Flotation experiments were performed, batchwise, on l-liter samples having an Fe 3+ concentration of 100 ppm. Iron was estimated by colorimetry using the phenanthroline method (8). The experiments were performed in the pH range 4 to 12. Figure 1 shows the flotation efficiency for iron for a number of pH values. Above pH = 4.5 this plot remains virtually flat, right up to pH = 12, which was the highest pH studied. On the other hand, on going from pH 6 to pH 4 the curve shows a considerable dip. As the pH is lowered to 4 more and more of iron is present as Fe 3+ ions rather than as colloidal Fe(OH)3 needed Journal of Colloid and Interface Science, Vol. 89, No. 1, September 1982

56

MANOHAR, KELKAR, AND YAKHMI

100

I

•

.

il 0

80 z uJ

~_

6o

u_ ui,i z

2

0 _.1

u.

3

40

20

o

I

I

I

I

I

I

I

I

I

4

5

6

7

8

g

10

11

12

13

I)H FIG. 1. Flotation efficiency of Fe(OH)3 without surfactants. Note that in contrast to Ref. (6) the efficiency does not go to zero at pH 9.5.

for flotation. Time required for flotation was 5 min and the current density was 0.02 A / cm 2. These results in Fig. 1 should be compared with the results of Grieves et al. (9), who floated Fe(OH)3 by using the conventional sintered disk cell and employing the surfactants sodium lauryl sulfate (SLS) and ethylhexadecyl dimethylammonium bromide (EHDA-Br). The flotation behavior using SLS starts out similar to that of Fig. 1 at pH 4 and follows our curve up to pH 9, but the efficiency goes to zero sharply at pH 9.5. Alternatively, the curve for EHDABr also follows that of Fig. 1 for pH values above 10. This has been attributed to a PZC of 8.5. The results in Fig. 1 are proof that the colloid flotation efficiency is independent of the surface charge on the colloid and the mechanism of the colloid-bubble attachment is not electrostatic in nature. There seems to be no need for making the colloid surface hydrophobic to enable bubble attachment. In fact, it is shown later (also see Ref. (10)) that partly due to surface tension a bubble attachment always lowers the surface energy and therefore is favored energetically. So, intuitively one should anticiJournal of Colloid and Interface Science, Vol. 89, No. 1, September 1982

pate that if a sufficient number of small bubbles attach to the colloid then it should be floated easily. This can happen if the colloidal particles are large enough to provide a sufficient number of sites for bubble attachment, i.e., if the colloid coagulates. Further, when the colloid coagulates in the presence of gas bubbles, the latter are "trapped" inside the coagulating floc and the flotation of the colloid occurs. It was tempting to check the possibility that coagulation by itself may imply flotation--provided there are sufficient number of bubbles. For this purpose the system of powdered activated charcoal (PAC) was chosen. The choice of this was influenced by several considerations. First, PAC mixes and disperses easily in water and can be coagulated using alum or ferric nitrate. Second, it has been shown that using a sintered disk, PAC cannot be floated unles surfactants are used (11). Third, the use of PAC is becoming unavoidable in effluent treatment whenever one has to deal with effluents containing nonbiodegradable chemicals and other organic chemicals, like phenol, which gives an unpleasant taste to water at low levels of concentration (12). However,

ELECTROFLOTATION OF COLLOIDS

3. Time required for flotation was about 5 min and the residual concentration of P A C was measured.

,100 ppm CHARCOAL + 2 5 p p m Fe

100 O

O

c//z/~ ,.////..

~,'////,

8O

////// /////i

8z

so

;/////, ~,'////,

4.0

~,'////

I/J

_~ It. la. tl.I

////////// ///// ///// / / / / / ///// /////

0

DISCUSSION

"/'//,4, ///// /I///

/ / / / / /////

O I/All. 4

5

6

I 7

I 8

I 9

10

11

57

12

FIG. 2. Results of jar test and flotation on powdered activated carbon (PAC) indicating that coagulation is sufficient for flotation. No coagulation or flotation took place in the shaded region.

using P A C is already suggested as a costly process unless regenerated and in addition using surfactants for flotation, as in Ref. (11 ), means additional cost. For the present experiments P A C m a r keted by E. M e r c k (India) Pvt. Ltd., was used as received, without passing through sieves. Potassium aluminium sulfate ( L R ) and ferric nitrate ( A R ) were chosen as coagulants. Two sets of parallel experiments were performed. In the first set, jar tests with varying amounts of one of the coagulants were performed to determine the m i n i m u m amount needed for coagulation. In the second set the flotation experiments were conducted with the same amount of coagulant as used in jar tests. Briefly the main conclusions were that wherever the system coagulated it also floated and the m i n i m u m concentration of coagulant required for coagulation was also the m i n i m u m concentration for flotation. For example, for a suspension of 200 m g / l i t e r P A C the m i n i m u m concentration of potash alum needed was between 640 to 680 rag/liter for both coagulation and flotation. It was found that ferric nitrate was a better coagulant and the results for this case are shown in Figs. 2 and

From the above experiments it was clear that in electroflotation, where the bubble size is small, no surfactant need be added and that the coagulation is sufficient condition for flotation. There has been brief mention of these important points in literature but detailed study does not seem to have been made (13). It can be shown that the bubble a t t a c h m e n t to a colloid reduces energy. For a bubble of radius r and the surface tension % the reduction in energy AW, when the bubble sits on a surface making a contact angle 0 is given by (10) AW : 4rrr23,{1 - l / f } ,

[11

where f =

2

4

+ 3 COS 0 -- COS3

0] 1/3

;

[2]

AWis positive for 0 > 0 and therefore energy is decreased on adsorption of a bubble. Suppose the same bubble is now broken into n

100 ppm

CHARCOAL, p H = 7

100

eo z 0 14. w N 4O

0

10

20

30

40

50

Fe CONCN.

60

70

80

90 100

(ppm)

FIG. 3. R e s u l t s o f j a r test a n d f l o t a t i o n a t fixed p H = 7 on P A C . N o c o a g u l a t i o n a n d f l o t a t i o n is s h o w n in

the shaded region. J o u r n a l o f C o l l o i d a n d I n t e r f a c e Science.

Vol. 89, No. 1, September 1982

58

MANOHAR, KELKAR, AND YAKHMI

smaller bubbles of radius rl, then 4

- Trr 3 =

4

nTrr 3

[3]

3

and if all these bubbles are adsorbed on the surface, then the reduction in the energy on adsorption will be AWl =

n.4r~3,{1 - l / f } .

[4]

Using Eq. [3], we obtain AWl

= nl/3.4rrr27{1

-- I / f } .

[5]

This shows that it is advantageous energetically to adsorb same volume of gas as smaller bubbles. As indicated above it is preferable to coagulate the colloid in the presence of small gas bubbles. First, the gas bubbles are adsorbed on the outer surface of the coagulated colloid. Second, the gas bubbles are "trapped" inside the coagulating floc. Floc is a loose structure and the mechanism to hold the gas bubbles inside the structure has to be looked at, perhaps, as adsorption of small bubbles on the inner surface of the floc as required by the energy minimization mentioned above. It has been already noted in literature that gas dispersion before adding coagulants increases the efficiency of the flotation (14). Thus co~igulation provides a sufficient number of sites for adsorption of gas bubbles. In the above arguments it is assumed that there is sufficient area on the colloid surface for the adsorption of n bubbles and that these bubbles remain attached to the surface. This latter assumption is valid if there is a large density of bubbles in the colloid surroundings such that despite the adsorption-desorption equilibrium, n bubbles, on the average, would remain adsorbed on the surface. In electroflotation, generation of a large density of bubbles is decided by the current density and the surface structure of the electrodes used for electrolysis. Preparation of the electrodes with suitable surface Journal of Colloid and Interface Science, Vol. 89, No. 1, September 1982

structure and ability to withstand hostile environments in actual plant circumstances is a topic by itself. Assuming that there is always a sufficient density of bubbles, the availability of sufficient surface sites is the deciding factor for the flotation efficiency in the laboratory experiments. Making available sufficient surface sites can be achieved by coagulation and possibly by floculation. From the above arguments and evidence, it is clear that coagulation itself is sufficient for flotation. Therefore, in the following pages a theoretical model dealing with the variation of flotation efficiency as a function of coagulant concentration on the basis of DLVO theory is described (5). A colloidal suspension does not coagulate because of the mutual repulsion between any two colloidal particles. The potential energy per unit area between any two flat parallel colloidal surfaces separated by a distance 2x is (5)

V = V A + VR -A 64nokT e_2X~ - 48~rx ~ + X

[6]

The first term represents the London attractive energy per unit area witth A being the Hamaker constant. The second term represents screened Coulomb repulsive energy per unit area; X is the inverse Debye radius and is given by X2 - 87re2no z2

DkT

'

[71

where no is the coagulant concentration and z is the valency of the coagulant ion and D is the dielectric constant of water. The potential energy per unit area has a peak value Vm which is a function of the temperature and the coagulant concentration no. Assuming that the colloidal particles before coagulation have a Maxwellian distribution of velocities, all the particles which have an energy E greater than energy corresponding to Vm coagulate and if this happens in the presence of small bubbles (sufficient in num-

ELECTROFLOTATION OF COLLOIDS

Equation [ 11 ] has to be solved for x to obtain x m at which the potential becomes maximum, i.e., Vm. In order to do this, we rewrite Eq. [ l l a ] using Eq. [7] as

COLLOID SIZE : 5000 ~2 100

I

I

!

I

80 z tu 60 o hLL ~40

N

7J

°

a3 e x p ( - 2 a ) -

Io II

"~1

20--

I

x

/

--1 10 O ---tl~

10-

30727rnokT [12]

In Eq. [12] a = X(Xm), A is in ergs, and C is no expressed in mole/liter. Now Vm can be written as

i

/

1 0

A)t 3

= 0.146A(C) 1/2 × 1012.

51

I I

59

V m = V A ( X m ) Jr V R ( X m )

FIG. 4. Flotation efficiencycalculated from Eq. [9] using Eqs. [12] and [13]; colloid size 5000/~. ber), as in flotation experiments, the bubbles adhere to the coagulated particles and float them. Therefore, the percentage fraction of the particles which coagulate and therefore float is given by f-

N(Em) - X 100 N

fE~ dE(.E) 1/2 e x p ( - E / k T ) = 100×

m

[8]

L

~ dE(E) ~/2 e x p ( - E / k T )

r

= IOOL1 - erf ( y ) + 2y exp(_y2)l, [91

= - 7 . 0 9 AC ~5 - ~5 ×

Now the plan is to use Eqs. [12] and [13] to evaluate Em and use this in Eq. [9] to obtain flotation efficiency. First a value for C is chosen and from Eq. [ 12] ~ is evaluated. This value is substituted in Eq. [ 13] to obtain Vm and thus Em. Figures 4 and 5 show the flotation efficiency calculated as the function of C for different values of A and colloid size. It is noted that the flotation efficiency rises abruptly and that on increasing the value of A, the minimum concentration decreases. Similar features have been observed in experiments on flotation using surfactants

where y = Em/kT, Em = Vm X area of colloid and e r f ( y ) is the error function. The value of Vm can be determined, as a function of the coagulant concentration, using Eq. [6] by solving for Vm from

dV = o. dx

--

Equation [10] can be rewritten as

dx

80 ~d.~;, , z uJ

6O

20

[ 64nok T'~ 2x x

+(-2X)~----~-)e

2 -

X

VA - 2hVR = O.

/

10"-12 erg

[llb]

!

-

/ ,

[]la]

/

_ A=lXl0 12erg

t, 40 tL hi

x

-

COLLOID SIZE = 1000 ~2 100

/ [lO]

[13]

10-z

10-1 C ~

/ 100

FIG. 5. Flotation efficiencycalculated from Eq. [9] using Eqs. [12] and [13]; colloid size 1000 A. Journal o f Colloid and Interface Science, Vol. 89, No. 1, September 1982

60

MANOHAR, KELKAR, AND YAKHMI

and have been interpreted as due to formation of hemimiceUes on the colloid surface. However, it should be realized that surfactants also can act as coagulants and further they also reduce the bubble size. So flotation efficiency would rise abruptly there also. This effect of varying chain length changes the nature of colloidal surface, thus changing A. CONCLUSION

In the present work it is shown that flotation could be effected without surfactants by using coagulants. Examples of Fe(OH)3 and powdered activated carbon are given in support of this. It is shown that coagulation is the sufficient condition for flotation provided gas bubbles are small and in a large number. A theoretical model based on DLVO theory of coagulation is formulated. It is argued that the role played by surfactants could be different from what it was presumed earlier. ACKNOWLEDGMENTS The authors are grateful to Dr. R. M. Iyer for encouragement. We are grateful to the referees, whose comments have put the manuscript in the proper perspective. REFERENCES 1. Lemlich, R. (Ed.) "Adsorptive Bubble Separation Techniques," Academic Press, New York, 1972; Wilson, D. J., Sep. Purif Meth. 7, 55 (1978).

Journal of Colloidand InterfaceScience. Vol.89, No. 1, September1982

2. Wilson, D. J., "EPA Technology Series EPA-600/ 2-77-072 (1977). 3. Wilson, D. J., Sep. Sci. 12, 447 (1977). 4. Fuerstenau, D. W., Healey, T. W., and Somasundaran, P., Trans. AIME, 229 (1964), and Ref. (3). 5. Rubin, A. J., and Ericksen, S. F., Water Res. 5, 437 (1971). 6. Derjaguin, B. V., and London, L., Acta. Phys. Chem. URSS 14, 433 (1941); Verwey, E. J. W., and Overbeek, J. T. G., "Theory of Stability of Lyophobic Colloids," Elsevier, New York, 1948; Shaw, D. J., "Introduction to Colloid and Surface Chemistry." Butterworths, London. 7. Kuhn, A. T., in "Electrochemistry of Cleaner Environment" (J. O'M. Bockris, Ed.), Chap. 4, Plenum, New York, 1972; See, for example, Shvedev, V. P., and Yakushev, M. F., Radiochimiya 12, 871 (1970). 8. "Standard Methods for Examination of Water and Waste Water," 16th ed. Amer. Publ. Health Assoc., Washington, D. C. 9. Grieves, R. B., and Bhattacharya, D., or. AppL Chem. 18, 149 (1968); Grieves, R. B., in "Adsorptive Bubble Separation Techniques," Chap. 12. Academic Press, New York, 1972. 10. Guadin, A. M., "Flotation," p. 153. McGraw-Hill, New York, 1957. 11. Bishop, P. L., Sep. Sci. Technol. 13, 47 (1978). 12. Culp, G. L., and Culp, R. L., "New Concepts in Water Purification," p. 230. Van NostrandReinhold, New York, 1974. 13. Reay, D., and Ratcliff, G. A., Canad. J. Chem. Eng. 51, 178 (1973). 14. Van Vuuren, L. R. J., Stander, G. J., Henzen, M. R., Meiring, P. G. J., and Van Blerk, S. M. V., "Water Research," Vol. 1, pp. 463474. Pergamon Press, Elmsford, N. Y., 1967.