Foam separations: A review

93

The Chemical Engineering Journal, 9 (1975) 93-106 @Eisevier Sequoia S.A., Lausanne. Printed in the Netherlands

Foam Separations: A Review ROBERT

B. GRIEVES

The University of Kentucky,

Lexington,

Ky: 40506‘ (USA)

(Received 4 February 1975)

Abstract

nants. In these cases, the process is generally termed

“foam fractionation”.

A review is presented of foam separations reported in the world literature in the period 19 70-I 9 74, Processes are included the objective of which is the removal and concentration from dilute aqueous solution of surfaceactive-agents; or of charged inorganic species (colhgends) foam separated by surfactants. The 200 references which are cited encompass foam fractionations, ion flotations, precipitate flotations, and adsorbing colloid flotations, but do not include processes whose primary aim is the flotation of particulates. Four separations are described in some detail: cationic surfactant selectivity in the foam fractionation of inorganic anions; the foam fractionation of surfactants and of Cu(II) from seawater desalination brines; the ion flotation of Cr( VZ) and precipitate flotation of Cr(IIZ), with application to electroplating waste treatment; and the adsorbing colloid flotation of trace metallic and non-metallic species from seawater.

INTRODUCTION

Foam separation processes have been utilized to separate and concentrate effectively a variety of constituents from dilute aqueous solutions. Foam separations rely on the adsorption of a surface-active-agent (ionic or nonionic) at the gas-aqueous solution interfaces of generated bubbles, which are either dispersed through a porous-plate-type of gas diffuser or precipitated by throttling a pressurized fraction of the aqueous phase. The surfactant is concentrated in a foam or froth formed on top of the bulk solution, and the separation is accomplished in batch operation by physically separating the foam and the bulk solution phases and in continuous operation by taking off steady-state foam (overflow) and underflow streams. Foam separations have been used to remove surfactants from process waste streams and for the purification of surfactants from non-surface-active contami-

Foam separations have also to remove and concentrate a charged, non-surface-active species (termed colligend) from dilute aqueous solutions by relying on the interaction of that species with an ionic surfactant. been utilized commonly

(1) If the species is an inorganic or organic cation or anion (colligend) which forms an ion pair or soluble complex with the surfactant ion, the process is termed “foam fractionation”; (2) if the ionized species (colligend) is precipitated by the surfactant, either in the bulk solution or at the gas bubble interfaces, and the precipitate is collected by the bubbles and thereby concentrated in the foam phase, the process is termed “ion flotation”; (3) if the colligend is first precipitated by a nonsurface-active ion, and then floated and carried into the foam phase, the process is termed “precipitate flotation”; “precipitate flotation of the first kind” requires the use of a surface-active collector for particle flotation while “precipitate flotation of the second kind” requires no collector; (4) if the colligend is adsorbed onto the surface of colloidal particles and then the particles floated with a surface-active collector and carried into the foam phase, the process is termed “adsorbing colloid flotation.” During the period 1970-1974, approximately 200 studies have been reported of foam fractionations, ion flotations, precipitate flotations, and adsorbing colloid flotations, which will be included in this review. Other reviews have appeared in the literature’-16, including a comprehensive, multi-authored text’ edited by Robert Lemlich, “Adsorptive Bubble Separation Techniques,” which covered the literature prior to 1970. This review will detail four applications of foam separations which should be of particular interest to chemical engineers and industrial chemists. In addition,

94 the 200 post-1970 references which involve species intially dissolved in aqueous solution will be cited. Processes in which the primary objective is the separation and concentration in a froth of particles from aqueous suspension will not be included. These include froth flotations, colloid flotations, solids flotations, and microflotations, which have been covered in other reviewsl’16-19. Other adsorptive bubble separation processes which do not utilize a foam or froth, such as solvent sublation, emulsion fractionation, and bubble fractionation, will also not be included. In the foam separations which have been reported, a number of experimental configurations have been utilized, including batch and continuous operation with and without refluxl, and with single and multiple equilibrium stages. Numerous studies have analyzed the effects of geometric variables (foam column height and diameter), operational variables (gas rate, feed rate, etc.), and solution variables (ionic strength, pH, etc.). A large number of parameters have been utilized to describe the extent of separation achieved, but many investigators have not reported a sufficient number of parameters or dependent variables, in order to assess adequately the success or feasibility of the separation that was achieved. For a foam separation involving a surfactant and an additional species (colligend) such as an inorganic anion, three parameters must be specified: the surfactant removal (percent flotation) in terms of concentrations, the removal of the colligend (percent flotation) in terms of concentrations, and the flow rate or volume of the foam stream or phase (relative to the feed rate or volu?e) in which the surfactant and colligend are concentrated.

R. B. GRIEVES

I

IO psig. Air

saturator

Fig. 1. Schematic diagram of experimental apparatus for continuous flow foam fractionations

EHDA+, and b, in Br-. The assumption could be made2O that the foam consisted of entrained bulk liquid (of concentration c,, e,, and b,) in equilibrium with surface liquid containing surfactant of surface concentration re, plus the fixed and diffuse layers of counterions, of surface concentrations, rc (Clog) and rb (Br-). As each bubble rose through the bulk solution, there may have occurred an exchange reaction, (EHDA-Br),

FOAM FRACTIONATION OF INORGANIC ANIONS: SELECTIVITY OF CXTIONIC SURFACTANTS

Model of surfactant as soluble ion exchanger for SCN-, I-, ClO, NO, BrO$ or NO; versus BrSteady state, single equilibrium stage experiments were conducted in the foam fractionation unit shown schematically in Fig. 120*21. For a feed stream containing, for example, NaC103, of concentration ci in ClO; and containing the quaternary ammonium surfactant, ethylhexadecyldimethylammonium bromide (EHDA-Br), of concentration ei in EHDA+ and bi in Br- (NaBr was also present in some experiments), there were produced by aeration of the bulk solution a steady-state residual stream lean in surfactant and a steady-state foam stream rich in surfactant. The residual stream concentrations are designated c, in ClO;, er in

Moisture Tro D

+ (ClO;),

t

(EHDA-ClO,),

+ (Br-),

in which the subscript, s, designates the surface layer or phase. The equilibrium selectivity coefficient is defined as,

+c$ b

r

the ratio of the distribution coefficients of the exchanging anions. The ratio r& in eqn. (1) could be replaced by(Ci - c,)/(bi - b,) to enable the direct experimental determination of K’. If no surface exchange reaction occurred, the selectivity was probably determined by ion pair formation in the bulk solution, K” = (EHDA-C103),(Br3,/(EHDA-Br),

(ClO&

(2)

FOAM SEPARATIONS

95

in which K” is the ratio of the ion pair formation constants of EHDA-ClOs and EHDA-Br. An initial series of experiments was carried out to determine, if possible, the extent of the surface exchange reactionzO. The feed EHDA-Br concentration was 1.6 x 10S4 M and that of the colligend salt NaNOs (or NaI) was held at 1.8 x 10 “4 M. The height of the liquid level in the column, and thus the height of bubble rise through the completely-mixed bulk solution, was varied from 8.5 to 58.5 cm. The foam height was held at 15.8 cm, the liquid feed rate at 0.056 l/min, and the air rate at 0.4 l/min. Figure 2 (bottom) indicates that surfactant adsorption, measured by Fe, increased with liquid height, more closely approaching equilibrium adsorption as the bubble contact time with the bulk liquid increased. The effects of liquid height on anion exchange were not significant, clearly in the case of iodide, and probably in the case of nitrate. Either the surface exchange reaction occurred rapily and reached equilibrium at a short distance above the air diffusers, or no surface exchange occurred, and the selectivity was determined by ion pair formation in bulk solution. Khentov, et al., in a series of studies of the separation of Cl- from SOi-

2 o-

0 D

0 l 0

0

I IO

I

IS-

---

I 20

I 30

I

I

I 40

I 50

I 60

I

I

I

40 cm

50

60

0 Iodide 0 Nitrcte

c,=I 8xlo-~M e,=I6x10-‘M

0 Chlorate n Nitrate

I.4

4 Bmmote 1.2

i

1.0 5 Re

0.8

r’ c 0.6

0

0

0.

I

0.2

03 r,/b,

0.4

0.5

Fig. 3. Relations between distribution coefficients of ClOj, NO;, and BrOi and distribution coefficient of Br-.

in solutions 0.1 M each in NaCl and Na2S0422-25, reported that their separation criterion continued to increase with increasing liquid height up to 10 cm and then at heights greater than 10 cm, there was no further change24. In a second series of experiments the liquid height was maintained at 28.5 cm (at which 95+% of equilibrium surfactant adsorption was achieved), the feed surfactant concentration, 6, ranged from 1.O x 1OT4 to 4.0 x lob4 M, and that of each of the salts NaClOs, NaNOa, or NaBrOs was within the range 0.4 x 10e4 to 6.6 x 10V4 M. In some experiments, NaBr was added to the feed stream in concentrations from 1.O x 10m4 to 7.8 x 10e4 M in order to broaden the range of values of Fb/br (and of I’,/c,). Results are given in Fig. 3.in terms of the distribution coefficient of each co&end versus that of bromide. The average bubble diameter, which remained rather constant over the range of variables covered experimentally, was 600 E.tm.For each colligend, K' is the slope of the straight line.

95% confidence 20 Liquid

30 Height.

Fig. 2. Effect of liquid column height on surface concentration of surfactant and on selectivity coefficients of I- and NOi versus Br-.

0.6

x103,cm

Colligend

K’

2.2 1.6 0.95

limits for K

’

2.2cl.O f 0.058) 1.6(1.0 f 0.18) 0.95(1.0 i 0.084)

Correlation coefficient, r, for K’

0.98 0.93 0.98

96

R. B. GRIEVES

45

4.0

3.5

30

2.5

2.0

-1 .5

I .o

0.5

0

P~JoCI]

Fig. 4. Effect of the negative logarithm of the sodium chloride concentration on the percent flotation of each of fne metal oxyanions.

The scatter in Fig. 3 is not excessive, considering that K’ is determined by two concentrations and two concentration differences, and the model, based on a surface exchange reaction, appears to be valid. Values of the selectivity coefficient were also established for SCN-, I-, and NO;, each versus Br-, over similar concentration ranges. The values of K’ were 15.1 5.85, and 0.73, respectively21. For each of the six .systems which were investigated, K’ was independent of the feed surfactant concentration, ei, and was rather independent of ionic strength and of the fraction of the “exchanger” occupied by the preferred ion, rJre2eJr. Batch foam fractionation of a solution containing five transition metal oxyanions (five colligend system)

A unique application of foam fractionation is to the separation of five colligends from each other2628 by exchanging one or mpre with a surfactant counterion and by concentrating one or more in the foam while leaving the others in the residualsolution. The oxyanions of Re(VII), Mo(VI), Cr(VI), W(VI), and V(V), all of which form MeO, or MeO$- species, were selected. Foam fractionations of several of these oxyanions have been reported, but the studies were limited to a maximum of two colligends: Re(VI1) and Mo(VI)~+~~; W(VI)33; and V(V)34, 35. Experiments were carried out at constant 1.O x ,10e6 M Initial concentration of each oxyanion (of Re(VII), MoOrI), Cr(VI), W(VI), and V(V)), with all five metal oxyanions always present2628. The initial solution contained 5.0 x 10m5M hexadecyldimethylbenzylammonium chloride (and was thus 5.0 x 10e5 M in chloride, at least). The ionic strength was modified

by the addition of NaCl, with the pH controlled at 6.0 + 0.2. As the solution was aerated and thus foam fractionated, at an air rate of 0.01 l/min through a 2030 micron glass frit, the concentration of each metal oxyanion in the bulk solution was recorded continuously by means of radioanalytical tracers and gamma radiation spectrometry. At each initial NaCl concentration, five identical experiments were run: in each of the five a different radiotracer was used and a different metal oxyanion was monitored. Experimental results are given in Fig. 4, with the flotation, at foam cease, of each metal oxyanion related to the negative log of the activity of NaCl in the initial solution. At low chloride concentration, p[NaCl] > 4.0, all five metal oxyanions were floated completely, in spite of chloride competition: note that at p [NaCl] = 4.0, the initial solution was 100 times more concentrated in chloride than in each of the metal oxyanions. The surfactant’s chloride counterions evidently were preferentially exchanged with the metal oxyanions, forming surfactant-oxyanion ion pairs, even in an excess of chloride. As the p [NaCl] was reduced below 4.0, the very large excesses of chloride began to destroy the surfactant’s selectivity, and first the flotation of V(V) decreased, and then that of Cr(VI), and W(VI), with zero flotation of all three at p[NaCl] < 1.5. Finally, Re(VI1) and Mo(VI) oxyanion flotation fell off, with less than 50% flotation of each in the presence of a 10,000 fold excess of chloride. Similar experiments were also carried out over variable, acidic pH and similar behavior was observeds’. Figure 4 indicates that foam fractionation can be used to separate completely one or more of the metal oxyanions from a mixture of all five. Figure 5 presents

97

FOAM SEPARATIONS

Prediction of surfactant selectivity Efforts have been made to predict the selectivity of anionic surfactants for a series of cations363g, and of cationic surfactants for a series of anions21*2a*30~40~41. The extent of interaction of a catiqnic surfactant with an anion, for example, should be determined by the anion’s charge, structure, and degree of hydration. The absolute partial molal entropy of the anion in aqueous solution, S&a abs, was suggested as a possible selectivity criterion 40. The absolute partial molal entropy did provide an indication of foam fractionation selectivity for anions of similar structure28, for example ReO;, HMoO;, HCrOi, and H2 VO,, or MoOi- and HVOi-, as long as the comparison was made of oxyanions with a wide spread of values of S&a abs. Segments of the selectivity sequence determined from the continuous flow experiments, SCN- > I- > ClO; > NO; > Br- > BrO; > NO;, were identical to those reported by Shinoda and Fujihira36 who measured relative anion adsorbabilities at an air-solution interface charged by2dsorption of a cationic surfactant, and by Moore and Phillips 41 who measured surfactant-anion pair formation constants. However, Moore and Phillips used an extremely questionable experimental procedure and also reported a dubious dissociation constant for their cationic surfactant, which should have behaved as a strong electrolyte in dilute solution.

I 3.0

2.0

2.5

I .5

I .o

0.5

0

[NaCI] Fig. 5. Effect of the negative logarithm of the sodium chloride concentration on the mole fraction of each of five metal oxyanions in the residual solution after foam fractionation. P

the mole fraction in the residual solution after flotation of each metal oxyanion in the five component mixture, related to p[NaCl] . Figure 6 represents the same relationship for the foam. Figure 5 shows that at p[NaCl] = 3.25-3.75, the residual solution contained only V(V) oxyanions (with chloride also present, of course); the other four metal oxyanions, together with some V(V) were separated completely into the foam. The optimum point of operation for V(V) separation, using both Figs. 4 and 5, would be at p[NaCl] = 3.3. Figure 6 shows that at p [NaCl] < 1.5, Re(VI1) and Mo(V1) oxyanions could be separated completely from V(V), Cr(VI), and W(VI), with the foam being approximately equimolar in Re(VI1) and Mo(V1) and containing none of the other oxyanions. The optimum point of operation, using both Figs. 4 and 6, would be at p[NaCl] = 1.4.

0 40

35

30

2.5

2.0

I5

IO

05

P[N~CII

Fig. 6. Effect of the nebtive logarithm of the sodium chloride concentration on the mole fraction of each of fwe metal oxyanions in the foam after foam fractionation.

FOAM FRACTIONATION OF SURFACTANTS AND OF Cu(II) IONS FROM SEAWATER DESALINATION BRINES

The work of Valdes-Krieg et al. originated with the

0

development of a surfactant-recovery process to use in conjunction with the interface enhanced vertical tube evaporation (VTE) process of Sephton42, in which lo-15 mg/l of an anionic surfactant were added to the seawater feed of a VTE desalination plant. Significant increases in heat transfer coefficients and reductions in pressure drops along the tubes resulted from the development of foamy flow in such tubes43. To make these processes environmentally acceptable, the surfactant removals would be required to increase with increasing sdrfactant concentration used in the evaporator. Removal efficiencies above 95% would be necessary, which would be substantiallyabove those which has been attained in the 1960’s with detergent bearing waste water. High ionic strength and moderately high temperatures in desalination plant blowdown brines would tend to lower the critical micelle concentration for surfactants, providing for high distribution coef-

98

~000’0

R. B. GRIEVES

’

’

IO

I

’

I

20

Flow

’

30

Ratio,

’

’

40

I

’

50

I

1

60

G/L

Fig. 7. Effect of gas and liquid flows on the pilot foam fractionator. o countercurrent flow with partial back-mixing conditions; o single stage conditions; e gross circulation conditions. From ref. 45.

ficients between air-liquid interfaces and bulk solution, with a consequent increase in separability. These distribution coefficients were experimentally established through measurement of concentration profiles in cocurrent-flow bubble columns. This procedure involved the determination of the difference in concentration between the feed liquid and the liquid at a sufficient height in the column for the concentration to remain essentially constant44. Using a “single stage” foam fractionator with an .enlarged liquid pool, it was shown that a significant part of the separation could be accomplished in the liquid pool of the column, a mode of operation designated as “bubble fractionation”. The mechanism of separation was enhanced by the use of a diffuser plate issuing very small bubbles, which tended to reduce the extent of back-mixing in the column and provided for partially countercurrent flow conditions even in a column with a relatively large cross-section (1 ft2)45. Process capabilities for surfactant removal in a 1 ft* square column with an aerated liquid height of 5.5 ft are depicted in Fig. 7, in which the ratio of surfactant concentration in the treated effluent to the concentration in the feed to the column is plotted against the gas-to-liquid volumetric flow ratio. The surfactant was Neodol25-3A (Shell Chemical Co.). As expected, removal efficiencies increased with increasing air flow per unit liquid treated, but they did so in different ways depending on the absolute magnitude of the flows of both phases: (1) Single-stage behavior: These conditions corresponded to simple equilibration between the bubbles and the bulk liquid, and prevailed at low liquid throughputs and moderate-to-high air throughputs.

(2) Countercurrent flow with partial back-mixing behavior: These conditions were observed to prevail at moderate air flows and moderate-to-high liquid throughputs. Axial concentration profiles measured for the liquid phase showed a concentration gradient along the column height at all surfactant concentrations. The magnitude of this gradient increased with increasing surfactant concentration in the feed. Analysis of such profiles through mass balances allowing for equilibrium operation opposed by back-mixing, and using independent equilibrium data, yielded effective axial diffusivities, which were used to draw the solid line joining this set of points. A comparison of the air throughput per unit liquid flow required to achieve the same removal under single stage and counter-current conditions revealed the desirability of the latter mode of operation. (3) Gross circulation behavior: This flow pattern could arise at moderate-to-highliquid throughputs accompanied by high air throughputs. This pattern was not the consequence of flooding but rather of flow instability and placed an upper limit on column capacity for efficient removal. Studies with controlled degrees of tilt in a smaller 24 inch diameter column showed that gross circulation significantly lessened the separation attained, even for inclinations from the vertical of less than one degree46. The presence of an anionic surfactant at an airliquid interface brings about the formation of a diffuse layer of cations in the vicinity of the interface. This layer is transported into the foam generated upon sparging air through the solution, enabling simultaneous cation and surfactant removal. The relative amounts of different cations in the layer reflect the selectivity of foam fractionation for separation of one cation from others. Copper analyses of blowdown brines reveal that concentrations (0.3-l mg/l) above the safe environmental limit can result from plant corrosion. Removal of this copper by foam and bubble fractionation

0

I

0 Flow

IO Rotlo. G/L

I

20 (Slmulaied

I 30 Desalmaiwn

1

1 43 Bran.?)

Fig. 8. Effect of gas and liquid flows on the simultaneous removal of the surfactant Neodol and Cu(I1) from 10.5 weight oercent NaCl solutions.

FOAM

SEPARATIONS

99

requires selectivities of the order of 105, because of the large amounts of other solutes present in seawater concentrates. A plot of copper removal and surfactant removal as a function of volumetric flow ratio is given in Fig. 8 for an anionic surfactant (Neodol). The fact that copper was removed indicates that the high selectivities necessary for the separation could be achieved. Similar results for removals as a function of surfactant concentration in the feed indicated nearly constant, high surfactant removal, accompanied by a gradual increase in the removal efficiency of copper ions with increasing surfactant concentrations. This resulted from the dependency of cation concentration near the bubble surfaces upon surfactant loading at the interface. Axial concentration profdes measured under these conditions showed that the contact between the phases approached countercurrent flow with partial back-mixing, conditions which were favorable for efficient surfactant stripping, but unfavorable for selective stripping of cations. The lack of selective removal of counterions resulted from the reduction of the surfactant concentration below the feed level. An improvement of the process was accomplished on the basis of this, by introducing a surfactant stream (Le. collapsed foam stripped of the cation of interest) low enough in the liquid pool so as to yield a more uniform surfactant loading per bubble throughout the column height. The increased surface capacity for cations along the column resulted in a marked improvement in fractionation effects. The results obtained for removal of copper from a feed containing 1 mg/l copper ion in otherwise pure water are shown in Fig. 947. Foam

Llqud

416

&liter

I 0

mg/llter

1000 0

mg/hter

mg/htsr

0.35 246

gpm/ft* mg/liler

Neodol

6 IO mg/hter

Copper

Feed

Neodol

1

copper

Neodol Copper I66

sctm/ft2

Fig. 9. Improvement column operation.

I.91 gpm/ft* 40.0 mg/lh 0 01 mg/llter

Neodol copper

it! Cu(I1) removal

provided

by modified

ION FLOTATION FLOTATION PLATING

OF Cr(V1) AND PRECIPITATE

OF Cr(II1): APPLICATION

TO ELECTRO

WASTE TREATMENT

An interesting comparison can be made between two .foam separations, both with application to the removal and concentration of heavy metals from electroplating rinse baths. Hexavalent chromium, Cr(VI), can be ion floated, with a cationic surfactant required in approximately stoichiometric concentrations4*53; or it can be reduced to trivalent chromium and floated as precipitated Cr(II1) hydroxide with an anionic surfactant required in concentrations only about 0.1 the stoichiometric48y 54-56. A precipitation reaction between a cationic surfactant and HCrOi has been reported by several investigators52, for example, HTA+ + HCrOi t

(HTA-HCrO&,iid

in which HTA+ is the hexadecyltrimethylammonium cation. Before precipitation, molecular clusters which are smaller than the critical cluster or nucleus may be formed, leading to the formation of soluble complexes, a HTA+ t b HCrOi

= (HTA, HCr04b)-(b-a)

Once the precipitate is formed, there may be adsorption on the particle surfaces of (HTA, HCr04b)-(b-a) or of unreacted HTA+ or of HCrOi. The adsorbed HTA species may stabilize the precipitate and may prevent the further aggregation which is essential for efficient flotation. The adsorbed HTA species and/or HTA which is part of the precipitate structure also act as a collector, promoting the attachment of the particles to generated gas bubbles. The unreacted and nonadsorbed HTA (“free”) acts as a frother to provide the foam which carries the particles from the flotation column. Thus in an ion flotation process, the surfactant plays four roles: precipitant, dispersant, collector, and frother, Experiments were conducted with 9.3 x 10m4 M solutions of sodium dichromate, with HCrOi as the dominant species at pH 4.1, and the surfactants were tetradecyltrimethylammonium bromide (TTA-Br), hexadecyltrlmethylammonium bromide (HTA-Br), and octadecyltrimethylammonium bromide (OTA-Br). The detailed experimental procedure has been reporteds3 for the batch flotation experiments at a 10 minute foaming time. The effect of the molar surfactant to Cr(V1) ratio (M) on the percent flotation of acid chromate is given in Fig. 10. The optimum surfactant was clearly HTA-Br

R. B. GRIEVES

0

I 0

04

I

I 12

08 M=MOlar

Fig. 10. Ion flotation

I 16

I

Surfactont/Cr

of HCrO;

I 24

20 (VI)

I 28

32

Rot10

with three surfactants.

(Cr,) yielding 90 percent flotation at approximately the stoichiometric ratio (M = 1.0). TTA-Br (C,,) yielded almost the same flotation, but approximately twice the stoichiometric surfactant concentration was required. OTA-Br (C ra) behaved similarly to HTA-Br , but provided a maximum flotation of only 50%. The abrupt decreases in flotation with HTA-Br and OTABr were brought about by excess surfactant adsorbing on the particles, stabilizing them and preventing further aggregation. The effects of particle size and surface charge have been Evaluated in an effort to establish the controlling flotation mechanisms3. From an extensive series of ion flotation experiments with the three surfactants at M = 1.04, but with variable temperature and surfactant-HCr04 mixing time, the extent of flotation could be related qualitatively to particle size. Some results are given in Fig. 11. The particle size distributions were established by filtering the feed suspensions through a series of membrane filters. From Fig. 11, it can be hypothesized

that all particles larger than 25 pm were floated and that few particles smaller than 7 pm (including the nonprecipitated acid chromate) were floated. Temperature, mixing time, and surfactant chain length effects could be explained in general on the basis of their impact on the particle size distributionss. Ion flotation provides an advantage over foam fractionation because the precipitation reaction between surfactant and colligend generally requires only slightly above the stoichiometric surfactant concentration; frequently, foam fractionation requires surfactant concentrations well in excess of the stoichiometric, due to ion competition. Surfactant utilization may be reduced considerably further by first precipitating the colligend and then by floating the precipitate48. For example, a three stage process of reduction of HCrOi with NaHSOs, followed by precipitation of Cr(II1) with NaOH, followed by batch flotation with an anionic surfactant, sodium dodecylsulfate (NaDS), provided 97% removal of chromium from suspensions 9.3 x 10e4 M in Crs4. The process was strongly pH dependent: below pH 6.3, soluble chromium species became appreciable and the flotation results tended to parallel the decrease in the fraction of Cr present as precipitate; above pH 9.7, the charge of the precipitate was reversed, as indicated by surface potential measure. ments, and the flotation fell off abruptly. For suspensions with a doubled Cr concentration (1.9 x 10e3 M), the optimum flotation range was narrowed and lowered to 6.3-6.5, indicating modifications in particle surface characteristics: the flotation was reduced tol87%, but the surfactant requirement was only 0.093 mole dodecylsulfate per mole Cr, less than 10% of the “stoichiometric.”

60

60

0

20 Percent

Fig. 11. Relations temperatures.

40

60

of Cr (VI)>25

between

80

100

,~rn

flotation

0

0

20 Percent

of surfactant-HCrO;

40 of

60 CrWlb7

80

100

pm

particulates

and particle

size cuts for several surfactants

and

FOAM SEPARATIONS

Because any precipitate flotation process of commercial significance must be carried out on a continuous flow basis, the Cr(II1) hydroxide suspensions were floated in a unit similar to that depicted in Fig. 1 55. For 9.3 x 10m4 M suspensions, the effluent Cr concentration, the effluent surfactant concentration,. and the fraction of the feed stream retained as the effluent (underflow) stream were related to five independent variables by stepwise, linear multiple regression analysis. The dependent variables were strong functions of the feed surfactant concentration and were rather weak functions of the feed rate, liquid column height, and air rate. Only the fraction of the feed retained as effluent varied with foam height. All of the dependent variables were independent of the precipitate suspension aging time for aging times greater than six hours and were independent of the precipitate-surfactant mixing time. For more concentrated Cr suspensions (1.9 x 10e3 M), the decrease in flotation efficiency could be overcome readily by using two column, series operation, with no chemical addition to the feed to the second column. The required surfactant concentration was 0.11 mole dodecylsulfate per mole .Cr to achieve 98% flotations5. Pearson and Shirleys7 applied precipitate flotation to solutions (1) containing 6.3 x 10s3 M Cu(II), as the sulfate; (2) containing 6.8 x 10m3 M Ni(II), as the sulfate; (3) containing 6.6 x 10m3 M Ni(II), prepared by diluting a spent, Watts-type plating solution; and (4) containing 1.6 x 10T4 M Cu(II), 2.2 x 10d4 M Ni(II), 1.6 x 10m4 M Zn(I1) and 1.9 x 10m4 M Cr(III), as a simulated effluent from copper fabrication. The metals were floated as the hydroxides and/or carbonates over pH 8 + 10, with tallow diamine acetate as the collector and isopropanol and Dowfroth 250 as frothers. The authors concluded that Ni(I1) was more difficult to float than Cu(II), that the effect of frother concentration was more significant than the effect of collector concentration, that dissolved (precipitated) air operation was more efficient than dispersed air, and that mixed wastes were easier to float than the simpler ones. They also concluded that, although being fairly costly, the value of the collector and frother used was much lower than the recovered materials, and the process appeared suitable for the full-scale treatment of metal-bearing effluents. Additional precipitate flotation studies have been reported,with the precipitates containing the following colligends: COAX; Zn(II)59; Pb(II)60; Cd(II), or Cu(II), or Hg(II)61; Cd(H), Cu(II), and Zn(II)? and orthophosphate and fluoridees. Koyanaka and TsutsuiM floated radioactive sludges produced by precipitation.

101 Kalman and Ratcliffes investigated the underlying mechanism of precipitate flotation and applied their theory to the flotation of Mg(I1) hydroxide with sodium dodecylbenzenesulfonate at pH 11.8.

ADSORBING COLLOID FLOTATIOtiS: THE EXTRACTION

APPLICATION TO

OF INORGANIC IONS FROM SEA-

WATER

During the past six years, Zeitlin, ef al. have conducted studies to remove and concentrate a series of inorganic anions and cations in pgram/l quantities from seawater. They adsorbed the ions onto a hydrophobic colloid of opposite charge and then floated the colloid with an ionic surfactant. Work by Kim and Zeitlinee on the role and behavior of Fe(II1) hydroxide and Th(IV) hydroxide as “collectors” of molybdenum present in seawater as molybdate (Moo:-) eventually led to the combined use of Fe(II1) hydroxide at pH 4.0 together with sodium dodecylsulfate to float the colloid and provide 93-99% flotation of 1.3 x lo-’ M molybdenum6’. The effectiveness of Fe(II1) hydroxide was attributed to the formation at the colloid surfaces of an Fe3+-MoOi- compound of low solubility. The positively-charged Fe(II1) hydroxide was effectively floated by the dodecylsulfate anions. Uranium, present in seawater as the stable UOz(C03)$-, was adsorbed by Fe(II1) hydroxide with a maximum separation of 82%, again using sodium dodecylsulfate as the surfactant68. Th(IV) hydroxide was found to be more effective for UOa(C03)j-, providing 91% separation, with sodium decanoate as the surfactanVj9. Adsorbing colloid flotation was next extended to the trace cations Zn(I1) and Cu(II), using Fe(II1) hydroxide which is negatively charged at pH 7.6 and dodecylamine as the surfactant70*71. Hg(I1) was adsorbed onto CdS and the colloid was floated with octadecyltrimethylammonium chloride, yielding 83-92% flotation from 1.O x 1O-lo M Hg(I1) solutions in seawater72. Phosphate and arsenate were floated from solutions containing each oxyanion and from a solution containing both oxyanions, using Fe(II1) hydroxide at pH 4 and sodium dodecylsulfate73. Flotations in the range 8294% were achieved for seawater solutions in the concentration range 3.0 x IO-’ to 3.0 x 10m6 M in phosphate and/or arsenate, utilizing 4.0 x lOa M FeC13 to produce the Fe(II1) hydroxide. Matsuzaki and Zeitlin74 evaluated nine different surfactants to float the colloids Fe(II1) hydroxide, Th(IV) hydroxide, Al(II1) hydroxide, HgS, Cd& and MnOa, with all the colloids having potential for adsorp

R. B. GRIEVES

102 tion of inorganic ions from seawater. In all of the studies by Zeitlin, et al., relatively large quantities of colloid and concentrations of surfactant were required to separate effectively the trace concentrations of the colligends which they studied. However, it should be noted that they were faced with a difficult problem of ion competition (in seawater) and they successfully carried out difficult separations, always including careful statistical analyses of their data. Ferguson, Hinkle, and Wilson75 found that the adsorbing colloid flotation of CdS and PbS, with adsorption onto FeS and flotation by the cationic hexadecyltrimethylammonium bromide, was superior to the precipitate flotation of CdS and PbS. Wilson76 has also began to develop some of the theoretical aspects of adsorbing colloid flotation. Other experimental studies have been conducted by Karger”, with Cr(II1) as the colligend, by Nazarov, ef al. 78, with W(VI), Mo(VI), Ni(II), and Cr(II1) as the colligends, and by Ichiki7g, with Cd(R) as the colligend.

TABLE

FOAM SEPARATIONS CATIONIC

OF ANIONIC

COLLIGENDS,

AND

SURFACTANTS

Tables 1 and 2 present the remaining references reported in the literature during 1970-1974 for anionic colligends and cationic colligends, respectively. Table 1 is arranged alphabetically according to the central metal (or non-metal, in a few cases) in the anion. Table 2 is arranged by metal oxidation state and then alphabetically by metal. References listed and/or detailed in previous sections are not relisted in Tables 1 and 2. In addition to the work on the foam fractionation of surfactants from seawater desalination brines outlined in a previous section, 3 1 investigations have been reported of the foam fractionation of surfactants from dilute aqueous solutions, with no colligends directly involved. These include 16 studies of a largely experimental nature146161, and 15 studies that involve modeling and theory validation, in addition to presenting experimental dataie*i76. Several references have applied foam separations directly to municipal or

1

Foam separation arrangement)

of anionic

co&ends

Colligend 3AgWi>

k&203)2

Ag(CN);, A&I;

(Alphabetical

TABLE

Foam separations of cationic colligends (Arrangement metal oxidation state and then alphabetical)

80

Colligend

Reference

Cs(l), Co(l) complex cation Be(H) Be(H), Fe(H) Ca(lI), Fe(H), Mn(ll) Cd(H), Co(H), Ni(l1) Cd(H), CuW), Pb(H) Co(H) complex cations Co(H), Ni(ll) Co(H), Cu(ll), Ni(ll) Cu(ll) Cu(lI), Pb(lI), Zn(ll) Ni(l1) Pb(Il) Zn(l1) Series of divalent cations Actinides (Ill) Ce(lll) Ce(lll), Pm(lll) Ce(lll). YHII) cr(lllj. Y(lll) Ru(lV) UWl) Flotation reagents Mathematical model

110 111 112 113 114 39 110 115-118 119-121 122-125 126 127-129 130 131 132-134 135 136 137 138, 139 140 141 142 143 144 145

82 83

Co(CNS):-

84, 85

Co(CNS)i-, Cu&-,

CoClj-,

Ni(CNS):-

86, 87

Cu(CNS):-

Fe(CN)z-,

88 89

Fe(CN)z-

and other Ge species

Hg(NO&, IMOO:-

90-93 94

Hg(NO&

95 and other

MO species

96 97,98

H2 PO; PdCli-

99

Ptc1:Pt&,

100 PtClf-,

101

Pt(CN):-

102

sotuo2(co&,

UO2(CO3)2(H20)2

uo260&-,

uo2(so4)~-

Large group

of complex

103 104

anions

105 106, 107

Phenolate Theoretical

2-

and review studies

2

Reference

81

Au(CN);

Co(CN):-

GeO$-

ADDITIONAL COLLIGENDS,

108, 109

by

103

FOAM SEPARATIONS industrial wastes. These include primary and secondary sewage effluents*7F18r; acid mine drainager82; laundry wastesls3; textile wastesr84~185; the wastewaters from tanneries”% wastes containing heavy metalsrs7; and low-level radioactive wastes’ 88, l 89. The remaining foam separation studies have included applications to seawater*9*193, and the removal and concentration of ligninr94, methacrylic acidr9s, and polyvinylalcoholr96.

ACKNOWLEDGEMENTS The author acknowledges gratefully the contribution to this paper by Mr. Ernest0 Valdes-Krieg, and his coauthors, Dr. H. H. Sephton and Prof. C. J. King, all of the University of California, Berkeley. Mr. Ralph R. Huffsey assisted with the literature review. The author also wishes to acknowledge the scientists and engineers from all over the world who have helped to make foam separation a powerful and intriguing separation technique. Thanks are also due to Ms. Vi&i McDuffee for the superb job of typing the manuscript.

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