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Extraction of chromium and zinc from cooling tower blowdown by liquid membranes
of Membrane Science, 18 (1984) 251-271 Science Publishers B.V.. Amsterdam - Printed
Journal Elsevier
EXTRACTION BLOWDOWN
E.J.
251 in The Netherlands
OF CHROMIUM AND ZINC FROM BY LIQUID MEMBRANES
COOLING
TOWER
FULLER
Exxon
and Engineering
Research
and NORMAN
Co., Linden,
NJ 07036
(U.S.A.)
N. LI
UOP Inc.,
Des Plaines,
(Received
February
IL 60016
10, 1983;
(U.S.A.) accepted
in revised
form
June
6, 1983)
Summary Since the invention of liquid membrane technology in 1968, a number of potential applications have been investigated at Exxon Research and Engineering Company, This paper reviews the recovery of metal ions from an aqueous solution, with specific details of research on the simultaneous extraction of chromium(VI), chromium(III), and zinc from a cooling tower blowdown stream. The important factors affecting the extraction of chromium and zinc are discussed.
Introduction The liquid membrane technology involves making an emulsion of either oil-in-water or water-in-oil [ 11. The emulsion is made so as to disperse tiny droplets of one liquid phase in a continuous, immiscible second phase. The critical elements here are (1) the introduction of high and uniform shear to the mixture of the two phases, such that the droplet size will be as small as desired, and (2) the presence of some stabilizing surfactant such that the dispersed droplets will not at once coalesce. The emulsion is then added to a relatively large volume of a liquid phase which is immiscible with the continuous phase of the emulsion, and dispersed as globules by the application of low shear. In this form, ions or dissolved species may be transferred across the liquid membrane, which is the continuous phase of the emulsion, into or out of the droplet or the internal phase of the emulsion. It is important to note that unless the oil phase is properly formulated to impart a high degree of stability to the emulsion, the rate of breakage of the droplets of internal phase may be significant, or the oil phase may tend to emulsify more aqueous phase during mixing conditions. Compositions and procedures have been worked out over a number of years at Exxon which provide highly reliable extraction systems. For oil-type liquid membranes, the key to emulsion stability is the use of one of a class of oil-soluble surfactant species developed by Exxon. These surfactants are characteristical-
0376-7388/84/$03,00
o 1984
Elsevier
Science
Publishers
B.V.
252
ly made from polyisobutylene as starting material, with a variety of polar groups bonded to the end of the chain [ 201. The substance to be extracted from the feed solution, after transport across the oil membrane, is stripped by the internal reagent and collected inside the droplets of the emulsion globules in a form which is not soluble in the oil membrane. In contrast to the normal solvent extraction scheme, extraction and stripping are not done in separate units but take place simultaneously in the course of liquid membrane contacting in the same vessel. Once the emulsion has contacted sufficient feed solution to “load” the internal reagent phase with desired product, the globules are allowed to settle into an emulsion phase, leaving treated feed solution. A separate unit to coalesce the emulsion into oil and rich extract phase is required; however, because of the high degree of concentration ordinarily attainable by these systems, the emulsion breaking step deals with streams much smaller in size than the feed solution. This implies that emulsion-making and emulsion-breaking equipment need not be very large. The capacity of the emulsion depends on the internal reagent, not on the oil phase as in solvent extraction. The consequence is markedly reduced holdup of oil phase in the process and whatever oil-soluble ligands or “carriers” are necessary to transport the extracted species across it. Another advantage of liquid membrane operations is the potential for greatly reduced entrainment losses relative to solvent extraction. Not only does the system inherently contain less oil phase, but it may happen that the low shear of the contacting step produces larger globules than would be allowable if the oil phase is to be “loaded” in solvent extraction. Entrainment is a major source of loss of oil phase in solvent extraction [2], and generally is worse for more intensely mixed phases where the oil phase is finely dispersed in order to achieve a “loaded” state. Depending on the system, it is commonly measured that entrainment losses of oil phase in liquid membranes extraction are extremely low - below 50 ppm. Liquid membrane separation mechanisms Much has been learned over the past several years as to the influence of emulsion properties on transport across liquid membranes [ 31 and on the accumulation of extract in the internal phase 1211. It is required that the water-in-oil emulsions of interest for metals extraction be stabilized by surfactants with the following properties: (1) They must be oil-soluble and water-insoluble. (2) They must be moderately inert chemically toward ion-carrying substances. (3) They must confer high stability on the interfaces between oil and internal reagent and between oil and feed solution. Ordinarily it is observed that increasing the concentration of surfactant in the oil membrane produces a corresponding increase in stability; that is, the containment of internal phase of the emulsion as the emulsion is dispersed in an external aqueous phase is improved. The limit of surfactant concentra-
253
tion is set mainly by incipient emulsification of feed, cost of surfactant, difficulty of emulsion breaking, and kinetics of extraction. Organic to aqueous phase ratios in the emulsion for an extraction scheme may vary from around 2/l to l/2. In these systems, it is highly desirable to avoid significant breakage of the emulsion during contacting of the feed solution. If the organic/aqueous ratio is below l/2, more breakage is observed; while a ratio over 2/l invites emulsification of feed. The actual ratio of feed to emulsion may be quite high when recycle schemes are used. Inside the feed/emulsion contactor, the limit of feed to emulsion ratio at any given time will be set by the efficiency of phase separation of feed from emulsion [ 221. Mathematical modeling of liquid membrane extraction systems has correlated experimental results for the extractions of uranium, copper and phenol quite well [ 3-51. The models indicate no internal circulation of the droplets within globules of emulsion. The liquid membrane configuration lends itself readily to systems which are kinetically limited, such as the extraction of copper(I1) ion from streams containing large amounts of iron. In solvent extraction, the complexing agent used to concentrate copper in the oil phase may quickly load copper which is slowly replaced by the iron. In liquid membrane operations, a low concentration of complexer (carrier) in the membrane usually suffices to promote rapid loading of the internal phase with copper, even in the presence of large amounts of iron [6,7]. It appears that the diffusion of complexes through the membrane and their stripping are quite rapid, effectively keeping a supply of stripped carrier available for the feed solution at all times [ 81. Water can be purified of phenol [ 5,9] or ammonia [lo] impurities by contacting with liquid membrane emulsions. The stripping agent (caustic in the case of phenol, acid in the case of ammonia) converts the extracted species to an ion which is insoluble in the oil and therefore remains in the internal phase. In contrast to ammonia and phenol, metal ions in aqueous solution ordinarily are present as ions which do not dissolve appreciably in oil. For liquid membrane extraction to operate, a carrier agent must be supplied which reacts with the metal ion to form an oil-soluble species [ 1,4,7,8,9], Treatment of cooling tower blowdown with liquid membranes The effluent from cooling tower blowdown contains chromium(VI), chromium(III), and zinc. The increased awareness in society of the potentially adverse affects on the environment caused by uncontrolled dumping of waste imposes strict limitations on the disposal of cooling tower blowdown. This work was undertaken to evaluate the possibility of utilizing liquid membrane technology to treat water containing low but environmentally intolerable levels of chromium and zinc. Operation of industrial-scale cooling tower circuits necessarily implies
254
evaporation of water and the addition of fresh makeup water. By and large, the dissolved salts are not lost during the cycles of cooling (some spray, or “windage” loss does occur) and are therefore concentrated. A purge stream, or “blowdown”, is removed to control incipient solid precipitation. Because the solids originally came from the environment as dissolved ions in the makeup water, their reintroduction into the water table causes no problem. However, the addition of toxic corrosion inhibitor to the cooling water is normally necessary and its removal from the blowdown is desired. In the case of zinc chromate inhibitor, government regulations limit the allowable amounts of zinc and chromium which can be discarded with the purge stream. The regulation limits which were used as targets for these extraction studies are 1 ppm Zn, 0.5 ppm total chromium, and 0.05 ppm hexavalent chromium. Recovery of hexavalent chromium from water by liquid membranes has been previously reported [ 11,121. Removal from several hundred ppm to less than 1 ppm in the effluent has been demonstrated [ll] . The approach to an extraction system for cooling tower blowdown used in this work involved (1) formulation of the emulsion to remove CrV’, Crm, and Zn” from blowdown to the desired levels, and (2) adaptation of the emulsion toward a viable process for not only extracting the ions of interest but recovering them for reuse in the cooling tower circuit. Experimental Materials used in experiments
The membrane phase used was a mixture of surfactant, solvent extractant and extractant stabilizers. The surfactants used was ENJ, 3029 polyamine with average molecular weight of 1500, manufactured by Exxon. The solvents used were SlOON, S600N and LOPS, either singly or as a mixture of SlOON with either S600N or LOPS. All these solvents are basically isoparaffin hydrocarbons manufactured by the Exxon Chemical Company (Table 2). The extractant used was Aliquat, made by General Mills*. C.P. grade tributylphosphate and nonyl and decyl alcohols were used to ensure the solution of Aliquat in the solvent oil phase. The concentration ranges of these compounds are: Solvent 100N: 60-90 wt.%; TBP: lO-15% (or nonyl/decyl alcohols: 3%); ENJ 3029 surfactant: lo-15%; Aliquat extractant: 2-5%. The emulsified internal phase was 5% caustic solution. Emulsion
preparation
in laboratory
Emulsions for use in bench-scale batch tests were made by extensive mixing of the organic membrane phase with the internal aqueous phase in a Waring Blender. A mixing time of at least six minutes was used to make batch emulsions in this way. The membrane (oir oil) phase to internal phase ratio was in the range of 1 to 2. *Now
Henkel
Corporation.
Test procedures Batch stir tests were carried out by contacting emulsion and aqueous feed in a 1000 ml baffled resin kettle stirred by two marine propellers at ZOO-400 rpm. When the mixing speed was within this range, high extraction rates with low entrainment (aqueous samples were visually clear) were obtained. The ratio of aqueous feed to emulsion was in the range of 4 to 5 by volume. Samples were taken periodically by pipette and the emulsion and aqueous feed rapidly separated by filtering through a filter paper. The aqueous phase was then analyzed. Analytical methods Analysis of the aqueous feed being treated was done by an atomic absorption technique for zinc and total chromium. Hexavalent chromium was determined by calorimetry; trivalent chromium was taken as the difference between total chromium and the hexavalent chromium content. Chloride ion was determined by an X-ray method (for concentrations of several thousand ppm or greater) or by a specific ion electrode. Samples of the aqueous phase were taken after a few minutes’ settling time to avoid contamination by the emulsion. Carrier It was taken as a starting point that an agent capable of complexing with anions such as CrOi- or Cr,Ot- would be needed. The high (up to 1000 ppm) chloride ion concentration in the waste water might be expected to produce anionic species of Cr3+ and Zn2’ complexes with chloride, which might be taken into the oil phase by the R3NCH3+ carrier (e.g., CrCI,-, CrCl:-, CrCli-, ZnCl,-, ZnCli-). The exact ionic species present are functions of not only Crv’, Cr”‘, Zn, and Cl concentrations, but hydroxide ion concentration as well [13]. Previous work demonstrated that Alamine 336 and Aliquat 336-S (General Mills liquid ion exchange agents; tri-n-octylamine and methyltri-n-octylammonium chloride, respectively) were capable of extracting chromate or dichromate ions into an oil phase, and that the chromium was effectively stripped from the oil by sodium hydroxide [ 14,151. Chromium(VI), cadmium, and zinc have been shown to be simultaneously extracted from water containing chloride ions by Alamine 336 in xylene, and stripping was again demonstrated with sodium hydroxide solutions [ 161. Liquid membrane extraction of chromium(VI) using tridodecylamine as a carrier and 0.1 M sodium hydroxide as strippant has been reported [ 121. Mention has already been made of the extraction of numerous metal species from wastewater using liquid membranes [ 4,6,7,11]. This background served as a starting point for this research, although it is noted that no previous research has demonstrated the removal of chromium down to the range of 0.05 ppm, the simultaneous extraction of Crvl, Cr”‘, and Znn ions, and the extraction of CrV’ alone in the presence of high concentrations of chloride ion.
256
Experiments were carried out using Alamine 336 as a carrier, to investigate the possibility that zinc and chromium(II1) might be extracted along with CrV1. This carrier must be protonated to be active. 2 RJNH+ + Cr, Ot-
+ (Rg NH), Cr207
(oil-soluble)
(1)
In general, extraction with Alamine is ineffective above pH 3. This was observed in tests with a model feed containing CrV1 and Zn. At low pH the extraction of hexavalent chromium could be accomplished with a liquid membrane containing Alamine, but at higher pH no extraction was obtained. Unfortunately, zinc extraction folIotied a reverse trend. This means that zinc was extracted at pH > 7.0. The results, shown in Figs. 1 and 2, appear to rule out the use of Alamine in this separation. The most promising carrier for this separation proved to be Aliquat 336. Figure 3 shows results of an extraction done under conditions comparable to those of Figs. 1 and 2. Chromium is evidently extracted more completely, but more slowly using Aliquat carrier than with Alamine (where Alamine is effective at all). The results of Figs. 1, 2 and 3 also suggest the effects of
Membrane: (MI
90% SIODN 5% Surfactant 2% Alamine 3% Nonyl Alcohol 5% NaOH
Internal: (1) Feed: (F)
1; 1000
M/IR
=
TIME,
1,
pdbm, c;,“’
pH 1.0
ppm Cl
F/E
= 4,
MINUTES
400
rpm - PH
(Feed)
I
I
I
I
0
10
20
30
EXTRACTION
Fig. 1. pH 1.0 Liquid
TIME,
membrane
MINUTES
extraction.
-
-
I-
30
20
ID
EXTRACTION
TIME,
MINUTES
1xl 30:6
-
MlNUTES
6.0 7.0 7.5
TlME,
-
i
Conditions similar to Fig. 5, of Feed 3.0 at Start, 500
Fig. 2. pH 3.0 Liquid membrane extraction.
r: t; I
G50 z t 4 5 40 z
\
i
\ \ 0 \
M/l
F:
\
l
I
20 30
1051” 7
EXTRACTION
I
TIME,
200
11.5 11.5
I
ppm Cl,
pH (Peed) 11.1 1016 I:*: 6.0
MINUTES
rpm
500
5% NaOH
5 ppm 2n,
MINUTES
= 4,
or”‘,
F/E
;pm
SlOON I: Surfactant Nonyl Alcohol Aliquat
CHROMIUM
= I,
,‘t
80% 15% 3% 2%
Fig. 3. Aliquat carrier for CrVr and Zn.
10
20
8C
9t
M:
258
chloride ion competition for available carrier, and limitations of the carrier because of its interaction with surfactant. This will be discussed further later. At present, the carrier Aliquat is seen to allow simultaneous extraction of CrV1 and Zn. Tests with a typical blowdown sample, containing Cr”* as well as Cr v1 Zn and chloride ion, were done to determine whether Cr I” could be extracted. This proved to be the case (Fig. 4) with a sample of real blowdown water where extraction to the desired low target levels was demonstrated by the liquid membrane technique for all three metal species. 100.0
I
1
total $ /
I
I
I
I
Blowdown Feed Extracted: Total Cr 18.65 pp~ Cr”’ 14 p pm Zn 3.74 ppm Cl- 230 ppn pH 6.2
Cr Cl”’
10.0
M:
I:
71”,:SlOON 15% TBP 104a Surfactan! 4% Aliquat
5% NaOH 25000 ppm Cl
M/l=2,F/E=5 390
rpm
ZE
B
2
-0
-
1.0
Zn Targe!
ti t;
Total Cr Target
2
.lO
ppmafter
(Zn < 0.005
.Ol
1 1
1
2
EXTRACTION
Fig. 4. Simultaneous extraction
I
I
3
4 TIME,
I
5
2 minute
I
b
1 J 4
MINUTES
of Crvl, Cr”I, and Zn.
A limited examination of liquid-liquid equilibria using solvent extraction methodology was done to define the extraction chemistry. Some of these results are shown in Table 1. They indicate significant extraction of CrV1 by Aliquat, somewhat less effective extraction of Crm, competition by chloride ion, and no extraction of zinc. The zinc results present no conflict with those of Figs. 3 and 4 since liquid membrane extraction employs stripping as well as extraction at the same time: a small concentration of zinc in the oil
259 TABLE
1
Liquid-liquid
equilibria
Substance
Charge (ppm)
CrV1 Cr’r’ Zn Cl
using Aliquat Effluents
(ppm)
Blowdown, pH 5.8
Oil
Aqueous, pH 5.8
Oil (by difference)
5.1 3.8 7.78 395
0 0 0 3210
0.078 0.442 7.32 550
50.2 33.6 4.6 1660
Conditions: 25”C, 15 minute oil = 10 (by weight).
equilibration,
Distribution ratio oil/aqueous
644 76 0.63 3.0
oil phase 4 wt. % Aliquat
in SlOON;
aqueous/
membrane might be effectively stripped at an appreciable rate into the internal phase of the emulsion to produce an efficient overall extraction of zinc. The large concentration of chloride ion in the oil phase in this test reflects the fact that Aliquat is presented as the chloride salt. This leads to a definition of the extraction stoichiometry for hexavalent chromium: it was observed for less than 200 ppm of CrV’ in the aqueous feed that two moles of counter-ion (chloride or hydroxide) move to the aqueous phase for each mole of CrV1 extracted into the organic phase over the pH range of 6 to 12. This is in accord with the reported chemistry of Aliquat extraction [14] :
7%
p
2 [R3NCllo,g
[(R3N)ZCr04]0rp
+ CrOimaq +
+ 2 Cl,,
(2)
MAXIMUM
/
0
5
(e
I
I
I
I
1
lo
15
20
25
30
Aliquat Fig. 5. Aliquat/Cr
= Observed)
in
for acid feed.
Oil
Phase,
mmoles
3!5
260
However, for more acidic systems (pH 6.0) the Aliquat/Crv” ratio was observed to be l/l, as shown in Fig. 5. This is explained by the aqueous solution behavior of hexavalent chromium [l&--20] : CrOi-
(yellow)
2 CrO, (OH)
f
+ H’
+ HCrO,-
Cr, Ot-
(orange)
CH, 2 [R&l]
= CrO,(OH)(3)
+ H, CH,
erg + CrzOt,,
+
[(R3h2Cr20710,
+
2 clas
(4)
Strippan t This component is a water-soluble substance able to remove the extracted species from the oil phase (membrane) and concentrate them in the internal phase. Because the chromium and zinc extracted from blowdown should ultimately be recycled to the cooling water circuit, an additional set of requirements are placed on the stripping chemistry. Compatibility with the corrosion-inhibiting nature of chromium and zinc is an obvious requirement, and irreversible chemical changes such as those implied by strong complexing agents are ruled out. No water-soluble substance in the emulsion which is not acceptable in the chromium and zinc-free blowdown stream is allowed. The literature of chromium and zinc extraction by Alamine and Aliquat [12,14--161 indicates that effective stripping should be accomplished either by strong sodium hydroxide solutions or by sulfuric acid. The extraction chemistry of Cr v1 by Aliquat, using hydroxide strippant above pH ‘7, is probably represented by the following two equations (assuming the solution of RqNOH in oil is maintained by the addition of alcohols): CH, 2
CH3
[R3~OHlo, + CrOi-cfeedj=+ [(%hCr0410u CH,
[(R&
+ 2OH&,,,
(5)
CH,
Cr%l oil + 2 OH&ernalj f 2 FdJOW ofi + CrO&hternalj
(6)
Overall, CrOt&?,, Aliquat-Crv’ CH3
+ 2 OH&ernan with sulfuric
+
CrOz&ternar)
acid strippant
+ 2 OH&,,, is described
as follows:
(7)
261 CH, [W3
h
CJ& Cr,
O&,
+ 2 HS04cintema,J+ 2 fR~~HS04l~il + Cr20:-c,,,,,,,
(9)
Overall, Crz 02?-(feed) + 2 HSoi
(internal)
+
Crz
o~
+
2 HSO~(feed)
(10)
Since the blowdown water samples examined in this work demonstrated no buffering action on addition of acid or base, even small additions of strong acid or base from the internal reagent to the feed can result in large changes in pH. The strippant-carrier combination of Aliquat-acid at a pH high enough to extract zinc was therefore not favored in view of acid build-up in the feed. As the pH of the aqueous feed rises during extraction by a liquid membrane emulsion using sodium hydroxide as strippant, the extraction suffers. Attempts to control this situation by reducing breakage will be discussed later. The other source of pH increase is, of course, the extraction chemistry itself, which M:
F:
73.0% 15 .O% 10 .O% 2 .O%
I:
SlOON TBP Surfactant Aliquat
19 ppm Cr”‘,
5% NaOH
l
5% NaOH . .705M - Cl
5 ppm Zn, 500
ppm Cl,
PH 6.0
i
\
0.1
0
I
1
I
2
.pH
I
3
EXTRACTION Fig. 6. Effect
of chloride
on hydroxide
I
4 TIME,
10.6
I
5 MINUTES
internal phase.
I
6
J 7
262
implies transfer of hydroxide ion from the internal phase to the feed as chromium is carried in the reverse direction (eqn. (‘7)). For membrane formulations in which the hydroxide form of Aliquat is marginally soluble, the transfer of hydroxide ion to the feed was observed to be somewhat reduced by the addition of chloride ion to the internal reagent phase (Fig. 6). The participation in stripping by the added chloride ion is indicated by the increased extraction of CrV’ and by the significant drop in final pH of the feed when chloride and hydroxide were used together. The difference in efficiency of stripping between hydroxide and chloride ion was not determined, largely because of emulsion instability problems encountered when hydroxide was absent from the internal phase altogether. It was observed that use of the chloride form of Aliquat did not result in phase separation from the membrane under conditions where this happened with the hydroxide form. High chloride in the internal phase should favor extraction and stripping by eqn. (4). Membrane
Once the blowdown feed solution was characterized as being free from buffering action by dissolved salts, the importance of reducing emulsion breakage became evident. The amount of breakage of the membrane which can be tolerated in this system varies inversely with the strippant concentration. Membrane stability improves as the viscosity of the membrane increases. These considerations led to a choice of SlOON as the oil base for membranes in much of this work. Table 2 shows typical oil base viscosity data. TABLE 2 Oil bases for liquid membrane extraction of blowdown Oil
Viscosity at 25°C (cP)
LOPS (Low Odor Paraffinic Solvent) SlOON (Isoparaffinic solvent) S600N (Isoparaffinic solvent)
2.4 19.5 210
S600N is so viscous that the droplet size for a given formulation is greater than if SlOON or LOPS were used under comparable conditions. This leads to lower rates unless the temperature is raised, so that the net effect of using S600N is largely cancelled. SlOON tended to minimize breakage (by comparison with LOPS) while maintaining small droplets of internal phase for high rates. With any of the oil bases considered, a tendency for derivatives of Aliquat to induce phase separation in the membrane was observed. It was, therefore, necessary to incorporate 3 wt.% of heavy alcohol (nonyl or decyl) in the membrane formulation to prevent phase separation by Aliquat in the hydroxide or dichromate forms.
In the formulation work, care must be taken to allow for interaction of the various ingredients in the oil phase. Carrier-surfactant interaction, to be discussed later as a limitation in process refinement, is an example of this. Results and discussion
Extraction of chromium from water containing no chloride The loading capacity of the emulsion in a liquid membrane extraction is of interest for process design. This was studied by extracting from quite concentrated feed solution, breaking the loaded emulsion, and characterizing the separated internal phase which then contained a significant amount of the metals, Analysis by X-ray methods of the internal phase showed 11,700 ppm chromium and around 3300 ppm zinc. Competition by chloride in extraction of chromium In Fig. 7 it is seen that a rapid extraction of hexavalent chromium from water can be accomplished. By comparison, Fig. 8 shows the results of an extraction performed under essentially identical conditions except that the feed contained 500 ppm of chloride ion. The rate is prohibitively slowed by the presence of chloride. The effect of chloride on chromium extraction is understood as a competition for the Aliquat carrier. The ratio of chloride to chromium in the fresh blowdown is SOO/lS = 28/l by weight, or 41/l on a mole basis. If chromium were reduced to 0.05 ppm this ratio becomes lo4 on a weight basis. Limitations on strippant In designing the separation process, the concentration of strippant cannot be very high. This is because if the zinc and chromium are added back to the cooling tower in a solution highly concentrated in strippant, the net effect could be an overall increase in dissolved solids. It is therefore important to choose a maximum acceptable ratio of strippant ion to chromium in the loaded internal reagent and work back to the extraction step from that choice. Obviously, there are many factors which have a bearing on this, such as the solids content of the makeup water, the ratio of hydroxide to chloride in the strippant, treating of the internal phase after it leaves the coalescer to adjust the pH or the ratio of CrV1/Crm, etc. All these considerations implied a value of around 7 for the strippant/ chromium mole ratio. As was previously discussed, no strong differences between hydroxide ion and chloride ion as to strippant efficiency were evident. The molar concentration of strippant was therefore taken as the sum of hydroxide and chloride. The first-order kinetic data of Figs. 9 and 10 show how the hydroxide ion concentration in the internal phase relates to the extraction of chromium(V1). Partial loading of the internal phase reduces the efficiency of the strippant (Fig. 10). The rate loss appears to be in excess of that expected from simple neutralization of the hydroxide by the chromium and zinc.
3SWHd
SnO3ll~W
-1
NI I”Jdd ‘&3
1
1
M: 80% SlOON 15 Surfactant 3 Nonyl Alcohol ! Aliquat
F: 18 ppm Cr 5 ppm Zn pH 6.0
1.25M OH-
I:
0.6252 M/I
= 1,
F/E
200
rpm
.
‘1.
W(
.
OH-.
= 4
.Nm\m
\
min.
-il.5
l
l
\.,
t$ = 5.3
min.
\ )-
1
,
I
30
20
10 EXTRACTION
TIME,
MINUTES
Fig. 9. Effect of strippant concentration
on extraction rate.
0.18 7
z 5 . X
O.lb0.140.12
P 2
0.08
”
0.06 0.10 I
v) 0’
(No Initial Loading
kT fk-
I
of Cr/Zn),m EXTRACTION CONDITIONS AS IN FIGURE 9
8
0.2
0.4
0.6
0.8
1.0
1.2
-M NaOHIN I
OH/C?
1.4
= 65
1.6
Of EMULSfON
Fig. 10. Effect of internal (I) loading on extraction rate,
1.8
2.0
2.2
2.4
266
Interaction of carrier and surfac tant If a surfactant molecule is ionized to produce an anionic polar group, may react with Aliquat to form an oil-soluble salt in the membrane:
it
CH, (R1);NCH3 Aliquat cation
+ -O=R2
+
(R,)&O=R2
(11)
Salt
Surfactant anion
Depending on the association/dissociation of the salt, Aliquat might be effectively removed from the membrane, to give lower extraction rates than if no salt formed. This possibility was investigated experimentally (Fig. 11); halving the Aliquat concentration in the membrane did not appear to change the initial extraction rate. On the other hand, a drastic increase in chromium content of the extracted feed after 5 minutes for the system containing higher carrier concentration was observed. This suggests, rather than a simple loss of carrier, that Aliquat is behaving as an oil-soluble surfactant antagonistic to
M: 0 High Aliquat
m Low Aliquat
71y SbOON 15 TBP 10 Surfactant 4 Aliquat 1698
ppm Cr”‘,
38 ppm Cr”‘, M/l
80.58: S600N 7.5 TBP 10 Surfactant 2 Aliquat
= 2,
F/E
425
11. Effect
2.5M-
NaOH
pti 6.0
= 2.5,
200
rpm
1
I
I
10
20
30
EXTRACTION Fig.
ppm Zn,
5 ppm Zn,
of surfactant/carrier
TIME, ratio
MINUTES
267
the effect of the surfactant already there. Microscopy of the two emulsions of Fig. 11 before the extraction tests showed that the higher proportion of Aliquat produced a “loose” appearance with droplets around six micrometers in diameter, while the droplets of the emulsion with half the Aliquat content were at most one micrometer in size. The conditions of extraction set by the system lead to a convenient measurement of breakage. Experimentally, samples of the aqueous phase were taken and the change in pH was related to a percentage loss of the internal reagent due to breakage. Figure 12 shows typical breakage test results at 25°C. For the process design of the potential application, it was permissible to produce a membrane which would lead to breakage release of around half a percent of the internal reagent in 30 minutes at 300 rpm stirring. Fifteen percent surfactant content in the membrane met this requirement. Extraction testing, using a model of the blowdown feed instead of pure water as in the breakage tests of Fig. 12 showed that 400 rpm stirring led to 50% swell as compared to 17% at 200 rpm, or 23% at 400 rpm in water. Breakage at 400 rpm in the model feed may have been worse than in pure 2.5 M:
2% Aliquat 3% Nonyl Alcohol 1 O/l 5% Surfactant 85;80% SlOON
2-
g $ . : :: w E
I: 5% F: M/I 400
NaOH
H20 = 1, rpm
1.5-
l-
0.5-
STIRRING
Fig. 12. Emulsion breakage.
TIME,
MINUTES
F/E
= 5
268
M: 80% SlOON 15 Surfactant 3 Nonyl Alcohol 2 Aliquat
I_ 0
1
I
10 EXTRACTION
Fig. 13. Stirring
1. 5% NaOH F: 18 ppm Cr”‘, 5 ppm Zn, pH 6.0 M/l = 1, F/E = 4
t
30
20 TIME,
MINUTES
rate.
water as well, since the chromium extraction rate dropped significantly at the higher stirring rate (Fig. 13). These results imply a maximum stirring rate of 200 rpm, at which the expected breakage would be lower than the 0.5% level found at 400 rpm for the same membrane. pH control
We have noted that Aliquat performs better as a carrier for CrV’ at low pH than at high pH. Since the rate of extraction of chromium from the blowdown appeared to limit our approach to a process scheme, and since control of the feed pH to a value less than 9 in the extractor was indicated by product quality specifications, it was worth examining the chromium extraction rate under conditions of controlled pH. The results, shown in Fig. 14, indicate that only a modest rate improvement was achieved.
269
(pH 9 - 11.5) NO ACID ADDED
O.l-
M: 80% SlOON 15 Surfactant 3 Nonyl Alcohol 2 Aliquat
200
EXTRACTION
rpm
TIME,
MINUTES
Fig. 14. pH Variations during test effect of pH control.
Conclusions Demonstration of extraction We have demonstrated that liquid membrane extraction can be used to extract hexavalent chromium to (0.05 ppm, trivalent chromium to <0.5 TABLE 3 Liquid membrane extraction of Cr and Zn from blowdown Stream
Total Cr (ppm) Crvl (ppm) Zn (ppm) PH
Blowdown
_
feed
Typical
Sample, sx 0315
10 7.5 5 6
18.65 14 4.63 6.5
Target effluent
Product achieved
0.25 0.05 1 6-9
0.030 0.030 0.005 11.3
Membrane: 71% SlOON; 15% TBP; 10% surfactant; 4% Aliquat. Internal: 5% NaOH. M/I = 2; 25000 ppm Cl-; Feed/Emulsion = 5. Extracted 5 min at 25”C, stirred 390 rpm.
270
ppm, and zinc to <1 ppm (Table 3). The results show that all three metallic species can be simultaneously extracted, and the formulation used provides a starting point for optimization work. Limiting constraints of process and product Future work is needed to investigate the extraction of chromium and zinc, using acceptable strippant concentrations in the emulsion and actual blowdown effluent. The limiting constraints for this particular application were found to be: (a) Competition for the carrier species between chromate and chloride ion in the blowdown; (b) Interaction between the surfactant and the carrier species employed; and (c) Limitations on strippant concentration in the emulsion and on allowable Crvl in the product water. Assessment The results show that if there is an appreciable amount of chloride in the waste effluent, it will interfere with the extraction of chromium and zinc. However, the liquid membrane process shows promise for effective removal and recovery of chromium and zinc ions from aqueous solutions of low chloride content. Acknowledgements This paper is dedicated to Professor Vivian Stannett in recognition of his pioneering research in the membrane separation field. Helpful conversations and support were provided by R.P. Cahn, J.W. Frankenfeld, S.E. Jaros, and A.L. Pozzi, Jr. Much credit for providing background information and economic analysis goes to A.J. Reitano, Jr. The laboratory results were obtained by L.W. Anderson, whose efforts are greatly appreciated. References 1 N.N. Li, Encapsulation and separation by liquid surfactant membranes, Chem. Eng. (U.K.), (July 1981) 325-327. 2 F.J. Hurst, D.J. Crouse and K.B. Brown, Recovery of uranium from wet-process phosphorus acid, Ind. Eng. Chem. Process Des. Dev., 11 (1972) 122. 3 W.S. Ho, T.A. Hatton, E.N. Lightfoot and N.N. Li, Extraction with liquid membranes: A diffusion-controlled model, AIChE J., 28(4) (1982) 662. 4 H.C. Hayworth, W.S. Ho, W.A. Burns and N.N. Li, Extraction of uranium from wet process phosphoric acid by liquid membranes, Sep. Sci. Tech., 18( 6) (1983) 493. 5 R.E. Terry and N.N. Li, Extraction of phenolic compounds and organic acids by liquid membranes, J. Membrane Sci., 10 (1982) 305. 6 R.P. Cahn, J.W. Frankenfeld, N.N. Li, D. Naden and K.N. Subramanian, Extraction of metals by liquid membranes, in: N.N. Li (Ed.), Recent Developments in Separation Science, Vol. VI, CRC Press, Boca Raton, FL, 1981, p. 51.
271 7 J.W. Frankenfeld, R.P. Cahn and N.N. Li, Extraction of copper by liquid membrane, Sep. Sci. Tech., 16(4) (1981) 385. 8 N.N. Li, Facilitated transport through liquid membranes, J. Membrane Sci., 3 (1978) 265. 9 E.S. Matulevicius and N.N. Li, Facilitated transport through liquid membranes, Sep. Purif. Methods, 4(l) (1975) 73. 10 J.W. Frankenfeld and N.N. Li, Wastewater treatment by liquid ion exchange in liquid membrane systems, in: N.N. Li (Ed.), Recent Developments in Separation Science, Vol. 3 (Part A), CRC Press, Boca Raton, FL, 1977, p. 285. 11 T. Kitagawa, Y. Nishikawa, J.W. Frankenfeld and N.N. Li, Wastewater treatment by liquid membrane process, Environ. Sci. Technol., 11 (1977) 602. 12 A.M. Hochhauser and E.L. Cussler, Concentrating chromium with liquid surfactant membranes, AIChE Symp. Ser., 71(152) (1975) 136. 13 R.W. Cattrall and M.Z. Ilic, Equilibrium constants for the extraction of iron(II1) from 6 M hydrochloric acid solutions by tri-n-octylammonium chloride in chloroform, J. Inorg. Nucl. Chem., 40(7) (1978) 1446. 14 General Mills Corp., Chromium, Tech. Bull. CDSl-61, 1961. 15 G.R. Smithson, Jr., Water Pollut. Control Res. Ser., 12OlElE (March 1971). 16 C.W. McDonald and R.S. Bajwa, Removal of toxir! metal ions from metal-finishing wastewater by solvent extraction, Sep. Sci., 12(4) (1977) 435. 17 G.P. Haight, Jr., D.C. Richardson and N.H. Coburn, A spectrophotometric study of equilibria involving mononuclear chromium(V1) species in solutions of various acids, Inorg. Chem., 3 (1964) 1777. 18 J.Y. Tong, Chromium(V1) species and spectra in acidic solutions, Inorg. Chem., 3 (1964) 1804. 19 R.C. Weast (Ed.), 1976-1977 Handbook of Chemistry and Physics, 57th edn., CRC Press, Cleveland, OH. 20 N.N. Li, Novel liquid membrane formulations, U.S. Patent 4,259,189, March 31, 1981. 21 M.N. Li, R.P. Cahn, D. Naden and R.W.M. Lai, Liquid membrane processes for copper extraction, Hydrometallurgy, 9 (1983) 277-305. 22 T.A. Hatton, E.N. Lightfoot, R.P. Cahn and N.N. Li, An internal recycle mixer for solvent extraction, mass transfer characterization with liquid surfactant membranes, Ind. Eng. Chem. Fundam., 22(l) (1983) 27.
Journal Elsevier
EXTRACTION BLOWDOWN
E.J.
251 in The Netherlands
OF CHROMIUM AND ZINC FROM BY LIQUID MEMBRANES
COOLING
TOWER
FULLER
Exxon
and Engineering
Research
and NORMAN
Co., Linden,
NJ 07036
(U.S.A.)
N. LI
UOP Inc.,
Des Plaines,
(Received
February
IL 60016
10, 1983;
(U.S.A.) accepted
in revised
form
June
6, 1983)
Summary Since the invention of liquid membrane technology in 1968, a number of potential applications have been investigated at Exxon Research and Engineering Company, This paper reviews the recovery of metal ions from an aqueous solution, with specific details of research on the simultaneous extraction of chromium(VI), chromium(III), and zinc from a cooling tower blowdown stream. The important factors affecting the extraction of chromium and zinc are discussed.
Introduction The liquid membrane technology involves making an emulsion of either oil-in-water or water-in-oil [ 11. The emulsion is made so as to disperse tiny droplets of one liquid phase in a continuous, immiscible second phase. The critical elements here are (1) the introduction of high and uniform shear to the mixture of the two phases, such that the droplet size will be as small as desired, and (2) the presence of some stabilizing surfactant such that the dispersed droplets will not at once coalesce. The emulsion is then added to a relatively large volume of a liquid phase which is immiscible with the continuous phase of the emulsion, and dispersed as globules by the application of low shear. In this form, ions or dissolved species may be transferred across the liquid membrane, which is the continuous phase of the emulsion, into or out of the droplet or the internal phase of the emulsion. It is important to note that unless the oil phase is properly formulated to impart a high degree of stability to the emulsion, the rate of breakage of the droplets of internal phase may be significant, or the oil phase may tend to emulsify more aqueous phase during mixing conditions. Compositions and procedures have been worked out over a number of years at Exxon which provide highly reliable extraction systems. For oil-type liquid membranes, the key to emulsion stability is the use of one of a class of oil-soluble surfactant species developed by Exxon. These surfactants are characteristical-
0376-7388/84/$03,00
o 1984
Elsevier
Science
Publishers
B.V.
252
ly made from polyisobutylene as starting material, with a variety of polar groups bonded to the end of the chain [ 201. The substance to be extracted from the feed solution, after transport across the oil membrane, is stripped by the internal reagent and collected inside the droplets of the emulsion globules in a form which is not soluble in the oil membrane. In contrast to the normal solvent extraction scheme, extraction and stripping are not done in separate units but take place simultaneously in the course of liquid membrane contacting in the same vessel. Once the emulsion has contacted sufficient feed solution to “load” the internal reagent phase with desired product, the globules are allowed to settle into an emulsion phase, leaving treated feed solution. A separate unit to coalesce the emulsion into oil and rich extract phase is required; however, because of the high degree of concentration ordinarily attainable by these systems, the emulsion breaking step deals with streams much smaller in size than the feed solution. This implies that emulsion-making and emulsion-breaking equipment need not be very large. The capacity of the emulsion depends on the internal reagent, not on the oil phase as in solvent extraction. The consequence is markedly reduced holdup of oil phase in the process and whatever oil-soluble ligands or “carriers” are necessary to transport the extracted species across it. Another advantage of liquid membrane operations is the potential for greatly reduced entrainment losses relative to solvent extraction. Not only does the system inherently contain less oil phase, but it may happen that the low shear of the contacting step produces larger globules than would be allowable if the oil phase is to be “loaded” in solvent extraction. Entrainment is a major source of loss of oil phase in solvent extraction [2], and generally is worse for more intensely mixed phases where the oil phase is finely dispersed in order to achieve a “loaded” state. Depending on the system, it is commonly measured that entrainment losses of oil phase in liquid membranes extraction are extremely low - below 50 ppm. Liquid membrane separation mechanisms Much has been learned over the past several years as to the influence of emulsion properties on transport across liquid membranes [ 31 and on the accumulation of extract in the internal phase 1211. It is required that the water-in-oil emulsions of interest for metals extraction be stabilized by surfactants with the following properties: (1) They must be oil-soluble and water-insoluble. (2) They must be moderately inert chemically toward ion-carrying substances. (3) They must confer high stability on the interfaces between oil and internal reagent and between oil and feed solution. Ordinarily it is observed that increasing the concentration of surfactant in the oil membrane produces a corresponding increase in stability; that is, the containment of internal phase of the emulsion as the emulsion is dispersed in an external aqueous phase is improved. The limit of surfactant concentra-
253
tion is set mainly by incipient emulsification of feed, cost of surfactant, difficulty of emulsion breaking, and kinetics of extraction. Organic to aqueous phase ratios in the emulsion for an extraction scheme may vary from around 2/l to l/2. In these systems, it is highly desirable to avoid significant breakage of the emulsion during contacting of the feed solution. If the organic/aqueous ratio is below l/2, more breakage is observed; while a ratio over 2/l invites emulsification of feed. The actual ratio of feed to emulsion may be quite high when recycle schemes are used. Inside the feed/emulsion contactor, the limit of feed to emulsion ratio at any given time will be set by the efficiency of phase separation of feed from emulsion [ 221. Mathematical modeling of liquid membrane extraction systems has correlated experimental results for the extractions of uranium, copper and phenol quite well [ 3-51. The models indicate no internal circulation of the droplets within globules of emulsion. The liquid membrane configuration lends itself readily to systems which are kinetically limited, such as the extraction of copper(I1) ion from streams containing large amounts of iron. In solvent extraction, the complexing agent used to concentrate copper in the oil phase may quickly load copper which is slowly replaced by the iron. In liquid membrane operations, a low concentration of complexer (carrier) in the membrane usually suffices to promote rapid loading of the internal phase with copper, even in the presence of large amounts of iron [6,7]. It appears that the diffusion of complexes through the membrane and their stripping are quite rapid, effectively keeping a supply of stripped carrier available for the feed solution at all times [ 81. Water can be purified of phenol [ 5,9] or ammonia [lo] impurities by contacting with liquid membrane emulsions. The stripping agent (caustic in the case of phenol, acid in the case of ammonia) converts the extracted species to an ion which is insoluble in the oil and therefore remains in the internal phase. In contrast to ammonia and phenol, metal ions in aqueous solution ordinarily are present as ions which do not dissolve appreciably in oil. For liquid membrane extraction to operate, a carrier agent must be supplied which reacts with the metal ion to form an oil-soluble species [ 1,4,7,8,9], Treatment of cooling tower blowdown with liquid membranes The effluent from cooling tower blowdown contains chromium(VI), chromium(III), and zinc. The increased awareness in society of the potentially adverse affects on the environment caused by uncontrolled dumping of waste imposes strict limitations on the disposal of cooling tower blowdown. This work was undertaken to evaluate the possibility of utilizing liquid membrane technology to treat water containing low but environmentally intolerable levels of chromium and zinc. Operation of industrial-scale cooling tower circuits necessarily implies
254
evaporation of water and the addition of fresh makeup water. By and large, the dissolved salts are not lost during the cycles of cooling (some spray, or “windage” loss does occur) and are therefore concentrated. A purge stream, or “blowdown”, is removed to control incipient solid precipitation. Because the solids originally came from the environment as dissolved ions in the makeup water, their reintroduction into the water table causes no problem. However, the addition of toxic corrosion inhibitor to the cooling water is normally necessary and its removal from the blowdown is desired. In the case of zinc chromate inhibitor, government regulations limit the allowable amounts of zinc and chromium which can be discarded with the purge stream. The regulation limits which were used as targets for these extraction studies are 1 ppm Zn, 0.5 ppm total chromium, and 0.05 ppm hexavalent chromium. Recovery of hexavalent chromium from water by liquid membranes has been previously reported [ 11,121. Removal from several hundred ppm to less than 1 ppm in the effluent has been demonstrated [ll] . The approach to an extraction system for cooling tower blowdown used in this work involved (1) formulation of the emulsion to remove CrV’, Crm, and Zn” from blowdown to the desired levels, and (2) adaptation of the emulsion toward a viable process for not only extracting the ions of interest but recovering them for reuse in the cooling tower circuit. Experimental Materials used in experiments
The membrane phase used was a mixture of surfactant, solvent extractant and extractant stabilizers. The surfactants used was ENJ, 3029 polyamine with average molecular weight of 1500, manufactured by Exxon. The solvents used were SlOON, S600N and LOPS, either singly or as a mixture of SlOON with either S600N or LOPS. All these solvents are basically isoparaffin hydrocarbons manufactured by the Exxon Chemical Company (Table 2). The extractant used was Aliquat, made by General Mills*. C.P. grade tributylphosphate and nonyl and decyl alcohols were used to ensure the solution of Aliquat in the solvent oil phase. The concentration ranges of these compounds are: Solvent 100N: 60-90 wt.%; TBP: lO-15% (or nonyl/decyl alcohols: 3%); ENJ 3029 surfactant: lo-15%; Aliquat extractant: 2-5%. The emulsified internal phase was 5% caustic solution. Emulsion
preparation
in laboratory
Emulsions for use in bench-scale batch tests were made by extensive mixing of the organic membrane phase with the internal aqueous phase in a Waring Blender. A mixing time of at least six minutes was used to make batch emulsions in this way. The membrane (oir oil) phase to internal phase ratio was in the range of 1 to 2. *Now
Henkel
Corporation.
Test procedures Batch stir tests were carried out by contacting emulsion and aqueous feed in a 1000 ml baffled resin kettle stirred by two marine propellers at ZOO-400 rpm. When the mixing speed was within this range, high extraction rates with low entrainment (aqueous samples were visually clear) were obtained. The ratio of aqueous feed to emulsion was in the range of 4 to 5 by volume. Samples were taken periodically by pipette and the emulsion and aqueous feed rapidly separated by filtering through a filter paper. The aqueous phase was then analyzed. Analytical methods Analysis of the aqueous feed being treated was done by an atomic absorption technique for zinc and total chromium. Hexavalent chromium was determined by calorimetry; trivalent chromium was taken as the difference between total chromium and the hexavalent chromium content. Chloride ion was determined by an X-ray method (for concentrations of several thousand ppm or greater) or by a specific ion electrode. Samples of the aqueous phase were taken after a few minutes’ settling time to avoid contamination by the emulsion. Carrier It was taken as a starting point that an agent capable of complexing with anions such as CrOi- or Cr,Ot- would be needed. The high (up to 1000 ppm) chloride ion concentration in the waste water might be expected to produce anionic species of Cr3+ and Zn2’ complexes with chloride, which might be taken into the oil phase by the R3NCH3+ carrier (e.g., CrCI,-, CrCl:-, CrCli-, ZnCl,-, ZnCli-). The exact ionic species present are functions of not only Crv’, Cr”‘, Zn, and Cl concentrations, but hydroxide ion concentration as well [13]. Previous work demonstrated that Alamine 336 and Aliquat 336-S (General Mills liquid ion exchange agents; tri-n-octylamine and methyltri-n-octylammonium chloride, respectively) were capable of extracting chromate or dichromate ions into an oil phase, and that the chromium was effectively stripped from the oil by sodium hydroxide [ 14,151. Chromium(VI), cadmium, and zinc have been shown to be simultaneously extracted from water containing chloride ions by Alamine 336 in xylene, and stripping was again demonstrated with sodium hydroxide solutions [ 161. Liquid membrane extraction of chromium(VI) using tridodecylamine as a carrier and 0.1 M sodium hydroxide as strippant has been reported [ 121. Mention has already been made of the extraction of numerous metal species from wastewater using liquid membranes [ 4,6,7,11]. This background served as a starting point for this research, although it is noted that no previous research has demonstrated the removal of chromium down to the range of 0.05 ppm, the simultaneous extraction of Crvl, Cr”‘, and Znn ions, and the extraction of CrV’ alone in the presence of high concentrations of chloride ion.
256
Experiments were carried out using Alamine 336 as a carrier, to investigate the possibility that zinc and chromium(II1) might be extracted along with CrV1. This carrier must be protonated to be active. 2 RJNH+ + Cr, Ot-
+ (Rg NH), Cr207
(oil-soluble)
(1)
In general, extraction with Alamine is ineffective above pH 3. This was observed in tests with a model feed containing CrV1 and Zn. At low pH the extraction of hexavalent chromium could be accomplished with a liquid membrane containing Alamine, but at higher pH no extraction was obtained. Unfortunately, zinc extraction folIotied a reverse trend. This means that zinc was extracted at pH > 7.0. The results, shown in Figs. 1 and 2, appear to rule out the use of Alamine in this separation. The most promising carrier for this separation proved to be Aliquat 336. Figure 3 shows results of an extraction done under conditions comparable to those of Figs. 1 and 2. Chromium is evidently extracted more completely, but more slowly using Aliquat carrier than with Alamine (where Alamine is effective at all). The results of Figs. 1, 2 and 3 also suggest the effects of
Membrane: (MI
90% SIODN 5% Surfactant 2% Alamine 3% Nonyl Alcohol 5% NaOH
Internal: (1) Feed: (F)
1; 1000
M/IR
=
TIME,
1,
pdbm, c;,“’
pH 1.0
ppm Cl
F/E
= 4,
MINUTES
400
rpm - PH
(Feed)
I
I
I
I
0
10
20
30
EXTRACTION
Fig. 1. pH 1.0 Liquid
TIME,
membrane
MINUTES
extraction.
-
-
I-
30
20
ID
EXTRACTION
TIME,
MINUTES
1xl 30:6
-
MlNUTES
6.0 7.0 7.5
TlME,
-
i
Conditions similar to Fig. 5, of Feed 3.0 at Start, 500
Fig. 2. pH 3.0 Liquid membrane extraction.
r: t; I
G50 z t 4 5 40 z
\
i
\ \ 0 \
M/l
F:
\
l
I
20 30
1051” 7
EXTRACTION
I
TIME,
200
11.5 11.5
I
ppm Cl,
pH (Peed) 11.1 1016 I:*: 6.0
MINUTES
rpm
500
5% NaOH
5 ppm 2n,
MINUTES
= 4,
or”‘,
F/E
;pm
SlOON I: Surfactant Nonyl Alcohol Aliquat
CHROMIUM
= I,
,‘t
80% 15% 3% 2%
Fig. 3. Aliquat carrier for CrVr and Zn.
10
20
8C
9t
M:
258
chloride ion competition for available carrier, and limitations of the carrier because of its interaction with surfactant. This will be discussed further later. At present, the carrier Aliquat is seen to allow simultaneous extraction of CrV1 and Zn. Tests with a typical blowdown sample, containing Cr”* as well as Cr v1 Zn and chloride ion, were done to determine whether Cr I” could be extracted. This proved to be the case (Fig. 4) with a sample of real blowdown water where extraction to the desired low target levels was demonstrated by the liquid membrane technique for all three metal species. 100.0
I
1
total $ /
I
I
I
I
Blowdown Feed Extracted: Total Cr 18.65 pp~ Cr”’ 14 p pm Zn 3.74 ppm Cl- 230 ppn pH 6.2
Cr Cl”’
10.0
M:
I:
71”,:SlOON 15% TBP 104a Surfactan! 4% Aliquat
5% NaOH 25000 ppm Cl
M/l=2,F/E=5 390
rpm
ZE
B
2
-0
-
1.0
Zn Targe!
ti t;
Total Cr Target
2
.lO
ppmafter
(Zn < 0.005
.Ol
1 1
1
2
EXTRACTION
Fig. 4. Simultaneous extraction
I
I
3
4 TIME,
I
5
2 minute
I
b
1 J 4
MINUTES
of Crvl, Cr”I, and Zn.
A limited examination of liquid-liquid equilibria using solvent extraction methodology was done to define the extraction chemistry. Some of these results are shown in Table 1. They indicate significant extraction of CrV1 by Aliquat, somewhat less effective extraction of Crm, competition by chloride ion, and no extraction of zinc. The zinc results present no conflict with those of Figs. 3 and 4 since liquid membrane extraction employs stripping as well as extraction at the same time: a small concentration of zinc in the oil
259 TABLE
1
Liquid-liquid
equilibria
Substance
Charge (ppm)
CrV1 Cr’r’ Zn Cl
using Aliquat Effluents
(ppm)
Blowdown, pH 5.8
Oil
Aqueous, pH 5.8
Oil (by difference)
5.1 3.8 7.78 395
0 0 0 3210
0.078 0.442 7.32 550
50.2 33.6 4.6 1660
Conditions: 25”C, 15 minute oil = 10 (by weight).
equilibration,
Distribution ratio oil/aqueous
644 76 0.63 3.0
oil phase 4 wt. % Aliquat
in SlOON;
aqueous/
membrane might be effectively stripped at an appreciable rate into the internal phase of the emulsion to produce an efficient overall extraction of zinc. The large concentration of chloride ion in the oil phase in this test reflects the fact that Aliquat is presented as the chloride salt. This leads to a definition of the extraction stoichiometry for hexavalent chromium: it was observed for less than 200 ppm of CrV’ in the aqueous feed that two moles of counter-ion (chloride or hydroxide) move to the aqueous phase for each mole of CrV1 extracted into the organic phase over the pH range of 6 to 12. This is in accord with the reported chemistry of Aliquat extraction [14] :
7%
p
2 [R3NCllo,g
[(R3N)ZCr04]0rp
+ CrOimaq +
+ 2 Cl,,
(2)
MAXIMUM
/
0
5
(e
I
I
I
I
1
lo
15
20
25
30
Aliquat Fig. 5. Aliquat/Cr
= Observed)
in
for acid feed.
Oil
Phase,
mmoles
3!5
260
However, for more acidic systems (pH 6.0) the Aliquat/Crv” ratio was observed to be l/l, as shown in Fig. 5. This is explained by the aqueous solution behavior of hexavalent chromium [l&--20] : CrOi-
(yellow)
2 CrO, (OH)
f
+ H’
+ HCrO,-
Cr, Ot-
(orange)
CH, 2 [R&l]
= CrO,(OH)(3)
+ H, CH,
erg + CrzOt,,
+
[(R3h2Cr20710,
+
2 clas
(4)
Strippan t This component is a water-soluble substance able to remove the extracted species from the oil phase (membrane) and concentrate them in the internal phase. Because the chromium and zinc extracted from blowdown should ultimately be recycled to the cooling water circuit, an additional set of requirements are placed on the stripping chemistry. Compatibility with the corrosion-inhibiting nature of chromium and zinc is an obvious requirement, and irreversible chemical changes such as those implied by strong complexing agents are ruled out. No water-soluble substance in the emulsion which is not acceptable in the chromium and zinc-free blowdown stream is allowed. The literature of chromium and zinc extraction by Alamine and Aliquat [12,14--161 indicates that effective stripping should be accomplished either by strong sodium hydroxide solutions or by sulfuric acid. The extraction chemistry of Cr v1 by Aliquat, using hydroxide strippant above pH ‘7, is probably represented by the following two equations (assuming the solution of RqNOH in oil is maintained by the addition of alcohols): CH, 2
CH3
[R3~OHlo, + CrOi-cfeedj=+ [(%hCr0410u CH,
[(R&
+ 2OH&,,,
(5)
CH,
Cr%l oil + 2 OH&ernalj f 2 FdJOW ofi + CrO&hternalj
(6)
Overall, CrOt&?,, Aliquat-Crv’ CH3
+ 2 OH&ernan with sulfuric
+
CrOz&ternar)
acid strippant
+ 2 OH&,,, is described
as follows:
(7)
261 CH, [W3
h
CJ& Cr,
O&,
+ 2 HS04cintema,J+ 2 fR~~HS04l~il + Cr20:-c,,,,,,,
(9)
Overall, Crz 02?-(feed) + 2 HSoi
(internal)
+
Crz
o~
+
2 HSO~(feed)
(10)
Since the blowdown water samples examined in this work demonstrated no buffering action on addition of acid or base, even small additions of strong acid or base from the internal reagent to the feed can result in large changes in pH. The strippant-carrier combination of Aliquat-acid at a pH high enough to extract zinc was therefore not favored in view of acid build-up in the feed. As the pH of the aqueous feed rises during extraction by a liquid membrane emulsion using sodium hydroxide as strippant, the extraction suffers. Attempts to control this situation by reducing breakage will be discussed later. The other source of pH increase is, of course, the extraction chemistry itself, which M:
F:
73.0% 15 .O% 10 .O% 2 .O%
I:
SlOON TBP Surfactant Aliquat
19 ppm Cr”‘,
5% NaOH
l
5% NaOH . .705M - Cl
5 ppm Zn, 500
ppm Cl,
PH 6.0
i
\
0.1
0
I
1
I
2
.pH
I
3
EXTRACTION Fig. 6. Effect
of chloride
on hydroxide
I
4 TIME,
10.6
I
5 MINUTES
internal phase.
I
6
J 7
262
implies transfer of hydroxide ion from the internal phase to the feed as chromium is carried in the reverse direction (eqn. (‘7)). For membrane formulations in which the hydroxide form of Aliquat is marginally soluble, the transfer of hydroxide ion to the feed was observed to be somewhat reduced by the addition of chloride ion to the internal reagent phase (Fig. 6). The participation in stripping by the added chloride ion is indicated by the increased extraction of CrV’ and by the significant drop in final pH of the feed when chloride and hydroxide were used together. The difference in efficiency of stripping between hydroxide and chloride ion was not determined, largely because of emulsion instability problems encountered when hydroxide was absent from the internal phase altogether. It was observed that use of the chloride form of Aliquat did not result in phase separation from the membrane under conditions where this happened with the hydroxide form. High chloride in the internal phase should favor extraction and stripping by eqn. (4). Membrane
Once the blowdown feed solution was characterized as being free from buffering action by dissolved salts, the importance of reducing emulsion breakage became evident. The amount of breakage of the membrane which can be tolerated in this system varies inversely with the strippant concentration. Membrane stability improves as the viscosity of the membrane increases. These considerations led to a choice of SlOON as the oil base for membranes in much of this work. Table 2 shows typical oil base viscosity data. TABLE 2 Oil bases for liquid membrane extraction of blowdown Oil
Viscosity at 25°C (cP)
LOPS (Low Odor Paraffinic Solvent) SlOON (Isoparaffinic solvent) S600N (Isoparaffinic solvent)
2.4 19.5 210
S600N is so viscous that the droplet size for a given formulation is greater than if SlOON or LOPS were used under comparable conditions. This leads to lower rates unless the temperature is raised, so that the net effect of using S600N is largely cancelled. SlOON tended to minimize breakage (by comparison with LOPS) while maintaining small droplets of internal phase for high rates. With any of the oil bases considered, a tendency for derivatives of Aliquat to induce phase separation in the membrane was observed. It was, therefore, necessary to incorporate 3 wt.% of heavy alcohol (nonyl or decyl) in the membrane formulation to prevent phase separation by Aliquat in the hydroxide or dichromate forms.
In the formulation work, care must be taken to allow for interaction of the various ingredients in the oil phase. Carrier-surfactant interaction, to be discussed later as a limitation in process refinement, is an example of this. Results and discussion
Extraction of chromium from water containing no chloride The loading capacity of the emulsion in a liquid membrane extraction is of interest for process design. This was studied by extracting from quite concentrated feed solution, breaking the loaded emulsion, and characterizing the separated internal phase which then contained a significant amount of the metals, Analysis by X-ray methods of the internal phase showed 11,700 ppm chromium and around 3300 ppm zinc. Competition by chloride in extraction of chromium In Fig. 7 it is seen that a rapid extraction of hexavalent chromium from water can be accomplished. By comparison, Fig. 8 shows the results of an extraction performed under essentially identical conditions except that the feed contained 500 ppm of chloride ion. The rate is prohibitively slowed by the presence of chloride. The effect of chloride on chromium extraction is understood as a competition for the Aliquat carrier. The ratio of chloride to chromium in the fresh blowdown is SOO/lS = 28/l by weight, or 41/l on a mole basis. If chromium were reduced to 0.05 ppm this ratio becomes lo4 on a weight basis. Limitations on strippant In designing the separation process, the concentration of strippant cannot be very high. This is because if the zinc and chromium are added back to the cooling tower in a solution highly concentrated in strippant, the net effect could be an overall increase in dissolved solids. It is therefore important to choose a maximum acceptable ratio of strippant ion to chromium in the loaded internal reagent and work back to the extraction step from that choice. Obviously, there are many factors which have a bearing on this, such as the solids content of the makeup water, the ratio of hydroxide to chloride in the strippant, treating of the internal phase after it leaves the coalescer to adjust the pH or the ratio of CrV1/Crm, etc. All these considerations implied a value of around 7 for the strippant/ chromium mole ratio. As was previously discussed, no strong differences between hydroxide ion and chloride ion as to strippant efficiency were evident. The molar concentration of strippant was therefore taken as the sum of hydroxide and chloride. The first-order kinetic data of Figs. 9 and 10 show how the hydroxide ion concentration in the internal phase relates to the extraction of chromium(V1). Partial loading of the internal phase reduces the efficiency of the strippant (Fig. 10). The rate loss appears to be in excess of that expected from simple neutralization of the hydroxide by the chromium and zinc.
3SWHd
SnO3ll~W
-1
NI I”Jdd ‘&3
1
1
M: 80% SlOON 15 Surfactant 3 Nonyl Alcohol ! Aliquat
F: 18 ppm Cr 5 ppm Zn pH 6.0
1.25M OH-
I:
0.6252 M/I
= 1,
F/E
200
rpm
.
‘1.
W(
.
OH-.
= 4
.Nm\m
\
min.
-il.5
l
l
\.,
t$ = 5.3
min.
\ )-
1
,
I
30
20
10 EXTRACTION
TIME,
MINUTES
Fig. 9. Effect of strippant concentration
on extraction rate.
0.18 7
z 5 . X
O.lb0.140.12
P 2
0.08
”
0.06 0.10 I
v) 0’
(No Initial Loading
kT fk-
I
of Cr/Zn),m EXTRACTION CONDITIONS AS IN FIGURE 9
8
0.2
0.4
0.6
0.8
1.0
1.2
-M NaOHIN I
OH/C?
1.4
= 65
1.6
Of EMULSfON
Fig. 10. Effect of internal (I) loading on extraction rate,
1.8
2.0
2.2
2.4
266
Interaction of carrier and surfac tant If a surfactant molecule is ionized to produce an anionic polar group, may react with Aliquat to form an oil-soluble salt in the membrane:
it
CH, (R1);NCH3 Aliquat cation
+ -O=R2
+
(R,)&O=R2
(11)
Salt
Surfactant anion
Depending on the association/dissociation of the salt, Aliquat might be effectively removed from the membrane, to give lower extraction rates than if no salt formed. This possibility was investigated experimentally (Fig. 11); halving the Aliquat concentration in the membrane did not appear to change the initial extraction rate. On the other hand, a drastic increase in chromium content of the extracted feed after 5 minutes for the system containing higher carrier concentration was observed. This suggests, rather than a simple loss of carrier, that Aliquat is behaving as an oil-soluble surfactant antagonistic to
M: 0 High Aliquat
m Low Aliquat
71y SbOON 15 TBP 10 Surfactant 4 Aliquat 1698
ppm Cr”‘,
38 ppm Cr”‘, M/l
80.58: S600N 7.5 TBP 10 Surfactant 2 Aliquat
= 2,
F/E
425
11. Effect
2.5M-
NaOH
pti 6.0
= 2.5,
200
rpm
1
I
I
10
20
30
EXTRACTION Fig.
ppm Zn,
5 ppm Zn,
of surfactant/carrier
TIME, ratio
MINUTES
267
the effect of the surfactant already there. Microscopy of the two emulsions of Fig. 11 before the extraction tests showed that the higher proportion of Aliquat produced a “loose” appearance with droplets around six micrometers in diameter, while the droplets of the emulsion with half the Aliquat content were at most one micrometer in size. The conditions of extraction set by the system lead to a convenient measurement of breakage. Experimentally, samples of the aqueous phase were taken and the change in pH was related to a percentage loss of the internal reagent due to breakage. Figure 12 shows typical breakage test results at 25°C. For the process design of the potential application, it was permissible to produce a membrane which would lead to breakage release of around half a percent of the internal reagent in 30 minutes at 300 rpm stirring. Fifteen percent surfactant content in the membrane met this requirement. Extraction testing, using a model of the blowdown feed instead of pure water as in the breakage tests of Fig. 12 showed that 400 rpm stirring led to 50% swell as compared to 17% at 200 rpm, or 23% at 400 rpm in water. Breakage at 400 rpm in the model feed may have been worse than in pure 2.5 M:
2% Aliquat 3% Nonyl Alcohol 1 O/l 5% Surfactant 85;80% SlOON
2-
g $ . : :: w E
I: 5% F: M/I 400
NaOH
H20 = 1, rpm
1.5-
l-
0.5-
STIRRING
Fig. 12. Emulsion breakage.
TIME,
MINUTES
F/E
= 5
268
M: 80% SlOON 15 Surfactant 3 Nonyl Alcohol 2 Aliquat
I_ 0
1
I
10 EXTRACTION
Fig. 13. Stirring
1. 5% NaOH F: 18 ppm Cr”‘, 5 ppm Zn, pH 6.0 M/l = 1, F/E = 4
t
30
20 TIME,
MINUTES
rate.
water as well, since the chromium extraction rate dropped significantly at the higher stirring rate (Fig. 13). These results imply a maximum stirring rate of 200 rpm, at which the expected breakage would be lower than the 0.5% level found at 400 rpm for the same membrane. pH control
We have noted that Aliquat performs better as a carrier for CrV’ at low pH than at high pH. Since the rate of extraction of chromium from the blowdown appeared to limit our approach to a process scheme, and since control of the feed pH to a value less than 9 in the extractor was indicated by product quality specifications, it was worth examining the chromium extraction rate under conditions of controlled pH. The results, shown in Fig. 14, indicate that only a modest rate improvement was achieved.
269
(pH 9 - 11.5) NO ACID ADDED
O.l-
M: 80% SlOON 15 Surfactant 3 Nonyl Alcohol 2 Aliquat
200
EXTRACTION
rpm
TIME,
MINUTES
Fig. 14. pH Variations during test effect of pH control.
Conclusions Demonstration of extraction We have demonstrated that liquid membrane extraction can be used to extract hexavalent chromium to (0.05 ppm, trivalent chromium to <0.5 TABLE 3 Liquid membrane extraction of Cr and Zn from blowdown Stream
Total Cr (ppm) Crvl (ppm) Zn (ppm) PH
Blowdown
_
feed
Typical
Sample, sx 0315
10 7.5 5 6
18.65 14 4.63 6.5
Target effluent
Product achieved
0.25 0.05 1 6-9
0.030 0.030 0.005 11.3
Membrane: 71% SlOON; 15% TBP; 10% surfactant; 4% Aliquat. Internal: 5% NaOH. M/I = 2; 25000 ppm Cl-; Feed/Emulsion = 5. Extracted 5 min at 25”C, stirred 390 rpm.
270
ppm, and zinc to <1 ppm (Table 3). The results show that all three metallic species can be simultaneously extracted, and the formulation used provides a starting point for optimization work. Limiting constraints of process and product Future work is needed to investigate the extraction of chromium and zinc, using acceptable strippant concentrations in the emulsion and actual blowdown effluent. The limiting constraints for this particular application were found to be: (a) Competition for the carrier species between chromate and chloride ion in the blowdown; (b) Interaction between the surfactant and the carrier species employed; and (c) Limitations on strippant concentration in the emulsion and on allowable Crvl in the product water. Assessment The results show that if there is an appreciable amount of chloride in the waste effluent, it will interfere with the extraction of chromium and zinc. However, the liquid membrane process shows promise for effective removal and recovery of chromium and zinc ions from aqueous solutions of low chloride content. Acknowledgements This paper is dedicated to Professor Vivian Stannett in recognition of his pioneering research in the membrane separation field. Helpful conversations and support were provided by R.P. Cahn, J.W. Frankenfeld, S.E. Jaros, and A.L. Pozzi, Jr. Much credit for providing background information and economic analysis goes to A.J. Reitano, Jr. The laboratory results were obtained by L.W. Anderson, whose efforts are greatly appreciated. References 1 N.N. Li, Encapsulation and separation by liquid surfactant membranes, Chem. Eng. (U.K.), (July 1981) 325-327. 2 F.J. Hurst, D.J. Crouse and K.B. Brown, Recovery of uranium from wet-process phosphorus acid, Ind. Eng. Chem. Process Des. Dev., 11 (1972) 122. 3 W.S. Ho, T.A. Hatton, E.N. Lightfoot and N.N. Li, Extraction with liquid membranes: A diffusion-controlled model, AIChE J., 28(4) (1982) 662. 4 H.C. Hayworth, W.S. Ho, W.A. Burns and N.N. Li, Extraction of uranium from wet process phosphoric acid by liquid membranes, Sep. Sci. Tech., 18( 6) (1983) 493. 5 R.E. Terry and N.N. Li, Extraction of phenolic compounds and organic acids by liquid membranes, J. Membrane Sci., 10 (1982) 305. 6 R.P. Cahn, J.W. Frankenfeld, N.N. Li, D. Naden and K.N. Subramanian, Extraction of metals by liquid membranes, in: N.N. Li (Ed.), Recent Developments in Separation Science, Vol. VI, CRC Press, Boca Raton, FL, 1981, p. 51.
271 7 J.W. Frankenfeld, R.P. Cahn and N.N. Li, Extraction of copper by liquid membrane, Sep. Sci. Tech., 16(4) (1981) 385. 8 N.N. Li, Facilitated transport through liquid membranes, J. Membrane Sci., 3 (1978) 265. 9 E.S. Matulevicius and N.N. Li, Facilitated transport through liquid membranes, Sep. Purif. Methods, 4(l) (1975) 73. 10 J.W. Frankenfeld and N.N. Li, Wastewater treatment by liquid ion exchange in liquid membrane systems, in: N.N. Li (Ed.), Recent Developments in Separation Science, Vol. 3 (Part A), CRC Press, Boca Raton, FL, 1977, p. 285. 11 T. Kitagawa, Y. Nishikawa, J.W. Frankenfeld and N.N. Li, Wastewater treatment by liquid membrane process, Environ. Sci. Technol., 11 (1977) 602. 12 A.M. Hochhauser and E.L. Cussler, Concentrating chromium with liquid surfactant membranes, AIChE Symp. Ser., 71(152) (1975) 136. 13 R.W. Cattrall and M.Z. Ilic, Equilibrium constants for the extraction of iron(II1) from 6 M hydrochloric acid solutions by tri-n-octylammonium chloride in chloroform, J. Inorg. Nucl. Chem., 40(7) (1978) 1446. 14 General Mills Corp., Chromium, Tech. Bull. CDSl-61, 1961. 15 G.R. Smithson, Jr., Water Pollut. Control Res. Ser., 12OlElE (March 1971). 16 C.W. McDonald and R.S. Bajwa, Removal of toxir! metal ions from metal-finishing wastewater by solvent extraction, Sep. Sci., 12(4) (1977) 435. 17 G.P. Haight, Jr., D.C. Richardson and N.H. Coburn, A spectrophotometric study of equilibria involving mononuclear chromium(V1) species in solutions of various acids, Inorg. Chem., 3 (1964) 1777. 18 J.Y. Tong, Chromium(V1) species and spectra in acidic solutions, Inorg. Chem., 3 (1964) 1804. 19 R.C. Weast (Ed.), 1976-1977 Handbook of Chemistry and Physics, 57th edn., CRC Press, Cleveland, OH. 20 N.N. Li, Novel liquid membrane formulations, U.S. Patent 4,259,189, March 31, 1981. 21 M.N. Li, R.P. Cahn, D. Naden and R.W.M. Lai, Liquid membrane processes for copper extraction, Hydrometallurgy, 9 (1983) 277-305. 22 T.A. Hatton, E.N. Lightfoot, R.P. Cahn and N.N. Li, An internal recycle mixer for solvent extraction, mass transfer characterization with liquid surfactant membranes, Ind. Eng. Chem. Fundam., 22(l) (1983) 27.