Ind. Eng. Chem. Res. 1996, 35, 1383-1394
1383
Novel Membrane-Based Synergistic Metal Extraction and Recovery Processes Zhi-Fa Yang, Asim K. Guha,† and Kamalesh K. Sirkar* Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102
Individual metal recovery and separation by solvent extraction has been an important technique in pollution control as well as in hydrometallurgical processes. Synergistic extraction and separation of two cations, e.g., Cu(II) and Zn(II), a cation and an anion, e.g., Cu(II) and Cr(VI), and two cations and one anion, such as Cu(II), Zn(II), and Cr(VI), have been demonstrated employing a novel hydrophobic microporous hollow fiber membrane-based (HFM) extraction technique. The extraction selectivity of Cu(II) and Zn(II) by LIX 84 (anti-2-hydroxy-5nonylacetophenone oxime) and bis(2-ethylhexyl)phosphoric acid (D2EHPA), respectively, in a two-fiber-set HFM extractor is significantly enhanced due to competitive extraction. The efficiencies of extraction of Cu(II) and Cr(VI) by LIX 84 and tri-n-octylamine (TOA), respectively, are increased due to the self-control of the aqueous feed pH. A simplified mathematical model for synergistic extraction of Cu(II) and Zn(II) has been developed. The model predicts the observed extraction and separation performance well. In the novel HFM extractor, the extraction rates of Zn(II) and Cu(II) by D2EHPA and LIX 84, respectively, were controlled by the aqueous and organic boundary layer resistances as well as the interfacial reaction resistances. Introduction
Cu2+(aq) + 2RH(org) h R2Cu(org) + 2H+(aq)
Solvent extraction is an important unit operation in hydrometallurgy. It is also widely employed in the separation of organic acids and purification of bioproducts, e.g., antibiotics, amino acids, peptides, etc. Conventionally-practiced solvent extraction relies on dispersion which causes loss of extractants as well as solutes. Microporous membrane-based nondispersive solvent extraction techniques have been developed recently (Kiani et al., 1984; Prasad and Sirkar, 1988; Yun et al., 1993; Seibert and Sengupta, 1994). These membrane-based techniques overcome the above shortcomings and possess many other advantages. We propose to explore how membrane-based extraction may be employed to solve other important problems: efficient individual metal removal/recovery from mixtures of metals present either as mixtures of cations or mixtures of cations and anions, etc. Heavy-metal-contaminated wastewaters contain usually a mixture of different cations and anions. For example, Cu, Zn, Ni, etc., are present as cations, whereas Cr(VI), Hg, Cd, etc., are commonly found as anions. To prevent pollution and achieve resource recovery/recycling, heavy metals are to be recovered individually from such waste streams and concentrated. Selective solvent extraction/concentration of individual heavy metals is an attractive option. Selective extraction of one metal over another having the same ionic form in an aqueous waste stream by a given organic extractant is highly pH dependent (Ritcey and Ashbrook, 1984). Even the extraction of an individual metal depends strongly on the aqueous solution pH. Suppose copper or zinc or nickel (all present as cations) is to be extracted by an acidic or a chelating extractant represented as RH. Then for Cu2+ * To whom all correspondence should be addressed. Fax: (201) 596-8436. † Current address: Department of Environmental Health, Air Pollution Control Program, County of Middlesex, 841 Georges Rd., North Brunswick, NJ 08902.
0888-5885/96/2635-1383$12.00/0
(1)
As the proton is released to the waste solution, the effective distribution coefficient mi of copper between the organic extractant and wastewater is reduced; this can significantly and, depending on the pH range, drastically reduce copper extraction efficiency (Yun et al., 1993). The same is true of Zn2+ and Ni2+, etc. If, however, a heavy metal present as an anion in the wastewater is simultaneously extracted into a basic amine-containing organic extractant via an ion-pair formation mechanism, the proton released by reaction (1) will be consumed; the pH range in the wastewater may be controlled. An example of such a synergy is provided if Cr(VI) (as HCrO4-), say, is simultaneously extracted via the reaction
HCrO4-(aq) + H+(aq) + 2R3N(org) h (R3NH)2CrO4(org) (2) which consumes the protons released by reaction (1). The amine is represented here as R3N. It is important to recognize here that individual recovery of each of the metals, Cu(II) and Cr(VI), in this fashion requires that we have two separate organic streams, one containing an acidic/chelating extractant and the other containing a basic amine extractant, contact the aqueous waste stream locally. Such an extractor configuration is conveniently realized via microporous hollow fiber membrane-based solvent extraction (Yun et al., 1993) and a two-separatefiber-set-based membrane extractor schematically shown in Figure 1. Through the bore of one set of microporous hydrophobic hollow fibers flows an acidic organic extractant, e.g., LIX 84 in a diluent; through the bore of the other set of microporous hydrophobic hollow fibers flows a basic organic extractant, such as TOA in a diluent. The aqueous waste stream is allowed to flow on the shell side of the membrane extractor. Metals present as cations (Cu2+, Zn2+, Ni2+, etc.) are extracted nondispersively into the acidic extractant stream flow© 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
Figure 1. Setup of a novel synergistic membrane-based extractor for the removal of heavy metals from wastewater.
Figure 2. Schematic of the synergistic extraction of Cu2+ and Cr6+ in a module containing two sets of hollow fibers.
ing through the bore of the fiber set 1; metals present as anions (e.g., HCrO4-) are extracted into the basic extractant stream in fiber set 2, simultaneously and locally. The synergy due to pH control achieved in such a configuration is illustrated schematically in Figure 2. We have studied experimentally the removal of Cu2+ and Cr6+ (as HCrO4-) from an aqueous waste stream by the synergistic solvent extraction scheme shown in Figure 2. We demonstrate here the synergy due to the simultaneous removal of a cation (Cu2+) and an anion (HCrO4-) from the mixed-waste stream. We have, in addition, explored the individual removal of three different heavy metals Cu2+, Zn2+, and Cr6+ present in an aqueous waste stream to illustrate the ultimate utility of such membrane extraction schemes (Figure 3). Hollow fiber membrane-based solvent extraction and the device shown in Figure 1 can achieve another new dimension in metal extraction. Consider the individual recovery of two cations, Cu2+ and Zn2+, often present simultaneously in leach streams in hydrometallurgical processes. It is known that LIX 84 and D2EHPA can extract individually and selectively Cu2+ and Zn2+, respectively, under particular conditions (Lo et al., 1983). In reality, a lot of Cu2+ and Zn2+ will be
coextracted by D2EHPA and LIX 84, respectively, leading to a poorer separation of Zn2+ and Cu2+. If, however, the hollow fiber membrane device of Figure 1 is employed where LIX 84 in a diluent flows through the bore of one set of fibers and D2EHPA in a diluent flows through the bore of the other set of fibers as the aqueous feed containing Cu2+ and Zn2+ flows on the shell-side, a new level of selective extraction is achieved due to synergy. As Cu2+ is extracted by the extractant LIX 84 in fiber set 1, less Cu2+ is available for extraction by D2EHPA in fiber set 2 (Figure 4). Similarly, as Zn2+ is extracted by the extractant D2EHPA, less Zn2+ is available in the shell side aqueous stream for extraction by LIX 84 in fiber set 1. Thus, the LIX 84 stream contains much less coextracted Zn2+ as the D2EHPA stream coextracts much less of Cu2+. A much higher selectivity is achieved in the individual extraction of both Cu2+ and Zn2+. We have experimentally demonstrated here the synergy in the individual extraction of Cu2+ and Zn2+ into the two sets of extractants flowing through the bores of two sets of hollow fibers from the aqueous feed flowing on the shell side. A simplified model has also been developed to predict the extent of extraction of each of
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1385
Figure 3. Schematic of the synergistic extraction and separation of Cu(II), Zn(II), and Cr(VI) in the HFM extractors.
Figure 4. Setup of a novel synergistic membrane-based extractor for the removal and separation of Cu(II) and Zn(II) from wastewater.
Cu2+ and Zn2+ from the feed solution into the respective organic extractants. Experimental Section Chemicals used: LIX 84 (Henkel, Tucson, AZ); trin-octylamine (TOA) (Fluka, Ronkonkoma, NY), 2-ethyl1-hexanol (Aldrich, Milwaukee, WI); copper sulfate pentahydrate, potassium dichromate, zinc sulfate (ZnSO4‚7H2O), xylene, heptane, kerosene, sulfuric acid (Fisher Scientific, Springfield, NJ); bis(2-ethylhexyl)phosphoric acid (D2EHPA) (Sigma, St. Louis, MO). All chemicals are ACS reagent grade except LIX 84 and TOA. Extractors and Flow Loop. We have built two extractors using hydrophobic microporous hollow fiber membranes (HFM) of polypropylene (Celgard X-10, 100
µm i.d., 150 µm o.d.; Hoechst Celanese SPD, Charlotte, NC). The detailed geometrical characteristics of all extractors are presented in Table 1. A transparent Teflon FEP tube (1.27 cm i.d. and 1.43 cm o.d.; Cole Parmer, Chicago, IL) was used as the shell for extractor 1, whereas for extractor 2 an opaque Teflon pipe (0.61 cm i.d. and 1.03 cm o.d.; Cole Parmer, Chicago, IL) was used along with polypropylene Y-fittings at two ends. The transparent shell had an added advantage of allowing the observation of any change in the extractor during operation. We have built flow loops for simultaneous flow of three streams, two organic extractant streams, and one cocurrent shell-side aqueous waste stream as shown in Figures 1, 3, and 4. The pumps used were Masterflex Model 7518-60 (Cole Parmer). Analytical Procedures. A Thermo-Jarrel Ash Model 12 atomic absorption spectrophotometer (AAS) located at HSMRC, NJIT, Newark, NJ, was used for measuring
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Table 1. Characteristics of Hollow Fiber Extractors Fabricated module no.
extractor type
length (cm)
extractor diameter (cm)
no. of fibersa
surface area/ volume (cm-1)
fiber packing fraction
1 2
two set fiber one set fiber
29.2 32.0
1.27 0.61
1000 600
74.4 96.8
0.28 0.363
a
The fiber number on each side or set.
concentrations of Cu2+, Cr6+, and Zn2+ in the aqueous solutions using individual hollow cathode lamps. A flame condition with a fuel (acetylene) to air ratio of 3:9 was used for Cu2+ and Zn2+. For Cr6+, a more reducing flame condition was used. For Cu2+, high concentration samples were diluted to the linear calibration range of 1-40 mg/L; measurements were made at a wavelength of 216.5 nm and a slit width of 0.15 nm. The measurement of Zn2+ was carried out at a wavelength of 307.6 nm and a slit width of 0.5 nm having a linear calibration range of 1-4000 mg/L. In case of Cr6+, a linear calibration range of 1-20 mg/L was used at a wavelength of 425.4 nm and a slit width of 0.5 nm. The pH of the aqueous phase was measured using a Corning pH-meter (Model 250, relative accuracy ) 0.001 pH error, Fisher Scientific, Springfield, NJ). Experimental Details. We will now describe the three systems studied. 1. Copper(II) and Chromium(VI) System. The experimental setup is illustrated in Figure 1. Through the bore of one set of fibers flowed extractant LIX 84 diluted in kerosene to extract Cu(II) from the aqueous feed solution flowing cocurrently in the shell side of the extractor; through the bore of the other set of fibers flowed another organic extractant, tri-n-octylamine (TOA), with octanol as the phase modifier diluted in kerosene to extract Cr(VI) as well as the protons released by copper extraction simultaneously and locally. Thus, Cu(II) and Cr(VI) in the aqueous feed solution were concentrated into two solvents LIX 84 and TOA, respectively and simultaneously. Cupric sulfate and potassium dichromate were dissolved in deionized water to prepare the synthetic wastewater feed solution. The pressures of the two organic extraction phases were always maintained lower than that of the shell-side aqueous phase. 2. Copper(II), Chromium(VI), and Zinc(II) System. The extractor setup is illustrated in Figure 3. Through the bore of one set of fibers flowed extractant LIX 84 diluted in kerosene to extract Cu(II) from the aqueous feed solution containing Cu(II), Zn(II), and Cr(VI) which flowed cocurrently in the shell side of the extractor and had a pH value at which the zinc may be extracted neither by LIX 84 nor by TOA. Through the bore of the other set of fibers flowed another organic extractant tri-n-octylamine (TOA), with octanol as the phase modifier diluted in kerosene to extract Cr(VI) and the protons released by copper extraction simultaneously and locally. Thus, Cu(II) and Cr(VI) in the aqueous feed solution will be concentrated into solvents LIX 84 and TOA, respectively and simultaneously. The pH of the aqueous raffinate containing only zinc(II) was next adjusted; this solution was pumped into the fiber bores of the one-fiber-set HFM extractor in which D2EHPA in kerosene flowed in the shell side of the fibers to extract zinc(II). Cupric sulfate, potassium dichromate, and zinc sulfate were dissolved in deionized water to prepare the synthetic wastewater feed solution. 3. Zinc(II) and Copper(II) System. The HFM extractor setup is illustrated in Figure 4. Through the bore of one set of fibers flowed an organic extractant
LIX 84 diluted in kerosene to selectively extract Cu(II) from the aqueous feed solution flowing cocurrently in the shell side of the extractor; through the bore of the other set of fibers flowed another organic extractant D2EHPA diluted in kerosene to preferentially extract Zn(II). Thus, Cu(II) and Zn(II) in the aqueous feed solution were concentrated into solvents LIX 84 and D2EHPA, respectively and simultaneously. Cupric sulfate and zinc sulfate were dissolved in deionized water to prepare the synthetic wastewater feed solutions; the pH of feed solution was adjusted with sulfuric acid. 4. Measurement of Extraction Distribution Coefficient of Zinc(II). Distribution coefficients of zinc (II) between the aqueous phase and 10 v/v % D2EHPA in kerosene were measured by stirring 20 mL of the aqueous phase containing zinc(II) at pH around 2.0 with an equal volume of the extractant for 1 h and then separating out the aqueous and organic phases for 12 h. The zinc concentration in the aqueous phase was analyzed with the AAS method described before. The zinc concentration in the organic phase was calculated via material balance. A Model for Simultaneous Extraction of Zinc(II) and Copper(II) in a Hollow Fiber Membrane Extractor. The individual transport of Cu(II) and Zn(II) into LIX 84 in kerosene and D2EHPA in kerosene, respectively and simultaneously, in a hollow fiber membrane (HFM) extractor (Figure 4) has been modeled as follows. The aqueous feed containing Zn(II) and Cu(II) flows in the shell side; LIX 84 in kerosene flows in the tube side of one set of microporous hollow fibers; in the bore of the other set of fibers flows D2EHPA in kerosene (Figure 4). These two sets of hydrophobic fibers have the same physical properties and the same number of fibers. The micropores of the fibers are filled with the respective organic solvent, and the extraction reactions take place on the aqueous-organic interfaces at the outside wall (d0) of the fibers. The schematic of reaction profiles of different species in such a synergistic extraction of Cu(II) and Zn(II) in the HFM extractor is illustrated in Figure 5. The copper and zinc ions from the bulk aqueous phase (shell side) diffuse through the boundary layers to the aqueousLIX 84 and aqueous-D2EHPA interfaces, respectively and simultaneously. At the aqueous-LIX 84 interface, 1 mol of Cu(II) reacts with 2 mol of free oxime (active component) in the organic phase to form the copperoxime complex. While at the aqueous-D2EHPA interface, 1 mol of Zn(II) reacts with 1.5 mol of the dimer of D2EHPA in the organic phase to form a zinc-D2EHPA complex. The hydrogen ions are exchanged with Cu(II) and Zn(II) ions and are released into the aqueous phase. The copper-oxime and zinc-D2EHPA complexes diffuse from the aqueous-organic interfaces to the inside wall of the fibers through the organic-filled membrane pores and then to the respective bulk organic phases (tube side). In the development of the extraction model, the following assumptions were made: (1) steady state has been reached, namely, the mass-transfer rate of the
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1387
Figure 5. Schematic of the selective extraction of Cu(II) and Zn(II) with LIX 84 and D2EHPA in a HFM extractor.
metals through the aqueous boundary layer is equal to the formation rate of the respective extracted complex (copper-oxime or zinc-D2EHPA) at the interface which is also equal to the transport rate of the extracted complexes through the membrane and through the organic boundary layer; (2) LIX 84 extracts only Cu(II); Zn(II) may only be extracted by D2EHPA; (3) the complex formations at the aqueous-organic interfaces obey Eigen’s mechanism. The justification for assumption (2) will be discussed later. 1. Transport of Copper, Oxime, and CopperOxime Complex. At low pH values, Cu(II) reacts with LIX 84 according to eq 1, releasing H+ (Pearson, 1983). The equilibrium constant for this reaction may be written as
KCu eq )
[CuR2][H+]2
[H+]2 ) mCu , [HR]2[Cu2+] [HR]2
)
[Cu2+]
(3)
2+
- [Cu ]FI)πdoN
(4)
The formation rate of the copper-oxime complex at the feed-solvent interface (the outside mouths of the micropores of the hydrophobic fibers) is
RCu T
(
)
)
kCu f
2+
[Cu ]FI
Define
[HR]FI [H+]FI
kCu′ f
)
+ 1 [CuR2]FI[H ]FI - Cu πdoN (5) Keq [HR]FI
kCu f
[HR]FI [H ]FI +
,
(8)
The diffusion rate of the copper-oxime complex through the organic boundary layer is
(9)
The overall Cu(II) transport can be expressed as
[CuR2]
At steady state, the mass-transfer rate of Cu(II) per unit extractor length (RCu T ) through the feed-side boundary layer is described by 2+ kCu c ([Cu ]Fb
Cu RCu T ) km ([CuR2]FI - [CuR2]i)πdlmN
Cu RCu T ) ko ([CuR2]i - [CuR2]Ob)πdiN
where mCu )
RCu T
Equation 5 is valid for oxime concentrations greater than 0.012 mol/L (Komasawa et al., 1983). A similar expression was also used for extraction of Cu(II) by LIX 84 in n-decane or heptane in a hollow fiber membrane extractor (Haan et al., 1989; Yun et al., 1993) and hollow fiber contained liquid membrane permeator (Guha et al., 1994). The diffusion rate of the copper-oxime complex through the pores of the fibers filled with LIX 84 in kerosene is
kCu′ r
)
kCu f
KD )
[H+]FI
,
KCu eq [HR]FI Cu′ kr 1 ) (6) Cu′ m kf Cu
Then eq 5 becomes Cu′ 2+ RCu T ) kf ([Cu ]FI - KD[CuR2]FI)πdoN
(7)
Cu 2+ RCu T ) K ([Cu ]Fb - KD[CuR2]Ob)πdoN
(10)
The total resistance may be written as
KDdo KDdo 1 1 1 ) Cu + Cu′ + Cu + Cu Cu K kc kf km dlm ko di
(11)
The diffusion rates of LIX 84 (HR) through the organic boundary layer and the pores of the fibers are given respectively by
R(HR) ) k(HR) ([HR]Ob - [HR]i)πdiN T o
(12)
R(HR) ) k(HR) T m ([HR]i - [HR]FI)πdlmN
(13)
Carrier balance:
[HR]T ) [HR]FI + 2[CuR2]FI
(14)
HR 2RCu T ) RT
(15)
2. Transport of Zinc, D2EHPA, and ZincD2EHPA Complex. The reaction stoichiometry of Zn(II) extraction with D2EHPA has been extensively studied (Grimm and Kolarik, 1974; Komasawa et al., 1981; Huang and Juang, 1986a,b; Juang, 1993). The extraction equilibrium at low organic loading can be represented by the following reaction:
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3 Zn2+ + (HA)2 h ZnA2(HA) + 2H+ 2
(16)
The extraction equilibrium constant KZn eq is given by
KZn eq )
[ZnA2(HA)][H+]2 [Zn2+][(HA)2]3/2
[H+]2 ) mZn , [(HA)2]3/2
where mZn )
[ZnA2(HA)] [Zn2+]
2[HA] h [(HA)2]
(17)
(19)
2
The extraction equilibrium may be expressed by the following reaction as well: 2+
Zn
KZn ex )
+ 3(HA) h ZnA2(HA) + 2H
+
[ZnA2(HA)][H+]2 [Zn2+][HA]3
The diffusion rate of the zinc-D2EHPA complex through the pores of the fibers filled with D2EHPA in kerosene is
[H+]2 ) mZn [HA]3
Zn HA 3/2 KZn ex ) Keq (Keq )
The diffusion rate of the zinc-D2EHPA complex through the organic boundary layer is Zn RZn T ) ko ([ZnA2(HA)]i - [ZnA2(HA)]Ob)πdiN
(18)
[(HA)2] [HA]
)
1 [ZnA2(HA)]FI πdoN (26) mZn
Zn RZn T ) km ([ZnA2(HA)]FI - (ZnA2(HA)]i)πdlmN (27)
The association equilibrium for D2EHPA is
KHA eq )
(
Zn′ RZn [Zn2+]FI T ) kf
An overall mass transfer coefficient KZn may be defined by
(
Zn [Zn2+]Fb RZn T ) K
)
1 [ZnA2(HA)]Ob πdoN (29) mZn
The total resistance may be written in terms of component resistances as
(20)
do do 1 1 1 ) Zn + + + (30) Zn Zn K kc mZndlmkm mZndikZn kZn o f [HA]FI
(21)
3. Counterdiffusion of Hydrogen Ions during the Extraction.
(22)
It is worth noting that, based on the equilibrium studies of metal extraction, the reaction stoichiometry expressed above is applicable only over a range of loading ratio of D2EHPA, defined as [ZnA2(HA)]/[(HA)2], less than 0.16 (Huang and Juang, 1986a,b; Komasawa et al., 1981). With an increase in the loading ratio, the monomeric species [ZnA2(HA)] tends to be broken up and some aggregated species appear (Komasawa et al., 1981). The transport of Zn(II) through the aqueous boundary layer may be written as: Zn 2+ 2+ RZn T ) kc ([Zn ]Fb - [Zn ]FI)πdoN
(23)
+ Zn + ) kH RH,Zn T c ([H ]FI - [H ]Fb)πdoN
(31)
+ Cu + RH,Cu ) kH T c ([H ]FI - [H ]Fb)πdoN
(32)
RH,Zn ) 2RZn T T ;
RH,Cu ) 2RCu T T
The formation rate of the zinc-D2EHPA complex at the aqueous-organic interface may be described by (Miyake et al., 1990; Huang and Juang, 1986a,b):
)
kZn f
-QF
d[Cu2+]Fb ) RCu T dz
(34)
-QF
d[Zn2+]Fb ) RZn T dz
(35)
d[H+]Fb Cu Q ) RH,Zn + RH,Cu ) 2(RZn T T T + RT ) dz
(
[Zn ]FI[HA]FI -
2 KZn ex [HA]FI
πdoN (24)
During extraction, the dimerization reaction of D2EHPA is assumed to be in equilibrium (Huang and Juang, 1986a,b; Komasawa et al., 1981).
Define Then eq 24 becomes
kZn′ f
)
kZn f [HA]FI
[Cu2+]Fb ) [Cu2+]in;
[Zn2+]Fb ) [Zn2+]in; [H+]Fb ) [H+]in (37)
)
[ZnA2(HA)]FI[H+]FI2
(36)
The boundary conditions are:
z ) 0,
2+
(33)
The differential mass balances for copper, zinc, and hydrogen ions along the length of the extractor for the individual extraction may be written as:
F
RZn T
(28)
(25)
z ) L,
[Cu2+]Fb ) [Cu2+]out; [Zn2+]Fb ) [Zn2+]out;
[H+]Fb ) [H+]out (38)
The mass-transfer coefficients through the membrane substrate may be obtained by:
kim )
2Di τ(do - di)
(39)
The mass-transfer coefficients in the tube and shell sides may be obtained from eqs 40 and 41, respectively.
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1389
For the tube side:
NSh ) ∞
βj ∑ j)1
-1/3
∞
∑ j)1
2
exp
[ [
βj-7/3 exp
Table 2. Parameters Used for Simulation parameter
] ]
-βj2(2z/di) NReNSc
-βj2(2z/di) NReNSc
,
where βj ) 4(j - 1) +
8 3
(40)
Extraction of Zinc(II) with 10 v/v % D2EHPA in Kerosene DZn 6.42 × 10-10 m2/sc DZnA2 5.84 × 10-10 m2/sd DHA 1.22 × 10-9 m2/sd Kex 0.0001 mol/m3 g kf 8.40 × 10-7 m (mol/m3)1/2/se νHA 1.63 × 10-6 m2/se,f
For the shell side:
NSh ) β[dH(1 - φ)/L]NRe0.6NSc1/3
(41)
Equation 40 is the local expression of the Graetz solution which is theoretically obtained using the constant wall concentration assumption (Skelland, 1974) for the tubular flow, and eq 41 is Prasad and Sirkar’s correlation (1988) for the shell side.
value
Extraction of Copper(II) with 10 v/v % LIX 84 in Kerosene 7.2 × 10-10 m2/sa DCu DCuR2 1.46 × 10-10 m2/sb DH 6.0 × 10-9 m2/sa Keq 1.7a kf 9.0 × 10-8 m/sa DHR 9.7 × 10-10 m2/sa νHR 9.90 × 10-7 m2/s τ 3.5 0.3
a Yun et al. (1993). b Haan et al. (1989). c Awakura et al. (1988). Miyake et al. (1990). e Huang and Juang (1988). f Huang and Juang (1986a,b). g The present paper.
d
Solute balance for the module as a whole: for copper:
QHR([CuR2]out - [CuR2]in) ) QF([Cu2+]in - [Cu2+]out) (42) for zinc:
QHA([ZnA2(HA)]out - [ZnA2(HA)]in) ) QF([Zn2+]in - [Zn2+]out) (43) In the present work:
[CuR2]in ) 0;
[ZnA2(HA)]in ) 0
(44)
Local solute balance in the module at axial location z: for copper:
Figure 6. Plot of [HA]3(org)/[H+]2(aq) versus mZn.
values of the parameters used in the simulation are listed in Table 2.
QHR([CuR2]z+∆z - [CuR2]z) ) QF([Cu2+]z - [Cu2+]z+∆z) (45)
Results and Discussion
for zinc:
QHA([ZnA2(HA)]z+∆z - [ZnA2(HA)]z) ) F
2+
2+
Q ([Zn ]z - [Zn ]z+∆z) (46) At z ) 0,
[CuR2] ) [CuR2]in ) 0; [ZnA2(HA)] ) [ZnA2(HA)]in ) 0 (47)
Equations 34-36 are ordinary differential equations with initial values identified in eqs 37 and 38. The Cu expressions for RZn T and RT appearing in eqs 34-36 are given by eqs 10 and 29. The quantity [Cu2+]Fb in eq 10 is merely [Cu2+]z in eq 45, similarly for the other quantities in eqs 10 and 29. They are employed in the numerical solution of eqs 34-36 with the help of eqs 45-47. These equations have been solved using the Runge-Kutta-Verner fifth-order and sixth-order method (IVPRK method) (Ramirez, 1989) in a VAX/VMS computer. In the model computations, the constant β ) 0.60 (in eq 41) was used to simulate the extraction process. The aqueous feed pH value was kept below 2.0 to achieve higher extraction selectivity. The numerical
We first present the results of synergistic extraction of two cations Zn2+ and Cu2+ in a two-fiber-set module shown in Figures 4 and 5. The computational results from the simplified model are also illustrated along with the experimental data. We focus next on the results of synergistic extraction of a cation and an anion from an aqueous waste feed stream by simultaneous flow of two different organic extractant streams in module 1 schematically shown in Figure 3. After Cu2+ and Cr6+ (as HCrO4-) were separated in this module, the raffinate pH was changed and the raffinate fed into module 2 for Zn2+ extraction. Results for this experiment are presented next to illustrate how Cu2+, Zn2+, and Cr6+ (as HCrO4-) are individually separated. The experimental data of the distribution coefficient of zinc (II) extraction by 10 v/v % D2EHPA in kerosene are plotted according to eq 21 and illustrated in Figure 6. From this plot it may be observed that, at the aqueous feed pH around 2.0, the slope, namely, KZn ex , is equal to 0.0001. The distribution coefficient of zinc, mZn,
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Table 3. Synergistic and Selective Extraction of Copper and Zinc by LIX 84 and D2EHPA, Respectivelya inlet
outlet feed
series no.
D2EHPA in kero. (v/v %)
LIX 84 in kero. (v/v %)
1 2 3 4
10 10 10 0b
10 10 0b 10
D2EHPA in kero.
LIX 84 in kero.
pH
Zn (ppm)
Cu (ppm)
Zn (ppm)
Cu (ppm)
[Cu]/[Zn] (%)
Zn (ppm)
Cu (ppm)
[Zn]/[Cu] (%)
1.59 1.96 1.98 1.98
469 466 482 482
688 536 540 540
444 1070 832
18.4 29.8 48.7
4.1 2.8 5.9
30.0 43.4
876 914
3.4 4.7
594
26.6
a
158
kero. ) kerosene; the flow rate of the extractants in the fibers is about 0.5 mL/min; the feed flow rate is about 1.5 mL/min. kerosene flows in this set of fibers.
Figure 7. Exit concentration of copper in the aqueous phase from the HFM extractor (module 1) as a function of the aqueous-phase flow rate.
Figure 8. Exit concentration of zinc in the aqueous phase from the HFM extractor (module 1) as a function of the aqueous-phase flow rate.
may be expressed as
mZn ) 0.0001 ×
[HA]3 [H+]2
(48)
Simultaneous and Selective Extraction of Cu(II) and Zn(II). Part of the results of the experiments conducted in the two-fiber-set hollow fiber membrane (HFM) extractor with LIX 84 in diluent (or only diluent) flowing in the bore of one set of fibers and D2EHPA in diluent (or only diluent) flowing in the bore of the other set of fibers and the synthetic wastewater containing zinc(II) and copper(II) flowing cocurrently on the shell side are shown in Table 3. These results indicate that there is significant synergy in the selective extraction and separation of zinc(II) and copper(II) in the HFM
b
100%
Figure 9. Exit concentration of copper in the aqueous phase from the HFM extractor (module 1) as a function of the LIX 84 phase flow rate.
Figure 10. Exit concentration of zinc in the aqueous phase from the HFM extractor (module 1) as a function of the D2EHPA phase flow rate.
module with LIX 84 in kerosene and D2EHPA in kerosene flowing in the two sets of fibers, simultaneously and respectively; the coextraction of zinc(II) by LIX 84 or copper(II) by D2EHPA appears to have been greatly reduced when there is simultaneous and separate extraction compared to the case of individual extraction of Cu(II) and Zn(II) in experiment numbers 4 and 3, respectively. Determination of Rate-Control Steps in Zn(II) and Cu(II) Extraction in the HFM Extractor. In order to determine the rate-control step in the simultaneous and selective extraction of zinc and copper in the HFM extractor, a variety of experiments were carried out with different concentrations of LIX 84 and D2EHPA in kerosene, different feed and solvent flow rates, and different feed concentrations. Figures 7-10 show that the experimental results are described well
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1391
Figure 11. Effect of flow-rate variations of the solvents on the mass-transfer rate of metal extraction in the HFM extractor (module 1).
Figure 12. Effect of the concentration of the solvents on the masstransfer rate of metal extraction in a hollow fiber membrane extractor (module 1).
by the results predicted by the mathematical model. This suggests that the simple model which neglects coextraction may be used in predicting the observed behavior. Figures 9 and 11 show that increasing the flow rate of LIX 84 will slightly reduce the outlet concentration Cu at of copper (Figure 9) or slightly increase RCu T and K higher flow rates (Figure 11), implying that the masstransfer resistance in the organic boundary layer has very little effect on the total resistance if the flow rate of LIX 84 is higher than 1.0 × 10-8 m3/s. Furthermore, in the concentration range of 156-500 mol/m3 of LIX 84, copper extraction rate RCu T and the mass-transfer coefficient KCu increase slightly with an increase of LIX 84 concentration (Figure 12); this indicates that the interfacial reaction rate between copper and LIX 84 also plays a role in the total extraction rate of copper at lower LIX 84 concentration. Figure 13 shows that the copper extraction rate RCu T and the mass-transfer coefficient KCu are strong functions of the aqueous feed flow rate; however, the variation of copper concentration in the aqueous feed has little effect on the mass-transfer coefficient KCu. From these results, it may be concluded that, in the HFM extractor, copper extraction rate is controlled by the aqueous boundary layer resistance, the interfacial reaction resistance, and the organic boundary layer resistance, of which the first one plays the major role. This conclusion is in agreement with other studies (Yun et al., 1993).
Figure 13. Effect of aqueous-phase flow rate on the mass-transfer rate of copper extraction in a HFM extractor (module 1).
Figure 14. Effect of aqueous-phase flow rate on the mass-transfer rate of zinc extraction in a HFM extractor (module 1).
For zinc(II) extraction, an increase in the flow rate of D2EHPA in kerosene significantly increases the zinc extraction rate RZn T (Figure 11) and reduces zinc concentration in the aqueous outlet solution (Figure 10), implying that the organic boundary layer resistance plays an important role in the zinc extraction rate. The zinc extraction rate RZn T and the mass-transfer coefficient KZn increase significantly with an increase of D2EHPA concentration if the concentration of D2EHPA is lower than 600 mol/m3 (Figure 12), implying that, at lower D2EHPA concentrations (
