Ind. Eng. Chem. Res. 1991, 30,1231-1239
Yazdi, P.; McFann, G. J.; Johnaton, K. P.; Fox, M. A. Reverse micellee in supercritical fluids. 2. Fluorescence and absorption spectral probes of adjustable aggregation in the two-phase region. J. Phys. Chem. 1990,94,7224. Ziger, D. H. Solid-SupercriticalFluid Equilibrium: Experimental
1231
and Theoretical Studies of Partial Molar Volumes of Solubilities; Ph.D. Dissertation, University of Illinois, 1983.
Received for review July 9, 1990 Accepted October 23, 1990
Continuous Membrane Dialysis Using Ion-Exchange Resin Suspension for Extracting Metal Ions Xinming Shao, Shangxu Hu, and Rakesh Govind* Department of Chemical Engineering, Uniuersity of Cincinnati, Cincinnati, Ohio 45221
A continuous membrane dialysis process using ion-exchange resin suspension has been developed for extracting metals from industrial waste streams. The basic idea is to use a chelating ion-exchange resin in conjunction with a semipermeable membrane. The membrane is capable of retaining the resin and its metal complex while allowing the free metal ion to permeate freely. The process consists of a chelating unit and a stripping unit. In the chelating unit, the resin reacts with certain metal ions and forms a metal complex. In the stripping unit, metal ions are released and the resin is regenerated a t a lower pH. The resin suspension is continuously circulated between the chelating and stripping units. Experimental studies have been conducted on the removal of copper ion. The results show that the extraction of metals can be significantly enhanced in comparison with a nonchelating system. In addition to experimental studies, a mathematical model for the process has been proposed and verified on the basis of system identification of the overall physicochemical parameters by fitting the experimental data. The basic process has also been optimized by using the validated model to explore the optimal (minimum operating cost with a specified feed stream and output) process structure and operating conditions.
Introduction There is a growing need for developing novel separation techniques for selectively extracting metals from industrial proceas streams. This is required not only to remove water bearing ion contaminants to meet environmental legislation for wastewater discharges, but also to selectively recover valuable metals from hydrometallurgical liquors, spent electroplating baths, and metal finishing wastewaters. Solvent extraction is a commonly used conventional technique. It is well-established and widely used. However, in handling of large volume solutions containing low metal concentrations, the solvent extraction process becomes uneconomical. Recently, there have been several membrane separation processes under development, which are as follows: supported liquid membrane (Lee et al., 1978; Komasawa et al., 1983;Danesi, 1985),liquid surfactant membrane (Gu et al., 1985), membrane-based solvent extraction (Strathmann, 1980;Kim, 1984;Heuckroch et al., 1986;Prasad and Sirkar, 1987),and affinity dialysis using dialysis membrane and water-soluble polymers (Davis et al., 1988;Hu and Govind, 1988). Due to their potential for lower energy utilization and significant increases in throughputs, these novel methods are attractive. However, these techniques still have some problems to overcome, such as solvent entrainment or washout, poor system durability, low extraction rate, and inadequate stability of membrane (Kordosky et al., 1987; Tavlarides et al., 1987). Generally, these membrane techniques remain immature and need more effort for industrial applications. Ion-exchange resins including chelating ion-exchange resins (Grinstead, 1978;Loureiro et al., 1988) and impregnated ion-exchange resins (Warshawsky, 1981)have been widely studied for the recovery of metals from dilute
* Correspondence should be directed to this author.
leach liquors. In most commercial ion-exchange processes such as fEed bed or continuous packed bed,feed solutions directly contact with resins. Fine particles in the feed solutions may contaminate the resin and tend to block the bed and increase the pressure drop. This requires that feed solutions must have a low suspended solids content. Though a backwash cycle may be incorporated, it is still undesirable to use these processes with more than lo00 ppm undissolved solids (Streat and Naden, 1987). This limits the flexibility of using ion-exchange resins in most metal recovery processes. In addition, fixed bed or packed bed ion-exchange operations require high mechanical strength and toughness of the ion-exchange particles. The resin should be resistant to abrasion and crushing, and it also should withstand continuous rapid swelling when loaded with hydrated ions and shrinking with less hydrated ions. Resin attrition, abrasion, and fracture may cause serious problems in actual process operations. Even in fluidized beds or continuous ion-exchange beds, resin particle attrition is still one of the major concerns. These requirements greatly increase the manufacturing cost of ion-exchange resins. Thus, any losses of the resin during regeneration are undesirable. In this work, by employing the ion-exchange chelating resin as an alternative for extractant agent in solvent extraction, a continuous membrane dialysis process using ion-exchange-resin suspension has been developed. This new process consists of a chelating unit and a stripping unit. Both of them are membrane hollow fiber cartridges operating at different pHs. The original resin was ground to form a resin suspension which can be circulated along the chelating unit where the resin reacts with metal ions selectively and the stripping unit where the resin is regenerated. The transport of free metal ion across the membrane is controlled by its concentration gradient across the membrane. The process combines membrane
0888-5885/91/2630-1231$02.50/00 1991 American Chemical Society
1232 Ind. Eng. Chem. Res., Vol. 30,No. 6, 1991
.C W
tL
29.0 30.0
28.0
-
27.0
-
.-e
'CI
0
Table I. Characteristics and Dimensions of Membrane Hollow Fiber Cartridge cutoff molecular weight 10Ooo surface area of membrane 0.03 m2 20.3 cm length of cartridge 0.03-0.07 L/min water flux inner diameter 0.105 cm outer diameter 0.178 cm number of hollow fibers per cartridge 55
26.0
-
LJ.U
I
0.0
El
e
El
D
original size reduced size
e I
1 .0 2.0 Cu aqueous ( g i l )
I
3.0
Figure 1. Chelating ability of resin of original and reduced sizes (pH = 2.0).
dialysis and the ion-exchange-resin technique. Differing from the common membrane dialysis processes, the semipermeable membrane does not necessarily function to separate metal ions. It may only be designed as an interface barrier to retain the ion-exchange resin and the formed resin-metal ion complex but allows the free metal ions to freely permeate. It may also prevent the resin from contamination caused by particulate suspension in feed solution, commonly found in processing of low-grade ores. The proposed process is expected to have advantages over membrane dialysis and broaden the application range of ion-exchange resins. Furthermore, loss of resin can be prevented. Mathematical modeling and parameter identification technique were used in this study. To fit the experimental data of cocurrent flow, the overall physicochemical parameters involved in the process can be estimated. By comparing the agreement of the identification results for different operating conditions, the modeling assumptions and identification procedure can be justified. The mathematical model with the identified parameters can be further verified by comparing the simulation and the experimental results in the case of countercurrent flow.
Experimental Studies Resin Suspension Preparation. Experimental chelating ion-exchange resin XFS43084.00 from Dow Chemicals U.S.A. was used in this experiment. Originally, the resin was designed for use in the processing of dilute sulfuric acid dump leaches in metallurgicaloperations, and it could be readily regenerated with dilute sulfuric acid. or ammonia. It was reported that the resin exhibits a high selectivity of copper over other ions (Grinstead, 1979). The general structure of the resins of this type is brised on picolylamine (2-(aminomethy1)pyridine).The resins are macroporous polystyrene/divinylbenzene copolymers to which has been attached a chelating functional group (Grinstead and Nasutavicus, 1978) as shown below. Copper ions are coordinated to the two nitrogen a t o m and the oxygen atom of the hydroxyl group.
N-(2-hydroxypropyl)picolybmine
To suspend solid resin in water, the size of the resin has to be reduced. The average original size of the resin is 1 mm. Since the functional groups are believed to be distributed fairly uniformly throughout the surface of resin particles, grinding the resin to a smaller size does not affwt the chelating ability. It is noted that at steady-state operation the smaller particle size probably allows a slightly
higher loading (Grinstead, 1988). In our experimental results of equilibrium study shown in Figure 1, our data agree well with Grinstead's results. The resin size distribution was narrowed by only using the preparation left after allowing the ground resin to settle over 2 h. The reaction mechanism of metal ion for this kind of chelating resin can be simply expressed in the following form:
Am+ + RnH+ s ARm++ nH+
(1)
and the reaction equilibrium constant
K' =
(AR"+)*(H+)" (RnH+)*(A"+)
or the conditional chelating constant K (AR"+) K = - - K' (H+)" (RnH+)*(Am+)
(2)
where R represents the resin structure radical, Am+denotes the free metal ions, and ARm+and RnH+ are of the resin phase. K depends on concentration of H+ in the solution and can be used to illustrate the dependency of metal ion chelating ability as a function of pH. Increasing the concentration of H+ in the solution reverses the reaction. The resin chelates metal ions from the solution or releases metal ions to the solution with changing the pH. Experimental Setup. Figure 2 is a schematic drawing of a countercurrent experimental setup of the membrane dialysis process. In the chelating unit, the metal ion diffuses from the feed stream across the membrane and reacts with the resin. When the metal ion is chelated by the resin to form the resin-metal complex, it cannot migrate back across the membrane. In the stripping unit, with a change of pH in the resin suspension with the acid mixer, the ion is released from the resin phase and diffuses across the membrane to the stripping stream while the resin is regenerated. The regenerated resin suspension is pumped to the base mixer for readjustment of pH and then recycled back to the chelating unit. Feed (CuS04 solution; pH was adjusted to 2.0) was pumped through the shell side of the membrane with the resin suspension passing through the tube side. A multichannel peristaltic pump (Ismatec IPS,Cole-Parmer) was used to provide flows on both sides of the membrane. Four membrane cartridges (HlP10-43, AMICON) containing polysulfone hollow fibers with a surface area of approximately 0.03 m2 each were connected in sequence as the chelating and stripping units. The membrane system parameters have been summarized in Table I. Solutions of 4 M HCl and 50% NaOH were used for pH adjustment in the alkalification and acidification steps, respectively, to minimize the amount of water added to the system. The stripping stream was HC1 solution at pH adjusted to 0.5. pH was measured with a calibrated pH meter (pH meter 107, Fisher Scientific). Data on the free copper concentrations were obtained by sampling at both ends and at the midpoints of the tube connecting the two
Ind. Eng. Chem. Res., Vol. 30, No. 6,1991 1233 Stripping Stream
Resin Suspcnsion
Rcgeneaatd Resin Suspension
IFdSbeam
I
Treated
I
PFd
3
Acid Feed
Figure 2. Schematic of membrane dialysis using ion-exchange resin suspension.
=
500h 4 \:
400
e a v
300
200
c
100:
0.00
I3
0
feed solution
4001
slripping slreaiii resin suspensioii
resin suspension
computation
'
I
0.20
'
I
'
0.40
I
0.60
'
I
1
0.80
1.00
Z
O
0.00
0.20
0.40
0.60
0.80
I 1.00
Z
Figure 3. Experimental results versus simulation of continuous operation mode of the membrane dialysis process, chelating unit, pH = 2.0.
Figure 4. Experimental results versus simulation of continuous operation mode of the membrane dialysis process, stripping unit, pH = 0.5.
adjacent membrane modules. Analysis of copper ion concentration was performed by a colorimetric method using a copper analysis kit (DR 100, Hach Co). Two pH controllers (Model 5997-20, Cole-Parmer) were used to record and control the pH in acid mixer and base mixer. The pH in acid mixer and base mixer was set at 0.5 and 2.0, respectively, and controlled within 0.2 unit of each set point. Two all-TEFLON pumps from Cole Parmer were slaved to the pH controllers to ensure proper rates of additional reagents (HC1and NaOH) to the acid mixer and base mixer. Two stirrers (Model 120M, Fisher Scientific) were used in these mixers for better mixing. A series of experiments were run at various operating conditions. Data on free copper ion concentration in feed and resin suspension were plotted along the dimensionless length 2,which represents the sequence of four membrane cartridges and connecting tubes for both the chelating and the stripping unit. Figures 3 and 4 are typical plots of copper concentration profiles in the system under steady state. Figure 3 shows the data of chelating unit, while Figure 4 gives results for the stripping unit. It is noted that a fairly long time is needed to obtain steady state if the resin suspension is fresh. In Figure 3, the free copper concentration in resin suspension seems to approach a constant. It means that the
amount of copper diffusing through the membrane is equal to the amount chelated by the resin at some distance along the chelating unit. This is reasonable because the chelating reaction rate is smaller compared with the diffusion rate and the chelating process is limited by the slow reaction. The process is reaction rate controlled. The slow chelating reaction rate also affects the efficiency of the resin (Grinstead, 1979). With a fast chelating reaction, the concentration of free copper in the resin suspension will be decreased, and the concentration gradient along the membrane will be high and hence the removal of copper from the feed stream would be enhanced. Data in Figure 4 shows that the free copper ion in resin suspension has a maximum between the first and the second membrane cartridges in the stripping unit, and after that maximum, the concentration decreased along the membrane sequence. This implies that the reverse reaction is faster in comparison with the forward reaction, and that causes the copper ion concentration to achieve a maximum. The stripping process is more likely restricted by the copper diffusion taking place over a definite membrane area rather than the reaction rate of resin. With more membrane cartridges or greater concentration gradient across the membrane, more copper will be stripped from the resin suspension. It seems that, in the stripping unit, large membrane area is helpful for the regeneration
1234 Ind. Eng. Chem. Res., Vol. 30,No. 6,1991
x
A
+ dxA
........................................
z=z+dz z=l
I
......................
A
I
Figure 5. Diagram of a differential section of a membrane module.
of resin suspension, while the chelating unit, more residence time is needed for chelating reaction. Mathematical Model The chelating or stripping unit in our process can be mathematically modeled as one system since they have exactly the same configuration. In a situation like flow in membrane hollow fibers, the small radius generally makes the velocity profile inconsequentialand the convective flow is often dominant. Furthermore, the small amount of resin (around 1% in weight percent) suspended in the suspension can be assumed distributing uniformly throughout the bulk flow and does not interfere with the flow. According to these arguments, we make the following assumptions in our mathematical formulation: 1. The chelating or stripping unit operates under isothermal condition with a plug flow on both sides of the membrane. 2. Axial or radial dispersion is negligible. 3. The complex heterogeneous ion-exchange reaction can be simplified as a pseudohomogeneous reaction with the reaction equilibrium given by ~*YA*YR
yoRA= -- ~A*YOA*YOR
(H+)
librium and reaction constants, which are functions of pH in the solution. Ion-exchange reaction changes the concentrations of the metal ions as well as the pH of the solution. Any uptake of metal ions should cause a change in H+concentration along the length of the membrane module. Chelating reaction rate constant kA and the equilibrium constant eA are dependent on H+concentration and thus in principle will vary along the length of the membrane module where reaction occurs. Making the material balance around the differential section shown in Figure 5 from z = z to z = z + dz and defining the following dimensionless groups:
P R A
Pm=-
xAO
z = z/l
(3)
where y”ru,yOA, and y”R are the equilibrium concentrations at certain pH. 4. The chelating reaction kinetics follows dyRA/dt = kA*
