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Ind. Eng. Chem. Process Des. Dev. 1980, 19, 522-526
Experimental Study of Dispersion and Separation of Phases in Liquid-Liquid Extraction of Copper by LIX 64N in Various Types of Mixers Jose C. Merchuk' Depariment of Chemical Engineering, Iowa State University, Ames, Iowa 5007 7
David Wolf, Rachamim Shai, and D. H. White Ben-Gurion University of the Negev, Beer Sheva, Israel
Dispersion band heights and separation times are reported for dispersions obtained in motionless mixers and packed tubes during extraction of Cu of aqueous solutions by LIX 64N. The results are compared with data obtained with a conventional mixer-settler. Considerabledifferences were found in the behavior of organic continuous dispersions and aqueous continuous dispersions. It was found that the material the packings are made of can determine which of the phases will be continuous. Results of dispersion band height vs. settling time confirm that static mixers give a narrow range of drop diameters.
Introduction
In liquid-liquid extraction, the mixer causes the dispersion of one of the phases in the other so as to get good mass transfer, while the settler allows the separation of the two phases after the mass transfer is achieved. Good dispersion in the mixer and short separation times in the settler are required. However, the better the dispersion is the longer it may take to separate the two phases, thus increasing considerably the holdup of the settler. Therefore there must be a compromise between the degree of dispersion and the settling time. Manufacturers of motionless mixers claim that their products can be a good solution for this problem since they create a narrow drop size distribution and a thorough mixing of the phases (Mutsakis, 1976; Streiff, 1977). Experimental findings seem to confirm the narrowness of the drop size distribution (Middleman, 1974). In this research work data were obtained on dispersion heights and separation times for several types of mixers using copper extraction by L E 64N as the experimental system. The dispersion height and separation times depend on several factors: type of mixer or agitator, type of impeller (if used), operating conditions, the properties of the solutions such as surface tension, density, viscosity, etc., flow conditions, and which of the two phases is the continuous one and which the dispersed one. The common unit used in the copper extraction process is the mixersettler. The performance of this unit, the effect of several parameters on the design, and the analysis of the dispersion and settling rates for this system were studied by Tunley and Birch (1970). The performance of a rotating disk contactor was studied by Paynter et al. (1970). In this work, the performance of static mixers of the Koch type and packed tubes are tested and compared to the classical mixed tank used in the mixer-settler unit. Replacing the mechanical mixer by a motionless mixer has the effect of reducing the solvent inventory, which is much larger in the former than in the latter mixer (Merchuk et
* J.C.M. was on sabbatical leave from Ben-Gurion University of the Negev, Beer-Sheva, Israel. 0196-4305/80/1119-0522$01.00/0
al., 1980). The reduced inventory of solvent, the reduced equipment size, and the good mass transfer obtained with motionless mixers in comparison to mechanical mixers are the main reasons for considering the static mixer for liquid-liquid extraction. Equipment, Materials, and Procedure The equipment used in this work was described in detail elsewhere (Merchuk et al., 1980). Koch-type motionless mixers and packed tubes filled with saddle-type packings made of ceramics and polypropylene were tested. The solutions used were an aqueous solution of Cu2+and an organic solution of LIX 64N in kerosene, both a t various concentrations as reported in relation with the experimental data and results. The detailed experimental setup has been reported elsewhere (Shai, 1979). The dispersion band or height of dispersed layer of the two immiscible liquids in the settler was found experimentally as follows: the organic and aqueous solutions were pumped at the desired ratios through the mixer and then to the settler. When steady state was reached with regard to all operating conditions, the height of the dispersed layer was measured. The settling time was then obtained by stopping the flow to the settler and allowing the two phases to separate into two layers. The separation time was quite well defined when the aqueous phase was the continuous one, but it was difficult to determine the separation time when the organic phase was the continuous one. Moreover, the separation time was longer when the organic phase was the continuous one, as will be seen later on. Measurements were made of the conductivity of the mixture in order to determine which of the two phases is the continuous one and which the dispersed one. The determination of the continuous phase was done by testing the electrical conductivity of the mixture using an electrode, as described by Merchuk et al. (1980). An alternative method for the determination of the dispersed phase is described by Selker and Sleicher (1965). The experimental results of the dispersion band and separation times are given for the following mixing units: (1)Koch motionless mixer, packing made of 316L stainless 0 1980
American Chemical Society
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This phenomenon concurs with the predictions of Middleman (1974), who developed a theory based on Kolgomoroff s theory of isotropic turbulence (Hinze, 1955, 1959). Middleman's deduction made for Kenics static mixers can in principle be applied to any motionless mixer to predict the following relationship between the Sauter mean diameter and the Weber and Reynolds dimensionless groups
D32= CDoWe-3/5Re1i10
(1)
An experimental determination in Koch motionless mixers of the exponents in eq 1 (Streiff, 1977) led to the following expression
D32 = 0.21DhWe-0.5Re0,15 1
4
J
L
i?Ok RATE.
~/rnli
F i g u r e 1. Dispersion band height VI. flow rate for various lengths of Koch motionless mixer, two LIX 64N concentrations and phase ratio O/A 1:l. 1hC
1 2
-
PHASE 2ATIO O / A 1 : l ( A Q . CGNT.! PHASE R A T I O O f A S . 5 : 1 (ORG. CONT.)
GRG PHASE:
13' L I X 64N
m
2
50
100
I
I
I
3 FLOW RATE, t h i n
4
5
I
150 SETTLING VELOCITY,
F i g u r e 2. Dispersion band height vs.
I
1
I
200
250
L/m2 min
flow rate for Koch motionless
mixers with 10% LIX 64N and three phase ratios.
steel, pipe of X2GrNiMo 1812 stainless steel; (2) packed tubes with polyethylene saddles; (3) packed tubes with ceramic saddles; (4) continuous stirred vessel. The above results are used for analyzing and comparing the various types of mixers. Results Koch Motionlesig Mixers. The diagrammatic representation of the dispersion bands for the Koch type motionless mixers vs. flow rates for concentrations of LIX of 5% and 10% and for a phase ratio of 1:1, 2.5:l and 4:l (organic to aqueous) is given in Figures 1 and 2, respectively. The figures show the effect of flow rate on the height of the dispersion band, and one can see that all dispersion bands increase significantly-even sharply-with increase in flow rate. This is because of the generation of smaller drops at higher flow rates and the smaller residence time in the settler. Both factors tend to increase the height of the dispersion band. No differences were found in the height of the dispersion band for the various numbers of mixing elements in the Koch type of motionless mixers investigated. This implies that the velocity of the liquid flowing through the motionless mixer determines the characteristics of thLe dispersion, and the residence time in the mixer has no influence.
(2)
Since in our experiments the tube diameter remains constant, as do the physical properties (with exception of the surface tension, which depends on the-concentration of LIX 64N), the Sauter mean diameter D32is expected to depend solely on the liquid velocity, and not on the residence time in the mixer. For the same flow rate, different lengths of mixer will produce drops of the same size and therefore the heights of the dispersion band will not change. This is indeed observed in the data presented. Figure 1 shows the effect of LIX concentration in kerosene on the height of the dispersion band for a phase ratio O/A 1:l. One can see that an increase in the concentration of LIX causes an increase in the height of the dispersion band, as expected from the relationship between surface tension and D,, given by either eq 1 or 2. Data were obtained for other phase ratios, with similar results. As the phase ratio O/A increases, the effect of the LIX concentration on the dispersion band height is greater, but its absolute value decreases. Detailed experimental data were reported by Shai (1979). In Figure 2 the height of dispersion band is plotted against the flow rate for three different phase ratios: 1:1, 2.5:1, and 4:l and 10% of L E in the organic phase. Higher dispersion band values correspond to lower phase ratios. It must be emphasized, however, that we are neglecting here an additional factor that plays an important role in the dispersion phenomenon, namely, which of the two phases is the continuous one. Curve 1in Figure 2 represents the dispersions for an aqueous continuous phase, while the other two curves represent heights of dispersion bands where the organic phase is continuous. Here, however, the leading factor is the phase ratio. Comparison of behavior of solutions of equal phase ratio and different continuous phase is shown in Figure 3, where data of dispersion band heights obtained for the same motionless mixer (20 elements) and four different phase ratios are presented, this time with 5% LIX in the organic phase. Figure 3 shows a different behavior of the system for different phase ratios. The curve for O/A 1:l increases monotonically with flow rate as seen in previous figures and represents dispersions where the aqueous phase is continuous. The curve corresponding to phase ratio O/A 4:l increases also with flow rate within the range studied, but at much lower levels of dispersion band heights. The three curves corresponding to phase ratios that are between the two cases mentioned above show a different behavior and exhibit a maximum flow rate in the range of 2.8 to 3.5 L/min. This maximum coincides with a discontinuity in the slope of the curve, which corresponds to phase inversion. It can be seen that a phase inversion occurs at different flow rates for each phase ratio, and the dispersion turns from aqueous continuous to organic continuous as the flow rate increases.
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 4, 1980
524
0
OIA : : I
PbASE R A T I O
D PHPSE RATLO @ / A 1 . 5 : l 0
3HASE RAT:O
0 PHASE
?HASE
Of;
2:l
VATIC O / R 2 . 8 : l RATIC @ / A 4::
L I X 64N - - ORG,5.. COIIT. GRG. PHASE: E
12-
AC. CONT. TEV’F:
jc O c
1
2
13-
z
0-
2 8-
P
5-
42-
O
I
I
2
3 :Lsb :;-L, L‘1
-
Figure 5. Dispersion band height vs. flow rate for packed tubes, Koch motionless mixers and stirred vessel, for 10% LIX 64N and phase ratio 1:l: 0,tube filled with polypropylene Intalox saddles 50 cm; A, tube filled with polypropylene Intalox saddles 18 cm; V,tube filled with polypropylene Intalox saddles 10 cm; 0 , tube filled with ceramic Intalox saddles length 50 cm; 0,tube filled with ceramic Intalox saddles length 20 cm; 0,tube filled with ceramic Intalox saddles 10 cm; M, Koch motionless mixer with 20 mixing elements; A, Koch motionless mixer with 15 mixing elements; 0 , Koch motionless mixer with 10 mixing elements; V, Koch motionless mixer with 5 mixing elements; +, conventional mixer, agitation speed: 470 rpm.
1 2 5 3 RPM
I 01
ORGAhIC PHASE: I 2
I
3
I 4
560 RPM 47C RPM 3 9 0 RPM 1 : l (AG. C O N T . ) 10’ iIX 64N 1
5
I
1
FLOW RATE, Jmin
Figure 4. Dispersion band height vs. flow rate for various agitation speeds in a stirred vessel, with 10% LIX 64N.
It seems therefore, on the basis of the experimental evidence, that higher organic to aqueous phase ratios will give higher dispersion bands as long as the aqueous phase remains continuous. However, at a given flow rate, phase inversion will occur and the dispersion band height will drop to low values that characterize continuous organic phases at that phase ratio. Comparison between the curve of phase ratio O/A 1:l and that of O/A 4:l in Figure 3 shows a large difference in the measured height of dispersion that is not predictable from eq 1 or 2. It should be mentioned that in the experiments carried out both in Middleman (1974) and Streiff (1977) the phase ratio was rather low, with a maximum value of O/A 1:4. Continuous Stirred Tank. The dispersion bands vs. flow rates for the continuous stirred tank at 390,470, 560, and 1250 rpm of impeller speeds, for a LIX concentration of 10% and an organic to aqueous phase ratio of 1:1,are shown in Figure 4. One can see in the figure that there is a sharp increase in the height of the dispersion band as the flow rate increases. The effect of the rotational speed on the dispersion band is more moderate than the effect of flow rate at low flow rates. In fact, the increase in the rotational speed has almost no effect on the dispersion band up to a flow rate of about 3 L/min and becomes significant only above the value of the flow rate. It is a recognized fact that the increase in the agitation speed has a great positive effect on the mass transfer and equilibrium
time and also causes a better dispersion which in turn gives longer settling times (Tunley and Birch, 1970). This concurs with results in Figure 4, which show that higher agitation speeds lead to higher dispersion bands. Packed Tubes. The effect of flow rate on the height of the dispersion bands for the tubes packed with both polypropylene and ceramic saddles can be seen in Figure 5 . One can see a sharp increase in the dispersion band caused by the increase of flow rates. In the figure, a comparison between the dispersion bands obtained from the packed tubes, the Koch type of mixers, and the continuous mixed vessels can also be seen. There is a higher dispersion band for the polypropylene than for the ceramic packing. The difference between the two packings can be explained by the fact that the polypropylene type produces an organic continuous phase while the ceramic type produces an aqueous continuous phase. It can also be seen that the polypropylene packed tube 50 cm high gives the same dispersion as the continuous stirred vessel with organic continuous phase, while the ceramic type of packing gives results in the same range as the Koch type of motionless mixer, giving all aqueous continuous dispersions. The organic continuous dispersions in the stirred vessel were obtained by just changing the location of the stirrer which was initially at the interface of the two liquids before the agitation was started (Nishikawa, 1979). An important fact observed here is that the material from which the packing elements are made can determine which of the two phases will be continuous. This is important in the case of copper extraction with LIX 64N where the total rate of the process is controlled by diffusion in the organic phase (Fleming et al., 1978). In any dispersion, only the turbulence of the continuous phase can be manipulated, since we can hardly exert any influence on the turbulence inside a single drop. Therefore in the case of the Cu extraction by LIX 64N, the organic phase should be forced to be continuous by a convenient choice of the materials of the mixer, allowing for the creation of more turbulence in the organic phase where it is needed in order to decrease
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 4, 1980
COhTINUOLS
CONTI'IbOLS
SETTLING TIME
525
-L3W ? A T E
(8)
Figure 7. Dispersion band height vs. settling time for organic continuous and aqueous continuous dispersions for three flow rates.
Figure 6. Total separation time for dispersions generated in the various mixers used, for 10% LIX 64N and phase ratio O/A 1:l: +, conventional mixer, agitation speed 470 rpm; 0,tube filled with polypropylene Intalox saddles, length 50 cm; A, tube filled with polypropylene Intalox saddles, length 18 cm; V, tube filled with polypropylene Intalox saddles, length 10 cm; 0 , tube filled with ceramic Intalox saddles, length 50 cm; 0,tube filled with ceramic Intalox saddles, length '20 cm; 0 , tube filled with ceramic Intalox saddles, length 10 cm; 11, Koch motionless mixer with 20 mixing elements; A, Koch motionless mixer with 15 mixing elements; V, Koch motionless mixer with 10 mixing elements; 0 , Koch motionless mixer with 5 mixing elements.
the mass transfer resistance. Another point of interest in Figure 5 is that while all the data obtained with IKoch motionless mixers of different lengths give one single line, each of the packed tubes shows two different groups of data: one for the 50 cm long tube and another for all ,the other lengths. This means that expressions of the type of eq 1 or 2 cannot be applied to packed tubes. Channeling could be thought of as the main difference between randomly packed tubes and motionless mixers and would invalidate the assumption of isotropic turbulence made by Middleman (1974). Channeling would also affect mostly the longer columns, and would give lower dispersion band heights. This can be observed in the case of the 50 cm long tube packed with ceramic saddles. The higher dispersion band produced by the 50 cm long tube packed with polypropylene saddles cannot, however, be explained on the basis of channeling effects. Settling Time. Rtelations between dispersion bands and settling times are shown in Figure 6. The experimental procedure consisted in running the equipment at a prefixed flow rate until a stable height of dispersion was measured in the settler, stopping the flow rate at time zero and measuring the time, t , needed for complete separation. There are several points of interest in Figure 6: with the exception of the data for the 50 cm long column filled with polypropylene saddles and for the conventional mixer, all of the data show a similar trend. In the range of low dispersion bands, thie relationship between the height of the band, d, and the time for complete separation, t , is almost linear. As tlhe height of the dispersion band increases, however, the slope of the d vs. t curves, which is the rate of separation, increases from about 0.2 to 0.6 cm/s. This is even more aclcentuated in the curves corresponding to the conventional mixer and the 50 cm long tube filled with polypropylene saddles, which gave dispersions where the organic phase is continuous. These two curves are far apart from all the others, since the rate of separation of the phases is much slower.
Figure 6 shows again the difference between Koch motionless mixers and the packed tubes. While all the data obtained with Koch motionless mixers of different lengths give one single line, each of the packed tubes shows two different groups of data; one for the 50 cm long tube and the other for all the other tube lengths. The 50 cm long tubes gives suspensions that separate slower, and this is more accentuated in the case of the polypropylene saddles which give an organic continuous phase. The difference in the settling process for organic and aqueous continuous dispersions can be seen in Figure 7, where data of dispersion band height versus time are presented for the same flow rates, LIX concentration (lo%), and phase ratio 1:l. One set of data was obtained with a 10 mixing-unit Koch motionless mixer, which gives aqueous continuous dispersion, the second with a tube 18.5 cm long packed with polypropylene saddles, giving organic continuous dispersions. The height of the dispersions is normalized in respect to the height at time zero, d*. The figure shows that the profiles of dispersion are straight lines, implying that the settling velocity of a given dispersion is independent of the height. Data published on dispersions generated in conventional mixers (Paynter et al., 1970) show an exponential type profile that may be attributed to a wide range of drop sizes. The largest drops would separate sooner, and the very small ones would take a much longer time. The straight profiles reported here may be attributed to a narrow range of drop sizes, and this concurs with the experimental findings of Middleman (1974) and Streiff (1977). In addition, Figure 7 shows clearly the larger settling time for organic continuous dispersions. The difference due to the flow rates is much smaller but clearly indicates that lower flow rates produce larger mean drop sizes and therefore a shorter settling time. It is worth stressing that this occurs with the trend found by Merchuk et al. (1980) in mass transfer experiments, who reported that higher liquid flow rates lead to a higher efficiency in the extraction process in static mixers, although the residence time in the mixer was shorter. Only the creation of a much larger interfacial area can explain this fact, since changes in the mass transfer coefficient should not be so drastic in the range reported. Such increase in interfacial area is the consequence of the formation of smaller drops that need larger times for separation. Conclusion Motionless mixers have been reported to be a good choice for the process of copper extraction by LIX 64N. In the present work, data are reported about the characteristics of the dispersions produced by various types of
Ind. Eng. Chem. Process Des. Dev. 1980, 19, 526-530
526
motionless mixers and by a stirred vessel. It can be concluded that the height of a dispersion band generated by motionless mixers is of the same order as that produced by stirrers. Studies of the settling times also show similar behavior. For all the types of mixers, the flow rate is one of the main variables affecting the dispersion. It was found that dispersions with continuous organic phase give much higher dispersion bands and much longer settling times regardless of the type of mixer. Which phase is the continuous one can in some cases be determined by the position of the stirrer in a conventional mixer or by the material of which the packings are made in a motionless mixer. Nomenclature
A = cross-sectional area of the settler, cm2 d = height of the dispersion band, cm Dh = hydraulic diameter, cm D32 = Sauter mean diameter, cm Re = pVDh/p, Reynolds number t = disengagement time for the dispersion band, s V = velocity of the dispersion, cm/s We = p v D h / k = Weber number
Greek Letters
p = CL =
density of the continuous phase, g/cm3 viscosity of the continuous phase, g/s cm
L i t e r a t u r e Cited Fleming, Ch. A., Nicol, M. J., Hancock, R. D.,Finkelstein, N. P., J. Appl. Chem. Biotechnol., 28, 443 (1978). Hinze, J. O., AIChE J., I, 289 (1955). Hinze, J. O., “Turbulence”, McGraw-Hili, New York, 1959. Merchuk, J. C., Shai, R., Wolf, D.,Ind. Eng. Chem. Process Des. Dev., I S , 91 (1980). Middleman, S.,Id.Eng. Chem. Process Des. Dev., 13, 78 (1974). Mutsakis, M., “Static Mixing in the Chemical and Petrochemlcal Industries”, Koch Engineering Co., fic., 1976. Nishikawa, M.,Ashiwake, K., Hashimto, N., Nagata, S., Int. Chem. Eng.,19, 153 11979). Paynter,‘J. C.,’ Tunley, T. H., Birch, C. P., “The LiquibIon Extraction of Copper with LIX 64N in a Rotating Disc Contactor”, National Institute of Metallurgy, Johannesburg, S.A., Research Report 940, May 1970. Seiker, A. H., Sleicher, C. A., Can. J. Chem. Eng., 43, (1965). Shai, R., M.S. Thesis, BenGurion University of the Negev, Israel, 1979. Streiff, F., Sotzer Tech. Rev., 3, 108 (1977). Tuniey, T. H., Birch, C. P., “The Recovery of Copper from Sulphate Leach Liquors by Liquid in Exchange with LIX 64N”, National Institute for Metallurgy, Johannesburg, S. A., Research Report 964, June 1970.
Received f o r review February 27, 1979 Accepted April 24, 1980
Funding for this research was provided in part by the Engineering Research Institute of Iowa State University.
Recovery of Fluoboric Acid from Waste Copper Fluoborate-Fluoboric Acid Solutions by Ion Exchange Parvir Ghossl and Alfred A. Donatelll’ Chemical Engineering Department, University of Lowell, Lowell, Massachusetts 0 1854
In the search for a method to recover fluoboric acid from copper fluoborate-fluoboric acid waste solutions, produced predominantly by the electronic industry, the technique of ion exchange has been considered. The system used was a laboratory size fixed-bed ionexchange column. The first part of the study was devoted to finding an adequate ion-exchange resin, and the second part was used to determine a set of optimum operating conditions. Amberlie 200, a macroreticular cation-exchange resin, was found to be the best material. The copper concentration in the waste solution was reduced from 21 to 1 g/L so that the fluoboric acid could be recycled into the electroplating operation. For the process evaluated in this study, the regeneration cycle was the most time-consuming step.
Introduction
Electroplating is a basic step in the production of printed circuit boards, and the metals involved in the process are usually copper, gold, nickel, rhodium, silver, and tin. Among these, copper has met with wide acceptance as the base conductor metal in printed circuit manufacture (Coombs, 1967). One of the commonly used solutions is copper fluoborate in which addition of fluoboric acid to the fluoborate bath is necessary for obtaining acceptable deposits (Lowenhein, 1974). At the present time, there is no prominent technique available to reclaim the copper or the fluoboric acid from the waste generated by electroplating. Every year thousands of pounds of valuable fluoboric acid and recoverable copper are allowed to go to waste after they have been used in electroplating operations. Often such wastes also constitute a serious pollution problem (Nachod and Schubert, 1956). In searching for alternatives to recover and recycle the valuable material from waste copper fluoborate-fluoboric acid solutions, one can utilize the technique of ion exchange 0196-4305/80/1119-0526$01.00/0
which is used predominantly in the treatment of water. It will be shown in this investigation that an ion-exchange process can effectively separate copper from a copper fluoborate-fluoboric acid waste solution in the form of copper sulfate and the acid. This will enable the fluoboric acid to be recycled into the electroplating operation. E x p e r i m e n t a l Section
The experimental procedure was divided into two parts: (1) screening tests where the best ion-exchange material wa? selected, and (2) optimizing the operating conditions for the best resin. The solution that was used in the study was an actual waste product from an electroplating plant. It had a pH close to zero and copper, fluoborate, and fluoboric acid concentrations of 21, 57, and 307 g/L, respectively. There were no other cations competing with copper for the ion-exchange sites in the resin. Screening Tests. In this part of the investigation, the goal was to obtain an ion-exchange resin that could withstand the chemical attack of fluoboric acid and also show satisfactory exhaustion and regeneration ability. From the principal producers of ion-exchange material, the cation(C 1980 American Chemical Society
