14
Ind. Eng. Chem. Fundam. 1981, 20, 14-20
Extraction Kinetics of Copper-LIX65N System. 1 Forward Extraction Rate Toshinori KoJlma and Terukatsu Miyauchi* Department of Chemical Engineering, Universw of Tokyo, Hongo, Bunkyoku, Tokyo, 113, Japan
Extraction rate of copper by the chelating extraction reagent, LIX65N, was measured with an extraction apparatus which contains a porous Teflon membrane. The results at (M2+),,/(H+)b >> 1 agree well with the theory by assuming that diffusion of chelating reagent and copper ion is accompanied by a complex-forming reaction in the aqueous phase. (M2+), and (H+A are respectively the concentration of copper ion and hydrogen ion in the bulk of the aqueous phase. The data deviate from the theory for the range (M2+)b/(H+)bS I. The deviation is closely related to the stripping process as explained in a companion paper.
Introduction Lately, the extraction of metals with chelating reagents has come to be used in copper mines and for the disposal of waste water, as a result of the development of commercial reagents such as LIX, SME, Kelex, and so on. Compared with the chelating reagents used in analytical chemistry, the general properties of these reagents are as follows: liquid type, highly soluble in organic solvents (e.g., kerosene), low solubility in water (Ashbrook, 1972), high copper selectivity from other metals, and high extraction rates (Flett, 1977). The controlling reaction mechanism of metal extraction with these commercial reagents is believed to be interfacial reaction (Flett, 1977). Chapman et al. (1975) theoretically analyzed the kinetics of extraction assuming a fast interfacial reaction and mass transfer limiting extraction rate. Flett et al. (1973) measured the extraction rate of the system LIX64N-copper using AKUFV (Reinhardt and Rydberg, 1970) and an analysis based on the assumption of the interfacial reaction limiting to obtain the following equation.
with an analysis and discussion based on a diffusion model accompanied by the formation of a chelating complex. LM65N contains 34% of 2-hydroxy, 5-nonylbenzophenone oxime (Taylor, 1978) with kerosene as the other main component. Dispersol (Shell Chemistry Co., Ltd.), a sort of kerosene, was chosen as diluent.
Properties and Extraction Equilibria of t h e System LIX65N-Dispersol-Copper Physical properties and equilibria were studied and presented in a previous paper (Kojima et al., 1979). The molecular weight of the active component of LM65N, HR, is 339, the molecular volume 440, and the density of LIX65N (Lot No. 4~10015)0.9091 g/cm3. The mean molecular weight of Dispersol was estimated to be about 180. The viscosity of Dispersol was measured to be 1.90 CP. The theory of equilibria is summarized in Table I. The concentration of active component of LIX65N in the aqueous phase, (HR), was measured in equilibrium with LIX65N solution in Dispersol. The partition coefficient K1 is apparently not constant due to dimerization of HR r = K(CU'+)(LIX~~N)(LIX~~)~/'/(H+) in the organic phase; as a result of the apparent partition coefficient was defined as K1* = CR/(HR). Values of KO L (C U - ( L I X ~ ~ N ) ~ ) . ( L'/'(Ht) I X ~ ~/ (LIX65N) ) = (=),/((HR)2, and K1for monomer were determined Atwood et al. (1975) measured the rate of extraction by from Figure 5 in the previous paper, yielding KO = 0.09 measuring the change in pH in a single-drop reaction cell mol/L and K1 = 1.1 X lom4.The data for CR < 0.091 and found the interfacial reaction to be rate controlling. mol/L (= 10%) were well expressed by eq 14 in the paper; The rate equation was r = ~ ( C U ~ + ) ( L I X ~ ~ N )however, ~ ' ~ - for CR 2 0.091 mol/L, the existence of polymer (LIX63)1/2(H+)o.Flett et al. (1977) suggested the uncerwas found. Values of and ((HR),) are calculated as tainties of the results of Atwood et al. (1975) from the (E) = (HR)/Kl and 2((HR),) = CR - (E). These valstandpoint of equilibrium. Whewell et al. (1975) found the ues are summarized in Table 11. Values of K z , K5,and rate of extraction a function of (Cu2+)/(Ht),employing a K,, cited in Table I were determined, yielding Kz= 1.8 X single drop method, but no quantitative analysis was made. lo4 mol/L, K5= 2.5 X lo5 and K,, = K1*2K22K&4K5= Thus, the expressions for the rate of extraction are dif2.27 (LIX65N 2.5 vol %) with an ionic strength of I = 0.2 ferent among investigators and definite conclusions have mol/L (M/15 Na,SO,). K,, was not found to be constant yet to be obtained. since K1* is not constant; however, K$K3K4 is constant On the other hand, in the field of analytical chemistry, and equals 1280 irrespective of the concentration of extraction of metals with chelating reagents has been LIX65N. studied for a long time and has been analyzed using the Theory mechanism of reactioa-limited monocomplex formation When the limiting reaction is the mono-complex for(Irving and Williams, 1949; McClellan and Freiser, 1964). mation reaction between a monovalent chelating reagent, Kondo et al. (1978) studied the kinetics of extraction of HR (with coordination number 2) and a divalent metal, copper with benzoylacetone using a stirred transfer cell and based their analysis on a diffusion model accompanied by M2+(with coordination number 4),in an aqueous phase, the formation of a chelating complex. the extraction kinetics may be expressed as follows. This particular study concerns the kinetics of copper Basic Equations and Solutions for the Aqueous extraction with LIX65N, a chief component of LIX64N, Phase. With the assumption that equilibria of K2,K4 are + -
(m)
0 196-4313/81/ 1020-0014f01 .OO/O
0 1981 American Chemical Society
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Table 11. Nature of LIX65N in Dispersol
CH x 103 LIX65N, vol %
m 0.5 1 2.5 5
(HR) x
mol/L
103 mol/L
4.56 9.12 22.8 45.6 91.2
4.15 7.65 17.1 28.8 42.8
((HR),) x 103
(FIX) x io6
mol/L
mol/L
K,* x 105
0.20 0.73 2.8 8.4 24.1
0.46 0.85 1.90 3.2 4.8
11.0 10.1 9.3 8.4 7.0 5.2 0
attained instantly, the reaction rate in the aqueous phase is presented below.
-.-
r = r - r = ~,(MZ+)(R-) - &MR+) = k3K2KM2+)(HR)/(H+)- (H+)(MR2)/Kz2K3K,(HR))(1) where the reaction species are M2+and R- according t o classical mechanism (Irving and Williams, 1949; McClellan and Freiser, 1964). This point will be discussed later. With the assumption (M2+) >> (MOH+) (when M2+ represents the copper ion, this assumption is valid at pH < 5 (Kagaku-Benran, 197511, from a mass balance of the metal, the following equation is obtained. d2(M2+) d2(MR+) d2(MRz) M 7 = -DMR (2) -D m * 2 dx dx2 dx With the assumption (HR) >> (R-) (when HR represents LM65N, this assumption is valid at pH > 1 in Figure 9; therefore the following equation is obtained. MH* = [ ~ ~ ~ K ~ O H R ( M ~ ' ) ~ /'( IH2' )/*k] =~ ~ 0.18[(M2+)b/(H+)*]'/2/kHR (20) The calculated values of MH* for all data in Figure 9 are much larger than 1. Therefore the assumption of Ei - 1 >> MH* >> 1 is satisified. Comparison of the Data at (M2+)b/(H+)*>> 1 with van Krevelen and Hoftijzer's Equation. All data at (M2+)b/(H+)*>> 1are expected to be expressed by eq 5-8, irrespective of the values of Ei and MH*. The values of MH* is calculated from eq 20 and the following equations. E = kR*/kHR = 2NM/(CR,b - ~ " ~ / ~ H R ) K I * ~ H R
E, = 1 + 2&(M2+)b/D~~(HR)i = 1+ 2&(M2+)b/D~~(cR,b
- 2NM/kE)Ki*
Figure 10 presents the data for (M2+)b/(H+)*>> 1 compared with van Krevelen and Hoftijzer's equation (1948). Good agreement may be seen in Figure 10. Discussion A value of It3 can be calculated as 2.0 X 1015cm3/mol s from eq 7 and 20 using values of K2 = 1.8 X 10-l2mol/cm3 (Kojima et al., 1979) and D m = 4.4 X lo4 cm2/s. Actually this value is not reasonable because it is larger than 1.4 X 1014cm3/mol s (Eigen and De Maeyer, 1955), the rate constant for the reaction between H+ and OH-, which is the fastest reaction in the aqueous phase. On the other hand, the complex forming rate constant of copper ion is
Ind. Eng. Chem. Fundam., Vol. 20, No. 1, 1981
4
L
Mi
10'
1 o3
Figure 10. Comparison of data with van Krevelen and Hoftijzer's equation.
usually 10" to 10l2cm3/mol s, e.g., 5.2 X 10" cm3/mol s at 30 "C for the benzoylacetone-copper (Kondo et al., 1978). There are several interpretations for such results. One is that the species are not M2+and R-. If the species are MOH+ and HR, the reaction equation becomes MOH+ + HR
-% MR+ + H2O
(21)
and ? = k,'(MOH+)(HR) = kiKMKw(M2+)(HR)/(H+) (22) where Kw = (H+)(OH-),KM = (MOH+)/(M2+)(0H-)and K i = (MR+)/(MOH+)(HR).Namely, k a 2 is substituted by k,'KMKw and K2K3by KMKWKi. The calculation of k i yields k i = 3.6 X 1013 cm3/mol s using Kw = mo12/cm6and KM= 1O'O cm3/mol (Kagaku Benran, 1975). This value is smaller than 1.4 X 1014cm3/mol s for H+ and OH- and is acceptable. On the other hand, consecutive reaction, for instance eq 23 and 24, may also be possible. M(H20)2++ HR
-
& M(H20),R+ + H+ k3))
+
(23)
M(H2O),R+ M(H2O),R+ (n - m)H2O (24) If the outer-sphere complex formation reaction (eq 23) is fast enough and the inner-sphere complex formation reaction (eq 24) is limiting, the forward reaction rate, ?, is proportional to (M2+)(HR)/(H+)because (M(H20),R+)= k,"'(M(H20),2')(HR)/k,"(H+). The value of the reaction rate constant k3/' cannot be calculated but will be much smaller than that of k , because (HR) is much larger than (R-). Therefore, the reaction mechanism by eq 23 and 24 is also acceptable for the present analysis. (In this present discussion, M(H20),2+and M(H20),R+ are used instead of M2+and MR+, in order to show the dehydration reaction clearly.) Further interpretation to such results is that the reaction takes place at the interface, because the concentration of HR in the organic phase is much higher than in the aqueous phase. Even if the reaction takes palce at the interface, the data obtained cannot be analyzed by the model proposed by Flett et al. (1973). Consideration will then have to be given to diffusion accompanied by reaction at the interface because our data for NM were proportional to [(M2+)b/(H+)*]1/2, but a reasonable model cannot be proposed since physical data at the interface, e.g., concentration, diffusion coefficient and so on are still unknown. Conclusion The extraction rates of copper by LIX65N were measured with the following results obtained.
1s
(1)In the region of very fast reaction, the limiting step is the diffusion of copper in the aqueous phase and the film coefficients for mass transfer have been calculated. (2) In the region where stirring speed does not affect the extraction rate, the mass transfer resistance of HR in the organic phase is eliminated. Thus the extraction rate is proportional to (HR)i raised to the first power, (M2+)b raised to the one-half power, and (H+)braised to the minus one-half power. At high pH values, the diffusion of hydrogen ion comes into effect and a modified (H+)*is used instead of (H+)b (3) All data for (M2+)b/(H+)b >> 1 agree well with the model assuming that diffusion of HR and M2+accompanied by a monocomplex forming reaction in the aqueous phase is controlling. However, the reaction mechanism is not clear yet, and several possible reaction equations are described. (4) The results for (M2+)b/(H+)b 5 1disagree with the theory. This point will be explained elsewhere in relation to stripping.
Nomenclature A = effective area of membrane, cm2 CR = concentration of activecomponentof LIX65N in organic phase = (E) + 2((HR),), mol/L D = diffusion coefficient, cm2/s DM* = distribution ratio of metal DR*= distribution ratio of active component of LIX65N E = enhancement factor = kg*/kHR (Hatta number) Ei = enhancement factor for instantaneous reaction K~ = partition constant of HR = (HR)/(HR) K1* = apparent partition constant of HR = (HR)/CR K2 = dissociation constant of HR in aqueous phase, mol/L K 3 = formation constant of MR+ in aqueous phase, L/mol K4 = formation constant of MR2 in aqueous phase, L/mol K5 = partition constant of MR2 K,, = extraction constant KM = (MOH+)/(M2+)(OH-), L/mol Kw = (H+)(OH-),mo12/L2 12 = mass transfer coefficient, cm/s kn* = 2NM/(HR);,cm/s kOR = Nh;j&, Cm/S ' k3 = forward reaction rate constant of (M2+)and (R-), cm3/mol S
k,' = forward reaction rate constant of (MOH+)and (HR), -cm3/mol s kgl' = forward reaction rate constant of (M2+)and (HR), -cm3/mol s MH = [ ~ ~ ~ ~ ~ H R ( M ~ + ) ~ / ( H + ) ~ ~ " ~ / K H R MH* = modified MH (eq 20) N = mass transfer rate, mol/cm2 s n = stirring speed, rpm r = reaction rate, mol/L s t = time, s V = volume of vessel, cm3 x = distance from interface, cm xL = thickness of boundary film, cm (X) = concentration of chemical species, X, mol/L Greek Letters 6 = thickness of membrane, cm t = porosity of membrane x = tortuosity factor of membrane Subscripts and Species b = bulk H = hydrogen ion HR = monomer of active component of LIX65N (HR)2= dimer of HR in organic phase i = interface M = copper ion MOH = Cu(OH)+ion MR = copper mono-complex ion with HR
Ind. Eng. Chem. Fundam. 1981, 20, 20-25
20
MR2 = copper bis-complex ion with HR 0 = initial value OH = hydroxyl ion R = anion of HR Superscripts
_ -- organic phase
--
= forward reaction = reverse reaction
Literature Cited
Ashbrook, A. W. Anal. Chim. Acta 1972, 58, 115. Atwood, R. L.; Thatcher, D. N.; Miller, J. D. Metall. Trans. 8 1975, 68, 465. ChaDman, T. W.; Caban, R.; Tunison, M. E. AIChf Svmt). ~. Ser. 1975. 71. No. 152, 128. Eigen, M.; DeMaeyer, L. 2. flektrochem. 1955, 59, 986. Fielt, D. S. Acc. Chem. Res. 1977, 70, 99. Flelt, D. S.; Melling, J.; Spink, D. R. J. Inorg. Nwl. Chem. 1977, 39, 700. Fielt, D. S.;Okuhara, D. N.; Spink, D. R . J. Inorg. Nucl. Chem. 1973, 35, 247 1
Irving, H.;Williams, R. J. P. J . Chem. SOC.1949, 1841.
Johnson, A. I.; Huang. ChenJung AIChf J. 1958. 2, 412. "Kagaku-Benran", The chemical Society of Japan, Maruzen, Tokyo, 1975. Kojima, T.; Tomb. J.; Mlyauchi, T. Kagaku Kogaku Ronbunshu 1979, 5 , 476. Kojima, T.; Miyauchi, T., Ind. fng. Chem. Fundam. following article in this issue, 1980. Kondo, K.; Takahashi, S.;Tsuneyuki, T.; Nakashio, F. J. Chem. f n g . Jpn. 1978, 1 7 , 193. McCielian, 8. E.; Freiser, H. Anal. Chem. 1984, 36, 2262. Moore, J. H.;Schechter, R. S. AIChf J. 1973, 19, 741. Reinhardt, H.; Rydberg, J. Chem. Id.1970, 7 1 , 488. Sherwood, T. K.; Pigford, R. L.; Wiike, C. R., "Mass Transfer", p 35, McGraw-Hili: New York, 1975. Taylor, R. Lectured at Shell Chemical Co. Ltd., Tokyo, Sept 12, 1978. van Krevelen, D. W.; Hoftijzer, P. Red. Trav. Chlm. 1948, 67, 563. Watanabe, H.; Mjauchi, T. Kagaku Kogaku Ronbunshu 1978. 2 , 262. Wheweii, R. J.; Hughes, M. A.; Hanson, C. J. Inorg. Nucl. Chem. 1975, 3 7 , 2303. Wilke, C. R.; Chang, P. AIChf J. 1955, 7 , 264.
Received for review June 20, 1979 Accepted July 16, 1980
Extraction Kinetics of Copper-LIX65N System. 2. Stripping Rate of Copper Toshlnori Kojlma and Terukatsu Mlyauchl' Depaflment of Chemical Engineering, University of Tokyo, Hongo, Bunkyoku, Tokyo, 113, Japan
The stripping rate of copper from the complex, copper-LIX65N, by sulfuric acid is measured with the same apparatus as that described in the companion paper. The results are explained well by assuming that diffusion of the monocomplex, MR, accompanied by dissociation reaction in the aqueous phase is the rate-controlling step. The experimental rate of extraction obtained at (M2+)b/(ti+)b 5 1 in the companion paper a p e s well with values calculated from theory using data from distribution equilibria, extraction rates at (M2+lb/(ti)b >> I , and stripping rates.
Introduction The physical properties and partition equilibrium of the LIX65N-Dispersol-copper system and extraction rates of copper have been reported (Kojima et al., 1979; Kojima and Miyauchi, 1980). Most of the rate data for (M2+)b/ (H+)b2 1 are explained by the diffusion-reaction mechanism between HR (active component of LIX65N in the aqueous phase) and M2+(copper ion). However, some data obtained for (M2+)b/(H+)b5 1could not be explained by the mechanism developed for (M2+)b/(H+)b >> 1. In this paper, stripping rates are studied using the same system as above. Also, the extraction rate data for (M2+)b/(H+)b 5 1are reassessed using the theory developed in this paper. In the companion paper, we state that the forward reaction rate constant value of k3 for the reaction of M2+and R- is too large and suggested the following alternative reaction mechanisms to explain this anomaly. First, the reaction species are MOH and HR or M2+and HR instead of M2+and R-. Second, the reaction takes place at the Dispersol-water interface instead of in the aqueous phase. However, the experimental data could not be explained by the mechanism proposed by Flett (1977). The information about the interface is not yet so sufficient as to analyze the interfacial reaction mechanism. Therefore the data are analyzed here by assuming that the reaction takes place in the aqueous phase with M2+and R- as the reactive 0196-4313/81/1020-0020$01.OO/O
species, because this analysis is also applicable to the case where the reaction species are MOH+ and HR or M2+and HR (Kojima and Miyauchi, 1980). Theory Equations 1-3 given in the companion paper (Kojima and Miyauchi, 1980) are also used for the analysis of stripping of copper from the organic phase. Equations 1 and 2 give the following relation for the rate of copper stripping
-r = -r - r' = k3K2((H+) (MR2)/K22K3K4(HR)-
Because these equations cannot be solved directly as expressed, the following simplificationsare assumed (1)the film theory is applicable for mass transfer; (2) quasi-steady state is realized, because of the relatively large amount of species in each phase; (3) (H+) is constant throughout the aqueous phase; (4) (HR) is also constant throughout the aqueous phase; (5) (M2+)is constant or zero depending on reaction conditions selected; and (6) equilibria of the complex formation reactions are attained in the bulk of the aqueous phase. Assumption (3) is reasonable due to @ 1981 American Chemical Society
