I n d . Eng. Chem. Res. 1990,29, 601-606
601
SEPARATIONS Modeling of Extraction Equilibrium and Computer Simulation of Extraction-Stripping Systems for Copper Extraction by 2-Hydroxy-5nonylbenzaldehydeOxime Mariusz B. Bogacki and Jan Szymanowski* Poznari Technical University, P1. Skbdowskiej-Curie 2, 60-965 Poznari, Poland
A model for computer simulation of extraction-stripping systems is presented in which the extraction equilibrium is computed by very precise smoothing spline functions. Extraction of copper from acidic sulfate solutions by the pure fraction of 2-hydroxy-5-nonylbenzaldehydeis discussed. Extraction of copper decreases in the following order: systems with cross-current flow > systems with combined flow > classical systems with sequential stages of extraction and stripping. For each process, an appropriate extraction-stripping flow sheet with various combinations of extraction and stripping stages and various solvent flows can be selected. Hydroxy oximes are well-established extractants for copper, and they are used in several industrial installations. It is now assumed that about 15% of all the copper produced in the world is produced by hydrometallurgical methods, i.e., is extracted by hydroxy oximes. In the first installations, LIX 64N extractant was usually used, and the extraction and stripping were carried out in countercurrent cascades having three and two stages, respectively. Recently, Acorga reagents or similar ones of LIX series containing 2-hydroxy-5-nonylbenzaldehydeoxime or 2hydroxy-5-dodecylbenzaldehydeoxime as the active substances were used often. These last extractants are much stronger than 2-hydroxy-5-nonylbenzophenone oxime applied in LIX 64N. As a result, they can extract copper from more acidic and concentrated solutions in the countercurrent cascades containing only two extraction stages. However, the extraction strength is not the only factor having an influence on the design and development of the industrial process. A susceptibility to stripping is another important factor. Due to the small influence of sulfuric acid concentration upon copper extraction by 2-hydroxy5-nonylbenzaldehyde oxime, high levels of sulfuric acid must be used to strip copper from organic solutions. These high concentrations are not practical on a commercial scale. So the extraction properties of 2-hydroxy-5-alkylbenzaldehyde oxime are modified in the industrial products with an nonylphenol aliphate alcohol or appropriate ester to decrease its extraction strength and to increase its susceptibility to stripping. Some improvement of the extraction-stripping process can also be obtained using the cross-current and combined solvent flows as was demonstrated recently for metal extraction; see Rod (1984) and Hughes and Parker (1985, 1987). Flow sheets with a cross-flow of solvent between the sections are compared there with those of the conventional countercurrent flow processes. In these new processes, the loaded extractant is returned to strip immediately after each extraction stage or after two extraction stages. It is demonstrated that such flow sheets are more efficient, as they allow an achievement of the same
* Author to whom correspondence should be addressed.
degree of separation with lower solvent flow rates. Hughes and Parker (1985,1987) pointed out that the differences between various considered processes are not always great. However, even small advantages which result from the application of the combined schemes to copper extraction may contribute significantly to the economics of copper recovery. Very precise models of the equilibrium extraction data must be used to obtain realistic results by means of computer simulations. Rod (1984) and Hughes and Parker (1987) used the chemical models discussed previously by Forrest and Hughes (1975a,b) and Hughes et al. (1975). The accuracy of these models is not sufficiently good. The modeling of the extraction isotherm for copper extraction from acidic sulfate solutions by hydroxy oximes was discussed in several independent papers. Forrest and Hughes (1975a,b), Hughes et al. (1975), and Robinson and Paynter (1971) discussed empiric, semiempiric, and chemical models and proposed using them to correlate the equilibrium copper concentration in the organic phase with its equilibrium concentration in the aqueous phase. Szymanowski and Jeszka (1985) applied polynomials to modeling the equilibrium of copper extraction by 2hydroxy-5-nonylbenzaldehyde oxime and 2-hydroxy-5nonylbenzophenone oxime, and on the basis of such functions, they discussed classical multistage and countercurrent multistage processes. Szymanowski and Atamakzuk (1982) used the modified Couchy distribution to model the equilibrium of copper extraction from very diluted solutions. Piotrowicz and Wasylkiewicz (1986) and Piotrowicz et al. (1989) proposed the chemical approach in which the activity coefficients of inorganic species present in the aqueous phase were computed according to the modification of the Pitzer method (Wasylkiewicz and Piotrowicz, 1988). Similar conclusions were obtained by using chemical models applying concentrations or activities. The accuracies of these models are not sufficient to use them for computer simulation of various extractionstripping flow sheets. Especially important deviations of these models from the experimental data are observed in regions of low and high copper concentrations. The chemical model proposed by Piotrowicz and Wasylkiewicz
0888-5885190 12629-0601$02.50/0 R 1990 American Chemical Societv
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(1986) and Piotrowicz et al. (1989) is very sophisticated, and several physicochemical data for the extraction system are necessary for computing. It does not give more accurate results in comparison with other models. The aim of this paper is to present an empirical and very precise model of copper extraction from acidic sulfate solutions by pure isolated fractions of 2-hydroxy-5nonylbenzaldehyde oxime and to use this model for discussing various unconventional extraction-stripping systems. Thus, this is the continuation of our previous articles (Szymanowski and Jeszka, 1985; Piotrowicz et al., 1989) in which empirical and chemical models were discussed and used for modeling copper extraction by 2-hydroxy-5nonylbenzaldehyde oxime and by 2-hydroxy-5-nonylbenzophenone oxime.
Equilibrium Extraction Data 2-Hydroxy-5-nonylbenzaldehydeoxime, the same as described in our previous papers (Szymanowski and Jeszka, 1985; Piotrowicz et al., 19891, was used. Extraction data were obtained a t 18-20 “C for different hydroxy oxime concentrations and various initial concentrations of sulfuric acid in an aqueous phase. The oxime concentrations were equal to 5%,11.3%, 1570, and 2070,while the amount of sulfuric acid added to the aqueous phase varied from 0 to 250 g dm-3. The equilibrium copper concentrations in the aqueous phase varied from 0 to =50 g dm-3. Thus, the initial extraction data are similar to those discussed in the papers mentioned above. Some experimental data for higher concentrations of copper and sulfuric acid were added in this case.
Mathematical Model of an Extraction-Stripping System The discussed extraction-stripping flow sheets are presented in Figure 1. The classical scheme used in industrial processes is presented in Figure la. In such process, copper is first extracted by hydroxy oxime into the organic phase in the three-stage countercurrent cascade and then stripped to the aqueous phase by sulfuric acid solution. Apart from this conventional countercurrent flow arrangement, flow sheets with total or partial cross-flow of the solvent between the extraction and stripping stages can be constructed. The total cross-flow, demonstrated in Figure Ib, can be implemented only for an identical number of stages in each section, while the partial crossflow, illustrated in Figure IC,can be used both for an equal and a different number of stages in each section. The mathematical model for metal extraction was presented by Rod (1984). It is assumed that extraction equilibrium is quickly achieved and theoretical stages are considered. The differential material balance for the ith stage of the extraction section is described by eqs 1 and 2. At steady dxM,i/dt + uy,r dyM,i/dt = F * ( X M , ~ - l - x M , ~ ) + s*(YM,kCi) - YM,i) (l) -u, dxM,i/dt = 2u, dxM,l/dt, i = 1, 2, ...*n ( 2 ) state, the concentrations do not depend on the volumes of both phases in the stage considered. Hence, for simplicity, we assume, uy = 0 and u, = V. We can also introduce the dimensionless time T = tF/ V. Each extraction stage can be described by the following set of equations:
4
‘“i
tOf’
P--
Figure 1. Extraction-stripping flow sheets: (a, top) classical countercurrent flows with sequential stages of extraction and stripping; (b, middle) cross-current solvent flows with the same number of stages for extraction and stripping; (c, bottom) combined solvent flow.
Analogously, we obtain similar differential equations for the stripping; we assume that V / F = V’/F’, where the superscript ’ denotes stripping:
To simplify computation, sequential stages of extraction and stripping were numbered according to the flow of the aqueous phase (Figure 1). Hence, depending upon the considered extraction-stripping flow sheet, the function k ( i ) , which denotes the number of the stage feeding on stage “i” or “ j ” of the extraction-stripping system by the organic phase, equals classical countercurrent flow (Figure l a ) k(i) =
cross-current flows (Figure l b ) k ( i ) = n + h - i , i = 1, 2, ..., n
+k
(10)
combined solvent flows (Figure IC) i = 1, 2, ..., 1 - 1 k(i) =
i i i i
=1
+ 1, 1 + 2, ..., n + m - 1 (11) + m + 2, ..., n + k - 1
=n+m = n + m + 1, n = n + k
Ind. Eng. Chem. Res., Vol. 29, No. 4, 1990 603
Model for Extraction Equilibrium In this paper, we apply the smoothing spline functions for modeling the extraction equilibrium. These functions were used previously by us for modeling the adsorption of hydroxy oxime extractants a t water/organic solvent interfaces (Bogacki e t al., 1988). The model of extraction equilibrium can be described as follows: r
Table I. Analyzed Extraction-Stripping Systems” type of extr-strp. system no. of stages C extr, strp., system n k a b l m 1 1 2 + 2 2 2 3 2 3 + 1 2
+
+
-
+
-
1
2
+
+
2
1
1
s
FhH, x M ) = C C Y ~ : ~ F ~ ( ~ H ) G , & M ) (12) a=08=O
4 5
2 3
4 3
1
1
where y z j denotes experimental copper concentrations in the organic phase for various initial concentrations of sulfuric acid and various equilibrium concentrations of copper in the aqueous phase, and r and s denote the number of various initial concentrations of sulfuric acid and equilibrium concentrations of copper in the aqueous phase, respectively. The functions Fa(xH)and G & x M ) are of the form
(
“Symbols used are the same as in Figure 1; + denotes that the system was computed.
Table 11. Initial Data for Considered Extraction-Stripping Systems” ratios of flow of aq phase to org phase xM,O, g dm-3 extr, F / S strp., F‘IS 5.0 1.0, 1.5, 2.0 2.0, 1.0, 0.5, 0.4, 0.25, 0.2, 0.125, 0.1 10.0 1.0, 1.5, 2.0 2.0, 1.0, 0.5, 0.4, 0.25, 0.2, 0.125, 0.1 15.0 1.0, 1.5, 2.0 2.0, 1.0, 0.5, 0.4, 0.25, 0.2, 0.125, 0.1 a Initial copper concentration in stripping, x ~ , = ~ 1.0 + ~g dm”; initial sulfuric acid concentration in extraction, x ~ = ,5.0~ g dm-3; initial sulfuric acid concentration in stripping, x H , ~ +=~ 250.0 g dm-3.
- MY?)
xH - xH,y-l
hH,, a = 1 , 2,..., r, y = l , 2 ,..., r
p = 1 , 2 ,..., s, p = l , 2 ,..., s
where
(13)
Computing Procedures The extraction equilibrium was computed according to the presented model using the method described previously by Bogacki et al. (19881, where the smoothing spline functions were applied. The Adams-Moulton method (Bogacki et al., 1989) was used to compute the model of extraction-stripping from various considered flow sheets (Figure 1 and Tables I and 11). The number of systems given in Table I are connected with the total number of extraction and stripping stages: 3,4, and 5 stages for systems 1, 2, and 3, respectively, and 6 stages for systems 4 and 5. The type of extraction system denotes the classical countercurrent flow with sequential stages of extraction and stripping (type a), cross-current flow (type b), and combined solvent flow (type c). For this last type for systems 3 and 5, two subtypes can be considered: 3C1 for which 1 = 1 and m = 2; 3Cz for which 1 = 1 and m = 1;5C1 and 5C2for which 1 = 2 and m = 1 and 1 = 1 and m = 2, respectively. The meaning of 1 and m is presented in Figure 1.
Results 1, i = j
Figure 2 presents the isotherm of the equilibrium surface computed by means of the spline function. A very good adjustment of the model and the experimental data is observed in the whole region of copper and sulfuric acid concentrations. Similar good approximations were also obtained for other considered concentrations of 2hydroxy-5-nonylbenzaldehydeoxime in the organic phase. The dynamics of the extraction-stripping systems are demonstrated in exemplary Figures 3-5. The attainment time of the steady state is similar for each flow sheet considered and equals about 12 units. During this time, constant values of copper concentrations are achieved both in the organic and aqueous phases. The changes in the mass balance calculated as the difference between outlet and inlet values for each stage or for the whole system ( A x ) converge quickly to zero (Figure 5). The maximal error of the total balance of the extractionstripping system does g dm-3. Hence, solution drifting is not not exceed 5 X observed as in the case when using the simulation proce-
604
Ind. Eng. Chem. Res., Vol. 29, No. 4, 1990
r
9c
3
2
1
F i g u r e 2. Equilibrium of copper extraction by 2-hydroxy-5-nonylbenzaldehyde oxime (c = 11.3%) for various initial concentrations of sulfuric acid; 1, 0 g dm-3; 2, 10 g dm-3; 3, 20 g dm-3; 4, 50 g dm-3; 5, 100 g dm-3; 6, 250 g dm-3.
-1
-2
1 1
I/
I
,
-3
Figure 5. Mass balance of successive extraction and stripping stages for combined solvent flow (type 3C). Number of the curve denotes the number of the stage, and stands for the total balance ( x M , ~= 10 g dm-3; X H , = ~ 5 g dm-3; XH,"+~= 250 g dm-3; F / S = 1.5; F ' / S = 0.2).
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F i g u r e 4. Dynamics of extraction-stripping system with crosscurrent flow (3, extraction and stripping stages; X M , ~= 10 g dm"; XH,O = 5 g dm-3; xH,n+l= 250 g dmS3;F / S = 1.5; F ' / S = 0.2).
dure proposed by Komatsu (1977), who solved quite a similar reaction-distillation problem. Figures 6-9 demonstrate the percentage of copper extracted from the aqueous phase in the system considered for various initial copper concentrations, while Table I11 presents the observed results for various systems considered for the same starting conditions ( X M , =~ 10 g dm-3, FIS = 1.5, F ' I S = 0.2). The obtained results demonstrate that the highest extraction percentage is obtained for the system with
~~u
~ ~
~ (*
T
F i g u r e 6. Percentage of copper extracted in extraction-stripping system (A, B, and C correspond to F / S = 1.0, 1.5, and 2.0, respectively; top, middle, and bottom rows correspond to lA, 2A, and 2B extraction-stripping systems, respectively).
cross-current flow (type b), while the lowest ones are obtained for classical countercurrent extraction-stripping systems (type a). Thus, the percentage of copper extracted varies in the following order for the considered systems: b > c > a. By an appropriate choice of the flow for the aqueous and organic phases in systems with cross-current or combined flow, it is possible to obtain a higher extraction of copper in comparison with a classical countercurrent extraction-stripping flow sheet. Even in the
Ind. Eng. Chem. Res., Vol. 29, No. 4, 1990 605 x , , ~ :ZOfdm'
Xnpi
100qd"
Z
rfl,,=t5o9&'
20 0
2
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6
8
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Figure 7. Percentage of copper extracted in extraction-stripping system (A, B, and C correspond to F I S = 1.0, 1.5, and 2.0, respectively; top, middle, and bottom rows correspond to 3A, 3C1, and 3Cz extraction-stripping systems, respectively).
.
I
4
6
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. 8
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.
8
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Figure 8. Percentage of copper extracted in extraction-stripping system (A, B, and C correspond to F / S = 1.0, 1.5, and 2.0, respectively; top, middle, and bottom rows correspond to 4A, 4C, and 5A extraction-stripping systems, respectively).
Table 111. Comparison of Various Extraction-Stripping Systems for F / S = 1.5, F / S ' = 0.2, x ~= 10.0 , ~g dm-3, x ~ , ~ + ~ = 1.0 g dm-3, x H , ~= 5.0 g dm-3, x ~ , ~= +250.0 ~ g dm-3 system 1 2a 2b 3a 3c1 3c2 4a 4c
5a 5b 5C1 5c2
XM,nr
XM,n+kr
(XM,O - X M . n ) /
XH,nr
XH,n+k,
g dm-3 5.489 5.270 4.233 4.822 3.615 3.814 4.581 3.228 4.807 2.414 3.668 3.229
g dm+
XM n
34.830 36.472 44.251 39.835 48.889 47.395 41.642 51.794 39.946 57.898 48.492 51.783
0.4511 0.4730 0.5767 0.5178 0.6385 0.6186 0.5419 0.6773 0.5193 0.7587 0.6332 0.6771
gdm-3 11.962 12.300 13.901 12.992 14.855 14.548 13.364 15.453 13.015 16.709 14.774 15.451
gdm-3 197.785 195.251 183.244 190.060 176.086 178.392 187.272 171.603 189.889 162.179 176.699 171.619
case of a lower total number of extraction and stripping stages, e.g., in system 3C1with two extraction and three stripping stages, a higher extraction of copper is obtained in comparison with the classical countercurrent scheme with three extraction stages and three stripping ones (system 5a, Table 111). Thus, such a change may contribute significantly to the economics of copper recovery. For such a strong extractant as the pure fraction of 2-hydroxy-5-nonylbenzaldehydeoxime, the conclusions are somewhat different than in the case of LIX 64N belonging to the class of relatively weak extractants. In this last case, Hughes and Parker (1987) proposed carrying out the process according to scheme c in which the first loop consists of one extraction stage and one stripping stage, while the second loop consists of two extraction stages and one stripping stage. Now, for 2-hydroxy-5-nonylbenzaldehyde oxime, quite an opposite combination is proposed with two extraction stages and three stripping ones combined according to the 3C, scheme. However, when the number of extraction and stripping stages are the same, the scheme with cross-current flow (type b) is preferred. In a hydrometallurigical process, the concentrations of the metal and sulfuric acid in the aqueous feeds of the extraction section and of the stripping section are usually determined by the technology of the preceding leaching and subsequent treatment of the enriched electrolyte, re-
I
40
Figure 9. Percentage of copper extracted in extraction-stripping system (A, B, and C correspond to F I S = 1.0, 1.5, and 2.0, respectively; top, middle, and bottom rows correspond to 5B, 5C1, and 5C2 extraction-stripping systems, respectively).
spectively. As a result, they can vary in different installations in which various resources are reprocessed. Conclusions Systems with cross-current flow give higher extraction percentage of copper with such a strong extractant as 2-hydroxy-5-nonylbenzaldehydeoxime than the typical countercurrent extraction-stripping systems. The choice of an appropriate cross-current flow system depends upon the type of extractant and extraction conditions, i.e., the strength of the extractant and the metal and sulfuric acid concentrations in the aqueous feeds and of the extraction and stripping sections. Thus, for each process, an appropriate extractionstripping flow sheet can be selected, and appropriate flows of both phases can be estimated by means of computer simulation, assuming that equilibria are achieved in each extraction and stripping step. However, due to the relatively slow stripping rates,
606 Ind. Eng. Chem. Res., Vol. 29, No. 4, 1990
computer simulation should be experimentally verified.
Acknowledgment This work was supported by Polish Research Program CPBP 03.08.
Nomenclature F = flow rate of the aqueous phase (feed) t = time T = dimensionless time S = solvent flow rate v, = volume of the aqueous phase uy = volume of the organic phase V = total volume of the stage xH = equilibrium sulfuric acid concentration in the outlet of stage i xM = equilibrium copper concentration in the aqueous phase y M = equilibrium copper concentration in the organic phase Y E P = experimental copper concentration in the organic phase for various initial concentrations of sulfuric acid and different equilibrium concentrations of copper in the aqueous phase k ( i ) = number of stages feeding stage i of extractionstripping systems by the organic phase ’ = stripping section i, 1, n = stage number in extraction j , k , m = stage number in stripping r , a , ’y = number of various initial concentrations of sulfuric acid s, 8, p = number of various equilibrium concentrations of copper in the aqueous phase F”’(x+), F”’(x-) = denote the left and right derivative, respectively Registry No. Copper, 7440-50-8; 2-hydroxy-5-nonylbenzaldehyde oxime, 50849-47-3.
Literature Cited Bogacki, M. B.; Szymanowski, J.; Prochaska, K. Application of Spline Functions in Calculating the Surface Excess Isotherm According to the Gibbs Isotherm. Anal. Chim. Acta 1988,206, 215-221.
Bogacki, M. B.; Alejski, K.; Szymanowski, J. The Fast Method of the Solution of a Reacting Distillation Problem. J . Comp. Chem. Eng. 1989,in press. Forrest, C.; Hughes, M. A. Modeling of Equilibrium Data for the Liquid-Liquid Extraction of Metals, Part I. Survey of Existing Models. Hydrometallurgy 1975a, I, 25-37. Forrest, C.; Hughes, M. A. Modeling of Equilibrium Data for the Liquid-Liquid extraction of Metals. Part 11. Models for the Copper/LIX 64N and Chromate/Aliquat 330 Systems. Hydrometallurgy 1975b,I , 139-154. Hughes, M. A.; Parker, N. A Computer Study of Liquid-Liquid Stage-Wise Calculation in Typical and New Counter Current Contacting. J . Chem. Technol. Biotechnol. 1985,35A,255-262. Hughes, M. A.; Parker, N. A Practical Proof of New Contacting Schemes for Copper Extraction. In Separation Processes in Hydrometallurgy; Davies, G. A., Ed.; Wiley: London, 1987; pp 161-173. Hughes, M. A,; Anderson, S.; Forrest, C. Distribution Surfaces in Solvent Extraction Systems. Int. J . Miner. Process 1975, 2, 267-276. Komatsu, H. J. A New Method of Convergence for Solving Reacting Distillation Problems. J . Chem. Eng. Jpn. 1977,10, 292-297. Piotrowicz, J.; Wasylkiewicz, S. Modeling of Equilibrium Data for the Solvent Extraction of Copper from Multi Component Aqueous Solution of Weak Electrolytes by Hydroxyoximes. Prepr. ISEC‘86 1986,2,205-231. Piotrowicz, J.; Bogacki, M. B.; Wasylkiewicz, S.; Szymanowski, J. Chemical Model for Copper Extraction from Acidic Sulfate Solutions by Hydroxy Oximes. J . Ind. Eng. Chem. Res. 1989,28, 284-288. Robinson, C. G.; Paynter, J. C. Optimization of the Design of a Counter Current Liquid-Liquid Extraction Plant. h o c . ISEC”71, Soc. Chem. Ind. 1971,2 1 , 1416. Rod, V. Unconventional Extraction-Stripping Flow Sheets for the Separation of Metal by Liquid-Liquid Extraction. Chem. Eng. J . 1984,29,77-83. Szymanowski, J.; Atamabczuk, B. A Modified Couchy Distribution Model for the Extraction of Copper by Hydroxyoximes for Very Dilute Acidic Sulfate Solutions. Hydrometallurgy 1982,9,29-36. Szymanowski, J.; Jeszka, P. Modeling of Simple Multistage and Counter Current Multistage Copper Extraction by Hydroxyoximes. Ind. Eng. Chem. Process Des. Dev. 1985,24, 244-250. Wasylkiewicz, S.; Piotrowicz, J. Deviation from Ideality in Weak Electrolyte Solutions. J . Comp. Chem. Eng. 1988,12. 141-145.
Received for review July 10, 1989 Revised manuscript received December 12, 1989 Accepted December 21, 1989
