Ind. Eng. Chem. Res. 1992,31, 1768-1780
1768
DECHEMA: Frankfurt/Main, Germany, 1977. Hala, E.; Pick, J.; Fried, V.; Vilim, V. Vapour-Liquid Equilibrium, 2nd ed.; Pergammon: New York, 1967. Hawley, G. G., Ed. The Condensed Chemical Dictionary, 8th ed.; Van Nogtrand Reinhold New York, 1971. Hodgman, C. D.; Weast, R. C., Shankland, R. S., Selby, S. M., Eds. Handbook of Chemistry and Physics, 44th ed.; Chemical Rubber Co.: Cleveland, OH, 1962. Hwang, Y.-L.; Keller, G. E., II; Olson, J. D. Steam Stripping for Removal of Organic Pollutants from Water. 1. Stripping Effectiveness and Stripper Design. Znd. Eng. Chem. Res. 1992,preceding paper in this issue. Isnard, P.; Lambert, S. Aqueous Solubility and n-Octanol/Water Partition Coefficient Correlations. Chemosphere 1989, 18, 1837-1853. Langen, F. H. M. M.; Paul, P. G.; Booren, R. v. Steam Stripping Organic Compounds from Contaminated Waters. In Enuironmental Technology, Proceedings of the European Conference, De W d , K. J. A., Van den Brink, W. J., Us.; Nijhoff: Dordrecht, The Netherlands, 1987;pp 513-519. Livengood, S. M. Higher Glycols. In Glycols; Curme, G. O., Jr., Ed.; ACS Monograph Series; Reinhold: New York, 1952;p 286. Mackay, D.; Shiu, W. Y. A Critical Review of Henry’s Law Constants for Chemicals of Environmental Interest. J. Phys. Chem. Ref. Data 1981,10,1175-1199. Magnuseen, T.; Galindez, H.; Castier, M. Calculation of Actiuity Coefficients by Means of UNIFAC or UNIQUAC, Manual 8103; Instituttet for Kemiteknik, Technical University of Denmark: Lyngby, Denmark, 1985. Mailhot, H.; Peters, R. H. Empirical Relationships between the 1Octanol/Water Partition Coefficient and Nine Physical Properties. Enuiron. Sei. Technol. 1988,12,1479-1488and supplementary material. Munz, C.; Roberta, P. V. Air-Water Phase Equilibria of Volatile Organic Solutes. J.-Am. Water Works Assoc. 1987, 79 (51, 62-69.
Prausnitz, J. M.; Lichtenthaler, R. N.; de hevedo, E. G.; Molecular Thermodynamics of Fluid-Phase Equilibria, 2nd ed.; PrenticeHall: Englewood Cliffs, NJ, 1986;Chapter 8,Figure 8-12. Robbins, L. A. (Dow Chemical Co.). Method of Removing Contaminants from Water. US Patent 4236973,Dec 1980. Shiu, W. Y.; Mackay, D. A Critical Review of Aqueous Solubilities, Vapor Pressures, Henry’s Law Constante, and Octanol-Water Partition Coefficients of The Polychlorinated Biphenyls. J. Phys. Chem. Ref. Data 1986,15,911-929. Solomon, R. L.; Peterson, D. J. An Innovative Solution for Steam Stripping of Volatile Organic Compounds and High Boiling Point Pollutants from Groundwaters with The AquaDetox Stripping System. Proceedings-Znternational Water Conference; Engineers’ Society of Western Pennsylvania: Pittsburgh, PA, 1987; Vol. 48, pp 296-306. Ssrensen, J. M., Arlt, W., Eds. Liquid-Liquid Equilibrium Data Collection, Part 1 in Chemistry Data Series, Vol. V., Part I; DECHEMA: Frankfurt/Main, Germany, 1979. Tsonopoulos, C.; Prausnitz, J. M. Activity coefficients of Aromatic Solutes in Dilute Aqueous Solutions. Znd. Eng. Chem. Fundam. 1971,10,593-600. Tsonopoulos, C.; Wilson, G. M. High-Temperature Mutual Solubilities of Hydrocarbons and Water. AZChE J . 1983,29,990-999. Weast, R.C., Astle, M. J., Beyer, W. H., Eds. Handbook of Chemistry and Physics, 65th ed.; CRC Press: Boca Raton, FL, 1984. Yaws, C. L.; Yang, H.4.; Hopper, J. R.; Hansen, K. C. Hydrocarbons: Water SolubiliQ Data. Chem. Eng. 1990 (Apr), 177-182; Organic Chemicals: Water Solubility Data. Chem. En& 1990 (Jay), 115-118. Zwolinski. B.. Ed. Pure ComDonent ProDertv Tables: Thermodvnamics’&ch Center, T&aa A&M Lfnivekty: Cohege Station, TX, 1975. Received for review December 10,1991 Revised manuscript received April 9,1992 Accepted April 27, 1992
Modeling of Extraction Equilibrium and Computer Simulation of Extraction-Stripping Systems for Nickel Extraction by Di-n -butyl Phosphorodithioate Mariusz B. Bogacki,? G6rard Cote,$Jan Szymanowski,*it and Denise Bauert Institute of Chemical Technology and Engineering, Poznaii Technical University, PlSklodowskiej-Curie 2, 60-965Poznaii, Poland, and Laboratoire de Chimie Analytique (Unit6 associde au CNRS No. 4 3 3 , E.S.P.C.I., 10, rue Vauquelin, 75231 Paris Cedex 05, France
A chemical model for nickel(I1) extraction from acidic sulfate solutions with phosphorodithioic acid di-n-butyl ester (0,O-di-n-butyl phosphorodithioate) is presented and used for discussion of extraction-stripping systems with either conventional sequential stages of extraction and stripping or with unconventional cross-current and combined flows. A good agreement of the model with experimental extraction data is observed. The numbers of extraction and stripping stages necessary to obtain a given yield of extraction of nickel(I1) have been found to be lower in unconventional extraction-stripping systems than in conventional systems with sequential stages of extraction and stripping. Nickel(I1) is present in several acidic and ammonium solutions, e.g., in spent electrolyte after copper refining. It can be recovered with hydroxy oximes after separation of copper remains and neutralization with ammonium (Davies, 1981). From an economic point of view it is essential that the separation of nickel does not require a
* Author to whom correspondence should be addressed. t Poznan
Technical University.
t E.S.P.C.I.
0888-5885/92/2631-1768$03.00/0
previous neutralization, in particular to allow the recycling of sulfuric acid. However, another extractant must be used for nickel extraction. In such a case it is possible to use the scheme with the cross-current flows with hydroxy oxime in the first loop to extract copper and phosphorus compound in the second loop to extract nickel. The laterites of New Caledonia are an important source of nickel, and sulfuric acid leaching is considered as the best way for the treatment of such low-grade ores. However, in this case the separation of nickel from cobalt is the crucial problem. 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 1769 Table I. Experimental and Calculated Equilibrium Concentrations (kg m") for Nickel(I1) Extraction with Di-m-butyl PhorphorodithioateResults (log BH= 1.99, log 9, = 2.36. K .. = 319. [HLl. = 0.431 mol dm-') nickel concn init HaO, error, % exptl calcd concn, kg m+ aq org org re1 av 0.0 8.39 12.15 12.24 0.74 6.33 12.16 12.16 0.00 4.38 12.06 0.08 12.05 2.62 0.94 11.76 11.87 0.96 11.36 11.38 0.18 0.182 10.11 0.20 10.09 7.590 0.047 8.17 8.79 0.007 6.15 6.19 0.40 0.65 12.17 4.9 5.370 8.99 11.55 12.05 0.42 6.49 12.99 11.92 4.56 11.88 0.34 11.71 2.75 11.63 0.69 2.11 11.21 11.19 0.18 10.02 9.90 1.20 0.246 8.15 8.34 2.33 0.064 6.14 6.24 1.63 0.015 4.10 4.11 0.88 0.24 0.0036 9.8 11.97 12.07 0.89 8.57 11.92 11.94 0.17 6.57 11.83 11.79 0.84 4.61 11.59 11.69 0.86 2.79 11.16 1.19 10.96 1.79 8.42 0.094 8.24 2.14 6.14 0.022 5.93 3.42 4.10 1.51 4.02 0.0064 1.95 12.01 14.7 0.67 11.93 8.53 11.88 0.51 6.61 11.82 11.76 0.85 11.66 4.68 11.46 2.92 11.40 0.52 10.99 1.36 10.84 1.84 9.84 0.91 9.75 0.434 8.10 1.36 7.99 0.117 6.12 0.65 6.16 0.037 1.46 4.04 4.10 0.92 0.0105 11.89 0.50 11.83 8.65 19.6 0.76 11.71 11.80 6.69 11.63 4.81 0.77 11.54 11.32 11.26 3.06 0.53 10.87 1.46 1.66 10.69 9.80 0.475 4.080 9.50 0.12 8.05 0.164 8.06 6.11 0.050 0.49 6.08 2.20 4.09 0.0166 4.18 2.04 2.45 2.09 0.0041 1.32 1-06 1.34 3.82 0.0015 ~
at
i
m
IH25O4It
mot d d
Figure 1. Comparison of percentage of extraction computed for various ~ e t Of s KH and B (1,log KH = 1.99 and 1% fi = 2.35;2,log KH = 1 and log 9, = 1; 3,log KH = 1.0 and log B = 2.0; initial nickel(I1) concentration, 0.01 mol dm"; extractant concentration, 0.2 mol dm-3 (I) and 0.43 mol dm-3 (11)); phase ratio A I 0 = 1.0.
It has been shown that the dithiophosphorus compounds, and more especially the phosphorodithoic acids dialkyl esters, are the key reagents for nickel(II) extraction; see Bohm et al. (1979),Nedjate et al. (Nedjate, 1977, Nedjate and Sabot, 1977a,b;Nedjate et al., 19781, and Sabot et al. (Sabot, 1978; Sabot and Bauer, 1978,1979). However, it should be pointed out that only the chemical aspects of the problem have been investigated in the works cited above. In particular, no attempt at modeling and optimization of the separation process has been made in spite of the interest of such a question from a chemical engineering point of view. Recently we have presented a model for computer simulation of extraction-stripping systems in which the extraction equilibrium is computed by a chemical model and by very precise smoothing spline functions. This model has been tested in the case of the extraction of copper by 2-hydroxy-5-nonylbenzaldehyde oxime and 2-hydroxy-5nonylbenzophenone oxime; see Bogacki and Szymanowski (1990,1991),Piotrowicz et al. (1989),and Szymanowski and Jeszka (1985). The aim of this paper is to present a chemical model of nickel extraction from sulfuric acid solutions by di-n-butyl phosphorodithioate and to use this model for discussing various unconventional extraction-stripping systems. Di-n-butyl phoephorodithioate has been chosen as an example of a dithiophoephorus compound, but other reagents such as bis(2,4,4-trimethylpentyl)phosphinodithioicacid, which is the active substance of a commercially available product, Cyanex 301 (American Cyanamid Company), could be used (Cote and Bauer, 1989). Experimental Section Di-n-butyl phosphorodithioate was synthesized from pure n-butanol (Hopkin and Williams) and pure phosphorus pentasaide (Fluka) (Cote and Bauer, 1984). Prior to its use, di-n-butyl phosphorodithioate was purified by two successive treatments, each consisting in dissolving its sodium salt in an aqueous sodium hydroxide solution and then in extracting its molecular form into an organic diluent after acidification of the aqueous phase by addition of sulfuric acid. Di-n-butyl phosphorodithioate was used in solution in a mixture of Solvesso 150 (Esso Chimie) and 160 kg m-3 octanol (Prolabo). At room temperature the organic solutions of the extractant are stable, which allowa their use in the hydrometallurgical processes. Neither air
Deviations observed, average error without deviated results.
oxidation nor acidic hydrolysis significantly occurs, but contact with strong oxidizing agents such as chromium(VI) should be avoided (Cote and Bauer, 1984,1989). All the other reagents were analytical grade. Di-n-butyl phoephorodithioate in the organic phases was determined by potentiometric titration with silver nitrate (Kolthoff and Elving, 1966). Nickel determinations in the aqueous phases were carried out either by titration with EDTA (Charlot, 1966)or by atomic absorption spectrophotometry with a Varian Techtron AA6 spectrophotometer. The concentration of nickel in the organic phases was usually calculated from maas balance, but occasionally was also determined by atomic absorption spectrophotometry. An agreement between nickel content in the organic phase calculated from the mass balance and that one determined directly by atomic absorption was observed. All extraction experiments have been carried out at 22 f 1 "C. The experiments in batch were performed by mixing equal volumes of aqueous and organic solutions
1770 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
---
1
r i
-1-
Yf
I I I I
I
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I ~
I
I
L
n
e
I I
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I
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*
I
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I
I I Y,,sr
n4
L
1
I Xrnm.1 -I--
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-_x,,,
inside a pear-shaped vessel, the movement of which was reproducible (Agitelec Equipment, Ste.Toulemonde et Cie, Paris). The countercurrent extractor consisted of glass mixer-settler equipment. Each mixer compartment was fitted with a mechanical stirrer operated by an electric motor, the speed of which could be controlled. In the settling compartment, it was possible to precisely adjust the interface level. The circulation of the aqueous and organic phases was a result of the pumping action of the stirrers.
%+&
c
n*m
;x
a
n +I
x,'
Chemical Model of Extraction Equilibrium It should be pointed out that the dialkyl phosphorodithioate are monomeric even in nonpolar organic diluents (Zucal et al., 1963; Rodina et al., 1973). The formula of the nickel species extracted into the organic phase has been investigated by the slope analysis method, and it has been concluded that extraction of nickel(I1) from acidic sulfate solution with dibutyl phosphorodithioate (HL) can be
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1771
P $ c
10
0
20
Nickel concentration in aqueous p h o s e , kq m" Figure 3. McCabe-Thiele graphs for classical extractionatripping system (four extraction and six stripping stages): feed, 5.15 kg m-3 Ni(I1) and 20.5 g dm-3 HzSO,; stripping, 490.4 kg m-3 HzS04;phase ratio A I 0 (extraction) = 2.174 and A'/O (stripping) = 0.243).
represented as follows (Sabot and Bauer, 1978). Ni2+, + 2HL, = NiLzo+ 2H+,
(1)
The extraction constant is given by
It is of interest that no solvating mechanism interferes with the cation exchange mechanism represented by reaction 1, not even at high ionic strength. Such an observation is in agreement with the fact that the trialkyl thiophosphates cannot extract nickel(I1) (Handley and Dean, 1960). In the aqueous phase the following equilibria must be considered:
and Ni2++ S042- = NiS04
'
[NiSO41a = [Ni2+],[S042-],
The dissociation of H2S04into H+ and HS04- can be considered to be complete under the present experimental conditions. Additional equations are given by the mass balance of nickel, hydrogen, and sulfate ions in the aqueous phase: [Ni2+],= [Ni2+],+ [NiS04], (5) [S042-]t= [S042-],
+ [HS04-], + [NiS04],
(6)
[H+It = W + l a + [HSO4-1, (7) and extractant concentration in the organic phase: [HL], = [HLI, + 2[NiL2], (8) where t is the abbreviation of total. The set of equations (1)-(8) defines the chemical model of the process. The parameters of the model (extraction
Table 11. Computed Values of Extraction Constant K,, and Total Extractant Concentration [HL], with Statistical Assessments of Models (Analytical Extractant Concentration, 0.475 mol dm-s) log KH = 1.99 log K, = 1.0 log KH = 1.0 parameter log @ = 2.35 log @ = 2.0 log @ = 1.0 319 265 541 Kex 0.431 0.428 0.433 [HL],, mol dm-3 F Snedecor function 1.703 1.572 2.004 0.171 0.203 0.115 significance level
equilibrium constant, K,,, and the extractant total concentration, [HL],) were estimated by the least squares method using the experimental data, i.e., equilibrium nickel concentration in the extraction phases and initial sulfuric acid concentration and three different sets of KH and 6 given in Table I1 (Piotrowicz et al., 1989). The minimum square deviation between complex concentration calculated from the model and determined experimentally was determined. F(K,,,[RH],) = C([NiLz],"'"i - [NiLz]ppt1i)2 (9) The solving of eqs 2-8 does not create any serious mathematical problem; however, precise values of KH and 6 constants should be known. According to Zonisation Constants of Inorganic Acids and Bases in Aqueous Solution (Perrin, 1982) and Stability Constants of Metal-Ion Complexes (Sillen and Martell, 1964,1971;Hagfeldt, 1982),it is possible to assume log KH = 1.99 and log 0 = 2.35, when the ionic strength is close to 0. However, it should be noticed that these two constants significantly decrease when the electrolyte content increases. Pesavento (1989) proposed the following relation for log KH calculation in systems of various ionic strength (I).
"his means that log KH decreases from about 2 to 1.2 when the ionic strength increases from 0.0 to 1.0. No appropriate relation was found for log 8.
1772 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
")
t
Extraction
Nickel concentration in aqueous phose, kg m' Figure 4. McCabe-Thiele graph for classical extraction-stripping system (two extraction and six stripping stages): feed, 5.15 kg and 20.5 kg m-s H2S04;stripping, 490.4 kg rn-' HzSO4; phase ratio A I 0 (extraction) = 2.174 and A'/O (stripping) = 0.243).
t ?E
m-3
Ni(I1)
Ex t roc ti on
I
a
$1
s,
: 10
Nickel
:
:
20
concentration
30
in
aqucou~ phose, kg
rn-'
Figure 6. McCabe-Thiele graph for atypical extraction-stripping system (two extraction and three stripping stages): feed, 5.15 kg m-3 Ni(I1) and 20.5 kg m-' HzSO,; stripping, 490.4 kg m-3 HzSO4; phase ratio A I 0 (extraction) = 2.174 and A'/O (stripping) = 0.243).
For the concentrations of nickel(I1) and sulfuric acid typically found in the extraction stages (Table I) the ionic strength of the aqueous phase changes from 0.3 to 1.8, which corresponds to a variation of log KH from 1.42 to 1.20. Under such conditions, the average values of ionic strength and log KH are assumed to be equal to 1.06 and 1.31, respectively. In the stripping stages nickel concentration changes from 0 to 40 kg mVsand sulfuric acid from 490 to 420 kg m-3. Such values give an ionic strength greater than 10. Thus, it is obvious that the assumption consisting of giving constant values to KH and j3 over the whole extraction-stripping domain is a rough simplification. However, such assumptions are often accepted and quite satisfactory agreements between calculated and experimental values are observed. Commercial but also model extractants, including di-nbutyl phosphorodithioate, can contain some impurities. In
particular, extractants very often contain residual reagents used for their synthesis and/or appropriate intermediates. Thus, di-n-butyl phosphorodithioate can contain small amounts of n-butanol used in excess for its synthesis. Moreover, extractants can also contain modifiers added to change their extraction properties and/or to enhance their solubility and the solubility of their metal complexes in the organic phase. For instance, in the system described in the present work, the addition of hydrophobic alcohols (e.g., octanol) significantly increases the rate of stripping. In two-phase solvent extraction systems, the molecules of extractant distribute between the two phases and, depending upon their hydrophobicity, this phenomenon can more or less affect their concentration in the organic phase. Hence, in extraction models it is very convenient and justified not to fix the extractant concentration but to calculate it together with extraction constants. All the considered effects can then compensate themselves, and
Ind. Eng. Chem. Res., Vol. 31,No. 7, 1992 1773 ,.n,o - ”.-
. .,
...
. n.u
Xr
Figure 6. Percentage of nickel(I1) extraction versus flow ratio of organic to aqueous stripping phase ( O I A ’ ) in various extractionatripping ) aqueous phase (flow ratio O / A during extraction: (a) 1.0, (b) 0.67, and (c) 0.5; systems for different initial metal concentrations( x ~ , in~ the data in successive rows from top to bottom correspond to systems la, IC,2a, and Zb, respectively).
a good agreement of the model with experimental data can be observed. The values of the extraction constant K,, and calculated total extractant concentration for three different sets of
KH and 6 values and the total analytical concentration [ H L I t 4 = 0.475 mol dnr3are given in Table II. All three models are statistically significant with similar values of
F Snedecor function and significance level. For each model
1774 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 Table 111. Analyzed Extraction-Stripping Systems" type of extr-strip. system no. of stages C system n k a b 1 m 1 4 6 + 2 3 2 4 4 3 3 3 + 1 16 2 2c 4 2 4 + 1 2 5 2 3 1 26 1 lC 6 2 6 + 1 3
+
a + means that the system was considered. a = classical countercurrent flow with sequential stages of extraction and stripping (Figure 2a). b = cross-current flow (Figure 2b). c = combined solvent flow (Figure 2 4 . k = total number of stripping stages. 1 = number of extraction stages in the first process loop. m = number of stripping stages in the second process loop. n = total number of extraction stages. System cl. System c2
almost the same values of the total extractant concentration, 0.431,0.428, and 0.433 mol dm-3,were obtained. They are 10% lower than the expected value of 0.475 mol dm-3. Similar results were obtained previously for copper extraction with hydroxy oximes; see Bogacki and Szymanowski (1990, 1991) and Piotrowicz (1989). The extraction constants are different. This is caused by various B values taken for computing and the corresponding concentration of free nickel(I1) ions in the aqueous phase. However, this will not affect further computing and final conclusions as shown below. The comparison of the extraction percent computed for different sets of KHand @ constants is given in Figure 1. Details of calculation are given in Table I for log KH= 1.99 and log B = 2.35. For all three different sets of KH and /3 similar values of the extraction percent were obtained. Differences between results are small and can be neglected. Results presented in Table I demonstrate a good agreement between the experimental data and computed results. The relative error is low. Only in three cases out of about 50 considered important deviations of 443% are observed, probably caused by some experimental errors. The error of estimation depends upon nickel(I1) concentration in the aqueous phase, and higher errors are observed at low nickel concentrations. The average error is about 1% . However, important deviations are observed in the region of sulfuric acid concentration used for stripping. For this concentration of 6.0 and 10.0 mol dm-3 H2S04and extractant concentration of 0.2 mol dm-3, the computed extraction percent equals to 35% and lo%, respectively, while the experimental data are below 1%. It was impossible to obtain better fitting in this region using other reasonable sets of KHand 0. To efficiently improve the fitting, it would be necessary to determine and to use the activities of the individual species instead of their concentrations, in both the aqueous and organic phases. However, such a determination was beyond the scope of the present work. These deviation will affect the computing results obtained for stripping. However, it is worthwhile to point out that the equilibrium model used for computing gives lower values of stripping percent in comparison to those experimentally determined. Using the equilibrium model proposed here, the estimated equilibrium nickel concentrations in the organic and aqueous phases are higher and lower, respectively, than thoee experimentally determined. This means that the model gives the overestimation of the required number of stages.
Table IV. Extraction of Nickel with Di-n -butyl Phosphorodithioate and Mass Balance of Extraction Stages (Flow Ratio of Aqueous to Organic Phase, 2.174; Concentrations in kg m-a;Experimental Data Described Previously by Sabot (1978)) aqueous phase organic phase nickel nickel stream exptl calcd H2S04calcd stream exptl calcd 20.5 yo 12.0 11.20 xg0 5.15 5.15 3.38 23.47 y1 11.0 7.33 XI 3.9 0.16 ~2 1.9 Y2 4.6 0.36 28.83 313 0.25 0.01 Y3 0.6 0.015 29.09 Y4 0.0 0.0 x4 0.05 0.0003 29.1 inlet outlet 2.714~;-1+ yi 2.174~;+ yi-1 stage exptl calcd exptl calcd 1 22.20 18.53 20.48 18.55 2 13.08 7.71 15.13 7.68 3 4.73 0.36 5.14 0.38 4 0.54 0.02 0.71 0.02 Table V. Stripping of Nickel with Sulfuric Acid and Mass Balance of Stripping Stages (Flow Ratio of Aqueous to Organic Phase, 0.255; Concentrations in kg m-$; Experimental Data Described Previously by Sabot (1978)) aqueous phase organic phase nickel nickel stream exptl calcd H2S04calcd stream exptl calcd XO 0.0 490.4 Yo 5.6 3.28 Xl 0.8 4.16 484.0 Y1 5.8 4.35 Y2 6.5 5.03 x2 1.6 6.83 474.0 X3 4.0 9.24 475.9 Y3 7.0 5.65 3c4 6.8 12.06 470.3 Y4 7.4 6.36 ~g 10.4 16.57 463.3 Y5 8.1 7.51 X6 14.8 28.28 443.2 Ye 10.5 10.50 inlet outlet O.255Xi-1 + yi 0.255~i+ yi-1 stage exptl calcd exptl calcd 1 5.80 4.35 5.80 4.34 6.09 2 6.70 6.21 6.09 3 7.41 7.38 7.52 7.39 8.72 4 8.42 8.73 8.72 10.59 10.05 5 9.83 10.59 6 13.15 14.73 11.87 14.72
Taking all these observations into account, it was decided to use the computed equilibrium data for further discussion of conventional and unconventional extraction-tripping systems. The values log KH = 1.99 and log B = 2.35 have been used.
Mathematical Model of an Extraction-Stripping System The discussed extraction-stripping flow sheets are presented in Figure 2. The classical scheme used in industrial processes is presented in Figure 2a. In such a traditional process, metal is first extracted into the organic phase in a multistage countercurrent cascade and then stripped into the aqueous phase using an appropriate concentration of mineral acid. Apart from this conventional countercurrent flow arrangement, flow sheets with total or partial cross-flow of the solvent between the extraction and stripping stagea can be constructed. The total cross-flow, demonstrated in Figure 2b, can be implemented only for an identical number of stagea in each section,while the partial cross-flow, illustrated in Figure 2c, can be used both for an equal and a different number of stages in each section.
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1775 Table VI. Extraction Yield (Y= (xoo- x,,)/xoo)and Nickel Concentration in Final Extract (x,,+~)in Various Extraction-Stripping Schemes (Extraction Conditions Same as in Tables IV and V) scheme Y. 73 zmAL. ke m-3
A
71.68
32.86
78.32
36.01
76.12
35.00
73.75
33.91
n+k
The mathematical model for metal extraction was presented by Rod (1984) and used in our previous works (Bogacki and Szymanowski, 1990,1991),in which copper extraction with hydroxy oxime was discussed. The Adams-Moulton method (Bogacki et al., 1989) was used for computing. Table I11 presents the systems considered with various flow sheets and different numbers of extraction and stripping stages. 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). In this last case 1 denotes the number of extraction stages in the first extraction-stripping loop, while m stands for the number of stripping stages in the second extraction-stripping loop.
Nickel(I1) Extraction in Classical Countercurrent Process In Tables IV and V, experimental data and calculated values are compared for both extraction and stripping of nickel(I1) in a classical countercurrent process. Four se-
quential stages were considered for the extraction of nickel (Table IV) whereas six sequential stages were considered for its stripping (Table V). The mass balance of each extraction/stripping stage (Tables IV and V) demonstrates good agreement between calculated nickel amounts in inlet and outlet streams. This proves the validity of the numerical method used for computing. The differences observed in computed mass balance are much lower than those of experimental data. In the last case the highest mass imbalance is observed in stage 2 in extraction and in stages 1 and 6 in stripping. The error of experimental nickel mass balance is about 20%. In the extraction system the computed and experimental concentrations of nickel in the final extract are equal to 11.2 and 12.0 kg m-3, respectively. From the calculation, the f d raffinate should contain only 0.0003 kg m-3 Ni(II), which corresponds to a computed yield of extraction of 99.99%. In fact, the experimental yield of extraction is equal to 99.03%. The number of theoretical stages is approximately equal to the actual number of extraction stages (i.e., 4).
1776 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
Xr
'Ot
Figure 7. Percentage of nickel(I1) extraction versus flow ratio of organic to aqueous stripping phase ( O / A ? in various extraction-etripping systems for different initial metal concentrations (xM,J in the aqueous phase (flow ratio O / A during extraction: (a) 1.0, (b) 0.67, and (c) 0.5; data in successive rows from top to bottom correspond to systems 3a, 3b, 3c2, and 3cl, respectively).
Results given in Table IV also demonstrate that nickel is mainly extracted in the first two or three stages. Thus, the number of extraction stages can be decreased. For the stripping of nickel (Table V) the differences between experimental data and results of computing are
more important than those observed for extraction. Moreover, the amounts of nickel actually stripped are much lower than those expected from the computing. In six stages the computed stripping yield is 68.8%, whereas it has been experimentally found to be equal to 46.7%.
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1777
x ~= SO, k9 ~ m3
x,
0
= f0.0
kg m'-
Xk.0
sf50
k g ~ '
-C
3
*t
20
Figure 8. Percentage of nickel(I1) extraction versus flow ratio of organic to aqueous stripping phase (O/A') in various extraction-stripping systems for different initial metal concentrations (xM,Jin the aqueous phase (flow ratio O / A during extraction: (a) 1.0, (b) 0.67, and (c) 0.6; data in successive rows from top to bottom correspond to systems 5cl, 5c2, 5a, and 4c, respectively).
Such values correspond to an overall efficiency of 68%. This is a result of a very slow rate of stripping (Nedjate, 1977; Nedjate and Sabot, 1977). As was presented before, one could expect to obtain deviations in the opposite direction because the equilibrium model predicts lower
stripping of nickel. However, the equilibrium is not achieved under experimental conditions of the countercurrent stripping: the kinetics effect is dominant and under experimental conditions the nickel stripping is lower than expected from the equilibrium model. The stripping
1778 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
Figure 9. Dynamics of classical countercurrent extraction-stripping system (data as in Figure 3).
I
x4
2
40
20
30
dimanaionless time
Figure 10. Dynamics of extraction-stripping system with combined solvent flow (data as in Figure 5).
40
*
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1779 efficiency is low and below that expected. Hence, theoretical stages must be taken under consideration. Figures 3 and 4 show the McCabe-Thiele graphs for extraction-stripping systems with classical countercurrent flows. In the system containing four theoretical extraction and six stripping stages, the extraction occurs mainly in stages 4 (88.75%) and 3 (10.82%). Hence, stages 2 (1.04%) and 1 (0.05%) can be neglected. Nickel is stripped mainly in stage 10 (45.3%),while the amounts of nickel stripped in the other stages are comparable and equal to 7.8-15.6%. The lowest amounts are stripped in stages 6,7,and 8. The overall transfer of nickel from the aqueous feed to the enriched aqueous phase is 72.43%. In the system containing two extraction and six stripping stages this transfer is comparable to the previous one and equal to 71.68%. Nickel concentrations in the enriched aqueous solutions are also similar and equal to 33.30 and 32.86 kg m9, respectively. Again the greatest amount of nickel (44.9%) is stripped in stage 8 and the amounts of nickel stripped in other stages are comparable and equal about 10%.
in the nickel distribution curve for extraction and stripping. However, such character of an extraction and stripping isotherm is typical for metal extraction with several acidic and acidic-chelating extractants for which the equilibrium is highly affected by mineral acid concentration caused by the proton liberation. Thus, the cross-current flow method of contacting will be more efficient than conventional countercurrent flow when liquid phases are highly concentrated. However, this effect will decrease significantly with lower phase loading. Finally, the dynamics of two selected extraction-stripping systems are demonstrated in exemplary Figures 9 and 10. For computing it was assumed that at the starting point the whole system is filled with the aqueous feed and the organic phase contains nickel in its equilibrium concentration. The steady state is obtained quickly after 8-10 and 12-14 undimensional time units for systems presented in Figures 10 and 9,respectively. Thus, the situation is similar to that observed previously for copper extraction with hydroxy oxime extractant (Bogacki and Szymanowski, 1990,1991).
Nickel(I1) Extraction in Extraction-Stripping Systems with Cross-Current and Combined Solvent Flows Table VI shows the yield of the overall transfer of nickel from the feed to the enriched aqueous phase and the nickel content in the f d extract for various extraction-tripping schemes. Examination of this table shows that improvement of the separation process is possible by using atypical schemes with cross-current and combined solvent flows. Thus, using systems with combined solvent flows, it is possible to decrease the number of stages down to two extraction and three stripping stages without any significant decrease of the technological parameters discussed here (Le., flow ratios in both extraction and stripping). In this last case (i.e., two extraction and three stripping stages), all stripping stages operate in a comparable way giving similar amounts of nickel stripped in each stage (Figure 5). Figures 6-8 show the percentage of nickel extraction in systems with different flows for various nickel concentrations and various flow ratios ( O / Aand OIA’) of organic to aqueous phase. In the extraction steps the ratio O / A ranges from 0.5 to 1 whereas in the stripping steps the ratio OIA’ varies between 0.5 and 10. However, to implement these results into practice, it is necessary to improve the kinetics of extraction and especially of the stripping. This can be partly achieved by adding an appropriate modifier of alcohol type to the extraction system. The obtained results demonstrate that the highest percentage of extraction is obtained for systems with cross-current flow (type b), while the lowest ones are obtained for classical countercurrent extraction-stripping systems (type a). Thus,the percentage of nickel extracted varies in the following decreasing order for the considered systems: b > c > a. By an appropriate choice of the flow rates of the aqueous and organic phases, it is possible to obtain a higher extraction of nickel in systems with cross-current or combined flow than in a classical countercurrent extraction-tripping flow sheet, even in the case of lower total numbers of extraction and stripping stages. Thus, such a change from classical to atypical systems may contribute significantly to the economics of nickel recovery. It is necessary to stress that the cross-current flow method of contacting is more efficient than conventional counter-current flow because of the marked nonlinearity
Acknowledgment This work was supported by Polish Research Grant KBN No. PB 84313191.
Nomenclature a = water phase A, A‘ = volume or flow rate of the aqueous phase (A, feed; A‘, stripping) I = ionic strength KH = formation constant of HS04- from H+ and SO4K,, = extraction constant k = total number of stripping stages 1 = number of extraction stages in the first process loop m = number of stripping stages in the second process loop n = total number of extraction stages o = organic phase (subscript) 0 = volume or flow rate of the organic phase R = percentage of extraction t = total T = dimensionless time 6 = formation constant of NiS04 from Ni2+and SO4= feed concentration (superscript) Registry No. Ni, 7440-02-0;(BuO)~P(S)SH,2253-44-3.
Literature Cited Bogacki, M. B.; Szymanowski,J. Modeling of Extraction Equilibrium and Computer Simulation of Extraction-Stripping Systems for Copper Extraction by 2-Hydroxy-5-nonylbenzaldehydeOxime. Znd. Eng. Chem. Res. 1990,29,601-606. Bogacki, M. B.; Szymanowski,J. Modeling of Extraction Equilibrium and Computer Simulation of Extraction-Stripping Systems for Copper Extraction by 2-Hydroxy-5-nonylbenzophenone. Znr. Chem. R o c . 1991,4,115-129. Bogacki, M. B.; Alejski, K.; Szymanowski,J. The Fast Method of the Solution of a Reacting Distillation Problem. Comput. Chem. Eng. 1989,13,1081-1085. Bohm, 0.;Sabot, J. L.; Bauer, D. Kinetics of Solvent Extraction of Nickel(I1) by 0,O’-Dialkylhydrogen Dithiophosphates. J . Chem. Res., Synop. 1979, 90-91; J . Chem. Res., Miniprint 1979, 1236-1257. Charlot. G. Analvse Minerale, 5th ed.: Masson et Cie: - Quantitative Paris’, 1966;p 980. Cote. G.: Bauer. D. Hvdrolvsis of the 0.0-Dialkvl Phoeohorodithioic Acids’ Used ’as E k r a c h t s in Liquid-Liqiid Systems. Anal. Chem. 1984,56,2153-2157 and references cited therein. Cote, G.; Bauer, D. Metal Complexes with Organothiophosphorus Ligands and Extraction Phenomena. Rev. Znorg. Chem. 1989,lO (1-3),121-144 and references cited therein.
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Davim, G. G. Unit Operations in Hydrometallurgy, Chem. Znd. 1981, 420-427. Handley, T. H.; Dean, J. A. Trialkyl Thiophosphates. Selective Extractants for Silver and Mercury. Anal. Chem. 1960, 32, 18761883. Hagfeldt, E. Stability Constants of Metal-Zon complexes, Part A: Inorganic Ligands; IUPAC Chemical Data Series 21; Pergamon Press: Oxford, 1982. Kolthoff, I. M.; Elving, P. J. Treatbe on Analytical Chemistry; Interscience, New York, 1966; Vol. 13, p 331. Nedjate, H. Etude CinBtique des RBactions d'Extraction des Ions Zn2+et Ni2+par les Acides Dialkyl- et Diaryldithiophoriques. C. R. Acad. Sci. Paris, Ser. C 1977,284,885-887. Nedjate, H.; Sabot, J. L. Extraction du Nickel(I1) par les Acides Dialkyldithiophosphoriques: Etude des Conditions de Wxtraction. C. R. Acad. Sci. Paris, Ser. C 1977a, 285,141-144. Nedjate, H.; Sabot, J. L. Selectivite de la SCparation Nickel(I1)Zinc(I1) au Cours de leur Extraction par l'Acide Di6thyl-2hexyldithiophosphorique. Bull. SOC.Chim. Fr. 1977b, No. 11-12, 1089-1092. Nedjate, H.; Sabot, J. L.; Bauer, D. Extraction du Nickel par lea Acides Dialkyldithiophosphoriques: RBle de la Nature du Groupement Alkyle. Hydrometallurgy 1978,3, 283-295. Perrin, D. D. Zonisation Constants of Inorganic Acids and Bases in Aqueous Solution, 2nd ed.; IUPAC Chemical Data Series 29; Pergamon Press: Oxford, 1982. Pesavento, M.; Profumo, A.; Biesuz, R. Exchange of Protons Between Some Poly(amido-amine) Resins and Aqueous Solutions: a Thermodynamic Interpretation. React. Polym. 1989,11,37-45. Piotrowicz, J.; Bogacki, M. B.; Wasylkiewicz, S.; Szymanowski, J. Chemical Model for Copper Extraction from Acidic Sulfate Solutions by Hydroxy Oximes. Znd. Eng. Chem. Res. 1989, 28, 284-288. Rod, V. Unconventional Extraction-Stripping Flow Sheets for the
Separation of Metal by Liquid-Liquid Extraction, Chem. Eng. J. 1984,29, 77-83. Rodina, T. F.; Varentsova, V. I.; Kolyshev, A. N.; Levin, I. S. Determination of the Association, Distribution and Acid Diasociation Constants of Bis(2-ethylhexy1)hydrogen Phosphorodithioate in Some Diluenta. Zzv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk 1973,6,14-18. Sabot, J. L. Extraction Liquide-Liquide du Nickel en Solution Acide par les Acides Dialkyldithiophosphoriques. These de Doctorat d'tEtat, Paris VI, 1978. Sabot, J. L.; Bauer, D. Liquid-Liquid Extraction of Nickel(I1) by DialkylphosphorodithioicAcids. J . Znorg. Nucl. Chem. 1978,40, 1129-1134. Sabot, J. L.; Bauer, D. Liquid-Liquid Extraction of Nickel(I1) by Dialkylphosphorodithioic Acids. Proceedings ZSEC'77; CIM Special Volume 21; Canadian Institute of Mining and Metallurgy: Montreal, 1979; Vol. 2, 509-516. Sillen, L. G.; Martell, A. E. Stability Constants of Metal-Zon Complexes, Section I: Inorganic Ligands; Special Publication 17;The Chemical Society: Burlington House, London, 1964. Sillen, L. G.; Martell, A. E. Stability Constants of Metal-Zon Complexes, Supplement No. l., Part I: Inorganic Ligands; The Chemical Society: Burlington House, London, 1971. 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. Zucal, R. H.; Dean, J. A.; Handley, T. H. Behaviour of Dialkyl Phosphorodithioic Acids in Liquid Extraction Systems. Anal. Chem. 1963,35,988-991. Received for review October 16, 1991 Revised manuscript received March 24, 1992 Accepted April 13, 1992
GENERAL RESEARCH Separation of Liquid Mixtures of p -Xylene and o -Xylene in X Zeolites: The Role of Water Content on the Adsorbent Selectivity Luis T. Furlan,t Beatriz C. ChavesVtand Cesar C. Santana* Polymer Division, Petrobrcis Research Center, Rio de Janeiro, RJ 2oo00,Brazil, and Department of Chemical Engineering, State University of Campinas, Campinas, 560 Paulo, 13081 Brazil
The selectivity and purity enhancement related to the separation of the isomers p-xylene and o-xylene were studied on a laboratory unit using the liquid-phase adsorption in fixed beds containing zeolite type X whose cation Na+ was replaced by Ba2+ and K+. Using adsorbents with preestablished contents of water and prepared on a special laboratory setup, experiments of the type stimulusresponse using the technique of pulses were conducted, being aimed at the determination of the selectivities on the separation of p-xylene from mixtures with ethylbenzene and also on the separation of p-xylene from mixtures with o-xylene. It is shown that an optimum water content in X zeolites with a value around 3.25 w t 3' % occurs for the selectivity in separating p-xylenes and o-xylenes. Concluding the work, we verified that it is possible to obtain with a good yield p-xylene from a stream of isomers through adsorption in zeolite type X in the liquid phase and that the developed methodology gave precise and repetitive results.
Introduction An investigation of the state of the art of adsorption processes applied to liquid-phase separation shows that
* To whom correspondence should be addressed at the State University of Campinas. PetrobrL Research Center.
most of the information is concerned with patents (Cier, 1970) with only a few studies involving details about laboratory Units to study the influence of the several variables that affect the separation process, such as temperature, . of . flow, feed concentration, and water content in the rate adsorbent. Among typical works that consider experimental results to analyze the separation of xylene isomers
0888-5885/92/2631-1780$03.00/00 1992 American Chemical Society
