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The kinetics of the solvent extraction of copper(II) with LIX 64N reagents—II
7. inorg, nucl. Chem,, 1976, Vol. 38, pp, 2067-2069, Pergamon Press. Printed in Great Britain
THE KINETICS OF THE SOLVENT EXTRACTION OF COPPER(II) WITH LIX% 64N REAGENTS--II ACTIVATION ENERGIES M. A. HUGHES, J. S. PRESTON and R. J. WHEWELL Schools of ChemicalEngineering,Universityof Bradford, Bradford BD7 1DP
(Received2 February1976) Abstract--Studies of the effect of temperature on the rate of extraction of copper from acidic sulphate media by LIX 64N in Escaid 100 are compared with studies of the temperature dependence of the viscosity of the phases involved. The comparison shows that a chemical step must at least partially control the initial rate of the extraction.
INTRODUCTION FOLLOWINGthe work described in a previous paper[l], a number of experiments have been carried out on the effect of temperature on the rate of extraction of copper(II) from sulphate media by LIX (fiN in Escaid 100, and on kinetic properties of high concentrations of the reagent. EXPERIMENTAL The chemicals and the single drop apparatus used in these experiments were exactly as described in Ref. [1], except for the replacement of the weighted syringe organic phase delivery system for the rising drop apparatus by a more convenient and controllablesystem driven by an adjustable pressure of nitrogen.
organic phase and the glassware show that corrections to the results on this account would be less than 1%, so that in view of the experimental errors such corrections are negligible. A feature of the experiment was the change in the upward motion of the drops as the temperature was increased. A drop which rose with an almost straight path at 18°C had acquired visible zig-zag motion at 38°C, although the contact times between drop and continuous phase did not appear significantly altered. Activation energies were evaluated from Arrhenius plots; a slight systematic curvature of the plots was noted so that the values quoted in Table 2 are appropriate to
RESULTS
Temperature dependence of the rate of extraction
Table 2. Measurementsof viscosity
An activation energy for the extraction of copper from the standard aqueous phase with 20 vol % LIX (fiN in Escaid 100 has already been reported[l]. Further results obtained with extreme concentrations of commercial LIX (fiN in Escaid 100 are reported in Table 1, together with results for a 20% LIX (fiN solution contacted with a high copper, low acid aqueous phase. Conditions corresponding to the extremes of both organic phase and aqueous phase concentration have thus been studied. In these experiments, the solutions were made up and the analyses carried out at room temperature (25 +-2°C), irrespective of the temperature of the kinetic experiment. Calculations of the effect of thermal expansion of the Table 1. Effect of tern ~eratureon the rate of extraction
Vol % LIX 64N
Aqueous Phase Concentrations x dm~ mmolCopper
Acid
5
41.5
17.9
20
4115
17.9
I00
41.5
20
14/.6
Viseosity/cP VoI % LIX 64N
18%
28°0,
3~°c
Activ~tio Energy x kJ-~ol
o
1.608
1.288
1.040
16.3
5
1.731
1.383
1.105
~.0
20 I00 Water [31
2.249 20.38 1.053
1.743 12.40 0.8327
1.366 9,02 0.6783
18.7 (> 25) 16.4
28°C and are subject to errors of _+2-3 kJ mol '. No such curvature was noted in previous work[2], but the data in Table 1 (while giving only three points for each Arrhenius plot) encompass a temperature range rather greater than
in Ref. [2]. Initial Bates x ~uol-lm2s
Activatic~
18%
28%
38°C
gne~gy x kJ-A me]
0.014
0 •027
0 •045
46
Temperature dependence o/viscosity see reference 1
45
17.9
0.049
0.iii
0.i~
51
15
0.i01
0.184
0.292
41
tLIX 64N is a registered trade mark of General Mills Inc., U.S.A. 2067
Although the viscosities of the aqueous phases used can be approximated to those of water, the viscosities of the organic phases require separate measurement. These measurements were made with an Ubbelohde viscometer thermostatted at 18.00, 28.00 or 38.00-+0.03°C and calibrated at each temperature with distilled water. Densities were measured with a specific gravity bottle, and the viscosities quoted in Table 2 obtained with a precision better than 0.5%.
2068
M. A. HUGHES et al.
When an Arrhenius treatment was applied to the data in Table 2, apparent activation energies were obtained which may be compared with those in Table 1. The plots were good straight lines with the exception of that for 100% LIX 64N.
Dependence of rate on organic phase composition Drops of organic phases containing varying concentrations of LIX 64N in Escaid 100 were allowed to rise through a standard aqueous phase containing 41.5 mmol dm 3 copper sulphate and 17.9 mmol dm -3 sulphuric acid at 28°C. The resulting rates of extraction are shown in Table 3. A comparison of these results with the rates reported earlier in Ref. [1] is shown in Fig. 1, and it is
Table 3. Effect of variationof LIX 64N concentrationon the rate of extraction Initial ~ e x mmol -I m S
Vol % LIX 6 ~
E 0.10 7 "6 E E o
~- 0 0 5 c
0
0
5
0.027
i0
0.034
20
0.055
50
0.088
75
0.i01
I00
0.ii0
/Z" ~
4'0 Vol%
~'o
8o
,oo
LIX64N
Fig. 1. Dependenceof rate on LIX 64N concentrationat 28°C. A, from Ref. [1];®, this work.
noticeable that the earlier values are greater than the more recent data. Repeated experiments on both occasions show that the differences between the data are greater than the experimental error of the determinations. This may be due to changes in any one of the components of the organic phase over the period of time. DISCUSSION The important conclusion from the results in Table 1 is that over a wide range of extraction conditions, the activation energies at 28°C are sensibly constant. The same fundamental mechanism should therefore account for the initial rate of extraction with high and low LIX
concentrations, and at high and low ratios of copper to acid concentration in the aqueous phase. The gradient of an Arrhenius plot for a wholly diffusion controlled reaction is expected to be very similar to that of the corresponding plot for the viscosity of the appropriate medium, irrespective of the site of the actual reaction. Thus, in a recent study of a fast homogeneous reaction[4], the temperature dependence of the rate in glycerol (known by several criteria to be diffusion controlled) was very similar to that of the viscosity of glycerol. A change of solvent to an ethylene glycol/water mixture (in which the rate was known to be activation controlled) resulted in an Arrhenius energy of about 33 kJ tool-', compared with viscosity results of 16.4 (for water) and 27 kJ mol-' (for ethylene glycol, Ref. [3]). Properties of the bulk phases will be important in diffusion control, despite uncertainties regarding the properties of any thin interracial zone separating the phases. The activation energy for a diffusion controlled reaction is therefore expected from Table 2 to be close to 17kJ tool-', whereas the values found in Table l are higher by a factor of about 3. It is, however, unsafe [5, 6] to conclude from these magnitudes alone that the rate determining factor in the extraction is a chemical one, although this may of course be true. An approach more likely to be appropriate is one which combines chemical and diffusional rate determining steps, and in which diffusion is of greater or lesser importance depending upon the conditions of an individual experiment. If diffusion as well as chemical reaction were important in determining the rate of extraction, then a change in the relative importance of the two may account for the curvature of the Arrhenius plots and the apparent decrease in activation energy as temperature is raised. In view of the changes in the motion of the drop referred to earlier, the possibility that hydrodynamic changes contribute to the increase of the rate of extraction with temperature must be considered, but it would be an unfortunate combination of effects that caused such similar values of apparent activation energies to emerge from experiments involving phases of such disparate viscosities and densities. The coincidence would be particularly great in the case of 100% LIX 64N, for which the high viscosity made necessary the substitution of a 29 g needle for the 33 g needle at the organic phase inlet, resulting in larger (15 mm3) drops than the normal range (5.5-7.5 mm3). No discontinuity is visible in Fig. 1, despite this change in drop size. It is significant that the results presented here confirm the difference between our previous value[l] of the activation energy and that of Flett [2]. Flett's experiments involved the addition of traces of 64Cu to a copper-LIX system at equilibrium in an AKUFVE apparatus, and observation of the rate of return to the equilibrium condition. The results of the present study indicate at least partial participation of a chemical step in the rate determining mechanism appropriate to unloaded or partially loaded LIX 64N. It is feasible, however, that there could be a gradual change of mechanism as the phases approach equilibrium, so that diffusional resistances become more important than chemical and that at equilibrium diffusion control of the rate of copper exchange consistent with Flett's value of 15 kJ mol -' may obtain. Study of diffusivity values suggests that diffusion of the copper loaded reagent away from the interfacial zone is most likely to be the slowest diffusional process; this will be discussed in a later paper.
Activation energies of LIX 64N reactions
Acknowledgements--We thank Nchanga Consolidated Copper Mines of Zambia for the financial support of this project. REFERENCES 1. R. J. Whewell, M. A. Hughes and C. Hanson, J. Inorg. Nucl. Chem. 37, 2303 (1975). 2. D. S. Flett, D. N. Okuhara and D. R. Spink, J. Inorg. Nucl. Chem. 35, 2471 (1973).
JINC Vol. 38 No. l l - - J
2069
3. Handbook of Chemistry and Physics, Chemical Rubber Company, Cleveland, Ohio (1973). 4. E. F. Caldin and B. B. Hasinoff, J. Chem. Soc. (Faraday Trans.) I. 71, 515 (1975). 5. L. F. Albright, Ind. Eng. Chem. 57, 53 (1965). 6. C. Hanson, J. G. Marsland and G. Wilson, Chem. Eng. Sci. 26, 1513 (1971).
THE KINETICS OF THE SOLVENT EXTRACTION OF COPPER(II) WITH LIX% 64N REAGENTS--II ACTIVATION ENERGIES M. A. HUGHES, J. S. PRESTON and R. J. WHEWELL Schools of ChemicalEngineering,Universityof Bradford, Bradford BD7 1DP
(Received2 February1976) Abstract--Studies of the effect of temperature on the rate of extraction of copper from acidic sulphate media by LIX 64N in Escaid 100 are compared with studies of the temperature dependence of the viscosity of the phases involved. The comparison shows that a chemical step must at least partially control the initial rate of the extraction.
INTRODUCTION FOLLOWINGthe work described in a previous paper[l], a number of experiments have been carried out on the effect of temperature on the rate of extraction of copper(II) from sulphate media by LIX (fiN in Escaid 100, and on kinetic properties of high concentrations of the reagent. EXPERIMENTAL The chemicals and the single drop apparatus used in these experiments were exactly as described in Ref. [1], except for the replacement of the weighted syringe organic phase delivery system for the rising drop apparatus by a more convenient and controllablesystem driven by an adjustable pressure of nitrogen.
organic phase and the glassware show that corrections to the results on this account would be less than 1%, so that in view of the experimental errors such corrections are negligible. A feature of the experiment was the change in the upward motion of the drops as the temperature was increased. A drop which rose with an almost straight path at 18°C had acquired visible zig-zag motion at 38°C, although the contact times between drop and continuous phase did not appear significantly altered. Activation energies were evaluated from Arrhenius plots; a slight systematic curvature of the plots was noted so that the values quoted in Table 2 are appropriate to
RESULTS
Temperature dependence of the rate of extraction
Table 2. Measurementsof viscosity
An activation energy for the extraction of copper from the standard aqueous phase with 20 vol % LIX (fiN in Escaid 100 has already been reported[l]. Further results obtained with extreme concentrations of commercial LIX (fiN in Escaid 100 are reported in Table 1, together with results for a 20% LIX (fiN solution contacted with a high copper, low acid aqueous phase. Conditions corresponding to the extremes of both organic phase and aqueous phase concentration have thus been studied. In these experiments, the solutions were made up and the analyses carried out at room temperature (25 +-2°C), irrespective of the temperature of the kinetic experiment. Calculations of the effect of thermal expansion of the Table 1. Effect of tern ~eratureon the rate of extraction
Vol % LIX 64N
Aqueous Phase Concentrations x dm~ mmolCopper
Acid
5
41.5
17.9
20
4115
17.9
I00
41.5
20
14/.6
Viseosity/cP VoI % LIX 64N
18%
28°0,
3~°c
Activ~tio Energy x kJ-~ol
o
1.608
1.288
1.040
16.3
5
1.731
1.383
1.105
~.0
20 I00 Water [31
2.249 20.38 1.053
1.743 12.40 0.8327
1.366 9,02 0.6783
18.7 (> 25) 16.4
28°C and are subject to errors of _+2-3 kJ mol '. No such curvature was noted in previous work[2], but the data in Table 1 (while giving only three points for each Arrhenius plot) encompass a temperature range rather greater than
in Ref. [2]. Initial Bates x ~uol-lm2s
Activatic~
18%
28%
38°C
gne~gy x kJ-A me]
0.014
0 •027
0 •045
46
Temperature dependence o/viscosity see reference 1
45
17.9
0.049
0.iii
0.i~
51
15
0.i01
0.184
0.292
41
tLIX 64N is a registered trade mark of General Mills Inc., U.S.A. 2067
Although the viscosities of the aqueous phases used can be approximated to those of water, the viscosities of the organic phases require separate measurement. These measurements were made with an Ubbelohde viscometer thermostatted at 18.00, 28.00 or 38.00-+0.03°C and calibrated at each temperature with distilled water. Densities were measured with a specific gravity bottle, and the viscosities quoted in Table 2 obtained with a precision better than 0.5%.
2068
M. A. HUGHES et al.
When an Arrhenius treatment was applied to the data in Table 2, apparent activation energies were obtained which may be compared with those in Table 1. The plots were good straight lines with the exception of that for 100% LIX 64N.
Dependence of rate on organic phase composition Drops of organic phases containing varying concentrations of LIX 64N in Escaid 100 were allowed to rise through a standard aqueous phase containing 41.5 mmol dm 3 copper sulphate and 17.9 mmol dm -3 sulphuric acid at 28°C. The resulting rates of extraction are shown in Table 3. A comparison of these results with the rates reported earlier in Ref. [1] is shown in Fig. 1, and it is
Table 3. Effect of variationof LIX 64N concentrationon the rate of extraction Initial ~ e x mmol -I m S
Vol % LIX 6 ~
E 0.10 7 "6 E E o
~- 0 0 5 c
0
0
5
0.027
i0
0.034
20
0.055
50
0.088
75
0.i01
I00
0.ii0
/Z" ~
4'0 Vol%
~'o
8o
,oo
LIX64N
Fig. 1. Dependenceof rate on LIX 64N concentrationat 28°C. A, from Ref. [1];®, this work.
noticeable that the earlier values are greater than the more recent data. Repeated experiments on both occasions show that the differences between the data are greater than the experimental error of the determinations. This may be due to changes in any one of the components of the organic phase over the period of time. DISCUSSION The important conclusion from the results in Table 1 is that over a wide range of extraction conditions, the activation energies at 28°C are sensibly constant. The same fundamental mechanism should therefore account for the initial rate of extraction with high and low LIX
concentrations, and at high and low ratios of copper to acid concentration in the aqueous phase. The gradient of an Arrhenius plot for a wholly diffusion controlled reaction is expected to be very similar to that of the corresponding plot for the viscosity of the appropriate medium, irrespective of the site of the actual reaction. Thus, in a recent study of a fast homogeneous reaction[4], the temperature dependence of the rate in glycerol (known by several criteria to be diffusion controlled) was very similar to that of the viscosity of glycerol. A change of solvent to an ethylene glycol/water mixture (in which the rate was known to be activation controlled) resulted in an Arrhenius energy of about 33 kJ tool-', compared with viscosity results of 16.4 (for water) and 27 kJ mol-' (for ethylene glycol, Ref. [3]). Properties of the bulk phases will be important in diffusion control, despite uncertainties regarding the properties of any thin interracial zone separating the phases. The activation energy for a diffusion controlled reaction is therefore expected from Table 2 to be close to 17kJ tool-', whereas the values found in Table l are higher by a factor of about 3. It is, however, unsafe [5, 6] to conclude from these magnitudes alone that the rate determining factor in the extraction is a chemical one, although this may of course be true. An approach more likely to be appropriate is one which combines chemical and diffusional rate determining steps, and in which diffusion is of greater or lesser importance depending upon the conditions of an individual experiment. If diffusion as well as chemical reaction were important in determining the rate of extraction, then a change in the relative importance of the two may account for the curvature of the Arrhenius plots and the apparent decrease in activation energy as temperature is raised. In view of the changes in the motion of the drop referred to earlier, the possibility that hydrodynamic changes contribute to the increase of the rate of extraction with temperature must be considered, but it would be an unfortunate combination of effects that caused such similar values of apparent activation energies to emerge from experiments involving phases of such disparate viscosities and densities. The coincidence would be particularly great in the case of 100% LIX 64N, for which the high viscosity made necessary the substitution of a 29 g needle for the 33 g needle at the organic phase inlet, resulting in larger (15 mm3) drops than the normal range (5.5-7.5 mm3). No discontinuity is visible in Fig. 1, despite this change in drop size. It is significant that the results presented here confirm the difference between our previous value[l] of the activation energy and that of Flett [2]. Flett's experiments involved the addition of traces of 64Cu to a copper-LIX system at equilibrium in an AKUFVE apparatus, and observation of the rate of return to the equilibrium condition. The results of the present study indicate at least partial participation of a chemical step in the rate determining mechanism appropriate to unloaded or partially loaded LIX 64N. It is feasible, however, that there could be a gradual change of mechanism as the phases approach equilibrium, so that diffusional resistances become more important than chemical and that at equilibrium diffusion control of the rate of copper exchange consistent with Flett's value of 15 kJ mol -' may obtain. Study of diffusivity values suggests that diffusion of the copper loaded reagent away from the interfacial zone is most likely to be the slowest diffusional process; this will be discussed in a later paper.
Activation energies of LIX 64N reactions
Acknowledgements--We thank Nchanga Consolidated Copper Mines of Zambia for the financial support of this project. REFERENCES 1. R. J. Whewell, M. A. Hughes and C. Hanson, J. Inorg. Nucl. Chem. 37, 2303 (1975). 2. D. S. Flett, D. N. Okuhara and D. R. Spink, J. Inorg. Nucl. Chem. 35, 2471 (1973).
JINC Vol. 38 No. l l - - J
2069
3. Handbook of Chemistry and Physics, Chemical Rubber Company, Cleveland, Ohio (1973). 4. E. F. Caldin and B. B. Hasinoff, J. Chem. Soc. (Faraday Trans.) I. 71, 515 (1975). 5. L. F. Albright, Ind. Eng. Chem. 57, 53 (1965). 6. C. Hanson, J. G. Marsland and G. Wilson, Chem. Eng. Sci. 26, 1513 (1971).