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An integrated electrochemical reaction and supported liquid membrane approach to recycling
EkctmchimicaAda. Vd. 38. No. 6, pp. 847-850.1993 Printed in Great Britaia
0013~4686p3 s6.00 + 0.00 Q 1993. Pergmon PIUS Ltd
SHORT COMMUNICATION AN INTEGRATED ELECTROCHEMICAL REACTION AND SUPPORTED LIQUID MEMBRANE APPROACH TO RECYCLING K. SCOTT and
A. 0. IBHADON
Department of Chemical and Process Engineering. Merz Court, The University of Newcastle upon Tyne NE1 7RU, U.K. (Receiwd 9 March 1992; in revisedform 3 Nooember 1992) Abstract-The application of membrane separation integrated into an electrochemical technique for recycling of industrial effluent containing dissolved chromium is presented. Hexavalent chromium, produced by the anodic oxidation of Cr(III) in an undivided electrochemical batch reactor equipped with membrane separation, is extracted into a strip phase by a solvent loaded microporous flat sheet polypropylene membrane containing Aliquat 336. This process is compared with oxidation carried out in a divided and an undivided cell without membrane separation.
INTRODUCIION The electrochemical valent chromium
oxidation
of Cr(II1) to hexa-
2Cr3+ + 7H,O - 6e- + Cr,O:-
+ 14H+
has been investigated by many workers[l, 23. An application of the reaction is its use as a regenerable oxidant in the manufacture of pharmaceutical intermediates, anthraquinone and related compounds. In these processes the chromic acid, after reaction with the substrate, is reduced to chromic sulphate and recycled to the electrolysis plant for the cycle to begin again[3]. The effect of electrolyte concentration, pH, temperature, current density and potential on the above process carried out in a divided cell electrolysis have been investigated and considerable disagreement exists in the literature[l]. Attempts have been made to reconcile the discrepancies, but this has met with limited success especially with regard to the effect of electrode material on the process. Platinum and titanium have been found not capable of withstanding the aggressive conditions in which the oxidation occurs due to the problem of dissolution. Magnetite, MnO, , Ni, stainless steel, Al, Cr, Fe, ferrosilicon and carbon were not effective in the oxidation of Cr(II1). Antonov et a/.[41 have investigated the electrochemistry of chromium using lead anodes. They have described steady state results on a rotating disc at various Cr(II1) concentrations. At low Cr(II1) concentrations, a linear relationship between efficiency, current and rotation speed was observed. However, at higher concentrations, a plateau was observed to develop where efficiency was independent of rotation speed indicating that the reaction was no longer diffusion controlled. The linear variation in efficiency with rotation speed is observed early in the experi-
ment while the decrease in efficiency with increased rotation has been attributed to retarded desorption of chromate ions, decrease in the surface area of lead dioxide and an increase in potential with current being redistributed in favour of oxygen evolution which is a competitive reaction. According to Blasiak[S] maximum conversion efficiency is obtained at an acid level of lOOgl_ ’ while Antonov et aL[4] reported a pH of 1.8. He pointed out that as the acid strength increases, viscosity rises fairly steeply and the rate of reaction becomes diffusion controlled. The rate dependence on chromium concentration is linear from 5 to 15gl-’ with fractional orders above these concentrations. Zabotin et al.[6] have reported an increase in current efficiency from about 3 to lOAdm_’ and a subsequent decrease between 10 and 100Adm-2 for Cr3+ solutions between 30-8Ogl-’ and 5gl-’ sulphuric acid in the temperature range 20-80°C. With a lead dioxide-coated titanium anode, Laskorin et aL[7] obtained current elliciencies lower than values obtained using massive lead anodes under identical experimental conditions. This was attributed to the non-porous nature of lead dioxide coated Ti anodes and their lower surface area. Soluble chromium species are used in electroplating, metal finishing processes, electrolytes in flow-through batteries and as oxidizing agents in organic synthesis. The disposal of the resulting waste baths is difficult and their regeneration is preferred. This depends on an understanding of the electrochemistry. However, processes have been developed which use an electrochemical cell to oxidize Cr(III)Cr(VI), regenerating the etchant in a convenient way by recycling the liquor through the anode compartment of the cell. A new method for separating and recycling the metal ions from solution is coupled transport using supported liquid membranes, SLM. This communication reports the application of this 847
848
Short Communication
technique with regard to the electrochemical oxidation of Cr(II1) and the recycling of the product Cr(V1).
several days before extraction. Filling the pores of the membranes was straightforward. The extraction of Cr(V1) as the chromate ion from the aqueous feed with Aliquat 336, a long chain amine, is described[8] by the chemical reaction ZR,CH,NCl
EXPERIMENTAL Electrochemical experiments were carried out either in a two compartment divided or an undivided glass H-cell. The working electrode was lead dioxide coated on titanium while the counter electrode was nickel. Another compartment containing the strip solution (100 cm3) was separated from the electrolyte compartment (2OOcm’) by a solvent loaded microporous polypropylene membrane, area 25 cm*. Experiments were conducted at 22°C at low pH and reasonably high Cr(II1) concentration. The solution pH was monitored using a pH meter, Beckman Selection 2000. The source of chromium was either Cr(II1) sulphate, chloride or oxide and the extractant used was Aliquat 336, and solvent, HPLC grade ortho-xylene, all supplied by Aldrich Chemicals Ltd. The concentrations of Cr(V1) were obtained by titration of the electrolyte and strip phases separately with Fe(H) ammonium sulphate using l,lO-phenanthroline ferrous sulphate as indicator. current eficiencies were determined on the basis of the total amount of Cr(V1) produced compared to the Faradaic maximum values. Polypropylene, flat sheet membranes were used to extract the oxidation product. They were impregnated with a solution of Aliquat 336 in o-xylene for
+ CrO:-
= (R,CH,N),CrO,
+ 2Cl-.
The stripping of CrO:- ion was carried out using a 1 M lithium hydroxide solution. High flux values for CrO:- are obtained in 25% vol. Aliquat 336 in oxylene, about 3 pgcrn-* min-‘[8]. In order to prevent the cathodic reduction of Cr(VI), the electrolysis is usually carried out in a divided cell and a reasonable anodic current emciency is maintained by using a high concentration of Cr(III). In this work an undivided cell is used for the oxidation and the Cr(V1) is extracted the moment it is produced. This would obviate the need for a separator or a solvent for an in situ extraction of the chromium species.
RESULTS
AND DISCUSSION
The electrochemical oxidation of Cr(HI)-Cr(VI) was conducted in three separate batch reactors-an undivided cell, and undivided cell equipped with membrane separation and a divided cell. In each case, the oxidation is characterized by a rapid build up of Cr(V1) with time especially at the early stages of the experiment. The aqueous and organic phase concentrations of Cr(V1) resulting from the oxidation
60
50
0
10
20
30
40
50
60
70
-
und.cell SLM
-
divided cell
-
und.cell
--0--
org.amc
60
Timrlmin
Fig. 1. Variation in chromium (VI) concentration with time. Current density 10.8mAcm_‘. Anolyte 0.1 moldme Ct(III) and 3moldm-’ H,SO,.
Short Communication
849
loo ,
. .
90 --
.
0
80 --
0
70 --
n
Ll
0
g ._ iu = 3 2
60--
.
:
l
50 --
.
l
.
.
.
0
0
.
40 --
.
.
0 divided cell
. 0
.
30 -20 -10 -04
0
I
,
5
10
15
I
I
20
25
Current density mAtan
Fig. 2 The variation of current dfkiency of Cr(III) oxidation with current density.
of Cr(II1) are shown in Fig. 1. The build-up of Cr(V1) with time remained constant after approximately an 80% conversion of Cr(III)-Cr(VI) had occurred. The Cr(VI) concentration-time curves for the three reactors are different. For the undivided cell without membrane separation, the conversion fell with time. The concentration-time plots for the divided and the undivided cells with SLM are similar. The decrese in the rate of oxidation of Cr(III) with time as observed in the undivided cell without the SLM, is attributable to the reduction of Cr(V1) at the counter electrode proceeding at a much faster rate than would
have been the case for an undivided cell equipped with membrane separation. At current densities of 4OmAcm-’ and greater, the oxygen evolution reaction and the cathodic reduction of Cr(V1) occur at high rates with resultant low etliciency for the anodic oxidation of Cr(III). Experiments were therefore conducted at lower current densities between 8 and 25rnAc~n-~. The effect of anodic current density on the oxidation process is shown in Fig. 2. Generally, the etkiency dropped with an increase in current density, and was lower in the undivided
90
l
undcell SLM
0 divided cell
I
0
10
20
30
40
I
50
60
70
I a0
Timdmin
Fig. 3. The variation of current e8iciency with tie.
_.Current density 10.8 mA cm-*.
Short Communication
30
20
0
0
20
10
30
50
40 Convemion
60
70
80
90
X
Fig. 4. The variation of current efficiency with conversion of Cr(III) for the SLM cell. Current density 10.8mAcm-2.
cell without
the supported liquid membrane compared to the divided cell and the undivided cell with
the SLM. The probable implication of this result is that for prolonged recycling of chromium in the undivided cell the efficiency of the operation falls as the reduction of Cr(VI) at the cathode occurs more easily. The variation in the current efficiency with time and conversion is shown in Figs 3 and 4. In all cases current efficiency falls with time due to several factors which includes a depletion of the reactant Cr(III). It can be seen that the oxidation is more eficient in the undivided cell with the SLM. This system thus may provide a better and more eficient means of recycling chromium. At a given current density, the conversions obtained were higher by about 510% compared to the values obtained for the divided cell. The undivided cell without the liquid membrane is clearly not efficient. Current elliciencies obtained with this cell are lower than the values obtained for the divided and undivided cell with SLM by more than 35%.
CONCLUSION The primary obtain it in a environment. resulting from waste bath is
objective in recycling chromium is to form useful and less harmful to the In an undivided cell, the Cr(V1) the anodic oxidation of chromium reconverted
to Cr(II1)
by cathodic
reduction. The efficiency of the process is therefore low. However in an undivided cell with a supported liquid membrane, the Cr(V1) is extracted by the SLM and separated into a strip phase where it can be recycled as a regenerable oxidant. The efficiency of this process is maintained by equalizing the generation of Cr(V1) by the anodic oxidation of Cr(II1) with its removal by the SLM. The efficiency of this operating is significantly higher than values obtained with a divided batch cell. Acknowledgement-The authors wish to thank the SERC for the funding provided to A. 0. Ibhadon.
REFERENCES 1. A. Kuhn and R. Clarke, J. Appl. Chem. Biotechnol. 26, 407 (1976). 2. R. J. Vora, S. R. Taylor and G. E. Stoner, Proceed. Elect. Sot. @-IO,279 (1989). 3. D. Pletcher and F. Walsh, Industrial Electrochemistry, 2nd Edition. Chapman and Hall, London (1990). 4. S. P. Antonov, D. P. Zosimovich and A. S. Gumen, Ukr. Khim. Zhur. 36,793 (1970). 5. R. Blasiak, Przemysl Chem. 7 (1967). 6. P. I. Zabotin, N. F. Razina and G. R. Kiryakov, Trans. Inst. khim. Acad. 9.49 (1962). 7. B. N. Laskorin and N. M. Simora. Prikl. khim. 47. 1920 (1974). 8. 0. Loiacono, E. Drioli and R. J. Molinari, Mem. Sci. 2% 123 (1986).
0013~4686p3 s6.00 + 0.00 Q 1993. Pergmon PIUS Ltd
SHORT COMMUNICATION AN INTEGRATED ELECTROCHEMICAL REACTION AND SUPPORTED LIQUID MEMBRANE APPROACH TO RECYCLING K. SCOTT and
A. 0. IBHADON
Department of Chemical and Process Engineering. Merz Court, The University of Newcastle upon Tyne NE1 7RU, U.K. (Receiwd 9 March 1992; in revisedform 3 Nooember 1992) Abstract-The application of membrane separation integrated into an electrochemical technique for recycling of industrial effluent containing dissolved chromium is presented. Hexavalent chromium, produced by the anodic oxidation of Cr(III) in an undivided electrochemical batch reactor equipped with membrane separation, is extracted into a strip phase by a solvent loaded microporous flat sheet polypropylene membrane containing Aliquat 336. This process is compared with oxidation carried out in a divided and an undivided cell without membrane separation.
INTRODUCIION The electrochemical valent chromium
oxidation
of Cr(II1) to hexa-
2Cr3+ + 7H,O - 6e- + Cr,O:-
+ 14H+
has been investigated by many workers[l, 23. An application of the reaction is its use as a regenerable oxidant in the manufacture of pharmaceutical intermediates, anthraquinone and related compounds. In these processes the chromic acid, after reaction with the substrate, is reduced to chromic sulphate and recycled to the electrolysis plant for the cycle to begin again[3]. The effect of electrolyte concentration, pH, temperature, current density and potential on the above process carried out in a divided cell electrolysis have been investigated and considerable disagreement exists in the literature[l]. Attempts have been made to reconcile the discrepancies, but this has met with limited success especially with regard to the effect of electrode material on the process. Platinum and titanium have been found not capable of withstanding the aggressive conditions in which the oxidation occurs due to the problem of dissolution. Magnetite, MnO, , Ni, stainless steel, Al, Cr, Fe, ferrosilicon and carbon were not effective in the oxidation of Cr(II1). Antonov et a/.[41 have investigated the electrochemistry of chromium using lead anodes. They have described steady state results on a rotating disc at various Cr(II1) concentrations. At low Cr(II1) concentrations, a linear relationship between efficiency, current and rotation speed was observed. However, at higher concentrations, a plateau was observed to develop where efficiency was independent of rotation speed indicating that the reaction was no longer diffusion controlled. The linear variation in efficiency with rotation speed is observed early in the experi-
ment while the decrease in efficiency with increased rotation has been attributed to retarded desorption of chromate ions, decrease in the surface area of lead dioxide and an increase in potential with current being redistributed in favour of oxygen evolution which is a competitive reaction. According to Blasiak[S] maximum conversion efficiency is obtained at an acid level of lOOgl_ ’ while Antonov et aL[4] reported a pH of 1.8. He pointed out that as the acid strength increases, viscosity rises fairly steeply and the rate of reaction becomes diffusion controlled. The rate dependence on chromium concentration is linear from 5 to 15gl-’ with fractional orders above these concentrations. Zabotin et al.[6] have reported an increase in current efficiency from about 3 to lOAdm_’ and a subsequent decrease between 10 and 100Adm-2 for Cr3+ solutions between 30-8Ogl-’ and 5gl-’ sulphuric acid in the temperature range 20-80°C. With a lead dioxide-coated titanium anode, Laskorin et aL[7] obtained current elliciencies lower than values obtained using massive lead anodes under identical experimental conditions. This was attributed to the non-porous nature of lead dioxide coated Ti anodes and their lower surface area. Soluble chromium species are used in electroplating, metal finishing processes, electrolytes in flow-through batteries and as oxidizing agents in organic synthesis. The disposal of the resulting waste baths is difficult and their regeneration is preferred. This depends on an understanding of the electrochemistry. However, processes have been developed which use an electrochemical cell to oxidize Cr(III)Cr(VI), regenerating the etchant in a convenient way by recycling the liquor through the anode compartment of the cell. A new method for separating and recycling the metal ions from solution is coupled transport using supported liquid membranes, SLM. This communication reports the application of this 847
848
Short Communication
technique with regard to the electrochemical oxidation of Cr(II1) and the recycling of the product Cr(V1).
several days before extraction. Filling the pores of the membranes was straightforward. The extraction of Cr(V1) as the chromate ion from the aqueous feed with Aliquat 336, a long chain amine, is described[8] by the chemical reaction ZR,CH,NCl
EXPERIMENTAL Electrochemical experiments were carried out either in a two compartment divided or an undivided glass H-cell. The working electrode was lead dioxide coated on titanium while the counter electrode was nickel. Another compartment containing the strip solution (100 cm3) was separated from the electrolyte compartment (2OOcm’) by a solvent loaded microporous polypropylene membrane, area 25 cm*. Experiments were conducted at 22°C at low pH and reasonably high Cr(II1) concentration. The solution pH was monitored using a pH meter, Beckman Selection 2000. The source of chromium was either Cr(II1) sulphate, chloride or oxide and the extractant used was Aliquat 336, and solvent, HPLC grade ortho-xylene, all supplied by Aldrich Chemicals Ltd. The concentrations of Cr(V1) were obtained by titration of the electrolyte and strip phases separately with Fe(H) ammonium sulphate using l,lO-phenanthroline ferrous sulphate as indicator. current eficiencies were determined on the basis of the total amount of Cr(V1) produced compared to the Faradaic maximum values. Polypropylene, flat sheet membranes were used to extract the oxidation product. They were impregnated with a solution of Aliquat 336 in o-xylene for
+ CrO:-
= (R,CH,N),CrO,
+ 2Cl-.
The stripping of CrO:- ion was carried out using a 1 M lithium hydroxide solution. High flux values for CrO:- are obtained in 25% vol. Aliquat 336 in oxylene, about 3 pgcrn-* min-‘[8]. In order to prevent the cathodic reduction of Cr(VI), the electrolysis is usually carried out in a divided cell and a reasonable anodic current emciency is maintained by using a high concentration of Cr(III). In this work an undivided cell is used for the oxidation and the Cr(V1) is extracted the moment it is produced. This would obviate the need for a separator or a solvent for an in situ extraction of the chromium species.
RESULTS
AND DISCUSSION
The electrochemical oxidation of Cr(HI)-Cr(VI) was conducted in three separate batch reactors-an undivided cell, and undivided cell equipped with membrane separation and a divided cell. In each case, the oxidation is characterized by a rapid build up of Cr(V1) with time especially at the early stages of the experiment. The aqueous and organic phase concentrations of Cr(V1) resulting from the oxidation
60
50
0
10
20
30
40
50
60
70
-
und.cell SLM
-
divided cell
-
und.cell
--0--
org.amc
60
Timrlmin
Fig. 1. Variation in chromium (VI) concentration with time. Current density 10.8mAcm_‘. Anolyte 0.1 moldme Ct(III) and 3moldm-’ H,SO,.
Short Communication
849
loo ,
. .
90 --
.
0
80 --
0
70 --
n
Ll
0
g ._ iu = 3 2
60--
.
:
l
50 --
.
l
.
.
.
0
0
.
40 --
.
.
0 divided cell
. 0
.
30 -20 -10 -04
0
I
,
5
10
15
I
I
20
25
Current density mAtan
Fig. 2 The variation of current dfkiency of Cr(III) oxidation with current density.
of Cr(II1) are shown in Fig. 1. The build-up of Cr(V1) with time remained constant after approximately an 80% conversion of Cr(III)-Cr(VI) had occurred. The Cr(VI) concentration-time curves for the three reactors are different. For the undivided cell without membrane separation, the conversion fell with time. The concentration-time plots for the divided and the undivided cells with SLM are similar. The decrese in the rate of oxidation of Cr(III) with time as observed in the undivided cell without the SLM, is attributable to the reduction of Cr(V1) at the counter electrode proceeding at a much faster rate than would
have been the case for an undivided cell equipped with membrane separation. At current densities of 4OmAcm-’ and greater, the oxygen evolution reaction and the cathodic reduction of Cr(V1) occur at high rates with resultant low etliciency for the anodic oxidation of Cr(III). Experiments were therefore conducted at lower current densities between 8 and 25rnAc~n-~. The effect of anodic current density on the oxidation process is shown in Fig. 2. Generally, the etkiency dropped with an increase in current density, and was lower in the undivided
90
l
undcell SLM
0 divided cell
I
0
10
20
30
40
I
50
60
70
I a0
Timdmin
Fig. 3. The variation of current e8iciency with tie.
_.Current density 10.8 mA cm-*.
Short Communication
30
20
0
0
20
10
30
50
40 Convemion
60
70
80
90
X
Fig. 4. The variation of current efficiency with conversion of Cr(III) for the SLM cell. Current density 10.8mAcm-2.
cell without
the supported liquid membrane compared to the divided cell and the undivided cell with
the SLM. The probable implication of this result is that for prolonged recycling of chromium in the undivided cell the efficiency of the operation falls as the reduction of Cr(VI) at the cathode occurs more easily. The variation in the current efficiency with time and conversion is shown in Figs 3 and 4. In all cases current efficiency falls with time due to several factors which includes a depletion of the reactant Cr(III). It can be seen that the oxidation is more eficient in the undivided cell with the SLM. This system thus may provide a better and more eficient means of recycling chromium. At a given current density, the conversions obtained were higher by about 510% compared to the values obtained for the divided cell. The undivided cell without the liquid membrane is clearly not efficient. Current elliciencies obtained with this cell are lower than the values obtained for the divided and undivided cell with SLM by more than 35%.
CONCLUSION The primary obtain it in a environment. resulting from waste bath is
objective in recycling chromium is to form useful and less harmful to the In an undivided cell, the Cr(V1) the anodic oxidation of chromium reconverted
to Cr(II1)
by cathodic
reduction. The efficiency of the process is therefore low. However in an undivided cell with a supported liquid membrane, the Cr(V1) is extracted by the SLM and separated into a strip phase where it can be recycled as a regenerable oxidant. The efficiency of this process is maintained by equalizing the generation of Cr(V1) by the anodic oxidation of Cr(II1) with its removal by the SLM. The efficiency of this operating is significantly higher than values obtained with a divided batch cell. Acknowledgement-The authors wish to thank the SERC for the funding provided to A. 0. Ibhadon.
REFERENCES 1. A. Kuhn and R. Clarke, J. Appl. Chem. Biotechnol. 26, 407 (1976). 2. R. J. Vora, S. R. Taylor and G. E. Stoner, Proceed. Elect. Sot. @-IO,279 (1989). 3. D. Pletcher and F. Walsh, Industrial Electrochemistry, 2nd Edition. Chapman and Hall, London (1990). 4. S. P. Antonov, D. P. Zosimovich and A. S. Gumen, Ukr. Khim. Zhur. 36,793 (1970). 5. R. Blasiak, Przemysl Chem. 7 (1967). 6. P. I. Zabotin, N. F. Razina and G. R. Kiryakov, Trans. Inst. khim. Acad. 9.49 (1962). 7. B. N. Laskorin and N. M. Simora. Prikl. khim. 47. 1920 (1974). 8. 0. Loiacono, E. Drioli and R. J. Molinari, Mem. Sci. 2% 123 (1986).