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The treatment of cupric chloride solution after the etching process by ion exchange membrane electrodialysis
Desalination, 62 (1987) 251-257 Elsevier Science Publishers B.V., Amsterdam -
251 Printed in The Netherlands
The Treatment of Cupric Chloride Solution after the Etching Process by Ion Exchange Membrane Electrodialysis* XUE DEMING, SONG DEZHENG and LIU XIAOYING The Development Center of Seawater Desalination and Water Treatment Technology, The Second Institute of Oceanography, The State Oceanographic Administration, P.O. Box 75, Hangzhou, Zhejiang (China). Tel. 057186924; telex 35035 NBOHZ CN
SUMMARY
The cupric chloride solution whih is discharged as wastewater after the etching process was treated by ion exchange membrane electrodialysis. This method not only reuses the water by desalination, which may or may not be connected with ion exchange, it also concentrates the cupric chloride and metal copper can be recovered by electrolysis or some other suitable method, making it a closed system. The results of practical operation show that the electrodialysis method is comparble with precipitation from the point of view of cost and efficiency. We demonstrated that the plant could be operated successfully without damage to the home-made ion exchange membrane and without electrodeposition of the metal copper on the cathode. Furthermore, the energy consumption was less than 3 kWh/t water. The cupric ion concentration of 2,000 mg/l in the wastewater was reduced to less than 1 mg/l at discharge. Keywords: cupric chloride solution, electrodialysis, ion exchange, wastewater, electronics industry, recycling, effluents, metal recovery
INTRODUCTION
There ufacture etching provide
are now a great many production lines around the world which manprinted circuit boards (PCB ) . The process involves the removal by of parts of a layer of copper from an insulating substrate in order to the desired pattern of conducting links on the surface of the insulating
*Presented at the International Symposium on Synthetic Membrane Science and Technology, Dalian, China, April 13-181986.
OOll-9164/87/$03.50
0 1987 Elsevier Science Publishers B.V.
252
substrate. The solution containing the cupric ions which have been rinsed away by water from the boards is discharged as wastewater. This is not only a waste of valuable copper resources, it is also one of the principal sources of environmental pollution. Therefore it is a matter of some urgency to find a better process for treating the etching effluent. Several processes have been reported so far, of which the commonest is the neutralization precipitate process. It consists of adding alkaline agents such as sodium hydroxide, lime, etc. to the wastewater to adjust pH and precipitate the cupric hydroxide and carbonate basic. Although the process works, it has some disadvantages; for example, its high consumption of chemicals, expensive running costs, the need for waste sludge treatment and so on. These drawbacks make it impractical to employ this process in small or medium capacity plants. Another process is the ion exchange method; however, this method is not yet feasible either from a technical or an economic point of view. Because the concentration of cupric ions in the effluent is usually more than 1,000 mg/l, and sometimes as high as 3,000 mg/l, the total ion concentration is about 6,000 mg/l. This means it is necessary to regenerate the ion exchange resin frequently. Fortunately there is another possibility; the effhrent can be desalted by ion exchange membrane electrodialysis within the ion concentration range. A desalination system which combines electrodialysis, to remove most of the cupric ions, and ion exchange, to remove the rest, will make it possible to treat solutions with a considerable ion content. It should be noted however that research into this process as reported in various publications, has not yet led to any clear results; this is why the authors of the present paper chose to investigate the subject further. EXPERIMENTAL
Apparatus The experimental plant was provided with an electrodialyzer ( SHD-01) , recirculating store tanks, pumps, motors, cation exchange resin (DK 110) column and various instruments such as a Type-721 spectrophotometer, a Type S-3 pH meter, etc. Home-made cationic (3361) and anionic (3362) ion exchange membranes and a ruthenium-coated titanium electrode were used in the experimental runs. The effective desalting area per membrane was 640 cm2, the space between membranes 0.85 mm. The membrane stack consisted of 30 cell pairs. Flow sheet The flow sheet of the electrodialysis plant is similar to the one we published previously [ 1 ] .
253
The raw water was clarified, filtered, had oil removed from it, and was then fed into the tanks. The electrode rinse was made of special chemical agents with additive to prevent the deposition of the metals and/or their hydroxide on the cathode. The linear velocity of the solution in the desalting chamber was 6 cm/s. The pressure drop in the concentrating and desalting- and electrode chambers was kept in equilibrium. The temperature of the solution was 28-30 oC. During operation, the dialysate and the concentrated brine were recirculated separately through their compartments. The voltage was kept at a constant value for each run. As the desalination progressed, the current density decreased in proportion to the ion concentration of the water being dialyzed. However, it must always be kept within limiting current density so as to ensure troublefree operation during a run. Solution samples were taken to measure the cupric ion concentration and pH at certain intervals. The cupric ion concentration in both the dialysate and the concentrated brine were measured with the complexometric titration method. After most of the cupric ions had been removed by electrodialysis, the solution was fed into an ion exchange column to remove the rest of them and the filtrate was measured by spectrophotometric method and pH meter. RESULTS AND DISCUSSION
Desalting and concentrating the effluent CuCl, Cupric chloride solutions with a cupric ion concentration of 0.031 N, 0.126 N and 0.208 N were desalted in batches at 15.0 and 18.0 V successively. The changes in cupric ion concentration with runing time are shown in Fig. 1. The curpic ion concentration of the dialysate was reduced rapidly at the beginning of the dialyzing process, as can be seen in Fig. 1, and it was reduced to 14-17 mg/l by the end of the run. Next, and effluent with a cupric ion concentration of 0.033 N (concentration stream 0.40 N) and an effluent with a cupric ion concentration of 0.21 N (concentration stream 0.41 N) were recirculated separately through the stack compartments at 18.0 V. The results of desalting and concentrating these effluents are illustrated in Fig. 2 and Fig. 3 respectively. There was a rapid increase in the cupric ion concentration of the concentrated brine at the beginning of the running period, matching the reduction in cupric ion concentration of the dialysate. The desalination and concentration characteristics of the cupric chloride solution are similar to those of zinc chloride solution; in fact the effluent containing cupric chloride can be both desalted and concentrated better [ 11. It has been reported that the maximum concentration of 4.9 N in cupric chloride solution can be reached by using an extremely high current density [ 21. How-
0
: ! ! 01020304050607060901oo
time
(min)
Fig. 1. Cupric ion concentration of dialysate vs. running time.
ever, the cupric ion content was kept at 20 g/l in the concentrated brine and 20 mg/l in the dialysate in the present study, since from the point of view of power consumption these concentrations are quite adequate for the post-treatment. Of course a higher or lower content could be easily obtained by electrodialysis. It can also be seen from Fig. 3 that the cupric chloride solution still shows good concentrated and desalted characteristics, as long as the concentration ratio p between concentrated brine and the dilution stream reaches about 900. (b)
_o_~-o-o-o-o
Fig. 2. Cupric ion concentration trated brine. Fig. 3. Cupric ion concentration trated brine.
0.5
I
in the ED outlet vs. running time. (a) Dialysate,
(b) concen-
in the ED outlet vs. running time. (a) Dialysate,
(b) concen-
255 TABLE I MEMBRANE
CHARACTERISTICS
Item
Water content Exchange capacity Electric resistance Transport number
Dimension
g/g dry medg dry Q cm-*
BEFORE AND AFTER THE RUN Cationic
Anionic
Before
After
Before
After
49.98 2.23
50.20 3.12
37.42 2.13
39.57 2.23
8.32 0.968
9.14 0.970
12.28 0.973
9.00 0.981
Membrane suitability The membranes were stable for eight months continuous running. There was no deterioration at all in the membranes, although they did change color. Both cationic and anionic membranes changed from their original color to a light green. We know that the cupric ions in the acidic solution exist not only in the form of the cation Cu2+ but also as the complex anion ( CuCl,) 2-, so they may pass through either cationic or anionic membranes. The color change results from the counter ion changing to Cu2+ or ( CuC14) 2-. However, simply immersing the membranes in a dilute acidic and alkali solution and washing them with deionized water restores their original color. This means that the cupric ions which have been changed to counter ions within the membrane can be easily changed back. Samples of the membranes were taken from the stack and inspected to confirm their suitability; Table I shows the characteristics of the membranes before and after the run. Evaluation of the energy consumption of the electrodialysis process Figures 4 and 5 relate the concentration ratio to the energy consumption of the stack. Fig. 4 shows that the energy consumption is comparatively low, on average about 1.0 kWh per ton of effluent when the cupric ion is desalted from 0.031 N to 0.00045 N in the dialysate and concentrated from 0.060 N to 0.114 N in the concentrated brine stream. Although a slight increase in energy consumption is noticed when the cupric ion content increases (Fig. 5 ) it is possible to treat the effluent at 3.0 kWh per ton within the concentration range from that the water can be reused and the copper can be recovered effectively. Moreover, there is hardly any further increase in energy consumption once the concentration ratio p has reached a certain value (see Fig. 5). This means that the major factor determining energy consumption is the dialysate concentration, sop should be as high as possible. The fact that the energy consumption
=: o/--o-o1.0
t
g c
0
.z a 0.5
E
/
I
0
i
E F i : 0 0
100
xx)
x0
concentration
Fig. 4. Energy consumption vs. concentration ratio
ratio
p
p.
is lower in the acidic solution with pH less than 3 is consistent with the higher electrical current efficiency associated with these solutions [ 21. Consequently, treating the effluent containing cupric chloride by electrodialysis is also attractive for its economy in power consumption. Operating stability The key to preventing metal hydroxide scaling on the membrane interface and the deposition of metals on the electrode surface is to treat solutions containing metallic ions such as ZIP+, Cu2+, Co2+, Ni2+ etc., with the electrodi-
oo+xlo~
concentration
mtio
p
Fig. 5. Energy consumption vs. concentration ratio
p.
257
alysis method. To ensure trouble free operation, the following steps should be taken: (1) The raw water should be loaded into the stack only after a thorough pretreatment. ( 2) Reasonable operating parameters should be chosen. ( 3 ) A suitable electrode rinse solution should be used. Thanks to these precautions, our electrodialysis plant has been in constant operation without giving any trouble. Post-treatment of the dialysate and the concentrated brine Dialysate with a cupric ion content of less than 40 mg/l can be either reused to wash the product or passes through a cation exchange column to be discharged. The cupric ion concentration in the column outlet is almost zero, with pH 6-7 during a run under normal conditions. The regeneration period of the resin is usually more than 60 h. It is also easy to recover metal copper from the concentrated brine with a cupric ion content of more than 20 g/l, at a total cost of 1 yuan per kg of metal copper. Thus the combined system is closed not only to eliminate sources of pollution, but also to facilitate copper recovery. We are now working on scaling up the process from pilot scale to commercial scale. CONCLUSIONS
The feasibility of the electrodialysis process for treating effluent which contains cupric chloride after the etching process has been confirmed from both a technical and an economic point of view. Home-made ion exchange membranes have been found suitable for desalination and concentration of cupric chloride solution. The suitability of the combined system consisting of electrodialysis, ion exchange and post-treatment has been demonstrated.
REFERENCES 1 Xue Deming, Liu Jingqing. Song Dezheng, Huang Linjin and Ding Hui, The study of treatment of wastewater containing zinc by electrodialysis, Technology of Water Treatment, 10 (1) (1984) 44. 2 Takatugu Azumi and Akio Yoneda, Himeji Kogyo Daigaku Kenkyn Hokoku, 30A (1977) 117-121.
251 Printed in The Netherlands
The Treatment of Cupric Chloride Solution after the Etching Process by Ion Exchange Membrane Electrodialysis* XUE DEMING, SONG DEZHENG and LIU XIAOYING The Development Center of Seawater Desalination and Water Treatment Technology, The Second Institute of Oceanography, The State Oceanographic Administration, P.O. Box 75, Hangzhou, Zhejiang (China). Tel. 057186924; telex 35035 NBOHZ CN
SUMMARY
The cupric chloride solution whih is discharged as wastewater after the etching process was treated by ion exchange membrane electrodialysis. This method not only reuses the water by desalination, which may or may not be connected with ion exchange, it also concentrates the cupric chloride and metal copper can be recovered by electrolysis or some other suitable method, making it a closed system. The results of practical operation show that the electrodialysis method is comparble with precipitation from the point of view of cost and efficiency. We demonstrated that the plant could be operated successfully without damage to the home-made ion exchange membrane and without electrodeposition of the metal copper on the cathode. Furthermore, the energy consumption was less than 3 kWh/t water. The cupric ion concentration of 2,000 mg/l in the wastewater was reduced to less than 1 mg/l at discharge. Keywords: cupric chloride solution, electrodialysis, ion exchange, wastewater, electronics industry, recycling, effluents, metal recovery
INTRODUCTION
There ufacture etching provide
are now a great many production lines around the world which manprinted circuit boards (PCB ) . The process involves the removal by of parts of a layer of copper from an insulating substrate in order to the desired pattern of conducting links on the surface of the insulating
*Presented at the International Symposium on Synthetic Membrane Science and Technology, Dalian, China, April 13-181986.
OOll-9164/87/$03.50
0 1987 Elsevier Science Publishers B.V.
252
substrate. The solution containing the cupric ions which have been rinsed away by water from the boards is discharged as wastewater. This is not only a waste of valuable copper resources, it is also one of the principal sources of environmental pollution. Therefore it is a matter of some urgency to find a better process for treating the etching effluent. Several processes have been reported so far, of which the commonest is the neutralization precipitate process. It consists of adding alkaline agents such as sodium hydroxide, lime, etc. to the wastewater to adjust pH and precipitate the cupric hydroxide and carbonate basic. Although the process works, it has some disadvantages; for example, its high consumption of chemicals, expensive running costs, the need for waste sludge treatment and so on. These drawbacks make it impractical to employ this process in small or medium capacity plants. Another process is the ion exchange method; however, this method is not yet feasible either from a technical or an economic point of view. Because the concentration of cupric ions in the effluent is usually more than 1,000 mg/l, and sometimes as high as 3,000 mg/l, the total ion concentration is about 6,000 mg/l. This means it is necessary to regenerate the ion exchange resin frequently. Fortunately there is another possibility; the effhrent can be desalted by ion exchange membrane electrodialysis within the ion concentration range. A desalination system which combines electrodialysis, to remove most of the cupric ions, and ion exchange, to remove the rest, will make it possible to treat solutions with a considerable ion content. It should be noted however that research into this process as reported in various publications, has not yet led to any clear results; this is why the authors of the present paper chose to investigate the subject further. EXPERIMENTAL
Apparatus The experimental plant was provided with an electrodialyzer ( SHD-01) , recirculating store tanks, pumps, motors, cation exchange resin (DK 110) column and various instruments such as a Type-721 spectrophotometer, a Type S-3 pH meter, etc. Home-made cationic (3361) and anionic (3362) ion exchange membranes and a ruthenium-coated titanium electrode were used in the experimental runs. The effective desalting area per membrane was 640 cm2, the space between membranes 0.85 mm. The membrane stack consisted of 30 cell pairs. Flow sheet The flow sheet of the electrodialysis plant is similar to the one we published previously [ 1 ] .
253
The raw water was clarified, filtered, had oil removed from it, and was then fed into the tanks. The electrode rinse was made of special chemical agents with additive to prevent the deposition of the metals and/or their hydroxide on the cathode. The linear velocity of the solution in the desalting chamber was 6 cm/s. The pressure drop in the concentrating and desalting- and electrode chambers was kept in equilibrium. The temperature of the solution was 28-30 oC. During operation, the dialysate and the concentrated brine were recirculated separately through their compartments. The voltage was kept at a constant value for each run. As the desalination progressed, the current density decreased in proportion to the ion concentration of the water being dialyzed. However, it must always be kept within limiting current density so as to ensure troublefree operation during a run. Solution samples were taken to measure the cupric ion concentration and pH at certain intervals. The cupric ion concentration in both the dialysate and the concentrated brine were measured with the complexometric titration method. After most of the cupric ions had been removed by electrodialysis, the solution was fed into an ion exchange column to remove the rest of them and the filtrate was measured by spectrophotometric method and pH meter. RESULTS AND DISCUSSION
Desalting and concentrating the effluent CuCl, Cupric chloride solutions with a cupric ion concentration of 0.031 N, 0.126 N and 0.208 N were desalted in batches at 15.0 and 18.0 V successively. The changes in cupric ion concentration with runing time are shown in Fig. 1. The curpic ion concentration of the dialysate was reduced rapidly at the beginning of the dialyzing process, as can be seen in Fig. 1, and it was reduced to 14-17 mg/l by the end of the run. Next, and effluent with a cupric ion concentration of 0.033 N (concentration stream 0.40 N) and an effluent with a cupric ion concentration of 0.21 N (concentration stream 0.41 N) were recirculated separately through the stack compartments at 18.0 V. The results of desalting and concentrating these effluents are illustrated in Fig. 2 and Fig. 3 respectively. There was a rapid increase in the cupric ion concentration of the concentrated brine at the beginning of the running period, matching the reduction in cupric ion concentration of the dialysate. The desalination and concentration characteristics of the cupric chloride solution are similar to those of zinc chloride solution; in fact the effluent containing cupric chloride can be both desalted and concentrated better [ 11. It has been reported that the maximum concentration of 4.9 N in cupric chloride solution can be reached by using an extremely high current density [ 21. How-
0
: ! ! 01020304050607060901oo
time
(min)
Fig. 1. Cupric ion concentration of dialysate vs. running time.
ever, the cupric ion content was kept at 20 g/l in the concentrated brine and 20 mg/l in the dialysate in the present study, since from the point of view of power consumption these concentrations are quite adequate for the post-treatment. Of course a higher or lower content could be easily obtained by electrodialysis. It can also be seen from Fig. 3 that the cupric chloride solution still shows good concentrated and desalted characteristics, as long as the concentration ratio p between concentrated brine and the dilution stream reaches about 900. (b)
_o_~-o-o-o-o
Fig. 2. Cupric ion concentration trated brine. Fig. 3. Cupric ion concentration trated brine.
0.5
I
in the ED outlet vs. running time. (a) Dialysate,
(b) concen-
in the ED outlet vs. running time. (a) Dialysate,
(b) concen-
255 TABLE I MEMBRANE
CHARACTERISTICS
Item
Water content Exchange capacity Electric resistance Transport number
Dimension
g/g dry medg dry Q cm-*
BEFORE AND AFTER THE RUN Cationic
Anionic
Before
After
Before
After
49.98 2.23
50.20 3.12
37.42 2.13
39.57 2.23
8.32 0.968
9.14 0.970
12.28 0.973
9.00 0.981
Membrane suitability The membranes were stable for eight months continuous running. There was no deterioration at all in the membranes, although they did change color. Both cationic and anionic membranes changed from their original color to a light green. We know that the cupric ions in the acidic solution exist not only in the form of the cation Cu2+ but also as the complex anion ( CuCl,) 2-, so they may pass through either cationic or anionic membranes. The color change results from the counter ion changing to Cu2+ or ( CuC14) 2-. However, simply immersing the membranes in a dilute acidic and alkali solution and washing them with deionized water restores their original color. This means that the cupric ions which have been changed to counter ions within the membrane can be easily changed back. Samples of the membranes were taken from the stack and inspected to confirm their suitability; Table I shows the characteristics of the membranes before and after the run. Evaluation of the energy consumption of the electrodialysis process Figures 4 and 5 relate the concentration ratio to the energy consumption of the stack. Fig. 4 shows that the energy consumption is comparatively low, on average about 1.0 kWh per ton of effluent when the cupric ion is desalted from 0.031 N to 0.00045 N in the dialysate and concentrated from 0.060 N to 0.114 N in the concentrated brine stream. Although a slight increase in energy consumption is noticed when the cupric ion content increases (Fig. 5 ) it is possible to treat the effluent at 3.0 kWh per ton within the concentration range from that the water can be reused and the copper can be recovered effectively. Moreover, there is hardly any further increase in energy consumption once the concentration ratio p has reached a certain value (see Fig. 5). This means that the major factor determining energy consumption is the dialysate concentration, sop should be as high as possible. The fact that the energy consumption
=: o/--o-o1.0
t
g c
0
.z a 0.5
E
/
I
0
i
E F i : 0 0
100
xx)
x0
concentration
Fig. 4. Energy consumption vs. concentration ratio
ratio
p
p.
is lower in the acidic solution with pH less than 3 is consistent with the higher electrical current efficiency associated with these solutions [ 21. Consequently, treating the effluent containing cupric chloride by electrodialysis is also attractive for its economy in power consumption. Operating stability The key to preventing metal hydroxide scaling on the membrane interface and the deposition of metals on the electrode surface is to treat solutions containing metallic ions such as ZIP+, Cu2+, Co2+, Ni2+ etc., with the electrodi-
oo+xlo~
concentration
mtio
p
Fig. 5. Energy consumption vs. concentration ratio
p.
257
alysis method. To ensure trouble free operation, the following steps should be taken: (1) The raw water should be loaded into the stack only after a thorough pretreatment. ( 2) Reasonable operating parameters should be chosen. ( 3 ) A suitable electrode rinse solution should be used. Thanks to these precautions, our electrodialysis plant has been in constant operation without giving any trouble. Post-treatment of the dialysate and the concentrated brine Dialysate with a cupric ion content of less than 40 mg/l can be either reused to wash the product or passes through a cation exchange column to be discharged. The cupric ion concentration in the column outlet is almost zero, with pH 6-7 during a run under normal conditions. The regeneration period of the resin is usually more than 60 h. It is also easy to recover metal copper from the concentrated brine with a cupric ion content of more than 20 g/l, at a total cost of 1 yuan per kg of metal copper. Thus the combined system is closed not only to eliminate sources of pollution, but also to facilitate copper recovery. We are now working on scaling up the process from pilot scale to commercial scale. CONCLUSIONS
The feasibility of the electrodialysis process for treating effluent which contains cupric chloride after the etching process has been confirmed from both a technical and an economic point of view. Home-made ion exchange membranes have been found suitable for desalination and concentration of cupric chloride solution. The suitability of the combined system consisting of electrodialysis, ion exchange and post-treatment has been demonstrated.
REFERENCES 1 Xue Deming, Liu Jingqing. Song Dezheng, Huang Linjin and Ding Hui, The study of treatment of wastewater containing zinc by electrodialysis, Technology of Water Treatment, 10 (1) (1984) 44. 2 Takatugu Azumi and Akio Yoneda, Himeji Kogyo Daigaku Kenkyn Hokoku, 30A (1977) 117-121.