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Nanofiltration of nitric acidic solutions from picture tube production
DESALINATION Desalination
ELSEVIER
145 (2002) 65-68 www.elsevier.com/locate/desal
Nanofiltration of nitric acidic solutions from Picture Tube production Dirk Jakobs*, Goetz Baumgarten AMAFILTER Deutschland GmbH, Am Pferdemarkt II, 30853 Langenhagen, Germany Tel. +49 (511) 726099-o; Fax +49 (511) 726099-60; e-mail: d.jakobs@amafilter-hannovetzde Received 1 February 2002; accepted 6 March 2002
Abstract In this paper the pilot tests of a recycling process for removing lead form nitric acid solutions have been presented. At first batch tests were performed. Permeate yields up to 80% could be achieved in these tests. Corresponding to these yields the lead concentrations rose up to 70 g/l. Due to the saturation limit of lead salts at about 75 g/l no higher yields could be achieved without precipitation of the lead salts. Altering a washing step of the production avoided the input of free silicic acid into the nitric acid solution. This step ensured that there was no fouling of the membranes due to the precipitation of silicon dioxide. Continuous tests showed that the permeate flows and the lead rejection rates remained stable over a long period of time. After successful completion of the pilot tests the concept has been realized on technical scale. Using the nanofiltration it is now possible to recycle approximately 80-90% of the nitric acid, which formerly had to be exchanged. In addition to the reduced need for fresh acid for the etching process, the alkali needed for neutralizing the waste acid stream could also be drastically reduced. As an added benefit the nitrate load of the remaining waste water was also decreased. Keywords: Nanofiltration;
Nitric acid; Heavy metal ions; Lead
1. Introduction It is often the case that picture tubes can be deemed to be fault free only after a final functional test. If they fail this test the cone is separated *Corresponding
accumulation
author.
Presented at the International July 7-12, 2002.
from the screen and the pieces are recycled. The disassembly of the picture tubes is performed in a multistage etching process using 11 wt.% nitric acid. This process is characterized by a continuous
Congress on Membranes
and Membrane
of soluble Processes
0011-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO01 l-9164(02)00387-9
heavy (ICOM),
metal
salts in the
Toulouse, France,
D. Jakobs, G. Baumgarten
66
etching solution. As the salt concentration increases there is a concomitant decrease in the degree of disassembly of the product. Therefore the picture tubes have to be run through the etching bath several times and the complete acid solution must often be changed. In fact only 30% of the nitric acid can be used for these chemical reactions in this process, leading to the possibility of malfunctioning in the system. In order to precipitate the heavy metal salts out of the acid waste large amounts of chemicals are required. The aim of the membrane process to be developed for this application was a continual acid recycling. Thereby both the yield of the chemical etching reaction and the quality of the disassembly can be improved. The production capacity can also be extended by a continuous operation mode. 2. Pilot scale tests for nitric acid recycling The disassembly of the picture tubes is performed in two consecutive plants. In the first, pre-etching plant, most of the lead-glass solder between the screen and cone is etched away. After this treatment the tubes are cut off and the remaining solder and coating rests are removed in a second etching plant [l]. As shown in Fig. 1, only a rather small amount of the nitric acid is transferred in a chemical reaction to lead nitrate. More than two thirds of the acid needed is either lost or exchanged. Due to the accumulation of soluble salts, which have a negative impact on the separation process, only a small amount of the exchanged acid (61 m3/a) can be recycled. chemical reaction 67.8 m’,a
/Desalination
145 (2002)
65-68
To date the etching baths were completely refilled with fresh acid after a given time. Especially at the end of such acycle this batch operation mode results in quality problems as the increasing concentrations of metal ions cause a decreasing etching speed. Because of the drawbacks mentioned above, a continuous process for the nitric acid regeneration is economically feasible and productively desirable. High rejection rates for bivalent ions are characteristic for nearly all nanofiltration membranes, yet their use is limited by their instability to highly oxidizing mediums such as 11% nitric acid [2,3]. In laboratory scale screening tests a nanofiltration membrane was discovered, which has a high rejection for lead ions while maintaining stability against the oxidizing medium. Pilot tests were carried out with this membrane type in the plant shown in Fig. 2. Prefiltration of the nitric acid solution was performed with a filter cartridge. The filtrate was passed to a feed and working tank from where it was transported to the membrane module using a high pressure pump. The retentate, passing the 2.5” spiral wound element, was led back to the feed tank. For continuous testing, it is possible to split off a certain amount of the retentate stream and remove this concentrate from the system. Two different operation modes were investigated during the test period. Originally batch tests were made, i.e. the solution was concentrated after beginning with a given fixed feed volume. After these batch experiments continuous tests were carried out with different permeate yields to obtain information about the membrane performance over a longer time period. Prefiltratlon
\j(HNOJ=654ti/a c(HNOJ=ll% 584 mYa fresh. 61”?/a recycled
-
exchange 8 losses 450 5 m’,a
chemical reaction 135 7 rrlia
Fig. 1. Nitric acid needs for the picture tube assembly.
Feed
Membrane
Fig. 2. Flow chart of the pilot plant.
module
D. Jakobs, G Baumgarten
3. Batch
tests
In the batch tests the entire retentate was led back to the feed tank. Removing the permeate increases the salt concentration in the remaining solution, but these salts are rejected by the membrane. After reaching the desired yield the permeate stream was also recycled into the feed tank. In this manner steady state investigations at higher salt concentrations could be performed. As in all other tests the nitric acid solution used as feed in these experiments contained about 11 g/l of lead. The tests showed that even for high feed concentrations it was possible to achieve very high rejection rates for Pb2+-ions. As can be seen in Fig. 3 even if the feed contained 80 g/l lead almost 90% of the ions were rejected by the membrane. It is not possible to concentrate the solution any further. Further concentration overaturates the solution and lead salts precipitate, causing irreversible damage to the membrane module. In a first batch experiment the membrane was already completely blocked at a much lower lead concentration. A detailed examination of the damaged module identified silica scaling as the major cause of the module blocking. The coating layer of the picture tubes contained a certain amount of potassium silicate. If this layer is not completely removed, free silicic acid is generated and dissolved in the nitric acid. This silicic acid cannot permeate through the membrane and accumulates at the membrane surface. At higher
/ Desalination
67
14.5 (2002) 65-68
concentrations the formation of polycondensations occurs, finally leading to crystal silicon dioxide as the final product. Complete removal of the coating layer can be achieved by modifying the preceding washing step for the cones. In this manner the formation of free silicic acid can be avoided [l]. 4. Continuous
tests
In addition to the batch experiments described above, continuous tests have also been performed. In these tests for a given feed flow a fixed ratio of permeate and retentate was selected and both streams were removed from the system. The values given in Fig. 4 were observed at a temperature of 20°C and a transmembrane pressure difference of 30 bar. The permeate yield was set to 80% of the feed stream. At the beginning of the run the permeate flow rapidly declined. After some time it stabilized at 10 l/m2h. During the next days only a slight decline to 8 l/m2h was observed. If the yield was reduced to 50%, the permeate flow could be doubled to J,= 16 l/m2h. Neglecting some unavoidable variations, this value remained constant for several days. Rejection rates of lead ions which have been measured during these experiments were on the same order as those from the corresponding batch tests. Other tests (not shown) had no significant variations of permeate quality and quantity. 30
1
Pressure = 30 bar Temperature = 20-C 25 .-~~
Ii
WOki~WOX.Bo* 0
48
96
YUdtmn)rSOXr
144
192
240
Time [h]
Fig. 3. Rejection and permeate lead concentrations correlated to retentive concentrations.
Fig. 4. Time dependence of the permeate flow at two different yields.
D. Jakobs, G. Baumgarten / Desalination
68
145 (2002)
65-68
Therefore it can be assumed that the nanofiltration membrane used during these tests is stable for an extended period in the oxidizing medium. 5. Process
integration
The engineering of the technical plant is based upon the results of the pilot tests described in this paper. Fig. 5 shows the basic structure of the plant. In this figure the volumetric flows and lead loads processed in the technical plant are also shown. As can be seen in this figure, only half of the lead load leaving the final etching tank is removed from the system together with the concentrate using the nanofiltration process. However at permeate yields of 50% this load reduction is sufficient to limit the lead concentration in the recycled stream to 11 g/l. During the former operation mode the acid had to be exchanged after a certain time. This led to rather large lead concentrations at the end of such a cycle, corresponding to a reduced driving force of the etching process. Often more than one process run was needed to completely cut off the tubes. The efficiency of the etching baths has been increased by establishing the continuous operation
5 g/lPb*’ 2.25 kg!h Pb”
70 gnPb” 3.2 kglh Pd
Fig. 5. Basic flow chart of the technical plant.
concept presented here. In this steady state process almost no changes of either the acid or the lead concentrations occur. The picture tubes are completely disassembled in only one etching run.
References 111 F. Brounen and U. Schneider, ijkologieorientiertes Bilanzieren und Controllen der Bildrohrenfertigung, Abschlussbericht zum Industrieprojekt Nr. 113387 Philips GmbI-IEraunhofer IPT, Aachen, 1999. PI M. Mulder, Basic Principles of Membrane Technology, 2nd ed., Kluwer Academic Publishers, Dordrecht, 1997. 131 S.P. Nunes and K.-V. Peinemann, MembraneTechnology in the Chemical Industry, Wiley-VCH, Weinheim, 2001.
ELSEVIER
145 (2002) 65-68 www.elsevier.com/locate/desal
Nanofiltration of nitric acidic solutions from Picture Tube production Dirk Jakobs*, Goetz Baumgarten AMAFILTER Deutschland GmbH, Am Pferdemarkt II, 30853 Langenhagen, Germany Tel. +49 (511) 726099-o; Fax +49 (511) 726099-60; e-mail: d.jakobs@amafilter-hannovetzde Received 1 February 2002; accepted 6 March 2002
Abstract In this paper the pilot tests of a recycling process for removing lead form nitric acid solutions have been presented. At first batch tests were performed. Permeate yields up to 80% could be achieved in these tests. Corresponding to these yields the lead concentrations rose up to 70 g/l. Due to the saturation limit of lead salts at about 75 g/l no higher yields could be achieved without precipitation of the lead salts. Altering a washing step of the production avoided the input of free silicic acid into the nitric acid solution. This step ensured that there was no fouling of the membranes due to the precipitation of silicon dioxide. Continuous tests showed that the permeate flows and the lead rejection rates remained stable over a long period of time. After successful completion of the pilot tests the concept has been realized on technical scale. Using the nanofiltration it is now possible to recycle approximately 80-90% of the nitric acid, which formerly had to be exchanged. In addition to the reduced need for fresh acid for the etching process, the alkali needed for neutralizing the waste acid stream could also be drastically reduced. As an added benefit the nitrate load of the remaining waste water was also decreased. Keywords: Nanofiltration;
Nitric acid; Heavy metal ions; Lead
1. Introduction It is often the case that picture tubes can be deemed to be fault free only after a final functional test. If they fail this test the cone is separated *Corresponding
accumulation
author.
Presented at the International July 7-12, 2002.
from the screen and the pieces are recycled. The disassembly of the picture tubes is performed in a multistage etching process using 11 wt.% nitric acid. This process is characterized by a continuous
Congress on Membranes
and Membrane
of soluble Processes
0011-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO01 l-9164(02)00387-9
heavy (ICOM),
metal
salts in the
Toulouse, France,
D. Jakobs, G. Baumgarten
66
etching solution. As the salt concentration increases there is a concomitant decrease in the degree of disassembly of the product. Therefore the picture tubes have to be run through the etching bath several times and the complete acid solution must often be changed. In fact only 30% of the nitric acid can be used for these chemical reactions in this process, leading to the possibility of malfunctioning in the system. In order to precipitate the heavy metal salts out of the acid waste large amounts of chemicals are required. The aim of the membrane process to be developed for this application was a continual acid recycling. Thereby both the yield of the chemical etching reaction and the quality of the disassembly can be improved. The production capacity can also be extended by a continuous operation mode. 2. Pilot scale tests for nitric acid recycling The disassembly of the picture tubes is performed in two consecutive plants. In the first, pre-etching plant, most of the lead-glass solder between the screen and cone is etched away. After this treatment the tubes are cut off and the remaining solder and coating rests are removed in a second etching plant [l]. As shown in Fig. 1, only a rather small amount of the nitric acid is transferred in a chemical reaction to lead nitrate. More than two thirds of the acid needed is either lost or exchanged. Due to the accumulation of soluble salts, which have a negative impact on the separation process, only a small amount of the exchanged acid (61 m3/a) can be recycled. chemical reaction 67.8 m’,a
/Desalination
145 (2002)
65-68
To date the etching baths were completely refilled with fresh acid after a given time. Especially at the end of such acycle this batch operation mode results in quality problems as the increasing concentrations of metal ions cause a decreasing etching speed. Because of the drawbacks mentioned above, a continuous process for the nitric acid regeneration is economically feasible and productively desirable. High rejection rates for bivalent ions are characteristic for nearly all nanofiltration membranes, yet their use is limited by their instability to highly oxidizing mediums such as 11% nitric acid [2,3]. In laboratory scale screening tests a nanofiltration membrane was discovered, which has a high rejection for lead ions while maintaining stability against the oxidizing medium. Pilot tests were carried out with this membrane type in the plant shown in Fig. 2. Prefiltration of the nitric acid solution was performed with a filter cartridge. The filtrate was passed to a feed and working tank from where it was transported to the membrane module using a high pressure pump. The retentate, passing the 2.5” spiral wound element, was led back to the feed tank. For continuous testing, it is possible to split off a certain amount of the retentate stream and remove this concentrate from the system. Two different operation modes were investigated during the test period. Originally batch tests were made, i.e. the solution was concentrated after beginning with a given fixed feed volume. After these batch experiments continuous tests were carried out with different permeate yields to obtain information about the membrane performance over a longer time period. Prefiltratlon
\j(HNOJ=654ti/a c(HNOJ=ll% 584 mYa fresh. 61”?/a recycled
-
exchange 8 losses 450 5 m’,a
chemical reaction 135 7 rrlia
Fig. 1. Nitric acid needs for the picture tube assembly.
Feed
Membrane
Fig. 2. Flow chart of the pilot plant.
module
D. Jakobs, G Baumgarten
3. Batch
tests
In the batch tests the entire retentate was led back to the feed tank. Removing the permeate increases the salt concentration in the remaining solution, but these salts are rejected by the membrane. After reaching the desired yield the permeate stream was also recycled into the feed tank. In this manner steady state investigations at higher salt concentrations could be performed. As in all other tests the nitric acid solution used as feed in these experiments contained about 11 g/l of lead. The tests showed that even for high feed concentrations it was possible to achieve very high rejection rates for Pb2+-ions. As can be seen in Fig. 3 even if the feed contained 80 g/l lead almost 90% of the ions were rejected by the membrane. It is not possible to concentrate the solution any further. Further concentration overaturates the solution and lead salts precipitate, causing irreversible damage to the membrane module. In a first batch experiment the membrane was already completely blocked at a much lower lead concentration. A detailed examination of the damaged module identified silica scaling as the major cause of the module blocking. The coating layer of the picture tubes contained a certain amount of potassium silicate. If this layer is not completely removed, free silicic acid is generated and dissolved in the nitric acid. This silicic acid cannot permeate through the membrane and accumulates at the membrane surface. At higher
/ Desalination
67
14.5 (2002) 65-68
concentrations the formation of polycondensations occurs, finally leading to crystal silicon dioxide as the final product. Complete removal of the coating layer can be achieved by modifying the preceding washing step for the cones. In this manner the formation of free silicic acid can be avoided [l]. 4. Continuous
tests
In addition to the batch experiments described above, continuous tests have also been performed. In these tests for a given feed flow a fixed ratio of permeate and retentate was selected and both streams were removed from the system. The values given in Fig. 4 were observed at a temperature of 20°C and a transmembrane pressure difference of 30 bar. The permeate yield was set to 80% of the feed stream. At the beginning of the run the permeate flow rapidly declined. After some time it stabilized at 10 l/m2h. During the next days only a slight decline to 8 l/m2h was observed. If the yield was reduced to 50%, the permeate flow could be doubled to J,= 16 l/m2h. Neglecting some unavoidable variations, this value remained constant for several days. Rejection rates of lead ions which have been measured during these experiments were on the same order as those from the corresponding batch tests. Other tests (not shown) had no significant variations of permeate quality and quantity. 30
1
Pressure = 30 bar Temperature = 20-C 25 .-~~
Ii
WOki~WOX.Bo* 0
48
96
YUdtmn)rSOXr
144
192
240
Time [h]
Fig. 3. Rejection and permeate lead concentrations correlated to retentive concentrations.
Fig. 4. Time dependence of the permeate flow at two different yields.
D. Jakobs, G. Baumgarten / Desalination
68
145 (2002)
65-68
Therefore it can be assumed that the nanofiltration membrane used during these tests is stable for an extended period in the oxidizing medium. 5. Process
integration
The engineering of the technical plant is based upon the results of the pilot tests described in this paper. Fig. 5 shows the basic structure of the plant. In this figure the volumetric flows and lead loads processed in the technical plant are also shown. As can be seen in this figure, only half of the lead load leaving the final etching tank is removed from the system together with the concentrate using the nanofiltration process. However at permeate yields of 50% this load reduction is sufficient to limit the lead concentration in the recycled stream to 11 g/l. During the former operation mode the acid had to be exchanged after a certain time. This led to rather large lead concentrations at the end of such a cycle, corresponding to a reduced driving force of the etching process. Often more than one process run was needed to completely cut off the tubes. The efficiency of the etching baths has been increased by establishing the continuous operation
5 g/lPb*’ 2.25 kg!h Pb”
70 gnPb” 3.2 kglh Pd
Fig. 5. Basic flow chart of the technical plant.
concept presented here. In this steady state process almost no changes of either the acid or the lead concentrations occur. The picture tubes are completely disassembled in only one etching run.
References 111 F. Brounen and U. Schneider, ijkologieorientiertes Bilanzieren und Controllen der Bildrohrenfertigung, Abschlussbericht zum Industrieprojekt Nr. 113387 Philips GmbI-IEraunhofer IPT, Aachen, 1999. PI M. Mulder, Basic Principles of Membrane Technology, 2nd ed., Kluwer Academic Publishers, Dordrecht, 1997. 131 S.P. Nunes and K.-V. Peinemann, MembraneTechnology in the Chemical Industry, Wiley-VCH, Weinheim, 2001.