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Chapter 6 Diffusion Dialysis
Chapter 6
Diffusion Dialysis 6.1.
OVERVIEW OF TECHNOLOGY
Diffusion dialysis is a separation process using the ionic diffusion caused by the concentration difference across a membrane. The phenomenon is governed by the Fick’s law and the diffusion velocity is generally low, so that in order to promote the process efficiency it becomes necessary to decrease membrane thickness and increase the membrane area. The feature of diffusion dialysis process using ion exchange membranes, however, is to utilize high mobility of H+ ions across an anion exchange membrane, and it is applied to recover acid from an electrolyte solution in the following instances (Itoi and Mochida, 1985):
Treatment of waste solutions from an aluminum foil etching process. Composition control in an aluminum anodizing bath. Acid separation in a metallic rust removing process. Acid separation in a chemical reaction process. Acid concentration control in a metal surface treatment process. Purification of crude acid. Treatment of waste acid in a stainless steel washing process.
6.2.
TRANSPORT PHENOMENA IN DIFFUSION DIALYSIS
In the process illustrated in Fig. 6.1, a high concentration salt solution including acid (feed) is supplied to the bottom of the feeding cell that flows upward in the cell and flows out at the top of the cell (deacid). A low concentration solution (water) is supplied to the top of the recovering cell that flows down in the cell and flows out at the bottom of the cell (recovery). In this system, the acid transfers across the anion exchange membrane because of its high mobility in the membrane. However, the salt transfer is restricted because of Donnan exclusion due to the interaction between the salt cations and the functional groups (quaternary ammonium groups) in the membrane. At the steady state the flux of acid or salt J (mol h1) is defined by the following Fick’s law: J ¼ US DC av 2
where S (m ) is membrane area. DOI: 10.1016/S0927-5193(07)12020-9
(6.1)
491
Diffusion Dialysis
acids is relatively short. The process costs are, therefore, determined by costs and life of the membrane. 6.4.
PRACTICE
6.4.1
Composition Control in an Anodized Aluminum Processing Bath Fig. 6.4 gives an aluminum sash manufacturing process consisting of the aluminum surface treatment by alkali etching and anodic oxidation. The solution in the aluminum anodizing bath is taken out and treated in a diffusion dialyzer to separate Al and H2SO4. Fig. 6.5 shows the material balance in the diffusion dialysis process operating in the aluminum sash manufacturing plant. In this process, a part of an anodic solution in the bath is fed continuously to the diffusion dialyzer. 75–85% of H2SO4 in the feeding solution is altogether recovered from the solution in the dialyzer, which is returned to the anodizing bath with newly supplied H2SO4 to maintain constant acid concentration in the bath. The above mentioned process is developed by Asahi Glass Co., and it not only improves the product quality but also decreases the quantities of new H2SO4 addition and alkali supplement for neutralization of discharged acid (Kawahara, 1984b).
Greese removing
Alkali etching
Alkali recovery
Anodic oxidation
H2SO4 recovery
Coloring
Electrodeposition painting
Figure 6.4
Aluminum sash manufacturing process (Kawahara, 1984b).
488
Ion Exchange Membranes: Fundamentals and Applications
Deacid
C′out
C ′′in = 0
Q′out
Q′′in
Water
M+
H+ Feeding cell
Recovering cell
A Feed
C′in
C ′′out Recovery
Q ′′out Q′in A: Anion exchange membrane
Figure 6.1 Mass transport in diffusion dialysis.
U (mol (h m2)1 (mol l1)1) in Eq. (6.1) is the overall dialysis coefficient of solutes (acid or salt) defined by 1 1 1 1 ¼ þ þ U K k0 k00
(6.2)
where K is the diffusion coefficient for the anion exchange membrane. k0 and k00 are the diffusion coefficients for the boundary layer formed, respectively, on the feeding and recovering surfaces of the membrane. Table 6.1 shows the overall diffusion coefficient U measured for Neocepta AFN (Noma, 1991). U is influenced by temperature and solute concentration in a feeding solution. Membranes having larger Uacid and smaller Usalt show excellent performance in diffusion dialysis. The effects of acids on the degrees of acid permeabilities are arranged as HCldHNO3>H2SO4>HF>H3PO4. Usalt for larger charge number is increased. Usalt for HNO3–Cu(NO3)2 and HNO3–Zn(NO3)2 systems takes larger values because they form anionic complex salt. DCav (mol l1) in Eq. (6.1) is the average concentration difference of solutes between the feeding and the recovering cells. DC av ¼
ðC 0in C 00out Þ C 0out lnððC 0in C 00out Þ=C 0out Þ
(6.3)
489
Diffusion Dialysis
Table 6.1
Overall diffusion coefficient of Neocepta AFN at 251C
Aciddsalt mixture
Acid concentration (N)
Salt concentration (N)
Uacid (mol/ hm2)/(mol/l)
Usalt (mol/ hm2)/(mol/l)
Usalt/Uacid
HCldNaCl HCldFeCl2 HCldFeCl3 H2SO4dNa2SO4 H2SO4dFeSO4 H2SO4dZnSO4 H2SO4dAl2(SO4)3 HNO3dAl(NO2)3 HNO3dZn(NO3)2 HNO3dCu(NO3)2 H3PO4dMgHPO4
2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.5 1.5 1.5 3.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.5 1.5 1.6 0.2
8.6 8.6 8.5 3.5 3.6 3.6 3.6 9.3 9.8 9.6 0.85
0.47 0.17 0.055 0.14 0.037 0.053 0.004 0.048 0.14 0.17 0.018
5.5 102 2.0 102 6.5 103 4.0 102 1.03 103 1.5 102 1.1 103 5.2 103 1.4 102 1.8 102 2.1 102
Source: Noma (1991).
The solute concentrations in the recovered solution C 00out and in the deacid solution C 0out are expressed by the following equations: Q00in USðð1=Q0in Þ ð1=Q00in ÞÞ 1 0 C out ¼ 1 0 00 C 0in Qin ðQin =Q0in Þ exp USðð1=Q0in Þ ð1=Q00in ÞÞ 1 (6.4) C 00out ¼
exp USðð1=Q0in Þ ð1=Q00in ÞÞ 1 C 0in ðQ00in =Q0in Þ exp USðð1=Q0in Þ ð1=Q00in ÞÞ 1
(6.5)
Membrane area S is obtained by the following equation introduced from Eq. (6.5): S¼
1 1Y ln Uðð1=Q0in Þ ð1=Q00in ÞÞ 1 ðQ00in =Q0in ÞY
(6.6)
where Y ¼ C 00out =C 0in (acid recovering ratio). The performance of diffusion dialysis is given by the following equations: Acid recovering ratio or salt moving ratio ¼
C 00out Q00out C 0in Q0in
Acid remaining ratio or salt remaining ratio ¼
6.3.
C 0out Q0out C 0in Q0in
(6.7)
(6.8)
DIFFUSION DIALYZER AND ITS OPERATION
A practical scale diffusion dialyzer consists of gaskets (feeding and recovering cells) and anion exchange membranes (10–1000 sheets) as shown in Fig. 6.2. Fig. 6.3 gives the process flow which consists of mainly a diffusion dialyzer itself
490
Ion Exchange Membranes: Fundamentals and Applications
Deacid
Water
Recovery
Feed Feeding cell
Figure 6.2
Membrane Recovering cell
Membrane
Feeding cell
Cell arrangement in a diffusion dialyzer (Kawahara, 1984a).
Head tank
Head tank
Deacid
Feed Filter P Feed tank
Figure 6.3
Water
Dialyzer Recovery
P Recovery tank
P
Heater Water tank
Diffusion dialysis process flow (Kawahara, 1984a).
and a filter to remove sludge or oil slicks in the feeding solution (Kawahara, 1984a). The velocities of the feeding solution and water are controlled using control valves and head tanks adjusting the level to 2.5 m. Flow resistance in the dialyzer is low because linear velocity is in the range of 50–300 cm h1. In winter season water is heated to prevent lowering of operating performance. When substances in the feeding solution are precipitated on the membrane surface, the stack is disassembled and the membranes are washed about one to two times in a year. The dialyzer is operated continuously and stably. Energy consumption is very low since electric energy consumption is only for pumping the solutions through the stack. Operating process is quite simple and operating costs are low. The main cost factor is charges related to the capital investment which are considerably high because the diffusion of the acids is slow and a large membrane area is required. The useful membrane life under operating condition in an environment of strong
492
Ion Exchange Membranes: Fundamentals and Applications
Al 4720 g/h 275.5 l/h
H2SO4 150 g/l Al 18 g/l
275.5 l/h
Aluminum anodizing bath
32.3 l/h
243.2 l/h H2SO4 127.4 g/l Al 1.0 g/l 98 % H2SO4 H2SO4
257 l/h 289.3 l/h
Diffusion dialyzer
Water Deacid
H2SO4 35.7 g/l Al 16.3 g/l
Figure 6.5 Material balance in a H2SO4 recovering process by diffusion dialysis in a 1000 t month1 aluminum sash manufacturing factory (Kawahara, 1984b).
Running costs of the process are: a. b. c.
Ion exchange renewal: 880,000 yen year1. Electric power: 104,000 yen year1. Others: 72,000 yen year1. Total: 1,056,000 yen year1. Cost saving merits in chemical reagent consumption are:
a. b.
98% H2SO4 160 l year1: 2,400,000 yen year1. 48% NaOH 265 l year1: 8,500,000 yen year1. Total: 10,900,000 yen year1.
6.4.2
Recovery of Nitric Acid in an Acid Washing Process Pretreatment in a plating process, surface treatment of stainless steel or etching treatment of electronic parts includes an acid washing process using acid such as H2SO4, HCl, HNO3, HF, etc., or their mixed acid. In these processes, metal is dissolved into the acid solution and its washing performance is gradually lowered. In order to prevent such a problem, Tokuyama Inc. developed diffusion dialysis technology for recovering HNO3 from the acid washing solution as shown in Fig. 6.6. The specifications and performance of the process are enumerated as follows (Motomura, 1986):
493
Diffusion Dialysis
Flocculant precipitation tank
Head tank
Air Steam Water
Back flow pump
Flocculant tank
Water tank
Dialyzer From acid washing tank
Cooler To acid washing tank
Mud pump Filter
Feed tank
Filtrate tank
Figure 6.6
To drainage
Recovery tank
Waste tank
Diffusion dialysis process for recovering HNO3 (Motomura, 1986).
100
HNO3 Recovery (%)
90 80 70 5 4 3 2 1 0
Al Leak (%)
1 1983
Figure 6.7
(1)
2
3
4
5
6
7 month
8
9
10
11
12
13 1984
Performance of HNO3 diffusion dialysis (Motomura, 1986).
Diffusion dialysis a. Feeding solution 6.8 m3 day1, HNO3 100 g l1, Al(NO3)3 100 g l1, SS 200–500 ppm. b. Diffusion dialyzer Neocepta TSD-50-400. c. Process performance See Fig. 6.7.
494
(2)
(3)
Ion Exchange Membranes: Fundamentals and Applications
Running cost and merits a. Running cost Ion exchange membrane renewal ¼ 75,000 yen month1. Filter ¼ 33,000 yen month1. Electric power ¼ 20,000 yen month1. Water supply ¼ 12,000 yen month1. Flocculant ¼ 4,000 yen month1. Total ¼ 144,000 yen month1. b. Merits HNO3 consumption per 1 t of steel material: Before diffusion dialysis adoption ¼ 365 kg t1. After diffusion dialysis adoption ¼ 213 kg t1. Steel material treated ¼ 100 t month1. HNO3 unit cost (as 65% HNO3) ¼ 55 yen kg1. Gain for recovering HNO3 ¼ (365 213) kg t1 100 t month1 55 yen kg1 ¼ 836,000 yen month1+58,000 yen month1 (cost saving for neutralization agent Ca(OH)2) ¼ 894,000 yen month1. Accordingly, cost merit ¼ (894,000 144,000) yen month1 12 month year1 ¼ 9,000,000 yen month1. Maintenance a. Renewal of filter materials: one time during four to five months (acid washing solution feeding side) and one time in a month (water supplying side). b. Heat exchanger washing: one time during five to six months. c. Diffusion dialyzer: no disassembly and no washing during two years.
REFERENCES Itoi, S., Mochida, M., 1985, Present status of the dialysis technique, Ionics, Ionics Co., Tokyo, No. 120, pp. 171–176. Kawahara, T., 1984a, Industrial diffusion dialyzer, In: Shimizu H., Nishimura, M. (Eds.), The Latest Membrane Treatment Technology and its Applications, Fuji Techno System Co., Tokyo, pp. 248–252. Kawahara, T., 1984b, Recovery of waste acid by diffusion dialysis, In: Shimizu, H., Nishimura, M. (Eds.), The Latest Membrane Treatment Technology and its Applications, Fuji Techno System Co., Tokyo, pp. 455–463. Motomura, H., 1986, Recovery of nitric acid and fluoric nitric acid by diffusion dialysis, In: Industrial Application of Ion Exchange Membranes, Vol. 1, Research Group of Electrodialysis and Membrane Separation Technology, Soc. Sea Water Sci., Jpn., 223–233. Noma, Y., 1991, Diffusion dialysis membrane, In: Nakagaki, M., Shimizu, H. (Eds.), Membrane Treatment Technology, Part I, Fundamentals, Fuji Techno System Co., Tokyo, pp. 174–179.
Diffusion Dialysis 6.1.
OVERVIEW OF TECHNOLOGY
Diffusion dialysis is a separation process using the ionic diffusion caused by the concentration difference across a membrane. The phenomenon is governed by the Fick’s law and the diffusion velocity is generally low, so that in order to promote the process efficiency it becomes necessary to decrease membrane thickness and increase the membrane area. The feature of diffusion dialysis process using ion exchange membranes, however, is to utilize high mobility of H+ ions across an anion exchange membrane, and it is applied to recover acid from an electrolyte solution in the following instances (Itoi and Mochida, 1985):
Treatment of waste solutions from an aluminum foil etching process. Composition control in an aluminum anodizing bath. Acid separation in a metallic rust removing process. Acid separation in a chemical reaction process. Acid concentration control in a metal surface treatment process. Purification of crude acid. Treatment of waste acid in a stainless steel washing process.
6.2.
TRANSPORT PHENOMENA IN DIFFUSION DIALYSIS
In the process illustrated in Fig. 6.1, a high concentration salt solution including acid (feed) is supplied to the bottom of the feeding cell that flows upward in the cell and flows out at the top of the cell (deacid). A low concentration solution (water) is supplied to the top of the recovering cell that flows down in the cell and flows out at the bottom of the cell (recovery). In this system, the acid transfers across the anion exchange membrane because of its high mobility in the membrane. However, the salt transfer is restricted because of Donnan exclusion due to the interaction between the salt cations and the functional groups (quaternary ammonium groups) in the membrane. At the steady state the flux of acid or salt J (mol h1) is defined by the following Fick’s law: J ¼ US DC av 2
where S (m ) is membrane area. DOI: 10.1016/S0927-5193(07)12020-9
(6.1)
491
Diffusion Dialysis
acids is relatively short. The process costs are, therefore, determined by costs and life of the membrane. 6.4.
PRACTICE
6.4.1
Composition Control in an Anodized Aluminum Processing Bath Fig. 6.4 gives an aluminum sash manufacturing process consisting of the aluminum surface treatment by alkali etching and anodic oxidation. The solution in the aluminum anodizing bath is taken out and treated in a diffusion dialyzer to separate Al and H2SO4. Fig. 6.5 shows the material balance in the diffusion dialysis process operating in the aluminum sash manufacturing plant. In this process, a part of an anodic solution in the bath is fed continuously to the diffusion dialyzer. 75–85% of H2SO4 in the feeding solution is altogether recovered from the solution in the dialyzer, which is returned to the anodizing bath with newly supplied H2SO4 to maintain constant acid concentration in the bath. The above mentioned process is developed by Asahi Glass Co., and it not only improves the product quality but also decreases the quantities of new H2SO4 addition and alkali supplement for neutralization of discharged acid (Kawahara, 1984b).
Greese removing
Alkali etching
Alkali recovery
Anodic oxidation
H2SO4 recovery
Coloring
Electrodeposition painting
Figure 6.4
Aluminum sash manufacturing process (Kawahara, 1984b).
488
Ion Exchange Membranes: Fundamentals and Applications
Deacid
C′out
C ′′in = 0
Q′out
Q′′in
Water
M+
H+ Feeding cell
Recovering cell
A Feed
C′in
C ′′out Recovery
Q ′′out Q′in A: Anion exchange membrane
Figure 6.1 Mass transport in diffusion dialysis.
U (mol (h m2)1 (mol l1)1) in Eq. (6.1) is the overall dialysis coefficient of solutes (acid or salt) defined by 1 1 1 1 ¼ þ þ U K k0 k00
(6.2)
where K is the diffusion coefficient for the anion exchange membrane. k0 and k00 are the diffusion coefficients for the boundary layer formed, respectively, on the feeding and recovering surfaces of the membrane. Table 6.1 shows the overall diffusion coefficient U measured for Neocepta AFN (Noma, 1991). U is influenced by temperature and solute concentration in a feeding solution. Membranes having larger Uacid and smaller Usalt show excellent performance in diffusion dialysis. The effects of acids on the degrees of acid permeabilities are arranged as HCldHNO3>H2SO4>HF>H3PO4. Usalt for larger charge number is increased. Usalt for HNO3–Cu(NO3)2 and HNO3–Zn(NO3)2 systems takes larger values because they form anionic complex salt. DCav (mol l1) in Eq. (6.1) is the average concentration difference of solutes between the feeding and the recovering cells. DC av ¼
ðC 0in C 00out Þ C 0out lnððC 0in C 00out Þ=C 0out Þ
(6.3)
489
Diffusion Dialysis
Table 6.1
Overall diffusion coefficient of Neocepta AFN at 251C
Aciddsalt mixture
Acid concentration (N)
Salt concentration (N)
Uacid (mol/ hm2)/(mol/l)
Usalt (mol/ hm2)/(mol/l)
Usalt/Uacid
HCldNaCl HCldFeCl2 HCldFeCl3 H2SO4dNa2SO4 H2SO4dFeSO4 H2SO4dZnSO4 H2SO4dAl2(SO4)3 HNO3dAl(NO2)3 HNO3dZn(NO3)2 HNO3dCu(NO3)2 H3PO4dMgHPO4
2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.5 1.5 1.5 3.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.5 1.5 1.6 0.2
8.6 8.6 8.5 3.5 3.6 3.6 3.6 9.3 9.8 9.6 0.85
0.47 0.17 0.055 0.14 0.037 0.053 0.004 0.048 0.14 0.17 0.018
5.5 102 2.0 102 6.5 103 4.0 102 1.03 103 1.5 102 1.1 103 5.2 103 1.4 102 1.8 102 2.1 102
Source: Noma (1991).
The solute concentrations in the recovered solution C 00out and in the deacid solution C 0out are expressed by the following equations: Q00in USðð1=Q0in Þ ð1=Q00in ÞÞ 1 0 C out ¼ 1 0 00 C 0in Qin ðQin =Q0in Þ exp USðð1=Q0in Þ ð1=Q00in ÞÞ 1 (6.4) C 00out ¼
exp USðð1=Q0in Þ ð1=Q00in ÞÞ 1 C 0in ðQ00in =Q0in Þ exp USðð1=Q0in Þ ð1=Q00in ÞÞ 1
(6.5)
Membrane area S is obtained by the following equation introduced from Eq. (6.5): S¼
1 1Y ln Uðð1=Q0in Þ ð1=Q00in ÞÞ 1 ðQ00in =Q0in ÞY
(6.6)
where Y ¼ C 00out =C 0in (acid recovering ratio). The performance of diffusion dialysis is given by the following equations: Acid recovering ratio or salt moving ratio ¼
C 00out Q00out C 0in Q0in
Acid remaining ratio or salt remaining ratio ¼
6.3.
C 0out Q0out C 0in Q0in
(6.7)
(6.8)
DIFFUSION DIALYZER AND ITS OPERATION
A practical scale diffusion dialyzer consists of gaskets (feeding and recovering cells) and anion exchange membranes (10–1000 sheets) as shown in Fig. 6.2. Fig. 6.3 gives the process flow which consists of mainly a diffusion dialyzer itself
490
Ion Exchange Membranes: Fundamentals and Applications
Deacid
Water
Recovery
Feed Feeding cell
Figure 6.2
Membrane Recovering cell
Membrane
Feeding cell
Cell arrangement in a diffusion dialyzer (Kawahara, 1984a).
Head tank
Head tank
Deacid
Feed Filter P Feed tank
Figure 6.3
Water
Dialyzer Recovery
P Recovery tank
P
Heater Water tank
Diffusion dialysis process flow (Kawahara, 1984a).
and a filter to remove sludge or oil slicks in the feeding solution (Kawahara, 1984a). The velocities of the feeding solution and water are controlled using control valves and head tanks adjusting the level to 2.5 m. Flow resistance in the dialyzer is low because linear velocity is in the range of 50–300 cm h1. In winter season water is heated to prevent lowering of operating performance. When substances in the feeding solution are precipitated on the membrane surface, the stack is disassembled and the membranes are washed about one to two times in a year. The dialyzer is operated continuously and stably. Energy consumption is very low since electric energy consumption is only for pumping the solutions through the stack. Operating process is quite simple and operating costs are low. The main cost factor is charges related to the capital investment which are considerably high because the diffusion of the acids is slow and a large membrane area is required. The useful membrane life under operating condition in an environment of strong
492
Ion Exchange Membranes: Fundamentals and Applications
Al 4720 g/h 275.5 l/h
H2SO4 150 g/l Al 18 g/l
275.5 l/h
Aluminum anodizing bath
32.3 l/h
243.2 l/h H2SO4 127.4 g/l Al 1.0 g/l 98 % H2SO4 H2SO4
257 l/h 289.3 l/h
Diffusion dialyzer
Water Deacid
H2SO4 35.7 g/l Al 16.3 g/l
Figure 6.5 Material balance in a H2SO4 recovering process by diffusion dialysis in a 1000 t month1 aluminum sash manufacturing factory (Kawahara, 1984b).
Running costs of the process are: a. b. c.
Ion exchange renewal: 880,000 yen year1. Electric power: 104,000 yen year1. Others: 72,000 yen year1. Total: 1,056,000 yen year1. Cost saving merits in chemical reagent consumption are:
a. b.
98% H2SO4 160 l year1: 2,400,000 yen year1. 48% NaOH 265 l year1: 8,500,000 yen year1. Total: 10,900,000 yen year1.
6.4.2
Recovery of Nitric Acid in an Acid Washing Process Pretreatment in a plating process, surface treatment of stainless steel or etching treatment of electronic parts includes an acid washing process using acid such as H2SO4, HCl, HNO3, HF, etc., or their mixed acid. In these processes, metal is dissolved into the acid solution and its washing performance is gradually lowered. In order to prevent such a problem, Tokuyama Inc. developed diffusion dialysis technology for recovering HNO3 from the acid washing solution as shown in Fig. 6.6. The specifications and performance of the process are enumerated as follows (Motomura, 1986):
493
Diffusion Dialysis
Flocculant precipitation tank
Head tank
Air Steam Water
Back flow pump
Flocculant tank
Water tank
Dialyzer From acid washing tank
Cooler To acid washing tank
Mud pump Filter
Feed tank
Filtrate tank
Figure 6.6
To drainage
Recovery tank
Waste tank
Diffusion dialysis process for recovering HNO3 (Motomura, 1986).
100
HNO3 Recovery (%)
90 80 70 5 4 3 2 1 0
Al Leak (%)
1 1983
Figure 6.7
(1)
2
3
4
5
6
7 month
8
9
10
11
12
13 1984
Performance of HNO3 diffusion dialysis (Motomura, 1986).
Diffusion dialysis a. Feeding solution 6.8 m3 day1, HNO3 100 g l1, Al(NO3)3 100 g l1, SS 200–500 ppm. b. Diffusion dialyzer Neocepta TSD-50-400. c. Process performance See Fig. 6.7.
494
(2)
(3)
Ion Exchange Membranes: Fundamentals and Applications
Running cost and merits a. Running cost Ion exchange membrane renewal ¼ 75,000 yen month1. Filter ¼ 33,000 yen month1. Electric power ¼ 20,000 yen month1. Water supply ¼ 12,000 yen month1. Flocculant ¼ 4,000 yen month1. Total ¼ 144,000 yen month1. b. Merits HNO3 consumption per 1 t of steel material: Before diffusion dialysis adoption ¼ 365 kg t1. After diffusion dialysis adoption ¼ 213 kg t1. Steel material treated ¼ 100 t month1. HNO3 unit cost (as 65% HNO3) ¼ 55 yen kg1. Gain for recovering HNO3 ¼ (365 213) kg t1 100 t month1 55 yen kg1 ¼ 836,000 yen month1+58,000 yen month1 (cost saving for neutralization agent Ca(OH)2) ¼ 894,000 yen month1. Accordingly, cost merit ¼ (894,000 144,000) yen month1 12 month year1 ¼ 9,000,000 yen month1. Maintenance a. Renewal of filter materials: one time during four to five months (acid washing solution feeding side) and one time in a month (water supplying side). b. Heat exchanger washing: one time during five to six months. c. Diffusion dialyzer: no disassembly and no washing during two years.
REFERENCES Itoi, S., Mochida, M., 1985, Present status of the dialysis technique, Ionics, Ionics Co., Tokyo, No. 120, pp. 171–176. Kawahara, T., 1984a, Industrial diffusion dialyzer, In: Shimizu H., Nishimura, M. (Eds.), The Latest Membrane Treatment Technology and its Applications, Fuji Techno System Co., Tokyo, pp. 248–252. Kawahara, T., 1984b, Recovery of waste acid by diffusion dialysis, In: Shimizu, H., Nishimura, M. (Eds.), The Latest Membrane Treatment Technology and its Applications, Fuji Techno System Co., Tokyo, pp. 455–463. Motomura, H., 1986, Recovery of nitric acid and fluoric nitric acid by diffusion dialysis, In: Industrial Application of Ion Exchange Membranes, Vol. 1, Research Group of Electrodialysis and Membrane Separation Technology, Soc. Sea Water Sci., Jpn., 223–233. Noma, Y., 1991, Diffusion dialysis membrane, In: Nakagaki, M., Shimizu, H. (Eds.), Membrane Treatment Technology, Part I, Fundamentals, Fuji Techno System Co., Tokyo, pp. 174–179.