Diffusion Dialysis

Chapter 6

6.1. OVERVIEW OF TECHNOLOGY

Diffusion dialysis is a separation process using the ionic diffusion causedby the concentration difference across a membrane. The phenomenon is gov-erned by the Fick’s law and the diffusion velocity is generally low, so that inorder to promote the process efficiency it becomes necessary to decrease mem-brane thickness and increase the membrane area. The feature of diffusiondialysis process using ion exchange membranes, however, is to utilize highmobility of H+ ions across an anion exchange membrane, and it is applied torecover acid from an electrolyte solution in the following instances (Itoi andMochida, 1985):

�

DOI

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 solutionincluding acid (feed) is supplied to the bottom of the feeding cell that flowsupward in the cell and flows out at the top of the cell (deacid). A low con-centration solution (water) is supplied to the top of the recovering cell that flowsdown 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 highmobility in the membrane. However, the salt transfer is restricted because ofDonnan exclusion due to the interaction between the salt cations and the func-tional groups (quaternary ammonium groups) in the membrane. At the steadystate the flux of acid or salt J (mol h�1) is defined by the following Fick’s law:

J ¼ US DCav (6.1)

where S (m2) is membrane area.

: 10.1016/S0927-5193(07)12020-9

Diffusion Dialysis 491

acids is relatively short. The process costs are, therefore, determined by costs andlife 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 thealuminum surface treatment by alkali etching and anodic oxidation. The solution inthe aluminum anodizing bath is taken out and treated in a diffusion dialyzer toseparate Al and H2SO4. Fig. 6.5 shows the material balance in the diffusion dialysisprocess operating in the aluminum sash manufacturing plant. In this process, apart 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 solutionin the dialyzer, which is returned to the anodizing bath with newly supplied H2SO4

to maintain constant acid concentration in the bath. The above mentioned processis developed by Asahi Glass Co., and it not only improves the product quality butalso decreases the quantities of new H2SO4 addition and alkali supplement forneutralization of discharged acid (Kawahara, 1984b).

Alkali recovery

H2SO4 recovery

Electrodeposition painting

Greese removing

Alkali etching

Anodic oxidation

Coloring

Figure 6.4 Aluminum sash manufacturing process (Kawahara, 1984b).

Deacid Water

H+

M+

A: Anion exchange membrane

A

Feeding cell

Recovering cell

C ′out

Q ′out

C ′′in = 0

Q ′′in

Feed RecoveryC ′inQ ′in

C ′′out

Q ′′out

Figure 6.1 Mass transport in diffusion dialysis.

Ion Exchange Membranes: Fundamentals and Applications488

U (mol (h m2)�1 (mol l�1)�1) in Eq. (6.1) is the overall dialysis coefficientof solutes (acid or salt) defined by

1

U¼

1

Kþ

1

k0þ

1

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 thefeeding and recovering surfaces of the membrane.

Table 6.1 shows the overall diffusion coefficient U measured for NeoceptaAFN (Noma, 1991). U is influenced by temperature and solute concentration in afeeding solution. Membranes having larger Uacid and smaller Usalt show excellentperformance in diffusion dialysis. The effects of acids on the degrees of acidpermeabilities are arranged as HCldHNO3>H2SO4>HF>H3PO4. Usalt for largercharge number is increased. Usalt for HNO3–Cu(NO3)2 and HNO3–Zn(NO3)2systems takes larger values because they form anionic complex salt.

DCav (mol l�1) in Eq. (6.1) is the average concentration difference ofsolutes between the feeding and the recovering cells.

DCav ¼ðC0

in � C00outÞ � C0

out

lnððC0in � C00

outÞ=C0outÞ

(6.3)

Table 6.1 Overall diffusion coefficient of Neocepta AFN at 251C

Aciddsalt mixture Acidconcentration

(N)

Saltconcentration

(N)

Uacid (mol/hm2)/(mol/l)

Usalt (mol/hm2)/(mol/l)

Usalt/Uacid

HCldNaCl 2.0 1.0 8.6 0.47 5.5� 10�2

HCldFeCl2 2.0 1.0 8.6 0.17 2.0� 10�2

HCldFeCl3 2.0 1.0 8.5 0.055 6.5� 10�3

H2SO4dNa2SO4 2.0 1.0 3.5 0.14 4.0� 10�2

H2SO4dFeSO4 2.0 1.0 3.6 0.037 1.03� 10�3

H2SO4dZnSO4 2.0 1.0 3.6 0.053 1.5� 10�2

H2SO4dAl2(SO4)3 2.0 1.0 3.6 0.004 1.1� 10�3

HNO3dAl(NO2)3 1.5 1.5 9.3 0.048 5.2� 10�3

HNO3dZn(NO3)2 1.5 1.5 9.8 0.14 1.4� 10�2

HNO3dCu(NO3)2 1.5 1.6 9.6 0.17 1.8� 10�2

H3PO4dMgHPO4 3.0 0.2 0.85 0.018 2.1� 10�2

Source: Noma (1991).

Diffusion Dialysis 489

The solute concentrations in the recovered solution C00out and in the deacid

solution C0out are expressed by the following equations:

C0out ¼ 1�

Q00in

Q0in

�USðð1=Q0

inÞ � ð1=Q00inÞÞ � 1

ðQ00in=Q

0inÞ expUSðð1=Q0

inÞ � ð1=Q00inÞÞ � 1

� �� C0

in

(6.4)

C00out ¼

expUSðð1=Q0inÞ � ð1=Q00

inÞÞ � 1

ðQ00in=Q

0inÞ expUSðð1=Q0

inÞ � ð1=Q00inÞÞ � 1

� C0in (6.5)

Membrane area S is obtained by the following equation introduced from Eq. (6.5):

S ¼1

Uðð1=Q0inÞ � ð1=Q00

inÞÞ� ln

1� Y

1� ðQ00in=Q

0inÞY

(6.6)

where Y ¼ C00out=C

0in (acid recovering ratio).

The performance of diffusion dialysis is given by the following equations:

Acid recovering ratio or salt moving ratio ¼C00

outQ00out

C0inQ

0in

(6.7)

Acid remaining ratio or salt remaining ratio ¼C0

outQ0out

C0inQ

0in

(6.8)

6.3. DIFFUSION DIALYZER AND ITS OPERATION

A practical scale diffusion dialyzer consists of gaskets (feeding and recov-ering 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

P P P

Deacid

Feed

Feed tank

Filter

Recovery

Recovery tank

Dialyzer

Water tank

Heater

Water

Head tankHead tank

Figure 6.3 Diffusion dialysis process flow (Kawahara, 1984a).

Feeding cell Feeding cellRecovering cell

Recovery

Water

Feed

Deacid

Membrane Membrane

Figure 6.2 Cell arrangement in a diffusion dialyzer (Kawahara, 1984a).

Ion Exchange Membranes: Fundamentals and Applications490

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 usingcontrol valves and head tanks adjusting the level to�2.5m. Flow resistance in thedialyzer is low because linear velocity is in the range of 50–300 cm h�1. In winterseason water is heated to prevent lowering of operating performance. When sub-stances in the feeding solution are precipitated on the membrane surface, the stackis 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 lowsince electric energy consumption is only for pumping the solutions through thestack. Operating process is quite simple and operating costs are low. The maincost factor is charges related to the capital investment which are considerably highbecause 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

Al 4720 g/h

275.5 l/h H2SO4 150 g/l 275.5 l/hAl 18 g/l

Aluminum anodizing bath 32.3 l/h

243.2 l/hH2SO4 127.4 g/lAl 1.0 g/l

H2SO4 35.7 g/lAl 16.3 g/l

98 % H2SO4

257 l/h Water

289.3 l/hDeacid

Diffusion dialyzer

H2SO4

Figure 6.5 Material balance in a H2SO4 recovering process by diffusion dialysis in a1000 t month�1 aluminum sash manufacturing factory (Kawahara, 1984b).

Ion Exchange Membranes: Fundamentals and Applications492

Running costs of the process are:

a.

Ion exchange renewal: 880,000 yen year�1. b. Electric power: 104,000 yen year�1. c. Others: 72,000 yen year�1.

Total: 1,056,000 yen year�1.Cost saving merits in chemical reagent consumption are:

a.

98% H2SO4 160 l year�1: 2,400,000 yen year�1. b. 48% NaOH 265 l year�1: 8,500,000 yen year�1.

Total: 10,900,000 yen year�1.

6.4.2 Recovery of Nitric Acid in an Acid Washing Process

Pretreatment in a plating process, surface treatment of stainless steel oretching treatment of electronic parts includes an acid washing process using acidsuch 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 grad-ually lowered. In order to prevent such a problem, Tokuyama Inc. developeddiffusion dialysis technology for recovering HNO3 from the acid washingsolution as shown in Fig. 6.6. The specifications and performance of the processare enumerated as follows (Motomura, 1986):

AirSteamWater

Water tank

To acidwashing tank

Head tank

Back flow pump

Dialyzer

Filter

Filtrate tank

Recoverytank

Wastetank

Flocculant precipitation tank

To drainage

Mud pump

Cooler

Feed tank

From acidwashing tank

Flocculanttank

Figure 6.6 Diffusion dialysis process for recovering HNO3 (Motomura, 1986).

100

90

80

70

543210

11983 1984month

2 3 4 5 6 7 8 9 10 11 12 13

Al Leak (%)

HNO3 Recovery (%)

Figure 6.7 Performance of HNO3 diffusion dialysis (Motomura, 1986).

Diffusion Dialysis 493

(1)

Diffusion dialysisa. Feeding solution

6.8 m3 day�1, HNO3 100 g l�1, Al(NO3)3 100 g l�1, SS 200–500ppm.

b. Diffusion dialyzerNeocepta TSD-50-400.

c. Process performanceSee Fig. 6.7.

Ion Exchange Membranes: Fundamentals and Applications494

(2)

Running cost and meritsa. Running cost

Ion exchange membrane renewal ¼ 75,000 yen month�1.Filter ¼ 33,000 yen month�1.Electric power ¼ 20,000 yen month�1.Water supply ¼ 12,000 yen month�1.Flocculant ¼ 4,000 yen month�1.Total ¼ 144,000 yen month�1.

b. MeritsHNO3 consumption per 1 t of steel material:Before diffusion dialysis adoption ¼ 365 kg t�1.After diffusion dialysis adoption ¼ 213 kg t�1.Steel material treated ¼ 100 t month�1.HNO3 unit cost (as 65% HNO3) ¼ 55 yen kg�1.Gain for recovering HNO3 ¼ (365� 213)kg t�1

� 100 tmonth�1

� 55yenkg�1¼ 836,000 yenmonth�1+58,000 yenmonth�1

(cost saving for neutralization agent Ca(OH)2) ¼ 894,000 yenmonth�1.

Accordingly, cost merit ¼ (894,000� 144,000) yen month�1� 12

month year�1¼ 9,000,000 yen month�1.

(3)

Maintenancea. Renewal of filter materials: one time during four to five months

(acid washing solution feeding side) and one time in a month (watersupplying 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 Sys-tem 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 Appli-cations, 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 Elect-rodialysis 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.