Toble Ill. Specificutions of commercial cell for f ield tests _--____-- -_I_

li. p p e of cell DCM 406 Anode compartment Titanium Cathode compartment Anode . DSAR Cathode Ni-based composite electroplating

on Ty JC 304 stainless steel me::h EffectiVf area of anode

Membranes S l X sheets O f h T a f i o J ~ ~ NS90209 Hated current 60 kA Opcrated current 40-60 kA Operated current density 22-3.3. kNm2

Type 3105 stainless steel

18.18 111’ and C>ltll(JdC

of high quality at high concentration (2,3). The perform- ance of the membrane-type chlor-alltali plant mainly de- pends on the cell design, the membrane, the quality of feed brine, and operating conditions. Of these, the brine quality is the key factor affecting the service life and the perform-

In the case of conversion from amalgam to membrane process, it is important to elucidate the effects of mercury in the brinc used in the amalgam process plant on the per- formance of chelate resin and membrane cell. This is nec- essary, to know whether it is possible to use the existing brine process facilities and the brine itself in the mem- brane process plant. If not, new brine process facilities and :; * Electrochemical Society Active Member.

ability. The closed points in the figure show the cathode potential in a laboratory cell. The potential in the commer- cial cell accords well with the laboratory results. On the other hand, the iron content in the activated cathode of the commercial cell was increased by electrolysis as shown by the triangular points in the figure. The figure illustrates the fact that the higher the iron content is the higher is the cathode overvoltage, an effect probably due to deac- tivation of the.coated cathode caused by iron powder de- posits.

Conclusion An activated cathode composed of electroplated Ni-

based composite has been developed and applied since 1984 in commercial-scale membrane cells for producing chlorine and caustic soda. At present the total installed production capacity in 12 plants is about 35,200 metric tons NaGH per month.

The cathode overvoltage, and hence the cell voltage, is low and stable for thousands of days on line. The cathode is unaffected by mercuric ions and the effect of ferrate ions in the electrolyte is limited to an acceptable level. The cathode is also resistant to anodic oxidation caused by short-circuiting of the cell.

The Ni-CE cathode is relatively cheap because inex- pensive raw materials are used and it is easy to manu- facture. ,

”*.. * I I I I ” ” I ““b,c;,y, 1111;. 1423

Acknowledgments The authors would like to thank Professor F. Hine :,f

Nagoya Institute of Tcchnologp for helpful comments I: 7 this paper.

Manuscript submitted June 2. 1989; revised inanuscr’ reccived Nov. 13, 1989. This \vas Papel, 370 prescntcd at t Los Angeles, CA, Meeting of the Society, May 7-12, 1989

nweting the pziblication costs of this article. Chlorine Engitteers C_cj,pocj,ation, I,imik?d, assisted .::

REFERENCES 1. K. Yamaguchi and I. Kumagai, “I)cvelopment %.f

Large Monopole Type Membrane Electrolyzer,” p-- sented at the 1988 London International Chlori Symposium, Electrochemical Technology Group the Society of Chemical Industry, London.

2. M. Seko, A. Yoniiyama. and S . Ggawa, in “Modc Chlor-Alkali Technology,” Vol. 2, C. Jackson, E tor, p. 97, Ellis Horwood, Chichester, England (i9SS .

3. US. Pat. 3,711,385 and 3,G32,498. 4. W. G. Grot, Cheir~.-lng.-Tecl~., 44, 167 (1972). 5. D. L. Caldwcll, in “Comprehensive Treatise of Electri--

chemistry, Val: 2, p. 105, J. O’M. Bockris, e. E. Cor.. way, E. Yeager, and R. E. White, Editors, Pleiiur. Press, New York (1981).

6. J. V. Winings and D. H. Porter, in “Modern Chlor- Alkali Technology,” M. 0. Coulter, Editor, p. 3:. Ellis Horwood, Chichester, England (1 980).

7. T. A. Liederbach, A.M. Greenberg, and V. H. Thomas. ibid., p. 145.

8. F. Hine, Soda to Enso (Soda and Chlorine), 38, 351 anc 397 (1987).

9. F. Hine, M. Yasuda, and M. Watanabe, Denki Kagakr.. 47,401 (1979).

10. H. E. G. Rommal and P. J. Moron, This Journal, 132. 325 (1985).

11. A. Nidola and R. Schira, Abstract 429, p. 612, The Elei- trochemical Society Meeting, Toronto, Gnt., Canad; Mav 12-17. 1985.

12. U S . Fat. 4,496,442. 13. F. Hine, “Electrochemical Processes and Elec.trochem-

ical Engineering,” p. 393, Plenum Press, New Yor;: (1985).

Effects of Mercury in Brine on the Performance of the Membrane-Type Chior-Alkali Plant

I 1 - _ _.

Kenzo Yomaguchi*

Chlorine Eizgzneers Corporation, Limited, 1 -I , Toranomon 2-cho7ne, Minato-ku, Tokyo 105, Japan

ABSTRACT

. .. _i

. L ‘ 1424 d -‘k j J. Electrochem. SOC., Vol. 137, No. 5, May 1990 0 The Electrochemical Society, Inc.

new brine, which are very expensive, must be prepared for the process conversion.

The effects of alkaline earth ions in feed brine on the per- formance of membrane have been investigated by several authors. On the other hand, brine contaminated with mer- cury has scarcely been discussed (4-9).

For the conversion from amalgam to membrane process, the brine for amalgam process containing calcium, magne- sium, and mercury has to be treated in the secondary brine purification system. Mercury in the brine can be removed almost completely by chelate resin, but the effect on the adsorption capacity and the irreversible deactivation of chelate resin is a concern because of the large equilibrium constant.

The effects of mercury in feed brine on the performance of membrane cell and cell parts are also a concern, in refer- ence to papers related to the effects of impurities in brine (7-8). In this paper, the effects of mercury in brine on the performance of the chelate resin and the membrane cell are discussed both through the laboratory and pilot plant tests, and industrial applications for the conversion from amalgam to membrane process.

Experimental Procedures I

Effects of mercury on the chelate resin.-Wo kinds of chelate resin, aminophosphoric acid (APA) type and imi- nodiacetic acid (IAI) type resins, were examined. Thirty ml of each resin was packed in 50-ml glass tubes for the labo- ratory experiment. The brine for these tests was specially prepared by dissolving extra-pure salt in deionized then distilled water and by the addition of a reasonable amount of reagent-grade CaC12, MgCI2, and HgClz in order to get specidcations similar to those in the existing amalgam- type process plant (for detailed specifications, see Table I).

To know the accumulation of mercury in the resin and to decide on the necessary number of tests, preliminary ad- sorption tests for calcium, magnesium, and mercury using the make-up brine at a S V (space velocity) = 10h-’, fol- lowed by regeneration of the resin at the conditions shown in Table 11, were repeated 20 times, and the mercury ad- sorbed in the resin was measured. As shown in Fig. 1, the accumulation of mercury in both types of resin gradually increased during adsorption-regeneration and reached a steady state after 9-10 cycles. Even so, the resin was still

- able to adsorb calcium and magnesium ions. In other words, both Types APA and IAA chelate resins are suit- able for the secondary brine purification system of the brine containing mercury as well as magnesium and calcium.

Detailed tests of adsorption and regeneration for these two resins were repeated nine times to elucidate the ef- fects of mercury in brine on the adsorption capacity of cal- cium and magnesium in the resins, and to investigate their durability under plant operating conditions. In each step of the tests, the mercury contents in the feed brine, the treated brine, and the regeneration solutions were meas- ured to determine the hold-up of mercury in the resins.

__ -_ .

/

Table 1. Composition of the feed brine

To the To the resin column membrane cell

NaC1, gn 300 300 Ca. mgn 19.6 11.7 Mg, mgll 1.3 1.2 Hg, mgn 13 (5-20)a PH 10 3-4

Note: HgClz was periodically added to the feed brine at the specified level as Hg2 .

Table II. Regeneration conditions

Volume Chemicals ratio Period Washing

Table 111. Specifications of-pilot membrane cells

Cell type DCM 406 Rated current load, kA 60 Number of unit cells 6 Effective area, m2 18.18

Cathode compartment Cathode coating

Rated current density, kA/m2 3.3 Type 3105 stainless steel

Activated with Ni-based composite Anode compartment Anode coating Membrane

Titanium DSAR

NafionR 90209

Adsorption tests with the make-up brine containing cal- cium and magnesium ions, but free of mercury, were con- ducted to compare the difference in adsorption capacity of the resin with and without mercury in the brine. The resin is typically regenerated with dilute HCl, say 2N, and de-

Effects of mercury on the performance of la membrane cells.-A rectangular laboratory cell,

distilled water. A reasonable amount of reagent-grade CaC12 and MgClz was added to th_e make-up brine for these tests (for detailed specifications, see Table I). The anolyte and the catholyte were fed to the respective compartments of cells from level tanks about one meter above the cells, and effluents were discharged to reservoirs by gravity, so that the solution compositions were almost unchanged by electrolysis during the experiment. During the operation of the two kinds of laboratory cells, one with the stainless steel mesh cathode and the other with the activated cath- ode, mercury in the form of HgClz was periodically added to the feed brine at the level of 20 ppm as Hg2+ to investi- gate the effects of mercury on the cell voltage and the cur- rent efficiency of the membrane cells. A membrane, util- ized for the test operation with mercury-contaminated brine for 72 days, was immersed in 2N HC1 solution at 90°C for 16h to desorb the accumulated mercury.

’ NafionR is the registered trademark of E. I. du Pont de Nemours and Company for its NafionR perfluorinated membranes.

10 20 30

Number of cycles Desorption: 2N HCl 5 2h Water Neutralization: 2N NaOH 5 2h Water

Fig. 1 . Amount of mercury accumulated in the resins through the ad- sorption and regeneration cycles. 6

iety, Inc..

5 r

4 -

3 -

2 -

1 -

0 .

. '

Resin: APA 6 Cycles

- Hg - c a - - - Mg

Resin: APA 1 Cycle ,

-_ I --

/ , L K ~ , & ! & : z n

4eel om p o s i t e

Resin: APA 9 Cycles

-/-

I I

ning cal- Jere con- pacity of rhe resin 1 and de- sins with Lhe same

5 - Resin: IAA 1 Cycle

/ 4 -

2

0 3 : f 1 I I $/ 1

boratory , 100" doyed. A I cathode roplated) ive com- rane was Je area of

Resin: IAA 6 Cycles

i : < I I I

3m extra- ized then ent-grade for these e anolytc Iartment: the cells ravity, s langed Is operatic stainlei,

ated cati illy adde to invest d the cu rane, ut; taminate on at 90'

d o t men secondar ~ S S as Ca:

le Nemou: 'S.

! : I 33

trough the od-

J. Electrochem. SOC., VOl. 137, NO. 5, May 1990 0 The Electrochemical Society, Inc.

3 , '

1

and free from mercury, was fed to the membrane cell in a pilot plant. The specifications of the membrane pilot cell are shown in Table 111. After 200 days of operation, HgClz was added to the feed brine at the level of 5-10 ppm as HgZt, and electrolysis was continued for another 120 days to learnthe effects of mercury on the membrane cell per- formance in a long term operation.

fied brine in the amalgam process plant was sent to the secondary brine purification system without removal of the mercury in the brine, and then it was fed to the mem- brane cell in the pilot plant, to confirm the effects of mer- cury on the performance of the chelate resin and the mem- brane cell.

3 Once-through tests in a pilot plant.-A part of the puri-

o

I Results and Discussion Effects of mercury on the chelate resin.-The break-

through curves of the columns packed with Types APA and IAA resins are shown on the top and the bottom graphs of Fig. 2, respectively. The ordinate is the leakage of ionic impurities, and the abscissa the bed volume. As shown in Fig. 2(a), mercury was undetected in the effluent ofthe column packed with virgin resin in the early stages. However, the mercury content in the effluent increased with the bed volumes greater than some 150 l/l-resin. The leakage of mercury increased with the repetition of ad- sorption-regeneration as shown in (b) and (c) of Fig. 2,until the mercury content in the effluent remained unchanged with the bed volume after 9 cycles, since the resin was sat- urated with mercuric ions as shown in Fig. 1.

However, it is important that calcium and magnesium ions did not leak at bed volumes lower than some 300-350 in Type APA resin and some 180-200 in Type IAA resin and the adsorption of alkaline earth metals was almost inde- pendent of the mercury content in the resin, though its ad- sorption capacity slightly decreased with the repetition of adsorption-regeneration.

The bed volume of Type IAA resin for Ca and Mg was small compared to that of Type APA resin [see (d), (e), and (f) of Fig. 21, but the effects of mercury on the performance of Type IAA resin seems to be similar. Table IV shows the results of these experiments.

It is clear that only a small fraction of adsorbed mercury in Type APA resin is dissolved by HCI, and hence the resin gradually becomes saturated with mercury. The ad- sorption of mercury is thus decreased. Calcium and mag-

,

1425

Table IV. Impurities odsorbed ond desorbed by 2N HCI (unit: mg)

APA IAA Type of resin Adsorbed Desorbed Adsorbed Desorbed

1st stage Ca2' 257.0 Mg2+ 18.5

6thstage Ca2* 264.9 Mg2' 15.1 Hg2' 16.0

9th stage Ca2' 213.8 Mg2+ 14.4 Hg2+ 11.6

HgZ+ 79.7

~ ~~ ~

232.0 254.0 215.0 16.8 13.0 12.2 1.0 33.7 0.7

251.2 140.1 137.3 14.1 8.4 7.9 7.0 23.0 22.7

210.5 143.6 136.1 13.8 9.2 8.8 3.6 24.2 23.0

nesium are adsorbed by Type APA resin, and dissolved al- most completely by dilute HC1 even if the resin is satu- rated with mercury. On the other hand, mercury adsorbed by Type IAA resin is more completely dissolved with di- lute HCI, as shown in Table IV, probably due to the differ- ent composition of the polymer.

Figure 3 shows the break-through curve of the resin col- umns packed with Type APA [(a) and (b)] and Type IAA [(c) and (d)] with the brine free of mercury. Experimental results with virgin resins are shown in (a) and (c) of Fig. 3. According to the comparison of data shown in (c) and (f) of Fig. 2 and (a) and (c) of Fig. 3, it was found that the ad- sorption capacity of Type APA resin for Mg and Ca in the brine containing 13 mg of mercury was reduced by about 15% compared to that in the brine free from mercury, and about 10% in the case of Type IAA resin.

To confirm the above results, 30 ml of Type APA resin was immersed in the brine containing 300 mg of mercury for 3 days, and then adsorption and regeneration of Ca2+ alone were repeated. The solutions were analyzed to ob- tain the material balance of mercury and calcium during the experiments. The resin was saturated with about 150 mg of mercury and 180 mg of calcium, calculated from the material balance, and hence the mole ratios of Hg and Ca in the resin were 0.14 and 0.86, respectively. This agreed with the data shown in Fig. 2 and 3, if the active sites of the chelate resin were exactly alike to Mg, Ca, and Hg.

Twenty ml each of Types APA and IAA new resin was immersed in one liter of the brine containing 100 ppm of mercury for three days, and then 5 ml of each resin was treated with 12N HCl instead of dilute soluticn (2N HC1) for

-

0 100 200 300 400 500 BED volume (Ill-R)

(c)

Resin: IAA 9 Cycles

I 8

0 100 200 300 400 500 BED volume (Ill-R)

(f)

d' 64 ,

Fig. 2. Breok-through curves of Type APA and Type IAA resins for mercury, calcium, and magnesium operoted in the brine containing mercury I- 6;'.

-~ "r

I426 J. Necfrochem. SOC., Vol. 137, NO. 5, May 1990 0 The Electrochemical Society, Inc. r t

Fig. 3. Break-through curves of Type APA and Type IAA resins for calcium and magnesium operated in the brine free from mercury.

, -., 24h at room temperature. Mercury was dissolved more ef- fectively by stronger acid, as shown in Table V. The ad- sorption capacity of the resins after the 9th cycle test was recovered to the level of virgin resin by the 12N HCl regen- eration, as shown in (b) and (d) of Fig. 3. However, the stronger acid regeneration'might reduce the resin life.

The volumes of the mercury desorbed during the regen- eration process in the 6th and 9th cycles are shown in Table VI. From these test results it was found that the mer- cury was dissolved not only with 2N HC1 solution but with 2N NaOH solution, and also by water washing.

Adsorption and regeneration of mercury with the che- late resin is sophisticated, because mercuric ions in NaCl solution -exist in the form of chloromercuric anions HgC1,-(m-2), where m is the coordination number (10). Therefore, the complex anions must be decomposed when mercury is adsorbed within the resin. This is possible in neutral NaCl solution since the formation constant of che- lated mercury is large enough. Mercuric ions are removed from the resin by HC1, and the extent of resin regeneration depends on the HC1 concentration.

The Type APA resin used in the test was examined by scanning electron microscopy (SEM) and XMA after nor- mal regeneration Figures 4 and 5 show the SEM phetogfaph and the x-ray chart of the surface and of the cross section of the specimen, respectively. The alkaline earth metals (magnesium, calcium, strontium, and barium)

'This experiment was conducted by T. Motohashi of Sumitomo Chemical Industry Company.

.

Fig. 4. a.) Photograph of surface of Type APA resin and b.) i ts XMA chart.

Toble V. Mercury adsorbed and desorbed by 12N HCI % (unit: mg) t

1 Type of Adsorbed in Desorbed in 12N HCl resin 5 ml resin 25 ml 50 ml 75ml

IAA 13.5 8.7 (64F 11.6 (86) 12.5 (93) APA 11.1 6.1 (55)

- v

9.0(81) 10.1 (91) -*

a Note: Number in ( ) shows desorption percentage of adsorbed I

mercury in the resins.

Toble VI. Impurities desorbed in regeneration ;&ions ~

Impurities desorbed - 9th stage Type of resin Regeneration 6th stage (unit mg) *

solution Ca Mg Hg Ca M g Hg 8 ~

APA 2NHCl 248.0 14.1 0.9 200.4 13.6 0.9 HzO(I) 3.2 0.0 1.1 1.1 0.2 0.9 2N NaOH 0.0 0.0 5.0 0.0 0.0 1.7 HLXII) 0.0 0.0 0.0 0.0 0.0 0.1 Total 251.2 14.1 7.0 201.5 13.8 3.6

IAA 2NHC1 135.3 7.9 2.7 134.9 8.6 1.3 . HzO(I) 2.0 0.0 3.8 1.2 0.2 1.4

2Y NaOH 0.0 0.0 14.7 0.0 0.0 20.0 HzO(I1) 0.0 0.0 1.5 0.0 0.0 0.3 Total 173.3 7.9 22.7 136.1 8.8 23.0

I

are almost completely desorbed from the resin by regener- ation. Mercury on the resin surface can be removed only incompletely, as shown in Fig. 4(b). On the other hand, mercury in the bulk of the resin is scarcely dissolved, as shown in Fig. 5(b), by the normal regeneration method within a limited period, probably because Qf slow diffu- sion; it will gradually accumulate during the repetition of adsorption-regeneration.

Effects of mercury on the performance of laboratory membrane cells.-Two laboratory cells, one with a Type 304 stainless steel cathode, and the other with an activated cathode, were operated at 3 kNm2 with the make-up feed brine shown in Table I. Mercuric chloride solution was added to the feed brine at intervals to keep the Hg concen-

stainless steel cathode

iety, Inc,

31

n 3.3 L

21 - 75 ml

(a) C.D. :3.OI<A/d Membrane : NafiinB90209

- Temp. :90°c Super purified mercury NaOH :32Wt%

process brine (10 ppm Hg - _

12.5 (93) 10.1 (91) -

adsorbed

ns 1

Itage mg) -

& H g

1.6 0.9 3.2 0.9 1.0 1.7 J.0 0.1 3.8 3.6 3.6 1.3 1.2 1.4 ).O 20.0 LO 0.3 $ 8 23.0

-

egener- ed only r hand. ved, as nethod v diffu- ition of

iratoq, a WPe tivated ip fee@ 3n was oncen-

pe 304 he cell 2d and age re ?d. The ned by "rent .hether or was

c Chlo-

n 8 v

J. Electrochem. ~ O C . , Vol. 1 3 7 , NO. 5, May 1990 0 The Electrochemical Society, Inc.

Hg addition Super purified mercury (SlOppm) process brine (10 ppm Ha:

(b) \

1427

It is noted that the catholyte liquor was contaminated by mercury at the level of ppb, probably caused by perme- ation from the anode side. It is well known that the mem- brane is exposed entirely to caustic soda liquor in the &lor-alkali cell under normal operating conditions. There- fore, chloromercuric ions react with caustic soda to form HgO on the surface of membrane facing the anode. Some mercuric ions permeate through the membrane since HgO has a solubility of some 10-2 g/l in 32% caustic soda.

The membrane of 22.5 cm2 area used for this test was im- mersed in 2N HCl solution at 90°C for 16h to dissolve the accumulated mercury. The accumulated mercury in the membrane was 0.13 pg!cm2, suggesting no effects of the mercury on the membrane performance, although a de- tailed discussion has not been made. Therefore, it is sug- gested that the mercury, which has come from the anolyte through the membrane, is deposited on the cathode, re-

C.D. -: 3 K " 2 Membrane :.NafionB90209 NaOH : 32wt% Temp.: 90°C

Hg addition : 20ppm

remove

Type 304 stainless steel cathode

Activated cathode

90 0 10 20 30 40 50

Days on line (days)

Fig. 6. Performance of laboratory membrane cell with 2.5 dm2 effec- tive area. Closed points: Type 304 stoinless steel cathode; open points: Ni-based composite electroplated cathode. a.) Cell voltage; b.) current

$: efficiency. I

Fig. 5. a.) Photograph of cross- sectional view of APA resin and b.) i ts XMA chart.

sulting in the increase of hydrogen overvoltage. However, the effect of mercury on the activated cathode was minor, since its effective area was large compared to that of the stainless steel cathode. Furthermore, there is no effect of mercury on the performance of the membrane itself.

Effects of mercury on the resin and membrane cell per- formance in the pilot plant.-As shown in Fig. 7, the per- formance of the membrane cell (for detailed specifications, see Table 111) in the pilot plant was not affected by the ad- dition of mercuric chloride at the level of 5-10 ppm as Hg2+ for 100 days to the feed brine, which was treated by the secondary brine purification system in the pilot plant.

Based on the results of the above tests for the resins and the membrane cells, a part of the feed brine containing about 10 ppm mercury from the amalgam process plant was fed to the secondary brine purification system without removal of the mercury. The mercury in the brine was ad- sorbed in the resin packed in the columns of a secondary brine purification system at the initial stage; therefore, no mercury was detected in the feed brine to the membrane

0

90 0 100 200 300 400

Days on line (days)

fig. 7. Performonce of Type DCM 406 membrane cell equipped with Ni-based composite electroploted cathode, 0.1 cell voltage; b.) current efficiency.

,

' f4B J. Electrochem. SOC., Vol. 137, No. 5, May 1990 0 The Electrochemical Society, Inc.

0

Membrane process 1.

Second Conversion

First Conversion

Amalgam process

1984 I End 1986 I March Year

Fig. 8. Schedule of the process conversion from an amalgam cell p!ant to a membrane cell plant.

cell at first. After partial saturation of the resin by mercury, mercury was observed in the feed brine, initially a trace, but gradually increasing to 9-10 ppm as in the laboratory tests (see Fig. 2). However, calcium and magnesium were well-treated to the level of 20-25 ppb total hardness as cal- cium in the secondary brine purification system, in spite of the leakage of mercury. This treated brine containing about 10ppm of mercury was continuously fed to the membrane cell. The cell operated at 3 kA/mZ for two months at these conditions, including periodic regenera- tion of the resin. The voltage and the current efficiency of the membrane cell equipped with the activated cathode were not affected by mercury in the feed brine, as shown on the right side of Fig. 7, and showed the same results as that in the laboratory tests.

i .

Industrial Application The conversion of an amalgam process plant with the ca-

pacity of 150 metric tons per day of caustic soda to a mem- brane process plant was undertaken with the knowledge and experiences gained in the laboratory and a pilot plant. A secondary brine purification system with the full capac- ity was newly constructed to purify the brine coming from the amalgam process plant. Type IAA resin was used be- cause of its easy regeneration.

The plant was converted by half in the first stage at the end of 1984, whereby the membrane cells were operated with the amalgam cells. The remaining amalgam cells were converted in the second stage, in the first quarter of 1986 as shown in Fig. 8. During the process conversion, a part of the brine purified in the existing primary brine pu- rification process for the amalgam process was fed directly to the secondary brine purification system to remove traces of impurities, and then fed to the membrane cells, and the balance of the purified brine was sent to the amal- gam cells, as shown in the block flow disgram of the plant (see Fig. 9). The impurities in the feed brine to the mem-

Fig. 9. Flowsheet of the com- bined operation of amalgam cells and membrane cells.

Table VII . Specifications of amolgom cell and membrone cell w

Amalgam cell Membrane cell

Cell type De Nora 14M2 DCM 408 x 2 Number of cells 22 12 (1st) + 12 (2nd) Current load, kA 200 85

Cathode materials Carbon steel Type 310s

Activated Anode material Ti Ti

Membrane NafionR 90209 Cell voltage, V 4.4 3.25 Current efficiency, 70 98 97 (at initial stagi Power consumption,

Product caustic soda

Product chlorine gas

Product hydrogen gas

Feed brine

Current density, kA/m2 13 3.5

coating stainless steel

coating DSA' DSA'

DC kWh/NaOH ton 1 3010 2250

NaOH, wlo 50 32 NaCl, ppm 15-20 20-30

Clz, VI0 98.5-99.0 98.5-99.0 0 2 , vlo 0.3-0.5 0.5-1.5

HP, VI0 99 9 99.9 .

NaCl, gR 300-305 300-305 Hardness, as Ca 10-20 ppm 20-25 ppb

brane cells were 10-15 ppm of mercury and 20-25 ppb calcium and magnesium, while the feed brine to the ama gam cells contained 10-15 ppm of calcium and mal nesium.

The depieted brine from the amalgam cells and tl- membrane cells was collected in the existing deplete brine reservoir and then sent back to the dechlorinatio system to remove completely active chlorine prior to re- saturation, since otherwise the chelate resin would be damaged. The 32% caustic soda produced i n t h e mem- brane cells was fed to the amalgam decomposers to be con- centrated to 50%. Therefore, no special evaporator was constructed in the first phase of conversion. The specifica- tions of the amalgam cells and of the membrane cells, op- erated in parallel during the process conversion, are shown in Table VII. Such combined operation of the amal- gam process and the membrane process in the plant was continued from the first step to the completion of the pro- cess conversion, in total about one and one-half years, as shown in Fig. 1. Figure 10 shows a view of the cell room during the combined operation of amalgam cells and membrane cells. The performance of the membrane cells, such as the cell voltage, the current efficiency, and hence power consumption, and the NaCl content in the product caustic soda, was not affected by the mercury in the feed brine, and was the same as other membrane cells operated with feed brine containing no mercury, and was also stable as shown in Fig. 11 and Table VII.

Conclusion The effects of mercury in brine on the performance of

the chelate resin in the secondary brine purification sys-

t ...................................................... Cl,gas

+ IEM cell Secondary brine purification

t 4 -

e v Primary brine purification Mercury cell

Dep. brine reservoir - =- 50wPh A

I )-4 NaOH

Mercury Brine satulation + Dechlorination --

ne cell

irane cell

408 X 2 + 12(2nd) 85 3.5 )e 310s less steel livated Ti

)SAR inR 90209 3.25 ;itid stage)

2250

32 20-30

1.5-99.0 ).5-1.5 ,

99.9

00-305 1-25 ppb

)-25 ppb of o the amal- and mag-

1s and the g depleted hlorination prior to re- would be

. the mem- 's to be con- )orator was ie specifica- ie cells, op- ersion, are of the amal- e plant was 1 of the pro- alf years, as le cell room n cells and ibrane cells, ., and hence the product

J in the feed 41s operated is also stable

-formanee of :fieation sys-

---t 32wPh NaOH

-(Pure water)

-----b Cl,gas ----* H,gas

+ 50wt% NaOH

J. Electrochem. SOC., Vol. 137, No. 5, May 1990 0 The Electrochemical Society, Inc.

la\

1429

tem and the membrane type chlor-alkali cell were dis- cussed, along with results of laboratory experiments and tests in the pilot plant. A case history of the conversion of an amalgam process plant to a membrane process plant,

-_ 98

j 9 4 . I= = = -: = = = O

%I 94

Fig. 10. View of cell room dur- ing combined operation of a.) membrane cells and b.) omolgam cells. Caustic sodo produced by the membrane cells was fed to c.) the amalgam decomposers and hy- drogen from membrane cells wos cooled by d.) individual hydrogen coolers, originally used for the de- composers.

based on test results, was also outlined. Although mercury in the brine coming from the amalgam process plant was adsorbed by the chelate resin at the initial stages of the op- eration, it soon broke through the resin column. However, other impurities such as calcium (Ca2+) and magnesium (Mg2+) were well adsorbed without any particular problem. The adsorption capacity of the resin for calcium and mag- nesium was reduced by 10-15% compared with resins used in brine containing no mercury, and depending on the type of resin and operating conditions. The decrement of the adsorption capacity of Type APA resin was, in general, larger than that of Type IAA resin. It was difficult to re- move mercury in Type APA resin with dilute HCl(2N) so- lution, but it could be dissolved by 12N HCl without any damage or loss of the resin.

Low levels (about 10 ppm) of mercury in the feed brine did not affect cell performance such as the cell voltage and current efficiency of membrane cells equipped with acti- vated cathodes. However, voltage escalation was observed in cells with stainless steel cathodes. This is because the mercury coming from the anolyte compartment deposits on the stainless steel cathode, resulting in an increase of the hydrogen overvoltage; on the other hand, an activated cathode having a large effective area was not affected by mercury. Consequently, the use of high-surface-area acti- vated cathodes is essential when the membrane cell is op- erated with the amalgam cell.

Based on the test results, an amalgam process plant was successfully converted to a membrane process plant, using the existing brine process facilities and the brine used for the amalgam process plant. The amalgam cells and the membrane cells were operated in parallel during the process conversion of one and one-half years, without any problem. These results are an example of an economi- cal and smooth process conversion from amalgam to mem- brane, or as an example of the combined operation of amalgam cellsand membrane cells in an expansion of an amalgam process plant by the addition of membrane cells, with minimum expense necessary for the installation of membrane cells.

Acknowledgment 0 100 200 300 400 The author would like to thank Professor F. Hine of Na-

goya Institute of Technology for his helpful comments on this paper.

script received Jan. 8,1990.

Days on line (days)

Fig. 1 1. Performance of membrane cells, Type DCM 408 x 2, oper- ated with amalgam cells. 0.1 Cell voltage; b.) current efficiency; c.) power consumption; d.) NaCl content in NoOH.

Manuscript submitted Sept. 28, 1989; revised manu-

1 , 1 * .

1430 J. Electrochem. Soc., Vol. 137, No. 5, May 1990 0 The Electrochemical Society, Inc.

Chlorine Engineers Corporation, Ltd., assisted in meet- 4-8, 1986. ing the publication costs of this nrticle. 6. Y. Masuda, J . Appl. Electrochem., 14,317 (1986).

7. K. Sato, M. Nakao, and T. Ishii, Paper 438 presented at The Electrochemical Society Meeting, Boston, Mass- achusetts, May 4-8, 1986.

8. K. Yamaguchi, T. Ichisaka, and I. Kumagai, Paper pre- sented at the Chlorine Institute’s Plant Operations

9. Y. Ogata, T. Kojima, S. Uchida, M. Yasuda, and F. Hine, This Journal, 136,91(1989).

10. F. Hine, “Electrode Process and Electrochemical Engi- neering,” p. 393, Plenum Publishing Corp., New York (1985).

11. A. Senda, A. Sakata, and K. Yamaguchi, in “Perform- ance of Electrodes for Industrial Electrochemical Processes,” F. Hine, J. Fenton, B. Tilak, and J. Tisius, Editors, p. 111, Electrochemical Society Pro- ceedings Series, PV 89-10, Pennington, NJ (1989).

12. T. Motohashi, Soda to Enso (Soda and Chlorine), 36, 291 (1985).

REFERENCES 1. K. Yamaguchi, in “Electrochemical Engineering in the

Chlor-Alkali and Chlorate Industries,” F. Hine, W. B. Darlington, R. White, and R. Varjian, Editors, p. 25, Electrochemical Society Proceedings Series, PV Seminar, Tampa, Florida, Feb. 1986. 88-2, Pennington, NJ (1988).

2. M. Seko, A. Yomiyama, and S. Ogawa, in “Modern Chlor-Alkali Technology,” Vol. 2, C. Jackson, Edi- tor, p. 97, Ellis Horwood, Chichester, England (1983).

3. K. Yamaguchi and I. Kumagai, presented at the 1988 London Int’l Chlorine Symposium, Electrochemical . Technology Group, SOC. Chem. Ind., June 1988.

4. T. C. Bissot, Paper 417 presented at The Electrochemi- cal Society Meeting, Toronto, Ontario, May 12-17, 1985.

5. T. C. Bissot, Paper 439 presented at The Electrochemi- cal Society Meeting, Boston, Massachusetts, May

Modification of Ion Exchange Membrane 1

. Surface- by Plasma Process I

, 1. H+ Ion Perm-Selective Membrane from Nafion f0.r Redox-Flow Battery

Z. Ogumi,* Y. Uchimoto, M. Tsujikawa, and Z. Takehara*

Dipartment of Industrial Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606, Japan

1

- I

I F. R. Foulkes* \ . Department of Chemical Engineering and Applied Chemistry, Unaversity of Toronto,

Toronto, Ontario, Canada M5S lA4

/ ABSTRACT

The proton perm-selectivity of a perfluorosulfonate cation exchange membrane, Nafion 117, was enhanced by de- positing on its surface an ultra-thin anionic exchange layer containing fixed pyridine rings. The thin layer was deposited from 4-vinylpyridine monomer vapor using a glow-discharge (plasma) polymerization technique. The resulting plasma polymer layers were found to be pinhole-free and of uniform thickness (-0.2 pm). The influence of the plasma parameters (monomer flow rate, applied power) on the plasma polymer deposition rate was investigated. The IR spectra of the plasma polymers showed the preservation of pyridine rings in the polymers; the lower the applied RF power, the greater the re- tention of pyridine rings. The proton perm-selectivity of plasma-modified Nafion membrane was determined by using it as the separator in a typical two-compartment cell (FeC12-HCVmembrane/HC1) and measuring tFe, the transference number of the Fe++ ion through the membrane. Pretreatment of the Nafion membrane by oxygen sputtering increased its proton perm-selectivity and enhanced the adhesion of plasma polymer onto its surface. Plasma-modified Nafion membranes ex- hibited very high proton perm-selectivities, but at the cost of high membrane resistances. For example, membranes hav- ing tFe values of 0.00034 and 0.012 had corresponding resistances of 40 and 4.1 R-cm2, respectively.

.-

In previous papers we have reported on the preparation of ionically conductive thin films (including lithium ion- conductive electrolyte films) by utilizing a glow-discharge (plasma) polymerization technique (1-8). Ion exchange membranes have been used in a variety of electrochemical applications, including, among others, water electrolyzers (9, lo), sensors (11, 12), and redox flow batteries (13, 14). Al- though such membranes have a high selectivity of coun- terions over co-ions, their selectivity among different counterions is generally low. The range of applications for ion exchange membranes would be Extended if such membranes had a higher selectivity among counterions. For example, the performance of a redox flow battery is strongly dependent on the separator perm-selectivity among ions of the same polarity. Thus, the development of the iron-chromium redox flow battery would be greatly aided by the availability of a cation exchange membrane having a high proton perm-selectivity (14). This battery usually consists of two solutions divided by a separator.

--.

*Electrochemical Society Active Member.

Each solution contains redox cations (e.g., Fe++/Fe+++, Cr”/Cr+++), protons, and anions. The transport of the redox ions through the separator decreases the cell effi- ciency and cyclicity. Although an anion exchange mem- brane might be suitable as a barrier against the redox cations, its electrical resistance is generally too high com- pared to that of conventional cation exchange membranes. Furthermore, some anionic complex ions containing the redox species may permeate through anion exchange membranes. Cation exchange membranes, on the other hand, generally exhibit lower electrical resistance, but are readily permeated by the redox cations.

In an earlier investigation, we reported on the enhance- ment of the proton perm-selectivity of a cation exchange membrane via the modification of the cation exchange membrane (15). In this communication, we present our findings in more detail.

The principle of the enhanced proton perm-selectivity is shown schematically in Fig. 1, using an HC1-FeC12 solu- tion, which is representative of the half-cell of an iron- chromium redox flow battery. A thin plasma polymer

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