-japanese-chinese symposium on electrodialysis- recovery

- Japanese-Chinese Symposium on Electrodialysis-

Original Paper

Recovery of Rare-metal Elements from

their EDTA Complex Solution by means of

Electrodialysis Accompanied by Metal Substitution Reaction

Hiroshi TAKAHASHI*, Sueji HIRAWATARI* and Ken-ichi KmucHi*

A novel electrodialysis process has been developed for the recovery of rare-metal elements from their EDTA

complexes. The electrodialyzer used for the experiments was composed of five compartments divided by a ca-

tion-exchange membrane, C, and an anion-exchange membrane, A. The compartments were set in the order of

Anode, Feed, Reaction, Strip, and Cathode compartments, and were partitioned with the A, C, C and A mem-

branes, respectively. The Feed solution, the Reaction solution, and the Strip solution, each of which flowed

through the corresponding compartment, contained CuCl2, rare metal-EDTA complex, and HCl solution, re-

spectively. The electrodialysis for the recovery of rare-metal elements was expected to proceed as follows: When

voltage is applied to the electrodialyzer, Cu2+ should proceed from the Feed compartment to the Reaction com-

partment, and act as the substitute for rare-metal elements in the EDTA complexes. The resultant free ions of

rare-metal elements move to the Strip compartment, and are recovered.

In the experiments, Co, La, Gd, Y, and Ni were selected as rare-metal elements. In the electrodialysis experi-

ment for the Co-EDTA system, Cu2+ from the Feed compartment was immediately substituted for Co, and part

of the resultant Co2+ moved to the Strip compartment. No permeation of Cu-EDTA and free Cu2+ was ob-

served during the dialysis. The flux of Co2+ increased with an increase in current density. For the systems of La-

EDTA, Gd-EDTA, and Y-EDTA, the permeation behavior of metal ions was almost the same as that for the

Co-EDTA system. For the Ni-EDTA system, however, Cu2+ as well as Ni2+ proceeded to the Strip compart-

ment: This suggests that for the Ni-EDTA system the metal substitution reaction is much slower than that for

the other metal elements. Moreover, the substitution reaction rates of Cu for the rare-metal elements in the

EDTA complexes were measured spectroscopically and analyzed with due consideration of dinuclear complex

intermediates. The rate constants were smaller in decreasing order- La-EDTA, Gd-EDTA, Y-EDTA, and Ni-

EDTA and were correlated with the stability constants for EDTA.

Key words: electrodialysis, metal substitution reaction, ion-exchange membrane, rare-metal, reare-earth,

EDTA

1.INTRODUCTION

Electrodialysis is one of the most useful separa-

tion processes for desalination of seawater, and is

also used in the demineralization of whey, the

purification of molasses, and the recovery ofmetal elements from electro-plating solutions. Re-

cently, a novel application of electrodialysis in

the selective separation of metal elements by

means of a masking effect of chelators was report-

ed by Bril et al.,1) Wallace et al.,7 Huang et al.,2)

and Takahashi et al.4,5) According to their stu-

dies, when the metal-element solution includes a

chelator and is electrically dialyzed, each uncom-

plexed-metal ion permeates a cation-exchange

membrane at different rates. Thus, selective sepa-

ration of metal elements is possible because of the

difference in the selectivity of metal ions to the

membrane and in the stability constant of their

complexes. Although this rate-governed separa-

tion process shows promise as an advanced sepa-

ration technique, it is difficult to achieve. The

complexed metal ions remain in the feed solution

without being dialyzed and their amount is esti-

mated to be large, especially with regard to the

separation of rare-earth elements; therefore,

these metal ions should be recovered from their

* Department of Materials Engineering and Applied Chemistry , Mining College, Akita University(1-1, Tegatagakuen cho, Akita city, Akita 010, Japan)

219

220 Bull. Soc. Sea Water Sci. Jpn. Vol.51 No.4 (1997)

complex. To overcome this difficulty, we

proposed an additional electrodialysis process ,6)in which the metal ions were recovered from their

EDTA complex solution by the substitution reac-

tion of Cu2+. However, there is no clear explana-

tion for the electrodialysis that accompanies the

metal substitution reaction.

In this study, we examined the transport charac-

teristics of metal ions in the electrodialysis that ac-

companies the metal substitution reaction under

various experimental conditions. Moreover, we

measured the metal substitution reaction rates, de-

termined the kinetic parameters, and discuss the

possibilities of its application to selective separa-tion.

2. PRINCIPLE OF RECOVERY

The metal element-EDTA complexes demon-

strate decreasing magnitudes of stability con-

stants in the order of Cu-EDTA, Ni-EDTA, Y-

EDTA, Gd-EDTA, La-EDTA, and Co-EDTA.

When a pH-controlled solution containing Cu2+

is mixed with a solution containing a complex of

a rare-metal element, Cu2+ is substituted for the

rare-metal element combined with EDTA, and

rare-metal ions are released. As shown in Fig. 1,

this substitution reaction could take place system-

atically in an electrodialyzer: The Cu2+ moving

from the Feed compartment reacts with the rare-

metal EDTA complexes in the Reaction compart-

ment. The resultant free rare-metal ions move

from the Reaction compartment to the Strip com-

partment, and are recovered. Moreover, in thecase of rare-metals solutions, a selective-separa-tion process is expected to occur simultaneouslywith the recovery of rare metals because of theirdifference in substitution reaction rates.

3. EXPERIMENTAL

3.1 MaterialsCobalt chloride hexahydrate, nickel chloride

hexahydrate, and copper chloride dihydrate wereobtained from Nakarai Tesque Inc. Lanthanum

chloride heptahydrate, gadolinium chloride hexa-hydrate, and yttrium chloride hexahydrate were

prepared from the oxides of their metal elements(Shin-etsu Chemical Co. Ltd.). Ethylenediamine-tetraacetic acid disodium salt, as a chelator, andsodium hydroxide were also obtained fromNakarai Tesque Inc.

3.2 Apparatus3.2.1 ElectrodialyzerFigure 2 shows a schematic diagram of the ex-

perimental apparatus. The electrodialyzer hasfive compartments, which were divided by cation-

exchange membranes (SELEMION CMV), C,and anion-exchange membranes (SELEMIONAMV), A. The compartments were set in the ord-

er of Anode, Feed, Reaction, Strip and Cathodecompartments, each of which was partitioned

Fig. 1 Transport of ionic species in electrodialyz-

er. Fig. 2 Schematic diagram of electrodialyzer.

Hiroshi Takahashi, et al.: Recovery of Rare-metal Elements from their EDTA Complex Solution 221

with the A, C, C, and A membranes, respec-

tively. The effective membrane area was 5 x 10-3m2 and the distance between them was 9 x 10-3m. The anode and cathode were platinum andnickel plates, respectively, of the same size as the

membranes.The dialysis solutions were fed into the com-

partments as follows: NaCl solutions as electrodesolutions to the Cathode compartment and theAnode compartment; CuCl2 solution as a reac-

tion solution to the Feed compartment; metal-EDTA complex solution to the Reaction compart-ment; and HCl solution to the Strip compart-ment. The volume of each solution was 2 dm3.The experimental solutions were pumped in circu-

lation from each reservoir to the compartment ata constant flow rate of 0.19 m/s. The metal ionswere quantitatively determined by means of atom-ic absorption spectrometry, chelometric titration,

and reversed phase HPLC.3.2.2 Measurement of substitution reaction

rateThe metal substitution reaction rates were fol-

lowed using the Hitachi U-2000 spectrophotome-ter for the Ni-EDTA system. As for the other sys-tems, the stopped-flow spectrophotometer, JAS-CO KS-100, which permitted reactants to be mix-

ed within a few milliseconds, was used. Thechange in absorbances corresponding to that inthe extent of metal substitution reaction was ana-

lyzed with a personal computer equipped with

GPIB board. All the calculations were done using

the algebraic equation Eq. (1). The parameters

used are listed in Table 1.

A Al=81 , 1/CI+81,21C2+•c+el,1n/Cn

Al2=82 , 1/CI+82,21C2+•c+82,nlCnEq.(1)

Arn=emi/CI+8,,, ,2/C2+•c+em,n/C,

4.RESULTS AND DISCUSSION

4.1 Recovery of rare metal elements from

their EDTA complexes

Figure 3 shows an example of the time

courses of the concentrations of ions in the

Feed, Reaction, and Strip compartments for the

Co-EDTA system. When voltage was applied to

Table 1 Molar absorptivities of elements

Fig. 3 Time courses of concentrations of ions for Co-EDTA system.


Fig. 4 Time courses of concentrations of ions for Ni-EDTA system.

the electrodialyzer, Cu2+ moved from the Feedcompartment to the Reaction compartment, andwas substituted for Co2+ in the EDTA complex.

The resultant dissociated-Co2+ moved to theStrip compartment, and was recovered. The Na+dissociated from EDTA-2Na also moved to the

Strip compartment through the cation-exchangemembrane. However, no movement of Cu2+ was

observed under these conditions. The sum of thedissociated-Co2+ concentrations in the Reactionand Strip compartments was almost equal to thetotal concentration of Cu, thus indicating thatthe metal substitution reaction between Co and

Cu proceeds quickly.Figure 4 also shows the time courses of concen-

trations for the Ni-EDTA system. Rhombus keysin the figure mean the sum of the concentrations

of Ni2+ and Cu2+, since their concentrations cannot be separately determined with each other bychelometric titration. As Cu2+ moved from theFeed compartment, Ni2+ moved to the Strip com-

partment after an induction period. However,Cu2+ also moved to the Strip compartment, thusindicating that the metal substitution reaction inthis system was relatively slow.

4.2 Recovery of rare-earth elements from

their EDTA complexesFigure 5 shows changes in the concentrations

of ions permeating the cation-exchange mem-brane in the system of La-EDTA. The circle keysin the figure represent the concentration of lantha-

num dissociated from the EDTA complex. Lan-thanum dissociated from the complex that movedfrom the Reaction compartment to the Strip com-

partment. The sum of the concentrations in theReaction and Strip compartments agree closelywith that of Cu. This result suggests that themetal substitution reaction reaches equilibrium

quickly, and proceeds stoichometrically.Figure 6 also shows changes in the concentra-

tions of permeable ions in the Gd-EDTA system.The permeation behavior of the ions is similar tothat for the La-EDTA system. However, the totalconcentrations of dissociated ions in the Reaction

and Strip compartments were smaller than thosefor the La-EDTA system. The behavior of the Y-EDTA system was similar to that for the Gd-EDTA system (the data are not shown).

4.3 Effect of current density on fluxFigure 7 shows the effect of current density on

the fluxes of the dissociated metal-elements mov-ing from the Reaction compartment to the Strip

compartment. The fluxes of dissociated metal-ele-ments increased in proportion to the current den-

sity except for the Ni-EDTA system. This

phenomenon may result from a linear increase


Fig. 5 Time courses of concentrations of ions for La-EDTA system.

Fig. 6 Time courses of concentrations of ions for Gd-EDTA system.

with an increasing current density in the flux of

Cu, which moved from the Feed compartment to

the Reaction compartment, and in the metal sub-

stitution rates. Figure 8 shows a comparison of

the rates of substitution reactions among metal

elements. The ordinate represents the reaction


Fig. 7 Effect of current density on flux.

rate standardized with the flux of Cu. The values

of the reaction rates were almost constant overthe range of 10 to 50 A/m2, and differed fromone metal element to another. This suggests that

the electrodialysis of the solution containingthese metal elements makes it possible to separatethe metal elements.

4.4 Separation of lanthanum and nickel

We attempted a novel method for the selectiveseparation of La and Ni during their recoveryfrom their EDTA complexes. Figure 9 shows theexperimental result of the selective separation of

Ni and La. Firstly, the dissociated-La transferredto the Strip compartment with due to the substitu-tion of Cu, Ni and Cu following after a long in-duction period. This means that La and Ni are

selectively separated by 120 minutes under theconditions studied. From the results in Figs. 7and 9, we can see that this separation behaviorcould result from the differences of La and Ni intheir selectivity to the membrane and in theirdifferent rates of substitution reaction. There-

fore, the electrodialysis that accompanied themetal substitution reaction is applicable to theselective separation of La and Ni from theirEDTA complexes.

4.5 Kinetic analysis of metal substitution reac-

tionBased on the results of the electrodialysis, the

metal substitution reactions seem to differ from

Fig. 8 Relation between rf/JcuS and current den-sity.

Fig. 9 Separation of lanthanum and nickel from

their mixed EDTA complexes.


one metal element to another. As the next step,we analyzed the metal substitution rates usingMargerum's model, which considers a dinuclearintermediate such as weak copper-EDTA-nickel

complex.4.5.1 Model analysisMargerum et al.3) analyzed the reaction

mechanism for the system of nickel-EDTA, and

proposed a kinetic model for the metal substitu-tion reaction. Under conditions of pH 3 to 5 anda copper concentration of 0.1 to 1 mol/ m3, thereaction rates were expressed by means of the two

site model as follows:

The scheme (A) described above has a rate expres-

sion of Eq. (2), which is derived using the steady-state approximation.

where k1 to k3 are rate constants, and are evaluat-ed using the Lineweaver- Burk method.

The experiments were carried out under the fol-lowing conditions: CEDTA 5 to 10 mol/m3, CCuO=2.5 to 10 mol/m3 , and pH=4.0, where Ccuo me-ans the initial concentration of Cu2+. The ob-served values of the reaction rates for the Ni-EDTA system agreed closely with Margerum'sresults3) despite of some differences in experimen-tal conditions. All the experimental results were

analyzed using Eq. (2); The kinetic parameters de-termined are listed in Table 2. The rate constants

Table 2 Kinetic parameters for Eq. (2)

Fig. 10 Relation between k3 and stability con-

stant.

Fig. 11 Relation between ke and stability con-

stant.

obtained were successfully correlated with the sta-

bility constants as shown in Figs. 10 and 11,

which could be available for estimating the rate

constants for the complex systems of other metal

elements. Figure 10 shows that the k3 for the

release of rare metal elements decreased with an

increase in the stability constants for the metal-

EDTA complexes. Furthermore, the values of k3

approached zero as the stability constants ap-

proached that for Cu-EDTA complex. The magni-

tudes of k3 for the rare earth-EDTA systems were


30~150 times as high as that for the Ni-EDTA sys-tem. This may explain why La was selectively

separated from the La-EDTA/Ni-EDTA mixturesolution as shown in Fig. 9.

Consequently, the electrodialysis that accompa-

nied the metal substitution reaction would resultfrom the differences in selectivity of rare-metalelements to the membrane and the differences in

the rates of substitution reaction. A quantitativemodel based on transport mechanisms is requiredfor feasibility studies and for optimization of theseparation process. We would like to report onthis in the next paper.

5. CONCLUSIONS

The following findings were obtained by ex-

perimental investigation of the recovery of raremetal elements by means of the electrodialysisthat accompanies metal substitution reaction:

The rare-metal elements of Co, Ni, La, Gd,and Y were successfully recovered from theirEDTA complex solutions by means of the elec-trodialysis. The rare-metal elements that were dis-

sociated by the substitution reaction of Cu thatmoved from the Reaction compartment to theStrip compartment, and were recovered there. Nomovement of Cu was observed except for the Ni-

EDTA system, where Cu was transported with Niand had a long induction period during permea-tion. The fluxes of the rare-metal elements in-creased linearly with an increase in current den-

sity.For the binary system of Ni-EDTA/La-

EDTA, La moved first, and Ni followed after along induction period, where the two metal ele-ments were selectively separated.

The metal substitution reaction rates were

measured spectroscopically, and their rate-con-stants were determined using the Margerum's

model with due consideration given to dinuclearintermediates. The magnitudes of the rate con-stants of the reactions when the metal elementswere released from their intermediates becamelarger in the increasing order of Ni, Y, Gd, and

La, and decreased with an increase in stabilityconstants for their EDTA complexes. The rateconstants were correlated with the stability con-stants; this correlation could be made available

for estimating rate constants for the complex sys-tems of other metal elements.

These experimental results lead to the conclu-sion that the electrodialysis that accompaniesmetal substitution reactions may be applicable to

the selective recovery of rare-metal elements fromtheir EDTA complex.

ACKNOWLEDGMENTS

The authors are grateful to the Salt ScienceResearch Foundation (No. 9318), Grant-in Aidfor Scientific Research (No. 06750785) by the Mi-

nistry of Education, Science and Culture, and theKuribayashi Academic Foundation (1995) fortheir support of this research. The authors wouldalso like to thank student colleagues (Messrs. T.

Ohira, M. Ban, Y. Okura, 0. Kitabayashi andMrs. T. Hayashi) for their assistance in the experi-ments.

NomenclatureC=concentration [mol/m3]e=molar absorptivity [m2/kmol]

I=current density [A/m2]J=flux [mol/(m2s)]KABS=stability constant [m3/mol]

ke=constant [mol/m3]ki=rate constant [s-1]k2=rate constant [s-1]k3=rate constant [s-1]r=width of optical cell [m]

r=rate of reaction [mol/(m3s)]S=effective surface area of ion exchange

membrane [m2]t=time [s] u=liquid velocity [m/s]

<subscripts>Co=cobalt

Cu=copperf=feedGd=gadoliniumM=metal elementNi=nickelY=yttriumT=total

0=initial

References

1) K. Bril and S. Bril, J. Phys. Chem., 63, 256 (1958)2) T. C. Huang and J. K. Waung, Desal., 86, 257

(1992)


3) D. W. Margerum, D. L. Janes and H. M. Rosen, J.Am. Chem. Soc., 83, 4463 (1965)

4) H. Takahashi, K. Miwa and K. Kikuchi,Kagakukogaku Ronbun-shu, 19(3), 418 (1993)

5) H. Takahashi, K. Miwa and K. Kikuchi, J. Ion-Ex-change, 4(3), 183 (1994)

6) H. Takahashi, S. Hirawatari and K. Kikuchi,Proceedings of International Conference of Materi-als Engineering for Resources, p. 49, (1994) AkitaJapan

7) R. M. Wallace, I.E.C. Proc. Des. Dev., 6, 423

(1967)8) C. P. Wen and H. F. Hamil, J. Member. Sci., 8, 51

(1981)

和文要旨

EDTA-レアメタル錯体からの金属の回収を目的と

した新しい電気透析法を提案した.実験で使用した電

気透析槽は,陽イオン交換膜および陰イオン交換膜で

仕切られた陽極室,フィード室,反応室,ストリップ

室及び陰極室の5室からなる回分循環型である。こ

の手法は,フィード室に塩化銅水溶液,反応室にレア

メタル-EDTA錯体を流し,反応室で銅とレアメタル

錯体との金属置換反応を生じさせ,EDTAから解離

した金属をさらにストリップ室で回収を行うものであ

り,実験に際してはコバルト,ランタン,ガドリニウ

ム,イットリウムおよびニッケルのEDTA錯体を用

いた。コバルト,ランタン,ガドリニウム,イットリ

ウムのEDTA単成分錯体を用いて実験を行った結果,

EDTA-金属錯体と銅との金属置換反応は比較的速や

かに進行し,それぞれの金属単成分として回収され,

各イオンの膜透過流束は電流密度の増加に伴い増加し

た.しかし,ニッケル-EDTA錯体の場合は銅イオン

との混合物として回収され,金属種によって金属置換

反応速度に違いがあることが示唆された.これらの結

果を基に,分光光度法によりEDTA一金属錯体と銅と

の金属置換反応速度を測定し,速度定数を決定した.

その結果,各金属種における金属置換反応速度はLa-

EDTA,Gd-EDTA,Y-EDTA,Ni-EDTAの順序で減

少した.また,得られた各速度定数はそれぞれの金属-EDTA

錯体の安定度定数との間に良好な相関関係が見られ

た.以上の結果を基に,本手法における金属の回収の

メカニズムについて考察を行った.

(平成9年6月27日受付Received June 27, 1997 )

-japanese-chinese symposium on electrodialysis- recovery

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