Purification of galvanic sewage from metals by electrodialysis

DESALINATION ELSEVIER

Desalination 126 (1999) 159-162

www.elsevier.com/locate/desal

Purification of galvanic sewage from metals by electrodialysis R. Klischenko, B. Komilovich*, R. Chebotaryova, V. Linkov A. V. Dumanskii Institute of Colloid and Water Chemistry, Ukrainian National Academy of Science, Vernadsky Av., 42, 252143 Kiev, Ukraine Tel. +38 (044) 444-2577, Fax +38 (044) 452-0276, E-mail: [email protected]

Abstract The electrodialysis process of solutions containing a salt mixture (Na2SO4 and ZnSO4, with concentrations of 0.05 N each) has been studied. The electric current density ranged from 5 to 25 mA/cm 2. The solution was fed at a rate of 0.01 cm/s. The electromigration flux of zinc is approximately twice lower than that of sodium ions. The transport of hydrogen ions does not exceed 20% of the overall electric current, and it comes to pass as a zincateanion. The dependence of the electromigration transport of zinc ions vs. the electric current density has a maximum in the range 10-15 mA/cm 2. In the presence of an ion-exchange fiber in the desalination chamber, the residual zinc concentration in the dialyzate does not exceed 2-3 mg/l.

Keywords." Desalination; Electrodialysis; Zinc removal

I. Introduction

The sewage discarded by industries with galvanic production is one of the most widespread sources of toxic non-ferrous and heavy metals enter the sewage system and natural reservoirs. The efficiency in the use of non-ferrous metals does not exceed 80% [1]. Therefore, a

* Corresponding author.

significant part of the raw material is not utilized and becomes solid waste or sewage. The purification of dilute solutions is the most difficult problem both from the point of view o f its industrial fulfillment and economic reasoning. In the first place, this is due to large volumes of such solutions. Despite the seeming simplicity of the reagent methods, some o f them are too cumbersome and do not remove heavy metals (HM) to the maximum contamination level (MCL) standards [2,3]. An imperfect level of sewage purification is mainly due to their multi-

Presented at the Conference on Desalination and the Environment, Las Palmas, Gran Canaria, November 9-12, 1999, European Desalination Society and the International Water Services Association. 0011-9164/99/$- See front matter © 1999 Elsevier Science B.V. All rights reserved PII: S0011-9164(99)00169-1

160

R. Klischenko et al. /Desalination 126 (1999) 159-162

component composition as well as an amphoteric character of HM compounds which form hydrous oxides at the different pHs. The increased salt concentration and the presence of organic admixtures are the factors making the reagent methods more complex. In addition, the necessity to provide a long-term settlement of the clarified solution and to use a number of various filters for their finishing purification makes reagent purification technology unacceptable in terms of current standards. It should also be noted that these filters use large water quantities for own needs. Presently, the schemes of local sewage purification immediately from collectors are increasingly used. To reach this objective, the methods of sorption and electrosorption [4], reagent ultrafiltration and microfiltration [5], and electrodialysis [6] are introduced. In electrodialysis, the desalinating chambers of the electrodialyzers are filled with ion exchangers [6]. This method appears to be the most attractive, as it does not require the application of special reagents and the use of water for its own service. It allows purification of water and, at the same time, concentration of the extracted component. To achieve removal of HM from a mixed electrolyte solution containing a background electrolyte and other components, experiments were carried out on the design of the desalting device, type of a load, the electric current density and feeding rate of a solution. The last two determine the overall process productivity to a great extent. The objective of this paper was to study the effect of electric current density and the presence of a load (filler) in the desalination chamber on the deionization process of the rinsing water formed in the bath of standing rinse by galvaniztion. We have examined the dependence of the specific electromigrative transport of zinc and sodium ions across both the cationexchange and anion-exchange membranes on the intensity of the polarization regime.

2. Experimental The studies have been performed in a fivechamber device with one deionization chamber whose lateral sides were the MK-40 cationexchange and MA-40 anion-exchange membranes. The scheme of this device is as follows:

(-)l

A K A K E I l ] 2 ] 3 [ E I(+)

The numbers denote the chamber number, letters A and K denote the anion-exchange and cation-exchange membrane, respectively. The chamber dimension was 2×5×0.5 cm. Conventional electrodialysis and electrodialysis in the presence of ion exchangers were used. In latter case, the cation-exchange fiber with a filling coefficient of 0.60 (in volume) was loaded into the central (desalinating) chamber. In the electrode chambers, 0.1 N Na2SO4 solution at a rate of 0.4 cm/s was circulated. The concentration chambers were fed with 0.05 N KNO3 solution at a rate of 0.02 cm/s. The desalination chamber was fed with the salt mixture (Na2SO4 and ZnSO4 with concentration of 0.05 eq/l each) solution. The feeding rate in this chamber was 0.01 cm/s, which allowed desalination through a single pass of the solution.

3. Results and discussion The range of the electric current density was selected based on the limiting electric current density, ilim. We have estimated it on the basis of current-potential curves (Fig. 1) for a chamber without (curve 1) and with (curve 2) the load/filling. As is known [7], the electric current significantly exceeding the limiting value is an essential intensifying factor of the electrodialysis process. In this context, a variation range of the electric current densities selected is higher than its limiting value. We also took into account that in the over-limiting range a generation of H ÷ and OH ions becomes more intense.

R. Klischenko et al. / Desalination 126 (1999) 159-162

161

25 3

2

lo"

20

i 8+

E

I

"'10

S

0

2

I-8

I 13

I 18

.....

I 23

I 28

U, V

-

•

0 5

?

9

11

13

I 15

I 1?

l~g

21

I 23

25

l, mN c~

Fig. 1. The current-potential curves of the elecrodialyzer without the filling fiber (I) and with the filling fiber (2).

Fig. 2. The dependence of the pH solution vs. the electric current density in chambers 1, 2 and 3.

This aids the formation of hydrous Zn(II) oxides as well as transformation of zinc ions into zincate anions. As can be seen in Fig. 1, the value o f limiting electric current in the electrodialyzer increases from 6.2 to 7.3 mA/cm 2 in the presence o f the fiber load. This is due to a high electric conductivity o f the ion-exchange fiber. Making allowance for the fact that in the concentration chamber from the anionexchange membrane side, the pH reaches values close to 12 already at i--5.0 mA/cm ~ (Fig. 2), we can suggest the following. The actual limiting electric current density reached during the desalination process is lower than the one that was established while determining the current-potential curves. Similarly, in the concentration chamber from the side of the cationexchange membrane, the actual pH of the solution is below 2.0. Thus, together with the salt transport, the involuntary expenditure of electric power on the transport o f H + and OH ions

process becomes more intensive (Fig. 3, curve 5). Here zinc removal with a stream decreases sharply (from 1.8 to 0.2 mg-eq/h) as the electric current density increases (see Fig. 3, curve 3). Nonetheless, zinc concentration in the dialyzate is about 170 mg/I. Thus its value is unacceptable for discard of such water and makes it difficult to use it repeatedly. Desalination chamber loading with the cation-exchange fiber physically expands the surface of the cation-exchange membrane and also affects the formation o f a boundary diffusive layer. These two factors have a positive effect on the kinetic parameters of the process. The transport of zinc cations across the cationexchange membrane grows with the increase of electric current density. This results in a sharp reduction in precipitation. Further, the residual zinc concentration in the dialyzate decreases sharply as well reaching 2-3 mg/1. The electromigrative zinc transport in the electrodialyzer in the presence of ion exchangers increases as high as 50% in comparison with the conventional electrodialysis (Fig. 4). Taking into account a comparatively low residual zinc concentration in the dialyzate and lower device voltage, the use of the former option for zinc concentration from a mixed electrolyte looks more preferable as compared with the latter. In this case, the process can be effectively realized

Occurs.

The specific zinc transport (q, mg-eq/h) across the cation-exchanger membrane reaches its maximum at an electric current density of 10 mA/cm z (Fig. 3, curve 1). q tends to decrease with the subseqent increase of the electric current density due to decrease in its relative concentration in entrain layer since the precipitation

162

R. Klischenko et al. / Desalination 126 (1999) 159-162

1.2 +

2.5~

1 2

~

1.5

~0.6

E

0,4 O.S"

0.2

~ 0

I 9

114

4 I 19

b 24

0

I

j

I

I

O

14

19

24

I, mA/o~n

Fig. 3. The dependence of the specific fluxes of zinc ions vs. electric current density across cation-exchange membrane (1), the anion-exchange membrane (2), in the dialyzate solution (3.chamber without the filling fiber; 4, chamber with the filling fiber), and in the precipitation (5).

Fig. 4. The dependence of the specific electromigrative ion transport vs. electric current density: H+(1), Na+(2), and Zn2÷(3,4). In the case of 3, there is filling fiber in the desalination chamber.

at the electric current density close to its limiting value when the transport o f hydrogen ions does not exceed 20% from the overall electric current.

increases and is equal to approx. 2 mg/1 when the electric current density is 25 m A / c m 2.

4. Conclusions The electrodialysis process o f zinccontaining solution in the presence of a background electrolyte has been studied. The results are as follows: 1. The dependence o f Zn 2+ electromigrative transport reaches its maximum at i = 10-15 mA/cm 2. 2. Up to 25 % o f zinc is transferred across the anion-exchange membrane as its anionic form. 3. The presence of the ion-exchange filler in the desalination chamber leads to an increase in zinc electromigrative transport and a decrease in its concentration in the dialyzate. 4. The residual zinc concentration in the dialyzate decreases as the electric current density

Acknowledgements The authors are grateful to Dr. Eugene Tsapiuk for his help in the design o f this paper and its translation into English.

References [1] [2] [3] [4] [5] [6] [7]

R.E. Tugushev, Galvanoengineering and Surface Treatment, 1 (1996) 37. V.A. Kolesnikov, Methods for regeneration of metals from the rinsing waters of galvanic productions, Metallurgia, Moscow, 1989. I.M. Vasserman, Chemical precipitation from solutions, Khimiya, Leningrad, 1990. L. Lanhecke, Int. J. Mater and Prod. Technol., 1 (1992) 119. M.I. Medvedev, V.M. Kochkodan and M.T. Bryk, Khim. Tecknol. Vody, 2 (1994) 159. [6] M.V. Pevnitskaja, A.G. Belobaba and K.A. Matasova, Khim. Tecknol. Vody, 8 (1992) 604. [7] V.I. Zabolotsky and V.V. Nikonenko, Ion transport in membranes, Moscow, Nauka, 1996.