Extraction of Ions from Solutions Using a Tubular Electrodialyzer

A. P. Vergun, I. A. Tikhomirov, V. T. Doronin

(Presented by the scientific seminar of the Faculty of Physics and Technology)

In carrying out certain electrochemical processes, ion-exchange membranes are beginning to become widely used. They are ionites in the form of sheets or plates. Ion-exchange membranes, owing to the presence in their structure of electrically charged groups, possess electrochemical activity. This property of membranes consists in the fact that, while offering no great resistance to electric current, they substantially change the transference numbers of ions. Thus, a cation-exchange membrane is permeable predominantly to cations, and an anion-exchange membrane to anions.

The indicated property of ionite membranes makes it possible to use them successfully for desalination of water [1], for purification of waste waters [2], for removal of excess acidity or alkalinity from various solutions [3], for separation of elements similar in properties [4], etc.

The present work describes the process of extracting the cations Na\(^+\), K\(^+\) from solutions simulating waste waters by the method of electrodialysis with ionite membranes. The electrical conductivity of ionite membranes in solutions of the indicated type was studied beforehand. The composition of the solution was as follows: \(6 \cdot 10^{-4}\) g/l Na\(^+\), \(4 \cdot 10^{-1}\) g/l K\(^+\).

To determine the specific electrical conductivity of the membranes, we used two methods:

1. Determination of the specific electrical conductivity from the difference in electrical resistances of a cell with electrolyte separated by a membrane and of a cell without a membrane.

2. Direct determination of the electrical resistance of wet, swollen membrane specimens.

The measurements were carried out with the aid of an alternating-current bridge, shown in Fig. 1. The numbers 1, 2, 3, 4 in the figure denote, respectively: the measuring cell, an oscillograph, a resistance box, and a capacitance box. An EO-7 electronic oscillograph was used as the null instrument. By selecting the magnitude of the resistance \(R\) and the capacitance \(C\), the minimum value of the amplitude of the sinusoid on the oscillograph screen was obtained.

As the measuring cell in determining the electrical conductivity of the membranes by the first method, a flow-through-type cell with electrodes of platinized platinum was used. In measurements by the second method, a strip of membrane was clamped by means of a spring between platinum electrodes and its electrical resistance was measured. First, the resistance of one membrane was measured. Then eva—

the electrical resistance of two identical membrane specimens of the same type was measured. The electrical resistance of the membrane was found as the difference between the resistances of the double and single membranes. If the resistance of a single membrane depends on the frequency of the alternating current of the measuring circuit (decreasing as the frequency increases), then the electrical resistance found as the difference between the resistances of the double and single membranes does not depend on the frequency of the alternating current of the measuring circuit [5].

Knowing the resistance \(R\) of a membrane specimen obtained on the measuring circuit, the specific electrical conductivity can be determined by the formula:

\[ \chi = \frac{RS}{l}, \]

where \(S\) is the membrane area,
\(l\) is the membrane thickness.

Fig. 1. Measuring circuit for determining the electrical conductivity of membranes

The results of measurements of the electrical conductivity of the membranes are given in Table 1.

Table 1

Electrical conductivity of membranes

It is evident from the table that the membrane made of oxidized fabric has the highest electrical conductivity. A cation-exchange membrane of this type is obtained by us by oxidizing cellulose materials with nitrogen oxides. However, the indicated membrane is insufficiently stable in an alkaline medium. From the standpoint of chemical stability during prolonged operation and high selectivity, a cation-exchange membrane of the MK-40 type was selected as the membrane for the electrodialyzer (this membrane is manufactured at the Scientific Research Institute of Plastics on the basis of the cation exchanger KU-2).

In contrast to ordinary filter-press-type electrodialysis units, which consist of a stack of alternating ion-exchange sheet membranes and gasket material, we designed and constructed a tubular electrodialyzer with ion-exchange membranes. The design of the electrodialyzer is shown in Fig. 2.

The body of the electrodialyzer, 1, is a tube 80 mm in diameter and 300 mm long, made of stainless steel. The body of the electrodialyzer also serves as the external cathode. Inside the body, four anode chambers, 2, are uniformly arranged; we made them in the form of tubes 20 mm in diameter from sheet cation-exchange membranes of the MK-40 type. In the center of each tube is an anode, 3, in the form of a metal rod. The cathode chamber is formed between the membrane tubes and the body of the electrodialyzer. For a more uniform distribution of the electric field over the electrodialyzer, a second, internal cathode, 4, was placed in the center of the apparatus. The internal cathode was at the same potential as the external one.

From the four anode chambers, the sodium and potassium ions contained in the initial solution (dialysate), under the action of the electric field, passed into the cathode chamber, where they were concentrated. In our experiments, both distilled water and the initial solution were fed as brine into the cathode chamber of the electrodialyzer. By changing the flow rate, and also the dialysate and brine circulating respectively through the anode and cathode chambers, it is possible to regulate the degree of ion extraction.

In the course of experiments on the tubular electrodialyzer, samples were taken from the anolyte and catholyte at definite time intervals; these samples were analyzed on a spectrophotometer constructed on the basis of a UM-2 monochromator. The error of individual measurements was about 10%. The results of the analysis are presented in Table 2. After 1.5 hours of operation of the electrodialyzer, the average degree of ion extraction was 84%. The experimental conditions were as follows: solution flow rate — 2.4 l/hour. Water flow rate in the cathode chamber — 0.6 l/hour. The current in the electrodialyzer was maintained equal to 75 mA. The consumption of electrical energy per 1 l of solution desalinated to the indicated degree was 0.74 watt·hour/l. When the initial solution was used as the brine, the consumption of electrical energy increased somewhat and amounted to 0.9 watt·hour/l.

Fig. 2. Diagram of a tubular electrodialyzer

Taking into account the specific features of the design of the tubular electrodialyzer, the coefficient of utilization of membrane area in it is about 90%. In filter-press-type apparatuses, however, it does not exceed 70% [1].

In addition to the indicated application, the tubular electrodialyzer can also be used for solving other problems, for example, for extracting iron ions from pickling effluents, obtaining acids, bases, etc.

Table 2

Change in the concentration of $K^+$ and $Na^+$ ions in the anode chambers as a function of electrodialysis time

Tubular membranes can be manufactured on the basis of an anion-exchange membrane. In this case, the tubular electrodialyzer can also be used to remove excess acidity from solutions.

Conclusions

1. A design has been developed for a tubular electrodialyzer with ion-exchange membranes.

2. In the tubular electrodialyzer, the consumption of electrical energy in the extraction of \( \mathrm{Na}^+ \) and \( \mathrm{K}^+ \) ions from solution was \(0.74\) W·h/liter (degree of extraction 84%).

3. The useful membrane area in the electrodialyzer is about 90%, whereas in filter-press-type apparatus it does not exceed 70%.

Literature

1. D. R. Wilson. Demineralization by the Electrodialysis Method. Moscow, Gosatomizdat, 1963.

2. A. S. Shubin, V. M. Novakovskii, L. M. Smirnova. In: Ion-Exchange Sorbents in Industry. Moscow, Publishing House of the Academy of Sciences of the USSR, 1963.

3. B. N. Laskorin et al. Ion-Exchange Membranes and Their Application. Moscow, Gosatomizdat, 1961.

4. E. D. Mason, E. D. Parsi. Proceedings of the Geneva Conference on the Peaceful Uses of Atomic Energy. Vol. 1, Atomizdat, pp. 457–460, 1959.

5. N. I. Isaev, V. A. Shaposhnik. Zavodskaya laboratoriya, XXXI, 10, 1965.