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New Anion Exchange Membranes for the Electrodialysis of Acids

P. Altmeier, A. Konradb

PCA - Polymerchemie Altmeier GmbH; D-66822 Lebach; Germany

bInstitut für chemische Technik, Universität Karlsruhe; D-76128 Karlsruhe

 

To obtain high current efficiencies by electrodialysis in acid medium, there is the need of special ion exchange membranes because of the high permeability of acids through standard anion exchange membranes.

In the first part of this work, the particular demands for membranes in different acids are pointed out and membranes developed in regard of these effects are presented. In hydrochloric acid, there is the demand of a membrane with a low proton leakage, which normally corresponds with low diffusion permeability. It is found that the factor a , defined as the molar ratio of water per fixed ions in the membrane, is the main component affecting the proton leakage. In sulfuric acid, some membranes show an unexpected high ohmís resistance depending on their chemical composition. A factor b is introduced, defined as the ratio of the ohmís resistance in sulfate to that of chloride containing solutions. b is found to be in the range of 8 to 12 in the case of membranes with high proton permeabilities, while otherwise it is found to be in the range among 4 and 6. Membrane materials for the use in sulfate solutions should show low b -values and have to be optimized with regard to a low a -value. The proportions in nitric acid, hydrofluoric acid and phosphoric acid are discussed briefly. For the electrodialysis of organic acids with higher molecular weight like gluconic acid the limiting effect for the application of electrodialysis is the low permeability of the membrane matrix for large anions. Membranes with an optimized matrix for large anions are presented.

In the second part of this work, optimized membranes are characterized in a electro-electrodialysis cell. A cation-, an anion- and a further cation exchange membrane are placed between the electrodes forming two electrode chambers and two middle chambers. In the middle compartment next to the cathode chamber a salt solution of the corresponding acid is circulated and split in course of the process to the product acid, which is circulated in the chamber next to the anode and to the product base. The concentration and amount of produced acid and voltage drops across the system compounds are analyzed in the test sequence. The following table shows some characteristic values obtained in the electrodialysis experiments.

Membrane

optimized for

transference number

(0,1 / 0,5 N KCl) t-

current efficiency

/ % *

ohm's resistance

/ W cm2 *

PC Acid 35

hydrochloric acid

>0,95

55

4

PC Acid 70

nitric acid

>0,93

50

5

PC Acid 100

sulfuric acid

>0,88 (1 / 2 N KCl)

41

4

PC 100D

acetic acid

> 0,96

91

2

PC 200D

gluconic acid

> 0,94

94

3,5

PC 400D

lactobionic acid

> 0,82

67

30

 

* 3 N Acid; 0,7 N Salt; current density: 1 kA m2

The results are discussed and the energy need and product purity of such processes using hydrogen consuming anodes or bipolar membranes respectively are compared. It is shown that the technology of producing the corresponding acids and bases from their salts could be effectively improved by the use of optimized anion exchange membranes which are one of the weak points of this technology up to now.

 

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