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Salt Metathesis by Electrodialysis

A Method to change the salt partners between two salts


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The Principle


Conventional electrodialysis changes concentrations of salt solution. Within this transport process, a separate movement of the cations is achieved towards the anions. To get the higher concentrated salt, both ions are collected together in one solution. However, it is also possible change the setup by using two different salts and collecting the anions and cations crosswise into two new salts, in which the ion pairs have changed.


MA + ma -> Ma + mA

How is this done?

As within a conventional electrodialysis unit, the solutions are separated by alternatively arranged anion and cation exchange membranes. The two kinds of compartments formed, are now driven as four compartments: Conc1, Dil1, Conc2, Dil2. The repeated unit into the cell now contains four units. Applying a current, cations within the first diluate (grey compartment set) move into a different concentrate compartment than the anions from this cell. And the cations in this cell, wherein the said anions arrive, are the cations from the second diluate compartment. These anions have changed their partner. Putting together all elementary processes, simply the salt pairs are changed.


Salt Metathesis Systems:

PCCell EDM System

EDM Cells
Laboratory size (up to 1 sqm)
PCCell ED 64 0 04
Small industrial size
PCCell EDQ380
Electrodialysis Systems
Bench scale
PCCell B-ED 1-4
Bench scale
PCCell EDM System






Running a Salt Metathesis Process


Salt metathesis is usually performed as a batch process.

An important point in running a salt metathesis process is the mass balance concerning the diluates. Application is very interesting in cases, a diluate stream is available with a high concentration and which has not to be completely desalted. This is the case e.g. for an fermentation, where the product should be removed by a certain extent. After ED-treatment the fermentation solution is recycled into the fermenter. The result is a continuos removal of product at a continuous high concentration level of the diluate. In such application, high current densities may be achieved.


Application examples of the salt metathesis


Typical metathesis applications are conversions of Calcium salts of organic acids into their acidic form by metathesis with hydrochloric acid. Often, calcium salts are obtained in fermentation processes or by the tartaric stabilization of wine. If the calcium salts are poorly soluble, they may also be processed into the sodium salts by processing with hot sodium carbonate solution. In each case, a metal free acid of a relatively high concentration is obtained. In cases of soluble calcium salts, a calcium chloride solution is obtained, which may be recycled into the process of obtaining the calcium salt.


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