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Salt Metathesis by Electrodialysis

A Method to change the salt partners between two salts

 

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The Principle

All kind of salt consits of ion pairs, which are formed by anions and cations. Electrodialysis is a process to move anions and cations separetely over ion exchange membranes into another solution. As described before, this is used in conventional electrodialysis to change the concentration of a salt. However, it is also possible to transform two different salts into two new salts, in which the ion pairs have changed.


 

MA + ma -> Ma + mA
 

Simply by using the fact, that after one ion is moved over a membrane, can be recombinated by any other kind of conter ion, which you are able to present to it.

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.


 

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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