Electrodialysis

A number of surface treating processes require a rinsing fluid, which dissolves salts produced in the process. It is possible to recover more than 80% of the salts in a rinsing fluid using electrodialysis equipment. This electrochemical process is particularly suitable for recovering salts of silver, copper, nickel and zinc. The method involves a large number of electrolytic cells in series. Each cell comprises a cationic membrane and an anionic membrane, with a spacer between each pair of membranes.

Positive ions (cations) pass through the cationic membranes towards the cathode, but they cannot pass through the anionic membranes. Similarly, negatively charged ions move in the opposite direction, towards the anode, passing through the anionic membranes but then being stopped by the cationic membranes. The result is that in one cell the solution is enriched both with positive and negative ions (i.e. with the whole salt), whilst in the adjacent cells the concentration of both positive and negative ions is reduced (i.e. the solution is deionized).

Applications for electrodialysis include:

mother liquor demineralization from crystallization
recovery and concentration of acids
treatment of fermentation liquors
ionic standardization control of ionic concentration, and
substitution and double decomposition reaction.

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