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With Electromembrane processes generally electrolyte solutions are processed between electrodes separated by membranes. Those membranes can be ion exchange membranes or microporous diaphragms. The membranes fulfill different functions:

  • as separators of the electrodes,
  • as selective separators of electrolyte solutions allowing some ions to pass and block others according to their charge and size and
  • as substitutes for electrolyte in solution (SPE processes).
electron microscopic view of electrodialysis membrane
 

 

 

The selectivity of ion exchange membranes results in a variety of different processes:

  • Electrodialysis: desalination and concentration of salt solutions
    (deutsche Beschreibung,
    version Francaise )
  • Membrane electrolysis: : e.g. caustic/chlorine production, liquid disinfectant production, oxidation of Fe(II) solutions etc. A combination of electrode reactions as a source of hydrogen ions and hydroxide ions with the selectivity of ion exchange membranes to split neutral salts into their corresponding acids and bases
  • Electrodialysis with bipolar membranes: splitting of neutral salts into their corresponding acids and bases using bipolar membranes as a source of protons and hydroxide ions
  • Salt metathesis: a special electrodialysis process in which two different diluates (salt ca and salt CA) and two different concentrates (salt Ca and salt cA) are processes: A change between the salt ions takes place. Example:

    Sodium Gluconate + Hydrochloric Acid ----> Sodium Chloride + Gluconic Acid

  • Diffusion dialysis
  • SPE processes including fuel cell and chemical reactors
The processes are designed for
  • a concentration and/or dilution of a solution or
  • a purification and/or isolation of a product from a mixture (removing of salt from sugar solution)
  • a chemical reaction of salts
  • the production of new material including hydroxide and protons or other electrode reaction products

     

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    Special Application Situations

     

    The classic application of Electrodialysis is the desalination of seawater and brackish water (see German version ) in which large areas of ion exchange membranes have been used for more than 40 years. Other examples are the desalination of whey and soy.[1]

    PCA deals with new applications: not only salts may be removed or reconcentrated but also acids. Examples are the process of recycling of pickling acid from rinsing water (German) in the hot dip galvanizing industry or the recycling of HNO3 / HF - mixed acid (German) from pickling acids.

     

    [1] Development and testing of a pilot-scale electrodialyser for desalination of fish sauce. J. Jundee et al. Porcedia engineering 32(2012) 97-103.

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