Electrodialysis Equipment
Electrodialysis equipment
Basic Principles and Characteristics
In an electrodialysis apparatus, many cation exchange membranes and anion exchange membranes are arranged alternately, dividing the device into small water compartments. When raw water enters these compartments, under the influence of a direct current electric field, the ions in the solution migrate in a directed manner. Cation exchange membranes only allow cations to pass through while retaining anions; anion exchange membranes only allow anions to pass through while retaining cations. As a result, some of these compartments become freshwater compartments with very few ions, and the outlet water is called freshwater. The adjacent compartments become concentrated water compartments with a large number of ions, and the outlet water is called concentrated water. Thus, ions are separated and concentrated, and the water is purified.
Compared with ion exchange, electrodialysis has the following similarities and differences:
(1) Although both methods use ion exchange resin as the working medium, the former is in the form of thin films, while the latter is in the form of spherical particles;
(2) From the perspective of mechanism, ion exchange belongs to ion transfer and replacement, where ion exchange resin undergoes ion exchange reactions during the process. Electrodialysis belongs to ion retention and replacement, where ion exchange membranes play a role in ion selective permeation and blocking. Therefore, more accurately, ion exchange membranes should be called ion selective permeable membranes;
(3) The working medium of electrodialysis does not need regeneration but consumes electrical energy; while the working medium of ion exchange must be regenerated but does not consume electrical energy. The characteristics of wastewater treatment by electrodialysis are: no need to consume chemical agents, simple equipment, and easy operation.
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Application of Electrodialysis in Wastewater Treatment:
Electrodialysis was initially used for seawater desalination to produce drinking water and industrial water, seawater concentration for salt production, and combined with other unit technologies to produce high-purity water. Later, it was widely applied in wastewater treatment.
In wastewater treatment, there are two types of electrodialysis operations based on process characteristics:
- Conventional electrodialysis process: Alternately arranged cation and anion exchange membranes are mainly used for simple separation of pollutant ions from wastewater, or to separate pollutant ions and non-electrolyte pollutants from wastewater for further treatment.
- Special electrodialysis process for separation and concentration: Composed of composite membranes and cation membranes, this process utilizes polarization reaction in composite membranes and electrode reaction in polar chambers to produce H+ and OH- ions, thereby producing acids and alkalis from wastewater.
Currently, the widespread applications of electrodialysis in wastewater treatment practice include:
- Treatment of alkaline papermaking wastewater: recovering alkali from concentrated liquid and lignin from dilute liquid.
- Separation and concentration of heavy metal ions from wastewater containing metal ions, followed by further treatment or recycling of concentrated liquid.
- Separation of radioactive elements from radioactive wastewater.
- Production of sulfuric acid and sodium hydroxide from mirabilite wastewater.
- Production of sulfuric acid and deposition of heavy metal ions from pickling wastewater.
- Treatment of electroplating wastewater and waste liquid, including wastewater containing Cu2+, Zn2+, Cr(IV), Ni2+ and other metal ions, which are suitable for electrodialysis treatment. Among them, nickel recovery from nickel plating wastewater is widely applied, and many factory practices have shown that this method can achieve closed-loop circulation.
