The principle of desalination by electrodialysis
1. The principle of electrodialysis desalination
Electrodialysis is a transfer process of solution separation, purification and concentration by utilizing the selective permeability of ion exchange membranes under the action of a DC electric field to generate directional migration of anions and cations. Electrodialysis (referred to as ED) technology is a kind of membrane separation technology, which alternately arranges anion and cation exchange membranes between positive and negative electrodes, and separates them with special separators to form two components: desalination (desalination) and concentration. Under the action of a DC electric field, the potential difference is used as the driving force, and the selective permeability of the ion exchange membrane is used to separate the electrolyte from the solution, so as to realize the concentration, desalination, purification and purification of the solution.
2. How to choose electrodialysis electrode
1. Electrodialysis electrodes are divided into: platinum-plated titanium electrodes, titanium-coated ruthenium electrodes, graphite electrodes, and stainless steel electrodes; electrodes have different design specifications according to the size of the electrodialysis device.
2. Different electrode materials have different characteristics.
Titanium platinized electrode: The corrosion resistance is quite good, and it can be used under very harsh conditions, but the price of platinum is expensive and the resources are few, which limits its promotion in China.
Titanium-coated ruthenium electrode: It is to coat ruthenium (Ru) and iridium (Ir) compounds on the titanium substrate, and form its mixed oxide after high temperature treatment; because the ionic radii of ruthenium (Ru) and iridium (Ir) are very close, the point The array structure and space group belong to the same type, so RuO2-IrO2 solid solution can be formed in the co-oxidation of heat treatment, which has excellent corrosion resistance and is very suitable as an electrode material.
Graphite electrode: Graphite electrode is easily corroded, mainly due to chemical corrosion and mechanical wear; when graphite is used as an anode, graphite is oxidized to CO2 or CO due to anodic oxidation, and its crystal structure is destroyed and damaged; in electrodialysis device The loss of medium graphite electrode is mainly caused by mechanical action. Water at high flow rate has a strong scouring effect on graphite. On the other hand, the gas generated by the electrode reaction has an impact on graphite. Coupled with electrochemical corrosion, graphite particles are often caused. Exfoliation pollutes the water quality and even blocks the anode water channel; with the advent of new insoluble titanium-coated ruthenium oxide electrodes, graphite electrodes have been gradually eliminated.
Stainless steel electrode: Generally speaking, stainless steel is only used as a cathode and cannot be used as an anode. Otherwise, because the natural water contains more chloride ions, the anode of stainless steel will dissolve to generate divalent iron, nickel and chromium ions.
The correct selection of electrode materials is of great significance to prolong the service life of electrodes, reduce system investment and operating costs, and choose electrodes of different materials for different water quality:
(1) For chlorides as the main component, titanium-coated ruthenium electrodes can be used.
(2) For those with sulfate as the main component, lead plate and titanium-coated iridium-tantalum-coated electrodes can be used.
(3) For calcium bicarbonate as the main component, titanium-coated ruthenium electrodes can be used.
(4) For mixed ions, titanium-coated ruthenium-iridium, iridium-tantalum, titanium-plating and graphite electrodes can be used.
