Chloride ion is the most common anion in water and wastewater. Excessively high concentration of chloride ion will cause bitter and salty drinking water, soil salinization, pipeline corrosion, difficult plant growth, and endanger human health. Therefore, chloride ion must be controlled emission concentration. If a large amount of chloride ions contained in wastewater are not effectively removed and discharged into water bodies, it will cause serious and lasting harm to human health, soil, and the ecological environment. Many newly-implemented local standards have stipulated corresponding chloride ion concentration discharge limits to limit the discharge concentration of chloride ions. According to different properties, they are roughly classified as follows: (1) Precipitation That is to use Ag+ or Hg+ to form a precipitate with Cl- to remove Cl-. Since chloride ions can form soluble salts with most metal ions, it is difficult to remove chloride ions from water by ordinary precipitation techniques. Although silver ions can form insoluble silver chloride precipitates with chloride ions, silver ions are difficult to recover and are too expensive for large-scale application. It is also possible to use cuprous ions and chloride ions to form cuprous chloride precipitation to realize the separation of chloride ions, but cuprous ions are easily oxidized, the conditions are difficult to control, and the treatment cost is also high. (2) Membrane separation Membrane separation technology is one of the commonly used technologies for water desalination, mainly including electrodialysis and reverse osmosis. At present, it is increasingly used in the field of wastewater desalination (dechlorination). Membrane separation technology can effectively remove chloride ions from wastewater, and is itself an excellent dechlorination technology. However, for most chlorine-containing wastewater, the chlorine content of the wastewater is usually high, which often exceeds the application limit of membrane separation technology, and these wastewater often contain a large amount of organic matter and other impurities, which will cause irreversible damage to the membrane module. pollution, which limits the application of membrane separation technology. (3) Evaporation When the content of chloride ions is high, evaporation technology can be used to concentrate chlorine-containing wastewater through evaporation and crystallize chlorine-containing salts to complete the separation of chloride ions and water. At present, the commonly used methods mainly include multi-effect evaporation, membrane distillation and molecular distillation. Although the treatment effect is better, due to the high content of chloride ions and the corrosion resistance requirements of the equipment, special alloys or even metals are usually required. Titanium is processed, so the equipment costs. At the same time, due to the high specific heat of water, the energy consumed by evaporating a unit of water is also huge, so the operating cost of evaporation technology is very high, usually ranging from tens to hundreds of yuan per ton of water. (4) Flocculation and precipitation, solvent extraction method Flocculation and precipitation mainly use flocculants to act on chloride ions to flocculate and remove them, such as composite flocculants; solvent extraction is to use extractants to extract and remove compounds containing chloride ions. (5) Ion exchange method When the content of chloride ions in wastewater is low, ion exchange resins can be used to remove them. Studies have shown that strong basic anion resins have a strong adsorption effect on chloride ions. (6) Redox method Cl- is removed by electrolysis, electrodialysis and reduction. The application methods include electrolysis, electrodialysis, adding oxidizing agent and so on. Electrolysis means that when the sewage is energized, a potential difference is generated between the negative and positive stages of the electrolytic cell, which tends to cause the anions in the sewage to move to the anode to undergo an oxidation reaction, and the cations to move to the cathode to undergo a reduction reaction, so that the pollutants in the wastewater are oxidized at the anode. It is reduced at the cathode, or reacts with the electrode reaction product, and is converted into harmless components, which are separated and removed. (7) Bismuth oxide method The bismuth oxide method is that after adding bismuth oxide reagent to the stock solution, the bismuth ions formed under acidic conditions will be hydrolyzed with bismuth ions and chloride ions within a certain pH range to form insoluble bismuth oxychloride precipitates. Remove the chloride ions in the stock solution. (8) Ultra-high lime aluminum method The ultra-high lime aluminum method is also called the Freund's salt method. The Freund's salt method is to add calcium oxide and sodium metaaluminate to chlorine-containing wastewater, and react under certain conditions to form calcium-chloro-aluminum compound precipitation. In order to achieve the purpose of removing chloride ions, in 1897, P. M. Friedel first discovered Freund's salt when he studied the chemical reaction of AlCl3. Ca4Al2Cl2(OH)12 is a kind of Freund's salt. 6]+ and negatively charged [Cl-, 2H2O] composed of layered compounds, in which [Ca2Al(OH)6]+ constitutes the main laminate, and [Cl-, 2H2O] is an interlayer anion.
