Production process of calcium hypochlorite
At present, there are three main types of calcium hypochlorite production methods, namely, calcium process, sodium process and hypochlorous acid process: the calcium process route is shown in Figure 1. First, a lime slurry is added to the chlorination tank, and then chlorine gas is introduced therein to carry out a chlorination reaction. The crystallized state of the product obtained at different times of the reaction is different, and the crystal state of the product is observed after a certain time interval to determine the progress of the reaction. When the reaction is completed, the chlorinated slurry is subjected to solid-liquid separation, and the bottom solid phase product is placed in a dry box for drying, and after drying, the product is obtained by cooling. The calcium chloride product produced by the calcium method generally has an effective chlorine content of about 60%, and the products are mostly irregularly shaped particles or powders, resulting in poor product usability. At the same time, the product contains more impurities, containing about 10% calcium chloride and 20% unreacted calcium hydroxide, resulting in strong hygroscopicity and poor stability. In the production of this method, about 10 tons of waste liquid is discharged per ton of product, containing about 10% of available chlorine, and the waste liquid is difficult to handle.
Figure 1 Calcium production of calcium hypochlorite process route
The sodium production process is shown in Figure 2. Firstly, calcium hydroxide is added to the slurry tank for slurrying, and then chlorine gas and sodium hydroxide are introduced for chlorination reaction, thereby making up for the defect of more impurities in the calcium production process, and the calcium hygrometer having strong hygroscopicity. And the conversion of the raw material calcium hydroxide into sodium chloride, while improving the stability of the product, increasing the utilization rate of the raw material, the content of calcium hydroxide in the product is less than 3%. Due to the low content of impurities, the effective chlorine content of the product can reach more than 70%. The product is regular granular and easy to use. After the chlorination is completed, the slurry obtained is separated to obtain a bottom solid phase, which is dried in a dry box, and after cooling, is cooled and recovered. At the same time, the supernatant liquid, that is, the mother liquor, can also be recycled and reused. The sodium chloride can be used for electrolytic production of chlorine and alkali in the chlor-alkali plant after purification treatment, thereby realizing cyclic production.
Figure 2: Sodium hypochlorite production route
The hypochlorous acid process route is shown in Figure 3. First, chlorine gas is introduced into the sodium carbonate aqueous solution to prepare hypochlorous acid, and then the lime slurry is chlorinated with hypochlorous acid to obtain calcium hypochlorite, which is then neutralized with hydrated lime to obtain a finished product. Since hypochlorous acid is an unstable inorganic weak acid, it is the main component of chlorine bleaching agent. It is oxidizing and unstable. It is easily decomposed at normal temperature. Therefore, it is usually used immediately after preparation, or it can be prepared first. The acid chloric acid is dichlorinated, and sodium carbonate is used as a raw material. The bleaching powder product produced by this process has high purity, its effective chlorine can reach more than 75%, the product is more stable, and the use is safer and more convenient. The three wastes in production are also well handled. However, hypochlorous acid is highly corrosive, requires high selection of materials for related equipment, and has good corrosion resistance. The use of the support material is generally required, which is expensive, resulting in high equipment investment, thereby increasing production costs.
Through the above comparative analysis, it can be known that the calcium method has low effective chlorine content, irregular products, many residues, and difficult treatment of waste liquid, which is not suitable for environmental protection requirements; while the hypochlorous acid process has high effective chlorine content, stable product, convenient use, etc. Advantages, but high cost, high equipment investment, and poor market competitiveness. Therefore, the sodium production process is currently the most suitable and can guarantee the quality requirements of calcium hypochlorite, and should be widely applied in actual production.