I. Introduction
Calcium hypochlorite, commonly referred to as calcium chlorate, is a versatile inorganic compound with the molecular formula NaClO. It has a rich history of applications in disinfection, bleaching, and water treatment. In recent years, calcium hypochlorite has gained prominence as an alternative to chlorine gas and sodium dioxide due to concerns about their environmental impact.
II. Properties of Calcium Hypochlorite
Calcium hypochlorite is a chemical compound, often encountered in the form of a pale yellow solution. It is sensitive to light and heat, leading to decomposition and the release of oxygen gas. Additionally, it is considered a hazardous chemical (at concentrations exceeding 5%), though not highly toxic.
III. Disinfection Mechanism
Disinfection: One of the primary uses of calcium hypochlorite is disinfection. Its disinfection mechanism is similar to chlorine gas. There are two main theories:Calcium hypochlorite hydrolyzes in water to form hypochlorous acid:
NaClO + H2O → NaOH + HClO
HClO → HCl + {O}
Calcium hypochlorite can interact with cell walls, penetrate cells due to its small size and lack of charge, and oxidize proteins, disrupting cellular processes.
It is widely accepted that both reactions play a role in disinfection.
When used as a drinking water disinfectant, typical dosages range around 2 mg/L of available chlorine for post-chlorination. Pre-chlorination doses depend on the characteristics of the source water and often require pilot testing.
IV. Oxidation, Coagulation, Algae Control, and Iron-Manganese Removal
Calcium hypochlorite’s strong oxidative properties also enable it to assist in coagulation, algae control, and the removal of iron and manganese. Its oxidative effect promotes the aggregation of impurities in the water.
V. Decolorization and Bleaching
Calcium hypochlorite is effective in decolorizing water and bleaching due to its strong oxidation capacity. It can oxidize various color-causing substances and organic dyes, resulting in improved water quality.
VI. Production
Large-scale production of calcium hypochlorite is typically achieved through the chemical reaction of chlorine gas and sodium hydroxide, resulting in a product with an effective chlorine concentration of 10-13%. The product can be categorized into Type A and Type B, with variations in heavy metal content.
In recent years, the development of on-site electrochlorination systems for producing calcium hypochlorite has gained popularity. These systems offer advantages in terms of cost-efficiency, safety, and ease of use.
VII. Storage and Dosage
Proper storage and dosing are crucial when handling calcium hypochlorite. The compound is light-sensitive and can decompose when exposed to light or heat. Recommended storage materials include light-protected, corrosion-resistant containers such as polyethylene (PE) drums. It is advisable to use calcium hypochlorite within a week of dilution to reduce the formation of chlorates and perchlorates. Additionally, undiluted calcium hypochlorite should be added separately to prevent pipe scaling.
VIII. Removing Ammonia Nitrogen
Calcium hypochlorite can react with ammonia in water, reducing ammonia nitrogen. The efficiency of this process is influenced by factors such as pH, temperature, and chlorine dosage. While it is a viable method for moderate ammonia nitrogen levels, severe cases may require excessive chlorine and produce unwanted by-products.
This comprehensive overview highlights the versatile applications of calcium hypochlorite in water and wastewater treatment, emphasizing its disinfection properties, oxidative abilities, and considerations for safe handling and storage.