Hemibasic calcium hypochlorite

Calcium hypochlorite is the principal commercial soHd hypochlorite it is produced on a large scale and marketed as a 65—70% product containing sodium chloride and water as the main diluents. A product with a significantly higher available chlorine, av CI2, (75—80%) has been introduced by Olin. Calcium hypochlorite is also manufactured to a smaller extent as a hemibasic compound (- 60% av Cl ) and to a lesser extent in the form of bleaching powder (- 35% av CI2). Lithium hypochlorite is produced on a small scale and is sold as a 35% assay product for specialty appHcations. Small amounts of NaOCl ate employed in the manufacture of crystalline chlorinated ttisodium phosphate [56802-99-4]. 

Chlorination of thick lime slurry at 40—45°C forms large crystals of hemibasic calcium hypochlorite. The fine crystals obtained under 30°C are difficult to filter and since they invariably contain occluded mother Hquor, they have frequently been incorrectly referred to as monobasic or two-thirds basic (187,188). The isolated hemibasic crystals are suspended in a thin chlorinated lime slurry and chlorinated, producing laminar crystals of Ca(OCl)2 2H20, which are filtered and dried. Mother Hquors are treated with a lime slurry to recover the dibasic crystals, which are then suspended in a Hquor of lower CaCl2 content and chlorinated to form the neutral salt (188—190). 

Hemibasic Calcium Hypochlorite. This basic hypochlorite [62974 2-9] is produced by chlorination of lime. It has a lime content of... 

Manufacture. Hemibasic calcium hypochlorite is manufactured by chlorination of lime slurry followed by filtration and dryiag of the filter cake. 

Economic Aspects. The approximate annual world capacity of hemibasic calcium hypochlorite, ca 60% av CI2, was 28,000 t ia 1988. The principal producers are Japan and The People s RepubHc of China Italy and India are minor producers (220). 

Applications. Because of its high lime content, the use of hemibasic calcium hypochlorite ia geaeral sanitatioa is limited. It is used primarily ia Japan and lesser developed countries as an alternative to bleaching powder. 

Hemibasic calcium hypochlorite, Ca(OCl)2-0.5Ca(OH)2, can be precipitated from a solution of calcium hypochlorite at 40-80°C. " These solutions are commercially made by chlorinating a slurry of hydrated lime. The precipitate is removed by filtration and dried. Although it may still be made in Asia, it has not been made in the United States or Germany since 1955. ... 

The precipitation of basic calcium hypochlorites can be nsed to ranove calcium chloride. The excess calcium that is not associated with calcium hypochlorite can be precipitated by adding sulfate or carbonate, but this is less effective, and does not remove other impurities that remain in the calcium hypochlorite after the solution is evaporated. In either case, the conversion of lime to calcium hypochlorite is reduced by the amount of calcium salts that are removed from the process. In an earlier process, a concentrated lime slurry is chlorinated at 40-45 C to form crystals of hemibasic calcium hypochlorite. These are ranoved by filtration and snspended in a slurry of chlorinated lime. Chlorine is then added to make calcinm hypochlorite dihydrate crystals that are removed by filtration. The filtrate is mixed with lime to precipitate dibasic calcium hypochlorite. The dibasic calcium hypochlorite is ranoved by filtration, snspended in recycled liquor, and chlorinated to form crystals of calcium hypochlorite dihydrate. Throughout the process, seed crystals are used and the filtrates are recycled. " Alternatively, slow chlorination at 15°C is used to directly produce calcium hypochlorite dihydrate instead of the hanibasic compound in the first step. Dibasic calcium hypochlorite is made from the filtrate and processed as before. " ... 

Intermediates may also be used to remove impurities from low-quality lime. Figure 24.10 shows the precipitation of dibasic calcium hypochlorite from mixing lime with recycled solutions of calcium hypochlorite and sodium chloride. The crystals are separated and purified by elutria-tion. Sodium hypochlorite is added and the mixture is chlorinated to make calcium hypochlorite dihydrate. This product is filtered and dried. ° For further purification, the dibasic calcium hypochlorite crystals may be reacted with chlorine or a solution of calcium hypochlorite to precipitate hemibasic calcium hypochlorite. These crystals are separated, mixed with water, and chlorinated. The resulting calcium hypochlorite dihydrate is separated and dried. Sodium hydroxide may be added before chlorination, or sodium hypochlorite may be mixed with the filter cake to convert calcium chloride to calcium hypochlorite. " ... 

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