Dibasic calcium hypochlorite

Dibasic Magnesium Hypochlorite. This salt Mg(OCl)2 2Mg(OH)2, [11073-21-5], is safer than calcium hypochlorite because of its higher... 

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). 

Other processes also use the dibasic salt as an intermediate. Dibasic calcium hypochlorite can be prepared from filtrates from chlorinated lime slurries in various ways. In one process, the filtrate is returned to the slurry being chlorinated to keep it thin. This is designed to improve crystal growth. The dibasic crystals, together with water, are added to the slurry during chlorination and some dibasic salt is prepared by chlorination in addition to the dibasic salt made from filtrates (188). In another process, dibasic crystals are separated, slurried in water, and chlorinated to obtain a slurry of neutral Ca(OCl)2 2H20 in a mother Hquor of reduced calcium chloride content which is then filtered and air dried (191,192). 

A number of patents utilize dibasic calcium hypochlorite and concentrated NaOCl solution to produce Ca(OCl)2 Coarse dibasic crystals... 

Dibasic magnesium hypochlorite is more thermally stable than neutral or dibasic calcium hypochlorite. In addition, its decomposition, which starts at - 325° C, is endothermic rather than exothermic as in the case of the Ca compounds. 

By contrast, decomposition of dibasic calcium hypochlorite begins at 265° C to give Ca(OH)2, CaCl, and O2. Dibasic magnesium hypochlorite exhibits a high degree of stabiUty to moisture as shown by the following relative available chlorine losses at 24°C and 80% rh for 60 d Mg(OCl)2 2Mg(OH)2 2%,... 

Commercial Processes. Olin s earlier triple salt process, originally commercialized in 1928, was modified in 1983. In the patented process, a slurry of dibasic calcium hypochlorite is mixed with a strong, low salt sodium hypochlorite solution and hypochlorite Hquors and chlorinated. The resultant Ca(OCl)2 2H20 slurry is filtered, the cake going to the dry-end and the filtrate to the dibasic precipitation step where it reacts with lime. 

Bleaching Powder. This material, known siace 1798, is made by chlorination of slightly moist hydrated lime, calcium hydroxide [1305-62-0] Ca(OH)2- It has the empirical formula Ca(OCl)2 CaCl2 Ca(OH)2 2H20. Its compositioa, loag a subject of coatroversy, was estabHshed by phase studies, microscopy, and x-ray diffraction techniques (241). The initial chlorination products are monobasic calcium chloride [14031-38-4] and dibasic calcium hypochlorite [12394-14-8] ... 

Othei hypochlorites include calcium hypochlorite, bleach liquor, bleaching powder and tropical bleach, dibasic magnesium hypochlorite, lithium hypochlorite, chlorinated tnsodium phosphate, and hypochlorous acid. 

Calcium hypochlorite solutions are made by suspending lime or hydrated lime in water and adding chlorine. However, reacting lime with water generates lots of heat, and this heat must be allowed to dissipate before chlorination. The maximum concentration is -85 g L of available chlorine, above which dibasic calcium hypochlorite begins to precipitate. 

Dibasic calcium hypochlorite, Ca(OCl)2 - 2Ca(OH)2, is commonly used as an intermediate in making calcium hypochlorite, CaCOCljj. It has never been successfully marketed as a product because it dissolves much too slowly. It is made by chlorinating a slurry of lime so that the optimum amount of unreacted calcium hydroxide remains to foster its crystallization. Adding sodium chloride to a chlorinated lime solution causes dibasic calcium hypochlorite to crystallize. Dibasic calcium hypochlorite also precipitates by adding Ume to saturated solutions of calcium hypochlorite as shown in the following reaction ... 

With poorer-quality lime, a gelatinous precipitate can be separated from the dibasic calcium hypochlorite crystals in a vertical classifier, flotation cell, hydrocyclone, or centrifugal classifier. When the magnesium content of the lime is high, calcium hypochlorite crystallizes better if the chlorination is stopped after most of the calcium hydroxide has been chlorinated but before magnesium hypochlorite begins to form. ... 

In a process used since 1984, a mixture of calcium hypochlorite dihydrate and larger sodium chloride crystals are formed by chlorination and then separated based on size and density. " Solutions of calcium hypochlorite and sodium chloride recycled from the process are mixed with lime to make a slurry of dibasic calcium hypochlorite and unreacted lime. Gravity sedimentation... 

Other commercial processes make, or have made, calcium hypochlorite dihydrate by chlorinating a slurry of dibasic calcium hypochlorite. The resulting calcium hypochlorite dihydrate is separated by filtration and dried. Lime is added to the filtrate to precipitate dibasic calcium hypochlorite, which is used as the starting material. The filtrate from this step can also be reused by adding sodinm hypochlorite or sodinm hydroxide and chlorine to convert calcium chloride into calcium hypochlorite and precipitate sodium chloride. The solution is then filtered and recycled into the process. In a process implemented in 1983, a mixture of dibasic calcium hypochlorite, lime. 

FIGURE 24.9 Continuous production of calcium hypochlorite from lime and dibasic calcium hypochlorite. (Redrawn from Sakowski, W.J., U.S. Patent 4,399,117, 1983.)... 

Instead of dibasic calcium hypochlorite, a triple salt was formed in a process that was used between 1928 and 1983. A slurry of lime in concentrated sodium hypochlorite is chlorinated. After cooling to -15°C, a triple salt, Ca(OCl)2 NaOCl NaCl -I2H2O, is easily ranoved by filtration. The triple salt is mixed with a slurry of chlorinated lime that contains enough calcium chloride to consume all of the sodium hypochlorite in the triple salt. This makes calcium hypochlorite dihydrate crystals that are easy to filter by the following reaction ... 

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. " ... 

The process is simplified by only isolating one type of intermediate. A lime slurry is chlorinated to produce a solution with equimolar amounts of calcium chloride and calcium hypochlorite. To remove insoluble impurities after filtration, sodium chloride is added to cause dibasic calcium hypochlorite to crystallize. The mixture is passed through a vertical classifier to separate a slurry of dibasic calcium hypochlorite. The overflow from the classifier is recycled into the beginning... 

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. " ... 

Initially, chlorination produces a mixture of dibasic calcium hypochlorite, CafOCljj 2Ca(OH)2, and monobasic calcium chloride dihydrate, CaClj 2Ca(0H)2-2H20, by the following reaction ... 

Further chlorination converts the dibasic calcium hypochlorite to a mixed crystal that is predominately calcium hypochlorite, CafOCOj. Some dibasic calcium hypochlorite remains as the minor component. The monobasic calcium chloride remains unchanged during this second stage of chlorination, and bleaching powder is typically a mixture of this compound and the calcium hypochlorite mixed crystal. - ... 

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