Stable Bleaching Powder

Stable bleaching powder is produced according to Backmann s method by using slaked lime with a minimum of moisture (which must not exceed 0.5 per cent) for the process. Also chlorine must be free of any moisture. 

Chlorination is carried out in tall, reinforced concrete towers protected inside by a bituminous coating (see Fig. 133). 

During chlorination the highest temperatures of 35 to 40 °C is reached on the middle floor, while on the bottom floor a temperature of approximately 

The Backmann apparatus is manufactured by the Krebs factory for a daily output of 3 to 5 tons of bleaching powder. 

Manufacture of stable bleaching powder according to Moore 

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Sodium Tetrahydroborate, Na[BH ]. This air-stable white powder, commonly referred to as sodium borohydride, is the most widely commercialized boron hydride material. It is used in a variety of industrial processes including bleaching of paper pulp and clays, preparation and purification of organic chemicals and pharmaceuticals, textile dye reduction, recovery of valuable metals, wastewater treatment, and production of dithionite compounds. Sodium borohydride is produced in the United States by Morton International, Inc., the Alfa Division of Johnson Matthey, Inc., and Covan Limited, with Morton International supplying about 75% of market. More than six million pounds of this material suppHed as powder, pellets, and aqueous solution, were produced in 1990. 

The finished bleaching powder usually contains 36 to 39 per cent total chlorine, 35 to 37 per cent active chlorine, 2 to 4 per cent calcium chloride, 0.5 per cent calcium chlorate, 2 to 3 per cent calcium carbonate, 8 to 16 per cent free calcium hydroxide, 0.05 to 0.15 per cent iron, 5 to 8 per cent moisture and 12 to 18 per cent total water. The product is not stable and the chlorine content gradually decreases. After being stored for 70 days, the average active chlorine loss amounts to some 11 per cent, after 120 days it reaches some 14 per cent. In order to reduce losses it is necessary to store the bleaching powder in a cool and dry place. 

Bleaching powder manufactured in chambers or in Hasenclever s apparatus is not stable and in the course of time the active chlorine content diminishes due to the following reactions ... 

A stability test is made after 2 hours drying at 100 °C hi order to ascertain the drop in active chlorine content. A stable product manufactured in Moore s equipment shows a loss of 3.5 per cent while an unstable product about 14 per cent. From this we can see that although decomposition is suppressed to a considerable extent in the case of a stable product, the active chlorine loss is still noticeable. The so called superstable bleaching powder is prepared by the addition of quick lime to the stable product. A stability test then shows active chlorine loss to be less than 0.75 per cent. 

Bleaching powder is packed in well sealed wooden barrels or in sheet iron drums. It is used mainly to bleach fabrics in the textile industry and cellulose in the paper industry. Bleaching powder is also a well known disinfectant and is used in purifying water for municipal purposes. Certain quantities are used for oxidation reactions in organic chemistry, for instance in the production of chloroform. The stable product is exported mainly to tropical countries. 

Efforts to produce a bleaching powder in a more concentrated and stable form resulted in the manufacture of calcium hypochlorite Ca(0 Cl)2. In pure form it contains 99.2 per cent active chlorine. It is manufactured in various ways and sold commercially under different names, e. g. Perchloron produced by I. G. Far ben, H. T. H. by Mathieson Alkali Works, Pittchlor by Pittsburg Plate Glass Co., Columbia. Chemical Division. 

The advantage of calcium hypochlorite, compared to standard bleaching powder, is a higher content of active chlorine. Moreover, it forms clear solutions in water and is much more stable particularly at higher temperatures. After 5 years storage in tropical countries, chlorine content will not drop below 55 per cent. Another advantage of the calcium hypochlorite is that it is less hygroscopic due to a lower content of calcium chloride. 

Tropical bleach is produced by blending bleaching powder with finely divided quicklime which converts any free water into calcium hydroxide. The resulting product is stable at temperatures up to 100 °C. 

Colorless to brown liquid with a fruity smell of bitter almond odorless in pure form boils at 240°C (464 F) freezes at -50 C (-58 F) vapor pressure 0.07 mm Hg [20° C (68°F)] density 1.077 at 20°C (68°F) miscible with organic solvents and water (hydrolyzes) solubility in water, 7.2 g/100 g at 20°C (68°F) rapidly hydrolyzed in basic solution, at 25°C (77°F) and pH 11, its half-life 1.5 minute decomposes when heated over 150°C (302°F) forming HCN, NOx, POx, and CO thermally stable below 49°C (120.2°F) decomposed by bleaching powder, forming cyanogen chloride synthesized by reacting sodium cyanide with dimethylamidophosphoryl dichloride in the presence of ethanol. 

Solid hypochlorites are stable up to 80 °C, their stability depending on the water content, which is < 1 % for bleaching powder, < 0.3 % for tropical bleach. They decompose by reaction with water or by heating to 180 °C. 

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