Chlorine gas does not react with quicklime at the ordinary temperature, but at a red heat oxygen is expelled and calcium chloride formed: 2CaO + 2Cl2 = 2CaCl2 + O2. If, however, chlorine is passed over slaked lime, Ca(OH)2, it is rapidly absorbed, forming a somewhat moist powder which smells of hypochlorous acid, and is called bleaching powder, or chloride of lime. The reaction is: Ca(OH)2 + Cl2 = CaOCl2 + H2O, the water formed remaining principally in the powder.
In the manufacture of bleaching powder the slaked lime is spread over the floors of closed lead chambers, so as to expose a large surface, and somewhat diluted chlorine gas admitted. At first the chlorine is rapidly absorbed, but the reaction afterwards slows down. The powder is then turned over with wooden rakes, and the action of the gas continued until absorption is complete, which takes 12-14 hours. The product usually contains 35-37 per cent, of chlorine present as CaOCl2, whereas that calculated from the formula CaOCl2 + H2O is 49. Some free lime is also present.
With very dilute chlorine, such as is produced by the Deacon process, it is necessary to provide a very intimate contact of the lime with the gas. This is effected by making the gas traverse lead or iron pipes placed horizontally one above the other, through which the lime is pushed in the opposite direction to the gas by means of Archimedean screws (Hasenclever screw-chambers)
Fig: Hasenclever bleaching powder apparatus
The lime drops from one pipe to the other and is withdrawn into casks at the bottom fully charged with chlorine. In modern works, large rotating inclined iron cylinders, cooled externally by water where the reaction is vigorous, are used, the slaked lime passing down in the opposite direction to the chlorine.