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The oxidation of hydrochloric acid gas by atmospheric oxygen in the presence of cupric chloride as a catalyst was applied by H. Deacon and F. Hurter in 1868 as a technical process for the preparation of chlorine:
4HCl + O2 <=> 2H2O + 2Cl2.
Hasenclever in 1883 improved the method, and chiefly in his hands the process became a successful technical operation, which almost completely displaced the older and wasteful Weldon process.
Hasenclever found that the contact mass impregnated with copper salt lost its activity slowly in any case, and had to be replaced from time to time. He used a decomposer consisting of an upright iron cylinder, 12-15 ft. wide, containing a ring of broken bricks, previously dipped into a solution of cupric chloride so as to contain 0.6-0.7 per cent. of copper in the mass, supported by iron shutters, and divided into six compartments, one of which can be emptied and refilled with fresh contact mass every fortnight.
 | Fig: Deacon converter |
He dissolved the crude gas from the saltcake furnaces in water in a tower, and ran the aqueous acid in a slow stream into concentrated sulphuric acid, blowing out the hydrochloric acid gas with a current of air. The mixture of 4 vols. of air and 1 vol. of hydrochloric acid gas is passed by a hot Roots' blower through a set of iron pipes heated in a furnace, called a preheater, where its temperature is raised to 450°. The gases then pass to the decomposer, which is kept at this temperature by hot flue gases from the preheater. About two-thirds of the HCl is decomposed, and the rest is washed out of the gas with water in a coke-tower. The gas, containing 5-10 per cent, of chlorine, diluted with nitrogen, is then dried in a sulphuric acid tower and used in making bleaching powder. The Deacon reaction is reversible, and the decomposition of HCl diminishes with rise of temperature. Below 350°, however, there is practically no decomposition, and the reaction becomes sufficiently rapid only at 425°-450°. There are two opposite conditions to satisfy: (i) the yield of chlorine, which decreases with rise of temperature; (ii) the speed of the reaction, which creases with rise of temperature. A technical balance is struck at about 450°, when about two-thirds of the HCl is decomposed. The Deacon process, which displaced the Weldon method, has now been rendered almost obsolete by the electrolytic processes.
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