Modern Gasworks Practice

538 MODERN GASWORKS PRACTICE venient chimney and so regulated that the carbon dioxide is present in the air to the extent of about 3 per cent. Hearing of the liquor up to 120° Fahr, accelerates the process considerably. ClSELET AND DeGUIDE’s PROCESS In this process1 the purification. of the gas is efl'ected by means of a limited quantity of ammonia which is constantly and automatically regenerated during the operation. For this purpose the gas is treated with. a liquid consisting of caust c ammonia in solution and lime in suspension. The ammonia reacts with the gas, the acid products of which it transforms into sulphide, cyanide and carbonate of ammonia, respectively, but in the presence of lime these ammonium salts give rise to a double decomposition with the lime and pass into calcium salts, while caustic ammonia is regenerated and thereupon contimies its purifying action. The ammonia has, therefore, the function of a mere vehicle between the gases from which it removes the acid products, and the lime to which it imparts these products imme-diately thereafter, which also retains them definitely. As long as the quantity of lime is not exhausted, the same quantity of ammonia may continually serve for the purification of a continuous current of gas. Briefly, the operation of the process is as follows : The cru.de gas coming from the retorts is in the first place freed from tar in the ordinary manner. The acid constituents are then extracted. To this end, the gases are passed through suitable apparatus such as atomizers, in which. they are placed in contact with. a liquid containing caustic ammonia in solution and hydrate of calcium in suspension. This liquid, which may be prepared in a very simple manner by mixing lime-water with a solution of caustic ammonia or with ammoniacal liquor, reacts with the gases in two phases. The first phase is the absorption of the acid products by the ammonia:— H2S +2 NH3=(NH4)2S. HCN +NH;i = NH1CN. CO2 + 2 NH3 +H2O = (NH4)2CO3. The second phase is a double decomposition between the ammonium salts and the lime :— (NH4)2S + Ca(OH)2=2 NH3 + CaS+2 H2O. (NH4)2CO3 + Ca(OH)2= CaCO3 + 2NH3 + 2 H2O. 2 NH4CN + Ca(0H)2= Ca(CN)2 + 2 NH3 + 2 H2O. The ammonia, therefore, does not remain in the state of a salt. It is con-stantly regenerated in the caustic state and can pursue its purifying action so long as the quantity of lime is not exh.au.sted. After some time it is advisable to eliminate the calcium carbonate, the precipi-tated mass of which increases rapidly. It is separated by filtration of the liquid which contains, in solution, the sulphide and cyanide of calcium and ammonia. 1 Eng. Pat. No. 9780, 1917.