Modern Gasworks Practice

THE PRELIMINARY PURIFICATION OF COAL GAS 539 If desired, the ammonia may then be distilled off. As, however, after one operation, the liquor is not sufficiently saturated with sulphide and cyanide of calcium, it is preferable to add to it a further quantity of lime and to return it to the gas purification vessels. This cycle can be contimied until the liquor is sufficiently saturated, after which the ammonia is expelled by distillation and used again in the purification of the gas. Direct Methods of Recovery During recent years some considerable attention has been given to the problem of recovering the ammonia resulting from the distillation of coal by direct means. In the ordinary way, this ammonia is wholly recovered in the form of an ammoniacal liquor, the liquor being subsequently worked up into sulphate of ammonia in a distinct plant. Chemists, particularly on the Continent, have en-deavoured to obviate the necessity for providing a separate unit for working up the ammonia into commercial form, and have introduced processes for the “ direct ” recovery of the by-product in the ordinary course of gas-making. The “ direct ” methods now employed for the recovery of ammonia may be classified under two headings, namely:— («) Those in which the acid impurity, sulphuretted hydrogen, is utilized to form sulphate of ammonia. (&) Those in which the crude gas is passed through a seal of sulphuric acid, thereby causing sulphate of ammonia to be deposited. These processes are sometijnes referred to as “ semi-direct ” methods. From the gas engineer’s standpoint the ideal process is undoubtedly that which renders the use of sulphuric acid unnecessary. Although the practical difficulties attending such methods are many, a great deal of headway has been made. In faet Cobb, Feld, and Burkheiser have devised processes which. from the theoretical standpoint are of exceptional interest, although they cannot be said to have as yet reached a stage which permits of their general application in practice. The Burkheiser Process In Burkheiser’s process, the usual practice of extracting ammonia prior to sulphuretted hydrogen is reversed—the sulphur impurities being removed first. The modus operandi of the plant can be followed best by referring to the diagram, Fig. 340. The crude gas coming away from the tar extractor is first pre-heated by means of a steam coil. It then travels through, a still, where the ammonia is expelled from any virgin liquor tlirown down in the condensers, etc., and is carried forward by the crude gas. The sulphuretted hydrogen is then extracted by passing the gas through purifiers containing a special concentrated oxide of iron. This oxide has been sub-jected to a special process of roasting, by means of which the organic matter and moisture are removed, leaving the material in the form of small nodules. The activity of the oxide is so inereased by this means that it is said to be capable of removing sulphuretted hydrogen at a speed thirty times greater than is the case with ordinary oxide. The apparatus is in two similar sections, one portion being used