Modern Gasworks Practice

572 MODERN GASWORKS PRACTICE Hydrated ferrie oxide (Fe2O3,H2O) . . . . . 60 to 65 per cent. Organic matter . . . . . . • ■ . 15 to 25 „ Silica . . . . . . . . • • . 3 to 6 „ Alumina ........... 1 „ The silica content of a natural oxide varies very miich. in accordance with tlie district from which it is taken. Best Dutch material seldom contains more than 4 to 5 per cent. Belgian oxide may contain from 10 to 12 per cent., whilst oxide occurring in this country has been known to contain so much as 20 per cent, of silica and alumina. The proportion of silica present has some considerable effect on the relative density of the various oxides. In general the following figures may be taken as representing the volume equivalent to 1 ton:— Dutch oxide......................45 cubic feet per ton. Belgian oxide ......... 40 „ „ „ Wiltshire oxide ........ 35 „ „ „ The active material in bog-ore is not the actual oxide, but the oxide plus a certain amount of water of combination, i.e. a hydrated ferrie oxide, Fe2O3,H2O. In addition to this moisture the material as used should contain a further quantity of water, usually from 20 to 30 per cent. For effective working the moisture should never be less than the lower limit given. In practice it is generally realized that the material should be in such a state as regards moisture that it will, when pressed in the hand, bind like snow. In common with natural oxides the artificial varieties are employed with. a similar degree of moisture. The made-up material is usually more active than the natural product, owing to a rather higher content of iron (which may reach so much as 75 per cent.), and on account of the elimination of vegetable matter. At the same time it is a fallacy to suppose that a material containing a high proportion of iron is better necessarily than one containing a lower proportion. The quality and condition of the iron is the main factor of efficiency. It is, as a matter of faet, possible to obtain natural bog-ore containing 90 per cent, of ferrie hydrate, but a good variety containing 70 to 75 per cent, is better suited for gas-works purposes. When foul gas containing sulphuretted hydrogen is passed through a vessel filled with. oxide of iron the sulphuretted hydrogen alone is extracted, the oxide having no affinity for carbon disulphide or carbon dioxide. The absorption of the sulphuretted hydrogen occurs in two distinct ways, which may be simply shown as foliows :— (1) Fe2O3,H2O + 3 H2S = FeaS3 + 4H,0 (ferrie sulphide). (2) Fe2O3,H2O + 3H2S = 2FeS+S+4:H2O (ferrous sulphide). It will be noted that in the first instance ferrie sulphide is formed, whilst in the alternative equation ferrous sulphide and free sulphur result. After a time the material ceases to absorb sulphuretted hydrogen, but can be returned to a state of activity by the process known as “ revivification.” For this purpose it is removed from the purifying boxes, spread. out in a thin layer, turned