544
MODERN GASWORKS PRACTICE
Fig. 342 shows the plant complete. The washing liquor is pumped to the Feld centrifugal washer, and, on leaving, passes through, the “ still,” being collected in the lower tank. Here (also in the still) the liquor meets witli SO2, which brings about the regeneration of the FeS into sulphates. As soon as the liquor contains 30 to 45 per cent, of ammonia sulphate, it is pumped to the upper f.a.nk; while the lower one is filled with. fresh liquor. The salts are converted wholly into sulphates in this upper tank. The sulphur is then separated out in the filter press, and the liquor coming away from this is continuously pumped round and round into the lowest bay of the washer. The object of this is to precipitate com-pletely the iron from the sulphate solution. The iron sulphide mucl is then filteret! out in the second press, and the liquor evaporated and' dried.
Fig. 343.— Diagkam showing Operation of Feld Tetbathionate Process.
Owing to the difficulties attending the considerable dissociation tension of sul-phites, Feld devised a process based on the oxidation of thionates, in its more recent form without the use of metal salts. In spite of the way in which. the process has been scientifically perfected, in practice it has proved too cumbersome.
In this process the gas is washed with a solution containing ammonium tetra-thionate which is prepared by the action of sulphur dioxide on ammonium sulphide (gas liquor).
The apparatus (Fig. 343) consists mainly of a washer, an acidifier, a boiling pan, an evaporator, and a sulphur (or pyrites) furnace. The gas, freed from tar and at a temperature of about 60° C., is admitted into the bottom of the washer, where it is washed by the descending stream of ammonium tetrathionate liquor. This takes up the ammonia and sulphuretted hydrogen from the gas, and is itself transformed