THE PRELIMINARY PURIFICATION OF COAL GAS 507
in the gas, moreover, would have detrimental effects from the point of view of CS2 removal; for it tends to dissociate ammonium thiocarbonate and to set free the sulpliur compound. Some express the opinion that ammonium polysulphide is alone the agent by means of which. the CS2 is removed.
Although all attempts at the reduction of sulphur compounds by treatment with liquor have been attended with somewhat erratic results, there seems little doubt that some reduction may be affected by dealing with the gas whilst in its hot state. One method of treatment is thoroughly to wash the gas with hot liquor in the hydraulic main, the liquor being circulated and recirculated so that only the surplus flows away. Lunge states that the liquor from the hydraulic main is especially suited for wet purification, for it contains some considerable quantities of free ammonia (i.e. NH3) owing to the far-going dissociation of tlie “ free ” salts—car-bonate and sulphide. It is probable, therefore, that some absorption of CS2 may take place owing to th.e presence of uncombined ammonia in. the liquor, and after the manner of Tilden’s equations already referred to. No great reliance can, however, be placed in the method. It has, in consequence, not been generally undertaken on gasworks.
So far as the difference of Constitution of the liquor from the hydraulic main and of the final product obtained in the storage well is concerned, Meyer and Hempel have given the following interesting table :—
Liquor from Storage Well. Liquor from Hydraulic Main.
Total ammonia .... 1200-6 grains per gallon 405-7 grains per gallon
“Free” „ .... 984-5 204-1
“ Fixed ” „ .... 216-1 201-6
Ammonium carbonate . 2468 grains per gallon 332 grains per gallon
„ sulphide . 193 „ „ „ 51
„ thiosulphate . 86 „ „ „ 30
„ sulphite Traces Nil
„ sulphate . 16 „ „ „ 16 „ „ „
„ chloride . 517 „ „ „ £05
„ sulphocyanide 127 „ „ „ 70 „ „
,, ferrocyanide . 6-2 „ „ „ 2-3 „
„ cyanide 8-3 „ „ „ 24 „ „ „
Uncombined ammonia 7-4 „ „ „ 73-0 „
METHODS OF OPERATING WET PURIFICATION PLANT
The most efficient manner of operating washers and serubbers is to ensure that the gas on first entering the apparatus is treated with the strengest liquor obtainable, whilst in each subsequent vessel the liquor circulated becomes gradually weaker until, in the final serubber, clean water is employed. Where circumstances permit it is preferable that the decarbonating vessel be worked with hydraulic main liquor,.