THE PRELIMINARY PURIFICATION OF COAL GAS 503
Actually, the acid salts are not formed direct, as shown above, but from further absorption of acid gases by the normal solutions, as follows :—
(NH4)2,CO3 + H3O + CO2= 2 NH4,HC03.
(NH4)2S + H2S= 2 NH4HS.
THE COMPOSITION OF GAS LIQUOR
The liquor deposited jn the hydraulic main and condensers is generally char-acterized as “ virgin. liquor,” which signifies that the water in which the salts are dissolved emanates from the coal and not from external sources. In the ordinary way the yield of virgin liquor varies from 10 to 14 gallons per ton of coal carbonized. The hydraulic main liquor dillers considerably from that obtained in the washing plant, chiefly owing to the faet that the temperature at this point (about 60° C.) renders many of the compounds which. are found at the lower temperatures unstable. Thus the salts formed as in the above reactions, i.e. the carbonates and sulphides, are present in only small quantities, there is little free ammonia in comparison. with the amount in the serubber liquor, and fixed ammonia salts may predominate.
Ammonia in gas liquor may be combined in two distinet ways, first as easily decomposable compounds such as carbonates and sulphides and, secondly, as more stable compounds which will not undergo decomposition by heating, but which require the addition of a caustic alkali to displace the ammonia from them. In the former case, the ammonia is combined with the weaker acidic gases CO2 and H2S and is known as “ free ” ammonia. In the latter case it is combined with stronger acids, such as hydrochloric and sulphuric, and is then denoted “ fixed ” ammonia. The “fixed” ammonium salts in gas liquor are the outeome of the combination of ammonia with the minor impurities in the erude gas and are derived to some extent from oxidation. In general, the final mixture of gas liquor obtained in the storage well contains from 75 to 80 per cent, of the “ free ” salts and from 20 to 25 per cent, of “ fixed ” salts, the possible constituents being as follows :—
The Composition of Gas Liquor
Water containing in solution :— Ammonium oarbonate \ „ bicarbonate
„ carbamate “Free” ammonia
„ sulphide > 75 to 80 per cent, of
„ hydrosulphide I total ammonia.
„ cyanide I
,, polysulphide '
„ chloride ■.
„ sulphocyanide I
„ sulphite “Fixed” ammonia
„ thiosulphate > 20 to 25 per cent, of
„ sulphate total ammonia.
„ ferrocyanide I
„ acetate /
Also phenols, amines and other nitrogenous bodies.