Purification of Coal-gas

When it leaves the hydraulic main the gas has a temperature of about 50°-60° C., and contains besides tar the following impurities, which must be removed:

Ammonia – 0.70-1.40 per cent, by volume.
Hydrogen cyanide – 0.05-0.15 per cent, by volume.
Sulphuretted hydrogen – 0.9 -1.70 per cent, by volume.
Carbon disulphide – 0.02-0.04 per cent, by volume.

The gas is cooled by passage through a series of vertical tubular condensers, which may be cooled with water. Here much of the remaining tar is condensed, though sometimes there is a special tar-extractor, where the remaining particles of this substance, which constitute " tar-fog," are removed mechanically. The efficient extraction of naphthalene at this stage is very important. The gas next passes through the exhauster, which is a pump by which the pressure and consequent flow of the gas are regulated. Then follow the scrubbers, which are towers filled with broken bricks, coke, or slanting boards. Down these towers water is sprayed to meet the up-coming gas and remove from it the last traces of ammonia. At the same time some sulphuretted hydrogen and carbon dioxide are removed and form ammonium sulphide and carbonate. Mechanical rotary washers have now largely replaced the scrubbers. Owing, however, to the trouble of recovering ammonia from dilute solution, processes for the direct absorption of this gas by acid, without the intermediate formation of ammoniacal liquor, are now being introduced.

The extraction of cyanide from coal-gas is a comparatively recent industry rendered important by the demand for cyanides in gold-mining, and also by the recognition of the corrosive action of hydrocyanic acid on iron and steel.

Cyanide is extracted in the form either of ferrocyanide or thio-cyanate by one or other of the following processes:

1. Ferrocyanide process (Bueb): Previous to entering the scrubbers for removal of ammonia the gas reacts with ferrous sulphate solution or ferrous sulphide suspended in water, ferrocyanides being produced as follows:
   
   FeS + 6NH₄CN = (NH₄)₂S + (NH₄)₄Fe(CN)₆
   FeS + 2NH₄CN = (NH₄)₂S + Fe(CN)₂
   2FeS + 6NH₄CN = 2(NH₄)₂S + Fe(NH₄)₂Fe(CN)₆.
   
   Sometimes the gas reacts with alkali and ferrous carbonate and hydroxide, hydrogen sulphide being at the same time absorbed. (Foulis process).
2. Thiocyanate process:
   
   
   1. British Cyanides Co.: The gas passes through ammoniacal liquor containing suspended sulphur, whereby ammonium polysulphide and thiocyanate are successively produced; the sulphur may, however, be derived from moist spent oxide (Williams):
      
      (NH₄)₂S (in gas liquor) + S = (NH₄)₂S₂.
   2. P. E. Williams (1909):
      
      (NH₄)₂S (in crude gas) + S (in moist spent oxide) = (NH₄)₂S₂ [(a) and (b)] (NH₄)₂S₂ + NH₄CN = (NH₄)₂S + NH₄CNS.

Sulphuretted hydrogen is removed from coal-gas by a dry absorption method. The absorbent may be: (i) lime; (ii) ferric oxide, as Irish or Dutch bog ore, precipitated hydroxide, or "Lux" from bauxite; (iii) manganese dioxide, as Weldon mud. Ferric oxide is the most general absorbent, and is most active when precipitated. It is placed on wooden grids contained in a series of cast-iron boxes or purifiers, through which the gas passes; and the following reactions take place:

Fe₂O₃.H₂O + 3H₂S = Fe₂S₃ + 4H₂O
and Fe₂O₃.H₂O + 3H₂S = 2FeS + S + 4H₂O.

When the oxide is "spent," it is removed from the purifiers, moistened, and exposed to the air, so that oxidation takes place as follows:

2Fe₂S₃ + 3O₂ = 2Fe₂O₃ + 6S
4FeS + 3O₂ = 2Fe₂O₃ + 4S,

and thus the "spent oxide" becomes "revivified."

The revivified oxide is used over and over again, whilst sulphur accumulates within it until 50-60 per cent, is present. It is then employed for the manufacture of sulphuric acid. Sometimes a little air is added to the coal-gas previous to the absorption of sulphuretted hydrogen, in order that oxidation may proceed simultaneously. In this way the ferric oxide lasts longer without requiring to be changed.

As it leaves the purifiers the gas should not discolour a strip of lead acetate paper when passed for three minutes through a vessel containing it. The gas will then contain not more than one volume of sulphuretted hydrogen in ten million.

Carbon disulphide vapour present in coal-gas is not absorbed by oxide of iron, and its removal, though desirable, is not always carried out. The only known absorbent for carbon disulphide is calcium hydrosulphide, which is produced from slaked lime when the latter is employed to remove sulphuretted hydrogen from the gas. The reactions may be represented as follows, calcium thiocarbonate being formed:

Ca(OH)₂ + 2H₂S = Ca(SH)₂ + 2H₂O
Ca(SH)₂ + CS₂ = CaCS₃ + H₂S.

Calcium thiocarbonate is an objectionable and almost useless byproduct, and on this account the process is often omitted.

An alternative process for eliminating carbon disulphide consists in passing the coal-gas at 230° C. over fire-clay surfaces impregnated with metallic nickel. The following reaction takes place with the hydrogen of the coal-gas:

CS₂ + 2H₂ = 2H₂S + C.

The hydrogen sulphide thus produced is absorbed by ferric oxide, and the carbon can be burned off from the nickel surface, which is thus renewed for further use.

After purification the gas passes through the meter-house to the holders, whence it is distributed to the consumers.