MODERN GASWORKS PRACTICE
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Percentage by Weight of Coal Carbonized. Percentage by Heat Units recovered from Original Coal.
Gas (made) 17-5 24-0
Gas (sold) 16-6 22-8
Coke (sold) 56-0 67-0
Tar 5-0 6-0
Ammonia 12-42
Cyanogen 0-02
Retort Carbon 0-06 —
COAL WASHING
All coals as won from tlie pit contain varying proportions of inert matter and undesirable impurities. Chief arnongst these are siliceous and calcareous matter, shale, and pyrites. The question of washing coal for the removal of these impurities is not one which directly concerns th.e gas engineer, who normally employs coal which has merely been picked over and in some cases screened. Düring the war the quality of coal deteriorated to such an extent that some of the larger gas under-takings seriously considered. the possibility of washing the coal before carbonizing it. Up to the present, however, no development of this suggestion has occurred.
The effect of washing on the quality of coal is well illustrated by the following figures which have been given by G. Knox 1:—
Inert Matter removed by Washing
Material Washed. Ash per cent, before treatment. Ash per cent, after treatment.
Eine coal 31-00 5-7
22-72 4-1
Slurry | in. to 0 23-70 2-2
„ A in. to 0 13-28 4-25
Belt pickings erushed 35-80 8-60
Slurry 30-25 4-83
18-42 5-21
16-00 5-03
Anthracite, shales, rubbish 49-07 5-46
Waste slack . 24-95 1-12
Fine coal 18-42 3-75
So far as the efficiency of washing apparatus is concemed it is usually specified that the treated coal shall contain not more than 2 per cent, of free inert matter, while the residue shall contain not more than 2 per cent, of free coal.
Theoretically, the princip] e of coal-washing clepends upon the faet that particles of differing specific gravity fall through water (or some other prescribed solution) at
1 Proc. 8. Wales Inst, of Engineers, 1919.