COKE 



885 



sidered preferable, in many respects, to that brought from Europe, especially the 

 advantage of floating in water. 



COKE. (Eng. and Fr. ; Coaks, Ger.) It is necessary to distinguish between 

 what is called gas-coke and oven-coke. The word ' coke ' applies, properly, to the latter 

 alone ; for in a manufacturing sense, the former is merely cinder. The production of 

 good coke requires a combination of qualities in coal not very frequently met with ; 

 and hence first-rate coking coals can be procured only from certain districts. The 

 essential requisites are, first, the presence of very little earthy or incombustible ash ; 

 and, secondly, the more or less infusibility of that ash. The presence of any of the 

 salts of lime is above all objectionable ; after which may be classed silica and alumina ; 

 for the whole of these have a strong tendency to produce a vitrification, or slag, upon 

 the bars of the furnace in which the coke is burnt ; and in this way the bars are 

 speedily corroded or burnt out ; whilst the resulting clinker impedes or destroys the 

 draught, by fusing over the interstices of the bars or air passages. Iron pyrites is a 

 common obstacle to the coke-maker ; but it is found in practice, that a protracted 

 application of heat in the oven dissipates the whole of the sulphur from the iron, with 

 the production of bisulphuret of carbon and metallic carburet of iron, the latter of 

 which alone remains in the coke, and, unless silica be present, has no great disposition 

 to vitrify after oxidation. Where the iron pyrites exists in large quantities it is sepa- 

 rated by the coal-washing machines, some of which will be described in a general 

 article. -See WASHING MACHINES. One object, therefore, gained by the oven-coke 

 manufacturer over the gas-maker is the expulsion of the sulphuret of carbon, and 

 consequent purification of the residuary coke. Another, and a still more important 

 consequence of a long sustained and high heat is, the condensation and contraction of 

 the coke into a smaller volume, which, therefore, permits the introduction of a much 

 greater weight into the same space ; an advantage of vast importance in blast furnaces, 

 and above all, in locomotive engines, as the repeated introduction of fresh charges of 

 coal fuel is thus prevented. Part of this condensation is due to the weight of the 

 superincumbent mass of coal thrown into the coke oven, by which (when the coal first 

 begins to cake or fuse together) the particles are forced towards each other, and the 

 cavernous character of cinder got rid of : but the chief contraction arises, as we have 

 said, from the natural quality of carbon, which, like alumina, goes on contracting, the 

 longer and higher the heat to which it is exposed. Hence, good coke cannot be made 

 in a short time, and that used in locomotive engines is commonly from 48 to 96, or 

 even 120 hours in the process of manufacture. 



The prospects of improvement in coke-making point rather to alterations in the 

 oven than in the process. Formerly it was not thought possible to utilise the heat 

 evolved by the gaseous constituents of the coal ; but now, as an example of the in- 

 correctness of this idea, it may be stated that at the Felling Chemical Works, 200 

 tons of salt per week are made by the waste heat alone, and it is also employed in 

 partially heating the blast for one of the furnaces. There appears no valid reason 

 why sets of coke-ovens might not be so arranged as mutually to compensate for each 

 other, and produce upon one particular flue a constant and uniform effect. Contri- 

 vances of this kind have been projected; but hitherto, we may suppose, without 



uniform success, as many of our large coke- 

 makers still continue the old mode of working. 

 The following figure, 489, represents a 



Shachtofen, or pit-kiln, for coking coals in 



Germany, a is the lining (chemise), made of 



fire-bricks ; the enclosing walls are built of the 



same material ; b, b, is a cast-iron ring covered 



with a cast-iron plate c. The floor of the kiln 



is massive. The coals are introduced, and the 



coke taken out, through a hole in the side d ; 



during the process it is bricked up, and closed 



with an iron door. In the surrounding walls 



are four horizontal rows or flues e, e, e, e, which 



are usually iron pipes ; the lowest row is upon 



a level with the floor of the kiln; and the 



others are each respectively one foot and a 



half higher than the preceding. Near the top 



of the shaft there is an iron pipe f, of from 



8 to 10 inches in diameter, which allows the 



incoercible vapours generated in the coking to escape into the condenser, which 



consists either of wood or brick chambers. For kindling the coal, a layer of wood is 



first placed on the bottom of the kiln. 

 The coking of small coal is performed upon vaulted hearths, somewhat like bakors' 



489 



