Page:The American Cyclopædia (1879) Volume V.djvu/38

34 railway lines in England, the use of the latter being retained only when flame and smoke require to be avoided. Coke has never been used on American railroads, and now that locomotives are so constructed that crude coal can be burned with facility and economy, the occasion will probably never arise. In consequence of its freedom from sulphur, coke is much better adapted to metallurgic processes, and therefore special attention has been directed in Europe, not only to the best methods of preparing it, but to the selection of that -kind of coal which is best adapted to the purpose; and it has been found in practice preferable to incur considerable expense to procure it of the best quality, some companies even preferring to obtain it from England rather than use the cheaper but inferior qualities made from the coals of France and Belgium. The cause of the superiority of the English coke is attributed to the coal beds of England containing but few seams of slate interstratified with the coal; so that this is obtained clear of the impurities which in the French and Belgian coals add largely to the proportion of ash, and render it necessary to subject the coal to processes of washing and sorting before coking. Iron pyrites is the principal objectionable material in coal for melting metals, and to get rid of it is the chief problem of the coke maker. A protracted application of heat expels a great part of the sulphur, with the formation of bisulphide of carbon and carburet of iron. The bisulphide of carbon, being volatile, passes off, and the carburet of iron which remains does no injury unless there is silica present, which is not the case in good coking coal. In the north of England it has been found that when the coal contains much pyrites, if it is first treated with a very strong brine the elimination of the sulphur is very greatly facilitated. The tendency to vitrifaction possessed by clays and lime salts renders these substances objectionable in coke, and therefore good coking coal is only found in certain districts. (See .) Two methods of coking are in use: one in ovens constructed for the purpose, and the other in large open heaps, upon the ground. The ovens are built of fire brick or stone of various sizes, and frequently arranged several together in one stack, in which case dimensions of 12 ft. square and 10 ft. high are found most convenient. They are arched over at the top, a hole being left for the exit of the gases, and another for the introduction of the coal, closed by an iron door in front. About two tons of coal are introduced through the latter, filling the oven to the springing of the arch, and leaving room above for the coal to swell. The charge is ignited by the heat left in the walls by the process just previously completed. Air is allowed to enter in proper quantity at the lower door, and the gases escape at the top. In 24 hours the air holes below are closed, and for 12 hours more the gases pass out at the top. The upper hole is then closed with a slab of stone or iron and covered with sand, and left for 12 hours more to partially cool down the charge. After this the door below is opened, the coke taken out quickly, quenched with water, and carried off in iron wheelbarrows. Where it is an object to save the coal tar, the ovens are provided with a flue at the top, through which the volatile products are conducted into a receptacle in which the liquid matters are condensed. The product of bituminous coal in coke, gas, and tar varies with its quality. As it approaches anthracite in quality, the yield of coke is large and of gas small, while the reverse is the case with the fat or highly bituminous coals. From 50 to V5 per cent, is the general range of the yield of coke. Upon some of the railroads in England the size of the ovens is stated to be 30 ft. square, and the charges about 8 tons each, spread in a thickness of about 4 ft. The duration of the process is 96 hours. With the same coals and ovens, by making the charges lighter and increasing the quantity of air admitted, and thus raising the temperature, so as to complete the process in 48, 24, or even 12 hours, the coke will be obtained lighter and more friable according as it may be desired. It has been found that the higher the heat of the oven the larger the yield of coke. This fact seemed for a long time anomalous, but is explained by well known chemical laws. When coal is melted, its hydrogen and carbon combine in the form of bicarburetted hydrogen, which in passing up through the red-hot coal above is decomposed into solid carbon and light carburetted hydrogen. Thus one half of the carbon of the gas is saved. The principle is illustrated by passing bicarburetted hydrogen through a red-hot tube, which after a while will become filled with a solid carbonaceous deposit. Coking in the open heap is the most common practice at the mines of bituminous coal of the United States, and this is the oldest method. The coal is piled up in long ranges, extending sometimes 200 ft. in length, with a width of 12 ft. at the base, and a height of 6 ft. The piles are made so that along the whole length an air passage extends through the centre on the ground. The largest lumps are placed in the middle portion, and smaller and smaller pieces toward the outside. Stakes are set up at intervals along the central line of the heap, which reach down to the base; when the pile is completed, these are taken out, and the passages they leave serve for the introduction of burning coals to fire the heap alopg its whole length. Whenever the thick black smoke and flame cease in any portion, and this begins to show appearance of ashes, the fire is kept down by the application of coke dust or ashes; and this goes on until the whole heap is thus covered. It is then left for a few days to cool. Portions exposed to the full action of the wind require, as in the process of making charcoal, a thicker cover of dust or ashes to check the consumption of the product. When sufficiently cool, the coke