Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/58

38 which in the electric furnace surrounds the ingot, yields an average of 5 cubic feet of gas per pound under proper conditions of generation. The volume of gas obtained, however, depends very largely upon the form of apparatus used, and while some will give the full 5 cubic feet, other apparatus will only yield, with the same carbide, 3f feet. The purity of the carbide entirely depends on the purity of the material used in its manufacture, and before this fact had been fully grasped by manufacturers, and only the purest material obtainable employed, it contained notable quantities of compounds which during its decomposition by water yielded a somewhat high proportion of impurities in the acetylene generated from it. Although at the Impurdt,espresent time a marvellous improvement has taken place all round in the quality of the carbide produced, the acetylene nearly always contains minute traces of hydrogen, ammonia, sulphuretted hydrogen, phosphuretted hydrogen, siliciuretted hydrogen, nitrogen, and oxygen, and sometimes minute traces of carbon monoxide and dioxide. The formation of hydrogen is caused by small traces of metallic calcium occasionally found free in the carbide, and cases have been known where this was present in such quantities that the evolved gas contained nearly 20 per cent, of hydrogen. This takes place when in the manufacture of the carbide the material is kept too long in contact with the arc, since this overheating causes the dissociation of some of the calcium carbide and the solution of metallic calcium in the remainder. The presence of free hydrogen is nearly always accompanied by siliciuretted hydrogen formed by the combination of the nascent hydrogen with the silicon in the carbide. The ammonia found in the acetylene is due to the presence of magnesium nitride in the carbide. This is formed by the metallic magnesium in the molten condition taking up nitrogen from the air. On decomposition by water ammonia is produced by the action of steam or of nascent hydrogen on the nitride, the quantity formed depending very largely upon the temperature at which the carbide is decomposed. The formation of nitrides by actions of this kind and their easy conversion into ammonia, will probably at some no very distant date prove a useful method for fixing the nitrogen of the atmosphere and rendering it available for manurial purposes, although it could never compete in price with the ammonia formed in the destructive distillation of coal for coal gas. Sulphuretted hydrogen, which is invariably present in commercial acetylene, is formed by the decomposition of aluminium sulphide. Murlot has shown that aluminium sulphide, zinc sulphide, and cadmium sulphide are the only sulphur compounds which can resist the heat of the electric furnace without decomposition or volatilization, and of these aluminium sulphide is the only one which is decomposed by water with the evolution of sulphuretted hydrogen. In the early samples of carbide this compound used to be present in considerable quantity, but now rarely more than per cent, is to be found. Phosphuretted hydrogen, one of the most important impurities, which has been blamed for the haze formed by the combustion of acetylene under certain conditions, is produced by the action of water upon traces of calcium phosphide found Calcium 62'5 per cent. in carbide. Although at first it was no uncommon thing Carbon. 37-5 to find a half per cent, of phosphuretted hydrogen present in the acetylene, this has now been so reduced by the use 100-0 of pure materials that the average quantity is rarely above Acted upon by water it is at once decomposed, yielding 0-15 per cent., and it is often not one-fifth of that

The yield acetylene and calcium hydrate. Pure crystal- amount. from line calcium carbide yields 5-8 cubic feet of

Generat loa calcium acetylene per pound at ordinary temperatures, carbide and water, all that has to be done is to of acetylene carbide. Put the carbide as sold commercially, being a bring these two compounds into contact, when from carmixture of the pure crystalline material with the crust they mutually react upon each other with the blde’ crucible is of metal and considerably larger than the ingot, the latter being surrounded by a mass of unreduced material which protects the crucible from the intense heat. When the ingot has been made and the crucible is full, the latter is withdrawn and another substituted. The process is not continuous, but a change of crucibles only takes two or three minutes under the best conditions, and only occurs every ten or fifteen hours. The essence of this process is that the coke and lime are only heated to the point of combination, and are not “ boiled ” after being formed. It is found that the ingot of calcium carbide formed in the furnace, although itself “carbide consisting of pure crystalline calcium carbide, is nearly always surrounded by a crust which contains a certain proportion of imperfectly converted constituents, and therefore gives a lower yield of acetylene than the carbide itself. In breaking up and sending out the carbide for commercial work, packed in air-tight drums, the worst of the crust is as far as possible picked out and rejected. A statement of the amount made per kilowatt hour may be misleading, since a certain amount of loss is of necessity entailed during this process. For instance, in practical working it has been found that a furnace return of 0‘504 lb per kilowatt hour is brought down to 0’406 Eb per kilowatt hour when the material has been broken up, sorted, and packed in air-tight drums. In the tapping process a fixed crucible is used, lined with carbon, the carbide electrode is nearly as big as the crucible, and a much higher current density is used. Fine grinding is unnecessary, as combination probably only takes place after fusion of the raw materials which mix more or less as liquids. The carbide is heated to complete liquefaction and tapped at short intervals. There is no unreduced material, and the process is considerably simplified, while less expensive plant is required. The run carbide, however, is never so rich as the ingot carbide, since an excess of lime is nearly always used in the mixture to act as a flux, and this remaining in the carbide lowers its gas-yielding power. Many attempts have been made to produce the substance without electricity, but have met with no commercial success. Carbide of calcium, as formed in the electric furnace, is a beautiful crystalline semi-metallic solid, having of calcium ^ density °f 2’22, and showing a fracture which carbide.

°ften shot with iridescent colours. It can be kept unaltered in dry air, but the smallest trace of moisture in the atmosphere leads to the evolution of minute quantities of acetylene and gives it a distinctive odour. It is infusible at temperatures up to 2000° C., but can be fused in the electric arc. When heated to a temperature of 245° C. in a stream of chlorine gas it becomes incandescent, forming calcium chloride and liberating carbon, and it can also be made to burn in oxygen at a dull red heat, leaving behind a residue of calcium

In the generation of acetylene from calcium carbonate. Under the same conditions it becomes incandescent in the vapour of sulphur, yielding calcium sulphide and carbon disulphide; the vapour of phosphorus will also unite with it at a red heat. It is a compound of