Page:Text-book of Electrochemistry.djvu/315

 ��DEVELOPMENT OF HEAT.

��CHAP.

��In the preparation of carborundum [Muhlhaeuser (IS)] the two thick electrodes, A and B (Fig. 53), are joined by a train 2 to 3 metres long of coke powder, C (of 4 to 5 mm.

���Fio. 53.

diameter granules). At the ends of this train there is placed some finer coke powder, D, in order to ensure good contact with the electrodes. The carbon electrodes are embedded, by means of asbestos packing, in the walls of the fire-proof furnace Z7. Under the influence of the current the train G more or less runs together to a conducting mass. The mixture to be heated, which consists of 100 parts of carbon (coke powder), 100 parts of sand, and 25 parts of common salt, is placed round C, Occasionally 12 parts of sawdust are added to the mixture, and the quantity of sand may then -be increased to 140 parts. Beaction occurs according to the equation —

SiOa + 3C = SiC (carborundum) + 200.

The salt serves to bake together the unattacked portions of the mixture. The process is generally carried out with an alternating current, and when it is finished it is found that round G there is an elliptical mass, H, of crystallised carborundum, but at the ends of C the substance is amorphous. Outside this kernel there remains a layer of unattacked mixture, and this in turn is surrounded by a layer of almost pure salt. Quite close to G there is generally a thin layer of graphite, which is probably produced as a decomposition product of the carborundum at the excessively high tempe- rature. After cooling, the carborundum is removed from the

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