Page:Popular Science Monthly Volume 80.djvu/48

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PSM V80 D048 Notodden portion of the saltpeter works near notodden.png

Fig. 8. Notodden. Portion of the Saltpeter Works near Notodden.

it to say that it is now accomplished industrially and it was my privilege to see the working of the whole process at Odda. The air is first liquified as in ordinary liquid-air machines, and then the constituent gases separated by rectification, using much the same process as that by which alcohol is separated from water. The difficulty of the process depends upon the low temperatures necessary. Oxygen boils at -182° and nitrogen at -194° Centrigrade. The air must of course be completely freed from moisture and also from carbon dioxid, for at the temperatures used both are solids and would clog the pipes. All the difficulties have been successfully overcome and from the stills the nitrogen is boiled off in an almost pure condition. Delicate tests in the laboratory show that on the average not over 0.2 per cent, of oxygen is present. The commercial weakness of the process is the fact that there is no use for the fairly pure oxygen which is left, which in many places would be very valuable and probably pay the whole cost of operating. The calcium carbid is ground and exposed for two days in an atmosphere of the pure nitrogen. While the absorption of nitrogen is an exothermic reaction, it must be started and supported in the initial stages by a supply of heat from some external source, and for this electric heating with carbon anodes is used. The resultant mass is a fairly pure cyanamid, with uniform nitrogen content of 20 per cent. From the cyanamid ammonia is easily obtained by the action of water, and this being absorbed by sulfuric acid gives the ammonium sulfate so extensively used as a fertilizer. At present the sulfuric acid for this absorption must be imported, but an electric zinc smelter is in process of