NITROGEN PRODUCTS 125 



and ammonia can be produced at reasonable cost only in pro- 

 portion as there is a demand for coke. 



The third source of these nitrogen compounds is atmos- 

 pheric nitrogen. During the last fifteen years there have been 

 developed a number of chemical processes by which the nitro- 

 gen of the air is " fixed," as we say, whereby ammonia, nitric 

 acid, or cyanide is produced. Only the three fixation processes 

 which had been operated before the war on a commercial scale 

 will be here briefly described. These are the cyanamide proc- 

 ess, the synthetic ammonia process, and the arc process. 



i. The cyanamide process starts with lime and powdered 

 coke. The first chemical reaction that takes place results in 

 the formation of calcium carbide (CaC 2 ), as follows: 



CaO-f 3C = CaC 2 + CO. 



This is the substance which is used so extensively in the manu- 

 facture of acetylene for use as an illuminant and in oxy-acety- 

 lene welding. The carbon monoxide escapes as a gas. The 

 first step in the cyanamicfe process is carried out in huge electric 

 furnaces. The charge of lime and coke in small lumps is fed 

 down through the furnace, in the center of which stands a 

 large carbon electrode ; the walls of the furnace form the other 

 electrode. The mixture is heated to a very high temperature, 

 and the melted carbide is tapped off at the bottom from time 

 to time, and allowed to solidify. 



The carbide is then crushed and subjected to the nitrifying 

 process. It is packed into large basket-shaped containers three 

 to six feet high and two to three feet in diameter. These 

 baskets, which are perforated with small holes, are enclosed 

 in an iron vessel into which is forced nitrogen made by dis- 

 tilling liquefied air. The chemical reaction is started by heat 

 produced by passing an electric current through a resistance 

 wire, placed in the axis of the basket. The reaction which 

 takes place is as follows : 



CaC 2 + N 2 = CaCN 2 + C. 



