214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



of carbide and to slake any free lime present. Sometimes it is also 

 treated with a small amount of oil to prevent dusting. It is then 

 known by the trade name " Cyanamid," and has a nitrogen content of 

 about 19 to 21 per cent. 



Cyanamid is extensively used as an ingredient in mixed fertilizers, 

 but during the past few years it has also been used to a great extent 

 as a source of nitrogen for making other products such as ammonia, 

 various ammonium compounds, nitric acid, etc. The next step for 

 either of these is the production of ammonia, which is obtained by 

 neating cyanamid with steam under pressure in so-called autoclaves, 

 when the nitrogen is given off as ammonia gas. This gas may be 

 absorbed in water, producing aqua ammonia, which is used in large 

 quantities for refrigeration and for many general chemical purposes. 

 It may also be used for producing anhydrous ammonia by drying 

 the gas and compressing it, Such ammonia is also used extensively 

 for refrigeration purposes. 



By absorbing the ammonia gas in sulphuric acid, sulphate of 

 ammonia is formed, which is separated out by crystallization in the 

 same manner as previously explained under the by-product coke oven 

 process. 



Similarly, by absorbing the ammonia gas in nitric acid, ammonium 

 nitrate is formed, which is used to a large extent in high explosives. 

 The nitric acid required for this product may also be obtained from 

 the ammonia gas by oxidation of the ammonia in the presence of a 

 heated catalyst, usually platinum, according to the following reaction : 

 4NH 3 +50 2 =4NO+6H 2 0. 



A mixture of about 10 volumes of ammonia gas and 90 volumes of 

 air is passed over a spongy platinum screen of very fine mesh, 

 heated electrically to a dull red heat, the temperature with the gas 

 passing over it being about 1,000° F. The gas velocity must be very 

 high so as to cut down the time of contact between the gas and the 

 catalyzer to about one one-hundredth of a second. The efficiency of 

 conversion is quite high, about 90 per cent, and the resulting strength 

 of the nitric oxide gas about 8 to 9 per cent, as compared to 2 per cent 

 with the arc process. 



The nitric oxide after leaving the catalytic burners is thereafter 

 treated in the same manner as with the arc process: that is, it is 

 passed through oxidation tanks, steam boilers, and aluminum coolers 

 and finally to the absorption towers. The absorption system is, how- 

 ever, much smaller than with the arc process, due to the much higher 

 concentration of the nitric oxide, and for the same reason the weak 

 acid will have a strength of about 45 per cent instead of 30 per cent 

 as with the arc process. 



There are a large number of cyanamid plants throughout the 

 world, their total annual productive capacity being possibly in the 



