90 CHEMICAL FERTILIZERS 



the process is extremely scanty. It has long been known, 

 that on leading a mixture of nitrogen and hydrogen over 

 certain catalysts, such as manganese, small traces of ammonia 

 are produced, even at atmospheric pressure, but it is only at 

 high pressures that commercially satisfactory results can be 

 obtained. The first step in the industrial synthesis consists 

 in the choice and adoption of appropriate means for manu- 

 facturing nitrogen and hydrogen in a fairly high degree of 

 purity, above all free from catalyst poisons of any nature. 

 Methods for producing mixtures of nitrogen and hydrogen 

 from producer gas suffer from the fault that the last traces of 

 carbon monoxide are difficult to remove. It is simpler 

 and more satisfactory to prepare nitrogen directly from 

 the air (see p. 77). The manufacture of hydrogen also 

 presents some difficulties. Electrolytic hydrogen is of little 

 use, owing to its high cost, but hydrogen can be manufactured 

 from water gas by a continuous method, in which water gas 

 and steam are passed together over a catalyst consisting 

 usually of an active form of oxide of iron. By the inter- 

 action of carbon monoxide with steam, carbon monoxide 

 is replaced by carbon dioxide and hydrogen, according to the 

 equation : 



C0+H 2 $ C0 2 +H 2 



The carbon dioxide thus produced is absorbed by com- 

 pression on to water, hydrogen sulphide and other impuri- 

 ties being removed by the usual iron-oxide purifiers of the 

 gasworks type. It is found in practice that considerable 

 quantities of carbon monoxide are still left in the gas, which 

 may be eliminated by treatment with calcium carbide, or 

 by soda-lime when the gas is heated and compressed. An 

 intermittent method consists in alternately passing steam 

 over red-hot iron and reducing the oxide so formed by water 

 gas. Owing to the instability of carbon monoxide at high 

 temperatures, it is decomposed partly into carbon dioxide 

 and carbon. The carbon produced is deposited on the iron 

 catalyst, interfeiing with its action, and leaving carbon 



