Formation of Nitrides

Many attempts have been made to fix atmospheric nitrogen in the form of metallic nitrides, e.g. AlN, Mg₃N₂, Ca₃N₂, etc., by the direct combination of the elements at a high temperature. The production of ammonia is then effected by the decomposition of these compounds by water or steam:

AlN + 3H₂O = Al(OH)₃ + NH₃.

When aluminium is heated in an atmosphere of nitrogen the metal increases in weight, and when the product is fused with potash, ammonia is evolved. Aluminium also forms a nitride when heated to 700° C. in an atmosphere of ammonia. These observations are the basis of the Serpek process of fixing atmospheric nitrogen gas, which was worked technically for a time in the Savoy.

Finely divided alumina, (Al₂O₃), heated by furnace gases, passes down an inclined tube, is mixed with powdered coke, and passes through a zone electrically heated to 1800° C. It there meets a current of nitrogen derived from the combustion of producer gas, with the formation of aluminium nitride and carbon monoxide:

Al₂O₃ + 3C + N₂ = 2AlN + 3CO.

The carbon monoxide, mixed with more air, is burnt to heat the first tube through which the cold alumina is introduced. The aluminium nitride readily hydrolyses - especially with water under pressure, and the hydroxide produced can be used for the production of metallic aluminium.

Originally the mixture of bauxite and carbon was heated in contact with nitrogen in rotating furnaces, but later Serpek used a short chamber in which the mixture, preheated to 1250°-1300° C., meets the current of nitrogen.

With regard to the mechanism of the reactions occurring during the process, Serpek assumed that aluminium carbide, Al₄C₃, was first formed, which reduced the remaining oxide to the metal,

Al₄C₃ + Al₂O₃ = 6Al + 3CO,

and the aluminium then combined directly with nitrogen to form the nitride,

6Al + 3N₂ = 6AlN.

The presence of certain impurities, such as oxides of iron and copper, acts catalytically, so that bauxite, which contains iron oxide, is a good raw material. The Badische Company accelerate the reaction by mixing with the alumina and coal 5 to 6 per cent, of oxides of silicon, titanium, vanadium, etc., which themselves form stable nitrides, from which the oxides can be recovered and used again.

The presence of such catalytic materials also enables the reaction to proceed at a lower temperature than would be possible with the pure alumina. The formation of aluminium nitride begins at 1100° C., and the yield increases with temperature to a maximum at 1800° to 1850° C. Above 1900° C., however, smaller yields are obtained owing to the dissociation of aluminium nitride into its constituents, and at 2000° C. no nitride was formed at all.

Although other metals, such as lithium, magnesium, calcium, etc., form nitrides readily, they are too expensive for technical use owing to the difficulty of recovering the metal from the hydroxide produced after hydrolysis. Hence aluminium is really the only suitable metal which has been the subject of technical investigation.

In actual practice the hydrolysis of the aluminium nitride is brought about by means of alkali,

AlN + 3NaOH = Al(ONa)₃ + NH₃,

and the sodium aluminate can be either converted by the Baeyer process into pure alumina, which is again used for making the nitride, or the metal. In the Serpek process potassium hydroxide solution was used in insufficient amount for the formation of the aluminate, so that the alumina remaining served to form fresh quantities of the nitride.