Fixation of Nitrogen

The set-back in Chilean nitrate seems to have been temporary, for there was a recovery in consumption in 1928. A further in crease took place in the fixation of nitrogen. Statistics given by F. A. Ernst and M. S. Sherman of the U.S.A. Fixed Nitrogen Laboratory (Industrial and Engineering Chemistry, Feb. 1927) assign the nitrogen fixed by synthetic processes in 1925 as follows: The figures for cyanamide are possibly too low.

According to Ernst and Sherman, fixation of nitrogen has attained the largest proportions in Germany, where the annual capacity of plants is stated to be :—(a) arc process, 4,500; (b) cyanamide, 94,000; (c) synthetic ammonia, 402,500 in tons of nitrogen. France, Italy, Japan, Norway and Canada are also noted for large plant capacity. Respecting the United States, M. S. Sherman (hid. and Eng. Chem., Jan. 1928) states that 12,800 tons were fixed in 1926 whilst the probable figure for 1927 is 23,000 tons. Production is rapidly increasing in Great Britain and the Billingham Works which were making about 65 tons of ammonia per diem in 1926 are expected to attain an annual output of 750,000 tons of ammonium sulphate (over 150,000 tons of nitrogen) in 193o. The sulphate radicle is now frequently obtained from natural calcium sulphate (anhydrite or gypsum) by utilizing the reaction, CaSO4+2NH3+ Combined nitrogen is also being supplied for agricultural purposes as urea (from ammonia and carbon dioxide) and in many fertilizers containing potash and phosphates.

Synthesis of Ammonia.

The combination of nitrogen and hydrogen is accompanied by the evolution of heat, the equation for the formation of ammonia at ordinary temperature being according to the measurements of Julius Thomsen. (See also papers by Haber and his co-workers between 1903 and 1915.) The heat evolved at higher temperatures is progressively less, and the amount of ammonia existing in true equilibrium with a mix ture of one volume of nitrogen and three volumes of hydrogen diminishes, as temperature is raised. If ammonia, nitrogen and hydrogen co-exist in an enclosed space, each gas can be considered as occupying the whole volume but exercising a partial pressure proportional to the number of its molecules present. For any given temperature, certain concentrations of the different gases will be in equilibrium when the p's representing the respective partial pressures of the gases whose formulae are put down as suffixes. It is more convenient in practice to take the square root and write If a mixture of these three gases be compressed, ammonia must be formed in order to preserve the above relationship, so that rise of temperature and of pressure act in contrary directions, the first diminishing the equilibrium amount of ammonia, the latter increasing it.

Since nitrogen and hydrogen react to form ammonia very slowly, no change is observed if these gases are mixed at ordinary temperature and pressure and left for years. In order to get

appreciable change the reaction must be accelerated, and the two ways in which this is possible are by increase in temperature and use of a catalyst. In Haber's earlier experiments (with G. van Oordt), ammonia was passed over a heated catalyst and the ammonia which had escaped decomposition absorbed and esti mated. The residual nitrogen and hydrogen were passed over another portion of catalyst at the same temperature and the ammonia formed estimated : the two sets of experiments showed fair agreement but were found later to be too high as regards ammonia formation. (Haber and R. le Rossignol, also F. Jost, 1908.) From a knowledge of heats of formation and specific heats of reacting gases, the equilibrium constant can be calcu lated theoretically.

Catalysts.

Ultimately agreement between theoretical and practical results was obtained and it became obvious that, for practical working, a catalyst would have to be found which was sufficiently rapid in its action about 500° C and not too sus ceptible to "poisoning" by impurities in the gases employed. Theoretically, the following approximate percentages of ammonia may be attained.

Haber used various catalysts in the course of his work. In the early stages, osmium and ruthenium were found to give good results but price and rarity preclude their use. The catalysts usually contain iron with a basic oxide (introduced by fusion) as a promoter. A large number of patents have been taken in the first quarter of the loth century, and the firms concerned do not divulge the nature of the catalysts they employ. The Badische Co. have been supposed to use an iron catalyst with (possibly) molybdenum as promoter. In the war plant of the U.S. Gov ernment, pumice was impregnated with nickel or iron sulphate, heated to 55o°, reduced by hydrogen and treated with sodium and ammonia gas at 4500 ; sodamide was formed in the spongy metal, and such a mass is said to act at 500° and 7o atmospheres. Synthetic Ammonia and Nitrates Ltd. claim that calcium ferrite, prepared in the electric furnace, gives an active catalyst on sub sequent reduction (Brit. Pat. 237,394). Norsk-Hydro mix corn pounds of metals of the iron group with cyanides in liquid an hydrous ammonia, remove the excess of ammonia and heat in a non-oxidizing atmosphere (U.S. Pat. 1,570,333). F. Uhde (Brit. Pat. 247,225) carefully avoids water. In an example, anhydrous ferric chloride and potassium ferrocyanide react in glycol solu tion, the precipitate is dried in vacuo, filled into the furnace and treated with nitrogen and hydrogen, first without and then with increased pressure at 350-450°. The hydrogen used in the syn thesis is chiefly produced from coke and steam but electrolytic, coke-oven and by-product hydrogen are also used. Nitrogen is obtained by rectification of liquid air or from producer-gas.