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Catalyst
Temper- ature C.
Pressure in atmos- pheres
S.V. of gas lo>X
Percent- age of ammonia formed
S.T.Y.
Osmium .
585
1 66
24 80 1 60
7-0
6-2
4-2
1-46
3'6
4-8
Uranium carbide .
515
II (I
113-6
II
5'2
28-5
74-4 174-9
7-63 6-42 4-78 4-18
0-28 1-3 2-5
0-8
2'5
4-3
6-2
8-2
9-2 10-8 11-6
Iron .
505 530
ti
ii
150
11 II
II
10
5
IOO 2OO 300 4OO 500 6OO
IM
6-9 5-6 4'3 3-8 3-2 3-o 2-7
of carbon dioxide, whereby ammonium sulphate is produced, cal- cium carbonate being precipitated. For the production of the chlo- ride, several modifications of the well-known Solvay ammonia-soda process have been suggested, by means of which ammonium chloride is formed from common salt, ammonia and carbon dioxide.
The direct synthesis of ammonia constitutes probably the most economical method of fixing nitrogen at present known. The cost of production is regulated principally by that of the hydrogen, the cost of compression being relatively low. On the other hand, the technical difficulties are probably more severe than in any other known industrial chemical operation. The high pressure, combined with a temperature sufficient to render steel of ordinary composition rapidly weakened by the hydrogen contained in the gas employed, has made necessary the construction of furnaces of special design. Further, all raw materials must be of a high degree of purity, by reason of the readiness with which the reaction is impeded and stopped by the presence of traces of catalyst poisons such as sulphur, arsenic, phosphorus, etc. Nitrogen also combines directly with hydrogen at the temperature of the electric arc and, further, under the influence of the silent electric discharge, but these methods have not up to the present given yields sufficient to justify their commercial application.
Immediately previous to and during the World War extensive factories were erected in Germany, at Oppau and Merseburg, for synthesizing ammonia, the tons of nitrogen fixed in this way being reported to have increased from 4,000 in 1913 to 100,000 in 1917. In Great Britain, Synthetic Ammonia & Nitrates, Ltd., was regis- tered by Messrs. Brunner, Mond & Co., Ltd., in 1920 with a capital of 5,000,000. In the United States, the synthesis of ammonia has been taken up on a large scale by the General Chemical Co., and plants exist at Sheffield, Alabama.
Cyanamide Process. A second highly important method of fixing nitrogen consists in forming calcium cyanamide (nitrolim), by the interaction of nitrogen with calcium carbide.CaCz-t-Nz =Ca :NCN+C. Absorption of nitrogen takes place readily at 1,000 to I, looC., with carbide of commercial quality. By the addition of catalysts such as calcium fluoride or calcium chloride, the combination may be carried out at 800 C. The reaction is exothermic, and the tem- perature of the charge rises considerably owing to the heat pro- duced. Temperatures exceeding 1 ,400 C. have a marked inhibitive effect on the yield, by .reason of the reversible nature of the reaction.
Two types of plant are employed in practice. In those at Odda in Norway, the charge of carbide is reduced to fine powder by grind- ing and placed in cylindrical firebrick furnaces, which are heated internally to the required temperature by means of carbon resistance rods, nitrogen being admitted under slight pressure. A period of about 36 hours is required for the completion of the reaction, at the end of which time the product contains upwards of 20 % of nitrogen. The charge shrinks away from the walls and forms a solid block, which is easily removed. It is the practice at certain other works, particularly those in Germany and Italy, to employ externally heated horizontal retorts. With these, the temperature of reaction is stated to be less easily controlled and trouble is experienced from the adhesion of cyanamide to the walls. Calcium cyanamide, in a finely ground condition, may be used directjy as an agricultural fertilizer, ammonia being produced in the soil by hydrolysis: CaN-CN+3H ? O = 2NH 3 +CaCO 3 .
The above hydrolysis is also effected by the action of super- heated steam, as an industrial operation for the manufacture of ammonia. The fixation of nitrogen by the cyanamide process is of considerable extent and importance. Factories exist at Odda (Norway), Piano d'Orta (Italy), Niagara, Wittenberg, Chorzow, Piesterloh and other places. It is stated that the total world pro- duction of cyanamide in 1916 amounted to nearly 1,000,000 tons, while Germany alone, owing to war-time extensions, is reported to have manufactured 886,000 tons in 1917.
Backer Process. The synthesis of sodium cyanide by the inter- action of sodium carbonate, carbon and nitrogen in the presence of
iron as a catalyst, according to the equation Na2CO 3 +4C+Nj = 2NaCN+3CO, constitutes a promising method of nitrogen fixa- tion, the commercial development of which is still in its infancy. The catalytic effect of iron in promoting this formation of cyanides at relatively low temperatures (8oo-i,oooC.) was noted by Thomp- son in 1839. Bucher (Jour. Indust. and Eng. Chem., 1917, 9. 233) drew renewed attention to the process, which has recently been developed industrially in the United States by the Nitrogen Products Co. According to the procedure adopted at Saltville, Virginia (Jour. Indust. and Eng. Chem., 1919, //. 1010), coke is ground to a fineness of 200 mesh, and after the admixture of a small quantity of iron the required quantity of soda ash is added. The charge is moistened slightly, kneaded, and extruded in the form of briquettes, which are dried by the action of flue gases. The briquettes are placed in vertical iron or nichrome retorts, which are heated externally in firebrick furnaces to a temperature of 900 to l,oooC., a current of nitrogen being led through the retorts. The briquettes, after treatment, contain about 20% to 30% of cyanide, which, in the plant in question, is removed in a somewhat novel manner by subsequent extraction with liquid ammonia, in which sodium cyanide is readily soluble. During this extraction process, the main structure of the briquette remains undcstroyed, and the uncombined residue may be used for further treatment with nitrogen. The chief technical difficulty lies in the rapid deterioration of the iron retorts at the temperature employed for fixation, the life of these being about 7 to 12 days. Nichrome retorts last longer, but are more expensive to replace. It has been proposed to use an electrically heated type of furnace in which the charge itself forms the resistance. Further, pure nitrogen, although conducive to a high yield of cyanide, is not essential for commercial success. Ferguson and Manning (Jour. Indust. and Eng. Chem., 1919, II. 946), in reviewing the replacement of nitrogen by producer gas containing carbon monoxide, state that at l,oooC. the presence of 15% of carbon monoxide in the nitrogen reduces the yield of cyanide by about 30%, while, if the producer gas contains 60% of carbon monoxide, the yield is one-half of the value obtained with pure nitrogen. This inhibitive effect of carbon monoxide, the reason for which lies in the reversibility of the equation Na 2 CO 3 +4C+Nj^I2NaCN+3CO, is even more pro- nounced at lower temperatures.
Numerous attempts have been made to synthesize barium cya- nide industrially from barium oxide or carbonate, carbon and nitrogen. Margueritte and Sourdeval in 1860 (Brit. Pat. 1,027/1860) appear to have been the first to suggest the process which was subsequently improved by Mond (Brit. Pat. 433/1882) and by Roadman (Brit. Pat. 6,621/1894). The optimum temperature is about 1,400 C., re- action taking place according to the equation: BaO+3C + N 2 = Ba (CN) 2 +CO. The above synthesis of barium cyanide was at one time worked on a considerable scale, but was not successful com- mercially, by reason of the deteriorating action of the fused cyanide on the walls of the furnace. It may be noted that the cyanides may readily be hydrolyzed to ammonia by means of superheated steam in an analogous manner to calcium cyanamide. G. W. Heise andH. E. Foote (Jour. Indust. and Eng. Chem., 1920, 12. 331) state that on treating briquettes containing synthetic sodium cyanide with steam at a pressure of 300 to 330 lb., a yield of ammonia amounting to over 90 % of that theoretically possible was obtained in 30 to 45 minutes.
Arc Process. The final method to be considered consists of the synthesis of nitric acid, either by the action of a high-tension elec- tric arc on air or by the explosion of compressed mixtures of air and a combustible gas in the cylinder of an internal combustion engine (Hausser's process). The production of oxides of nitrogen by either method depends on the reaction of nitrogen with oxygen at a high temperature, according to the reversible equation: N 2 +O Z ^2NO. The equilibrium percentage of nitric oxide formed by heating air to various temperatures has been measured by Nernst, Jellinek and Finckh (Gottinger Nachr. 1904, p. 261), Zeitschr.l f. anorg. Chem., I95. 45- n6; 1906, 49. 212, 229; Zeitschr. f. Elektrochem., 1906, 12. 527), the results being summarized in the following table:
Temperature C.
Equilibrium Percentage of NO ume (from air)
by vol-
i,538 1,604 1,760 1,922
2,307 2,402
2,927
o-37 0-42 0-64 0-97 2-05 2.23 5-00
Nitric oxide is formed, and consequently also decomposed, at a very high velocity, less than one-thousandth of a second being required for the attainment of equilibrium even at 2,000, so that, in order to preserve the products formed at arc temperature, these must be removed as quickly as possible from the arc flame; otherwise a less advantageous percentage, corresponding to a temperature lower than the maximum, is obtained. In practice, this rapid cooling is effected by employing a rapid flow of air, which is injected into a specially spread-out arc. Even with these precautions, however,
