This process was developed by the Badische Co. (now part of I. G. Farbenindustrie A.-G.) and is carried out at about 500° and 200 atmospheres. The necessary mixture of nitrogen and hydrogen is obtained from water-gas and producer-gas. Air is blown through a coke fire, shut off and steam blown through when the reaction, C-I-H20=CO-FH2, takes place. The mixture actually contains about 5o% hydrogen, 42 45% carbon monoxide and a small balance, chiefly carbon dioxide and nitrogen. The producer-gas from air and coke contains hydrogen, 5-15%; nitrogen, 65-50%; carbon monoxide, 20-30% and residue of carbon dioxide, methane, etc. Suitable proportions give a gas mixture with 22-23% nitrogen and remainder mostly hydrogen and carbon monoxide. By passage over a catalyst with excess of steam, the reaction CO-FH20=CO2-FH2 is driven as far as possible to the right to give a gas mixture with about 27% N2, 52% H2, 27 to 29% CO2, 2 to 4% CO and small amounts of argon, methane, etc. The carbon dioxide can be reduced to under 2% by water scrubbing under pressure, the remainder is washed out with cold alkali solution. Removal of carbon monoxide and residual sulphur compounds is essential as they are catalyst poisons; for the former, a solution of cuprous ammonium car bonate (or formate) may be employed. Treatment with ammonia is also used (e.g., Synthetic Ammonia and Nitrates Ltd., Slade and Parkes, Brit. Pat. 240,350; Casale, Brit. Pat. 231,417). F. Uhde finally cleanses and dries the gas-mixture by passage through a melt of sodium amide containing a metal of the alkali or alkaline earth class. 2o% sodium and 8o% sodamide can be used at 200-250° C, the contact of the gas and melt lasting 20 to 6o seconds. (Brit. Pat. 247,226.) According to Waeser, the gas mixture passes a three-stage corn pressor and the reaction is carried out in tubular vessels of special steel. Once the reaction is started, further heating is unnecessary and heat may have to be taken away. The gases leave the catalyst with about 20% of ammonia which is removed after cooling by washing with water.
G. Claude increases the pressure to 600 or 2,000 atmos.; this so displaces the equilibrium towards ammonia formation that higher temperatures (500-600°) can be employed, at which the catalysts are more active and less sensitive to poisoning. Up to 2o% of ammonia may be attained and if the issuing gases are cooled under this pressure, the ammonia is mostly liquefied and may be removed in an anhydrous condition. Usually three contact units are used in series. After passage through each unit, the gas is cooled and the liquefied ammonia removed, the remaining nitrogen and hydrogen being passed on to the next unit. A suitable steel for the high pressure apparatus is said to contain Si, 0.028%; S, 0.09%; C, 0.098%; and Mn, 0.93%. Chrome-vanadium steel is also employed for reaction chambers. The process is worked with hydrogen from water-gas or coke-ovens whilst electrolytic hydrogen is also employed, as at Bussi (Italy). Claude gives the following equilibria for ammonia
at 536°.
Discrepant statements have been made as to the pressure em ployed by L. Casale in the process named after him ; it is probably about 800 atmos., the gases circulating in a closed circuit. A characteristic of the process is that a certain percentage of am monia is returned with the nitrogen-hydrogen mixture in order to avoid overheating the catalytic mass. Only a portion of the ammonia is removed from the gas after leaving the synthetic tube and uncombined gases are made to pass repeatedly over the catalyst. Many important details have been worked out with respect to heat-interchangers and the maximum temperature of the pressure containers. This process has been largely adopted and is worked with nitrogen from various sources.
The ultimate raw materials are limestone, some form of carbon (charcoal, anthracite or coke) and atmospheric nitrogen. The limestone is burnt to lime, the latter heated in an electric furnace with carbon and the resultant calcium carbide brought to reaction with nitrogen (99.8% purity). Henri Mois san found that pure calcium carbide did not react with nitrogen on heating but Frank and Caro obtained cyanamide from corn mercial, impure carbide. The reaction is one which may be cata lysed and calcium fluoride has been used for this purpose. Calcium chloride is also effective but undesirable as it attracts moisture. The ovens in which the reaction between carbide and nitrogen is effected are either discontinuous or continuous in action. In the first case they are usually cylindrical in shape and charged with finely ground carbide around a carbon pencil. The covers are placed in position, nitrogen admitted under slight pressure and the carbon pencil heated electrically. The carbide surrounding the pencil soon attains the necessary temperature ; when the reaction has well started further electrical heating is unnecessary and the reaction spreads from the core outwards until the entire charge is converted. This may take 24 to 4o hours.
The crude product contains about 6o% calcium cyanamide (2I% nitrogen), 20% lime, 12% free carbon and a small amount of aluminium and iron oxides, silica, etc. Cyanamide is manu factured on a large scale in Germany and France and in sub stantial quantities in other countries. It is largely used as a fer tilizer; a certain amount is converted into ammonia and then into ammonium sulphate.
(a) At a high temperature, nitro gen and oxygen partially combine to form nitric oxide, the reaction being reversible.
The reaction is endothermic, i.e., heat is absorbed, so that the higher the temperature, the greater the quantity of nitric oxide in equilibrium with nitrogen and oxygen. At ordinary temperatures, the amount of nitric oxide is negligible for true equilibrium, but the rate of decomposition of nitric oxide is so slow that, if the mixture of gases be rapidly cooled, most of the nitric oxide formed at a high temperature may be preserved.