Calcium Cyanamide, CaCN₂

In 1895, after Moissan's and Willson's discovery made practicable the preparation of carbides on a commercial scale, Frank and Caro attempted the fixation of atmospheric nitrogen by heating alkali or alkaline earth carbides to a sufficiently high temperature in free nitrogen. It was found that, whereas the alkalies gave cyanide, the barium compound contained a large proportion of cyanamide, and calcium formed Calcium Cyanamide, CaCN₂, only, between 1100° and 1200° C. The discovery that calcium cyanamide could be used in agriculture as a nitrogenous fertiliser resulted in the rapid development of the cyanamide industry which began at Odda in 1908.

Calcium carbide is used as the raw material instead of lime and coke because the temperature required for the formation of the carbide, which appears to be necessarily an intermediate product, is above the decomposition temperature of the cyanamide.

The carbide is crushed to a fine powder, sieved, and packed in perforated steel retorts previously lined with corrugated paper. These are transferred to heat-insulated ovens lined with fire-brick and fitted with a carbon rod through the centre. The latter heats the furnace by the resistance it offers to an electric alternating current passed through it. When the necessary temperature is reached in the neighbourhood of the carbon rod, pure dry nitrogen is passed in and absorption begins in accordance with the equation -

CaC₂ + N₂ = CaCN₂ + C.

As the reaction is exothermic, the mass rapidly becomes incandescent, and an average temperature of 1100°-1200° C. is maintained until absorption is complete, but it must not be allowed to rise above about 1300°-1400° C., or the process will be reversed. The decomposition temperature varies with the purity of the carbide and the pressure of nitrogen.

The process may be made either continuous or discontinuous. The nitrogen is obtained by the fractional distillation of liquid air or by passing air over heated copper. Heat may also be applied to the furnace by a gas flame.

The cold product, containing about 20 per cent, of nitrogen, has a dark grey colour owing to the separated carbon. It is crushed, ground, and sprayed with a small quantity of water to decompose any unchanged carbide and slake any free lime. Finally, it is mixed with a little mineral oil to render it dustless. It is stated that sugar or similar solutions, derived from molasses or cellulose manufacture, are better than mineral oil for this purpose, as they combine with lime to form sucrates for example, which act as a binding agent for the dust and aid the action of the fertiliser by the development of bacteria.

The total yearly production in 1913 was 150,000-160,000 metric tons, but in 1918 it had reached nearly 800,000 metric tons.

There are several trade names for calcium cyanamide indicating products of slightly varying composition: -

Lime nitrogen is the crude calcium cyanamide simply ground to a fine powder after removal from the oven. It contains about 55 per cent, calcium cyanamide, 2 per cent, calcium carbide, and about 20 per cent, free lime.

Cyanamid is the trade name for the completely hydrated material prepared for use as a fertiliser in the United States, America. It contains about 48 per cent, calcium cyanamide, 27 per cent, calcium hydroxide, and no carbide.

Nitrolim or nitrolime is the trade name for the material sold in England for agricultural purposes. It is lime nitrogen with just enough water to flestroy the carbide, but practically all the free lime is present as calcium oxide.

Kalkstickstoff, the German commercial product, is similar to nitrolim.

Stickstojfkalk was formerly the crude calcium cyanamide made at Westeregeln by nitrifying calcium carbide containing about 10 per cent, of calcium chloride, according to Polzeniusz's process, whereby the temperature of reaction is lowered. Its manufacture was discontinued in 1910.

Calciocianamide is the carefully hydrated Italian commercial product.

Cyanamide de calcium is the corresponding French product.

The effect of the presence of various oxygen-free salts on the temperature of nitrogen absorption has been studied by several investigators. Calcium chloride, which has a lower melting-point than calcium fluoride, is more effective in reducing the temperature of the reaction and thus accelerating it, but a limit is reached. Sulphides, which have a high melting-point, are without influence. Potassium carbonate accelerates the reaction, the optimum concentration at 900° C. being 4 per cent. In general, bromides and iodides cause a greater acceleration than chlorides. Metals of low atomic weight have a greater influence than those of higher in the same periodic group. Bredig regarded the accelerating action as simply due to partial fusion by which the protective coating of calcium cyanamide is removed, and the unchanged carbide exposed to the action of the nitrogen. This seems plausible, but Polzeniusz suggested that, with added calcium salts, there is intermediate formation of calcium nitride by the action of nitrogen on the ions of the added salt dissociated in the fused mixture.

In connection with the influence of foreign salts on the rate of absorption, it is interesting to note that Moissan could observe no reaction between nitrogen and pure calcium carbide, at least up to 1200° C.

Calcium cyanamide may also be obtained by fusing calcium cyanate, or by fusing cyanamide, CN.NH₂, or urea, with lime. It forms colourless crystals which sublime at 1050° C. at atmospheric pressure.

The molecular heat of formation of calcium cyanamide from carbide diminishes with increasing temperature, as is seen from the following table: -

Temperature, ° C	1100	1200	1300	1400
Heat of formation in Cal.	63.9	27.0	18.8	15.2

The heat of formation from its elements is 94.820 Cal. if amorphous carbon be employed, or, if this be replaced by diamond, 91.480 Cal.

As there are four phases present, three solid and one gaseous, and three components, the system is monovariant, and for every temperature there should be a definite dissociation pressure of nitrogen. The following values have been found experimentally: -

Temperature, ° C.	1053	1114	1160	1223	1278	1308	1378	1448
Pressure, cm. mercury	2.3	6.8	12.5	22.4	27.3	28.3	37.8	44.4

Kameyama found that the temperature at which the dissociation pressure of nitrogen is one atmosphere is 1754° C. if the carbon be present as graphite, or 1690° C. for amorphous carbon.

The higher the pressure is raised above the dissociation pressure for any one temperature, the more readily will the formation of cyanamide take place at that temperature, but Pollacci found that a limit is reached at two atmospheres.

Oxygen reacts with calcium cyanamide at 420°-450° C. with the formation of calcium carbonate and nitrogen. Below 1070° C. carbon dioxide gives calcium carbonate or calcium oxide according to temperature, and nitrogen, whilst above 1110° C. the separation of carbon takes place owing to the action of the carbon monoxide produced.

CaCN₂ + CO = CaO + 2C + N₂. Carbon monoxide alone exerts no action on calcium cyanamide up to 1000° C.

Calcium cyanamide is insoluble in alcohol but easily soluble in water, about 2.5 grm. dissolve in 100 c.c. of water at 25° C. It is hydrolysed by water, forming the acid salt Ca(CN.NH)₂. A basic salt, (CaOH)₂CN₂.6H₂O, has also been separated from solutions containing excess of lime, and by passing carbon dioxide into a solution of calcium cyanamide a compound, Ca(CN.N.CO₂).5H₂O, is obtained.

Two structural formula have been given for calcium cyanamide, one representing it as the calcium salt of cyanamide, Ca=N-C≡N, and the other as the calcium salt of di-imide,

A crude cyanide mixture known as " surrogate," to be used for the extraction of metals from their ores, can be obtained by heating the commercial calcium cyanamide, which contains carbon, with sodium chloride, CaCN₂ + C + 2NaCl = CaCl₂ + 2NaCN,

or, better still, with sodium carbonate, because the process is not reversible. Calcium cyanamide may also be applied directly to the extraction of ores.

Calcium cyanamide forms an intermediate product in one of the methods of fixing atmospheric nitrogen. By treating with superheated steam, ammonia is obtained -

CaCN₂ + 3H₂O = CaCO₃ + 2NH₃.

It may also be used as the starting-point for the preparation of a number of organic compounds. By the action of water and carbon dioxide in the cold, cyanamide is formed, and by extracting with hot water, dicyano-diamide, (H₂NCN)₂, is obtained. The latter is used in the production of certain organic dyes and for reducing the temperature of combustion of explosives. By suitable treatment with dilute acids and a catalyst, urea, guanidine, and nitro-guanidine may be obtained.

Calcium cyanamide also forms one of the constituents of the preparation "Ferrodur" used for case-hardening and tempering iron and steel.

Perhaps the most important use of nitrolime is as a fertiliser, but the fact that it may do serious harm if improperly used has caused it to fall into disfavour to some extent. It should be applied to the soil at or before the time of sowing, and well distributed in not too large a quantity, and not as a top dressing. The nature of the soil should also be carefully considered. Nitrolime and its products do not readily disappear from clayey or sandy soils, but are decomposed in peaty soil.

Nitrolime in too large quantities is toxic to plants, especially if it contains dicyanodiamide. This compound is also destructive to nitrifying organisms, thus preventing the decomposition of the nitrolime from going further than the ammonia stage. Humus-containing soils, however, apparently absorb dicyanodiamide to such an extent that the poisonous action disappears, and presumably the nitrogen becomes available for plant life. Nitrolime may be used as a weed-killer if scattered on the leaves when wet. In the presence of the free lime, dicyanodiamide is thus produced.

It is evident that it is important to know the amount of dicyanodiamide present in commercial calcium cyanamide. Caro and others have worked out methods of estimation which depend on the fact that cyanamide is precipitated by a salt of silver in presence of excess of ammonia, whilst silver dicyanodiamide is soluble, but may be precipitated in the filtrate by the addition of a solution of potassium hydroxide.

The fertilising value of nitrolime nitrogen is somewhat less than that of either nitric nitrogen or ammonium nitrogen, but it has the advantage of forming a fine, dry, non-hygroscopic powder.

If used with mixed fertilisers, it is better with nitrates than with phosphates, which are said to be almost immediately reverted, and which may also cause the rapid formation of dicyanodiamide. This, however, is contradicted by Pranke, and nitrolime is apparently often used in conjunction with superphosphates. Mixed with nitrates, it checks their too rapid decomposition, increases their availability, and secures a more lasting and uniform effect. The resulting dilution of the cyanamide is also beneficial.

The first stage in the absorption of nitrolime by the soil is the removal of free lime by the action of carbon dioxide. By a complex process, which is variously ascribed to the agency of colloids, ferric oxide, zeolites, or bacteria, urea is produced, and, by bacterial action, is ultimately transformed, first into ammonium salts, and finally into nitrates.