CYANAMIDE forms white crystals, melting at 40° C. and readily soluble in water, alcohol and ether. It is the amide of normal cyanic acid, NC.OH, having the formula NC•NH.2. It is obtained by acting on cyanogen chloride, bromide or iodide with ammonia, or generally by desulphurizing thiourea with mercuric oxide. Heated above its melting point cyanamide polymerizes to di-cyandiamide which at 150°C. is transformed into the polymer n-tri-cyantriamide or melamine the mass solidifying. Nascent hydrogen reduces cyanamide to am monia and methylamine. It gives mono-metallic salts of the type NC.NHM when treated with aqueous or alcoholic solutions of alkalis. Di-metallic salts are obtained by heating cyanates alone, e.g., calcium, or cyanides in a current of nitrogen, e.g., barium.
Calcium cyanamide has assumed importance in agriculture since the discovery of its economic production in the electric furnace (see below) . It may also be produced by heating lime or chalk with charcoal to 2,000° in a current of air. It appears that with soils which are not rich in humus or not deficient in lime, calcium cyanamide is almost as good, nitrogen for nitrogen, as ammonium sulphate or sodium nitrate ; but it is of doubtful value with peaty soils cr soils containing little lime, nor is it use fully available as a top-dressing or for storing.
The metallic' cyanamides, of which calcium Cyanamide, is the only one of technical importance, are derivatives of cyana mide, In practice, calcium cyanamide is always formed by the action of nitrogen on calcium carbide. In 1892 Moissan showed that calcium carbide could be prepared easily and in quantity in the electric furnace, and in 1895 A. Frank and N. Caro began investigations on the absorption of nitrogen by metallic carbides which have led to the establishment of the modern cyanamide industry. Although pure calcium carbide does not absorb nitrogen even at 1,200° C. (Moissan), Frank and Caro found that technical carbides did so readily at much lower tem peratures, probably owing to their impurities.
Formation of Cyanamides by Nitrogen.—By the action of nitrogen on metallic carbides, mixtures of cyanamides and cya nides are obtained, the proportions depending on the metal and on the temperature. The general type of the reaction may be illustrated by the case of calcium carbide.
With alkali carbides, cyanide preponderates largely. With barium carbide about 30% cyanide and 70% cyanamide are obtained at 7oo°-800°C., whilst with calcium carbide cyanamide is practical ly the only product up to 1,1oo°C. In all cases the proportion of cyanide increases with the temperature. These facts explain the formation of cyanide by fusing calcium cyanamide and carbon with sodium chloride. The mechanism of the production of calcium cyanamide is by no means clear, in spite of numerous investigations. The chief difficulty in interpreting the results is the effect of impurities, always present in commercial carbide.
Absorption of Nitrogen.—The rate of absorption of nitro gen by carbide has been much studied. Pure calcium carbide does not absorb nitrogen at 1,200°C. With commercial carbide (con taining about io% lime) absorption is rapid and complete at Below i,000°C. the absorption is less rapid and is incomplete, the nitrogen absorbed tending to a limiting value, which is lower the lower the temperature. These limiting values do not represent true equilibria (Foerster and Jacoby). Pollacci found that the velocity of absorption is proportional to the nitrogen pressure up to two atmospheres, but that further in crease of pressure makes little difference.
Many substances when added to calcium carbide lower the temperature at which nitrogen absorption begins and increase the velocity of absorption for a given temperature. The most im portant of these substances are calcium chloride (Polzenius) and calcium fluoride (Carlson) . Many other substances, including lime, act similarly, and this explains the action of commercial carbide. The role of these additions has been investigated by G. Bredig, W. Fraenkel and E. Wilke, F. Foerster and H. Jacoby, Rudolphi, G. Pollacci and others, and has been connected with the lowering of the melting-point of the mass which they cause; but how this increases the velocity of absorption, whether by re moving protecting layers of reaction products from the carbide, by allowing the nitrogen to dissolve in the liquid mass, or by other means, has not been settled. Reaction (I) is strongly ex othermic. The heat of reaction is about 77,800 calories at 25°C.
Technical Production.—On the commercial scale a mixture of calcium cyanamide and carbon is produced by heating ground calcium carbide in an atmosphere of nitrogen to about r,000°-i,ioo°C. The nitrogen is generally prepared by the fractional distillation of liquid air by the Linde or Claude process, and should be as free as possible from moisture, oxides of carbon and especially oxygen, as these act on either the carbide or cyanamide and diminish the nitrogen content of the final product.
Discontinuous Process.—In the discontinuous process a ver tical steel drum lined with firebrick is fitted loosely with a thick paper or thin, perforated, sheet-iron cylinder. Down the centre of the retort passes a carbon rod contained in a cardboard tube to separate it from the carbide. This rod is connected to an al ternating current supply and is used as a heating resistance to start the reaction. The ground carbide is packed between the inner tube and the outer cylinder. The lid is luted on airtight. Nitrogen is admitted by a pipe through the outer casing. The charge consists of 300-800 kilos of carbide. A battery of such units is connected to the current supply. The heated carbon resistor starts the reaction at the centre. After a time the cur rent is cut off as the reaction produces sufficient heat to main tain the charge at about 1,000°C. and so complete the reaction. The drums are allowed to cool, the contents ground and treated with sufficient water to slake the lime and decompose any uncom bined carbide. The ground product is sometimes mixed with a little oil to render it dustless before packing in bags. The crude cyana mide, known as "nitrolim," "lime nitrogen," "kalkstickstoff" or "stickstoffkalk," contains 20-22% of nitrogen, corresponding to of and about 20% lime, 7-8% silica, iron oxide and alumina and 14% of graphitic carbon.
Continuous Process.—In one type of continuous furnace the carbide is packed in perforated metal boxes which are sent through a tunnel filled with nitrogen and heated, electrically or by ex ternal gas firing, to the necessary reaction temperature. Another common type is a vertical iron shaft lined with firebrick and di vided internally into sections by shelves. The carbide is fed in at the top and heated by arcs between carbon electrodes. It meets an ascending stream of nitrogen and is made to fall slowly from shelf to shelf by scrapers until it is discharged at the bot tom. In the Carlson process calcium fluoride is added to the car bide to increase the velocity of absorption and to lower the re action temperature.
BIBLIOGRAPHY. Final Report of Nitrogen Products Committee; Bibliography. Final Report of Nitrogen Products Committee; Joseph Knox Fixation of Atmospheric Nitrogen (1921) ; A. J. Allmand The Principles of Applied Electrochemistry, p. 643 (1924) ; J. R. Partington and L. H. Parker The Nitrogen Industry (1922) ; B. Waeser The Atmospheric Nitrogen Industry (trans. E. Fyleman, 1926) .
(J. KN.)