Calcium Cyanamide

The substitution of calcium carbide for the barium compound by Pfleger, Rothe, and Freundenberg caused the process to become more economical, and Frank and Caro patented the process during the years 1895-98. The real impetus given to the technical production of calcium cyanamide was the discovery in 1901 that this substance could be used directly as a fertiliser.

The essential materials for the manufacture of calcium cyanamide are limestone, carbon, and nitrogen. The production of calcium carbide is effected by first burning the limestone (CaCO₃) to lime (CaO), and then fusing of the lime with carbon (anthracite coal, or coke) in an electric furnace:

CaCO₃ = CaO + CO₂;
CaO + 3C = CaC₂ + CO.

The direct fixation of nitrogen is brought about by heating the powdered calcium carbide in an atmosphere of nitrogen:

CaC₂ + N₂ = CaNCN + C.

As in other fixation processes, one of the most important points with the cyanamide process is the purity of the reacting substances. Thus the initial raw material - limestone - must contain about 97 per cent, of calcium carbonate with relatively small amounts of magnesium carbonate and oxides of aluminium, otherwise the fluidity of the carbide is decreased. Silicon and iron tend to produce ferrosilicon, which gives furnace trouble and difficulty in grinding the carbide. Phosphorus in the form of calcium phosphate is very deleterious even in small quantities, as calcium phosphide is produced in the furnace, which with moisture produces phosphine, and is especially objectionable if the cyanamide is hydrolysed to ammonia which is subsequently to be oxidised to nitric acid.

Calcium carbide is finely crushed and heated in contact with nitrogen under slight pressure, when absorption begins at 700° to 800° C. and becomes rapid at 1000° C. The equation

CaC₂ + N₂ = CaNCN + C +97,800 calories

shows the reaction to be exothermic, and once started, the heat of formation is sufficient to carry the process to completion. It is obvious that an abundant supply of nitrogen is necessary, and this is generally obtained from the liquefaction of air by either the Linde or Claude rectification plant, although in part of the Niagara works nitrogen is obtained from air by the copper-cupric-oxide reaction. The absence of oxygen, moisture, carbon monoxide, and carbon dioxide is very desirable, otherwise various by-reactions take place, decreasing the yield of cyanamide:

CaC₂ + 2O₂ = CaCO₃ + CO;
CaC₂ + 2H₂O = Ca(OH)₂ + C₂H₂;
CaC₂ + CO = CaO + 3C;
2CaC₂ + CO₂ = 2CaO + 5C.

Moissan found that pure calcium carbide and nitrogen would not react at 1200° C., but Rothe discovered that nitrification proceeded rapidly if the crude carbide was used. It would appear that some form of catalytic material was necessary for the reaction, and Polzenius patented the use of calcium chloride in 1901. Thus, whereas with no calcium chloride there was less than 1 per cent, of nitrogen absorbed in twelve to fourteen hours at 730° C., yet under the same conditions with 10 per cent, of calcium chloride 18 per cent, of nitrogen was absorbed (as against 22 per cent, theoretical absorption).

The disadvantage in the use of calcium chloride, however, is the deliquescent nature of the final product. The action of water is to generate acetylene from the residual carbide and to form dicyano-diamide, (CN.NH₂)₂, from the cyanamide, which is deleterious when the cyanamide is to be used for fertiliser purposes. Dicyanodiamide is toxic to plant life, partly because it inhibits the action of nitrifying bacteria.

In 1906 the use of calcium fluoride, (CaF₂), was suggested in the form of crushed fluor-spar as a catalyst for carbide nitrification, and apparently most modern works now employ this substance.

The ovens used in the process of nitrification are either of the continuous or discontinuous type. The former type, as worked at Knapsack in Germany and Marignac in France, consists of metal boxes filled with carbide which are pushed through a long tunnel filled with nitrogen and heated either electrically or by means of producer gas. The Carlson Company in Sweden use cylindrical ovens, fitted with shelves, filled with nitrogen and heated at the top by arcs to a temperature of 950° C. The carbide is worked downwards by a mechanical scraper from shelf to shelf, and the cyanamide is discharged at the bottom of the oven after a period of two hours. The discontinuous type of oven is of the more common type, and may be externally fired or electrically heated. The electric furnace, such as is used at Odda in Norway, consists of a metallic cylindrical oven heavily lined with refractory material, so that the internal dimensions are about 5 feet by 3 feet. Nitrogen is forced in at the bottom, and the calcium carbide in a heavy container is heated by means of a carbon pencil passing through the centre, which carries a single-phase current of 100 volts and 200 to 250 amperes. After a few hours the current is switched off, as the heat of the reaction is then sufficient to carry the process to completion. Nitrification proceeds from the core outwards, the temperature in the furnace being about 1100° C. About 85 per cent, only of calcium carbide undergoes nitrification, and the rest is destroyed in some unexplained way during the reaction, as there is only. 1 to 2 per cent, of carbide in the crude cyanamide. Generally speaking, the nitrogen content of the cyanamide varies between 19 and 20 per cent, according to furnace practice, quality of raw materials, etc., in different countries. A typical analysis of cyanamide produced at Muscle Shoals in the United States is given:

	Per cent
Calcium cyanamide	61.2
Calcium oxide	20.0
Calcium carbide	1.5
Calcium sulphide	0.2
Calcium phosphide	0.04
Free carbon	12.5
Silica	2.4
Iron and aluminium oxides	1.8
Magnesium oxide	0.2

The crude product, which sinters to a solid greyish-black cake, is then crushed and milled. The small amount of residual carbide is very objectionable, and the only efficient way to remove same is to spray with water, when acetylene is generated. Insufficient water must be used than will form dicyanodiamide, and modern practice favours the use of a water-and-oil spray, especially if the cyanamide is to be used as a fertiliser. The water, to the extent of 6 to 7 per cent, of the weight of cyanamide, decomposes the carbide and hydrates the free lime, but does not remove dustiness, which is effected by a spray of mineral oil (3 to 4 per cent, of the weight of cyanamide).

Fertiliser

The crude calcium cyanamide, known as "nitrolim," "lime-nitrogen," "kalkstickstiff," etc., has the great advantage of cheapness. On the other hand, there is considerable difficulty in its storage owing to the action of atmospheric moisture and carbon dioxide, which results in the formation of dicyanodiamide. As has already been mentioned, this latter substance is not only directly toxic to plant life, but it also prevents the action of nitrification bacteria in the soil. Hence treatment and storage of cyanamide is a matter of first-rate importance; the water-and-oil treatment described above is very necessary, and when stored in double burlap bags it can be kept for six to twelve months without deterioration.