What is calcium cyanamide?
Calcium cyanamide, also known as lime nitrogen or nitrolime, is an inorganic compound with the formula CaCN2. It is a neutral salt of cyanamide that was first produced industrially in the late 19th century as a result of nitrogen fixation technology.
Industrial-grade calcium cyanamide is not pure CaCN2 but contains several additional components. Typically, it consists of about 20% calcium oxide and 10–12% free carbon, which gives it its characteristic gray-black appearance. Small amounts of nitrides formed from silica and alumina are also present.
The total nitrogen content of commercial calcium cyanamide ranges from 22 to 25%, depending on the raw materials and manufacturing conditions. Of this nitrogen, approximately 92–95% exists in the form of cyanamide, 0.1–0.4% as dicyandiamide, and the remainder as metallic nitrides.
Calcium cyanamide was first reported in 1877, when it was obtained in the laboratory by heating calcium carbamate to red heat. In 1889, larger quantities were prepared by heating finely ground mixtures of urea and calcium oxide.
The breakthrough in commercial production came with the development of the Frank–Caro process, a method for the direct nitrogenation of calcium carbide, which was patented in Germany in 1895.
The first industrial-scale plant for calcium cyanamide production, using a batch oven furnace, was established in 1905 at Piano d’Orta, Italy. Around the same time, the Polzeniusz–Krauss channel furnace was introduced, improving furnace design and efficiency.
By 1910, production facilities had been established in several countries, including Germany (Bayerische Kalkstickstoffwerke; AG für Stickstoffdünger), France, Japan, Sweden, Switzerland, and the United States (American Cyanamid).
Table of Contents
1. Physical properties of calcium cyanamide
Pure calcium cyanamide is a hygroscopic, colorless solid that crystallizes in the rhombohedral system. It melts when heated in a nitrogen atmosphere at about 1300 °C.
Important physical properties of calcium cyanamide are included in Table 1.
| Property | Value |
|---|---|
| CAS Number | 156-62-7 |
| Chemical Formula | CaCN2 |
| Molar Mass | 80.10 g·mol-1 |
| Appearance | Colorless, hygroscopic crystals |
| Crystal System | Rhombohedral |
| Density (25 °C) | 2.36 g·cm-3 |
| Melting Point (in N2) | ~1300 °C (with decomposition) |
| Heat of Fusion | 54 kJ·kg-1 |
| Specific Heat (20–100 °C) | 909 J·kg-1·K-1 |
| Standard Enthalpy of Formation (ΔH°298) | –348 kJ·mol-1 |
| Standard Gibbs Free Energy of Formation (ΔG°298) | –303 kJ·mol-1 |
| Standard Molar Entropy (S°298) | 87.1 J·mol-1·K-1 |
2. Chemical reactions of calcium cyanamide
The chemical behavior of calcium cyanamide varies with temperature, gas atmosphere, and impurities present.
Thermal decomposition:
Above 1000 °C, calcium cyanamide decomposes, and the products depend on the conditions. When heated in vacuum or in an inert atmosphere, the main products are calcium carbide, metallic calcium, and nitrogen gas:
2 CaCN2 → CaC2 + Ca + 2 N2
At elevated nitrogen pressures, decomposition favors the formation of cyanide compounds, while calcium nitride and elemental carbon are usually present as by-products.
Oxidation reactions:
Calcium cyanamide reacts with oxygen and carbon dioxide starting at approximately 475 °C to form nitrogen gas and calcium carbonate. At higher temperatures (above 850–900 °C), the principal product becomes calcium oxide. Carbon present as an impurity is not removed by oxidation, since the reaction preferentially consumes calcium cyanamide.
Reaction with carbon monoxide:
At temperatures above 1000 °C, carbon monoxide reacts with calcium cyanamide to yield calcium carbide and calcium oxide.
Reactions in aqueous media:
In water, the reactivity of calcium cyanamide depends strongly on temperature and pH:
- At room temperature, it forms monocalcium cyanamide Ca(HCN2)2.
- When heated at pH 9–10, it is converted into calcium hydroxide and dicyandiamide [(NH2)2CNCN].
- At pH 6–8 and below 40 °C, cyanamide is produced, while lime precipitates upon introduction of carbon dioxide.
- In acidic solution with suitable catalysts, urea can be formed; in the presence of sulfides, thiourea is obtained.
Hydrolysis to ammonia:
Under alkaline conditions at about 200 °C and elevated pressure, calcium cyanamide hydrolyzes to produce ammonia and calcium carbonate:
CaCN2 + 3 H2O → CaCO3 + 2 NH3
This hydrolysis reaction was historically used in the early 20th century as an industrial method for ammonia production, prior to the widespread adoption of the Haber–Bosch process.
3. Industrial production of calcium cyanamide
Calcium cyanamide is produced by nitrogenation of calcium carbide, which is a three stages process.
1. Lime production: High-purity limestone is calcined to form quicklime:
CaCO3 → CaO + CO2
2. Calcium carbide synthesis: Quicklime reacts with coke or coal in an electric resistance furnace to form calcium carbide. The process requires melting of the lime and absorption of the endothermic reaction heat:
CaO + 3 C → CaC2 + CO
3. Nitrogenation of calcium carbide: Calcium carbide reacts with nitrogen at 900–1000 °C to yield calcium cyanamide and carbon:
CaC2 + N2 → CaCN2 + C
This reaction is exothermic. Once initiated, it continues by controlled nitrogen addition without further external heating. The heat release amounts to 286.6 kJ·mol⁻¹ at 1100 °C and 295 kJ·mol⁻¹ at 0 °C.
Reaction mechanism
Calcium cyanamide formation involves intermediates such as Ca(CN)2, CaC2N2, CaC, and Ca2N2. The principal products of the nitrogenation are calcium cyanamide and carbon. At temperatures above 1000 °C, calcium cyanide forms in equilibrium with cyanamide and carbon:
CaCN2 + C ⇌ Ca(CN)2 ; ΔH298= +163 kJ/mol
This equilibrium is endothermic, in contrast to the exothermic nitrogenation reaction. Above 1160 °C, the CaCN2–C–Ca(CN)2 system melts with more than 60% cyanamide at equilibrium.
Small cyanide fractions generated at 1000–1100 °C are converted back to cyanamide by slow cooling. Rapid cooling favors cyanide retention, which formed the basis of the historical fusion cyanide process for sodium cyanide production.
By-products of nitrogenation
Impurities in technical-grade calcium carbide lower the yield of calcium cyanamide. Silica and alumina in the feed react with CaCN2 to form nitrides:
3 CaCN2 + Al2O3 → 3 CaO + 2 AlN + 3 C + 2 N2
3 CaCN2 + SiO2 → 2 CaO + CaSiN2 + 3 C + 2 N2
Carbide impurities such as calcium hydroxide and calcium carbonate decompose during heating to form water and carbon dioxide. These react with carbide to form acetylene. Decomposition of acetylene generates hydrogen, which is consistently detected in the exhaust gases of nitrogenation furnaces.
Oxygen introduced unintentionally during nitrogenation operation further reduces yield. Overall efficiency losses during the process reach about 10%. High-purity raw materials provide the most favorable results.
Catalysts and kinetics
Fluxes such as calcium chloride and calcium fluoride are used to enhance reaction rates or lower the nitrogenation temperature. Their exact role is not fully understood, but they are believed to create a transient liquid phase, consistent with the sintered appearance of the final product.
Studies show that calcium fluoride increases the reaction rate by a factor of 4.5 at 1000 °C and shifts the optimum temperature downward.
The nitrogenation rate depends on temperature, nitrogen partial pressure, carbide purity, additives, and crystallite size. Coarse-grained, high-purity carbide reacts more slowly than lamellar-structured carbide. Metal halides accelerate nitrogenation.
The reaction front progresses inward from the surface of carbide grains. At low temperatures, nitrogen diffusion through the porous product layer controls the rate, while at higher temperatures or in the presence of additives, the surface chemical reaction is rate-limiting.
3.1. Industrial processes
Both batch and continuous methods have been used for the industrial preparation of calcium cyanamide. The principal processes are the Frank–Caro batch oven process and the Trostberg rotary furnace process, in which the exothermic reaction of calcium carbide with nitrogen occurs at 1000–1150 °C.
In the rotary process, the heat of reaction sustains the operation after initiation, while the batch method requires ignition for each charge.
3.1.1. Frank–Caro batch oven process
The batch oven process dominated production during the first half of the 20th century but has since lost importance due to low nitrogenation yields, high labor requirements, and limited throughput.
In this method, reactors are charged with up to 10 t of ground calcium carbide. The reaction is initiated by electric heating or a pyrotechnic mixture, after which nitrogen gas is introduced through inlets in the furnace shell.
The nitrogenation proceeds spontaneously and lasts several days. The result is a solid sintered block of calcium cyanamide, which is subsequently broken and milled into a usable product.
3.1.2. Trostberg rotary furnace process
The continuous rotary furnace process was developed by SKW Trostberg (now Degussa AG). The furnace measures about 20 m in length and is lined with fireclay. The broadened head of the furnace serves as the main reaction zone.
Ground calcium carbide (55–60% CaC2 content) is blended with calcium fluoride and recycled lime nitrogen, then nitrogen gas is added. The solids remain in the furnace for 5–6 h, and the exothermic heat of reaction maintains the operating temperature at 1000–1100 °C.
Once started, the furnace can operate for many months without external heating. The output is a granular or powdered lime nitrogen that is transferred to a cooling drum. A single unit has a capacity of approximately 25 t per day of fixed nitrogen.
3.1.3. Alternative processes
Alternative routes have been studied to reduce the high energy consumption of conventional methods, particularly in calcium carbide production. Between 600 and 1000 °C, lime reacts with nitrogen-containing compounds to form calcium cyanamide. Examples include reactions with hydrocyanic acid, dicyanogen, urea, and dicyandiamide.
- Lime and urea initially form calcium cyanate, which on heating converts to calcium cyanurate and finally to calcium cyanamide.
- Lime reacts with hydrogen cyanide at 750–850 °C to form calcium cyanamide.
- Lime reacts with ammonia and carbon monoxide at 700–900 °C to give 99% calcium cyanamide.
These processes yield white calcium cyanamide, free from the carbonaceous impurities associated with the conventional route based on limestone and coal. However, none of these methods have reached large-scale commercialization due to economic or yield limitations.
3.2. Processing of Technical Calcium Cyanamide
Crude calcium cyanamide is ground in tube mills until the material passes through a 0.2 mm screen. For commercial products supplied in granular form, the grinding step is omitted, and the required particle size is obtained through screening.
When the residual calcium carbide content exceeds 0.1 %, as typically found in products from the Frank–Caro process, the material undergoes a degassing treatment with water. In this step, the carbide reacts to form acetylene and calcium hydroxide. For environmental and safety compliance, the acetylene-rich off-gases must be incinerated and subsequently scrubbed to eliminate acidic byproducts.
Granulation
The handling of finely divided calcium cyanamide as a fertilizer presents significant dust-related challenges. To mitigate these issues, the material can be treated with oil or transformed into a denser product via granulation or compaction.
Granulation using calcium nitrate solutions produces uniform, bead-like calcium cyanamide granules. These beads provide a dual-nitrogen system, combining nitrate nitrogen, which is immediately available to plants, with cyanamide nitrogen, which releases more slowly. It is stable in storage because the free calcium oxide is fully hydrated, preventing volumetric expansion.
In Europe, Degussa AG has been the principal manufacturer of beaded calcium cyanamide.
4. Uses of calcium cyanamide
Calcium cyanamide is used extensively in agriculture and horticulture, especially in Europe and Asia, as a slow-release nitrogen fertilizer. In addition to supplying nitrogen, it provides ancillary benefits by suppressing soilborne pathogens, slugs, and emerging weeds.
Upon contact with soil moisture, it decomposes into lime and free cyanamide. The cyanamide fraction is responsible for fungicidal and herbicidal effects. Microorganisms in the soil subsequently metabolize cyanamide to urea and then to ammonia.
An alternative transformation pathway leads to the formation of dicyandiamide, a known nitrification inhibitor. By delaying the oxidation of ammonium to nitrate, this process reduces nitrate leaching and prolongs nitrogen availability for several weeks. This makes calcium cyanamide particularly advantageous for intensively cultivated soils with high pathogen loads that cause root and stem rot.
Recent innovations in urea fertilizers involve incorporating a calcium cyanamide core within urea granules. This design stabilizes the nitrogen fraction by slowing nitrification.
Beyond crop fertilization, calcium cyanamide has applications in animal health management. When spread on pastures, it eradicates the dwarf water snail (the intermediate host of liver flukes) and destroys the eggs and larvae of intestinal and gastric parasites in grasses. It also suppresses salmonella in liquid manure.
In industrial and environmental contexts, calcium cyanamide effectively removes nitrogen oxides from exhaust gases, with efficiencies exceeding 99 % when used in scrubbing processes. In the cement industry, the addition of cyanamide and calcium cyanamide enhances setting behavior, improves compressive strength, and mitigates freeze–thaw deterioration.
Pharmaceutical-grade calcium cyanamide is used in the treatment of chronic alcoholism. In small daily doses, it induces adverse reactions, such as facial flushing, when alcohol is consumed, thereby discouraging intake.
In metallurgy, calcium cyanamide is used as a nitrogen donor in steel nitriding and as a desulfurization agent. Furthermore, it constitutes an important raw material for the chemical industry in the synthesis of cyanamide, dicyandiamide, and thiourea.
References
1. Güthner, T. and Mertschenk, B. (2006). Cyanamides. In Ullmann’s Encyclopedia of Industrial Chemistry, (Ed.). https://doi.org/10.1002/14356007.a08_139.pub2
2. Cameron, W. (2010). Cyanamides. In Kirk-Othmer Encyclopedia of Chemical Technology, (Ed.). https://doi.org/10.1002/0471238961.0325011416012005.a01.pub3
3. Klimek, A.; Yount, J.; Wozniak, D.; Zeller, M.; Piercey, D. G. “A Laboratory Preparation of High-Purity Calcium Cyanamide.” Inorg. Chem., 2023, 62 (40), 16280–16282. DOI: 10.1021/acs.inorgchem.3c02235
4. Dedman, A. J.; Owen, A. J. “Calcium cyanamide synthesis. Part 3.—The decomposition of ammonia and the mechanism of cyanamide ion formation.” Trans. Faraday Soc., 1961, 57, 684–693. DOI: 10.1039/TF9615700684
