Properties. Pure calcium cyanamide is a colorless crystalline solid, soluble in water (2.5g in 100 ml of water at 25C) and insoluble in alcohol. It is volatile at high temperatures, sublimes at 1090C (at atmospheric pressure), and can be melted under excessive pressure of nitrogen at 1340C. Dissolved in cold water, it is hydrolyzed forming a soluble acidic salt and calcium hydroxide (eq.1), while hot water decomposes calcium cyanamide with formation of urea and calcium hydroxide (eq.2), and action of water steam at 110-115C causes release of ammonia and formation of calcium carbonate (eq.3). At temperatures above 400C calcium cyanamide is readily oxidized by air (eq. 4).

2CaCN2 + 2H2O(cold) → Ca(CN2H)2 + Ca(OH)2
CaCN2 + 3H2O(hot) → Ca(OH)2 + CO(NH2)2
CaCN2 + 3H2O(steam) → CaCO3 + 2NH3
2CaCN2 + 3O2 → CaCO3 + 2N2

Pure calcium cyanamide contains 34.98% of nitrogen. The industrial product has greyish black color due to contamination by carbon (9-13%), contains 55-65% of pure cyanamide and 18-24% of nitrogen. Additional contaminants (~5%) contain mainly silicic acid, oxides of iron, magnesium and aluminum. In addition industrial calcium cyanamide contains 14-20% of calcium oxide and 2-5% of unreacted calcium carbide.

Calcium cyanamide slowly reacts with moisture in the air forming urea, cyanamide, dicyandiamide and ammonia. Calcium cyanamide is used in agriculture as a nitrogen fertilizer. In chemical industry calcium cyanamide was used to produce cyanides, guanidine and a large variety of organic compounds.

Calcium cyanamide is toxic and acts mainly as a skin and lung irritant. Toxicity is relatively low but can be raised greatly then combined with alcohol. Action of cyanamide dust followed by alcohol drinks cause severe poisoning accompanied by agonising suffocation. This specific toxic effect is reason why cyanamide solutions are proposed for use in anti alcoholism drugs.

Preparation. Calcium cyanamide is produced on an industrial scale by fixation of atmospheric nitrogen by calcium carbide at temperatures above 1000C in electric furnaces:

CaC2 + N2 → CaCN2 + C

Other methods of preparation include: action of ammonia on cyanogen chloride, melting of urea with metallic sodium, high temperature reaction of hydrocyanic acid with calcium oxide and thermal decomposition of calcium cyanate. It is clear that only last one is possible in home scale since calcium cyanurate can be readily produced by melting urea with calcium oxide or hydroxide, but this method require some work to gain access to high temperature.

The process consists of two main stages – in the first stage is formation of calcium cyanate by melting urea with calcium oxide. Thermal decomposition of urea is not a simple process and can produce wide variety of products, depending from reaction conditions and timing, but in presence of alkali it goes almost exclusively to cyanate direction. Mechanism of cyanate formation can be expressed by the following reactions:

1. 3CO(NH2)2 → 3HOCN + NH3

2. CaO + 2HOCN → Ca(OCN)2 + H2O

3. HOCN + H2O → CO2 + NH3

As one can see, theory requires 3 moles of urea for 1 mole of calcium oxide, since one molecule of cyanic acid is decomposed by water liberated in neutralization of calcium oxide. Reaction takes place at temperatures above urea melting point (120 - 350C) and causes formation lots of ammonia gas, accompanied by intensive foaming of the melted mixture. When the process is nearing completion the reaction mixture re-solidifies due to formation of calcium cyanate which have higher melting point (decompose without melting at higher temperatures). In order to produce calcium cyanamide, calcium cyanate produced is cooled, grounded in mortar and calcined at high temperature in absence of air oxygen.

The second stage (calcination of calcium cyanate) proceeds at temperatures about 700-900C, producing calcium cyanamide and carbon dioxide (Eq.1 below). It should be noted that that air oxygen oxidizes produced cyanamide to carbonate (Eq.2 below), and process should be carried out with very limited air contact or under protective gas atmosphere:

1. Ca(OCN)2 → CaNCN + CO2

2. 2CaCN2 + 3O2 → CaCO3 + 2N2

This reaction proceeds relatively slowly, and it takes about an hour to be substantially complete. Process also require use of high temperature for extended amount of time and in relatively large volume and ideally can be preformed in laboratory electric furnace. Since such equipment is not readily accessible for amateur chemist, below i will show you how you can build your own from commonly obtainable components.

In general the oven consists of ceramic fireproof muffle tube, covered by nichrome wire heating element, covered by radial fireclay layer which ensures that main heat flow is directed to opposite direction – into the muffle. To prevent heat losses to environment and allow heating to high temperature the whole construction is protected by heat shielding. Temperature in the muffle is measured by means of thermocouple. Heat wire is powered by AC power supply which allows voltage regulation – this will allow temperature regulation inside the muffle by regulating heat flow from heat element.

1. The muffle is fireproof ceramic tube, stable at high temperature, high electric resistance and able to withstand heat shock. Perfect materials include ZrO2/MgO/Al2O3 based ceramics. Tubes suitable for ovens can be obtained from many sources, for example from high voltage safety fuses and other high voltage electric components. Tube i used for my oven is zirconium oxide based fireproof ceramic tube with following dimensions: 90 mm long, 60 mm outer diameter, wall thickness 4 mm.

2. Nichrome heating element was made from 80:20 nickel-chromium alloy, with high electric resistance, with low temperature-resistance dependence. Nichrome is best for the job because of high electric resistance, high working temperature (about 1000C) and is stable to air oxidation at such temperatures. I used wire of 0.5 mm diameter, which was wound on the ceramic muffle with narrow equally spaced turns. To make turns equally spaced the wire is wound in pairs with filament having approximately the same diameter, turns are made contiguous to each other, and after all wire is coiled filament is carefully removed leaving coil with ideally spaced turns. Coil wire is fixed in place by two nichrome wire rings spaced near the top and bottom edges of the tube. Don’t forget to provide the coil with some free nichrome wire at the edges, to exit from the heating element and make contacts with the power supply output line. A photo of my coil is shown below, it has 81 Ohm of electric resistance.

3.The next thing to do is to determine working voltage and appropriate power supply – those can be calculated from required power of the heating element. If power is too high wire will simply burn out due to wire melting, if power is too low it won’t reach high temperature – so optimum values have to be determined. First we need to select appropriate output power for our heat element; for a small oven like ours 500 watts is a way to go. Now we can determine voltage required to reach desired level of power, by using Joule–Lenz and Ohm’s laws:

As shown by the calculation above to reach desired power we need to use regulated AC power supply, capable of 200 V output voltage and current of ~2.5A. To make sure our wire doesn't burn out at such current and determine maximum recommended wire temperature we must check out current value against maximum allowed current:

As we can see from the table above, for our wire (0.5 mm diameter) maximum allowed current is about 3.5A with recommended working temperature about ~1000C. Then combined with efficient heat shielding such heating a element would allow us to reach temperature around 900C. Now we can derive the voltage limit for our heating element and maximum possible power. To do this we must substitute maximum allowed current to Ohm’s formula to get voltage at Imax and then use the derived voltage to calculate power:

Resuming conclusions above we can now summarize parameters of our heating element: 500W voltage is ~200 V with current of 2.5A, maximum allowed voltage is 283.5V with current 3.5A and ~1 KW power, recommended working temperature is <= 1000C.

4. Now that our heating element is ready we must complement it with heat shielding and make the oven body. First we need to cover the nichrome wire with 0.5-1 cm layer of fireclay (I’ve used ~0.75mm thickness). Take some fire clay and add the least water needed to reach plasticity and cover wire with a clay layer, then the wire is covered and the clay layer is smoothed by rolling on a smooth surface. The whole device is placed into a kitchen oven to dry for 2 hours at ~250C. When initial drying is finished one can attach contacts of nichrome wire to power supply and heat it slowly rising voltage to nominal and allow element to sit at this voltage for a while (0.5-1 hour) to complete drying of the clay layer. Top and bottom caps of muffle can be made in the same manner, but can be worked out without such careful drying, sitting for 2 hours at 250C kitchen oven is enough. The oven body can be made from an iron can. Carefully remove can cover, remove can contents and drill two holes in can side walls, those will be required for nichrome wire contacts (my can was 10 cm in diameter and 12 cm height). Fill the bottom of the can with a layer of magnesium oxide, and place heating element and its clay bottom in the can center, leading wire contacts through the drilled side holes. Cover holes with contacts with clay to prevent wire short circuit over the oven body and fill the whole space between the heating element and walls of can with powdered magnesium oxide. To complete the oven you must cover muffle with the clay cover you made earlier and cover whole thing with iron cover of the can (this second cover will help in prevention of convective heat transfer caused by active air circulation), both covers are made coaxial and provided with a drilled hole in the center which will be used for thermocouple temperature control.

5. Your oven is now almost ready to use. Just place it into a saucepan of wider diameter filled with a wide layer of magnesium oxide to minimize heat transfer from oven can (I’ve used saucepan with ~15 cm diameter), place thermocouple to the top cover of can and attach your oven to the corresponding power supply. I’ve used a laboratory auto transformer (0-220V AC, with maximum secondary coil current of 4A). Now your heating device is fully operational. The device I’ve described is can reach a temperature of 900C in less then 1.5 hours. A photo of the working heating device is shown below:

Preparation of calcium cyanamide was performed according to US patent 5753199. Prepare a mixture of 56.4g (1 mol) of pure finely ground calcium oxide (Note №1) and 180g (3 mol) of pure fine urea. Mixture is placed to saucepan and heated on a hotplate. As soon as the urea melts the reaction starts, a lot of ammonia is evolved and mixture begin to foam (Note №2). After foaming subsides, mixture begins to solidify and became unstirable. Then reaction mixture is completely solidified, heat source is turned off and still hot product is grounded to fine powder. This yields about 134g of crude calcium cyanate (Note №3). Calcium cyanate is then placed to ceramic crucible, witch is covered by closely fit clay lid, placed to electric furnace (also provided with closely fit cover) and is heated up to 750-800C for one hour (Note №4). After this period furnace is turned off and is allowed to cool down, without opening furnace cover. Then temperature in furnace is downed to 150-200C, furnace cover and crucible is removed and placed to cool on the air. Opened crucible is filled with spongy pinkish-white calcium cyanamide, with small (about ¼ of crucible) bottom layer of product with grey color (Note №5). Total yield is 69.3g of high quality calcium cyanamide (86% from theory), containing almost theoretical amount of nitrogen (~33-34%).

1. It is essential to use pure calcium oxide, to prevent presence of alkali metal salts in reaction mixture. Those at first will form corresponding cyanates, witch are unstable at high temperature of second – calcinations stage and will decompose to form cyanide and oxygen. Even if relatively small amounts of sodium or potassium are present, final product can became much more dangerous since on action of acids it will realize extremely dangerous and toxic hydrogen cyanide, in quantities easily detectable by smell from such reacting mixture, or even from solid product itself.

2. Foaming of reaction mixture is very intensive and can easily overflow reactor wessel. This must be taken to account then choosing volume of reactor. Foam can be sufficiently settled down by intensive mixing. Note that moistured source materials will not result in even more intensive foaming but will also reduce overall yield due to increased moisture decomposition of cyanic acid.

3. This quantity is larger then theoretical (134g vs 124g theory), this is result of the fact that solidification and drop of reaction rate occur earlier that source products are completely reacted. It was found that continue of heating after solidification doesn’t lead to sufficient change of mass and complete transformation. Analysis of reaction product shown clear strong signal for cyanate, but some small amount of unreacted source products also present. This however is not critical for further calcinations step, since strong heat will cause them to react further, forming final reaction product.

4. Then this crude cyanate is heated up to about 300-400C, impurities of unreacted urea and calcium oxide will react/decompose, forming more ammonia. This additional amount of ammonia evolved is quite small but is easily detectable by smell, so it is advised to perform calcinations in place with good ventilation. Remember to use closely fit lids for both crucible and furnace to prevent oxidation by air, if such lids are used circulation of waste reaction gases is avoided, allowing them to act as protective atmosphere for main product.

5. Analysis of both colored layers shown that their constituents are exactly the same – both show strong and clear signal for cyanamide with ammoniacal silver nitrate and both contain only tiny traces of cyanate shown by reaction with cobalt salt solution. It appears that grey coloration of bottom layer is caused by charring of some organic impurities present in starting cyanate, and is not related to exact calcination temperature or speed of heating.

As mentioned earlier it was found that calcium cyanamide produced from precipitated CaCO3 and urea fertilizer was not essentially pure for production of aminoguanidine. So taking to account rrrocket's statement, big effort has been made to produce pure calcium cyanamide - this time from pure CaO and urea starting materials (both bought from chemical supplier and is no more impure fertilizer product). Decision to use chemically pure CaO was made due the statement of rrrocket and due to it's purity from alkali metal salts (those will produce cyanide on calcination stage).

First of all much more reference work has been made to gain information on detailed reaction mechanism and conditions and lead to first major improvement. It was found in some references, that 1 mole cyanamide actually require 3 moles urea, not 2 as basic theoretical view can predict, this is due to the following reactions:

So the synth i proposed (using 2 mole urea vs 1 mole of calcium oxygen compound) is no good due to lack of urea to fully complete conversion to cyanate. Another improvement to process was realized then more experimentation with new ratio has been done - it was found that then mixture is calcined in open crucible placed in closed electric furnace i've described - results in large of contamination by cyanamide oxidation product - calcium carbonate. Resulting product is snowy - white powder, then pure calcium cyanamide prepared under protective atmosphere was found to be pinkish. So great care should be taken to protect calcinating product in the reactor out of air contact, covering crucible with relatively tight lid and placing in to furnace covered in same manner found to be satisfactory to produce quite pure calcium cyanamide.

Here are results from second line of cyanamide experiments i've preformed: 56g of pure calcium oxide were mixed with 180g of pure crystalline urea and mixture was melted as mentioned in procedure in top of topic. Resulting first stage product is weighted 135g vs 124g theoretically predicted - this indicates that some ammount of unreacted urea and calcium oxide still present in solidified reactor mixture. Effort has been made to force this stage to complete by grinding solidified mixture and applying more heat for extended time period, it was found that reaction (and decrease of mixture mass) proceeds very slowly and it is not profitable to do this additional heating at first stage. Resulted product was analyzed for presence of cyanate and it was found that mixture consists mainly from calcium cyanate while some 5-6% of unreacted urea + CaO still present in the mixture. Photos of this reaction product as well as colored reaction it gives in cyanate test are shown below:

Calcination of resulting product out of air contact, as expected resulted in calcium cyanamide, however some interesting results have been obtained. Removing product from crucible after cooling shown that product consists from 2 layers, lower layer (~1/4 of crucible height) was filled with grey product, while upper layer (~3/4 crucible height) consisted from pinkish-white product. This was confusing at first and i thought it may be from some crucible impurity, later experiments shown that such stuff happens with any batch in more or less ammount, however ammount of grey product is always much smaller. After this to parts separated both vere analyzed for cyanamide pressence using ammoniacal silver nitrate and in both cases clear and strong signal of cyanamide was observed, as formation of bright pale-yellow silver cyanamide precipitate. In addition both samples vere tested for cyanate presence, and in both cases only traces of them were discovered. In total from 135g of starting material from first step (calcium cyanate) 68.9g of calcium cyanamide was produced, this corresponds to 87% theoretical based on starting urea and calcium oxide. Photos of 2 layers, final product and product test for cyanamide are shown below:

This alternative route to aminoguanidine is known to me for a long time, however it took time to find enough free time to do extensive experimentation like that, since it required not only extensive chemistry work but also construction of electric furnace suitable for production of cyanamide. Finally all this work arouse in success and pure cyanamide based aminoguanidine was obtained, leading alternative way to home made tetrazole compounds.

Preparation of aminoguanidine was carried out on base of procedure described in US Patent 3673253, and is based on condensation of cyano-group in free cyanamide with hydrazine. Reaction of hydrazine sulphate with calcium cyanamide in aqueous media results in formation of insoluble calcium sulphate and liberation of free cyanamide and hydrazine, witch react with each other in basic media forming aminoguanidine. Since cyanamide is quite reactive and can readily form dimeric form – dicyandiamide witch doesn’t produce aminoguanidine on reaction with hydrazine, this side reaction leads to decreased yield of final product. Reaction conditions, mixture pH and temperature are controlled in such manner witch allows to overcome this side process. This is achieved by keeping minimal concentration of cyanamide and decreasing pH by addition of sulpuric acid to level keeping conversion to dicyandiamide at minimum while still allowing fast condensation process for cyanamide and hydrazine. Reaction scheme is shown on picture below:

55.8g of monohydrazine sulphate (0.429 mol) and 40g of finely ground calcium cyanamide containing 30% N (~0.5 mol) were simultaneously and uniformly added with vigorous stirring to 200 ml of water at pH value of 9.5 (Note №1). The pH value was regulated by the addition of 50 percent sulfuric acid in an appropriate quantity (Note №2). The reaction temperature was kept at 40C. After reaction mixture had been stirred for 30 minutes at pH 9, the pH value was adjusted to 7 and the mixture was heated for about 1 hour at 80C at that pH value. After reaction mixture had been cooled to 60C, it was filtered and the filtration residue, consisting essentially of calcium sulfate, was washed with 200 ml of warm water (Note №3). Both the filtrate and water used for washing were collected in a precipitation vessel and were adjusted to a pH value of 6.5 with 50 percent sulfuric acid. 36.9g of sodium bicarbonate were then slowly and uniformly added at 30C as a result of which the aminoguanidine bicarbonate was separated as fine white precipitate. Precipitate is filtered, washed with small amount of cold water and dried. Product is white microcrystalline powder, yield is 49.1g (85% based on hydrazine, Note №4).

1. This pH range can be achieved without adding sulfuric acid – by adding components in corresponding rate, however it can also be justified by addition of acid. Addition can be carried out in portions, adding new portion, stirring for 1-2 minutes then adding new one e.t.c. Addition should not produce any gas or significant odor, if such are present – your cyanamide is impure and yield be lowered. Use of cyanamide contaminated with alkali metals on this step will lead to formation of hydrogen cyanide witch will be noticeable by odor, while cyanamide significantly oxidized by air during production (e.g containing carbonate) will form a lot of bubbles of carbon dioxide gas.

2. Total amount of sulfuric acid, consumed during whole reaction process is equivalent to 12-15 ml of concentrated 96% H2SO4. It should be noted that not uniform addition of sulfuric acid will cause high local concentrations of H2SO4 and formation of solid/gases, so if H2SO4 is added by portions for example from syringe, addition must be accompanied by vigorous stirring to prevent loss of reactants.

3. Precipitated calcium sulfate consist from particles of relatively large size and filtration proceeds with ease. Total amount of water used for washing is not critical however 200 ml is reasonable amount. One can notice that filtrate obtained has distinctive yellow coloration common for solutions of aminoguanidine salts.

4. Product yield can vary depending on quality of starting cyanamide and from care with witch correct pH ranges are controlled.

1. Left photo shows reagents – calcium cyanamide and hydrazine sulfate used for reaction, second photo is reaction mixture in the middle of reagent addition, third one is heating stage, last photo shows reaction mixture with calcium sulfate precipitate, obtained after heating period.

2. Left photo shows filtration from calcium sulfate, second is filtrate produced –note that this yellow color is common for solutions of aminoguanidine salts, third photo is precipitate of aminoguanidine bicarbonate formed after addition of sodium bicarbonate, fourth photo shows final filtering, last photo is white pure aminoguanidine bicarbonate produced by the process.

1. I managed to determine actual cyanate presence in Urea/CaO melting product, by means of flushing small sample with large amount of water, filtering and precipitation of cyanate by means of lead acetate. Insoluble lead cyanate is filtered, washed and dried. This resulted in 1.38g of lead cyanate from 5g crude cyanate/cyanurate sample. Product of first stage found to contain a lot of water insoluble product thought to be calcium cyanurate. This is in agreement with theoretical view of the article Roscoe posted, "Cyanuric Acid and Cyanurates" Table 4, witch shows that conversion to cyanate require better heating, and solidified product must be mostly cyanurate. Observation on loss of mass during the first stage, shown that reaction product obtained then reaction mixture solidifies has greater mass than assumed by theoretical view:

3CO(NH2)2 => 3HOCN + 3NH3
CaO + 2HOCN => [Ca(OCN)2]
HOCN + H2O => CO2 + NH3

For example fusion of 56g (1 mol) of CaO and 180g of urea (3 mol) should give 1 mol - 124g of cyanate/cyanurate, while experiment gives 132g of solidified product. However if product is finely grounded and reheated for more time on saucepan it found to loose mass further and theoretical mass of 124g may be obtained in such manner.

2. Previous runs of second stage - pyrolysis of cyanate shown that portion of product was colored due to presence of carbon, derived from organic impurities in source cyanate/cyanurate. I was able to completely evade carbonization of final product. I found that carbonization did not occur if final stage is carried out at 700C for 30 minutes. At first i was wondering if pyrolysis will be complete in such short period - however experimental data on weigh loss shown that conversion is essentially complete. Reaction equation:

Ca(OCN)2 (124g, 1 moll) => CaNCN (80g, 1 mol) + CO2

Shows that weight loss in such process must be 35.5% of mass of initial cyanate, experiment for 26g sample of cyanate/cyanurate shown weight loss to 14.2g (11.8g vs 9.24 theoretical). This indicates that conversion is essentially complete and that some cyanate/cyanurate impurities are also decomposed.

3. I've managed to preform quantitative analysis of product produced as described above. 1g of product was added to 150 ml of cold water and stirred for several minute, 2-3 ml of 70% HNO3 is added and mixture is stirred until all solid is dissolved and transparent solution is obtained. After this solution is complemented with 4 ml of 25% ammonia, and solution of 4.5g of silver nitrate in 50 ml of water with stirring. On addition of ~40 ml more of 25% ammonia silver cyanamide is precipitated as pale yellow mircrocrystalline solid. Mixture is stirred for 10 minutes, precipitate is filtered off, washed with water and alcohol and dried. Silver cyanamide is weighted and checked against theoretical of 3,197g for 100% calcium cyanamide. Analysis for my sample shown that is gives 2.85g of silver precipitate, indicating that sample contained 89.15% of pure calcium cyanamide, corresponding to nitrogen content of 31.2% (vs 34% calculated for formula CaNCN)