Cyanamid, manufacture, chemistry and uses, by Edward J. Pranke ...

Preface

This volume is intended to be a review of the present knowledge of Cyanamid, particularly its chemical and agricultural properties. Its purpose is to render some assistance to the investigator who has neither the time nor the library facilities to enable him to make a thorough study, yet who wishes to broaden his knowledge of Cyanamid. Most of the important literature on this subject is written in foreign languages, and many valuable papers occur in journals not found in the ordinary agricultural or chemical library. Moreover, the opinions that have been expressed on almost every phase of the behavior of Cyanamid are so diversified and frequently so deeply buried in controversy that the casual reader is at a loss to know what to accept as generally established facts. It is hoped that the present volume will give to the reader a consistent explanation of Cyanamid that will form the starting point for the acquisition of further knowledge.

In order to arrive at an understanding of the principles underlying particular phenomena it is necessary to adopt at the beginning of an investigation some sort of working hypothesis that will account for the observed facts. Every further fact that is acquired must then verify the original hypothesis or the latter must be modified to fit the facts. The constant remodeling of ideas to agree with observed facts finally leads to a system of knowledge, in which every fact explains to a certain extent every other fact, and in no case contradicts any of them. Such a system of knowledge of Cyanamid, it is believed, is now at hand. The pure chemistry of Cyanamid, its physico-chemical action in the soil, its biological behavior, and its agricultural properties, as presented in this volume, are consistent with each other. Such consistency is believed to induce confidence in the validity of the views expressed. Further experiments may make necessary some slight changes, but the general scheme of the properties of Cyanamid may now be considered as quite definitely established.

iv PREFACE

There is no question but that Cyanamid will play an important part in the future development of agriculture, and that a great deal of research will be undertaken to broaden the knowledge of its practical application. Much labor has been wasted in the past by the pursuance of faulty methods, and a great deal of work has been but the duplication of earlier efforts, and has contributed little that was not known before. If the publishing of this book will direct research into the fields that still remain more or less unexplored, and if it is helpful in avoiding the errors of past investigations, its purpose will have been accomplished.

Nashville, Tenn.

January, 1913.

Table Of Contents

Preface lu

CHAPTER I. Discovery and Manufacture of Cyanamid-. i History of Technical Process I

Nomenclature of Cyanamid Industry 3

Manufacture of Commercial Cyanamid 4

Preparation for Use as a Fertilizer 7

Commercial Derivatives 8

CHAPTER II. Preparation and Properties of Cyanamide io Preparation I o

Properties T i

Action of Heat 1 1

Action of Acids 12

Action of Alkalies 12

Action of Oxidizing and Reducing Agents 13

Other Reactions 13

Metal Salts 13

Dimetal Salts 13

Calcium Cyanamide 14

Acid Calcium Cyanamide 14

Basic Calcium Cyanamide 16

Calcium Cyanamide Carbonate 16

Silver Cyanamide 17

Dicyandiamidine 18

CHAPTER HI. Analytical Methods 19

Determination of Total Nitrogen 19

Determination of Cyanamide and Dicyandiamide 20

Caro Method 20

Brioux's Modified Caro Method 21

Determination of Urea 22

Identification of Amidodicyanic Acid 23

Identification of Ammeline 23

CHAPTER IV. Storage of Cyanamid 24 Factory Test on Large Scale 24

Test of Two Bags 25

Chemical Changes in Storage 28

Relative Amounts of Decomposition Products 29

CHAPTER V. Decomposition of Cyanamid in the Soie 32 Factors Involved 32

Experiments of Ulpiani 32

Experiments of Kappen 34

First Stage of Decomposition 37

Second and Third Stages of Decomposition 38

Influence of Concentration 40

Influence of Temperature 43

Influence of Soil at ioo°C 43 vi TABLE OF CONTENTS

PAGE

Nature of Products formed in Soil at Ordinary Temperatures- 44

Effect of Changing Ratio of Liquid to Soil 46

Influence of Aeration 47

Influence of Electrolytes 48

Nature of Effective Soil Constituents 48

Effect of Zeolites 49

Effect of Carbon 50

Experiments with Natural Colloids 51

Experiment with Sterilized Soil 5 b

Conclusions 57

CHAPTER I. Discovery and Manufacture of Cyanamid

The problem of the artificial fixation of atmospheric nitrogen has engaged the attention of scientists for the greater part of a century. The rapid growth of the fertilizer industry that has attended the development of agricultural science, and the great increase in the number and extent of chemical industries, during the past fifty years, have emphasized the necessity for artificial methods of maintaining and increasing the world's stock of combined nitrogen. One of the influences that stimulated immediate action was the introduction in 1887 by MacArthur and Forest, and at about the same time independently by Siemens & Halske, of Berlin, of the cyanide process for leaching gold and silver from their ores. This discovery produced a strong demand for cyanides, which had hitherto been used to the extent of only a few hundred tons a year, principally in the dye-industry and to a smaller extent in electroplating.

Attempts had been made early in the nineteenth century to living about the direct synthesis of cyanogen from atmospheric nitrogen and carbon. Among other processes, that worked out in 1847 by Bunsen and Playfair, in which barium carbonate was heated in an atmosphere of pure nitrogen, seemed promising, but did not prove to be commercially successful. The introduction of the electric furnace in 1894 by Moissan and by Willson, for the production of carbides on a large scale, afforded a new instrument for further research. Siemens and Halske, among others, at once adopted the use of the electric furnace for the working out of the problem of nitrogen fixation. In 1895, they worked on the process of Prof. H. Mehner, which consisted in fusing a mixture of sodium carbonate and carbon and conducting nitrogen through the hot mass. In the same year they took up the process of Prof. Adolph Frank and Dr. Nicodem Caro, which consisted in subjecting a mixture of barium carbide, sodium hydroxide, potassium hydroxide and carbon at a high temperature to the action of steam and nitrogen. Frank and Caro, with the co-operation of F. Rothe, found in 1895 that dry nitrogen is essential to successful absorption.

In 189S it was found that when barium carbide is heated to a temperature of 700 ° to 8oo° C, in the presence of nitrogen, about 30 per cent, of the carbide is changed into barium cyanide and the remainder into barium cyanamide. The reactions can be represented by the following simple equations :

BaC 2 + N 2 = Ba (CN)„ Ba(CN) 2 = BaCN 2 + C.

Since it was desired to have all the nitrogen in the form of cyanide, further operations were necessary. The product of the above reactions was fused with soda, when the carbon again reacted with the cyanamide group and produced the cyanide form. The cyanide was leached out with water, and treated with ferrous carbonate to form the ferrocyanide, which was sold as such or fused with sodium to form pure sodium cyanide. The barium carbonate residue was again used to produce barium carbide, as represented by the reactions : BaC0 3 + heat = BaO + C0 2 , BaO + 3C = BaC 2 + CO.

The fall in the price of cyanides due to the interruption in the production of gold during the Boer War in South Africa made it necessary to seek cheaper methods of manufacture. It was found that calcium carbide could be manufactured at less cost, and also had the advantage of possessing a lower molecular weight. This carbide required a temperature of from i,ioo° to 1,200° C. for the absorption of the nitrogen, but combined it entirely in the form of calcium cyanamide, without the formation of any cyanide. By fusion with alkaline salts, however, the cyanamide form, in the presence of carbon, readily goes over to the cyanide form, which can be leached out with water, if desired, and be further purified. When sodium chloride is used as the fluxing agent, the resultant mass contains about 30 per cent, sodium cyanide, and is known as a "surrogate." It is suitable for use directly for the extraction of gold ores.

Agricultural experiments with the crude calcium cyanamide showed that this material is suitable for use as a nitrogenous fertilizer, and patents were issued in 1910 to Dr. Albert R. Frank, son of Prof. Adolph Frank, and to Herman Freudenberg, a co-worker of A. R. Frank, protecting the use of Cyanamid for this purpose. The basic patent protecting the process of manufacture of Cyanamid was issued to Prof. Adolph Frank and Dr. Nicodem Caro in 1908.

The large demands of agriculture for cheap nitrogenous fertilizer materials have directed the efforts of the manufacturers toward the production of Cyanamid rather than of cyanides and other derivatives. At present, the total output of sodium cyanide derived from Cyanamid is only about 2,000 tons per annum, all made in Germany, while the world's production of Cyanamid is estimated at about 120,000 tons per annum. The factory of the American Cyanamid Company, at Niagara Falls, Canada, now has a capacity of 30,000 tons per annum, and extensions now under way will increase this to 60,000 tons per annum. There are thirteen Cyanamid factories abroad, located in Germany, Italy, France, Switzerland, Austria, Norway, Sweden and Japan.

NOMENCLATURE OF CYANAMID INDUSTRY.

With the development of the Cyanamid industry there has grown up a nomenclature that is often confusing to the uninitiated. The terms here defined will be understood to have the following meanings throughout this treatise :

Lime-nitrogen. — Crude calcium cyanamide, ground to a fine powder after removal from the ovens in which it is formed. It contains about 55 per cent, of calcium cyanamide, CN.NCa, about 2 per cent, calcium carbide, and about 20 per cent, of free calcium oxide.

Cyanamid. — This is a trade name for the completely hydrated material prepared for use as a fertilizer in the United States. It contains about 45 per cent, calcium cyanamide, 27 per cent, calcium hydroxide and no carbide. The name is always capitalized and has no final "e."

Cyanamide. — The compound represented by the formula CN.NHo. It is sometimes referred to as acid cyanamide, or free cyanamide.

Calcium Cyanamide. — The chemical compound of the formula CN.NCa, or CaCN 2 , as it is freqently written.

Calcium Cyanamid. — The name used by the United States Department of Agriculture and by some State Departments of Agriculture to designate commercial Cyanamid. It is sometimes used to indicate the substance represented by the formula CN.NCa, but for the sake of clearness the compound CN.NCa will be called calcium cyanamide in the present paper.

Nitrolim. — The trade name for the material sold in England for agricultural purposes. It is a lime-nitrogen to which has been added just enough water to destroy the carbide. Practically all the free lime is present as calcium oxide.

Kalkstickstoff. — The commercial material manufactured in Germany for use as a fertilizer. It is similar to nitrolim.

Stickstoffkalk. — A crude calcium cyanamide made by nitrifying a calcium carbide which contains about 10 per cent, of calcium chloride. Its manufacture in Westeregeln, Germany, under the Polzeniusz patents was discontinued in 1910.

Calciocianamide. — The Italian commercial product, completely hydrated.

Cyanamide de calcium. — The French commercial product, completely hydrated.

MANUFACTURE OF COMMERCIAL CYANAMID.

The first step in the manufacture of Commercial Cyanamid is the preparation of calcium carbide. This is brought about in the usual manner by fusing in an electric furnace a mixture of lime and coke in accordance with the following equation:

CaO + 3C — CaC 2 + CO.

The carbide is removed from the furnace at regular intervals, is cooled, crushed to a fine powder, and packed in the nitrifying ovens. These are cylindrical, perforated steel cans, set in heat-insulated brick ovens. A carbon pencil through the axis of the can is used to heat the carbide to the combining temperature. On admission of the nitrogen to the cans the following reaction takes place:

CaC, + N 2 — CaCN 2 + C.

This reaction is accompanied by an evolution of heat which is just about sufficient to maintain the mass at the combining temperature. The commercial calcium carbide used contains about 20 per cent, of impurities, which so influence its physical and chemical properties that the absorption of nitrogen takes place very readily at atmospheric pressure at a temperature of about i,ioo° C. The addition of catalytic agents, principally haloids, suggested by various investigators, is not necessary for the fixation of nitrogen, since the manufacturer can easily regulate the reactions by suitable disintegration of materials and by other mechanical means.

Nitrogen is obtained either by fractional distillation of liquid air, or by means of the copper oxide process. In the latter, air is passed through a red-hot mass of finely divided copper, suspended in asbestos or other inert material. The copper combines with the oxygen and allows the nitrogen to pass through. The copper oxide is easily recovered for use by reduction in situ with a suitable gas, such as natural gas.

The nitrogen used must be pure and dry, otherwise, at high temperatures, there is destruction of the carbon pencils, and of calcium carbide, according to the following reactions :

Carbon dioxide also destroys the calcium cyanamide with formation of calcium oxide, carbon monoxide and free nitrogen.

The reaction by which calcium cyanamide is formed is reversible:

CaC 2 + N 2 = CaCN, + C.

The temperature of reversal at atmospheric pressure varies greatly with the composition of the carbide used. Thus the temperature of reversal lies at about 1,360° C., 1 for a crude calcium cyanamide containing 21.1 per cent, combined nitrogen, and made from a commercial carbide of the following composition :

Per cent.

CaSi 0.06

Ferrosilicon 0.72

Not determined 0.80

An increase of the free lime in the carbide greatly lowers the critical temperature. Thus with a carbide containing 75 per cent. CaC 2 the equilibrium point lies at about 1150° C. 2

The effect of nitrogen pressure on the equilibrium point has been investigated by M. Thompson, who found that the temperature at equilibrium varies directly as the pressure. 3 Since calcium cyanamide is decidedly volatile at the equilibrium temperature, even as low as 1,050° C, and distils to the colder parts of the apparatus the determination of the equilibrium conditions is open to some errors, but these may not be large enough to vitiate the general conclusions that have been drawn.

It is owing to the reversibility of the reaction that nitrogen

1 Caro, Chem. Trade Jour., 1909, p. 622.

cannot be absorbed by liquid carbides as the latter leaves the furnace, since calcium cyanamide cannot exist at the temperature of liquid carbide. As the carbide cools it becomes practically impermeable to gases and absorption takes place only on the surface to a slight depth.

Processes for the nitrifying of a heated mass of lime and coke have not been commercially successful.

The energy consumption for the fixation of one ton of nitrogen as calcium cyanamide is about three horse power years, including the manufacture of the carbide and all subsequent factory operations.

PREPARATION FOR USE AS FERTILIZER.

Cyanamid finds its principal use in agriculture, as a source of nitrogenous plant food, and for this reason practically all the crude calcium cyanamide is converted into a form more suitable for its incorporation in complete fertilizers. To this end, water is added to the crude material in a rotating cylinder; the one or two per cent, of calcium carbide is decomposed and the lime slaked. This powdered Cyanamid is converted to granulated Cyanamid as follows : A small amount of water is mixed with it, and the damp material is run through brick presses. The resulting bricks harden rapidly, and are stored until the material is to be shipped, when they are run through a series of crushing rolls and screens. The coarse material, which passes through a 15-mesh standard screen and over a 60-mesh standard screen, is practically free from dust, and is known commercially as Granulated Cyanamid. The fine material, mostly smaller than 60-mesh, is either incorporated with fresh powdered Cyanamid and again run through the brick presses, or it is mixed with several per cent of an odorless oil to reduce the dustiness, and is sold without further treatment. Both grades of Cyanamid are packed in ordinary fertilizer bags, and are distributed in carload lots to manufacturers of mixed fertilizers. Material so prepared contains nitrogen equivalent to 18 to 20 per cent, of ammonia, and is 2 b CYANAMID — MANUFACTURE, CHEMISTRY AND USES

sold on the basis of its nitrogen content, as determined by analysis.

The following is a typical analysis of commercial Cyanamid.

Per cer.t.

Calcium cyanamide CaCN 2 45-92

Calcium carbonate CaCO s 4.04

Calcium sulphide CaS 1 .73

Calcium phosphide Ca 3 P 2 0.04

Calcium oxide, free CaO —

Calcium carbide CaC 2

Calcium hydroxide Ca(OH) 2 26.60

Free carbon C 13. 14

Iron and alumina R 2 3 1.98

Silica Si0 2 1.62

Magnesia MgO o. 15

Combined moisture — 3.12

Free moisture H,0 0.35

Undetermined — 1.31

100.00

COMMERCIAL DERIVATIVES.

Ammonia. — Steam, at a high temperature and pressure, converts calcium cyanamide quantitatively into calcium hydroxide and ammonia, thus forming a convenient source of ammonia for the manufacture of ammonium salts. The carbon, which is in the form of graphite, and the lime, can be used over again for the production of carbide.

Nitric Acid. — By the Ostwald process, ammonia can be oxidized to nitric acid, mixtures of thoria and ceria being used as catalyzers. No external supply of energy is required in this process.

Cyanides. — When calcium cyanamide and carbon are fused together with alkaline salts, in the absence of carbide the calcium cyanamide is converted into calcium cyanide:

CaCN 2 + C — Ca(CN) 2 .

The product of this reaction is called a "surrogate." It is used in the recovery of metals by the cyanide process.

The above reaction is completely reversed in the presence of carbides, hence their absence is imperative in this process.

Dicyandiamide. — This derivative is easily prepared by leaching the crude calcium cyanamide mass with hot water, precipitating the lime in the filtrate with carbon dioxide, and concentrating the filtrate. Dicyandiamide is used in the dye industry, and also as a deterrent in nitro-explosives, in place of ammonium oxalate.

Other derivatives, such as urea, guanidine, nitro-guanidine, are being made at the Spandau works, in Germany. A process has also been worked out for the production of synthetic indigo by the action of dialkylcyanamides on Phenylgycine and its derivatives.

Ferrodur and intensit are special mixtures prepared for metallurgical purposes. Ferrodur is a cementing powder used in place of potassium cyanide for hardening iron and steel in ovens. Intensit is a hardening powder for hardening iron and steel in open fires ; it is used in place of potassium ferrocyanide. There are other powders of a similar nature with special names differing only in the proportion of active ingredients that they contain. These products are of considerable importance to metallurgy, since they are cheap, yet efficient for the purposes for which they are sold.

CHAPTER II. Preparation and Properties of Cyanamide

PREPARATION.

Free cyanamide, CN.NH 2 , was first obtained by Bineau, in 1838, by the action of ammonia on chlorcyan, but it was not isolated by him from the ammonium chloride with which it was formed. The Italian chemists Cloez and Cannizzaro, 1 in 185 1, effected the separation, and gave the first description of the compound.

Their method consists in passing chlorcyan into a solution of ammonia in absolute ether, filtering off the crystalline ammonium chloride and evaporating the solution in vacuo below 40 . The reaction takes place according to the following equation :

2NH, + CNC1 — * CNC1 2 NH 4 + NH.

It can also be prepared by the action of freshly precipitated mercuric oxide on thio-urea, in the presence of a little ammonium thiocyanate, which dissolves some of the mercuric oxide as the double thiocyanate, and so renders it more active : NH 2 NH 2

NH 2 N

It is most conveniently prepared from either commercial sodium cyanamide or commercial calcium cyanamide.

From Commercial Sodium Cyanamide. 2 — Twenty-five grams of the salt are gradually added to 37 grams of hydrochloric acid (sp. gr. 1.19) with strong cooling, and the water is removed by distillation in vacuo below 40 C. The residue solidifies on cooling; it is extracted with ether, the ether distilled off from the solution, and the cyanamide caused to

2 Caro, Schiick, Jacoby, Zeit Angew Chem. 1910, XXIII, 2405, 2417.

CYANAMID — MANUFACTURE, CHEMISTRY AND USES II

crystallize by cooling. It is purified by recrystallization from ether. Yield about 5 grams.

From Commercial Calcium Cyanamide. — Fresh commercial Cyanamid or better, the unhydrated lime-nitrogen, is extracted with cold water (solubility about 0.9 grams nitrogen in 100 cc. water). The calcium is removed either with oxalic acid or aluminium sulphate, but preferably with the latter. After removal of the calcium sulphate and alumina by filtration, the filtrate is evaporated in vacuo below 40 , and the residue extracted with ether. It can be purified by recrystallization from ether.

PROPERTIES OF CYANAMIDE.

Cyanamide, 1 CN.NH,, most probably has the formula

,NH, C <^ , although in a very few reactions it seems to act as

if it were carbodiimide, Cx" . It is a colorless, crystal-

line solid, which melts at 41-42 C, as usually prepared. It can be undercooled to 12 without solidifying. On stirring with a sharp-pointed glass rod the undercooled liquid freezes. The carefully purified substance melts sharply at 46 C. 2 It is easily soluble in water, alcohol and ether, and is volatile in steam. It is slightly soluble in carbon disulphide, chloroform and benzol.

Action of Heat. — Pure cyanamide is perfectly stable at ordinary temperatures, but polymerizes slowly on heating above its melting point. Impure cyanamide polymerizes slowly at ordinary temperatures. The principal polymer is dicyandia-

/NH. mide, NH: C<( >CN, or (H 2 CN 2 ) 2 , which is probably

X NH X

cyan-guanidine. By strong heating, other derivatives are

1 Sidgwick, Organic Chemistry of Nitrogen, p. 216, (Oxford, 1910). - G. Henschel, Diss. Univ. of Leipzig, 1912.

formed, the most important of which are, the polymer

N

//\ Tricyantriamide or Melamine H,N — C C — NH.„ and Me-

I II

N N

C

NH 3 lam, C 6 H,N n , and Mellon, C 6 H 3 N a . Ammonia is evolved during the formation of these bodies. By the action of superheated steam the conversion of cyanamide to ammonia is almost quantitative.

Action of Acids. 1 — Cyanamide reacts readily with acids ; with nitric acid forming urea nitrate (95 per cent, conversion) ; with sulphuric acid and phosphoric acid giving mostly urea, (about 95 per cent, conversion) together with some ammeline, C 3 N 3 (NH 2 ) 2 OH; ammelide, C 3 N 3 (NH 2 ) (OH) 2 ; possibly cyanuric acid, C 3 N n (OH) 3 , and some ammonia.

Cyanamide combines directly with the haloid acids. It combines slowly with free H 2 S, readily with yellow ammonium sulphide, with formation of thio-urea. Thio-urea is also formed by the action of thioacetic acid on cyanamide in alcoholic solution. Acetic acid produces principally ammonium acetate (about 80 per cent, conversion) and some urea.

Action of Alkalies. 2 — The strong alkalies KOH or NaOH in aqueous solutions produce almost entirely urea, with no trace of dicyandiamide ; weak alkalies, NH 4 OH or MgO, produce dicyandiamide almost exclusively at first, and then ammonia. CaO, however, produces a mixture of urea, dicyan-

diamide, ammeline, amidodicyanic acid I O : Q.C ) ,.

v X NH - CN 7

ammonia and other bodies.

1 Ulpiani, Gas Chim., Ital. II, No. 4, 358-417.

Hence, with strong acids and strong bases, cyanamide in aqueous solutions forms principally urea ; with weak acids principally ammonium salts; with weak bases dicyandiamide, which decomposes further to ammonia; with lime, a mixture of urea, dicyandiamide and other derivatives.

Action of Oxidizing and Reducing Agents. — In the chapter on availability it will be shown that oxidizing agents convert the nitrogen of cyanamide or its derivatives into forms more insoluble in water and less easily decomposed by strong alkalies.

By the action of zinc and hydrochloric acid, cyanamide yields ammonia and methylamine :

On heating with potassium nitrite solution a violent reaction takes place, and C0 2 , N 2 and dicyandiamide are produced : 4 CN.NH 2 + 4 KN0 2 — * 2K 2 C0 3 + 4 N 2 + (CN.NH 2 ) 2 + 2 H 2 0.

Other Reactions. 1 — In cyanamide, either one or both of the hydrogen atoms can be displaced by metals, alkyl or aryl groups, or by alcohol or acid radicals. It combines with amino-acids, especially in the presence of ammonia. It combines with ammonium chloride at high temperatures, forming guanidine hydrochloride. Heated with ammonium sulphide it yields guanidine hydrosulphide. It combines directly with cyanogen to form a yellow, amorphous powder. With potassium cyanate it forms potassium amidodicyanate, K.C 2 H 2 N 3 0. It combines directly with chloral, and also with aldehydes, but with the separation of water.

Metal Salts. — The dimetal salts of the alkali metals can be prepared only in the dry way, since in aqueous solution they lose one of the metal ions by hydrolysis. Thus, Na 2 CN 2 in aqueous solution yields NaHCN 2 :

Na 2 CN 2 + H 2 = NaHCN 2 + NaOH 1 Beilstein, Handbuch der Org. Chem.

Na 2 CN 2 on fusion with carbon yields sodium cyanide: Na 2 CN, + C <~ 2NaCN.

,NCa Calcium Cyanamide, CaCN 3 or C^ , can be made by the

fusion of calcium cyanate: 1

Ca (CNO) 2 — CaCN 2 + C0 2

or by fusion of cyanamide or its polymers with calcium oxide. Calcium cyanamide forms colorless crystals which sublime at about 1,090° C. at atmospheric pressure. It is insoluble in alcohol, but easily soluble in water (about 2.5 g. in 100 cc. water at 25° C). Upon solution of the calcium cyanamide in water it is directly hydrolyzed into the acid calcium cyanamide and calcium hydroxide.

That such hydrolysis takes place as indicated by the equation is shown by the relative amounts of lime and nitrogen existing in solutions of calcium cyanamide. C. Ulpiani 1 investigated the relation of lime to nitrogen in a solution of calcium cyanamide kept at a constant temperature for several weeks. At intervals of several days determinations were made of total nitrogen, nitrogen in the form of cyanamide, and calcium in solution. It was noted that crystals of pure calcium hydroxide, as determined by analysis, were deposited on the walls of the vessel after a day or two. The quantities of lime and nitrogen found in the solution are shown in Fig. 1.

Since the solubility of calcium cyanamide is much greater than that of calcium hydroxide, a concentrated solution of calcium cyanamide is, after hydrolysis, saturated with respect to calcium hydroxide. In addition, there is present lime as a calcium compound of cyanamide. If this compound is calcium acid cyanamide, Ca(CN.NH)._,, there will be in solution one atom of calcium to four of nitrogen, or 56 parts by weight

1 Beilstein loc cit.

2 Rend. Soc. Chim. di Roma, n. 4 (1906).

of CaO to 56 of N, or equal weights of each. By reference to the curves in Fig. i it is seen that if the ordinate representing the amount of CaO present as Ca(OH) 2 is subtracted from the ordinate of total CaO, the ordinate of CaO combined in other forms (with cyanamide) would coincide with the ordinate of nitrogen present as cyanamide; that is, the amounts of CaO and N present are in the relation demanded by the formula Ca(CN.NH) 2 .

On long standing of the solution, the acid salt Ca(CN.NH) 2 decomposes, forming principally urea, some dicyandiamide,

«5

D-Total^

OaO- Total

H- aa t*

N \ N \

N \ a

CaO- as

~~ ----__

«a c

Days

Fig. I.— Variation of nitrogen and calcium in a solution of lime-nitrogen.

and small quantities of melamine, amidodicyanic acid and ammonia. The dicyandiamide diminishes slowly, and finally probably disappears entirely. This is shown in the following analyses by G. Liberi 1 of a solution made by extracting lime-nitrogen containing 18.63 P er cent, cyanamide nitrogen, with twenty times its weight of cold water. The nitrogen figures are given as a percentage of the dry lime-nitrogen.

1 Ann. R. Staz. Chim. Agrar. Sper di Roma., 191 1, Vol. V, Series II.

l6 CYANAMID — MANUFACTURE, CHEMISTRY AND USES

Nitrogen in solution

As cyanamide As dicyanamide

After Per cent. Per cent.

Basic calcium cyanamide is formed in solutions containing an excess of lime:

N N

C v + Ca(OH) 2 — * C v /CaOH

X NCa X N<

^CaOH

It can be obtained from lime-nitrogen by extracting with a small portion of water, filtering, and allowing the solution to stand several hours. Long, needle-shaped white to transparent crystals separate out on the walls of the vessel. Filter with suction in the absence of carbon dioxide (under a bell-jar). Dry under a bell-jar over caustic potash.

This salt is almost insoluble in water. In the dry condition it is stable at ordinary temperatures, but when heated to 120 C. it rapidly decomposes to dicyandiamide and calcium hydroxide.

Calcium cyanamide carbonate 1 is readily formed by the action of carbon dioxide on calcium cyanamide in the presence of moisture. It can be prepared by extracting lime-nitrogen with one and one-half times its weight of water, filtering and bubbling CO, through the filtrate. In about half an hour a white precipitate forms, which can be filtered and washed with alcohol or ether. 1 Ulpiani, loc cit.