US2431301A - Preparation of guanidine nitrate - Google Patents

Description

Patented Nov. 18, 1947 PREPARATION OF GUANIDINE NITRATE George F. Wright, Toronto, Ontario, Canada, as-

signor to The Honorary Advisory Council for Scientific and Industrial Research, Ottawa, 011'- tario, Canada, a corporation of Canada No Drawing. Application October 25, 1944.,

Serial No. 560,353

3 Claims. (Cl. 260-564) The present invention relates to the preparation of guanidine salts and more particularly to a new and improved method of preparing guanidine nitrate.

It has been known for many years that guanidine salts may be produced by fusing an anhydrous mixture of appropriate ammonium and cyanamide salts, this preparative method being illustrated by the preparation of guanidine nitrate from calcium cyanamide and ammonium nitrate according to the overall equation:

The process illustrated by the above equation requires excessively high temperatures in order to effect the fusion of the ammonium nitrate, and because of the explosive character of mixtures containing ammonium nitrate and carbonaceous materials, the anhydrous fusion procedure is hazardous to carry out. In 1939, British Patent 507,498 proposed a similar method wherein the fusion of the ammonium nitrate at more moderate temperatures was accomplished by the addition of water to the ammonium salt. A somewhat related method is described in U. S. Patent 2,109,934. By the use of water in accordance with these modified procedures, it was expected that the reaction temperature could be reduced to about IOU-125 C. It was found, however, that the presence of water in the reaction mixture promoted the formation of various reaction byproducts including dicyandiamide, melamine and also certain insoluble and infusible impurities which may have been formed from one or more of the hydrolytic by-products. Of these by-products, dicyandiamide may be converted to guanidine nitrate by treatment with an excess of ammonium nitrate but this transformation is not effected below about 160 C. (Compare Blair and Brahams, Ind. and Eng. Chem, 23, 1124 (1939).) Accordingly, in order to reduce the amount of dicyandiamide in the reaction product, it is necessary to carry out the above wet fusion process at a temperature of about 160 C. or above, under which conditions pressure equipment capable of withstanding 200225 lbs. pressure per square inch is required.

Although the wet fusion procedure described above represents an improvement over the earlier anhydrous fusion technique, in certain respects, the more recent of these procedures is not entirely satisfactory particularly for large scale manufacture. In the first place, under the high temperature conditions required to convert the dicyandiamide by-product to guanidine nitrate,

the reaction mixture has been found to be corrosive to most metals, even including l8-8 stainless steel. In the second place, the process requires the use of pressure equipment and this fact,

coupled with the corrosive character of the reaction mixture, renders the process inherently hazardous. In the third place, even though dicyandiamide may be eliminated from the reaction products by carrying out the reaction at 160 C.,

the high temperature employed tends to increase the amount of insoluble and infusible by-products formed in the presence of water. It is therefore apparent that the wet cyanamide fusion procedure is characterized by several serious disadvantages and limitations.

Broadly speaking, the object of the present invention is to provide an improved process for the production of guanidine nitrate, the improved procedure being so designed as to obviate the disadvantages of the methods heretofore employed.

A more particular object is the provision of a guanidine nitrate process that may be carried out at ordinary atmospheric pressure in thin walled vessels.

A further object is a process of the character described which permits the production of guanidine nitrate under conditions such that the reaction mixture is not corrosive to stainless steel equipment. p,

Still another object is to provide a process capable of producing guanidine nitrate that is free from dicyandiamide, melamine and from insoluble and infusible impurities.

A further object is to provide a process that enables the direct conversion of crude nitrolime to guanidine nitrate in yields of at least or better, calculated on the basis of the cyanamide content of the nitrolime.

Other objects and advantages will be apparent as the invention is hereinafter more fully described.

The foregoing objects may be attained in accordance with the present invention by effecting the fusion of the ammonium salt and the cyanamide salt in the presence of sufficient urea to reduce the melting point of the reaction mixture at all times to below about C. By this procedure, the reaction between the added cyanamide salt and the ammonium salt may be carried out under reasonably anhydrous conditions at a temperature within the range of about 110 to about C.

a The process of the present invention as briefly non-corrosive to stainless steel equipment within.

the relatively moderate temperature'range (110- 135 C.) employed during, the fusion operation. Other advantages of the process will .be apparent as the invention is described more fully.

The process of the present invention is CD11? veniently effected by slowly adding the nitrolime orany otherv convenient source of cyanamide salt to-a fused mixture of ammonium salt and urea, maintained at about 110-135 C., preferably at 120C. Thermo-regulation is necessary during the addition of the'nitrolim'e, inasmuch as the reaction is exothermic. With proper temperature control and with 'slowaddition of the cyanamide salt, the formationio'f by-products, especially dicyandiamide, is greatly minimized, :if not entirely prevented.

After the addition ofith'e 'cyanamide salt to the ammonium salt-urea mixture is 'completedthe. temperature of the reaction mixture is maintained at about 120 (3., preferably until the total reaction time is about two hours. The reaction mixture is then diluted with sufficient water to dissolve and retain in solution any unreacted ammonium salt remaining in the. mixture. Thereafter any limepresen'tin the mixture (when the cyanamide saltconsists of calcium cyanam ide) may be precipitatedin the form of themsoluble carbonate, and then-separated from the hot solution. The hot solution is then cooled'tocrystallize the guanidine salt. If desired, the mother liquors remaining after the separation of the guanidine salt may be evaporated to dryness and the recovered mixture of salts and urea may then be recycled in the processafter fortification with additional quantities of ammonium salt.

As pointed out above, in the process of the present invention, liquefaction of the ammonium salt is accomplished at moderate temperaturesby carrying it out in the presence of sufiicient urea materially to depress the fusion temperature of the mixture. The proportion of urea em-' ployed may obviously be-varied within very wide limits. However, thelower limit is that'necessary to reduce the fusion temperatureof the ammonium salt-urea mixture to-be1ow- 1 20 C; Where the ammonium salt consistsofammonium nitrate, the urea contentof' themixture shouldnot be permitted to fall materiallybelow "about 15% of the weight-of "the ammonium salt'employed. To ensure a reasonable safety factor; it

is preferable to use more than 15% urea, for example 25% or even larger proportions, The maximum ratio of urea to ammonium salt is -deter-' I mined largely by convenience inhandling the reaction mixture which of course becomes pro-' 1 gressively bulky as the-urea contentincreases. Using mixtures containing from about l to about 45% urea, yields approaching th'e theoretical, calculated on the :basis-ofthe calcium cyanamide content'of the crude nitrolime havebeen; obtained. Where larger amounts of-urea-=are employed, the effect of dilution 'of the reaction 4 mixture generally renders it desirable to increase the amount of ammonium nitrate employed per unit weight of nitrolime used, in order to achieve a satisfactory yield.

The ammonium nitrate-urea mixture employed in accordance with the present invention, although not necessarily'completely free'of water, should not contain a large proportion of water inasmuch as the presence of a few per cent of water causes arather marked diminution of yield to about -87% of the theoretical. For this reason it is preferable (but not essential) to employ a substantially anhydrous mixture of ammonium-nitrate and urea, this term being understood to mean not more than 0.5% moisture.

It is significant that dicydiamide is not present iinth'ereaction product, by qualitative tests, if the of the present invention-a specific-example illustrating the production of guanidinenitrate will. hereinafter be described in considerabledetail. It should be clearly understood, however, that this isdone solely by way of example and 'isn'ot intended to restrict the scope of the appended claims.

Example Crude lime nitrogen (4020 63.3%

CaCNz=0.3l7'mole) was added over a twenty minute period .to a fused mixture containing 180 g. (2.25 moles) of 'ammonium nitra'te andjnot less than '30 -g. o'f-urea1 (0:5 mole*) It is important that this urea con'tentnot drop below'15% of theurea-ammonium nitrate mixture during subsequent cycles in order that the 'mixture=melting1 point be maintained below about 120C.

The addition (which wascarried out in. ast'ir'red. vessel of capacity twice that required 'to hold the reaction mixture) was followed by a heating. period such thatthe total reaction time was two hours. The temperature was maintained throughout at about 120 C., "although this was somewhat 'di'fiicult during the addition stage since the initial reaction Thermoregulation was therefore employed in order to maintain the temperature at about At the end r this timethe reaction was diluted. with about 100 'cc. Watenthecalciumprecipitated with carbon dioxide and ammonium carbonate (42 g, -0.35-mo1e') and-the mixture-was'fi'ltered at about C. to remove the carbonaceous sludge. This precipitate was washed w'ithahoutllo '00. water, the wash waterbeing usedas-diluent fora successive run. Thehotsolution was cooled :to'

about 25 C. to crystallize the guan'idine" nitrate, which was removedand" dried.

The final filtrate was analyzed for ammonium nitrate, and then evaporated by a-single' pass through-astaiiiljess steel 'columnjacketed'at about 142 C., using boilingdsoa-myl acetate for this p n pose,- the internal-"pressure being-maintained'at about 1 0 mm. The ammonium nitrate-ureais slightly "endothermic;

guanidine nitrate solution was collected in a receiver (which served as the reaction vessel for a successive run) containin the fortifying ammonium nitrate, and maintained at about 120 C. For simplification purposes, the solution was not allowed to solidify before the subsequent run.

The average overall yield of guanidine nitrate for an entire series of runs was 92% of the theoretical. There was no tendency for this to decline, the final runs showing a slightly higher percentage conversion. The product showed an average purity of 90%, the remainder bein largely ammonium nitrate. The thiourea formed was destroyed during the evaporation of the recycled residue at 140 C, at such a rate that the maximum amount (11 g. in 200 cc. filtrate) did not reach saturation value, and only small amounts, not more than 1.5%, appeared in the product. The thiourea may be destroyed on standing in the ammoniacal solution of the reaction at 100 C., which provides a convenient method for its removal, should this be found desirable. Dicyandiamide was not detected in the product, nor was there any trace of insoluble matter present.

It will be apparent to those skilled in the art that many variations in proportions, reactants and conditions may be made in the procedure described in the foregoing example. Thus other ammonium salts may be substituted for the nitrate employed in the illustrative example, where it is desired to produce other guanidine salts. It will also be apparent that cyanamide salts other than the calcium salt may be used if desired, the procedure being modified where necessary or desirable to take care of such changes. All such variations and modifications are to be understood as included within the scope of the appended claims.

I claim:

1. In the production of guanidine nitrate by the fusion of ammonium nitrate with a cyanamide salt, the improvement which comprises effecting the fusion under substantially anhydrous conditions in the presence of urea in an amount substantially 15 to 45% by weight of the ammonium nitrate to provide a reaction mixture which is liquid at a temperature below 120 C.

2. A method of producing substantially pure guanidine nitrate which comprises mixing ammonium nitrate with at least substantially 15% by weight of urea to form a mixture which becomes liquid at a temperature not exceeding 120 0., heating the mixture to a temperature between substantially 110 and 135 C., adding calcium cyanamide to said mixture while maintaining the temperature within said limits to form guanidine nitrate, adding water to form an aqueous solution, converting lime in the solution to an insoluble carbonate, removing the insoluble carbonate and cooling the solution to a temperature between 0 and 30 C. to precipitate substantially pure guanidine nitrate.

3. A method of producing guanidine nitrate which comprises providing an anhydrous mixture comprising from about to about ammonium nitrate and from about 15% to about 45% urea, fusing said mixture by heating the same to a temperature between about C. and about 140 (3., slowly adding calcium cyanamide to said fused mixture while maintaining the temperature of the resulting reaction mixture at about 115 C. to about C., continuing the heating of said reaction mixture after the addition of said calcium cyanamide for a total heating period not exceeding about two hours, diluting the resulting crude product with water in a sufficient amount to dissolve and to retain in solution during subsequent purification a substantial portion of any unreacted ammonium nitrate present in the crude material, treating the resulting solution with ammonium carbonate in an amount sufficient to precipitate substantially all of the calcium contained therein in the form of the carbonate, separating the calcium carbonate from the hot solution, cooling the hot solution to about 0 C, to about 30 C. to crystallize guanidine nitrate therefrom and separating the crystallized guanidine nitrate from the cooled supernatant liquid,

GEORGE F. WRIGHT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Blair et a1 Jan, 9, 1923 Aldred Apr, 19, 1938 FOREIGN PATENTS Number Number