Patented Nov. 10, 1953 2,658,891 UNITED STATES PATENT OFFICE‘ 2,658,891 PREPARATION OF MELAMIN E AND / 0R GUANIDINE Johnstone S. Mackay, Old Greenwich, Conn, as signor to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Application February 6, 1950, Serial No. 142,705 15 Claims. (Cl. 260—-249.7) 2 The present invention relates to the prepara is that yields of melamine exceeding 100 percent, tion of melamine and/or a guanidine salt by based on the nitrogen available in the calcium reacting calcium cyanamide, ammonia, and car cyanamide used, are obtainable. This indicates bon dioxide under heat and pressure. that the added ammonia and carbon dioxide More particularly, the invention relates to the somehow contribute’ to the yield. However, the preparation of melamine and/or a guanidine salt exact chemical mechanisms have not yet been by heating lime nitrogen, ammonia, and carbon de?nitely established and the applicant does not dioxide at a pressure of at least 450 p. s. i. and at wish to be bound by any theory or supposed a temperature of at least 65° C. to produce a reac course of reaction. _ , tion mass containing a guanid'ine salt when the 10 The invention may be described in general as reaction temperature is within the approximate follows. Calcium cyanamide is placed in an range 65°-250° 0., or melamine when thereaction autoclave and ammonia and carbon dioxide are temperature is at least 125° C. forced in under pressure. The autoclave isthen It is an object of the present invention to ob heated to 300°-¢l00° C., cooled, opened, and the tain melamine from calcium cyanamide directly 15 melamine is recovered. It is not required to add without going through an intermediate step of ammonia and carbon dioxide as gases; ammo ?rst isolating cyanamide or dicyandiamide. A nium oarbamate, the primary reaction product‘ further object of this invention is to prepare of dry carbon dioxide and dry ammonia, may be melamine directly from lime nitrogen. A still added instead of or in addition to either or both further object is to prepare melamine simultane of the gases, since under the reaction conditions ously from mixtures of lime nitrogen, ammonia, carbon dioxide and ammonia, either as such or and carbon dioxide. chemically combined as ammonium carbamate, It is also an object of this invention to prepare are equally effective. Consequently, when car guanidine and its salts more cheaply than form bon dioxide and ammonia are referred to herein erly possible and without danger to the operator. after, it is to be understod that they may be pres It is a further object to make a product rich in ent as ammonium carbamate. guanidine salts by a novel gaseous reaction of The general features of the invention may be ammonia and carbon dioxide with calcium cyan illustrated by the following examples. amide or lime nitrogen, and to recover relatively pure guanidine salts, such as the carbonate and 30 Example 1 the like, from said product. 50 grams of calcium cyanamide (86% pure) Additional objects will become apparent from were reacted with 78 grams of ammonium car the discussion hereinafter. bamate and 20 grams of ammonia in a 300 cc. For convenience of discussion the procedure autoclave for 1 hr. at 375° C. The autoclave was directed mainly to the recovery of melamine will 35 cooled, opened, and the reaction product was re be discussed in a separate section herein, fol moved and leached with boiling water. The lowed by a discussion of the procedure directed slurry was ?ltered and melamine was crystallized mainly to the recovery of a guanidine salt. from the ?ltrate. Yield, 21.8 grams. (SECTION I), THE‘ MELAMINE PROCESS As the purer forms of calcium cyanamide are expensive, the applicant prefers to use the rela Although melamine has been known for many tively impure but cheap form known in commerce years, the commercially desirable methods for its as lime nitrogen‘. Lime nitrogen commonly ana production have utilized cyanamide or dicyandi lyzes from 60-65% calcium cyanamide, the re amide as starting materials. These latter com mainder being lime, graphite, and other impuri pounds yield melamine in fair to good yields by 45 ties. ' polymerization upon heating, either alone or with Widely varying ratios of reactants may be used several di?erent types and kinds of solvents, dilu in making melamine by the process of this inven ents, catalysts, etc. A few other organic nitro tion. Small quantities of ammonia and carbon gen compounds, for example, guanidine salts, dioxide with relatively large quantities of lime have been said to yield small percentages of mel 50 nitrogen result in small yields of melamine, and amine along with various other compounds when melamine is similarly obtained when the ammo heated at temperatures up to about 200° C. niazcarbon dioxide ratio is less than their ratio Prior to the present invention, so far as the in ammonium carbamate (2:1). For best re applicant is aware, no method was known for sults, however, the preferred proportions should obtaining melamine directly from calcium cyan 55 be calculated as follows. The initial carbon amide. By the method of the present invention dioxidezcalcium cyanamide ratio is’ preferably melamine can be produced more cheaply than by 1.5:1. If lime nitrogen is used instead of a purer any other known process. > calcium cyanamide, additional carbon dioxide is‘ A remarkable feature of the present invention added (in a carbon dioxidezcalcium oxide mole 2,658,891 4 ratio of 1:1) to neutralize the free calcium oxide were autogeneously produced. However, the re present in the lime nitrogen, whereby the total action is not restricted to conditions of autogen carbon dioxide:calcium ratio is reduced to about ous pressure. For example, melamine will be 1.4:1. This ratio will vary with the calcium obtained if an autoclave containing lime nitrogen oxide content of different lots of lime nitrogen, ca is heated above about 125° C. and gaseous am and may readily be determined by those skilled monia and carbon dioxide similarly heated are in the art. The lime nitrogen used in making forced into the autoclave at a pressure exceeding the experiments in developing the present inven about 450 Ila/sq. in. tion analyzed approximately 30.5% calcium pres Although some melamine is obtained by react ent as calcium cyanamide and 13.0% calcium ing lime nitrogen, ammonia, and carbon dioxide present as calcium oxide, a total of 43.5% cal at the relatively low temperature of 125° 0., the cium. The e?ect of the carbon dioxidezcalcium best yields are obtained at temperatures in the ratio on the yield of melamine is demonstrated range of 350°~400° C. The preferred tempera in the following tabulation. ture is approximately 375° C. 15 The following tabulation indicates the in?uence TABLE 1 of temperatures on the yield of melamine. , {Lime nitrogen and ammonium carbamate reacted in autoclave 1 hr. at 350° (3.] TABLE 3 [Lime nitrogen, ammonia. and carbon dioxide in Ca: NHgZ CO2 mole ratio of about 125.8: 1.8 reacted in Percent yield of melamine based on total N in lime 20 autoclave for 1 hr.] nitrogen Percent yield of Additional am melamine Total 0022011 mole ratio No ammonia monia present Temperature, Degrees O. based on present except in 60% stoichio total N in that in the metric excess lime nitrogen ammonium over that in carbamate ammonium carbamate

0. 94 ______. 1. 45_ 1. 69 1.80. The preceding examples were carried out in 1. 88. 2.81 a 300-cc. autoclave, and the melamine-contain 4. 80 ______ing product was removed. The product was leached with boiling water, the slurry was ?ltered, To obtain the best yields, ammonia should be and melamine was recrystallized from the hot present in excess over the amount in ammonium filtrate. carbamate. For example, the ammoniazcarbon In commercial-scale operations a variety of dioxide ratio or 3.211 gives a better yield of mel means for removing melamine from high pressure amine than the 2:1 ratio available in ammonium 40 apparatus can be employed, and those skilled in carbamate, as may be noted in the foregoing the art may adapt many of these for removing table. the melamine formed by the process of the pres Although there is a certain minimum period of ent invention. One such method comprises blow time necessary for the best yields of melamine, ing the reaction product from the autoclave. for example, about 1 hour when the autoclave is leaching said product with a suitable solvent. held at about 375° C., a good yield may be ob such as hot water, and recrystallizing the mel tained simply by bringing the autoclave to re amine from the solvent. action temperature (about 350° to about 400° (3.), (SECTION II) , THE GUANIDINE PROCESS followed by immediate cooling. At the higher temperature, however, the reaction time prefer 50 Various processes for the direct production of ably should not exceed 15 minutes. Beyond that guanidine salts from metallic cyanamides have length of time the yield is reduced. been reported. In one process lime nitrogen is The yield of melamine increases with increased fused with a low melting ammonium salt to yield pressure, as demonstrated in the following table. the corresponding guanidine salt. Under some The variation in pressure was obtained by Vary- ' conditions water is added for increased yields. ing the total amounts of reactants, i. e., lime Although the yields in processes involving am nitrogen, ammonium carbamate, and ammonia monium salts have been good, this general meth charged into a 300 cc. autoclave. 0d of preparing guanidine salts has the disad vantage of the relatively high cost oi‘ ammonium TABLE 2 60 salts as compared with the materials used in [Lime nitrogen, ammonia, and carbon dioxide in the the present invention. Also, in making guani Ca:NH8:C02 mole ratio of about 1:4.8:l.5 reacted in an autoclave at 375° C. for 1 hr.] dine nitrate by the above-named processes, it has been found that ammonium nitrate tends to Percent yield react explosively with the graphite contained in Autoclave of melamine 65 lime nitrogen. Total weight of reactants pressure,_ based on total lb.lsq., m. N in lime It has now been found that a product con nitrogen taining a guanidine salt or mixture or” salts, be lieved to be the carbamate possibly admixed with 8 the carbonate, may be obtained by reacting lime 75 So 25cog II.‘ nitrogen directly with ammonia and carbon di~ 500 oxide gases. The reaction does not require that the ammonia be present in the form of a salt, Melamine is also formed at pressures of 5000 nor is a solvent or melt required for the reaction. lbs/sq. in. and even higher. Also by the process of the present invention, In the examples cited in Table 2 the pressures 75 dangerous ammonium salts such as the nitrate aesegaerr 51’ are not required, and‘ the- reaction is conducted ofi ammonia-i. were. forced; into; a dim-cc. safely. autoclave which was heated‘: 101:2- hours:.at.;159f The following examples ill'ustratetheinvention. C. under the autogenously produced pressure, Example 2 yielding a mass from which guanidine carbon 5 ate. was obtained by leaching with hot water, 150 grams of lime; nitrogen was placed in a astin Example_2,,. the yield. being-1 18% of..' theory; 1300‘-cc;. authclave‘ which was then heated to The» screen analysis of the calcium.- cyanamide 150° C.,.and.ainmonia~ gas and carbon dioxide gas used was 41% remaining on a 20-mesh screen, in. an ammoniazcarbon dioxide mole ratio of 147.1% on‘a-40-meshscreen, 28.87% on a IOU-mesh 2374:]; werepassed into the autoclave under 650 screen. and the; remainder passing a 100-mesh lbz/sop in pressure‘ for 75 minutes. The lime screen. nitrogen increased} in weight by 55% and a re Example 5 action product was-obtained containing guanidine values-in ayield of 19.7% of. theoretical. The. The» preceding» example was repeated‘ with-the guanidine values were not positively identi?ed; sole- di?erencebeing that thecalciumcyanamide but were believed to. be mostly. guanidine cara was groundto pass a ZOO-mesh” screen. The yield. bamate. The percent yield‘ is based: on the. was 53%. amount of guanidine carbonate obtained. from The reactionmay be conducted inv various inert; said values. In determining a. basisv ior. calcu solvents, such as methanol or the like. However. lating percentage yields, the usual assumption 20 no solvent is required and it is preferably omit was made that’ one mole of; calcium» cyanamide ted if the guanidine is to be recovered as the car was. converted to one mole- ofj guanidine‘ or a bonatei. . . monoa'cidic guanidine“ salt; or. two moles for. a As relatively pure. calcium.- cyanamideiscostly, cliacidic guanidine‘ salt' such as the carbonate: it. preferable economically to . use , technicalv cal, Guanidine carbonate was‘ recovered from the . cium cyanamid'e, or “lime nitrogen” as it is bet reaction mass obtained, in thisandthe following ter known. This material iscommercially avail examples. by‘ leaching the mass.- with.v hot water. able in a ?ne-ground form suitable for the reac The guanidine carbam'atel presumed present tion- and. contains: 60-65%. calcium" cyanamide, would thus be hydrolyzed“ to guanidine carbonate, the remainder being mostly lime. and graphite. which dissolves in the water, and the carbonate 30 The lime nitrogen used in developing this inven may berecovered from its aqueous solution by tion contained about 43% total calcium. evaporation or'ley “salting ou "with ethanol and Intheiollowing exampleslwhich further illus ?ltering. Otherguanidine salts may be obtained trate. theinvention, the calcium cyanamide is al by leaching the reaction mass with the corre— __ ways‘ present as lime nitrogen, and when ‘cal sponding aqueous acid. cium is mentioned it includesboth that in the calcium cyanamide and that in the accompany-1 Ammonium. carbamate is. a, convenient source; i-ng impurities. " ' of ammoniaand carbon dioxide. It need not-be mixed with the lime nitrogen, as shown in the Example 6 following example. Lime nitrogen was heatedwith 76.8. grams, of Example 3 ammonia and 73.3 grams of carbon dioxide for A glass cylinder" open at both ends approxi~ 2. hours in a. BOO-cc. autoclave at.- 15.00 C. The mately 35mm. in diameterand 200- mm; long was“ pressure developed‘ was 1.35.0v lb../sq. The, auto prepared withv a porous glass=disc in the center; A clave was then opened and the reaction mass In one half of the tube was placed‘ 30T grams of was removed. The yield. of guanidine recovered ammonium carbamate, and‘ in the other half- 12 as the carbonate was 65%‘ of theoretical. grams of lime nitrogen. The tube with its con» Varying- the. temperature substantially from. tents thus separatedrwaslplaced in a 3001-00. auto-i that stated in the example decreases the yield clave and 20 grams of ammonia was forcedv into of guanidine salt as shown in Table 4 below. the vessel. The autoclave was heated for 2 hours ‘ TABLE 4" at 150° C. The pressure developed was 1000 lbs/sq. in. A reaction mass containing guanidine [The e?ect of: temperatureon. reacting 73:3’ grams of (1.02, values as de?ned in Example. 2‘ was obtained in, 76.8.; grams of NH3': and’_ 5.0, grams of‘v lime nitrogen (giving; a mole ratio, C02;:~,Ua ofv 3:1‘) in a 300-cc. a yield oi“ 25%. autoclave for" 2'» hours.- ]‘-' ' Accordingly, ity is within the scope. of the pres. ent invention, and’ the claims are to be so con Autogen strued, that ammonium. carbamate may serve. as: ously Yield of a source of at least, part of the ammonia. and. Temperature, °C. developed guanidine -‘ pressure, salt,1 percent carbon dioxide available for reaction. with. the. p. s. i. theory lime nitrogen. Said ammonium carbamate may be placed in the autoclave as. such. or may iorm. there (with or withoutv intimate contact. with lime nitrogen) by reaction of ammonia and‘ car bon dioxidev when these gases. are forced into. the. autoclave, or the. carbamatev may. be. present. as as aresultof both steps. _ 1 The guanidine salt as initially synthesized was. not identi?ed, but probably occurred mostly as the carbamate. In order to present a greater reaction sunface Yields recorded are based on guanidine carbonate recovered to the ammonia and carbon dioxide, it. is pre» from saidsalt. . ferred to use a ?ne-ground calcium cyanamide, 70 as shown in the following two examples. If the carbon dioxidezcalcium ratio is reduced to 1.5:1 instead of 3:1 as in the preceding exam, Example. 4 ples, the yield of guanidine salt is reduced appre ciably for the same temperatures,’ as shown in Fifty‘ grams of calcium cyanamide (91.3% Table 2. However, some guanidine salt is ob! pure), 67.0 grams of carbon dioxide, and‘ 72.3 75 tained regardless of what proportions of react 2,658,891 7 8 ants'are used, and the invention is not limited to TABLE 8 any speci?ed proportions. [The effect of time of reaction on reacting 73.3 grams of CO2, 76.8 grams of N113, and 50 grams of lime nitrogen TABLE 5 (giving a mole ratio, CO2:Ca of 3:1) in a 300-cc. 5 autoclave.) [The effect of temperature on reacting 7 3.3 grams of CO2, 76.8 grams of NHS, and 100 grams of lime nitrogen (giving a mole ratio, (1050a of 15:1) in a 300-cc. Yield of guanidine salt,1 autoclave for 2 hours] percent of theory

m Hours Reaction at Reaction at ‘d‘umlgen‘ Yield of 150° C. under 175° C.under one y- . . autogenou sly . autogenously Temperature, °O. developed 56121212113123‘: developed developed pressure ' p_ s_ i_ ’ of theory pressure pressure

60 ______0 450 4 l, 275 25 1, 400 23 1, can 25 l, 800 16

20 1 See de?nition in Table 4. 1 See de?nition in Table 4, An ammoniazcalcium mole ratio of about 8:1 In Table 6 below, it is shown further that vary to 12:1 is preferably employed in techniques ing the carbon dioxidezcalcium ratio from the using autogenously developed pressures, as will be optimum value of 3:1 results in decreased yields. noted from the following table. 2° TABLE 9 TABLE 6 [The ‘effect of variation of NHS : ‘Ca mole ratio. with [The effect of varying the C02:Ca mole ratio in reacting C02: Ca mole ratio constant at 3 : 1, on heating CO2, NH3, and lime nitrogen in a 300-cc. autoclave for ammonia, carbon dioxide, and lime nitrogen in a 300-00. '2 hours at 150° C.) autoclave at 150° C. for 2 hours] 30 Yield of - , . Agtogelnouglyeve ope cuaniYieldi mepf I 0022021 mole ratio (in each run the mole ratio guanidinc Mole “mun NH" 03 pressure, salt.l percent ~- 00::NH; is about 112.5) salt,1 percent of theory - l p. s. i. of theory I

28 _ 1,000 40 54 ______. 62 65 1,350 66 57 1,700 63 58 l, 750 6‘)

40 1 See de?nition in Table 4. 1 See de?nition in Table 4. In all of the preceding examples the autoclave Increasing the time of reaction beyond about remained sealed during the reaction. However, 4 hours for the lower ratio, even better results are obtained when the auto 45 clave is vented continuously or intermittently carbon dioxide : calcium: 1.5 : 1 during the reaction period, especially when the ammoniazcalcium mole ratio is increased con— does not su?ice to increase the yield appreciably siderably over that found suitable for non as shown in Table 7. vented reactions, as shown in the following example. 50 TABLE 7 Example 7 [The effect of time of reaction on reacting 73.3 grams of . CO2, 76.8 grams of NH3, and 100 grams of lime nitrogen 150 grams of lime nitrogen, 424 grams of car (giving a mole ratio, CO3: Ca of 1.5: 1) in a BOO-cc. autoclave at 150° C. under the autogenously developed 55 bon dioxide, and 826 grams of ammonia were pressure] heated in a 3-quart autoclave at 150° (3., with stirring, for 2 hours. At room temperature the pressure was 160 lb./sq. in., but rose within 10 Yield of guanidine minutes to nearly 1600 lb./sq. in. when the re salt,1 percent action temperature of 150° C. was reached. of theory 60 Venting-was begun immediately and continued at such a rate that within 55 minutes of the start 23 the pressure had fallen to 950 lb./sq. in. Con 25 28 tinued venting brought the pressure to 760 33 1b./sq. in. within 1 hour and 25 minutes of the 33 65 start, and within 1 hour and 55 minutes the pressure had dropped to 425 lb./sq. in. At the 1 See de?nition in Table 4. end of 2 hours and 10 minutes, the pressure had dropped to 350 lb./sq. in., and at this point the In Table 8 the effect of increased reaction autoclave was opened and guanidine carbonate time on the optimum carbon dioxidezcalcium 7° recovered from the reaction mass as in the ratio of 3:1 is shown. It will be noted that method of Example 2. The yield was 81% of yields increase rapidly with increase of reaction theoretical. . time up to about 2 hours at 150° C., or about 1%; Variations in the venting rate are possible, and hour at 175° C., after which longer reaction re may be combined with changes in the proportions sults in no increase in yield. ,75 of reactants as shown in the following example. 2,658,891 10 Example 8 bon dioxide, and calcium cyanamide at a tem 200 grams of lime nitrogen, 282 grams of car perature within the approximate range of bon dioxide, and 718 grams of ammonia were 65°-250° C. in a closed reaction zone under super heated in a 3-quart autoclave as in the preced atmospheric pressure whereby a substance con ing example. The autoclave was vented from taining a guanidine salt is produced, and recov 1800 lb./sq. in. to 480 lb./sq. in. over the 2-hour ering at least the guanidine portion of said salt. period, and guanidine carbonate was recovered 8. The process that comprises heating a reac— from the reaction mass as in Example 2 in a 77% tion mixture consisting essentially of ammonia, yield. carbon dioxide, ammonium carbamate, and cal When the reaction is conducted at a pressure 10 cium cyanamide within the approximate range of of about 450-2100 p. s. i. and at a temperature of 65°-250° C. in a closed reaction zone at super at least 125° 0., both melamine and a guanidine atmospheric pressure whereby a substance con salt are obtained in the reaction mass. These taining a guanidine salt is produced, and recover are separated by methods well known in the art. ing at least the guanidineyportion of said salt. For example, the reaction mass is leached with 15 9. The process that comprises heating a reac ' hot water to dissolve both melamine and to tion mixture consisting essentially of ammonia. hydrolyze the guanidine salt to the very soluble carbon dioxide, and calcium cyanamide at a carbonate. Melamine, being relatively insoluble, temperature within the approximate range‘ of especially in cold water, crystallizes out as the 65°-250° C. in a closed reaction zone under the ?ltrate cools, and is recovered. Guanidine car 20 autogenously developed pressure whereby a sub bonate remains in solution and is recovered as stance containing a guanidine salt is produced, described in the Guanidine Process above. and recovering at least the guanidine portion of This is a continuation-in-part of applicant’s said salt. co-pending applications, Serial No. 14,380 and 10. The method of claim 9 in which the calcium Serial No. 51,110, ?led respectively March 11, 25 cyanamide is present as lime nitrogen. 1948, and September 24, 1948, both now 11. The process that comprises heating a mix abandoned. ture consisting essentially of ammonia, carbon While the invention has been described with dioxide, and lime nitrogen in a closed reaction particular reference to speci?c embodiments, it 30 zone at a temperature and pressure within the is to be understood that it is not to be limited approximate range of 65°C. and 450 p. s. i. to 250° thereto but is to be construed broadly and re C. and 2100 p. s. i. stricted solely by the scope of the appended claims. 12. The process of claim 11 in which the reac tion is conducted for a period of time not exceed I claim: ing about 4 hours. 1. The method comprising heating a reaction 35 mass consisting essentially of calcium cyanamide, 13. The process that comprises heating a reac ammonia and carbon dioxide at a temperature tion mass consisting essentially of ammonia, car of at least about 65° C. and a pressure of at least bon dioxide, and lime nitrogen in the approxi 450 p. s. i. to produce a reaction mass containing mate NHs2CO22Ca mole ratio of 12:3:1 in a closed at least one member of the group consisting of 40 reaction zone at a temperature within the ap a guanidine salt and melamine. proximatel range of 125°-1'75° C., under the auto 2. The process for making melamine which genously developed pressure, whereby a substance comprises heating a reaction mass consisting containing guanidine in combined form is pro essentially of calcium cyanamide, ammonia, and duced, and recovering a guanidine salt from said carbon dioxide in a closed reaction zone at a 45 substance. temperature of at least about 125° C. and at a 14. The process that comprises heating a reac pressure of at least about 450 p. s. i. and recov tion mass consisting essentially of ammonia, ering the thus-formed melamine. carbon dioxide, and lime nitrogen in a closed re 3. The method according to claim 2 in which action zone at a temperature within the approxi the calcium cyanamide is present as lime nitro 50 mate range of 125°-175° C. under the autogen gen. ously developed pressure, and venting the zone 4. The method according to claim 3 in which over a period of time. the ammonia and carbon dioxide are initially 15. The process of claim 14 in which the present at least in part as ammonium carbam NH3:CO2:Ca mole ratio is within‘ the approxi ate. 55 mate range of 20-30:3-7:1. 5. The method according to claim 2 in which sui?cient carbon dioxide is initially present to JOHNSTONE S. MACKAY. make the carbon dioxidewalcium cyanamide mole ratio about 1.5:1, and then adding suf?cient References Cited in the ?le of this patent additional carbon dioxide to neutralize any free 60 calcium oxide present. UNITED STATES PATENTS 6. The method according to claim 2 in which Number Name Date the pressure is autogenously produced. 7 7. The process that comprises heating a reac 2,114,280 Aldred ______Apr. 19, 1938 tion mass consisting essentially of ammonia, car 65 2,545,480 Mackay ______-_ Mar. 20, 1950