DE1470032A1 - Process for the production of melamine - Google Patents

Description

Process for the production of melamine

The present invention relates to a method of extraction of melamine in particular from by pyrolysis of urea under Pressure Preserved Melamine · The present invention particularly provides a simplified technical process for the production of melamine with a purity Above 99 # starting from crude melamine obtained by pyrolysis of urea under pressure at high temperature

It is known that when one expands from urea Melamine produced at high pressure and temperature at normal pressure decomposes, with various impurities are formed, namely melam, merries, melon and isotriazine, which, as is known, can only be separated from the main product with great difficulty.

By using the method according to the invention, the decomposition -Positioning of melamine in comparison to the one that usually occurs Decomposition due to the direct expansion to normal pressure reduced.In addition, the by-products, which are difficult to separate from the main product, at the same time in the expansion device into easily removable products

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It is known that the by-products derived from the breakdown of melamine can only be separated from it with difficulty can be. However, it is now also known that these in. other products can be transformed which by exposure aqueous solutions of alkalis or ammonia can be separated more easily under pressure and at temperatures above 100 C. can·

The present invention is based on the surprising discovery that the conversion of the mentioned by-products in an advantageous manner in the presence of high concentrations of carbon dioxide (5-10 Grew ft of the solution) alongside or in addition to Ammonia and water, i.e. in the presence of ammonium carbonate and ammonium carbamate

The importance of this discovery becomes clear when one looks at the manufacture is considering the working process used by melamine from urea.

Urea is known in melamine according to the following Equation converted!

6KEL - 00 - HH ₀ , ^ melamine + 6NH- + 3CO ₀

from which heßvorgeht that only 50 are converted i> of the urea into melamine and the other 5O # into ammonia and carbon dioxide in the ideal case ·

The gas mixture obtained from the reaction described above is present in the stoohiometric proportions precisely suitable for use for urine synthesis, which represents a defined economic advantage for a melamine production cycle

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In a cycle in which the gases mentioned are used for synthesis If urea is reused, the simplest method is to recover it and after it has been separated from the Melamine in the same solution of ammonia and carbon dioxide which is returned to the urine synthesis plant to absorb.

To achieve this, it is necessary that the ammonia and carbon dioxide gases from the melamine plant are at a pressure of at least about 5 atm are available at the end of the quenching phase.

The quenching of the reaction mixture under pressure is carried out with saturated solutions of ammonia and carbon dioxide, so that the degradation of the melamine coming out of the reactor is reduced to a considerable extent or avoided entirely and immediately an aqueous solution of melamine in the presence of NH ,, ammonium carbonate and ammonium carbamate with the receives the aforementioned advantages.

The present invention provides an improved process for the recovery of melamine obtained by pyrolysis of urea under pressure. The invention is characterized in that the molten mixture containing the melamine coming from the reactor is quenched after expansion to a pressure above atmospheric pressure This quenching is carried out with an aqueous reflux solution (A) which is saturated with ammonia and coal dioxide and with their reaction products such as ammonium carbonate and ammonium carbamate, and the action takes place at a temperature of 100-200 ⁰ O for 10 60 min. The melamine solution obtained after this period is filtered, the insoluble products being removed and then the melamine solution, preferably at atmospheric

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Chilled under low pressure in order to crystallize out the Mela-mini afterwards the cooled solution is centrifuged off in order to separate the crystals which have a high content of pure melamine. The mother liquors are exposed to a stream of steam at low pressure to remove the carbon dioxide and ammonia, as well as their Remove the reaction products contained in the mother liquors mentioned. · The condensed and cooled vapors are returned to the deterrence stage as solution (A)

The impurities originally related to the melamine present, remain dissolved and / or suspended in the mother liquors.

The preferred working conditions are as follows:

Pressure of the mixture leaving the reactor of 60-100 atm; Quenching pressure of 5-95 atm, preferably 10-35 atm!

Absohreoktemperatur of 120 - 17O ⁰ C;

Residence time under the above-mentioned conditions of pressure and temperature! 20 - 50 min |

Concentration of melamine in the solution is 5-20 i "preferably 8-15 *.

Under these conditions it is possible to remove the unwanted by-products (which can only be separated from melamine with difficulty) into other products which are soluble in alkalis are, like amelline and amellide, convertible.

The eaktionsgase ^s, which can not be dissolved in the thus already saturated mother liquors by means of an automatic valve of the system off for synthesis zugefü Harnst, · - ;.

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The process according to the invention therefore combines the economical use of the ion from the reuse of _: ammonia; round Kohlenctiöiyd-ziii ^^ lifcheöe ^ ri- Härn © Jtof £ äerg ± bt; _r ittit the ycmfceifcn the extraction of pureii w ^ tÄins & Xt a salary of libev $ <£ ^C J> .i difficult « ¹ cleaning ^ g» ve * fahreii ^ e ^ en ^ ii ziräüa-sen »:::; ? /

of course, a number of changes can be made in the above method without departing from the scope of the present invention. For example, it was found that when treating the melara lounges coming from the quenching phase in the overflow of ancient Asian moniac in a Bactraktloass under pressure and under pressure and conditions similar to those in quenching are in such a way that the dissolved carbon dioxide removes ^: and hLor is obtained by a saturated solution which exclusively contains animonia (2 - 30 ^ and preferably 5-20 ^), then, after filtration and crystallization, crystals with a high content κη melamine can be obtained · In such a case, the liquids saturated with ammonia can ejo. as such after their Sntleerun ^ from the Absqhrockkolonne Recoveries ^ okreiot be * The total in the Bxtrakt ions zone in freedom .- - translated ammonia and carbon dioxide gases to the urea plant

su ^ ofUhrt // earth.

In a wide drainage chamber, the ammonia can be poured Mother liquors can be treated with carbon dioxide, for example by putting in them a gaseous stream of ammonia and Carbon dioxide from the extraction zone and from the suction back solution is initiated · The carbon dioxide falls in the ammoniacal Solution present hydroxylated by-products by adding it reduces their solubility, whereby the removal of the

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named by-products by FiltELreii: eric & faertamird ^ i Β ± «& - χ.Γ © 2: 3 clarifies the necessity of the; Lüoung (A); jv ^^ e ^ iWL ^ d.kr ^ L ^ i ^ circulating solution, otherwise a loss of usable product would occur.

Another variant of the method includes; the- Behänd! "ag: ar _r vo" quenching received: Melamtnlösung- im- G ^ gensisfc 'am toit>> £, · so · that ammonia and any carbon dioxide are eriit ^ -' . The ammonia and carbon dioxide can - then, cooled and ^! - be condensed in order to obtain solution (A) which is recirculated.

The Dampfbehaiidlimg is preferably in a • ■ 1-6 atm and in a ratio of 0.15-0, 3Q "kg steam and ^:

preferably 0.25 kg of steam per kg of the quenching stage s solution. In such a case, na η receives a ^ solution _i; .

or a slurry of melamine, which is free from IHL, / and _t COp and '«eiche danacli with an alkaline solution geiaäps, ,,.

the procedures normally used to purify melamine,

can be treated · - ο

A Apparabtir of stainless steel (18 4 chromium, 11 5 »iiickel _v and 3 # Moljrbdän) used and small amounts of molecular oxygen, the substance elbasen in the quench, a ^ is obtained a further increase in the degree of purity of the Mola ~ a. ns,

The aforementioned aims and purposes are conjoined with any one of the three

Yerfaitr ens schemata described below, as they are in the Pig, 1, 2 and 3 and which describe continuously operating cyclical processes.

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In detail, they are as follows t

Cycle 1 (this cycle is explained in Fig. I).

When the melamine leaves the urea pyrolysis boiler (not shown), it is introduced into the quenching apparatus 1 through the pipe 2. The Li el amini ö solution leaves the apparatus through the pipe 5, is filtered at 6, flows through the float valve 7 and is then expanded and cooled in the crystallization tank 9 to crystallize the melamine.

The melamine is separated from the mother liquors in the centrifuge 10 and the crystals are removed at 11. The mother liquors and the washing liquids are fed by means of the pump 12 and the line 13 to the top of a stripping column 14, into the bottom of which steam is blown at 17.

The ammonia and carbon dioxide escape at the top of the column 14 and are discharged through line 18 and into the apparatus 19 reabsorbed in water. The ones with ammonia and carbon dioxide saturated ^ solution is via line 23 by means of the Pump 20 returned to the quench column.

From the bottom of the stripping column 14 is made of ammeline and ammelide Existing suspension 15 withdrawn, which can be further treated to obtain the above-mentioned by-products "

The loss of kelamin in the outlet at 3 is proportionate low owing to the high concentration of melamine in the region described in Par Ireck apparatus 1 with 5 escaping solution.

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The working conditions for apparatus 15 and 19 are as follows i

The pressure is 1-20 atm and is preferably in the range of tone 1-5 atm at the corresponding temperatures Xs should be noted that although lower pressure increases the downforce, higher pressure the achievement of a higher flow temperature permitted for the mother liquors recirculated at 23 · This but has a saving in steam eur lolly.

Cycle 2 (this cycle is shown in Fig. 2) In this embodiment, the carbon dioxide is removed by stripping with ammonia before crystallization. The subsequent crystallization of the melamine is carried out exclusively in the presence of ammonia in order to keep the hydroxyl-containing earthquake products in solution.

The melamine solution leaving the quenching apparatus 2-1 is fed through the pipe 2-4 to the top of the stripping column 2-5, which is kept under the same pressure or a lower pressure than in the quenching stage · At 2-6, ammonia is blown to remove the dissolved carbon dioxide. A gaseous mixture is formed from the top of the column via the Eohr 2-16 removed from ammonia, carbon dioxide and steam. From the bottom of the column there is a slurry of melamine in water and

Ammonia emptied and then filtered at 2-7, expanded through valve 2-8 and crystallized by cooling in apparatus 2-9. The slurry is centrifuged at 2-10, the ammonia-containing ones Mother liquors are pressurized again by means of the pump 2-12 and via line 2-13 of the absorption apparatus

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2-14 for the removal of NH-, CO ₂ and steam, which are released from the top of the stripping column 2-5, supplied

As a result of the CO ₂ dissolved in it, the oxyaminotriazines, which are present in the mother liquors obtained by the centrifuge , fall out and are eliminated by filtration at 2-18.

By suitable Hege averaging of pressure and temperature in the Appara tur 2-14, it is possible at the bottom of a saturated under the output conditions with ammonia and carbon dioxide solution to obtain, which is wiedereingekreist by filtration at 2-18, at 2-19. From the top of column 2-14, pure ammonia is removed at 2-15 and then fed back to the stripping column 2-5.

The äinreichtungen 2-5 and 2-14 can at a temperature between 100 and 200 ⁰ C and are preferably operated between 120 -and 170 ⁰ C. The ammonia concentrations in it are 2 30 $> and ran preferably between 5 and 20 "/>,

The working temperature is ζ in Übereinstinmun with the pressure and the Pr ο ζ ent share of ^ eIostern. Ammonia selected.

Cycle ■ ') (this cross-process is shown in Fig. 3). The oil-amine pulp leaving the quenching apparatus 5-1 is expanded at 3-5 and fed to the stripping column 3-0, which is supplied with steam at 3-9 to remove the amino and carbon dioxide.

The ammonia mixture escaping at the top of the stripping column, Carbon dioxide and / or barrels is supplied via line 3-26 of the apparatus 3-27 for renewed absorption with water and thereafter

909822/1241 bad original _ _1o _

returned to the alarm system. The slurry discharged from the bottom of the column at 3-10 is fed to the dissolver 3-13; which the alkaline mother liquors containing KaOH, "at 3-23 in a for the production of a" at 10O ⁰ C saturated melamine. solution measured amount are recirculated.

The exiting at 3-14 alkaline melamine solution is filtered in the apparatus at 3-16 10O ⁰ C and then fed to a crystallizer 3-20. The melamine is separated from the alkaline mother liquors by centrifugation at 3-21. The mother liquors are returned to the solution apparatus after a suitable cleaning treatment and after replenishing or replenishing the amount of HaOH.

Devices 3-8 and 3-27 can operate at pressures between 1 atm and quenching pressure, with a pressure range between 1 and 6 atm being preferred.

The NaOH content in the backflowing mother liquors, which at 3-23 are re-circled, varies between 0.1 and 0.4 #

As regards the usable Konstruktionsniaterialien so is consisting of i / ater, ammonia, carbon dioxide and melamine ordered mixture very corrosive when heated to temperatures above 10O ⁰ C and the commonly used stainless steels are corroded By this means very quickly. However, the addition of small amounts of oxygen or air to the above-mentioned solutions is sufficient to prevent corrosion.

More expensive substances such as Ti, Ta, Zr, Ag withstand this corrosive medium very well even in the absence of O _2.

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The following examples serve only for illustration and are not to be understood as a limitation of the invention, since, as stated above, the working conditions can be changed sufficiently and appropriately over a wide range leaving mixture of melamine, ammonia and carbon dioxide to be changed

In the following part of the description, reference is made to the above-mentioned system diagrams Referenced.

example 1

Bs is added to fig. I referenced.

A mixture of ₅ NI i, 00 'and molten melamine, which

comes from a (not shown) reactor through line 2 with 380 ⁰ C and 100 atm and has the following composition (in parts by weight):

NH = 46- jS, CO ₂ = 28 #, melamine »2 6 #, is expanded in apparatus 1 to 20 atm and then with a solution introduced at 23 containing 20 # ammonia and 6 # CO« in a proportion of 8, 0 kg solution washed on 1 kg melamine

The collected at the bottom of column 1 ^ eIaminiösung is heated to 150 ⁰ C and 3O min-left.

Under these conditions, the total amount of ammonia and Carbon dioxide that accompanied the melted melamine was recovered and discharged from the upper part of the stripping column via a system 3 »4 for automatic control of the pressure · The Kelamin solution 5 is filtered at 6 and expanded through valve 7 un: i.T. "n-r etallizer 9 cooled» 909822/1241

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The centrifuge 10 becomes a melamine 11 with the following properties emptied!

Mosslamine content 99.1 Sew. f

insoluble in water 0.3 wt. S *

Isotriazine 0.6 wt. Ί

The mother liquors 13 containing NH, and CO _g are fed to the top of the stripping column 24, in which KH, and CO g are passed through. Injection of steam of atmospheric pressure and 10O ⁰ C removed at 17 and then reabsorbed in ifasser of 30 C in the apparatus 19, whereby a solution of the above-described composition for the mother liquors of the molten melamine

is obtained

The solution 23 is the deterrent via the pump 20 and a automatic system 21, 22 for regulating the liquid level rüokleitung ·

The milk content is determined in each pail by, potent iometric titration with HgSO. according to Dochlemann (Angew. Chem. 66 (1956),, ' n. 19,606). The proportion of isotr ^ iazine is determined by ultraviolet absorption measured·

Example 2

Bs reference is made to Fig. 2

Bs is at 2-2, a mixture of HH_ COP and molten melamine, which reactor (not shown) of at a temperature of 380 ⁰ C and a pressure of 100 atm, and which is the same composition as in Example 1 at 15 atm expanded and with an aqueous solution introduced at 2-19, which contains 8 # KH, and 2 fi CO ₂ and with melamine

909822/1241 ^^

■ is saturated at 25 ° C, in a ratio of 7 kg of solution each 1 kg of melted melamine, washed ^

The Helaminauf slurry is collected at the bottom of the quench 2-1 and heated to 17O ⁰ C, 'and maintained under these conditions for min 20th

Likewise, at 2-21 the mixture of KH and CO ₂ , which accompanied the molten melamine, is removed from the top of the column via valve 2-20. The melamine solution 2-4 is fed to a stripping column 2-5 filled with Raschig rings fed in and kept under the same pressure and temperature conditions as in the quenching apparatus.

At 2-6, ammonia is blown into the bottom of the stripping column in a ratio of 0.1 kg per 1 kg of solution.

The ammoniacal solution of melamine, which is free of carbon dioxide is collected at the bottom of the column, is filtered at 2-7, expanded at 2-8 and at 2-9 for crystallization des üelamina chilled.

Under the working conditions of the plant, the Centrifuge 2-10 at 2-11 removed melamine has the following properties

Melamine content 99.2% by weight »

insoluble in water 0.3 wt.

Isotriazine 0.5 wt. #

The ammonia mother liquors are pumped at 2-13 2-12 is returned to the absorption tower 2-14 at 15 atm, where it is chemically combined with that from line 2-16 from the stripping column 2-5 coming carbon dioxide through its reaction with the in the.

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Chemically combine dissolved ammonia contained in mother liquors From the bottom of the column there is a solution of ammonia and carbon dioxide which, in suspension, contain the oxyaminotriazines precipitated from the ammonia mother liquors by the action of CO 2 contains

The solution saturated with ammonia and carbon dioxide becomes Removal of these by-products, "filtered at 2-18 and thereafter repatriated at 2-19 of deterrence. That in the head of the column 2-14 collected ammonia is v / ied by means of a (not shown) Blower, which is connected to line 2-15, is fed to the stripping column 2-6 again.

Example 3

Bs is referred to Pig 3

The mixture of ammonia, carbon dioxide and molten melamine coming from the reactor (not shown) is expanded under the same conditions and with the same composition as described in the previous examples at 3-2 to 30 atm and with an aqueous solution of ammonia and carbon dioxide 3-32, containing 2 5 $> M- and 8 # CO ₂ , quenched in a ratio of 9 kg of solution to 1 kg of melamine.

The Melaminbrei is collected at the bottom of the quench column 3-1, heated to 15O ⁰ C and held there 60 min.

The gases originally coming from the reactor are combined with the melted melamine from the top of the column through the automatic System 3-3 »3-4 emptied to check the pressure.

The melamine solution is under atmospheric pressure via the ai: - '
Valve 3-5 expanded and fed to an output column 3-8

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whose bottom steam at 3-9 in a ratio of 0.25 kg / kg Mushroom fig bubbles will remove the ammonia and the dissolved carbon dioxide to remove·

From the bottom of the column 3-8 a _{slurry free of carbon and CO 2 is} obtained, which at 3-12 is emptied into the dissolving apparatus 3-13, where it is recirculated together with the alkaline mother liquors at 3-23, which with Melamine are saturated at 25 C and contain 0.2> UaOH, are dissolved until a ^{solution saturated at 100 0} O is obtained, which contains almost about 5 # melamine.

The removed at 3-14 solution is filtered in the apparatus 3-16 at 100 ⁰ C. and the crystallizer 3-20 fed in which it is cooled to 25 ° C.

The melamine separated by the centrifuge 3-21 is v / o at 3-22 removed and washed with 1 1/2 times its weight in water.

Under the working conditions of the plant, the crystals have the the following properties:

Melaminehs: < $ 99.0>

insoluble in 7 / water 0.2 #

Isotriazine 0.7 ^

The iuuttirlaugen together with those entering at 3-24

^ iiascl'iflüss: .j; kciten cleaned and again reduced to a NaOH content of 20 - / » jebraciit, the dissolving apparatus 3-13. the

· * ° ilischun-s 3-26 of ammonia and carbon dioxide and water, which ^ from the kbtriebsä ule 3-8 kaiunt, v / ird in the apparatus 3-27 in - * "absorbs water. LIan receives a saturated solution of ammonia

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and carbon dioxide 3-33 »which re-enter the quench column is circled.

Example 4

The quenching device is made of stainless steel and is located under operated under the following conditions!

Pressure 20 atm

20 56

CO ₂ 6 i

Melamine 10 i »

Temperature 150 ⁰ C

Dwell time 30 min

The melamine solution is emptied from the quenching apparatus and crystallized by cooling. In this way, a crystalline product is obtained which, according to the analysis, has an ash content of 0.20 - 0.3 i » , which consists of 80 - 90 # ^ ^e 2 ^Q 1 ^te "

According to a preferred alternative embodiment of the invention, oxygen is blown into the same quenching apparatus under the reaction conditions given above in a ratio of 0.012 ι 1 per 1 solution at 20 atm, resulting in a crystalline product which contains 0.01-0.005 1 ash.

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Claims (1)

- 17 - 'Patent claims I 4 7 U U 3 2 1 · Process for the extraction of by pjsrolysis under pressure from Urea obtained melamine, characterized in that the coming from the reactor under pressure and containing melamine molten mixture after expansion to above atmospheric Pressure lying pressure is quenched with an aqueous recirculated solution (A), which with ammonia and carbon dioxide and its reaction products is saturated and with which it is at a temperature of 100 - 200 C during Is brought into contact for 10-60 min; the melamine solution obtained after this time is filtered and the insoluble Products are separated, crystallized by cooling under atmospheric pressure and used to separate the high-percentage Centrifuged melamine crystals; from the mother liquors carbon dioxide and ammonia, as well as their reaction products, which contained in the mother liquors mentioned, recovered! the said vapors are after condensation and 'cooling returned as solution (A) to deterrence 2. Yerxanren according to claim 1, characterized in that the tibnc / irecken of the melted melamine at a temperature zv / is before η 120 and 170 ⁰ C is carried out. % 7or £ ju.-ren according to claim 1 and / or 2, characterized in that that the ^ ischunr ^ from the reactor with a pressure between 60 and 100 atm comes. 909822/12 * 1 »d omginm. -18- U70032 4. The method according to the preceding "claims, characterized in that that the reaction mixture prior to treatment with solution (A) is expanded to a pressure between 5 and 95 atm » 5. The method according to claim 4, characterized in that the quenching of the molten melamine is preferably under a pressure of 10 - 35 atm is carried out 6. The method according to the preceding claims, characterized in that that the melamine solution in the quenching column for leave for a period of preferably between 20 and 50 minutes will. 7. The method according to one or more of the preceding address, characterized in that the solution in the Absciirckturm has a concentration of 5 - 20 wt. $> Melamine. 8. The method according to the preceding claim, characterized by that the solution is preferably a concentration from 8 - 15 # melamine. 9 · A method according to the preceding claims, characterized in that the following composition from the reactor under a pressure of about 100 atm coming kischung in weight besitzil IEI, about $ 5>, CO ₂ and melamine ti about 27 about 27 10. The method according to claim 9, characterized in that said molten mixture makes expansion to 20 atm ntt an aqueous solution containing about 20 wt. ^C /> IXfI-, and 909822/1241 BATH -19- _ _lg _ Η7Ό032 8 'wt · # Op in a ratio of about 3 kg solution / kg Melanin is quenched and the resulting pulp is heated to 150 C and aged for 30 minutes. 11 · Process according to claims 1 to 4 »characterized in that that the helamin solution obtained from the quench is preferred under the same temperature and pressure conditions Ammonia in a ratio of about 0.1 kg HH ~ / kg solution is treated and the pure melamine from the carbon dioxide-free solution is crystallized. 12. The method according to claim 11, characterized in that the j pure melamine from a free of carbon dioxide, ammonia solution having a weight skonzentr ation of 2-30 wt. I »is crystallized to ammonia. 13. The method according to claim 12, characterized in that the ammonia solution preferably contains 5-20% ammonia. 14. The method according to claims 11 to 13 »characterized in that the molten mixture which contains the melamine, preferably after expansion to about 15 atm with an aqueous solution containing about 8 wt. I» NH and 2 wt. # COp, which with Melamine is saturated at 2 5 ⁰ O, is quenched with a ratio of 7 kg of solution / kg of molten melamine. 15 · The method according Ansprtich 14, characterized in that the Melaainlösung is heated to about 170 ⁰ C and is aged in the quench column for about 20 min · 909822/1241 -20 - BATH ORIGINAL 16 · The method according to any one of claims 11 to 15 »characterized in that marked »that the mother liquors, from« calibrate the pure melamine was separated, for the absorption of the HH, from the Downforce coming grasses are used to reijikr Eisung si ution (A) eu obtained. 17. The method according to claims 1 to 4, characterized in that the meliemin solution obtained from the quenching is treated with steam at a pressure between atmospheric pressure and the pressure during the quenching in order to remove the ammonia and carbon dioxide, and the The vapors containing the above-mentioned compounds are cooled and condensed to obtain the solution (A), which is recirculated while the ammonia and carbon dioxide free melamine pulp with the alkaline recirculation mother liquors saturated with melamine and about 0.2 grain in size. £ KaOH contained, is brought to a solution until a solution saturated with melamine at 100 - 130 ⁰ O is obtainedf the melamine is separated in a pure state by hot filtration, cooling and centrifugation 18 · The method according to claim 17, characterized in that the Treatment with steam, preferably in a pressure range between 1 and 6 «ta, and in proportions of 0.15-0.3 kg Dtmpf per kg from the AtosonreoJtuiig coming solution carried out "Fore n" oh the Annßrtiöhcm 17 and 18, thereby lost «It is advisable that the mixture containing dfts suitable melamine riaoh expansion to about JO atm with an aqueous solution, 909822/1241 BAD ORSGiNAL _ ₂₁ _ containing about 25 wt. ί ammonia and 8 sew. + Carbon dioxide is quenched in a ratio of about 9 kg solution / kg melamine 20. The method of NaOH to claims 17 and 19, characterized in that the melamine solution heated to about 150 ⁰ C and is aged in the quench column for about 60 min · 21 · Method according to any one of the preceding claims, d «- duroh marked that molecular oxygen in the Quenching column is blown in 909822/1241 L t e eersei