PROCESS FOR RECOVERING SOLID MELAMINE

The present invention relates to a process for recovering solid melamine from a gaseous mixture containing melamine, ammonia and carbon dioxide, and the integration of this process into a process for the production of melamine from urea.

BACKGROUND OF THE INVENTION

Processes for recovering solid melamine from a gaseous melamine containing mixture are known from the prior art.

US Patent 3,310,558 discloses a process for recovering melamine from a hot, melamine-containing gas mixture that further comprises ammonia and carbon dioxide, by cooling in direct contact with water in a cooling zone. According to the preferred embodiments described in that reference the cooling zone is constructed as a gas scrubber wherein the gaseous fluid from the reactor is contacted in countercurrent relation with a solution containing ammonia and carbon dioxide recycled from the process. Thereby a slurry of solid melamine in an aqueous ammonia and carbon dioxide containing phase is obtained. Such slurry is concentrated and the solid melamine is separated by means of a centrifuge. The aqueous phase from concentration and melamine separation is recycled to the gas scrubber. In the explicit example the aqueous phase that is recycled to the scrubber contains 11.5 wt-% of ammonia and 6.2 wt-% of carbon dioxide. Furthermore, that reference does not teach that the aqueous melamine containing slurry obtained from the gas scrubber according to the first embodiment or a combination of two gas scrubbers according to the second embodiment is further cooled to a certain temperature.

A similar process is disclosed in US Patent 3,598,818. According to this reference a gaseous mixture comprising melamine, ammonia and carbon dioxide is directly cooled with a circulating solution containing ammonia and carbon dioxide. The improvement compared to previous processes is that the system is still pressurized and the relatively thin suspension of melamine crystals in the

aqueous phase is first concentrated by means of one or more hydrocyclones to a concentration of 30 to 60 wt-% of melamine, whereupon the resulting concentrated suspension after expansion to atmospheric pressure, is fed to a stripping column in which, by applying heat the residual ammonia and carbon dioxide still present is stripped off and the melamine crystals are separated by, for example centrifuging or filtration. The resultant mother liquor is recirculated through the process, but the thus obtained melamine does in general not fulfill the high purity requirements of the industry. Consequently, according to a preferred embodiment described in US Patent 3,598,818 the concentrated melamine suspension from the stripper is again diluted with mother liquor and heated in order to dissolve the melamine. Thereafter the solution is filtered and treated with a decoloring agent, such as activated carbon, and subsequently recrystallized in order to obtain a melamine of higher purity. In the explicit example in US Patent 3,598,818 the melamine suspension leaving the quenching step has a concentration of 11.9 wt-% ammonia and 2.9 wt-% carbon dioxide.

The drawback of the process according to US Patent 3,598,818 is that additional purification steps are necessary, like redissolution of the solid melamine in order to filter impurities, subsequent treatment with carbon black and recrystallization. In addition, low pressure steam is necessary in order to strip ammonia and carbon dioxide in the system which increases the energy consumption of the entire process.

From US Patent 3,711 ,479 a process for recovering melamine from a gaseous mixture comprising melamine, ammonia and carbon dioxide is known whereby the gaseous mixture is fed to a cooler where the gaseous mixture is quenched with an ammonium carbamate solution resulting in a melamine suspension comprising 8 wt-% of ammonia and 2.7 wt-% of ammonium carbamate, based on the total weight of the aqueous phase. The thus obtained melamine suspension is diluted with very dilute melamine suspension and subsequently expanded to atmospheric pressure. The thus obtained suspension has a temperature of 85°C. The melamine suspension is subsequently concentrated in a thickener-cyclone and thereafter the melamine is recovered by filtration.

Although no further purification steps for the recovered solid melamine are described in US Patent 3,711 ,479 the melamine obtained by the process described therein still has a too high content of unwanted oxygen containing impurities.

According to US Patent 3,496,176 disclosing also a process where solid melamine is recovered from an aqueous suspension and the resulting mother liquor is recycled into the process, unwanted oxygen containing byproducts, such as ammeline and ammelide, build up in the recycled mother liquor and result in an unwanted impurity level of the resultant solid melamine. In order to overcome this problem US Patent 3,496,176 suggests to first strip the aqueous mother liquor after separation of the solid melamine to remove ammonia, cooling the liquid and precipitating ammeline and ammelide by acidification. After removal of the precipitated ammeline and ammelide by filtration the filtrate is recycled to the process. Although these measures result in a reduced impurity level the energy consumption is rather high.

In order to overcome this problem EP 91 174 teaches to treat only a portion of the recycled mother liquor by acidification by introduction of carbon dioxide and heating to dissolve any remaining suspended melamine whereby a precipitate of impurities is formed and filtered off. The thus purified part of the mother liquor is recycled into the process. The drawback of the process described in EP 91 174 is that still a purification treatment of a part of the mother liquor is necessary, resulting in energy consumption and the resultant purity is still not satisfactory.

WO 01/00596 also deals with the problem of formation of oxygen containing impurities, like ammelide and cyanuric acid, and suggests rapid cooling of a gaseous mixture comprising melamine, ammonium and carbon dioxide by direct contact with an evaporating medium that is sprayed in order to have a certain specific minimum area of liquid and a minimum impulse while having a minimum residence time within the cooling unit. The evaporating medium used for cooling may be ammonia, water or ammonium carbamate solution. Although the amounts of ammelide and cyanuric acid can be reduced by adjusting the surface area and impulse of the evaporating medium the best example still has a content of

0.021 wt-% of ammelide and 0.009 wt-% of cyanuric acid, based on the weight of the recovered melamine.

WO 02/14289 deals with a process wherein the gaseous effluents from a low pressure catalytic process for the production of melamine comprising melamine, ammonia and carbon dioxide are quenched with an aqueous ammonium carbamate solution recycled from a condensation absorption unit wherein the gaseous effluents from the quenching step are condensed and absorbed in an aqueous phase resulting in an aqueous carbamate solution. The advantage of that process is that solutions of a higher ammonium carbamate concentration are obtained in the absorption condensation step that can be directly exported without further concentration to a urea plant.

Thus, the object of the present invention is to provide a process for recovering solid melamine from a gaseous mixture containing melamine, ammonia and carbon dioxide which results in an improved purity of the recovered solid melamine, especially a reduced amount of oxygen containing impurities, without the necessity of energy consuming purification steps, like recrystallization of the recovered melamine or purification of the recycled mother liquor.

SUMMARY OF THE INVENTION

This object has been surprisingly attained by a process for recovering solid melamine from a gaseous mixture containing melamine, ammonia and carbon dioxide comprising: a) quenching said gaseous mixture with an aqueous ammonium carbamate containing solution thereby producing an aqueous melamine containing slurry or solution, said aqueous melamine containing slurry or solution comprising 24 to 80 wt-% of combined ammonia and carbon dioxide based on the total weight of the aqueous phase; b) cooling said aqueous melamine containing slurry or solution obtained in step a) to a temperature of 80 to 120 ⁰ C thereby precipitating solid melamine to form a melamine slurry; c) optionally concentrating the melamine slurry obtained in step b); and

d) separating the solid melamine from said slurry obtained in step b) or c) to obtain recovered solid melamine and a mother liquor.

By employing the process of the present invention it has been surprisingly discovered that no additional purification steps are necessary. Consequently, the separated solid melamine does not need to be recrystallized. Nor is a treatment with discoloring agents, like carbon black necessary. Additionally, there is no need to introduce alkali metal or alkali earth metal hydroxide into the quenching liquid. Thus, the aqueous ammonium carbamate containing solution used in the quenching step according to the present invention contains preferably less than 0.5 wt-%, more preferred less than 0.05 wt-%, and most preferred less than 0.005 wt-%, and most preferred substantially no alkali metal or alkali earth metal hydroxide.

Furthermore, according to a preferred embodiment of the present invention the recovered solid melamine obtained from step d) of the process of the present invention is not subjected to a recrystallization step.

According to another aspect the present invention relates to a process for the production of solid melamine comprising: i) obtaining a gaseous mixture containing melamine, ammonia and carbon dioxide from urea by a catalytic or non-catalytic reaction; and ii) subjecting the gaseous mixture containing melamine, ammonia and carbon dioxide to a process as defined above.

According to a preferred embodiment of the present invention step i) of said process comprises:

A) reacting molten urea in a reaction section at a pressure of 45 to 150 bar in a non-catalytic reaction to produce a reaction mixture containing molten melamine and reaction off-gases containing ammonia and carbon dioxide; and

B) vaporizing the molten melamine in said reaction mixture in a vaporization section to produce a gaseous mixture comprising the vaporized melamine and the reaction off-gases at a pressure of 45 to 150 bar.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

According to the process of the present invention a gaseous mixture containing melamine, ammonia and carbon dioxide is quenched in step a) with an aqueous ammonium carbamate containing solution thereby producing an aqueous melamine containing slurry or solution comprising 24 to 80 wt-% of combined ammonia and carbon dioxide based on the total weight of the aqueous phase. Consequently, in step a) if a melamine containing slurry is formed the precipitated melamine is not considered when calculating the weight percentage of ammonia and carbon dioxide. Thus, the weight percentage of ammonia and carbon dioxide is only based on the total weight of the aqueous phase that in addition to ammonia and carbon dioxide contains water, dissolved melamine and very limited amounts of byproducts.

The weight percentage of combined ammonia and carbon dioxide based on the total weight of the aqueous phase is 24 to 80 wt-%, preferably 27 to 75 wt-%, and even more preferably 30 to 70 wt-%.

Depending on which process is used in order to obtain said gaseous mixture containing melamine, ammonia and carbon dioxide, this gaseous mixture has a temperature between 300 ⁰ C and 500 ⁰ C, preferably 36O ⁰ C to 440 ⁰ C, and more preferred 400 ⁰ C to 430 ⁰ C, and a pressure between 3 and 150 bar. The gaseous mixture has a pressure at the lower end of the above range if a catalytic low pressure process is used for making melamine from urea, and has a pressure of preferably 45 to 150 bar if a non-catalytic high pressure process is used for providing the gaseous mixture.

In the quenching step a) according to the present invention the gaseous mixture is quenched with an aqueous ammonium carbamate containing solution as defined above, and thereby cooled. The quenching step can be conducted as a single step or as a multiple step. Thereby the aqueous ammonium carbamate containing solution can be brought into contact with the gaseous mixture either in cocurrent or countercurrent manner. It is also possible to contact the gaseous mixture in a multiple quenching step first in a cocurrent manner with the aqueous ammonium

carbamate containing solution and in a second quenching step in a countercurrent manner with the aqueous ammonium carbamate containing solution.

It is preferred to carry out the quenching step a) as fast as possible. The quench pressure is equal or lower than the reactor pressure.

The aqueous melamine containing slurry or solution as obtained in step a) has preferably a temperature of 100 ⁰ C to 179°C, more preferred of 121 ⁰ C to 169°C. The molar ratio of ammonia to carbon dioxide in said aqueous melamine containing slurry or solution obtained in step a) is preferably at least 2.1 , more preferred at least 2.5, even more preferred at least 3.0, and most preferred at least 3.4. Usually the molar ratio of ammonia to carbon dioxide in said aqueous melamine containing slurry or solution does not exceed 14 and is preferably less than 10 and more preferred less than 7. Higher ratios are not preferred since they result in the necessity of high pressure and thus increased process cost.

The pressure in the quenching step, of course, depends on the pressure of the melamine containing gaseous mixture that is provided to the quenching step and thus depends on whether the low pressure catalytic process is used for the production of melamine, or whether a non-catalytic high pressure process is used. Thus, the pressure can vary in a wide range and is preferably between 3 and 45 bar, more preferred between 14 and 45 bar, and most preferred between 17 to 35 bar.

The thus obtained aqueous melamine containing slurry or solution is subsequently cooled in a cooling step b) to a temperature of 80 ⁰ C to 120 ⁰ C. Cooling means that the temperature of the melamine containing slurry or solution obtained in quenching step a) is reduced in cooling step b) which is the common understanding of the term "cooling" by a person skilled in the art. Preferably the melamine containing slurry or solution is cooled to a temperature between 86°C and 115°C and more preferred 90 ⁰ C to 110°C. Cooling causes further precipitation of solid melamine. Thus, in case in quenching step a) a melamine containing slurry is obtained cooling causes further precipitation of solid melamine. In case in quenching step a) a melamine solution is obtained cooling

causes the formation of a melamine containing slurry by precipitation of solid melamine.

The cooling in step b) may be either effected by mixing the melamine containing slurry or solution obtained in the quenching step with a coolant or cooling by heat exchange and/or by depressurization. Thus, the cooling is preferably conducted by mixing and/or depressurization at a pressure of 0.5 to 8 bar, preferably 0.8 to 4 bar, most preferred 0.9 to 2 bar.

As a coolant recycled mother liquor obtained in the separating step d) of the process according to the present invention can be used. Optionally, at least a part of the recycled mother liquor is purified prior to recycling to the cooling step b). But this purification step is not mandatory in order to achieve the desired low level of oxygen containing impurities. If, however, extraordinarily high purity is required a purification step can be added.

The aqueous melamine containing slurry obtained from cooling step b) is preferably further concentrated by means known to a person skilled in the art, like a hydrocyclone to effect further precipitation of solid melamine.

Either the melamine slurry obtained in cooling step b) or the concentrated melamine containing slurry obtained in concentrating step c) is subjected to a separation step in order to recover solid melamine and mother liquor that may be recycled to cooling step b). Any known methods for solid/liquid separation, like centrifugation or filtration, may be used.

The thus obtained solid melamine already fulfills the purity requirements without any further purification steps, like recrystallization or treatment with discoloring agents, like carbon black. Surprisingly, the solid melamine obtained by the process according to the present invention has a very low content of oxygen containing impurities, like ammeline, ammelide or cyanuric acid.

According to a particularly preferred embodiment of the present invention the process for recovering solid melamine from a gaseous mixture containing

melamine, ammonia and carbon dioxide is combined with a process for the production of melamine that comprises:

A) reacting molten urea in a reaction section at a pressure of 45 to 150 bar in a non-catalytic reaction to produce a reaction mixture containing molten melamine and reaction off-gases containing ammonia and carbon dioxide; and

B) vaporizing the molten melamine in said reaction mixture in a vaporization section to produce a gaseous mixture comprising the vaporized melamine and the reaction off-gases at a pressure of 45 to 150 bar.

According to the process of said preferred embodiment of the present invention melamine is produced using urea as raw material. The urea is fed as a melt into a reaction section and is reacted at a pressure of 45 to 150 bar, preferably 50 to 80 bar at elevated temperature to form melamine and the by-products ammonia and carbon dioxide in accordance with the above mentioned reaction equation. The reaction conditions in the reaction zone are selected in order to obtain melamine in the liquid state. The reaction temperature is preferably 360 to 440 ⁰ C, more preferred 400 ⁰ C to 43O ⁰ C, even more preferred 401 ⁰ C to 419°C.

The thus obtained reaction mixture comprising molten melamine and the gaseous reaction by-products carbon dioxide and ammonia, is then subjected to a vaporization step whereby without separating the molten melamine from the gaseous reaction by-products, the melamine is vaporized in order to form a gaseous mixture comprising the vaporized melamine and the reaction offgases.

Vaporization can be achieved by any means known to the person skilled in the art, like increase of temperature or reduction of pressure. Preferably the pressure is kept approximately constant but vaporization is achieved by feeding ammonia into the vaporization section in order to reduce the partial pressure of melamine, thereby vaporizing the melamine. In accordance with the preferred embodiment of the present invention vaporization of the melamine can be achieved by feeding ammonia in an amount of 0.5 to 3 kg ammonia/kg urea to the vaporization section, preferably 1.05 to 1.9 kg ammonia/kg urea at an evaporator temperature between 401 ⁰ C and 419°C. Thus, only limited amounts of ammonia are needed,

resulting in smaller volumes of ammonia to be processed in process steps subsequent to the vaporization step of the present invention. Thus, smaller process units are sufficient to handle the ammonia streams generated in the process of the present invention, thereby further improving the economics of the present process. Preferably the melamine content in the gaseous phase from the evaporator is lower than the saturation pressure of melamine at the prevailing process conditions.

In general, the reaction step A) and the evaporation step B) according to the present invention can be conducted in different vessels, but it is preferred to conduct these process steps in different sections of the same vessel. Preferably the reactor/evaporator contains a draught tube for improving the contact between the melamine liquid and the ammonia gas. The ammonia may be split to the different sections, irrespective whether they are part of the same or different vessels.

The gaseous mixture obtained from the vaporization step B) comprising melamine, carbon dioxide and ammonia is directed to a cooling unit wherein the gas mixture is quenched by contact with an aqueous ammonium carbamate solution. The pressure in the quenching step a) is preferably at least 5 bar lower than in the evaporation step B). In order to achieve an even faster quenching it is preferred that the pressure in the quenching step is less than 75%, even more preferred less than 60% of the pressure in the evaporation step B). As will be discussed in more detail below with respect to the condensation/absorption step in accordance with a preferred embodiment of the present invention the pressure in the quenching step a) is preferably at least 16 bar, more preferred at least 19 bar, and most preferred at least 22 bar.

In the cooling unit upon quenching the gaseous reaction product a liquid aqueous phase comprising melamine and a gaseous phase comprising water, ammonia and carbon dioxide, is formed.

After quenching the liquid phase is separated from the gaseous phase for further processing.

The gaseous phase comprising water, ammonia and carbon dioxide separated from the quenching step is preferably directed to a condensation/absorption step d) where the gaseous phase is at least partially condensed, optionally in presence of additional water to form a concentrated aqueous ammonium carbamate solution and a gaseous fluid comprising ammonia. For example, additional water, optionally together with carbon dioxide and ammonia, may be introduced into the condensation/absorption step from stripping the aqueous ammonium carbamate solution in the work-up section to isolate solid melamine.

The pressure in the condensation/absorption step C) is virtually the same as in the quenching step a). Higher pressures in the absorption condensation step C) are preferred in order to produce higher concentrated ammonium carbamate solutions that can be directly used without any intermediate concentration steps in a urea plant. Thus it is preferred to operate the quenching step a) and thus the condensation and absorption step C) at a pressure of at least 16 bar, preferably at least 19 bar, and more preferred at least 22 bar, as already mentioned above.

Furthermore, it is an advantage of the process of the present invention that the pressure drop between the evaporation step B) and the quenching step a) can be adjusted to find an optimum balance between fast quenching (achieved by high pressure differences between the evaporation step and the quenching step) and high concentration of the ammonium carbamate solution obtained in the absorption condensation step C) (achieved by high pressure in the condensation absorption step). Thus, the process according to the present invention provides a highly concentrated ammonium carbamate solution that can be directly introduced into a urea plant without a further concentration step. Furthermore, part of the obtained highly concentrated ammonium carbamate solution is preferably recycled to the quenching step a) in order to obtain in the quenching step a) an aqueous melamine containing slurry or solution comprising the high level of ammonia and carbon dioxide as required by the present invention. The concentrated carbamate solution obtained in the condensation/absorption step contains less than 50 wt.-% water, preferably less than 30 wt.-% water.

The gaseous effluent from the condensation/absorption step C) consists essentially of ammonia and can be after optional separation or purification steps and after repressurization recycled to the reactor/evaporation unit. The gaseous ammonia from the condensation/absorption section may be condensed partially and used as a reflux to increase the purity of the gaseous ammonia. Virgin liquid ammonia may also be used as an absorption liquid for purification of ammonia gas.

The present invention will be discussed in more detail with reference to the following examples:

EXAMPLE 1

Melamine was produced from urea melt (1.4 t/h, 140 ⁰ C) at 55 bar in a combined liquid-phase reactor/evaporator, which was heated with molten salt. The liquid melamine was evaporated at 422 ⁰ C by introducing 1.6 t/h ammonia of 330 ⁰ C. The gas from the reactor/evaporator (containing mainly ammonia, CO ₂ and melamine vapor) was quenched rapidly with recycled aqueous carbamate solution (from the hydrocyclone and from the off-gas condensation) in a quenching tower at a temperature of 138 ⁰ C and a pressure of 19 bar. A melamine slurry in aqueous carbamate solution (CS1 ) and quench off-gas were produced. The quench off-gas was sent to the absorption column operating at almost the same pressure as the quenching tower. In the absorption zone water and CO ₂ was removed from the quench off-gas by partial condensation and by washing with liquid ammonia producing an aqueous carbamate solution (CS2) as a bottom stream and ammonia gas as a top stream. Part of the aqueous carbamate solution was returned to the quenching tower and used as a cooling agent. Water was mixed with this carbamate solution before returning to the quenching tower to balance the water export.

The composition of CS1 was: 23 wt-% ammonia, 12 wt-% carbon dioxide, 13 wt-% dissolved melamine and byproducts, the balance being water. The melamine slurry was concentrated in a hydrocyclone and cooled to 100 ⁰ C by mixing with a saturated melamine mother liquor of 95 ⁰ C recycled from the melamine filtration. After depressurization to atmospheric conditions the melamine

was filtered and dried. The melamine has less than 0.025 wt-% ARC (ARC = ammeline + ammelide + cyanuric acid).

EXAMPLE 2 Melamine was produced from urea melt (1.4 t/h, 140 ⁰ C) at 55 bar in a combined liquid-phase reactor/evaporator, which was heated with molten salt. The liquid melamine was evaporated at 422 ⁰ C by introducing 1.6 t/h ammonia of 330 ⁰ C. The gas from the reactor/evaporator (containing mainly ammonia, CO ₂ and melamine vapor) was quenched rapidly with recycled aqueous carbamate solution (from the hydrocyclone and from the off-gas condensation) mixed with additional water in a quenching tower at a temperature of 138 ⁰ C and a pressure of 19 bar. A melamine slurry in aqueous carbamate solution (CS) and quench off-gas were produced. The composition of CS was: 23 wt-% ammonia, 12 wt-% carbon dioxide, 13 wt-% dissolved melamine and byproducts, the balance being water. The melamine slurry was concentrated in a hydrocyclone and cooled to 86 ⁰ C by mixing with a saturated melamine mother liquor of 81 ⁰ C recycled from the melamine filtration. The melamine was filtered and dried. The melamine has less than 0.025 wt-% ARC (ARC = ammeline + ammelide + cyanuric acid).

EXAMPLE 3

Melamine was produced from urea melt (1.4 t/h, 140 ⁰ C) at 55 bar in a combined liquid-phase reactor/evaporator, which was heated with molten salt. The liquid melamine was evaporated at 422 ⁰ C by introducing 1.6 t/h ammonia of 330 ⁰ C. The gas from the reactor/evaporator (containing mainly ammonia, CO ₂ and melamine vapor) was quenched rapidly with recycled aqueous carbamate solution (from the hydrocyclone and from the off-gas condensation) mixed with additional water in a quenching tower at a temperature of 136 ⁰ C and a pressure of 26 bar. A melamine slurry in aqueous carbamate solution (CS) and quench off-gas were produced. The composition of CS was: 29 wt-% ammonia, 22 wt-% carbon dioxide, 13 wt-% dissolved melamine and byproducts, the balance being water. The melamine slurry was concentrated in a hydrocyclone and cooled to 83 ⁰ C by mixing with a saturated melamine mother liquor of 78 ⁰ C recycled from the melamine filtration. The melamine was filtered and dried. The melamine has less than 0.025 wt-% ARC (ARC = ammeline + ammelide + cyanuric acid).

COMPARATIVE EXAMPLE A

Melamine was produced from urea melt (1.4 t/h, 140 ⁰ C) at 55 bar in a combined liquid-phase reactor/evaporator, which was heated with molten salt. The liquid melamine was evaporated at 422 ⁰ C by introducing 1.6 t/h ammonia of 330 ⁰ C. The gas from the reactor/evaporator (containing mainly ammonia, CO ₂ and melamine vapor) was quenched rapidly with recycled aqueous carbamate solution (from the hydrocyclone and from the off-gas condensation) mixed with additional water in a quenching tower at a temperature of 120 ⁰ C and a pressure of 7 bar. A melamine slurry in aqueous carbamate solution (CS) and quench off-gas were produced. The composition of CS was: 10 wt-% ammonia, 2.6 wt-% carbon dioxide, 8.7 wt-% dissolved melamine and byproducts, the balance being water. The melamine slurry was concentrated in a hydrocyclone and cooled to 70 ⁰ C by mixing with a saturated melamine mother liquor of 65 ⁰ C recycled from the melamine filtration. The melamine was filtered and dried. The melamine has 0.064 wt-% ARC (ARC = ammeline + ammelide + cyanuric acid).

COMPARATIVE EXAMPLE B

Melamine was produced at 400 ⁰ C and 10 bar by introducing urea melt (1.4 t/h, 140 ⁰ C) and ammonia (2.6 t/h at 330 ⁰ C) in gas fluidized-bed reactor. The gas from the reactor (containing mainly ammonia, CO ₂ and melamine vapor) was quenched rapidly with recycled aqueous carbamate solution (from the hydrocyclone and from the off-gas condensation) mixed with additional water in a quenching tower at a temperature of 145 ⁰ C and a pressure of 10 bar. A melamine slurry in aqueous carbamate solution (CS) and quench off-gas were produced. The composition of CS was: 9.5 wt-% ammonia, 1.5 wt-% carbon dioxide, 13 wt-% dissolved melamine and byproducts, the balance being water. The melamine slurry was concentrated in a hydrocyclone and cooled to 85 ⁰ C by mixing with a saturated melamine mother liquor of 80 ⁰ C recycled from the melamine filtration. The melamine was filtered and dried. The melamine has 0.054 wt-% ARC (ARC = ammeline + ammelide + cyanuric acid).

COMPARATIVE EXAMPLE C

Melamine was produced at 400 ⁰ C and 23 bar by introducing urea melt (1.4 t/h, 140 ⁰ C) and ammonia (2.6 t/h at 330 ⁰ C) in gas fluidized-bed reactor. The gas from the reactor (containing mainly ammonia, CO ₂ and melamine vapor) was quenched rapidly with recycled aqueous carbamate solution (from the hydrocyclone and from the off-gas condensation) mixed with additional water in a quenching tower at a temperature of 158 ⁰ C and a pressure of 23 bar. A melamine slurry in aqueous carbamate solution (CS) and quench off-gas were produced. The composition of CS was: 18 wt-% ammonia, 5 wt-% carbon dioxide, 20 wt-% dissolved melamine and byproducts, the balance being water. The melamine slurry was concentrated in a hydrocyclone and cooled to 70 ⁰ C by mixing with a saturated melamine mother liquor of 65 ⁰ C recycled from the melamine filtration. The melamine was filtered and dried. The melamine has 0.061 wt-% ARC (ARC = ammeline + ammelide + cyanuric acid).