A multiplicity of processes for the production of melamine are already known from the literature. A preferred starting material is urea which is converted to melamine, ammonia and C02 either at high pressure and non-catalytically or at low pressure and using a catalyst. It is also known that in these processes, in particular in the high-pressure processes, various byproducts or contaminants, such as melem, melam, ammeline, ammelide or ureidomelamine are formed, which impair the purity of the melamine.
Melamine produced by a high-pressure process is worked up, for example according to US 3,116,294 (Montecatini), by separating off the C02 and NH3 off-gases from the liquid melamine which is then treated in counter-current with NH3 in order to remove still dissolved C02, collected in a further reactor and allowed to remain therein for a defined time. The separation column and the second reactor are held in this case under the same conditions of temperature and pressure as the urea reactor, that is at 250 to 500°C and a pressure of about 40 to 150 bar. The melamine thus obtained is removed from the second reactor and cooled rapidly by quenching with water or by mixing with cold gases. However, the purity of melamine which is obtained by this process is insufficient for many applications, for instance in the production of melamine-formaldehyde resins for surface coatings, since, in particular, the melem content is too high. As is known from CH 345.894, contaminants such as melem and melam are insoluble or slightly soluble in water and aqueous alkali solutions, so that these compounds can be removed only with difficulty, and with melamine losses from the melamine to be purified, in a recrystallization of melamine, for example following the workup according to Montecatini.
The object of the present invention was therefore to find a process in which the content of contaminants, in particular of the contaminants which are insoluble or
slightly soluble in water or in aqueous alkali solutions, can be markedly reduced without melamine losses.
Unexpectedly, it was possible to achieve this object by a process in which contaminated melamine is allowed to remain for a certain time at an ammonia partial pressure of above 150 bar at a temperature of above 280°C and then rapidly cooled.
The present invention therefore relates to a process for the purification of melamine which comprises allowing contaminated melamine to remain for 5 minutes up to 20 hours at an ammonia partial pressure of 150 to 400 bar within a temperature range from 280 to 430°C, whereupon the reaction vessel is first rapidly cooled to room temperature and then depressurized or simultaneously cooled and depressurized and pure melamine is obtained in powder form.
The process according to the invention is suitable for the purification of melamine which arises in any process known from the prior art and contains, in particular, contaminants such as melem and melam. The melamine to be purified is, in the case that it occurs in crystalline form or as powder, first heated to a temperature within the temperature range of 280 to 430°C, preferably to or above the melting point of melamine up to 400°C, and allowed to remain within this range at an ammonia partial pressure of above 150 bar for a defined time. If the melamine to be purified is already present as a melt or as a liquid phase, such as following a high-pressure reactor for melamine synthesis by means of urea conversion, this heating-up phase can be dispensed with and the temperature of the melt is brought to the desired value within the above defined temperature range.
The temperature of the melamine to be purified can be held constant during the residence time of the melamine, but it can also be changed within the above defined temperature range. Thus the temperature can be decreased or increased within the limits of the temperature range, for example continuously or stepwise. The residence time can be varied in a wide range. It princi-
pally depends on the desired final value of contaminants, and on the ammonia partial pressure and on economic factors and is between 5 minutes and 20 hours, preferably between 10 minutes and 10 hours, particularly preferably between 30 minutes and 4 hours. Longer residence times are also possible if desired. The ammonia partial pressure can likewise vary in a broad range and is between 150 and 400 bar. Preferably, an ammonia partial pressure up to 360 bar, particularly preferably up to 300 bar, is set. It is also possible to vary the pressure during the residence time. Furthermore, it is possible to admix other gases, e.g. nitrogen, which are inert under the reaction conditions. After completion of the residence time, depending on the technical conditions, the reaction vessel can first be cooled and then depressurized or simultaneously depressurized and cooled. The cooling to room temperature is performed, for example, by quenching with a cold liquid medium, for instance by means of water or liquid ammonia or by mixing with cold gases.
Melamine is obtained in crystalline form or as powder by the process according to the invention and has, in particular, a markedly reduced content of melem and melam.
The process according to the invention can be carried out both discontinuously and continuously.
The process according to the invention can be coupled to any melamine process known from the prior art. In a preferred embodiment, the process according to the invention is combined with one of the high-pressure processes known from the prior art, such as described in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, vol. A-16, pp 174-179. Particularly preferably, the process according to the invention is carried out subsequently to already known work-up steps of the high-pressure processes. These work-up steps include
a) separating off from the liquid melamine the NH3/C02 gas
mixture obtained in the urea conversion, and
b) reduction of the C02 dissolved in the melamine by
introducing NH3.
These work-up steps are described, for example, in US-3,116,294. The process according to the invention can thus be connected directly to the reactor for the urea conversion or be carried out subsequently to the above described work-up steps, in which case the work-up can include not only the steps a) in combination with b), but also only one of these steps. Examples 1-7
A defined amount (SW) of melamine having a defined initial content of contaminants and the amount of ammonia necessary to maintain a defined pressure p were introduced into a mini autoclave having a volume of 10 ml.
The autoclave was then rapidly heated to a temperature T by immersion in a thermal transport medium and allowed to remain at this temperature for t minutes. The autoclave was then rapidly cooled by immersion in cold water and then depressurized. The melamine thus purified was analyzed for the final content of contaminants, in particular melem.
The process parameters such as sample weight of melamine (SW), pressure p, time t, temperature T, initial content of melem (MEo), Melam (MAo), ammeline (ANo), ammelide (ADo), and ureidomelamine (UMo), and the final contents of melem (ME), melam (MA), ureidomelamine (UM) and in some cases of ammeline (AN) and ammelide (AD) can be seen in Table 1.
(Table Removed)

We Claims
1. A process for the purification of melamine which comprises allowing contaminated melamine to remain for 5 minutes up to 20 hours at an ammonia partial pressure of 150 to 400 bar within a temperature range from 280 to 430°C, whereupon the reaction vessel is first rapidly cooled to room temperature and then depressurized or simultaneously cooled and depressurized and pure melamine is obtained in powder form.
2. The process as claimed in Claim 1, wherein melamine is allowed to remain within a temperature range between the melting point of melamine and 400°C.
3. The process as claimed in Claim 1, wherein the ammonia partial pressure is set at up to 360 bar.
4. The process as claimed in Claim 1, wherein the residence time is 10 minutes up to 10 hours.
5. The process as claimed in Claim 1, wherein the process is connected to a process for the production of melamine.
6. The process as claimed in Claim 1, wherein the process is connected to a high-pressure process for the production of melamine from urea.
7. The process as claimed in Claim 1, wherein the process is connected downstream of the workup of a high-pressure process, where the workup includes the steps
a) separating off from the liquid melamine the NH3/C02 gas
mixture obtained in the urea conversion and/or
b) reduction of the C02 dissolved in the melamine by
introducing NH3.
8. A process for producing pure malamine substantially as herein described with reference to the foregoing examples.