A multiplicity of processes for the production of melamine are already known from the literature. A preferred starting material in this case is urea which is converted to melamine, ammonia and CO2 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 by-products or contaminants such as melem, melam, ammeline, ammelide or ureidomelamine are formed which impair the melamine purity.
However, purification methods conventional to date, such as recrystallization, suffer from disadvantages. Thus, particularly in recrystallization, there is the disadvantage that the contaminants accumulate in the mother liquor and therefore a considerable portion of the same, which also contains melamine, must be discarded or given an after-treatment. Furthermore, it is described in CH 345.894 that contaminants such as melem, melam or ureidomelamine are insoluble or sparingly soluble in water and aqueous alkali solutions so that these compounds can be removed by recrystallization only with difficulty and with losses of melamine from the melamine to be purified.
The object of the present invention was therefore to find a process in which the contaminants, in particular melem and ureidomelamine, which are insoluble or sparingly soluble in water or in aqueous alkali solutions can be considerably reduced in the melamine to be purified, while avoiding high melamine losses.
Unexpectedly, it was possible to achieve this object by a process in which contaminated solid melamine is held for a certain time in the presence of ammonia at an elevated temperature which is below the melting point of melamine.
The present invention therefore relates to a process for the purification of melamine, which comprises bringing the contaminated melamine to a temperature which is above 200°C and below the melting point of melamine/ which depends on the applied arrmonia pressure and holding the melaine which

is present as a solid in this temperature range for a period of 1 minute up to 20 hours at an ammonia pressure of 5 to 350 bar, whereupon, in any desired sequence, the reaction mixture is depressurized and cooled to room temperature 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, in particular, contains contaminants such as melam and ureidome1amine. It has been found that, in particular, the content of melam and ureidomelamine can be decreased by heating the contaminated melamine which is present in crystalline form or as powder and holding it in the presence of ammonia for a certain time in a temperature range below
the melting point of melamine which depends on the applied ammonia pressure (tempering).However, it is also possible to cool to this temperature range melamine which has a temperature above this range. The temperature range in which the tempering is carried out is in this case above 200°C and below the melting point of melamine. Preferably, the temperature range is between 240°C, particularly preferably between 250°C, and below the melting point of melamine. The melamine is brought in this case to an initial temperature which is in this range. The temperature can be kept constant during the tempering, 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 discontinuously. The period for which the melamine is kept in this temperature range can vary in a wide range. It principally depends on the desired final value of melam and ureidomelamine and on the particular ammonia pressure and on economic factors and is between 1 minute and 20 hours, preferably between 10 minutes and 10 hours, particularly preferably between 0.5 and 3 hours. The ammonia pressure can likewise- vary in a wide range and is between 5 and 350 bar. Preferably, a pressure between 10 and 100 bar is set. After ending the tempering, depending on the technical

conditions, the reaction mixture can first be cooled and then depressurized or, in reverse sequence, first depressurized and then cooled. These steps can, if appropriate, be carried out in a further reaction apparatus. Melamine is in this context cooled to room temperature, for example using heat exchangers, simple removal of the heating medium and allowing the medium to stand 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 melam and ureidomelamine which is sometimes even below the detection limit.
The process according to the invention can be carried out both discontinuously and continuously.
The process according to the invention can also 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 subsequent to previously known work-up steps of the high-pressure processes. These work-up steps comprise
a) separating off the NH3/C02 gas mixture obtained in the urea conversion from the liquid melamine, and
b) reducing the C02 dissolved in the melamine by introducing NH3, and
c) allowing the liquid melamine to remain in the presence of ammonia and
d) cooling the melamine, for example by quenching with cold water or liquid ammonia, or by mixing with cold gas. These work-up steps are described, for example in US 3,116,294.
The process according to the invention is carried out in this case subsequent to step d), the number of the preceding work-up steps a) to c) carried out being able to be varied according to the particular conditions. The work-up of the melamine produced from urea by a high-

pressure process up to the cooling phase d) can thus include the steps a) to c) , and any combination of two or else only one of these steps. Examples 1-20
A defined amount (SW) of melamine having a starting content of 18,300 ppm of melam and 10,600 ppm of ureidomelamine 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 brought to a temperature T and held 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 content of melam and ureidomelamine, the content of ureidomelamine always being below the detection limit of 100 ppm. The process parameters such as initial sample weight of melamine (SW), pressure p, time t, temperature T, final content of melam (MA) can be seen in Table 1.

Table 1
(Table Removed)

Examples 21-26:
A defined amount (SW) of melamine having a starting content of MA0 of melam and UM0 of ureidomelamine and the amount of ammonia necessary to maintain a defined pressure p were introduced into an autoclave having a
volume of 2 1. The autoclave was then brought to a temperature T and held at this temperature for t minutes. The autoclave was then cooled and then depressurized. The melamine thus purified was analyzed for the content of melam (MA) and ureidomelamine (UM) . The process parameters such as initial sample weight of melamine (SW) , pressure p, time t, temperature T, initial content of melam (MA0) and of ureidomelamine (UM0) and final content of melam (MA) and of ureidomelamine (UM) can be seen in Table 2.
Table 2:

(Table Removed)

We Claim:

1. A process for the purification of melamine, which comprises bringing the contaminated melamine to a temperature which is above 200°C and below the melting point of melamine which depends on the applied ammonia pressure and holding the melamine which is present as a solid in this temperature range for a period of 1 minute up to 20 hours at an ammonia pressure of 5 to 350 bar, whereupon, in any desired sequence, the reaction mixture is depressurized and cooled to room temperature and pure melamine is obtained in powder form.
2. The process as claimed in claim 1, wherein the solid melamine is held within a temperature range between 240°C
and below the melting point of melamine, which depends on the applied ammonia pressure.
3. The process as claimed in claim 1, wherein the ammonia pressure is between 10 and 100 bar.
4. The process as claimed in claim 1, wherein the process follows a process for the production of melamine.
5. The process as claimed in claim 1, wherein the process follows a high-pressure process for the production of melamine from urea.
6. A process for the purification of melaraine substantially
as herein described with reference to the foregoing examples.