Claims
- 1. A process for the preparation of substantially pure melamine, which comprises rapidly depressurizing liquid, ammonia-containing melamine frm an ammonia partial pressure p1 between 400 and 50 bar to an ammonia partial pressure p2 between 200 bar and 20 bar pressure, where p1 is always greater than p2, at a temperature which is 0 to 60° C. higher than the melamine solidification point which is dependent on the particular prevailing ammonia partial pressure, but is below 350° C., higher pressures permitting a greater temperature interval from the melamine solidification point than lower pressures, whereby pure melamine separates out in solid form, whereupon, in any sequence, optionally further depressurizing the resultant product to atmospheric pressure, cooling to room temperature and isolating the substantially pure melamine.
- 2. The process as claimed in claim 1, wherein a melamine to be purified which is already present as melt or as liquid phase is brought to the ammonia partial pressure desired for the depressurization between about 400 to 50 bar, and to a temperature which is about 0 to 60° C. above the melamine solidification point dependent on the particular prevailing ammonia partial pressure.
- 3. The process as claimed in claim 2, wherein, in the process, the temperature is decreased.
- 4. The process as claimed in claim 2, wherein, in the process, the temperature is decreased at a cooling rate of 0.8 to 10° C./min.
- 5. The process as claimed in claim 2, wherein, in the process, ammonia is fed.
- 6. The process as claimed in claim 2, wherein the NH3/CO2 gas mixture forming in the urea conversion is separated off from the liquid melamine and the CO2 dissolved in the liquid melamine is reduced by introducing gaseous ammonia.
- 7. The process as claimed in claim 1, wherein, before the depressurization, the liquid melamine is allowed to dwell for about 5 minutes to 20 hours.
- 8. The process as claimed in claim 1, wherein, before the depressurization, the liquid melamine is saturated with ammonia.
- 9. The process as claimed in claim 1, wherein the depressurization of the liquid, ammonia-containing melamine is carried out as closely as possible to or above the solidification point of the melamine dependent on the particular ammonia partial pressure.
- 10. The process as claimed in claim 1, wherein the liquid, ammonia-containing melamine is depressurized to a pressure between 20 bar and about 200 bar.
- 11. The process as claimed in claim 1, wherein the depressurization is performed in the vessel into which the liquid melamine was introduced.
- 12. The process as claimed in claim 1, wherein the depressurization is carried out by transferring or spraying into one or more vessels.
- 13. The process as claimed in claim 11, wherein an ammonia atmosphere is present in the vessels.
- 14. The process as claimed in claim 12, wherein approximately the same temperature prevails in the vessels as in the receptacle from which depressurization is performed.
- 15. The process as claimed in claim 12, wherein, after the depressurization, the then solid melamine is allowed to dwell at a pressure between 200 bar and 20 bar for about 1 minute to 20 hours.
- 16. The process as claimed in claim 1, wherein, after the depressurization, the then solid melamine is allowed to dwell at a temperature below about 290 ° C.
- 17. The process as claimed in claim 1, wherein the then solid melamine is allowed to dwell at a temperature between about 280 and 250° C.
- 18. The process as claimed in claim 1, wherein the already solid melamine is cooled to room temperature by quenching with a cold, liquid medium.
- 19. The process as claimed in claim 1, wherein it is carried out continuously.
- 20. The process as claimed in claim 1, wherein it is carried out immediately downstream of a melamine high-pressure process.
- 21. The process as claimed in claim 1, wherein the melamine to be purified which is initially present as solid is heated at an ammonia partial pressure between about 400 and 50 bar, to a temperature which is about 0 to 60° C. above the melamine solidification point dependent on the particular prevailing ammonia partial pressure.
Priority Claims (1)
Number |
Date |
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Kind |
A 1994/95 |
Dec 1995 |
AT |
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Parent Case Info
[0001] This is a continuation of Ser. No. 09/612,941, filed Jul. 10, 2000, now abandoned, which is a continuation of Ser. No. 09/414,633, filed Oct. 8, 1999, now abandoned, which is a continuation-in-part of Ser. No. 09/011,183, filed Sep. 23, 1998, now abandoned, which is a 371 of PCT/EP96/05389, filed Dec. 4, 1996.
Continuations (2)
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09612941 |
Jul 2000 |
US |
Child |
09839199 |
Aug 2001 |
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Parent |
09414633 |
Oct 1999 |
US |
Child |
09612941 |
Jul 2000 |
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Continuation in Parts (1)
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09011183 |
Sep 1998 |
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09414633 |
Oct 1999 |
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