Technical field The present invention refers to a process for the purification of melamine manufactured by synthesis from urea, according to a high pressure, non-catalytic process, and more particularly a process which is able to transform certain impurities, resulting from melamine synthesis or subsequent manipulations, essentially into Background art It is well known that in the high pressure melamine synthesis from urea a series of by-products is fonned which affects melamine purity.

Particularly in the reactor, during melamine synthesis, and subsequently in the containers wherein melted melamine resides at a temperature higher than 350°C, typical melamine deammoniating condensation products are fonned in a large amount. This category of condensation products are hereinafter collectively referred to as polycondensates The simplest product is melam resulting from two melamine molecule condensation and one ammonia molecule separation.

Melamine Melam From two melamine molecules another product can be formed by separation of tango ammonia molecules: <BR> <BR> MdamineMelem.

Another by-product resulting from melamine deammoniating condensation is melon:

Melon Generally liquid melamine (and therefore at a temperature higher than melting temperature of 350°C), both inside reaction vessel and during the melamine residence in downstream treatment equipment, is subject to a chemical condensation associated with ammonia separation resulting in the formation of abovementioned products or even more complex ones collectively referred to as polycondensates. Polycondensates formation increases when ammonia partial pressure is lower and residence time of melted melamine is longer. These products are undesired for melamine use in the manufacture of formaldeh) de/melamine condensation resins, therefore in the final melamine product a polycondensates content lower than 1000 ppm is requested as a minimum acceptable content.

Fortunately the rections leading to these by-products are equilibrium reactions.

Therefore melamine polluted by polycondensates may be purified, by subjecting said melamine to an ammonia action under suitable temperature and pressure conditions.

Then, the ammonia action not only results in a polycondensates elimination, but due to the polycondensate transformation into melamine, an increase of global yield is also obtained.

However the operative conditions necessary to reach the above results are quite heavy and expensive in that they imply to subject melted melamine to an ammonia partial pressure higher than 250 bars (2,5xl07 Pa), preferably above 400 bars (4x107 Pa), in order to obtain a melamine of an acceptable purity.

On the other hand the transformatioz of polycondensates to melami medium requires less severe and less expensive operating conditions.

In the industrial practice all melamine synthesis processes include a recovery and purification step of raw melamine coming out of the synthesis reactor in an aqueous medium.

Said process consists in sending the reaction product, comprising a raw melamine liquid phase and an essentially NH3-and CO2-containing gas phase, to an appropriate contacting equipment (quench tower) wherein the product is cooled down by means of water. Pressure is simultaneously reduced from reaction pressure (higher than 70 bars- 7x106 Pa) to a pressure of about 25bars (2, Sx106 Pa). and at a temperature lower than 165 °C to limit both corrosion problems and the amount of steam entrained in vapour phase.

As a result of the above contact, a gas phase containing NH3, C02 and steam and a liquid aqueous phase which contains melamine, polycondensates, unreacted urea as well as minor impurities are obtained. From the liquid phase, melamine is recovered, through several manipulation and crystallisation steps.

Typically, said solution has a melamine concentration of 5 to 15% w, and contains also some unreacted urea, dissolved gases as NH3 and C02 as well as other minor impurities including the polycondensates. The latter are present in the amount of 1 : 30 to 1: 70 with respect to dissolved melamine.

By subjecting this solution to ammonia action, the polycondensates transformation into melamine takes places ; however such reaction proceeds with a very low rate requiring a very long contact time to reach a polycondensates transformation degree matching the required purity specifications.

For this reason in the commercial plants, the liquid solution coming out of the quenching tower is kept as such, with or without any ammonia addition for a suitable period of time which may be of one to several hours. Said technique requires the use of large treatment volumes that means, taking into consideration the temperature and pressure conditions and the corrosive characteristics of the products under treatment, the use of special stainless steels, high apparats thickness and, therefore, higher investment costs. Moreover in the course of the above treatment an undesirable loss of product takes place because of the contemporaneous melamine hydrolysis reaction in presence of water, which loss Is higher, the longer is the treatment time necessary to reduce the polycondensates to an acceptable value.

There is not, at this moment, any available melamine purification process capable to reduce the impurity contents, particularly the by-products originated from the melamine deammoniating condensation, which do not require a melamine treatment under severe temperature and pressure conditions or capable to reduce the reaction time to an economically acceptable value. The present invention intends to solve the abovementioned problems Disclosure of the invention It has been surprisingly found that the conversion rate of polycondensates to melamine in an aqueous environment in presence of ammonia, which, as we saw, is very low, may considerably increase if the C02 content of the aqueous solution deriving from the water quenching of the reaction products is reduced to a value lower than 0.5 and preferably lower than 0.2 % wt.

Detailed description of the invention It is therefore a first object of the present invention a process for the purification of melamine, deriving from urea, according to a high pressure non-catalytic process, by means of polycondensates and other by-products transformation, which by-products derive from the melamine synthesis, comprising the following steps: -a) dissolving in water the raw liquid melamine coming out of the synthesis reactor and containing polycondensates and other melamine synthesis by-products; -b) subjecting such solution to a treatment capable to remove the dissolved CO2, limiting its concentration to a value lower than 0.5% wt, preferably lower than 0. 2% wt ; -c) reacting the obtained solution with ammonia in an amount in a range of 5 to 25%, preferably 12 to 15% by weight at a temperature in the range of 130 to 250CC, preferably 160 to 180°C, and keeping the mixture to react for some minutes; -d) collecting the solution having a polycondensates concentration lower than 1000 ppm, typically lower than 100 ppm.

The C02 content reduction, in the second step b), may preferably be obtained by stripping the solution with a gas that is inert with respect of the solution components.

Preferably the solution stripping gas is steam, which does not leave any extraneous component.

As a matter of fact by submitting raw melamine solution, coming out of quenching tower, to a steam stripping action, ammonia and COs are almost completely removed.

The operation is carried out in such a way as to reduce as much as possible any residual C02 concentration in the melamine solution and, in any case, to a C02 concentration lower than 0.5% by weight, preferably lower than 0.2% based on the total weight of the melamine solution.

By submitting raw melamine solution, coming out of quenching tower, to ammonia action, in an amount of 5 to 25% wt preferably 12 to 15% wt, at a temperature of 130 to 250°C, preferably 160 to 180°C and under the resulting equilibrium pressure, polycondensates originally present in the solution completely disappear in a few minutes of reaction and are not detected when analysed with ultraviolet spectrophotometer in the melamine final product (analtical limit < l Oppm).

From the abovesaid it results that is very convenient to submit the solution of raw melamine to the action of ammonia when C02 is practically absent or the relevant amount is very low (less than 0.5% wt or preferably less than 0.2% wt).

Best mode to carry out the invention The following examples show the extent of the advantages deriving from the practice of this invention, but are not to be interpreted as restricting the scope of the invention.

Example 1 Melamine non-catalytic synthesis from urea, according to the high pressure process is carried out in an internal circulation reaction vessel operating at 390'C and 8. Oxl06Pa (80 bars).

The reaction product coming out of the reactor consists of a liquid phase and a gas phase having the following compositions (by weight) : Gas phase (about 65% of total products coming out of the reactor): NH3 46.5% C02 50% Melamine (vapour) 3.5% Liquid phase (about 35% of total products coming out of the reactor): Melamine 88% OAT (oxyaminotriazines) 2.7% Polycondensates 2.2% Dissolved NH3 1. 3% Dissolved C02 0.8% Unreacted urea 5%

Both liquid and gas phases are contacted with water in the quenching. tower under 2.5xlO6Pa gauge (25 bars gauge) and at 162°C.

From quenching tower a gas phase containing water, NH3, and C02 is collecte with the following composition (by weight) : H20 20% C02 38% NH3 42% And a solution phase which contains all melamine having the following composition by weight : H20 68.5% Melamine 10% OAT 0. 3% Polycondensates 0.23% Urea 0.47% NH3 16. 7% C02 3.8% The above solution is subject to stripping with steam under the following conditions: 5 bars (5x105 Pa gauge) and 160°C at bottom column. The solution recovered from the bottom of the column has the following composition (by weight) : H20 88.7% Melamine 9.5% OAT 0.32% Polycondensate 0. 24% Urea 0.4% NH3 0.51 % C02 0.33% The above water solution is added with ammonia up to a concentration higher than 13% by weight and then is left to react in a suitable column under 2.5xlO6Pa gauge (25 bars) and 172°C. Residence time of the above solution added with ammonia was about 15 minutes and the polycondensates concentration at the column outlet was lower than the minimum detectable amount by the selected analytical method.

Since the minimum detectable value of the analytical method, based on the Ultraviolet spectrophotometric technique, is lOppm with respect to the amount of melamine, it

derives that with the inventive method a melamine solution is obtained, practically free from polycondensates.

Example 2 (comparative) Same ammonia treatment was made on the raw melamine solution of Example 1 before the steam stripping operation (i. e. in presence of CO2 in an amount corresponding to the conditions of quenching tower). Residual polycondensates concentration in the melamine product was clearly analytically detected and its concentration was always over 2000 ppm with respect to melamine, even after some hours of ammonia treatment at the conditions specified in the preceding example.