Such a process is known from WO 98/54160. In the process of WO 98/54160 an ammonia-containing melamine melt with a temperature between the melting point of melamine at the prevailing pressure and 450°C is sprayed via a spraying device and cooled using an evaporating cooling medium in a vessel in an ammonia environment, the melamine melt being converted into melamine powder (crystalline melamine) with a temperature between 200°C and the melting point of melamine, after which the ammonia pressure is relieved in the pressure adjustment step, and the powder is optionally cooled further. Keeping the crystalline melamine at a temperature above 200°C at an elevated pressure in an ammonia environment results in a reduction in the content of impurities such as melem and melam.

The process of WO 98/54160 further describes that the crystalline melamine is cooled, prior to the pressure adjustment step, to a temperature below 200°C by mechanically agitating and directly or indirectly cooling the crystalline melamine at a pressure between 4. 5 MPa and 25 MPa, because the colour of the crystalline melamine is adversely influenced if the melamine remains at a high temperature for too long.

A disadvantage of the known process is that mechanical agitation of the crystalline melamine at said high pressure, necessary in order to prevent an excessive adverse influence on the colour of the crystalline melamine, is technically complicated; as a result it is not always possible or desirable to carry this out.

It is therefore an object of the invention to provide an alternative process that also leads to an improvement of the colour of the crystalline melamine.

This object is achieved in that the pressure adjustment step is followed by: c) a dissolving step, in which the solid melamine is contacted with an aqueous phase

that has a pH between 6 and 13 and in which less than 5 wt. % ammonia is present, upon which a solution is formed, d) a crystallization step, in which the solution is cooled by at least 20°C, upon which crystalline melamine is formed, e) a separation step, in which crystalline melamine is separated from the solution.

The advantage of the process according to the invention is that crystalline melamine can be obtained that has a very good colour, suitable for virtually all applications. A very good colour means an APHA value lower than 20, preferably lower than 17, more preferably lower than 15. The APHA colour measurement is described inter alia in WO 98/55465, pages 4-6. In the APHA colour measurement crystalline melamine is processed into a melamine-formaldehyde resin with a molar formaldehyde-melamine (F/M) ratio of 3, use being made of a formaldehyde solution of 35 wt. %. The colour of the resin is calculated from the difference in the absorbance values between the wavelengths 380 nm and 640 nm. The APHA value is lower as less discoloration is present in the melamine.

In this description crystalline melamine is understood to mean melamine in crystalline form, in which the crystals may have all possible forms such as for example crystals consisting of a single unit or crystal agglomerates.

A further advantage of the process according to the invention is that the dissolving step and the crystallization step provide greater control of the particle size and particle size distribution of the crystalline melamine than the known process.

As a result the flow behaviour of the crystalline melamine can be controlled.

From US-3637686 a process for obtaining purified melamine is known in which a melamine melt is cooled with liquid ammonia so that solid melamine is formed. The solid melamine is then cooled further to a temperature between 100°C and 200°C at a pressure between 0.5 MPa and 10 MPa, use being made of an aqueous phase in which between 5 wt. % and 80 wt. % ammonia is present. US- 3637686 further discloses that it is possible to subsequently bring melamine into solution, with the object of converting impurities such as ammeline and ammelide into melamine.

In US-3637686 the ammonia content of the aqueous phase is higher than according to the invention, and preferably even between 20 wt. % and 50 wt. %.

This implies that the pressure of the solution with a temperature between 100°C and 200°C must be high, according to US-3637686 between 0,5 MPa and 10 MPa, to ensure that a liquid phase of said composition can exist at that temperature. It is an

advantage of the process according to the invention that the dissolving step and the crystallization step can be carried out at a relatively low temperature, and at a lower pressure than in US-3637686, with a lower ammonia content, while still yielding melamine of good purity and colour. As a consequence, an installation in which the process according to the invention is carried out can be simpler and cheaper than when the method according to US-3637686 is applied.

From WO 00/29393 a process for the preparation of pure melamine from a melamine melt is known in which the temperature of the melamine melt is first adjusted to between 1°C and 50°C above the melting point at the prevailing pressure.

The melamine melt is then either cooled with an aqueous phase to obtain solid melamine, or first cooled with ammonia to obtain solid melamine, and then further cooled with an aqueous phase. The composition of the aqueous phase of WO 00/29393 is not defined; neither is a solution according to the process of the invention formed.

According to the invention the crystalline melamine is obtained from a melamine melt. This is understood to mean a liquid phase (or a gas/liquid mixture) which contains liquid melamine. In addition, ammonia can be present, as well as by- products known per se that can be formed or can be present during the preparation of melamine, such as melam, melem, melon, ammeline, ammelide, urea and carbon dioxide. If the melamine melt is a gas/liquid mixture, the gas phase generally contains mainly ammonia, melamine vapour, while in addition it may contain other compounds such as carbon dioxide. The melamine melt can be obtained in a way known per se, such as for example by means of a high-pressure non-catalytic process for the preparation of melamine from urea as described in US 4,565, 867 A.

The pressure of the melamine melt may vary between wide limits. If the melamine melt has been obtained from urea by means of a high-pressure non- catalytic process the pressure usually lies between 5 MPa and 50 MPa. The temperature of the melamine melt is higher than the melting point of melamine at the prevailing pressure. The melting point of melamine is, as is known, influenced by the presence of other compounds. If for example ammonia is dissolved in the melamine melt, this leads to a reduction in the melting point of melamine ; the reduction is greater as more ammonia is dissolved in the melamine melt. As the pressure is higher, more ammonia can be dissolved in the melamine melt. The melamine melt will generally have a temperature between 270°C and 450°C.

In the cooling step according to the invention the melamine melt is

given a pressure between 0.1 MPa and 20 MPa and a temperature below the melting point of melamine at the prevailing pressure, upon which solid melamine is formed.

It can be advantageous to lower the pressure in the cooling step relative to the pressure of the melamine melt before the cooling step; as a result the melting point of the melamine melt can rise, for example due to evaporation of ammonia dissolved in the melt. It is possible that due to the said pressure reduction the melting point rises to above the prevailing temperature, so that the solid melamine is formed. It is, however, also possible that cooling is desirable or necessary. This cooling takes place in any suitable known way, for example by means of a coolant. Preferably liquid and/or gaseous ammonia is used for cooling. The coolant can be contacted with the melamine melt during the cooling step; it is also possible for the coolant to have already wholly or partially been incorporated in the melamine melt. A possible embodiment of the cooling step is by means of spraying the melamine melt in a cooling vessel. It may further be advantageous to agitate the melamine mechanically during the cooling step while cooling the melamine indirectly by means of contact with a cooled surface.

The solid melamine formed in the cooling step has a temperature below the melting point at the prevailing pressure. Preferably the temperature of the solid melamine is below 300°C, more preferably below 200°C. The advantage of this is that the formation of by-products such as for example melam or melem is prevented.

The temperature of the solid melamine is preferably higher than 30°C, more preferably higher than 60°C.

In the pressure adjustment step according to the invention the pressure of the solid melamine is adjusted to a value between 0.1 MPa and 5 MPa, preferably to between 0.1 MPa and 1 MPa, more preferably to between 0.1 and 0.5 MPa and most preferably to between 0.1 and 0.3 MPa. It is the object of the pressure adjustment step to adjust the pressure of the solid melamine to such a value that the dissolving step to be discussed hereafter can be carried out in a simple way. This can imply either a pressure increase or a pressure reduction, depending on the pressure at which the cooling step took place and the pressure at which the dissolving step will take place. The pressure adjustment can be carried out in any suitable known way, for example in the case of a desired pressure reduction by venting of gas that is present together with the solid melamine or, in the case of a desired pressure increase, by introducing a gas under pressure, for example air, nitrogen or ammonia.

After the pressure adjustment step the solid melamine is subjected

to a dissolving step according to the invention, which results in a solution in which the melamine is wholly or partially dissolved. The dissolving step according to the invention is carried out by contacting the solid melamine with an aqueous phase that has a pH between 6 and 13 and in which less than 5 wt. % ammonia is present, upon which a solution is formed. After contacting the solid melamine with the aqueous phase the pressure of the solution can, if desired, be increased again, for example using a pump.

The aqueous phase consists substantially of water but may additionally contain other substances. The aqueous phase may for example contain ammonia, although the quantity of ammonia is below 5 wt. %. This has the advantage that the pressure can remain low during the dissolving step, preferably between 0.1 MPa and 1 MPa, more preferably between 0.1 and 0.5 MPa. The quantity of ammonia in the aqueous phase is preferably below 3 wt. %, more preferably below 1 wt. %.

Depending on the circumstances, however, the presence of 0.3 wt%, preferably 0.5 wt. % of ammonia or more can be beneficial in order to obtain crystalline melamine of good colour. The aqueous phase may also contain for example melamine ; this may be the case when the aqueous phase is recirculated to the dissolving step from, for example, the separation step to be discussed later. If the aqueous phase contains ammonia, this will lead to an increase in the pH. In addition other components may also affect the pH of the aqueous phase, such as melamine, which likewise has a pH- increasing effect.

The pH of the aqueous phase lies between 6 and 13. The choice of the temperature during the dissolving step is determined on the one hand by the quantity of melamine that is to be dissolved: the higher the temperature, the higher the quantity of melamine that can be dissolved per quantity by weight of aqueous phase.

The choice of the temperature during the dissolving step is determined on the other hand by the starting point that the vapour pressure should not rise too much, so that the pressure during the dissolving step can preferably remain between 0.1 MPa and 1 MPa, more preferably between 0.1 MPa and 0.5 MPa, even more preferably between 0.1 MPa and 0.3 MPa. As a consequence of said factors the temperature during the dissolving step preferably lies between 75°C and 125°C, more preferably between 80°C and 115°C. Even more preferably the temperature during the dissolving step lies between 80°C and 105°C, most preferably between 80°C and 95°C ; this has the advantage that, at a low ammonia content, atmospheric conditions can be used, so that the required equipment can be technically simpler and thus cheaper. During the dissolving step the solid melamine wholly or partially goes into solution. Going partially

into solution means that at least 30 wt. %, preferably at least 50 wt. %, more preferably at least 70 wt. % of the added solid melamine goes into solution. The weight percentage of melamine that goes into solution, which therefore is at least 30 wt. % but may also be a higher weight percentage, such as 50 wt. %, 75 wt. %, 80 wt. % or even 90 wt. % or 100 wt. %, can be simply controlled by adjusting the quantity of the aqueous phase to the solubility of melamine that applies to the chosen temperature. The choice of the weight percentage of melamine that goes into solution is determined amongst other things by the desired product quality: as the weight percentage of melamine that goes into solution rises, the effects of the process according to the invention such as colour improvement will increase. The solubility of melamine in water is known ; US-3637686, for example, presents some basic data with regard to this, such as a solubility of 5 wt. % at 100°C. These and other known data can be taken as the starting point for the experimental determination of the optimal weight ratio in the dissolving step between the solid melamine feed and the aqueous phase. In doing so, account should be taken of any melamine already present in the aqueous phase, which may be the case if the aqueous phase is recirculated from a later step of the process according to the invention.

The solution formed in the dissolving step is subsequently cooled according to the invention in a crystallization step by at least 20°C, for example by means of a heat exchanger or by evaporation of part of the water under reduced pressure, preferably to a temperature between 35°C and 80°C. As a result part of the dissolved melamine crystallizes out. The percentage of dissolved melamine that crystallizes out increases with the degree of cooling of the solution. The melamine that has crystallized out, together with any undissolved melamine, is defined as the crystalline melamine that is present in the solution. The crystals may in principle have all known shapes. It is for example possible for the resulting melamine to have the shape of single crystals, resembling the crystals which are obtained from a low- pressure catalytic process for the preparation of melamine. It is also possible for the crystalline melamine to be obtained partially in the shape of multicrystalline particles; these are agglomerates, larger than 20p, consisting of a multiplicity of smaller (agglomerates of) single crystals. Preferably the crystallization step is carried out under reduced pressure, the cooling being caused by evaporation of a part of the water.

Reduced pressure here means a pressure which is lower than atmospheric pressure.

According to the invention the crystalline melamine that is formed in the crystallization step is subsequently separated from the solution in a separation

step. This separation can be effected using any technique known to one skilled in the art, by means of for example a centrifuge.

As indicated earlier the solution separated from the crystalline melamine can be recirculated in order to be used, after heating and optionally adjustment of pH, ammonia content and pressure, as the aqueous phase in the dissolving step. In doing so it may be advantageous to subject the separated solution to a purification step known per se, in which by-products such as for example melam, melem, ammelide and ammeline, as well as colour-forming compounds, are converted to melamine and/or are separated.

If ammonia has been used as coolant in the cooling step, it may be advantageous when applying the process according to the invention to carry out an ammonia removal step during the pressure adjustment step, or between the pressure adjustment step and the dissolving step, in which ammonia removal step a part or almost all of the ammonia that is present in the solid melamine is removed and replaced by another medium. This can be effected by, for example, passing a gas, for example air, steam or nitrogen, through the solid melamine. An advantage of the ammonia removal step is that ammonia management, including recirculation, is technically much simpler and thus cheaper, because ammonia does not need to be recovered from an aqueous phase.

The process according to the invention can be carried out as part of, for example, a high-pressure, non-catalytic process for the preparation of melamine from urea.

In the preparation of melamine use is preferably made of urea as raw material in the form of a melt. NH3 and C02 are by-products during melamine preparation, which takes place according to the following reaction equation: 6 CO (NH2) 2 o C3N6H6 + 6 NH3 + 3 C02 urea melamine ammonia carbon dioxide The preparation can be carried out at a high pressure, between 5 and 50 MPa, preferably between 5 and 25 MPa, without the presence of a catalyst. The temperature of the reaction varies between 325°C and 450°C and preferably lies between 350°C and 425°C. The NH3 and C02 by-products are usually returned to an adjoining urea plant.

The above-mentioned object of the invention can be achieved in an

installation suitable for the preparation of melamine from urea. An installation suitable for the present invention may consist of a scrubber unit, a reactor in combination with a gas/liquid separator or with a separate gas/liquid separator, optionally a post-reactor, a first cooling vessel and optionally a second cooling vessel, a dissolution vessel, a crystallizer, and a separator such as a centrifuge.

In one embodiment of the method melamine is prepared from urea in an installation consisting of a scrubber unit, a melamine reactor, optionally in combination with a gas/liquid separator or a separate gas/liquid separator, a cooling vessel, a dissolution vessel, a crystallizer and a centrifuge. This involves feeding urea melt from a urea plant to a scrubber unit at a pressure of preferably 5 to 25 MPa and at a temperature above the melting point of urea. This scrubber unit may be provided with a cooling jacket to provide additional cooling in the scrubber. The scrubber unit may also be provided with internal cooling bodies. It is also possible for cooling in the scrubber unit to be carried out wholly or partially by means of the introduction of an additional flow, for example liquid ammonia. In the scrubber unit the liquid urea comes into contact with the reaction gases from the melamine reactor or from a separate gas/liquid separator downstream of the reactor. The reaction gases consist mainly of CO2 and NH3 and also contain a quantity of melamine vapour. The melted urea scrubs the melamine vapour from the waste gas and entrains this melamine back to the reactor. In the scrubbing process the waste gases are cooled from the temperature of the reactor, i. e. from 350°C to 425°C, down to 170°C to 250°C, the urea being heated to 170°C to 250°C. The off-gases are removed from the top of the scrubber unit and are for example returned to a urea plant to be used as raw material for the urea production.

The preheated urea is withdrawn from the scrubber unit together with the scrubbed-out melamine and fed via for example a high-pressure pump to the reactor, which has a pressure of preferably 5 to 25 MPa. It is also possible to make use of gravity for the transport of the urea melt to the melamine reactor by placing the scrubber unit above the reactor.

In the reactor the melted urea is heated up to a temperature of 325°C to 450°C, preferably approximately 350°C to 425°C, at a pressure as indicated above, under which conditions the urea is converted into melamine, COZ and NH3. To the reactor a quantity of ammonia can be dosed, for example in the form of a liquid or hot vapour. The ammonia feed may for example serve to prevent the formation of melamine condensation products such as melam, melem and melon or to promote

mixing in the reactor. The quantity of ammonia fed to the reactor amounts to 0 to 10 mole per mole urea, preferably use is made of 0 to 5 mole ammonia and in particular of 0 to 2 mole ammonia per mole urea.

The C02 and NH3formed during the reaction as well as the extra ammonia feed collect in the separation section, for example in the top of the reactor, but a separate gas/liquid separator downstream of the reactor is also possible, and are separated in gaseous condition from the liquid melamine. When use is made of a separate gas/liquid separator downstream of the reactor it may be advantageous to dose ammonia to this separator. The quantity of ammonia amounts to 0.01-10 mole ammonia per mole melamine, preferably 0.1-5 mole. The advantage of this is the carbon dioxide is quickly separated and the formation of oxygen containing by-products such as ammeline and ammelide is prevented.

The gas mixture formed upon gas/liquid separation is led to the scrubber unit for removal of melamine vapour and for preheating of the urea melt.

The liquid melamine with a temperature between the melting point of melamine and 450°C is withdrawn from the reactor or from the gas/liquid separator downstream of the reactor and its temperature can, if required, be lowered prior to the cooling step to a value above the melting point of melamine. It is also possible for the pressure of the melamine melt to first be increased before the above-mentioned temperature reduction is carried out. The pressure increase can take place for example batchwise in 2 alternately operated vessels, which are pressurized with ammonia gas.

Preferably the temperature of the liquid melamine is lowered to a temperature that is 1-50°C, preferably 5-20°C higher than the solidification point of melamine at the prevailing pressure. The reduction in the temperature of the melamine melt can take place in the gas/liquid separator or in separate equipment downstream of the gas/liquid separator or reactor such as for example a second reactor or an ageing vessel The temperature reduction of the melamine melt can take place by injection of a medium, for example ammonia gas with a temperature lower than the temperature of the melamine melt, or by means of a heat exchanger.

Further, ammonia can be introduced into the liquid melamine in such a way that a gas/liquid mixture is sprayed in the spraying device, the ammonia serving as an incorporated coolant in the cooling step. The pressure of the introduced ammonia is higher than the pressure of the melamine melt.

The residence time of the liquid melamine between the reactor and the spraying device is preferably more than 10 minutes, in particular more than 30

minutes. The residence time will usually be shorter than 4 hours.

The melamine melt is transferred, optionally together with ammonia gas, to a cooling vessel in which the liquid melamine melt is sprayed via a spraying device in an ammonia environment and cooled with an evaporating medium at an ammonia pressure of 0.1-20 MPa, preferably 1-5 MPa, upon which solid melamine is formed with a temperature below the melting point at the prevailing pressure, preferably between 30°C and 300°C.

The spraying device is an apparatus that converts the melamine melt into drops or powder by allowing the melt to flow into the cooling vessel at a high velocity. The spraying device can be a nozzle or valve. The outflow velocity of the liquid from the spraying device is as a rule higher than 20 m/s, preferably higher than 50 m/s.

The melamine drops from the spraying device are cooled to form a powder by an evaporating cooling medium. This coolant may for example be liquid ammonia. The liquid ammonia may (partly) already be present in the melamine melt, and/or be sprayed into the cooling vessel.

Optionally the ammonia pressure is relieved and the product is cooled further, if desired.

The powder obtained by spraying can, prior to further cooling, be contacted with ammonia at a pressure of 0.1-20 MPa, preferably 1-5 MPa, preferably for 1 min-5 hours, with special preference for 5 min. -2 hours.

Preferably the quantity of ammonia gas is then lowered by blowing air through it, such that during contacting of the solid melamine with the aqueous phase the quantity of ammonia introduced in the solution in this way is smaller than 1 wt. %.

The product can simultaneously be cooled further.

Subsequently the product is contacted with an aqueous phase, preferably under atmospheric conditions. The weight ratio of aqueous phase to solid melamine usually amounts to between 5 and 100, preferably between 10 and 50.

Subsequently the solution can optionally be pressurized using for example a pump and optionally heated further using for example a heat exchanger to a temperature between 75°C and 125°C, preferably between 80°C and 115°C. More preferably the temperature during the dissolving step lies between 80°C and 105°C, even more preferably between 80°C and 95°C. The residence time in the dissolving vessel usually lies between 1 minute and 40 minutes.

If it is decided to dissolve all the melamine, the solution can optionally first be filtered, if desired preceded by the addition of activated carbon to the solution,

before it is transported to the crystallizer. In the crystallizer the solution is cooled by at least 20 C, preferably to a temperature between 35°C and 80 C.

The product can then be recovered further in the usual way, known to one skilled in the art. The crystals can be separated using for example a centrifuge and then optionally washed with an aqueous phase and subsequently dried in the ways known to one skilled in the art such as by means of a contact drier.

The method according to the invention will be explained further with the following comparative experiment and example.

Comparative experiment A Melamine melt with a temperature of 355°C and an ammonia pressure of 19.6 MPa is cooled to 160°C with liquid ammonia in a cooling vessel. The pressure in the cooling vessel is 2.1 MPa. The pressure is subsequently relieved to atmospheric pressure. The colour of the formed crystalline melamine amounts to 35 APHA.

Example I Melamine melt with a temperature of 355°C and an ammonia pressure of 19.6 MPa is cooled to 160°C with liquid ammonia in a cooling vessel. The pressure in the cooling vessel is 2.1 MPa. The pressure is subsequently relieved to atmospheric pressure. Then the product is dissolved completely in water at 95°C and a pH of 8.7, resulting in a solution with 4 wt. % melamine and an ammonia content of less than 50 ppm, at atmospheric conditions. Next, the solution is cooled for 40 minutes to a temperature of 55°C ; subsequently the formed crystals are filtered off. The colour of the formed crystalline melamine amounts to 10 APHA.

The above Example I demonstrates that the dissolving step, crystallization step and separation step according to the invention lead to an improvement in the colour of the crystalline melamine.