JP6268540B2 - Method for preparing melamine condensate - Google Patents

Description

Detailed Description of the Invention

  The present invention relates to a process for the preparation of melamine condensates, in particular melam.

Melamine forms melamine condensates when heated under certain reaction conditions. Ammonia is released in the reaction. Similarly, melamine salts form a condensate when heated. Melamine condensates include melem, melon, and melam, and their salts. In general, melam (C ₆ H ₉ N ₁₁ ) is formed by heating melamine and / or melamine salts below 325 ° C. in the presence of a catalyst and is a byproduct of melamine synthesis. Such products are very suitable for use as flameproofing agents in polymer compositions. The melamine condensate has high thermal stability compared to other flameproofing agents (for example, halogen compounds and melamine). Melam and higher order melamine condensates (eg melem, melon and methone) do not sublime and decompose in large quantities at temperatures below 350 ° C. As a result, such polymer compositions exhibit better thermal stability compared to compositions incorporating conventional flameproofing agents. A further advantage is that the formation of mold deposits during injection molding of melammed polymer compositions is suppressed.

  Specific methods for the preparation of melam by condensation are described, for example, in V.C. A. Gal'perin et al., Zhurnal Oranicheskoi Kimii, Vol. 7, no. 11, pp. 2431-2432 (November 1971), and Gavrilova et al., Zhurnal Oranicheskoi Kimii, Vol. 13, no. 3, pp. 669-670 (March 1977). In laboratory scale experiments, melamine was converted to melam salt using ZnCl2 condensing agent at temperatures between 290 ° C and 320 ° C.

  The proportion of zinc and chloride in the melam product so obtained is very high. The presence of zinc and chloride impurities is a significant drawback for the use of melam as a flame retardant in plastics. Both ions are difficult to wash off. Furthermore, washing off zinc and chloride from the product can result in a high degree of hydrolysis of melam.

  Alternatively, melamine can be converted to condensates such as melam on a laboratory scale (eg, milligram or gram scale) and in the presence of an inorganic acid as a condensing agent. Inorganic acids include HCl, HBr, sulfuric acid, phosphoric acid, and mixtures thereof. Ammonia or melamine salts of these acids can also be used.

A method for providing a melamine condensate is disclosed in WO 96/16948. This method comprises starting materials comprising melamine or melamine salts,
In the presence of (i) at least one organic acid, or (ii) at least one ammonia or melamine salt of an organic acid, or (iii) a combination of (i) and (ii). Heating under reaction conditions effective for formation.

  This method is suitable for commercial scale production.

  The method of WO 96/16948 shows an improvement in view of previously known methods, however, the problem with this method is that it still yields a melamine condensate with a high level of purity. It is difficult. This results in a reaction product mixture comprising melamine condensate and an organic or inorganic acid and / or its salt or other residue.

  In the examples of WO 96/16948, a method for washing the reaction mixture after the heating step with an ammonia solution in water is disclosed. Low levels of organic acid in the final mixture containing the condensate have been reported after, for example, washing 420 g of the reaction mixture with only 1 liter of 3% aqueous ammonia. However, these results are incorrect. This is because washing the reaction mixture after the heating step as shown in WO 96/16948 does not result in the desired low levels of organic acids in the final mixture containing condensate. Because. As observed by the inventors, such low levels of impurities would require much larger amounts of cleaning liquid and more stringent measures.

  V. B. Lotsch et al., Chem. Eur. The preparation of melam hydrate is described in a scientific paper by J, 2007, 13, 4956-4968. The melam hydrate was obtained from melam-NH 4 Cl adduct, which was synthesized by heating melamine and NH 4 Cl (ammonium chloride) at 723 K (382 ° C.) for 12 hours. The resulting melam-NH4Cl adduct (550 mg) was stirred in aqueous NH3 (60 mL, 25%).

  In WO 96/16948 referenced above, an experiment is described in which melam is similarly prepared by reaction of melamine (25.2 g) and ammonium chloride (5.4 g). The mixture was heated at 340 ° C. for 2 hours. The product was washed with 1 liter of 3% ammonia solution. The melam yield was 90% and the residual chloride content was 5.7%.

  Obviously, such a low concentration solution combined with a smaller amount of cleaning solution is not efficient in removing residual acid components from melam. Despite the relatively large volume of water still used for cleaning, it is not highly pure. Concentrated ammonia has been shown to be effective and may be useful for purifying such small amounts of melam for scientific research, but is not suitable for use on an industrial scale.

FIG. 1 shows a schematic diagram of a method for cleaning a concentrated slurry.

  Accordingly, it is an object of the present invention to provide a process that can be used on an industrial scale in which a high purity final mixture containing melamine condensate is easily obtained.

Surprisingly, this object is a process for the purification of a melamine condensate obtained, for example, by a process comprising the steps as defined above,
a) making a diluted slurry of a reaction product mixture comprising a melamine condensate in a base solution in water having a pH of at least 9;
b) holding the diluted slurry for at least 1 hour;
c) concentrating the diluted slurry, thereby obtaining a concentrated slurry and an eluent;
d) diluting the concentrated slurry with an aqueous liquid and washing the concentrated slurry by repeating step c, wherein the washing is performed by countercurrent washing and the eluent of the downstream concentration step is upstream This is achieved by the method used to wash the concentrated slurry in the concentration step.

  The effect of this purification method is, surprisingly, despite the fact that basic aqueous solutions with relatively low base concentrations can be used and the total amount of water required is kept within a reasonable range. In other words, a melamine condensate having a relatively high purity is obtained.

The method according to the invention preferably comprises a plurality of washing steps. The method has, for example, N cleaning steps, where N is a positive integer. A positive integer means all integers greater than zero, for example 1, 2, 3, 4, 5, etc. In this method according to claim 1, preferably in step n ₁ = x (x is an integer selected (selected) from 1 to N-1 including N-1), the mixture is Washing with the water used in ₂ = x + 1, and in step n ₃ = N, the concentrated slurry is washed such that the condensate is washed with a new aqueous liquid, preferably water.

  The number N of washing steps is at least 2, preferably in the range of 2-10.

  With respect to the concentration step in the method according to the invention, in principle any method that can be used in an industrial scale process can be applied in order to separate and obtain the concentrated slurry and the eluent from the diluted slurry. Such a step can be performed by filtration or sedimentation. These can be combined with centrifugation, for example in the case of centrifugal filtration or centrifugal sedimentation, to accelerate the process.

  Washing can be accomplished by filtration or sedimentation techniques. Examples of such techniques include decanting centrifuge, cross flow filtration, centrifugal filtration, and static filtration. These techniques are combined with countercurrent washing in which the eluent of the downstream concentration step is used to make a slurry in the upstream concentration step. Thus, a 75-85% savings in wash water liquid requirement can be achieved.

  Preferably, this concentration is combined with countercurrent washing, each by cross-flow filtration (microfiltration); centrifugal sedimentation (decanter centrifuge); centrifugal filtration (basket centrifuge) or total filtration (Nuetsche or belt filter). Done.

  Cross-flow filtration (eg, microfiltration) has the advantage that high filtration flux can be achieved and no cracking cake is formed, which can validate this filtration method.

  Centrifugal sedimentation (eg decanter centrifuge) has the advantage that the washing liquid does not have to permeate the filter cake. The sediment is re-slurried fairly easily with fresh aqueous liquid. Depending on the particle size, density and G force, the separation rate can be high.

  Centrifugal filtration (eg, a basket centrifuge) allows for intensive wash aqueous / cake contact resulting in a high concentration factor and low wash aqueous consumption.

  Total filtration (eg, Nuetsche, Büchner or belt filter) also provides an intensive wash water filter cake contact that results in a high concentration factor. This results in low wash aqueous liquid consumption.

  Most preferred is total filtration combined with countercurrent washing, which allows for the lowest wash water requirements. Also most preferred is a sedimentation centrifuge combined with countercurrent washing. This allows for very low cleaning aqueous liquid requirements and a very stable process, reducing the risk of cake cracking resulting in uneven cleaning.

  Preferably, step b) of holding the diluted slurry is performed at a temperature between 20 ° C and 70 ° C, more preferably between 30 ° C and 60 ° C. Step b) is preferably performed for at least 2 hours, more preferably for at least 4 hours. Step b) is preferably performed for up to 15 hours, more preferably up to 10 hours, even more preferably up to 8 hours, most preferably up to 6 hours. The pH of the base solution in water is preferably at least 9, more preferably at least 10, even more preferably at least 12, and most preferably between 12-14. By the pH of the solution is meant the pH of the starting solution, which is the solution before it is combined with the reaction mixture to make a diluted slurry in the solution. Preferably, the base is in an amount greater than the stoichiometric amount in light of the inorganic or organic acid used in the heating step, more preferably at least 1 of such stoichiometric amount. Used in amounts greater than 25 times. As the base, for example, ammonia or an alkali hydroxide (for example, sodium hydroxide or potassium hydroxide) can be used. The best results regarding the purity obtained in the final product are obtained with sodium hydroxide and potassium hydroxide.

  Preferably, the concentration of solids in the dilute slurry is between 5-25 wt.% Of the total weight of the dilute slurry, more preferably 10-18 wt. %.

  Good results are obtained when the slurry of the mixture and base solution is mixed in a stirred vessel and held in the stirred vessel at the temperature indicated above for the time indicated above.

  Preferably, the step a) of preparing the diluted slurry of the mixture and the step b) of retaining the diluted slurry are performed immediately after the heating step in which the condensate is formed, preferably before contacting the mixture with the base solution. The mixture is cooled to a temperature below 100 ° C.

  After step b) of holding the diluted slurry at a specific temperature for a specific time, the diluted slurry is concentrated. The result of such concentration is often a concentrated cake of a mixture containing the condensate. Such concentration can be done by using a filter, basket centrifuge or decanter centrifuge.

  If a filter or basket centrifuge (total filtration) is used, the concentrated slurry is preferably washed with water prior to removal of the concentrated slurry from the filter or basket centrifuge.

  When a decanter centrifuge (sedimentation) is used, washing is preferably performed by mixing the concentrated slurry with water in a container to obtain a diluted slurry again, and concentrating the diluted slurry after the washing step to obtain a concentrated cake again. Is done. Suitably the washing is carried out with water, preferably at a temperature of 20-70 ° C.

  Preferably, the washing of the concentrated slurry is performed by N washing steps, where n = x (where x is 1 (first washing step) to N-1 (the last washing step) In the previous washing step), the concentrated slurry is washed with the water used in step x + 1, and in the final washing step N, the condensate is washed with fresh water.

  A schematic diagram of such a method is shown in FIG.

  With this scheme, the net water demand is only a fraction of the amount when only fresh water is used in all washing steps, thus providing a very economical method.

  As the fresh water, any water can be used, but preferably water having a low mineral content (for example, drinking water, process water or demineralized water) is used.

  Preferably N = 2-10, ie N is an integer in the range 2-10. More preferably, N = 3-8, and most preferably N = 3-6. With this method, it is possible to use less than 50 liters of water per kg of the purified condensate. Preferably less than 30 liters, more preferably less than 20 liters, most preferably less than 10 liters of water are used per kg of condensate.

  After the washing step, the mixture can be dried.

  The melamine condensates according to the invention are the result of the self-condensation of starting materials comprising melamine or melamine salts resulting in the release of ammonia. Exemplary melamine condensates include melam, melem, and melon.

  Preferably, the melamine condensate contains at least 90% by weight melam.

  The amount of melamine condensate is suitably measured by HPLC (= high pressure liquid chromatography).

  Examples of melamine salts that can be used in the process according to the invention include salts prepared from phosphoric acid, sulfuric acid, nitric acid, fatty acids, and formic acid.

  The heating step in the process of the present invention is performed at temperatures and reaction conditions that result in the formation of a melamine condensate. For example, the heating step can be performed to produce a maximum temperature, for example, between about 250 ° C and about 350 ° C. Preferably, however, the heating step is performed to produce a maximum temperature between about 260 ° C and about 300 ° C.

  Preferably, ammonia is removed from the reaction site as it is released.

  Organic or inorganic acids are used as condensing agents. As the inorganic acid, phosphoric acid, sulfuric acid and nitric acid can be used. Preferably organic acids are used. The organic acid can be selected from various organic acids.

  In general, organic acids can have, for example, carboxyl groups, sulfone groups, or phosphorous groups in their structure. Other acidic groups are possible.

  Very good results with the process according to the invention are obtained when the organic acid used is a sulfonic acid, more preferably para-toluenesulfonic acid. This is because a high level of purity is obtained.

  In general, the amount of organic acid or salt of organic acid can be, for example, between about 0.05 and about 5.0 mol relative to the amount of melamine or melamine salt. Preferably, the amount of organic acid or salt of organic acid is between about 0.1 and about 3.0 mol relative to the amount of melamine or melamine salt.

  The heating step is preferably performed while applying at least some oscillation to the reaction mixture.

For example, the reaction mixture can be stirred. Preferably, the reaction is carried out in a stirred reactor mounted substantially horizontally. As a result of the condensation, ammonia NH ₃ is formed. Ammonia can be purged from the reactor using an inert gas such as nitrogen. The length of the heating step can be 1-6 hours, preferably 3-5 hours.

  Melamine condensates (eg, melam) can be mixed with the polymer to produce a flame retardant composition. Melamine condensates (eg melam produced using the method according to the invention) have been found to be very suitable for use as flameproofing agents in polymer compositions.

  Preferably, according to the method of the present invention, a final mixture containing a melamine condensate and an organic acid, wherein the organic acid content in the mixture is less than 0.1 pbw in 100 parts by weight (pbw) of the condensate, more preferably Gives a final mixture which is less than 0.05 pbw at 100 pbw condensate.

  Flameproofing polymer compositions (e.g., at least flame retardant) can be prepared by converting one or more polymers to a melamine condensate (e.g., melam) at an elevated temperature, e.g. And can be prepared by mixing together. This mixture is converted into granules (or other desired physical forms such as pellets, powders, flakes, etc.).

  The relative amounts of melamine condensate and moldable polymer are selected to impart flame retardancy and moldability to the final composition. The amount of the melamine condensate is, for example, 5 to 35 wt. %, Preferably 10-25 wt. %.

  Preference is given to powdered particles of the polymer composition having a relatively uniform composition. Conversion to the final physical form can be done in various ways as known to those skilled in the art.

The polymer that can be flameproofed with a melamine condensate is preferably a moldable polymer. Preferably, the polymer can be injection molded and is a thermoplastic polymer. However, in some cases, thermosetting polymers can also be used. Various polymers and polymer mixtures can be used. Examples include one or more of the following polymers. That is,
(1) Monoolefin and diolefin polymers (eg, polypropylene (PP), polyisobutylene, poly-lbutene, polymethyl-1-pentene, polyisoprene, polybutadiene, polyethylene (optionally crosslinked) (E.g., such as high density polyethylene (HDPE), low density polyethylene (LDPE) or linear low density polyethylene (LLDPE)) or mixtures thereof);
(2) copolymers of monoolefins and diolefins, which may optionally be copolymers with other vinyl monomers (eg ethylene-propylene copolymers, linear low density polyethylene and mixtures thereof with low density polyethylene, and ethylene) And terpolymers of propylene and dienes (eg hexadiene, dicyclopentadiene or ethylidene norbornene (EPT)); and also mixtures of such copolymers with the polymers described in (1) (eg polypropylene / ethylene- Propylene copolymers);
(3) Polystyrene, poly (p-methylstyrene), poly (a methylstyrene), and copolymers of styrene or amethylstyrene with diene or acrylic derivatives (eg, styrene butadiene, styrene acrylonitrile, styrene alkyl methacrylate, styrene butadiene alkyl) Acrylates, styrene-maleic anhydride, styrene-acrylonitrile-methyl acrylate, etc.), and block copolymers of styrene (eg, styrene-butadiene-styrene, styrene-isoprene-styrene, styrene-ethylene / butylene-styrene or styreneethylene / propylene- Styrene);
(4) Polybutadiene, polybutadiene-styrene copolymer or graft copolymer of styrene or a-methylstyrene on polybutadiene-acrylonitrile copolymer; graft copolymer of styrene and acrylonitrile (or methacrylonitrile) on polybutadiene (ABS); onto polybutadiene Graft copolymer of styrene, acrylonitrile and methyl methacrylate (xBS); graft copolymer of styrene and maleic anhydride on polybutadiene; graft copolymer of styrene, acrylonitrile and maleic anhydride or maleimide on polybutadiene; Graft copolymers of styrene and maleic imide; styrene and polymers on polybutadiene Graft copolymer of kill acrylate (or alkyl methacrylate); Graft copolymer of styrene and acrylonitrile (AES) on ethylene-propylene-diene terpolymer, graft of polyalkyl acrylate or polyalkyl methacrylate on acrylate-butadiene copolymer Copolymers and mixtures with the copolymers described in (3);
(5) Polymers derived from a, S-unsaturated acids and derivatives thereof (eg polyacrylates and polymethacrylates and polyacrylamides) and copolymers thereof with other unsaturated monomers (eg acrylonitrile-butadiene copolymers, acrylonitrile) -Alkyl acrylate copolymers, acrylonitrile-alkoxyalkyl acrylate copolymers or acrylonitrile-alkyl methacrylate-butadiene terpolymers);
(6) Polymers derived from unsaturated alcohols and amines or their acyl derivatives or acetals (eg, polyvinyl alcohol, polyvinyl acetate, polyvinyl stearate, polybenzoic acid vinyl, polymaleic acid vinyl, polyvinyl butyral, allyl polyphthalate) , Polyallylmelamine) and copolymers with the olefins described in (1);
(7) homopolymers and copolymers of cyclic ethers (eg polyalkylene glycols, polyethylene oxide, polypropylene oxide or copolymers thereof with bisglycidyl ethers);
(8) Polyacetals such as polyoxymethylene containing polyoxymethylene and a comonomer (for example, ethylene oxide); polyacetals modified with thermoplastic polyurethane, acrylate or NBS;
(9) polyphenylene oxides and sulfides and their mixtures with styrene polymers or polyamides;
(10) polyurethanes derived from polyethers, polyesters and polybutadienes having terminal hydroxyl groups on one side and aliphatic or aromatic polyisocyanates on the other side, and precursor products thereof;
(11) Polyamides and copolyamides derived from diamines and dicarboxylic acids and / or from aminocarboxylic acids or the corresponding lactams (eg polyamide 4, polyamide 6/6, 6/10, 6/9, 6/12) 4/6, polyamide 11, polyamide 12, aromatic polyamide based on aromatic diamine and adipic acid); made from hexamethylene diamine, isophthalic acid and / or terephthalic acid, and optionally an elastomer as a modifier Polyamides (eg, poly-2,4,4-trimethylhexamethylene terephthalamide, poly-m-phenyleneisophthalamide); polyamides and polyolefins, oleoin copolymers, ionomers or chemically Block copolymers with combined or grafted elastomers or with polyethers such as polyethylene glycol, polypropylene glycol or polytetramethylene glycol; furthermore, polyamides or copolyamides modified with EPT or ABS, and during processing Polyamide formed (RIM polyamide system);
(12) polyurea, polyimide, polyamideimide, polybenzimidazole, and polysiloxane;
(13) Polyesters derived from dicarboxylic acids and dialcohols and / or from hydroxycarboxylic acids or the corresponding lactones (eg polyethylene terephthalate, polybutylene terephthalate, poly-1,4-dimethylolcyclohexane terephthalate, polyhydroxybenzoate) ), As well as block polyether esters derived from polyethers having hydroxyl end groups; further polyesters modified with polycarbonate or MBS;
(14) polycarbonate, polyester carbonate, polysulfone, polyether sulfone and polyether ketone; and (15) thermosetting resins (eg, unsaturated polyester, saturated polyester, alkyd resin, polyacrylate or polyether, or polymers thereof) A composition comprising one or more of them and a cross-linking agent).

  Flameproofing polymer compositions are also components used in polymer compositions as known to those skilled in the art (eg, fillers, plasticizers, lubricants, stabilizers, flame retardants, synergists, processing aids. , And reinforcing fibers such as carbon fibers or glass fibers).

[Example]
The invention is illustrated by the following non-limiting examples.

[Comparative Experiment A]
A mixture of melamine (252 grams) (DSM) and para-toluenesulfonic acid monohydrate (174 grams) (Hoechst) was heated in a 2 liter flask in an oven at a temperature of 290 ° C. with stirring. . The condensation reaction yielded ammonia (NH ₃ ), which was removed from the reaction mixture with nitrogen. The ammonia was captured with 1 molar H ₂ SO ₄ solution. After a reaction time of 2 hours at 290 ° C., the mixture in the flask is cooled and the reaction mixture is mixed with the solution using 3.66 liters of 3% ammonia solution. Washed by stirring for 3 minutes. The combined slurry of the combined mixture and solution was then filtered and the residual filter cake thus obtained was dried. After filtration and drying, melam (241 grams, 100% yield) was obtained. The sulfur content is 0.5 wt. %Met. Therefore, the para-toluenesulfonic acid content is 2.5 wt. %Met. Comparative experiment A is the same as Example 2 of WO 96/16948.

[Comparative Experiment B]
A horizontally arranged double wall stirred reactor with an effective volume of 120 liters was heated by a thermostatically controlled oil heated reactor jacket set at 350 ° C. The reactor was charged with melamine (37.2 kg) and para-toluenesulfonic acid (25.3 kg). The reactor was operated under a weak nitrogen overpressure (0.6 m3 / hour) in order to remove all ammonia formed in the condensation reaction. The temperature of the reactor contents was gradually increased to 300 ° C., after which the contents were cooled. The total reaction time was 260 minutes, of which approximately 60 minutes was spent for heating. The product was washed by mixing the product with the solution using 538 liters of 3% C ammonia solution to obtain melam. The combined slurry of the combined mixture and solution was then filtered and the residual filter cake thus obtained was dried. The melam was dried at a temperature of 175 ° C. for 3 hours to obtain a dry melam powder. 59.4 kg (yield 99.18) of dry melam powder was obtained, and the content of paratoluenesulfonic acid was 42 wt. %Met. Comparative experiment B is the same as Example 3 of WO 96/16948.

[Comparative Experiment C]
Comparative experiment A was repeated. However, after the heating step, a slurry of the reaction mixture was made in 3.66 liters of sodium hydroxide solution in water having a pH of 13. The diluted slurry was stirred at a temperature of 50 ° C. for 6 hours. The diluted slurry was then filtered and the concentrated slurry cake on the filter was washed with 12 liters of water. After drying, 0.08 wt. 235 g of melam containing less than% para-toluenesulfonic acid was obtained.

[Comparison Experiment D]
Comparative Example C was repeated. However, after filtering the diluted slurry, the obtained cake was removed from the filter, and diluted with fresh water in the same amount as the obtained filtrate to obtain a new diluted slurry. The filtration step and reslurry step were performed three times, and each time the filtrate was collected in a separate beaker.

  The total amount of new water used was 10.9 L. After drying, 235 g of melam containing less than 0.05 w% paratoluenesulfonic acid was obtained.

[Comparative Experiment E]
Comparative experiment B was repeated, except that after the heating step, a slurry of the reaction mixture was made in 538 liters of sodium hydroxide solution in water having a pH of 13. The diluted slurry was stirred at a temperature of 50 ° for 6 hours.

  Prior to drying, the slurry was filtered and the resulting cake was diluted with 538 L of fresh water to obtain the initial volume. The filtrate was collected in a separate tank. The filtration and cake dilution steps were repeated three times, after which the cake was dried to obtain 34.1 kg of dry melam. The total amount of fresh water used was 2155L. The content of paratoluenesulfonic acid in the final mixture thus obtained was less than 0.05 w%.

[Example 1]
Comparative Example D was repeated several times, but during each iteration, three washing steps, each including filtration and reslurry, were collected from the previous iteration of this Example 1. It carried out using the beaker containing. Here, the beaker containing the filtrate of step x from the previous iteration was used for step x-1. A portion of fresh water was used in the last washing step.

  The total amount of fresh water used per iteration was 4.1 L. After drying, 235 g of melam containing less than 0.05 w% paratoluenesulfonic acid was obtained.

[Example 2]
Comparative Example E was repeated several times, except that during each iteration, three washing steps, each including filtration and reslurry, were performed with the recovered filtrate from the previous iteration of Example 2. This was done using the containing tank. Here, the tank containing the filtrate from step x of the previous iteration was used for step x-1. A portion of fresh water was used in the last washing step.

  The total amount of fresh water used per iteration was 550 L in total. After drying, 34.0 kg melam containing less than 0.05 w% paratoluenesulfonic acid was obtained.

  The following table shows some typical values for the relative amount of wash solution required to achieve an equivalent high level of purity for different wash conditions.

  Due to the high cake resistance of melam in total filtration, the wash fluid flux per unit cake volume is very low. This results in very long filtration times and / or the need for large impractical filter surfaces.

  In addition, it is specifically mentioned that a small amount of water for washing is achieved thanks to a special initial preparation of the diluted slurry in combination with a specific washing method. A decant centrifuge is most preferred because it uses a small amount of washing liquid and does not have a cake that prevents filtration, or because of crack formation.

Claims (11)

Starting materials containing melamine or melamine salts,
250 in the presence of (i) at least one organic or inorganic acid, or (ii) at least one ammonia or melamine salt of said acid, or (iii) a combination of (i) and (ii). Heating under reaction conditions to produce a maximum temperature between 0 ° C. and 350 ° C., resulting in a reaction product mixture comprising melam and said organic or inorganic acid and / or salt thereof A method for purifying melam obtained, the purification method comprising:
a) creating a diluted slurry of the reaction product mixture in a base solution in water having a pH of at least 9;
b) holding the diluted slurry for at least 1 hour;
c) concentrating the diluted slurry, thereby obtaining a concentrated slurry and an eluent;
d) diluting the concentrated slurry with an aqueous liquid and washing the concentrated slurry by repeating step c, wherein the washing is performed by countercurrent washing, and the eluent of the downstream concentration step Is used to wash the concentrated slurry in an upstream concentration step. The washing of the concentrated slurry is performed in N washing steps (N is a positive integer), where the steps n ₁ = x (x is from 1 to N-1 including N−1 ( The selected mixture is washed with the water used in step n ₂ = x + 1, and in step n ₃ = N, the melam is washed with fresh aqueous liquid. Item 2. The method according to Item 1.   The method of claim 2, wherein the aqueous liquid is water.   4. A method according to claim 2 or 3, wherein N is an integer in the range of 2-10.   The process according to any one of claims 1 to 4, wherein less than 50 liters of aqueous liquid is used per kg of purified condensate.   6. A method according to any one of the preceding claims, wherein the filtration is performed by total filtration combined with countercurrent washing.   7. A process according to any one of the preceding claims, wherein the amount of base is at least in a stoichiometric amount with the acid in the reaction product mixture.   The method according to claim 1, wherein sodium hydroxide is used as the base.   The method according to any one of claims 1 to 8, wherein the step b) of holding the diluted slurry is performed at a temperature of 20 to 70 ° C.   The method according to claim 1, wherein the step b) of holding the diluted slurry is performed for a maximum of 14 hours.   The method according to any one of claims 1 to 10, wherein the step b) of holding the diluted slurry is performed in a stirred vessel.

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