Process for purifying melamine

Abstract

Process for purifying melamine

Description

A process for purifying melamine Description of the invention Technical field of the invention This invention relates to the field of melamine production from urea. In particular, this invention relates to the purification of melamine melt obtained through a non-catalytic high-pressure process. Prior knowledge Melamine synthesis processes from urea include low-pressure catalytic processes and high-pressure non-catalytic processes. These processes are well known in the art. High-pressure non-catalytic processes typically operate at pressures above 7 MPa and temperatures of 300 to 450 °C. Both high-pressure and low-pressure processes essentially include the steps of melamine melt synthesis, melamine melt purification, and off-gas treatment. According to the technology used in most non-catalytic high-pressure processes, the conversion of urea to melamine melt also produces an off-gas consisting mainly of ammonia and carbon dioxide and a number of by-products including OATs and polycondensates. Carbon dioxide is typically removed prior to purification of the melamine melt, the purpose of which is to separate such by-products, unconverted urea and dissolved ammonia. Melamine purification generally occurs by dissolving the melamine melt and then crystallizing the solid melamine. Examples of melamine purification processes are given in US 7,176,309 and US 7,741,481. The melamine melt is placed in an alkaline aqueous solution in which the melamine, unconverted urea, and by-products dissolve, resulting in an alkaline aqueous solution of melamine. The placement in the alkaline aqueous solution results in an increase in pH, which is required for rapid hydrolysis of the by-products. Said alkaline aqueous solution includes, for example, sodium hydroxide (NaOH) or potassium hydroxide (KOH). The resulting alkaline melamine solution feeds a crystallizer, where the crystallized solid melamine is separated from the aqueous alkaline solution, also called the mother liquor. The liquid contains some by-product sediments and, according to prior art, is treated in a wastewater treatment unit as disclosed, for example, in US 7,445,722 and US 7,723,516. In the wastewater treatment unit, carbonates and bicarbonates are produced by the reaction between alkalis (such as NaOH or KOH) present in the mother liquor and carbon dioxide is released by hydrolysis of the by-product. Accordingly, the wastewater treatment unit returns a wastewater stream containing carbonates and bicarbonates. Even if the wastewater stream is not toxic, its disposal can be a significant issue if environmental regulations impose limits on the salinity of the wastewater. Furthermore, the removal of said wastewater requires the replacement of alkali and water to the melamine process, which involves high costs for providing fresh alkali and water. Summary of the invention The purpose of this invention is to address the aforementioned deficiencies in prior art. This objective is achieved by a process for purifying a melamine melt containing melamine and by-products and obtained from a high-pressure non-catalytic synthesis process, such a process comprising the steps of: (a1) Cooling said melamine melt (a2) alkaline decomposition of at least a portion of said by-products, providing an alkaline aqueous solution of melamine, (b) Removing the alkaline aqueous solution of melamine, resulting in a de-melamined solution. (c) Crystallizing melamine from the denatured melamine solution with an aqueous solution containing primary alkali and separating melamine from the mother liquor (d) treating at least a portion of the mother liquor to provide an aqueous solution containing carbonates (e) decomposing at least a portion of the carbonate present in said aqueous solution into carbon dioxide and alkali, providing an aqueous solution containing a secondary alkali and a waste water stream, (f) returning at least a portion of the aqueous alkaline solution to one of said steps (a1), (a2) and (c). According to some embodiments, the aqueous solution containing the secondary alkali is returned to two or all of steps (a1), (a2) and (c). Preferably, step (e) of decomposition is carried out by an electrolysis process of the aqueous solution containing carbonates provided from step (d) of operation. In most cases, the aqueous solution also contains bicarbonates. For ease of description, reference is made below to the aqueous solution containing carbonates and bicarbonates. Preferably, the aqueous solution resulting from step (d) is subjected to a high-efficiency filtration step before being sent to the said electrolysis process in order to separate suspended solids that may be present therein and have a negative effect on the electrolysis process. According to some embodiments of the invention, the secondary aqueous solution containing alkali resulting from the decomposition step (e) is at least partially subjected to a concentration step before being returned to at least one of said steps (a1), (a2) and (c). The term "high pressure non-catalytic synthesis process" refers to a non-catalytic process for the synthesis of melamine that operates at a pressure that is preferably equal to or greater than 7 MPa. By-products in melamine melts include, for example, OATs (i.e., amelin, amelide) and several melamine condensates (i.e., melam, melam, melon). Typically, said melamine melts contain dissolved ammonia and unconverted urea. The term "impurities" will be used to refer to said by-products, unconverted urea, and dissolved ammonia. According to different examples, the cooling step (a1) can be carried out with water (aqueous cooling) or with alkali (alkaline cooling). According to one embodiment of the invention, in the case of alkaline quenching, the secondary aqueous solution containing alkali is at least partially returned to the hardening step (a1) to act as a hardening agent. The decomposition process (a2), also called the hydrolysis step, proceeds faster the higher the pH of the solution. A pH between 9 and 12 is best, for which purpose the decomposition step (a2) is preferably carried out in the presence of an aqueous solution containing potassium hydroxide (KOH) or sodium hydroxide (NaOH). When aqueous quenching is carried out, said aqueous solution of potassium hydroxide or sodium hydroxide is, according to various embodiments, provided at least in part from a portion of the mother liquor resulting from the crystallization step (c) or at least in part from the secondary alkali solution from the decomposition step (e). According to this embodiment, the quenching steps (a1) and decomposition (a2) are preferably carried out in two separate means, called a hardener and a decomposer. When alkali quenching is carried out, a third aqueous solution containing alkali as a quenching medium is fed to the quenching step (a1) and an aqueous solution containing melamine and impurities is obtained. The third aqueous solutions containing alkali are preferably aqueous solutions of KOH or NaOH. According to the example of the alkali quenching, the alkali (e.g. KOH or NaOH) in the presence of which the decomposition step (a2) is carried out is provided at least in part by the aqueous alkali solution obtained from the quenching step (a1) which comprises alkali together with melamine and impurities. Preferably, said alkali is provided entirely by said alkali solution and no other alkali is fed. Preferably, the third aqueous alkaline solution comprises a portion of the mother liquor provided from crystallization step (c). According to some embodiments, the third aqueous alkaline solution comprises at least a portion of the alkaline solution from decomposition step (e). According to other examples, at least a portion of the second aqueous alkaline solution and a portion of said mother liquor are combined to form a third aqueous alkaline solution or a portion thereof. The alkaline quench and the decomposition step can be carried out in the same apparatus but are preferably carried out in a separate quench and separate decomposer. Usually, due to the introduction of the alkaline solution and the corresponding increase in pH, the decomposition of the melamine polycondensates is already initiated in the quench and proceeds in the decomposer, where the alkaline quench effluent remains for a suitable period of time until the hydrolysis is complete or most of it is carried out. During the stripping step (b), ammonia is at least partially stripped from the alkaline aqueous solution of melamine, resulting in the above-mentioned stripped aqueous solution and the aqueous ammonia solution. Steam is preferably used as the stripping medium. Preferably, said aqueous solution of ammonia is at least partially subjected to said step (d) of mother liquor treatment. The removed solution, free of ammonia and the aqueous solution containing the first alkali, is subjected to crystallization, so that the melamine crystallizes and is separated from the melamine mother liquor. Like the third alkaline solution, the first aqueous alkaline solution is preferably an aqueous solution of potassium hydroxide or sodium hydroxide. For ease of description, reference is given below on how to use sodium hydroxide for the aforementioned alkaline solutions. The examples explained below with the source of sodium hydroxide solution are also applicable to potassium hydroxide solution or other solution. Preferably, the first alkaline solution is formed at least in part by the second alkaline solution from the decomposition step (e). Preferably, the first alkaline solution is formed entirely, or almost entirely, by at least a portion of the second aqueous alkaline solution and no make-up solution is required. The mother liquor contains by-products, such as precipitated polycondensates and OATs, precipitated undissolved melamine and NaOH. In particular, sodium hydroxide is present as free sodium hydroxide or in the form of salts such as sodium salts of OATs or sodium carbonates. At least a first portion of said mother liquor is subjected to said step (d) above, during which said by-products are at least partially hydrolyzed to CO2 and NH3. According to preferred embodiments, when cooling is carried out in the presence of alkali, the second portion of said mother liquor is returned to said step (a2) or to step (a1). Preferably, step (d) is a thermal step. Preferably, the thermal step is carried out at a temperature of between 200 and 300 °C and a pressure of between 30 and 100 bar. The carbon dioxide obtained from the hydrolysis of the by-products reacts, at least partially, with the sodium hydroxide present in the mother liquor to produce carbonates and bicarbonates according to the following reactions: 2 NaOH + CO2→ Na2CO3+ H2O H2O + Na2CO3+ CO2→ 2 NaHCO3 Thus, an aqueous solution containing CO2 is separated in the form of carbonates and bicarbonates. Preferably, said aqueous stream has a total Na2CO3 and NaHCO3 content in the range of 0.1 to 10% by weight, and more preferably 1.5 to 3.5%. Preferably, the majority of the carbon dioxide reacts with ammonia, thus producing ammonium carbonate as a solution separate from the aqueous solution of sodium carbonates and bicarbonates mentioned above. Said ammonia is obtained by hydrolysis of the by-products occurring during step (d) and possibly from the aqueous ammonia solution resulting from step (b) of removal. The carbonates and bicarbonates in said aqueous solution are at least partially decomposed back into carbon dioxide and sodium hydroxide, and a second aqueous solution containing alkali and a desalinated water stream are obtained. As mentioned above, according to preferred embodiments, the decomposition of carbonates and bicarbonates is carried out by hydrolysis. Hydrolysis has several advantages, it is carried out at relatively low temperatures and with low energy consumption, and it provides pure or highly pure products. Due to the presence of (bi)carbonates in water, which have an alkaline pH above 7, the electrolysis process is advantageously carried out in an alkaline electrolysis cell. Typically, an alkaline cell consists of two electrodes (i.e. anode and cathode) operating in a liquid alkaline electrolytic solution, the electrodes being separated by a diaphragm. An example of an alkaline cell suitable for this invention is described in EP0212240. As mentioned above, in some examples, the aqueous solution containing sodium carbonates and bicarbonates is subjected to a high-efficiency filtration step before being fed to the electrolysis process to remove any suspended solids that may be present and have a negative impact on the operation of the electrolysis cell. A second aqueous solution containing alkali (e.g., hydroxide solution) is obtained from the cathode and a desalinated water stream from the anode. At least a portion of the second aqueous solution containing alkali is returned to at least one of the decomposition step (a2), the crystallization step (c) and the cooling step (a1) when carried out in the presence of alkali, thus leading to a significant reduction in the supply of fresh alkali or to the avoidance of providing it. In some embodiments, returning at least a portion of the second aqueous alkaline solution to at least one of the above steps is performed at a high concentration of said solution, or a portion thereof, at a suitable concentration unit. The desalinated stream may contain residual amounts of (bi)carbonates. Depending on the residual concentration of (bi)carbonates, said water stream may be at least partially discharged or returned to the melamine process, thus involving a significant reduction in water compensation. For example, the desalinated stream may be returned to the melamine purification process, e.g. to step (e) or to step (a1). According to a preferred embodiment, the desalinated water stream does not contain residual (bi)carbonates and is referred to as a clean water stream. Therefore, said clean water stream may be completely recycled to the melamine process. The electrolysis process produces both CO2 and H2 gas, which can be vented to the atmosphere, stored, or used for other purposes. For example, CO2 can be usefully reused as a feedstock for urea synthesis, while H2 can be reused as a feedstock for the ammonia process. The purification section for carrying out the aforementioned process and the reconstitution method according to the appended claims are also objects of this invention. Renovation of the existing melamine purification unit includes the following steps: Installing a decomposition unit downstream of the existing processing unit to decompose at least a portion of the bicarbonates in the aqueous solution leaving the processing unit into carbon dioxide and alkali to obtain an aqueous solution containing alkali. Installing one or more flow lines to return at least a portion of the aqueous alkaline solution to a minimum cooler, decomposer, or crystallizer. The newly installed decomposition unit is preferably an electrolysis cell. In some embodiments, said melamine purification section comprises a cooler operating with water or an aqueous ammonia solution, and the regeneration method comprises a cooler operating with an aqueous solution of soluble alkali metal hydroxides, preferably sodium or potassium. This can be accomplished by changing the existing cooler or installing a new cooler. The newly installed cooler advantageously replaces the existing cooler. This invention has the combined advantage of reducing or even eliminating the need for alkali and water compensation and reducing the concentration of (bi)carbonates in the wastewater. The first advantage is characterized by a significant reduction in environmental impact. In fact, the desalinated water stream is discharged with a low content of carbonates and bicarbonates, which is particularly advantageous in the case of strict restrictions on the concentration of (bi)carbonates in water. Even more advantageously, the desalinated water stream may contain no (bi)carbonates, resulting in the provision of clean water. A second advantage lies in the fact that the desalinated water stream can be recycled back into the melamine process, reducing or eliminating the need for water make-up. This is a significant advantage as it allows for the reduction of fresh water supplies, which are not always feasible and involve high costs. A third advantage is that the alkali is usefully recovered from the effluent of the processing unit and continuously recirculated to the melamine purification section, which reduces or eliminates the need for water make-up in the decomposer or crystallizer or cooler (when operating in the presence of alkali). The advantages will become even more apparent with the help of the detailed description of preferred examples below. Brief description of the maps Figure 1 is a simplified diagram of the melamine purification process according to the present invention. Figures 2 to 5 are block diagrams of melamine purification processes according to various embodiments of the invention. Figure 6 is a diagrammatic representation of the operation of an alkaline electrolysis cell. Detailed description of preferred examples Figure 1 shows a simple block diagram of melamine purification section 1. Section 1 consists of a primary block 2, which actually includes a cooler, a decomposer, a stripper and a crystallizer, an operation unit 3 and an electrolysis cell 4. Said block 2 receives a first input stream 5 and a second input stream 6. Said first stream 5 is melamine melt produced from the high pressure synthesis section (not shown) of a melamine plant and said secondary stream 6 is an aqueous sodium hydroxide solution. A portion 30 of said secondary input stream 6 is provided by an electrolysis cell 4. As shown in the example of Figure 2, said secondary stream 6 includes an inlet stream 6a to the cooler and an inlet stream 6b to the crystallizer. Said block 2 provides solid melamine 7 and mother liquor 8 containing, for example, polycondensates, OATs and NaOH. Said solid melamine 7 is collected and removed from the purification section and said mother liquor 8 is subjected at least partially to a suitable treatment in a treatment unit 3. Said unit 3 is also fed with ammonia solution 9, preferably provided with the stripper in the first block 2, and produces a stream of ammonium carbonate 10 and an aqueous solution containing sodium carbonates and bicarbonates. The ammonium carbonate-containing stream 10 is withdrawn and the solution 11 is fed to the electrolysis cell 4. The electrolysis cell 4 also receives the fresh water stream 12 and produces an aqueous sodium hydroxide solution 30 and a desalinated water stream. The solution 30 is returned to the first block 2 and the brine stream 13 is at least partially withdrawn from the purification section 1. Figure 2 shows the process in Figure 1 in more detail. According to Figure 2, the melamine purification section 1 basically includes a cooler 21, a decomposer 22, a stripper 23 and a crystallizer 24 (which are part of block 2 of Figure 1), an operation unit 3 and an electrolysis cell 4. Said melamine melt 5 is fed to the cooler 21 together with an aqueous solution 6a of sodium hydroxide. Said melamine melt 5 contains melamine, unconverted urea, undissolved ammonia and a number of by-products. Said by-products essentially include OATs and polycondensates. Within the cooler 21, the melamine, unconverted urea and by-products are not dissolved, thus forming an initial alkaline aqueous solution of melamine 25 containing by-products. Said solution 25 is then sent to the decomposer 22 where, due to the presence of alkali (NaOH) in said solution 25, the polycondensates are hydrolyzed at least partially to melamine and OAT, thus forming a secondary aqueous solution of melamine 26. Said solution 26 is fed to the stripper 23, where the ammonia is removed, thus forming an aqueous ammonia solution 27 and a stripped solution 28. Stream 29 is typically used as the stripping medium. According to the example in the figure, the aqueous solution of ammonia 27 is divided into two parts: the first part 27a is sent to the operation unit 3 and the second part 27b is removed from the purification section 1. The stripped solution 28 is further purified by, for example, filtration and clarification with activated carbon and then subjected to crystallization in a crystallizer 24, where melamine crystals 7 are separated from said mother liquor 8. Said crystallizer 24 is also fed with an aqueous solution 6b containing sodium hydroxide. According to the example of the figure, said solution 6b is formed with a portion 30b of the alkaline solution provided from the electrolysis cell 4 and with a compensation portion 6c. According to even more preferred embodiments, said solution 6b is formed entirely with a portion 30b of the alkaline solution from the cell 4, while a compensation portion 6c is not required. The first portion 8a of said mother liquor is combined with an aqueous solution 30a of sodium hydroxide provided from the electrolysis cell 4 to form the inlet stream 6a to the cooler 21, which is better explained below. The second part 8b of said mother liquor is subjected to a high temperature and pressure operation in unit 3 in which the by-products present in mother liquor 8 are hydrolyzed to CO2 and NH3, forming an aqueous solution containing sodium (bi)carbonates and an aqueous solution of ammonium carbonate 10. In more detail, part of the CO2 obtained in unit 3 reacts with the sodium hydroxide present in liquor 8 to form sodium carbonates (Na2CO3) and sodium bicarbonates (NaHCO3). Part of the CO2 also reacts with NH3 to form ammonium carbonate ((NH4)2CO3). Said NH3 is provided both by the hydrolysis of the by-products and by part 27a of the aqueous ammonia solution. An aqueous solution containing (bi)carbonates 11 is provided to the electrolysis cell 4 which is fed by a stream of fresh water 12. In said cell 4, the sodium carbonates and bicarbonates present in solution 11 are decomposed into CO2 and NaOH to form an aqueous sodium hydroxide solution 30 and a desalinated water stream 13. Part of the solution 30 is returned to the cooler 21 and part to the crystallizer 24, while the desalinated water stream 13 is at least partially discharged from the purification section 1. Figure 2 shows an example in which the solution 30 is divided into two parts 30a and 30b: part 30a is returned directly to the cooler 21, where it is formed by combining with the first part 8a of the mother liquor of the inlet stream 6a, and part 30b is returned directly to the crystallizer 24, where it is formed by combining with an optional alkaline solution 6c of the inlet stream 6b. More examples are shown in Figures 3 to 5. Figure 3 shows an example in which a sodium hydroxide solution 30 is concentrated in a suitable concentration unit 31 to provide a concentrated solution 32. Said solution 32 is divided into a first portion 32a and a second portion 32b: the first portion 32a is combined with the mother liquor portion 8a and returned to the cooler 21, and the second portion 32b is combined with an optional make-up alkali solution 6c and returned to the crystallizer 24. Said concentration unit 31 produces a steam stream 33 which can be returned to the plant steam system. Figure 4 shows an example in which the desalinated water stream 13 leaving the electrolysis cell 4 is divided into three parts: a first part 13a is returned to the cooler 21, a second part 13b joins the fresh water 12 to form a 13d which is returned to the electrolysis cell 4, a third part 13c is discharged. Figure 5 shows an example that is a combination of Figures 3 and 4 in which the sodium hydroxide solution 30 is concentrated in unit 31 before being returned to the cooler 21 and crystallizer 24, and the desalinated water stream 13 is divided into three parts 13a, 13b, 13c, as in the process of Figure 4. Figure 6 shows a schematic diagram of an electrolysis cell 4, which includes an anode section 41 and a cathode section 42. The sections are separated by a diaphragm. An aqueous solution 11 containing sodium carbonates and bicarbonates is fed to the anode section 41 of cell 4, while a fresh water stream 12 is fed to the cathode section 42. The general reactions that occur in cell 4 are: Na2CO3(aq) + 2 H2O (l) → 2 NaOH (aq) + CO2(g) + ½ O2(g) + H2(g) 2 NaHCO3(aq) + 2 H2O (l) → 2 NaOH (aq) + 2 CO2(g) + O2(g) + 2 H2(g) Oxygen and carbon dioxide are released from the anode section 41 as stream 34: cations (e.g. H3O+ and Na+) are transferred to the cathode section 42 to form sodium hydroxide and oxygen, the former being separated as stream 30 and the latter being extracted as stream 35. As a result, the aqueous sodium hydroxide solution 30 is generated by the cathode section 42, while the desalinated water flow 13 is generated by the aforementioned anode sections 41.

Claims

Claims 1. A process for purifying a melamine melt (5) containing melamine and by-products, which is a high-pressure non-catalytic synthesis process, such process comprising the steps of: (a1) cooling said melamine melt (5), (a2) alkalizing at least a portion of said by-products, providing an alkaline aqueous solution (26) of melamine, (b) stripping the alkaline aqueous solution of melamine (26), resulting in a stripped melamine solution (28). (c) crystallization of melamine (28) from the stripped melamine solution with a primary alkaline aqueous solution and separation of melamine from the mother liquor (8); (d) treatment of said mother liquor, and providing a waste water stream (11) containing carbonates; (e) decomposition of at least a portion of the carbonates in the waste water stream (11) into carbon dioxide and alkali, and providing a secondary alkaline aqueous solution (30); (f) recycling at least a portion of the alkaline aqueous solution (30) to at least one of steps (a1), (a2) and (c).

2. The process according to claim 1, wherein the decomposition step (e) is carried out by an electrolysis process of an aqueous solution (11) containing carbonates.

3. Process according to claim 2, wherein the aqueous solution (11) containing carbonates is subjected to a very strong filtration step before being sent to the electrolysis process.

4. A process according to the preceding claims, wherein the secondary aqueous alkali solution (30) provided by the decomposition step (e) is at least partially subjected to a concentration step before being returned to one of steps (a1), (a2) and (c).

5. A process according to any one of the preceding claims, wherein at least a portion of the secondary aqueous alkali solution is returned to said step (c) wherein the primary aqueous alkali solution (6b) is formed completely or almost completely with at least a portion (30b, 32b) of the secondary aqueous alkali solution.

6. The process according to any one of the preceding claims, wherein said cooling step (a1) is carried out in the presence of alkali and a third aqueous solution containing alkali (6a) is fed to said step (a1) as a cooling medium.

7. The process according to claim 6, wherein the third aqueous alkali-containing solution (6a) comprises a portion (8a) of said mother liquor provided from the crystallization step.

8. The process according to claim 7, wherein said portion (8a) of mother liquor and at least a portion of said secondary aqueous alkali solution (30) are combined to form a third aqueous alkali solution (6a) or a portion thereof.

9. The process according to any of the preceding claims, wherein said solutions contain alkali solutions of sodium hydroxide or potassium hydroxide.

10. The process according to any one of the preceding claims, wherein said water stream (13) is at least partially recycled to the melamine plant.

11. Melamine purification section (1) of a high-pressure melamine plant comprising: a cooler (21) receiving melamine melt containing melamine and by-products, a decomposer (22) in which at least a portion of said by-products is decomposed by alkali and provides an aqueous alkaline solution of melamine, a stripper (23) fed with said aqueous alkaline solution of melamine (26) and with a stripping medium (29) and providing a stripped melamine solution (28), a crystallizer (24) fed with said stripped melamine solution (28) and the initial aqueous alkaline solution (6b) in which melamine (7) is separated from the melamine mother liquor (8), a treatment unit (3) for at least a portion (8b) of said mother liquor providing an aqueous solution (11) containing carbonates, a decomposition unit (4) for at least a portion of The carbonates in said aqueous solution (11) are converted to carbon dioxide and alkali, which provides a secondary aqueous solution containing alkali (30) and a water stream (13), at least one line for returningAt least a portion of the secondary aqueous alkaline solution (30) provided by the decomposition unit (4) is subjected to at least one of a cooling unit (21), a decomposition unit (22), and a crystallizer (24).

12. Melamine purification section (1) of a high-pressure melamine plant comprising: a cooling / decomposition unit receiving melamine melt (5) containing melamine and by-products, wherein at least a portion of said by-products is decomposed by alkali to provide an aqueous alkaline solution of melamine (26), a stripper (23) fed with said aqueous alkaline solution of melamine (26) and with a stripping medium (29) to provide a stripped melamine solution (28), a crystallizer (24) fed with said stripped melamine solution (28) and the initial aqueous alkaline solution (6b) in which melamine (7) is separated from the melamine mother liquor (8), a treatment unit (3) for at least a portion (8b) of said mother liquor to provide an aqueous solution (11) containing carbonates, a decomposition unit (4) at least a portion of The carbonates in said aqueous solution (11) are converted to carbon dioxide and alkali, which provides a secondary aqueous solution containing alkali (30) and a water stream (13),At least one line for returning at least a portion of the secondary alkaline aqueous solution (30) provided by the decomposition unit (4) to at least one of the cooling / decomposition and crystallizer units (24).

13. A purification section according to claim 11 or 12, wherein said decomposition unit (4) is an electrolysis cell.

14. A purification section according to any one of claims 11 to 13, comprising one or more lines for returning at least a portion of said water stream (13) to said melamine plant.

15. A method for regenerating a melamine purification section of a high-pressure melamine plant comprising: a melamine purification section (1) of a high-pressure melamine plant comprising: a cooler (21) receiving a melamine melt containing melamine and by-products, a decomposer (22) in which at least a portion of said by-products is decomposed by alkali and provides an aqueous alkaline solution of melamine, a stripper (23) fed with said aqueous alkaline solution of melamine (26) and with a stripping medium (29) and providing a stripped melamine solution (28), a crystallizer (24) fed with said stripped melamine solution (28) and an aqueous alkaline initial solution (6b) in which melamine (7) is separated from the melamine mother liquor (8), an operation unit (3) for at least a portion (8b) of said mother liquor comprising an aqueous solution (11) provides carbonates, the method being characterized in that: a decomposition unit (4) is installed downstream of said operation unit (3) to at least partially decompose the carbonates present in said aqueous solution (11) into carbon dioxide and alkali and produce a secondary aqueous solution containing alkali (30) andTo provide a water flow (13), one or more lines are installed to return at least a portion of the secondary aqueous alkali solution (30) to at least one of a cooler (21), a decomposer (22), and a crystallizer (24).

16. The method according to claim 15, said plant comprising a melamine melt cooler operating with water or with an aqueous ammonia solution, and said method comprising replacing said cooler with a cooler operating with an aqueous solution of soluble alkali metal hydroxides, preferably sodium and potassium.

17. The method according to claim 15 or 16, wherein said decomposition unit (4) is an electrolysis cell.

18. The method according to any one of claims 15 to 17, wherein one or more flow lines are installed to return at least a portion of said water flow (13) to the melamine plant.

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