Method for removing biuret from urea

Abstract

A method for removing biuret from solid urea (UR), in which the solid urea is in particulate form or in powder form, comprises leaching the solid urea with a leaching solution (LS), which is a saturated aqueous urea solution, in which the biuret contained in the solid urea is transferred into the leaching solution during the leaching process, the purified solid urea is obtained.

Description

[0001] manual Technical Field

[0002] The invention relates to the field of urea production. Background Art

[0003] Urea is produced industrially by reacting ammonia and carbon dioxide under suitable urea-forming conditions, typically high pressure and temperature.

[0004] Urea is synthesized at a synthesis pressure higher than 100 bar, obtaining a reaction effluent containing urea, water and unconverted reagents mainly in the form of ammonium carbamate. The reaction effluent is usually treated in one or more recovery sections at a pressure lower than the synthesis pressure, obtaining a recycle solution containing the reagents removed from the effluent and a purified aqueous urea solution. The purified solution usually contains about 65% to 70% urea, the remainder being water and unavoidable impurities.

[0005] The recovery process generally involves heating the solution to decompose the ammonium carbamate into ammonia and carbon dioxide; the gas phase containing ammonia and carbon dioxide is then removed and condensed to yield a recycle solution that can be returned to the reactor.

[0006] In the widely used stripping process, the effluent of the high-pressure reactor is heated in a high-pressure stripping column (possibly in the presence of a stripping agent) to decompose the ammonium carbamate and extract gaseous ammonia and carbon dioxide. These gases are condensed in a high-pressure condenser and recycled to the synthesis reactor. When used, the stripping agent is typically gaseous carbon dioxide or gaseous ammonia.

[0007] The high pressure stripper and high pressure condenser may be operated at substantially the same pressure as the synthesis reactor, thereby forming a high pressure synthesis section or loop.The urea-containing effluent from the stripper is then treated as described above in one or more recovery sections.

[0008] Impurities found in the purified solution include biuret. Urea is thermally decomposed to form biuret and ammonia, so some biuret is inevitably formed in the process. In traditional urea production, biuret is an undesirable by-product and efforts are made to avoid its formation. Certain applications of urea require strict restrictions on the maximum content of biuret, for example not more than 1.5% by weight or 1.0% by weight, or even lower. Applications with such strict restrictions include, for example, foliar fertilizer grade urea, technical grade urea, and urea for the production of diesel exhaust fluid (DEF). The quality requirements for DEF are shown in the DIN 70070 standard. On the other hand, biuret itself also has some interesting applications, for example, urea with a high content of biuret can be used in cattle feed.

[0009] In most cases, the urea solution extracted from the recovery section is converted into a solid form. The technology for converting the solution into a solid product includes a concentration step by crystallization or evaporation and a subsequent product forming step by granulation or pelletization. Granulation requires a highly concentrated solution (at least 95% to 96% urea), while pelletization requires an almost pure urea melt (>99.5% urea). All of the above processes are well known to the skilled person.

[0010] The crystallization process involves heating the urea solution and reducing it to a vacuum, usually about 0.1 bar absolute pressure, to evaporate water from the solution; urea crystals are then produced by crystallization and separated from the aqueous phase (mother liquor) by appropriate means (such as a combination of screen bends and centrifuges). The crystals are then melted to produce a urea melt; the urea melt is fed to a pelletizing tower or a granulating device.

[0011] During the evaporation process, the solution is heated under vacuum to remove water and obtain directly (without solidification and subsequent melting of solid crystals) a concentrated solution or urea melt suitable for granulation or spheronization. In this process step, due to the high temperature close to the melting point of urea and the vacuum conditions, some biuret formation is inevitable.

[0012] The crystallization process can produce urea with a very low biuret content. If the biuret is kept below the saturation point in the urea solution, the biuret remains in the liquid and does not crystallize with the urea. Urea crystals with a biuret content of less than 0.9 wt% (weight %) can be obtained, which are then melted to produce a liquid urea melt and fed to a pelletizing tower or granulator. In contrast, concentrating the urea solution by simple evaporation is not always possible to obtain such low biuret urea, but is more common in modern urea plants and is generally more popular because it does not require the installation of a urea melter and the associated equipment for handling urea solid crystals.

[0013] In conclusion, the demand for low-biuret urea is growing. However, most urea plants have only one concentration section based on evaporation, which may not be able to achieve the required purity. Installing a dedicated crystallization section is very expensive and therefore not attractive. Summary of the invention

[0014] The present invention aims to improve the above-mentioned prior art. A first object of the present invention is to provide a novel method for producing urea with a low content of biuret (e.g. less than 1.0% by weight or less than 0.9% by weight). Another object of the present invention is to provide a method suitable for producing low-biuret urea, starting from a recovered urea solution, which can be implemented by evaporation concentration and then granulation or pelletization to obtain solid urea with a purity (in terms of low biuret content) comparable to that obtained by crystallization and subsequent concentration of melting urea crystals. Another object of the present invention is to provide a method that can produce low-biuret urea and high-biuret urea, as long as the latter is also of industrial value.

[0015] These objects are achieved by a method according to claim 1. The invention provides a method for removing biuret from solid urea by a leaching process. Leaching is carried out using an aqueous urea solution as the leaching solution. During the leaching process, the biuret contained in the solid urea is transferred into the leaching solution, thereby obtaining purified solid urea and a leaching solution rich in biuret.

[0016] In some embodiments, after the leaching process, a portion of the solution may be processed to obtain biuret or high biuret urea, which may be an additional valuable output of the process.

[0017] In the present invention, biuret is removed directly from solid urea, which may be in granular or powder form. The present invention does not require expensive equipment and can be implemented in a urea plant with an evaporation-based concentration section, obtaining urea with a low biuret content comparable to that obtained from a crystallization-based concentration section. DETAILED DESCRIPTION

[0018] The present invention relates to a method for removing biuret from solid urea, wherein the solid urea is in a granular or powdery form, the method comprising leaching the solid urea with a leaching solution, the leaching solution being a urea-saturated aqueous solution, wherein the biuret contained in the solid urea is transferred into the solution, thereby obtaining purified solid urea, which is then separated from the leaching solution.

[0019] Urea in granular form may be urea obtained from a pelletizing process or from a granulation process. In one embodiment, the solid urea is in the form of spherical particles having an average diameter of not more than 3.5 mm, for example in the range of 2.0 mm to 3.5 mm. In some embodiments, the granular urea may be urea obtained from a pelletizing machine, for example from a rotational molding machine.

[0020] The leaching process is preferably carried out at a controlled temperature. In a preferred embodiment, the temperature of the leaching process is no higher than 80°C, more preferably in the range of 20°C to 80°C, even more preferably in the range of 50°C to 70°C or 55°C to 65°C, for example 60°C or about 60°C.

[0021] Very preferably, the leaching process is carried out at a substantially constant temperature. The solid urea and the leaching solution are preferably both fed at the same temperature at which the process is carried out, which temperature may be referred to as the leaching temperature. In a preferred embodiment, the solid urea and the leaching solution are both at or about 60°C.

[0022] The appropriate contact time between solid urea and the leaching solution depends mainly on the size distribution of the urea particles, the hydrodynamic field and the leaching temperature. In a preferred embodiment, solid urea and the leaching solution are contacted under strong mechanical agitation (e.g., about 10hp / 1000gal) to enhance the mass transfer of biuret. In a preferred embodiment of the present invention, the contact time is 15 minutes to 120 minutes. For a given temperature, it has been noted that fine particle size makes the leaching process more effective, and therefore, a short contact time within the above range may be sufficient. In particular, for urea in powder form, the preferred contact time is 15 minutes to 45 minutes, while for urea in granular form, the preferred contact time is 60 minutes to 120 minutes.

[0023] The leaching process can be carried out in a suitable device (also referred to as a leacher) with solid urea and a leachate as feed. The leacher is preferably a stirred tank reactor. The above contact time can be regarded as the residence time of the solid urea and the leachate solution in the leacher.

[0024] In the leaching process of the present invention, the weight ratio between the solid urea to be treated and the leaching solution is preferably between 1.0 and 4.0, more preferably between 1.5 and 2.5.

[0025] After the leaching process, for example after a selected residence time in the leacher, the purified solid urea is preferably separated from the leaching solution in a suitable device by centrifugation and / or filtration. More preferably, the separation process is carried out at the same or substantially the same temperature as the leaching process to avoid precipitation of additional solid phases.

[0026] The leachate solution separated from the purified solid urea can be partly recycled into the leaching process, for example reintroduced into the leachator, if the biuret contained in the solution permits.

[0027] In one embodiment, the leach solution separated from the purified urea is divided into a first part and a purge solution, the first part is directly recycled to the leaching process to form the main recycle stream. The purge solution is a small portion of the leach solution separated from the purified urea, for example 1% to 20% or preferably 2% to 10%, typically about 5%.

[0028] The main recirculating solution forms the major part of the leach solution and contains a large amount of biuret (previously removed from the solid urea). It may therefore be necessary to take measures to control the amount of biuret in the leach solution.

[0029] For practical reasons, it may be preferred to control the concentration of biuret in the effluent stream during the leaching process. For example, the leaching process is carried out in a leaching unit; the solution is separated from the purified urea in a separator downstream of the leaching unit; and the amount of the purified solution is controlled to maintain a target concentration of biuret in the leaching solution at the outlet of the leaching unit below saturation.

[0030] The biuret content in the effluent of the leaching solution of the leaching process may preferably be controlled in the range of 15 wt% to 18 wt%. The target biuret concentration of 15% to 18% in combination with a leaching process temperature in the range of 50°C to 70°C, more preferably at or around 60°C, is particularly preferred.

[0031] According to a preferred embodiment, the concentration of biuret in the leaching solution can be controlled:

[0032] treating the purified solution to remove biuret contained therein and combining the resulting biuret-depleted secondary recycle stream of urea solution with the main recycle stream, and / or

[0033] The purified solution is recycled to the urea plant, forced through the urea reactor and a make-up solution of fresh aqueous urea solution saturated with urea and free of biuret or with a lower biuret content than the main recycle flow is added.

[0034] In a first option, the treatment of the purified solution can be carried out in a dedicated section. In a second option, the purified solution is preferably recycled to a connected urea plant. Recycling the purified solution to the urea plant preferably comprises passing the purified solution through a urea synthesis reactor.

[0035] When there is also a demand for biuret on the market (e.g. as cattle feed), the first option is particularly interesting. In this case, the biuret removed from the purified solution can represent a kind of added value. Therefore, the treatment of the purified solution can have the combined advantages of controlling the biuret in the leaching solution and obtaining the biuret as a more valuable product. The obtained biuret can be substantially pure biuret, or it can contain some urea and is called high biuret urea. In this selection, the biuret concentration in the purified stream is preferably controlled to be higher than 10wt%, more preferably higher than 15wt%, to improve the productivity of biuret.

[0036] A second option may include dissolving solid urea with water to obtain the replenishment solution. The solid urea may be part of the purified urea obtained after the leaching process. Alternatively, the replenishment solution may be a saturated urea aqueous solution obtained by diluting a high concentration urea melt with water.

[0037] The treatment of the purified solution to remove the biuret option may include a biuret crystallization step. Prior to the crystallization step, water may be added to the relatively hot purified solution containing little water and cooled to achieve conditions suitable for the crystallization of biuret. In a preferred embodiment, the biuret solution fed to the crystallization contains 40% to 60%, preferably 45% to 55% water and has a temperature of 10° C. to 30° C., preferably 10° C. to 20° C. The crystallization of biuret is preferably carried out at about 5° C.

[0038] The slurry obtained during the biuret crystallization process can be separated into a solid phase containing biuret crystals and a biuret crystallization liquid; the liquid can then be concentrated by evaporation to remove water and obtain a secondary recycle stream for the leaching process. The secondary stream can be combined with the main recycle stream obtained directly from the liquid-solid separation after leaching as described above.

[0039] The crystallization of the biuret releases heat, making the slurry cooler than the feed solution. In a preferred embodiment, the feed solution is about 15°C and the slurry is about 5°C.

[0040] The evaporation of the biuret crystallization liquid is preferably adjusted to obtain a liquid solution saturated with urea. The water removed from the liquid in the evaporation step can be condensed and used to dilute the purified solution before the biuret crystallization. Thus, at least part of the water stream added to the purified solution can be obtained from the water removed from the liquid.

[0041] The evaporation step may be adjusted to obtain a liquid solution saturated with urea at the temperature of the leaching process.Preferably, the evaporation step is performed under vacuum, more preferably at a pressure of 0.08 to 0.15 bar absolute, such as 0.10 bar absolute.

[0042] In a preferred embodiment, the purified solution is cooled before the biuret crystallization by transferring heat to the crystallization liquid so that the liquid is also preheated before the evaporation step. This can be done in an indirect heat exchanger, such as a tubular heat exchanger or a plate heat exchanger, one side of which is traversed by the hot purified solution and the other side by the liquid.

[0043] For example, the purification solution may be cooled from 60°C to about 15°C, while the liquid may be heated from about 5°C to 50°C.

[0044] The treatment of the purification solution, in particular with regard to the crystallization of biuret, can be carried out according to the various embodiments described in WO 2022 / 106083.

[0045] Some embodiments may include a startup phase in which an initially biuret-free leached urea solution is continuously recycled to the leaching process without removal of a purge stream until a target concentration of biuret is reached in the solution effluent of the leaching process. At the end of this startup phase, i.e., when the target concentration has been reached, the process may continue as described above by removing a purge stream to maintain the desired biuret concentration in the leaching solution.

[0046] In some embodiments, the process may be performed intermittently, for example, once a target concentration of biuret is reached due to recycling, the leachate solution may be treated separately to remove the biuret contained therein, thereby recovering a solution substantially free of biuret.

[0047] The invention can be applied to a urea production process, wherein the aqueous urea solution obtained from the recovery section is concentrated in the evaporation section, and the concentrated solution or urea melt obtained is fed to a pelletizing process or a granulation process. According to the invention, the obtained pellets or granules can be treated to remove the biuret contained therein.

[0048] One embodiment of the present invention is a method for producing low-biuret urea, comprising:

[0049] producing an aqueous urea solution by reacting ammonia and carbon dioxide under urea-forming conditions, preferably according to a urea stripping process;

[0050] Evaporating the solution to remove water and obtain a concentrated urea solution or urea melt;

[0051] Processing the concentrated urea solution or urea melt by pelletizing or granulating to produce solid urea;

[0052] At least a portion of the solid urea is treated with a leaching process as described above to remove biuret.

[0053] The production of an aqueous urea solution may comprise the production of a urea-containing solution in a high-pressure synthesis section and the subsequent treatment of said solution in one or more recovery sections, for example a low-pressure recovery section or a medium-pressure recovery section followed by a low-pressure recovery section.

[0054] The invention may also be implemented as a retrofit to an existing urea plant by installing suitable equipment for the leaching process as described above.

[0055] In particular, one aspect of the present invention is a method for modifying a urea plant, the urea plant comprising a synthesis section and a recovery section for producing an aqueous urea solution; an evaporation section for removing water from the solution to produce a concentrated solution or a urea melt; and a forming section for converting the concentrated solution or the urea melt into solid urea. The method is characterized by installing a leaching device for treating at least a portion of the solid urea by a leaching process to remove biuret, the device being configured to leach the solid urea using an aqueous urea solution as a leaching agent.

[0056] As described above, the retrofitting method according to an embodiment of the present invention may further include installing a biuret crystallization section configured to remove biuret from the purge stream removed from the leach solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The present invention and its advantages are now described with the aid of the accompanying drawings, in which:

[0058] Figure 1 An embodiment of the present invention is shown for producing a diesel exhaust fluid solution.

[0059] Figure 2 Shows something like Figure 1 An embodiment of the present invention for producing low-biuret urea.

[0060] Figure 3 Another embodiment of the present invention is shown.

[0061] Figure 1 The following main equipment is shown.

[0062] LSTR leaching stirred reactor.

[0063] U-SEP liquid-solid separator, used to separate purified solid urea from the leachate solution contained in the effluent of the reactor LSTR.

[0064] DISS dissolver, in which the low-biuret solid urea obtained in the reactor LSTR and separated from the solution in the separator U-SEP is dissolved with demineralized water DW to obtain a diesel exhaust fluid solution DEF. This solution DEF usually contains 32.5 wt% urea.

[0065] BCS Biuret Crystallization Section

[0066] V valve, used to control the amount of leach solution treated in the BCS section.

[0067] HE heat exchanger.

[0068] BCR Biuret Crystallizer.

[0069] The B-SEP liquid-solid separator is used to separate the solid biuret obtained in the crystallizer BCR.

[0070] DR dryer is used to remove residual water from solid biuret to obtain biuret product BIU.

[0071] P slurry pump, used to feed the slurry effluent from the crystallizer BCR to the separator B-SEP.

[0072] EVAP evaporator, used to remove water from the liquid fraction discharged from the separator B-SEP, which is then recycled to the leaching reactor LSTR.

[0073] COND condenser.

[0074] EJE air ejector is used to keep the condenser COND in vacuum state.

[0075] Solid urea UR, for example in granular or powder form, is leached in a reactor LSTR with a leaching solution LS, which is a saturated aqueous solution of urea.

[0076] The effluent 10 of the reactor LSTR contains purified urea and a leachate solution loaded with biuret removed from the urea. Said effluent 10 is separated in a separator U-SEP into solid urea and a liquid fraction 12 which is introduced via line 11 into a dissolver DISS.

[0077] The liquid fraction 12 is divided into a main part 121 which is directly recycled to the reactor LSTR and a minor part 122 , also called purge solution, which is treated in the biuret crystallization section BCS.

[0078] In the biuret crystallization section BCS, biuret is removed from the purified solution 122 to produce the biuret product BIU, and the amount of biuret in the solution LS is controlled. The flow rate of the purified solution 122 is controlled by a valve V to maintain a target concentration of biuret in the solution LS, and thus to maintain a target concentration of biuret in the effluent 10 of the reactor LSTR. It is particularly preferred that the amount of purified solution 122 is controlled according to the concentration of biuret in the effluent 10 to keep the concentration below the saturation level. The content of biuret in the stream 10 can be detected, and the valve V can be controlled accordingly to increase or decrease the purified solution 122. Typically, the purified solution 122 can be 2% to 10% of the entire solution 12.

[0079] The solution 122 is cooled in the heat exchanger HE and recirculated water 13 is added to reach conditions suitable for crystallization of biuret in the crystallizer BCR. The effluent 14 of the crystallizer BCR is fed to the separator B-SEP; the solid part 15 is dried in the dryer DR to form the biuret product BIU; the liquid fraction 16 is a biuret-depleted urea solution, which is preheated in the heat exchanger HE using the heat removed from the solution 122, and the preheated solution 17 is sent to the evaporator EVAP.

[0080] The effluent 14 is a slurry containing liquid solution and biuret crystals, which is sent to the separator B-SEP by a slurry pump P. The biuret product BIU is biuret of high purity (usually greater than 70%, the rest being urea, residual water and impurities).

[0081] In the evaporator EVAP, water is removed from the preheated solution 17 to reach a urea concentration suitable for the leaching process. The concentrated solution 18 discharged from the evaporator EVAP is combined with the solution 121 from the separator U-SEP to form the feed of the leach solution LS.

[0082] Solution 18 forms a secondary recycle stream which is combined with main recycle stream 121. Since solution 18 contains no or only a small amount of biuret compared to solution 121 which is relatively rich in biuret, it can adjust the biuret content in stream LS and thus in effluent 10.

[0083] The vapor stream 19 removed from the evaporator EVAP is condensed in the vacuum condenser COND to form a water-rich stream 13 which, as described above, dilutes the purified solution 122 upstream of the crystallizer BCR, preferably upstream of the heat exchanger HE, as shown.

[0084] The heat exchanger HE is an indirect heat exchanger (eg, a tube or plate heat exchanger) in which the hot solution 20 transfers heat to the cold solution 16, so that the solution 20 is cooled to a temperature suitable for crystallization, while the solution 16 is preheated before entering the evaporator EVAP.

[0085] The leaching process in the reactor LSTR and the separation in the separator U-SEP are preferably carried out at the same temperature, ie the leaching temperature. Very preferably, UR and LS are also input at the leaching temperature and the leaching process is substantially isothermal.

[0086] It will be appreciated that treating the purified solution 122 in the BCS stage has the dual advantage of controlling the amount of biuret during leaching and obtaining a more valuable product BIU.

[0087] The solid urea UG can be fed via a connected urea plant. For example, the outlet of the forming section of the urea plant can be directly connected to the leaching reactor LSTR.

[0088] It can be noted that water is continuously recycled in the section BCS. Any water losses in the circuit of the section BCS can be compensated with suitable supplements (not shown).

[0089] Figure 2 Shown with Figure 1 The final urea product is different embodiments. Figure 2 In the process, the solid part 11 from the separator U-SEP is introduced into the fluidized bed cooler FBC to produce granules of low-biuret solid urea LBU. The urea LBU may contain less than 1.0% biuret.

[0090] Figure 2 Also shown is the source of the urea granules UG, namely a pelletizing tower PT or a urea granulator GR, which is part of a connected urea plant.

[0091] It must be pointed out that Figure 1 and Figure 2 Given merely as an example, the present invention is equally applicable to processes for the production of urea for various applications.

[0092] Figure 3 A simplified embodiment is shown, in which no biuret crystallization section BCS is provided. The purified solution 122 is recycled directly to the connected urea unit, for example the synthesis reactor. The amount of biuret in the reactor LSTR is controlled by injecting a low-biuret urea solution 24 mixed with the main recycle portion 121.

[0093] Specifically, in Figure 3 In the example of the invention, the low-biuret urea solution 24 is obtained in the dissolver 21, wherein a portion 22 of the low-biuret solid urea in the cooler FBC is dissolved in fresh make-up water 23. In other embodiments, solid urea from different sources, such as solid urea from storage, can be used in the dissolver 21.

[0094] Example

[0095] The following examples provide some Figure 1 Relevant process conditions. The leaching process is carried out at 60°C. The purified solution 122 is 5% of the recirculated solution 12, and the stream 121 comprises the remaining 95%. After mixing with the water stream 13, the solution 20 contains about 50% water and is cooled to 15°C in the heat exchanger (cooler) HE. The temperature of the slurry 14 after crystallization is about 5°C. Reheating on the hot side of the heat exchanger HE brings the temperature of the solution 16 to 50°C.

Claims

1. A method for removing biuret from solid urea (UR), wherein the solid urea is in granular or powder form, the method comprising leaching the solid urea with a leaching solution (LS), the leaching solution being a urea-saturated aqueous solution, wherein biuret contained in the solid urea is transferred to the leaching solution during the leaching process to obtain purified solid urea, and separating the purified solid urea from the leaching solution after the leaching process.

2. The method according to claim 1, wherein the leaching is carried out at a controlled temperature not higher than 80°C, preferably in the range of 20°C to 80°C, more preferably in the range of 50°C to 70°C, even more preferably in the range of 55°C to 65°C.

3. The method according to any one of the preceding claims, wherein the solid urea is contacted with the leaching solution for a contact time of 15 minutes to 120 minutes.

4. The process according to any one of the preceding claims, wherein the solid urea is contacted with the leaching solution in a stirred tank reactor (LSTR).

5. A method according to any one of the preceding claims, wherein: In the leaching process, the weight ratio between the solid urea to be treated and the leaching solution is 1.0 to 4.0, preferably 1.5 to 2.

5.

6. A method according to any one of the preceding claims, wherein: After the leaching process, the purified solid urea is separated from the leaching solution by centrifugation and / or filtration.

7. The method according to any one of the preceding claims, wherein separation of the purified solid urea from the leach solution is carried out at the same or substantially the same temperature as the leaching process.

8. A method according to any one of the preceding claims, wherein the solid urea subjected to the leaching process is in the form of spherical particles having an average diameter not greater than 3.5 mm.

9. A method according to any one of the preceding claims, wherein: The leaching solution (12) separated from the purified urea is divided into a first part (121) and a purified solution (122), the first part (121) being directly recycled into the leaching process to form a main recycle stream.

10. The method according to claim 9, wherein the concentration of biuret in the leaching solution is controlled by: a) treating the purified solution (122) to remove the biuret contained therein and combining the resulting biuret-depleted secondary recycle stream (18) of urea solution with the main recycle stream (121), and / or b) adding a make-up solution of fresh urea-water solution to the main recirculation flow, the make-up solution being free of biuret or having a lower biuret content than the main recirculation flow.

11. The method according to claim 10, wherein: Processing the purified solution (122) according to option a) comprises: diluting the purified solution with a water stream (13) to reach a water concentration suitable for biuret crystallization, and subjecting the purified solution to biuret crystallization; Separating the slurry (14) obtained by the biuret crystallization treatment into a solid phase (15) containing biuret crystals and a biuret crystallization liquid (16); The liquid (16) is concentrated by removing water by evaporation, thereby obtaining the secondary recycle stream (18).

12. The method according to claim 11, wherein the evaporation of the biuret crystallization liquid is adjusted to obtain a liquid solution saturated with urea.

13. A process according to claim 12, wherein prior to crystallization of the biuret, the purified solution is cooled by transferring heat to the crystallization liquid so as to preheat the liquid prior to the evaporation step.

14. The method according to any one of claims 11 to 13, wherein: At least part of the water flow (13) added to the purification solution is obtained from water removed during evaporation of the liquid (16).

15. The method of claim 10, wherein option b) comprises dissolving a portion of the purified urea obtained after the leaching process with water to obtain the replenishing solution.

16. The method according to claim 10, wherein option b) comprises recycling the purified solution to a connected urea plant, wherein the purified solution passes through a urea synthesis reactor of the urea plant.

17. The method according to any one of claims 10 to 16, wherein the leaching process is carried out in a leaching unit, the separation of the solution from the purified urea is carried out in a separator, and the amount of the purification solution removed from the leaching solution and / or the amount of the make-up solution is controlled to maintain a target concentration of biuret in the leaching solution at the outlet of the leaching unit.

18. The method according to claim 17, wherein the target concentration of biuret is below saturation, preferably in the range of 15 wt% to 18 wt%.

19. The method according to any one of claims 9 to 18, wherein the purification solution is 1 to 20 wt. %, preferably 2 to 10 wt. % of the solution effluent of the leaching process.

20. A process according to any one of the preceding claims, wherein the initially biuret-free leach solution is continuously recycled to the leaching process without removal of a purge stream until the solution effluent from the leaching process reaches a target concentration of biuret.

21. A method for producing low-biuret urea, comprising: producing an aqueous urea solution by reacting ammonia and carbon dioxide under urea-forming conditions and subsequently purifying the reaction effluent, preferably according to a urea stripping process; Evaporating the urea aqueous solution to remove water to obtain a concentrated urea solution or a urea melt; Processing the concentrated urea solution or urea melt by pelletizing or granulating to produce solid urea; At least a portion of the solid urea is treated according to the process of any one of claims 1 to 20 to remove biuret.

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