IMPROVED PROCESS FOR THE TREATMENT OF RETENTATE IN A MELAMINE PRODUCTION PLANT

The present invention relates to a process for increasing the efficiency of a plant for the production of melamine with an overall increase in yield.

The present invention falls within the technical field of the production of melamine starting from urea.

In particular, the present invention proposes to improve the demolition operation of OATs (acronym indicating OxyAminoTriazines : i.e. the combination of ammelide and ammeline) with respect to the present state of the art, contemporaneously containing not only a reduction in the demolition costs but also an increase in yield of the whole production process of melamine. As is well-known, all the synthesis processes of melamine from urea operate at temperatures higher than

36O ⁰ C and are divided into two categories:

- high-pressure processes, in liquid phase, in which the reaction pressure is maintained at over 70 bars and reactions which lead to the formation of melamine take place without catalysts;

- low-pressure processes, in gaseous phase, in which the reaction pressure is lower than 10 bars and the reactions which lead to the formation of melamine take place in the presence of a solid catalyst.

For both process categories, the reaction which, starting from urea, leads to the formation of melamine is represented as a whole, by the following stoichiometric equation (1) : 6H ₂ NCONH ₂ --> (CN) ₃ (NH ₂ )S + 6 NH ₃ + 3 CO ₂ (I) urea melamine

The reaction is endothermic, almost totally shifted towards the right (practically irreversible reaction) and is characterized by the formation of a high quantity of gaseous by-products.

The stream containing the reaction products leaving the reactor contains, in addition to not completely converted urea, also a certain quantity of by-products consisting of reaction intermediates not completely transformed into melamine and compounds coming from subsequent reactions involving melamine itself.

Typical examples of the two species of by-products are respectively ammeline and ammelide (indicated with the term OAT, acronym of OxyAminoTriazines) and polycondensates (mainly melam and melem) , resulting from the condensation, with deammoniation, of two or more molecules of melamine .

These by-products accompany the melamine leaving the synthesis reactor and form its main impurities. They are separated from the melamine by means of specific

purification treatment which, characterize the various production processes.

In most processes currently used, the stream leaving the synthesis reactor is put in contact with an aqueous solution in which the melamine is dissolved and from which the pure melamine is then separated by crystallization. The resulting aqueous solution after the separation of the pure melamine, the so-called mother liquor, still contains however a significant quantity of residual melamine (normally from 5 to 10 g per litre) .

Consequently in order to effect a production process characterized by a high yield to melamine, it is necessary for said mother liquor to be either totally or partially recycled to a suitable point upstream of the crystallizer.

The recycling of the crystallization mother liquor, however, is conditioned by the presence of the byproducts mentioned above. As these by-products in fact, typically OATs and polycondensates, are also dissolved in the mother liquor, they potentially pollute the end- product .

Among these by-products, those coming from the subsequent reactions involving melamine (typically polycondensates) are normally eliminated in the purification processes of the water containing melamine

in solution and mostly transformed again into melamine with an overall increase in yield of the synthesis process, and partly into OATs in proportions depending on the specific purification cycle adopted. Vice versa, as the by-products consisting of reaction intermediates (typically OATs) , cannot be transformed into melamine in the aqueous treatment medium, they must be continuously extracted from the aqueous purification cycle to prevent their accumulation which would finally reach the saturation threshold. This should be prevented as, if the concentration of OATs were to reach the saturation value in the circulating aqueous solution, they would precipitate together with the melamine into the crystallizer with a consequent pollution of the end-product .

The extraction of OATs from the aqueous purification cycle is one of the greatest causes of losses in yield for synthesis processes of melamine starting from urea.

As previously pointed out, in production processes of melamine, the crystallization mother liquor normally contains a melamine residue ranging from 5 to 10 g per litre, whereas the OATs are generally present in quantities ranging from 3 to 4 g per litre.

If the OATs were eliminated through a simple flushing of the mother liquor, without any treatment,

there would consequently be a much higher additional loss in yield than that corresponding to the OATs alone .

The treatment of the flushing stream containing OATs, for recovering the melamine residue contained therein, is however a complicated and costly operation.

In order to limit the loss of melamine present in the flushing stream, in fact, it is necessary to concentrate the OATs as much as possible in the flushing stream itself or, even better, separate the OATs from the mother liquor by precipitation and subsequent filtration. This operation however, when applied in some plants, is extremely difficult due to the colloidal nature of OATs which requires resort to filtration aids (such as for example infusorial earth) with a result which is not entirely satisfactory, as better illustrated below.

According to the filtration procedure, the crystallization mother liquor is subjected to a distillation operation which provides for the complete recovery of the ammonia contained therein (deammoniation) , and the mother liquor leaving the bottom of the distillation column prepared for this specific operation is then cooled and acidified up to a pH lower than 7.5 by means of gaseous CO ₂ (neutralization) . Under these conditions, the OATs are almost totally separated from the aqueous solution in colloidal form. At pH values

lower than 7.5 and at a temperature of 50-60 ⁰ C the solubility of the OATs in aqueous solution is in fact lower than 100 ppm.

As already mentioned, as this suspension is of a colloidal nature, it is filtered on special filters in the presence of a layer of infusorial earth which acts as filtration aid. In this way, a clarified aqueous solution is obtained, containing residual crystallization melamine and less than 100 ppm of OATs (corresponding to the solubility of these at a normal filtration temperature of 50-60 ⁰ C) . Practically all the OATs in excess with respect to the soluble fraction are withheld in the filtration cake.

By applying this process, the OATs which are withheld in the filtration cake are present therein in a ratio of 30% by weight approximately, inevitably associated however with a certain quantity of solid melamine co-precipitated during the acidification with CO ₂ . In industrial practice, the quantity of melamine precipitated and present in the filtration cake is normally less than 50% of the quantity of OATs on a dry base, consequently considerably reduced with respect to the quantity of melamine present in the flushing as such, again with respect to the quantity of OATs . The presence in the filtration cake of a foreign material as

filtration aid, however, makes the recovery or upgrading of both the OATs and melamine associated therewith, difficult.

Using this separation method of OATs, in addition to the delicate nature and difficulty of the operation, the problem also arises of how to eliminate the filtration cake without polluting the environment.

In this respect, the use of the technology described in WO 01/46159, which illustrates a process for separating OATs in a colloidal suspension from aqueous streams containing melamine, is particularly advantageous . According to this technology, the separation of the colloidal suspension of OATs is effected by ultrafiltration on porous ceramic membranes, without the addition of any foreign substance.

According to the operation described in WO 01/46159 two aqueous streams are obtained: a "permeate" impoverished in OATs and a "retentate" containing practically all the OATs in the form of a concentrated colloidal suspension, but still pumpable.

The permeate, practically free of OATs as, again according to WO 01/46159, it contains less than 100 ppm of these in solution, can be totally recycled to the aqueous purification and separation cycle of the melamine, obtaining the complete recovery of the melamine

contained therein and, at the same time, avoiding the accumulation of OATs in the water of the purification cycle .

The retentate which, as already specified, contains, in the form of a colloidal suspension, the fraction of OATs in excess with respect to the soluble fraction, corresponding to the almost total quantity of OATs, can be subjected to treatment to destroy or transform the same OATs. In common industrial practice, the retentate is subjected to complete demolition by hydrolysis at a temperature of 270-300 ⁰ C, so that all the organic molecules present, consequently including OATs and melamine, are transformed into ammonia and carbon dioxide, suitable for being possibly recovered as starting material for the production of urea.

This ultrafiltration treatment on porous ceramic membranes followed by the thermal decomposition of the retentate, on the one hand, allows the organic material contained in the retentate to be recovered and, on the other, enables clean water to be discharged into the atmosphere, thus also solving the ecological problem. This treatment however is costly from the point of view of energy consumption, which leads to a significant loss in yield on the part of the melamine production process

and which also has operational problems as illustrated hereunder .

From an operational point of view, the retentate is sent to a thermal decomposer, after high-pressure pumping and heating to a high temperature by means of heat exchangers. In a first heat exchange station, the heat of the liquid leaving the decomposer is recovered, whereas in a second heat exchange station a suitable heating means is used (for example diathermic oil) for bringing the retentate to the temperatures required for the subsequent demolition treatment by hydrolysis.

In the heating operation, as a result of its colloidal nature, the retentate tends to dirty and even obstruct the pump, exchangers and relative connection pipes, creating various kinds of drawbacks, causing the stoppage of the operation for the necessary cleaning and restoration interventions . In order to reduce this problem, the retentate is normally diluted with an aqueous stream containing ammonia. This technical expedient however causes an increase in the volume of the stream containing the retentate to be treated and consequently an increase in the energy consumption associated with both its movement by pumping and heating.

As already mentioned, the melamine and OATs are transformed in the thermal decomposer by hydrolysis into

ammonia and carbon dioxide; this operation however causes a loss of melamine. The gas formed in the decomposer is appropriately collected and recycled, so as to recover ammonia and carbon dioxide to be possibly used as starting materials for the production of urea.

The liquid leaving the decomposer, saturated in ammonia and carbon dioxide, is treated in a stripping column (stripper) . The gas at the head is recycled so as to recover ammonia and carbon dioxide also here, as raw materials for the production of urea. The liquid at the bottom is clean water, which is discharged into the environment .

The demolition operation of the retentate described above consequently causes both a loss in melamine associated with the OATs, the lower the concentration of OATs in the retentate, the higher the loss, and also possible interruptions in the continuity of the operation itself due to the above-mentioned fouling and encrustations of pumps, exchangers and lines. An objective of the present invention is to overcome the drawbacks of the known art, mainly associated with the loss of melamine and fouling caused by the indirect heating of the retentate, i.e. by heat exchangers.

An object of the present invention therefore relates to a process for increasing the efficiency and overall

yield of the synthesis process of raelamine by treatment of the crystallization mother liquor of melamine, deammoniated and neutralized, characterized in that it comprises the following operational phases: a) subjecting the stream of said mother liquor containing OATs and melamine to one or more ultrafiltration steps, with the production of: (i) a permeate consisting of an aqueous solution containing melamine and the sole soluble fraction of OATs, i.e. substantially free of OATs, and : (ii) a retentate consisting of a concentrated colloidal aqueous suspension comprising the fraction of OATs in excess with respect to the soluble fraction, corresponding to almost the total quantity of OATs,- the purpose of step a) being to reduce the volume of retentate thus obtained to the minimum; b) subjecting the retentate deriving from phase a) to a decomposition treatment by hydrolysis at a temperature ranging from 270 ⁰ C to 300 ⁰ C, with the formation of a gaseous stream, consisting of NH ₃ , CO ₂ and water vapour, and a liquid stream; said retentate leaving phase a) being brought to the temperature of the decomposition treatment by mixing said retentate with an aqueous stream having a temperature which is such that the stream resulting from the mixing has a temperature ranging from 270 ⁰ C to 300 ⁰ C;

c) subjecting the liquid stream coming from phase b) to stripping treatment with the separation of a further gaseous stream consisting of NH ₃ , CO ₂ and water vapour, which can be optionally joined to that coming from phase b) , and a liquid stream substantially free of organic compounds .

In the process according to the present invention, in phase a) the retentate coming from a first ultrafiltration step is preferably subjected to a second ultrafiltration step, with the formation of a second permeate and a second retentate with a higher concentration of OATs, with a further reduced volume with respect to the retentate coming from the first ultrafiltration step. The retentate obtained after the second ultrafiltration step preferably has a volume whose value is reduced by 25 to 35% with respect to the volume of the retentate obtained after the first ultrafiltration step.

The ultrafiltration phase a) according to the present invention is in any case also considered complete when, in a single ultrafiltration step, a reduction in the volumetric capacity of the retentate is obtained which is substantially equal to that obtained with several ultrafiltration steps . The concentrated retentate deriving from phase a) is

then brought to the demolition temperature of 270-300 ⁰ C by mixing, in suitable proportions, with an aqueous stream having a suitable temperature, thus avoiding the indirect heating process by means of heat exchangers, whose heat exchange surfaces are subject to the deposit and accumulation of encrustations. Furthermore, the retentate is brought to the demolition pressure with a specific pump which operates under low temperature conditions and consequently does not have the tendency to become fouled.

The permeate obtained in the first ultrafiltration step is recycled to the aqueous purification and separation cycle of melamine, whereas the permeate obtained in the second ultrafiltration step can also be recycled to the aqueous purification and separation step of melamine, together with the permeate obtained in the first ultrafiltration step, or it can be recycled immediately upstream of the first ultrafiltration cycle.

The aqueous stream mixed with the retentate to raise its temperature to the desired value before being sent to the thermal decomposition section can be an aliquot of the liquid stream leaving the stripping treatment section of phase c) , i.e. leaving the bottom of the stripping column of phase c) , or an aliquot of the liquid stream leaving the decomposition treatment section of phase b) .

In both cases, the aqueous stream mixed with the retentate is added in a quantity ranging from 85 to 95% by weight with respect to the weight of the stream resulting from the mixing, in relation to the temperature of the heating fluid available.

The decomposition treatment is carried out at a temperature within the range of 270 to 300 ⁰ C; the treatment pressure is maintained at 75 to 90 bar; the treatment time of the retentate to obtain the almost complete demolition of all the organic molecules contained therein ranges from 1 to 10 hours, preferably from 2 to 5 hours .

The liquid stream leaving the decomposition section of phase b) can be used for heating an aliquot of the liquid stream leaving the stripping section of phase c) , said aliquot subsequently being mixed with the retentate.

The main advantage of the present invention is the complete prevention of fouling caused by the retentate.

As already mentioned, in fact, the pumping operation of the retentate takes place under low temperature conditions and therefore under stability conditions of the colloidal suspension of the OATs, which does not therefore have problems associated with fouling. Furthermore, the recovery heat exchanges and with thermovector fluids (for example diathermic oil) are

effected on an aqueous stream which does not contain colloidal substances in suspension and which consequently does not have the danger of fouling of the heat exchange surfaces . As already specified, said aqueous stream can conveniently consist of a part of the liquid stream leaving the bottom of the stripper. As this stream does not contain thermolabile substances as it is practically pure water, it can be used as thermovector liquid, with the advantage of already being at an intermediate temperature between the temperature of the retentate leaving the ultrafiltration and the temperature required for the thermal decomposition.

Alternatively, said aqueous stream can consist of a part of the liquid stream leaving the decomposer which, in addition to also having the characteristics of stability, as it is water practically free of colloidal substances in solution and also containing a certain quantity of ammonia, has the advantage of already being at a high temperature and pressure and reducing the consumption of vapour in the subsequent stripping column, as only a quantity equal to the retentate has to be treated.

Another advantage of the process according to the present invention is represented by a significant overall

energy saving, envisaging the heating and moving of a reduced volume of retentate to be treated.

A further and equally important advantage of the process according to the present invention lies in the increase in yield of the whole melamine plant . The additional permeate coming from the second ultrafiltration is in fact joined to that already existing, thus recovering a further quantity of melamine.

Without limiting the applicability of the present invention, this can be better understood and its advantages will be more evident from the following description and examples, in addition to the enclosed figures. Said description, examples and figures should be considered as being non-limiting of the scope of the invention.

Figure 1 is a block scheme which shows an ultrafiltration and thermal decomposition process according to the state of the art.

Figure 2 is a block scheme which shows an ultrafiltration and thermal decomposition process according to the present invention, in a first preferred embodiment .

Figure 3 is a block scheme which shows an ultrafiltration and thermal decomposition process according to the present invention, in a second preferred

embodiment .

In figure 1, which illustrates the state of the art, the stream to be treated (1) is sent to the ultrafiltration section U. As the permeate (2) is practically free of OATs (i.e. indicatively containing a concentration of 100 ppm, equal to its maximum solubility under the specific conditions in which the ultrafiltration is effected) , it is recycled to the aqueous purification and separation cycle of the melamine, where all the melamine contained therein is recovered. The retentate (3) , containing practically all the OATs to be eliminated, is diluted with the ammonia aqueous stream (4) , preheated with vapour at medium pressure MS in the heat exchange section El . The dilution with the ammonia aqueous stream has the purpose of minimizing the fouling and encrustations in the pump Pl and in the exchangers downstream of the pump. The resulting stream (5) is pumped at a pressure of 75-90 bar by means of the pump Pl and is heated to a temperature of 270-300 ⁰ C, first through passage in the heat exchange station E2 where the stream (5) recovers heat from the liquid stream (6) leaving the decomposer D, and then through passage in the heat exchange station E3 where the stream (5) is heated to the temperature of the thermal demolition process by means of diathermic oil (DT) as

thermovector fluid. The stream (5) at the preselected temperature of 270-300 ⁰ C enters the thermal decomposer D, where the melamine, OATs and all the organic substances present are transformed by hydrolysis into ammonia and carbon dioxide. The gas (7) leaving the decomposer D is appropriately separated, so as to recover ammonia and carbon dioxide to be possibly used as raw materials for the production of urea. The liquid stream (6) leaving the decomposer D is used for preheating the liquid stream (5) entering the decomposer itself, through the heat exchange station E2; said liquid stream (6), after heat exchange, is sent to the stripper S. The gaseous stream at the head

(8) of the stripper S, consisting of ammonia, carbon dioxide and water vapour, can be suitably joined to the stream (7) . The liquid stream leaving the stripper (9) consists of clean water which is discharged into the environment .

One of the possible embodiments of the process according to the present invention is illustrated with reference to figure 2. The stream to be treated (1) is sent to a first ultrafiltration step U. The primary permeate (2) is recycled to the aqueous purification and separation cycle of melamine, where all the melamine contained therein is recovered. The primary retentate (3) , on the other hand, is sent to a second

ultrafiltration step Ul, obtaining a further permeate (4) and a retentate (5) with a higher concentration of OATs and reduced volume with respect to the primary retentate (3) . The further permeate (4) can also be recycled to the aqueous purification and separation cycle of melamine, recovering all of the melamine contained therein.

As the further ultrafiltration step Ul is fed by the primary retentate (3) containing a greater quantity of OATs in suspension with respect to the deammoniated and neutralized stream (1) coming from the deammoniation column of the mother liquor, it operates under more critical conditions with respect to the ultrafiltration step U and could separate a permeate (4) which may not be directly recoverable as occurs, on the contrary, with the primary permeate (2) . In this case, the permeate (4) can in any case be recovered by recycling it immediately upstream of the ultrafiltration unit U, through the line (4 bis), indicated by a dashed line.

The retentate concentrated in OATs (5) coming from the ultrafiltration section Ul is pumped under cold conditions through the pump Pl directly to the decomposer D, after mixing with the preheated aqueous stream (6) .

The aqueous stream (6) consists of an aliquot of the liquid stream (11) leaving the stripper S, which is pumped at a high pressure through the pump P2 to the

decomposer D. It is heated to a high temperature first through the heat exchange station E2 (which recovers the heat of the stream (8) leaving the decomposer D) , and then through the heat exchange station E3 which uses a suitable thermovector fluid, for example diathermic oil (DT) .

The resulting stream (7) , the sum of streams (5) and (6) , is already at the decomposition temperature and enters the thermal decomposer D, where the melamine, OATs and all the organic substances present are transformed by hydrolysis into ammonia and carbon dioxide.

The gaseous stream (9) leaving the decomposer D is suitably separated so as to recover ammonia and carbon dioxide to be possibly used as raw materials for the production of urea.

The liquid stream (8) leaving the decomposer D is used for preheating the aqueous stream (6) through the heat exchange station E2. The liquid stream (8), after heat exchange in E2 , enters the stripper S . The gaseous stream (10) at the head of the stripper S, consisting of ammonia, carbon dioxide and water vapour, can be suitably joined to the stream (9) . The liquid stream (11) at the bottom of the stripper S is clean water which is partly discharged into the environment (12) and partly recycled (6) to the decomposer D through P2, E2 and E3 , as

previously described.

In another possible embodiment of the process according to the present invention, represented in figure 3, the stream (1) is sent to the ultrafiltration step and treated in two steps as described in figure 2, obtaining a retentate (5) with a higher concentration of OATs and reduced volume with respect to the primary retentate (3) .

The stream (5) is pumped under cold conditions through the pump Pl directly to the decomposer D, after mixing with a preheated aqueous stream (6) .

The aqueous stream (6) consists of an aliquot of the liquid stream (8) leaving the decomposer D, which is fed through the pump P2 downstream of the high-pressure pump Pl. It is heated to a suitable temperature higher than the decomposition temperature through the heat exchange station E3 which uses a suitable thermovector fluid, for example diathermic oil (DT) .

The resulting stream (7) , the sum of streams (5) and

(6) , is already at the decomposition temperature and enters the thermal decomposer D, where the melamine, OATs and the other organic substances present are transformed by hydrolysis into ammonia and carbon dioxide.

The gaseous stream (9) leaving the decomposer D is suitably recycled, so as to recover ammonia and carbon dioxide to be possibly used as raw materials for the

production of urea.

The liquid stream (8) leaving the decomposer D is partly used as aqueous stream (6) for preheating the liquid stream (5) through the heat exchange station E3. The remaining part (10) of the liquid stream enters the stripper S. The gaseous stream (12) at the head of the stripper S, consisting of ammonia, carbon dioxide and water vapour, can be suitably joined to the stream (9) .

The liquid stream (11) at the bottom of the stripper S is clean water which is discharged into the environment .

Practical examples are provided hereunder for a better illustration of the purpose and advantages of the present invention but should in no way be considered as limiting the scope of the claims. Example 1 (comparative)

A stream of 36,000 kg/h of an aqueous solution at

70 ⁰ C containing 1.15 by weight of melamine, corresponding to 414.0 kg/h, and 0.5% of OATs, corresponding to 180.0 kg/h, is sent to the ultrafiltration section in a 30,000 t/y of melamine run according to the state of the art.

The permeate separated in the ultrafiltration section consists of a stream of 28,378 kg/h which is totally recycled to the purification and separation cycle of the melamine. It contains 1.16% by weight of melamine,

corresponding to 327.9 kg/h of melamine which are then totally recovered, and 0.03% by weight of OATs, corresponding to 8.5 kg/h.

The corresponding retentate consists of a stream of 7,622 kg/h which is sent to the thermal decomposer. It contains 1.13% by weight of melamine, corresponding to 86.1 kg/h, and 2.25% by weight of OATs, corresponding to 171.5 kg/h which are thus totally destroyed in the decomposer. In order to minimize the fouling phenomena associated with the presence of OATs in colloidal form, the retentate, which is at 70 ⁰ C, is mixed with a stream of ammonia water having a flow-rate of 1,905 kg/h and a temperature of 170 ⁰ C. A stream is obtained having a flow- rate of 9,527 kg/h and a temperature of 9O ⁰ C, which is pumped at a pressure of 82 bar and then heated to 290 ⁰ C in two heat exchange stations in series, the first fed with the solution leaving the decomposer and the second with diathermic oil; said stream is finally introduced into the decomposer (see stream (5) of figure 1) .

The amount of melamine recovered in the ultrafiltration section is equal to 327.9/414.0*100 = 79%.

The capacity of 30,000 t/y of melamine corresponds, with an operating factor equal to 7,874 h/y, to an hourly

production of 3,810.0 kg/h of melamine. This production is obtained by feeding to the plant 12,306.3 kg/h of urea at 100%, with a specific consumption equal to 12,306.3/3,810.0=3.23 kg/kg of melamine. As the stoichiometric specific consumption is equal to 2.86 kg/kg of melamine (see formula 1) , the efficiency of the melamine production process from urea is equal to 2.86/3.23*100 = 88.5%. , Example 2 In a plant for the production of melamine having a potentiality equal to 30,000 t/y, equipped for activating the process according to the present invention, there are two ultrafiltration units.

A stream of 36,000 kg/h of an aqueous solution at 7O ⁰ C containing 1.15% by weight _, of melamine, corresponding to 414.0 kg/h, and 0.5% by weight of OATs, corresponding to 180.0 kg/h, is sent to an ultrafiltration section consisting of two units situated in series, as per figure 3. The sum of the permeates obtained from the two ultrafiltration units consists as a whole of a stream of 33,877 kg/h which is totally recycled to the purification and separation cycle of the melamine. This stream contains 1.16% by weight of melamine, corresponding to 391.4 kg/h which are then totally recovered, and 0.03% by

weight of OATs, corresponding to 10.2 kg/h.

The final retentate, leaving the second ultrafiltration unit, consists of a stream of 2,123 kg/h which is sent to the thermal decomposer. It contains 1.06% by weight of melamine, corresponding to 22.6 kg/h, and 8.00% by weight of OATs, corresponding to 169.8 kg/h which are thus totally destroyed in the decomposer.

The final retentate, which is at 70 ⁰ C, is pumped under cold conditions at a pressure of 82 bar and then mixed with a stream of water having a flow-rate of 15,568 kg/h and a temperature of 320 ⁰ C. Said aqueous stream consists of a partial amount equal to 90% of the liquid stream leaving the decomposer which is pumped at 113 bar and heated in a heat exchange station by means of diathermic oil. The mixture of the two streams has a flow-rate of 17,691 kg/h and a temperature of 29O ⁰ C, and is introduced directly into the decomposer (see stream (7) of figure 3) .

The amount of melamine recovered in the ultrafiltration section is equal to 391.4/414.0*100 = 95%, against 79% of the comparative example.

The supplementary recovery of melamine obtained with the process according to the present invention is therefore equal to 391.4 - 327.9 = 63.5 kg/h; the hourly production of melamine therefore rises to 3,810.0 + 63.5

= 3,873.5 kg/h, with an increase equal to 63.5/3,810.0*100 = 1.7% with respect to the hourly production value corresponding to that obtained according to the state of the art. This higher production is obtained again by feeding 12,306.3 kg/h of urea at 100% to the plant, with a specific consumption equal to 12,306.3/3,873.5 = 3.18 kg/kg compared with the value of 3.23 of the comparative example. As the stoichiometric specific consumption is equal to 2.86 kg/kg, the efficiency of the melamine production process from urea is equal to 2.86/3.18*100 = 89.9%, against 88.5% of the comparative example.