DE1231250B - Process for the production of melamine - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

CO7d

German class: 12 ρ - 10/05

Number:
File number:
Registration date:
Display day:

N20112IVd / 12p
May 29, 1961
December 29, 1966

The invention relates to a process for the production of melamine by heating a preheated solution of. Dicyandiamide and / or cyanamide in ammonia in an atmosphere composed mainly of ammonia gas and which is under a pressure of more than 50 kg / cm ² , especially 50 to 150 kg / cm ^2. _

Many processes for the production of melamine have already become known, for example by heating of dicyandiamide to a temperature above its melting temperature or by heating a liquid ammonia solution of dicyandiamide or cyanamide.

For example, a process for the production of melamine is known in which a mixture or solution of dicyandiamide is at least 0.6 times, preferably however, the same. The amount by weight of liquid ammonia is slowly introduced into a reaction vessel ^{kept at about 150 to 200 °} C., ie to the extent that the reaction mixture reacts, and the pressure is kept below 100 atm by continuously releasing the ammonia during the reaction.

According to a further known procedure for the production of melamine, a mixture or solution of dicyandiamide with or in liquid ammonia is slowly introduced into a reaction vessel which "is already at a temperature of 150 to 220 ^° C. and is maintained there, with pressures above 60 at be avoided by continuous expansion of ammonia. Here, the dicyandiamide-ammonia mixture or solution is not preheated temperature exceeding prior to their introduction to a 150 ⁰ C.

Furthermore, a process for the production of melamine is known in which melamine is ^{heated with anhydrous ammonia in a closed vessel at a temperature above 150 °} C. and preferably between 240 and 300 ^° C., at least twice the theoretically required amount of ammonia being present should be. However, the rate of conversion is ^{still so low below 300 °} C. that considerably long reaction times are required and no technically useful process can be derived from this.

However, in raw melamine made according to the known procedures are common contain various impurities, such as B. Melam, Mieren, Ammelin, Ammelid and Mellon.

Among these impurities, melam in particular exerts a very adverse influence on the Cleaning process or the resinification of melamine

Process for the production of melamine

Applicant:
• Nippon Carbide Kogyo Kabushiki Kaisha, Tokyo

Representative:

Dr. E. Wiegand and Dipl.-Ing. W. Niemann,
Patent Attorneys, Munich 15, Nussbaumstr. 10

Named as inventor:

Iwao Arai, Toyama-Ken (Japan)

Claimed priority:

Japan February 7, 1961 (3532)

the end. Although the solubility of melam in cold water is very low, it increases rapidly at high temperature when the melam content in the melamine is above about 1.5%. Therefore, the removal of melam in the cleaning of raw melamine by recrystallization from water is very difficult, and it is practically impossible to increase the purity of the recrystallized melamine to more than 99% ⁷ ^. Furthermore, in the production of melamine resin through the reaction of melamine with formalin, the stability of the aqueous solution of methylolmelamine or its precondensation product is impaired by the melam present as an impurity, and solutions with a milky white color are obtained. In this way, the presence of melam has various disadvantages.

Usually, the purification of raw melamine is most expedient on an industrial scale and most advantageously by recrystallization from water.

However, while other impurities present in raw melamine that are found in both cold and are extremely sparingly soluble even in hot water, relatively easily by a recrystallization process can be separated and removed,

609 749/412

the removal of melam is very difficult since the solubility of which in water, though very difficult in the cold, increases rapidly when the water is hot.

The object of the invention is therefore to create a process for the production of melamine in high purity, especially with a low melam content and moreover with high yield.

The process according to the invention for producing melamine by heating a preheated solution of dicyandiamide and / or cyanamide in ammonia in a pressure of over 50 kg / cm ² , especially 50 to 150 kg / cm ² , composed mainly of ammonia gas Gas atmosphere is characterized in that the solution is heated uniformly and rapidly to the reaction temperature in the range from 300 to 345 ° C by passing the solution against a fixed heating surface heated to over 300 ° C and at least while the solution is being fed to the solid heating surface scrapes off the melamine formed on it as far as possible.

Investigations have shown that in the production of melamine from a liquid ammonia solution of dicyandiamide or cyanamide a very close relationship between the reaction temperature and the degree of purity of the melamine obtained and further between the former and the by-product

ίο generated amount of melam consists.

This is explained in more detail below using an example: If a solution with a weight ratio of dicyandiamide to liquid ammonia from 1: 1.15 according to the following example 1, however, was treated at different temperatures and. the salary. the main impurities in the obtained crude melamine were examined, those in the following Results given in Table I were obtained.

Table I. 270 Reaction temp
280 erature (° C)
• 290 300: composition
of raw melamine
(Weight percent) 95.4 to 96.0
1.6 to 2.4
1.2 to 1.7
0.15 to 0.4 ■ 95.6 to 96.2
1.4 to 2.1
1.2 to 1.7
0.15 to 0.4 95.8 to 96.3
1.2 to 1.7
1.2 to 1.7
0.15 to 0.4 96.0 to 96.4
1.2 to 1.6
1.2 to 1.7
0.15 to 0.4 melamine Melam Mieren Ammelin

composition

of raw melamine

(Weight percent)

310 315

Reaction temperature ( ⁰ Q
320

330

340

melamine
Melam ..
Mieren ..
Ammelin

96.3 to 96.9
0.9 to 1.4
1.2 to 1.7

0.15 to 0.4 96.8 'to 97.4
0.7 to 1.0
1.2 to 1.7

0.15 to 0.4

97.5 to 98.9
0.4 to 0.7
1.2 to 1.7

0.15 to 0.4

98.2 to 98.5 0.1 to 0.3 1.1 to 1.5 0.1 to 0.2

98.3 to 98.7 below 0.1. 0.8 to 1.4 0.1 to 0.2

As is apparent from the above Table I, the crude melamine produced at a reaction temperature below 28O ⁰ C contains about 1.6 to 2.4% melam. In particular, it can be seen that at reaction temperatures above 300 ° C., the melam content of the raw melamine is markedly reduced.

Thereafter, it is particularly preferable that a reaction temperature of 310 ° C or higher for Timing of the formation of melamine is applied.

On the other hand, the melting point of pure melamine is known to be 354 ° C; the one of raw Melamine is usually a little lower than that due to the presence of impurities above, and in this way too high a temperature can sinter and solidify the produced melamine, as described below, causing the smooth progression the process is hindered. Furthermore, melting of the melamine produced must be prevented because otherwise it will stick firmly to the inside of the autoclave when it cools, causing emptying the autoclave would be very difficult. It is therefore necessary that the upper limit of the reaction temperatures used according to the invention is set to 345 ° C.

It can also be seen from the results in Table I that within the given reaction S " temperature range, d. H. above 300 ° C, but below a maximum of 345 ° C, as high as possible Reaction temperature is preferred.

It has also been found that the melem content in the product is not so pronounced is influenced by the level of pressure, as is done in the case of melam, and it is moreover it has been found that the ammelin content has essentially no relation to pressure.

To illustrate the correlation between the pressure, the purity of the raw The melamine product and the content of the raw melamine in melam, melamine and ammeline are examined,

d. H. in the case when the reaction using the conditions according to Example 1, such as is explained in more detail below, is carried out, but under different pressures of the ammonia gas in the autoclave adjusted by controlling the rate of discharge of ammonia gas from the autoclave. The scored Results are shown in Table II below.

Table II

composition

of raw melamine

(Weight percent)

43

51

Reaction pressure, kg / cm ²
55

80

melamine
Melam ..
Mieren ..
Ammelin

91.1 to 94.3
3.0 to 4.0
4.0 to 4.5
0.2 to 0.3

95.2 to 96.6
1.0 to 1.2
2.5 to 3.5
0.2 to 0.3

96.5 to 97.5
0.7 to 0.9
2.0 to 3.0
0.2 to 0.3

98.0 to 98.5
0.1 to 0.3
1.2 to 1.5
0.1 to 0.2

98.1 to 98.5
0.1 to 0.2
1.1 to 1.5
0.1 to 0.2

As can be seen from the above Table II, a pressure of the reaction system above 50 kg / cm ^a , in particular above 55 kg / cm ² , not only increases the degree of purity of the crude melamine obtained, but also increases its melam content rapidly This results in easy purification or refining of the raw melamine and, accordingly, the production of melamine with a high degree of purity.

It is also used in the production of melamine from a solution of dicyandiamide and / or Cyanamide in ammonia is already known to be the high pressure of the ammonia gas in the autoclave leads to an increase in the melamine yield, the high pressure causing the side reaction, d. H. the levy ammonia from melamine.

In this way, the maintenance of the pressure of the mainly ammonia gas in the autoclave above 50 kg / cm ² , in particular 55 kg / cm ² , according to the invention is a critical condition for the production of melamine in a high degree of purity, since on this Way, the production of melamine is made possible both in high yield and with a noticeable decrease in the melam content and, moreover, the purity of the crude melamine is improved.
According to the invention, it has also been found that an essential requirement for the production of crude melamine in a high degree of purity is that the preheated solution of dicyandiamide and / or cyanamide in ammonia as quickly and evenly as possible to above 300 to 345 ° C heated.

For this purpose, the preheated solution of dicyandiamide and / or cyanamide in ammonia against a fixed heating surface heated to over 300 ° C.

Fixed heating surfaces are those with a large heat capacity and, moreover, high resistance preferred over ammonia, for example those made of iron or corrosion-resistant steel.

According to the invention, such a solid heating surface can be placed in a suitable place in the autoclave be provided in the form of a plate, a rod or a ball, or it can also be the inner wall of the autoclave can be used as a fixed heating surface. As mentioned earlier, these need heating surfaces are heated to a temperature above 300 ° C by direct or indirect heating means, for example through electrical heat or »Dowtherm«.

If a preheated solution of dicyandiamide and / or cyanamide in ammonia on above of 300 ° C, but below the melting point of melamine, is heated quickly and uniformly, by blasting them against the inner surfaces of an autoclave operating at above 300 ° C is heated, out and in this way according to the preferred mode of operation of the invention in Is brought into contact, the product melamine adheres immediately after the start of the reaction and collects in a layer over the inner surface of the autoclave on which the ammonia solution hits. According to the investigations within the scope of the invention, this prevents this

ao way of adhering melamine the rapid and uniform heating of the preheated solution of Dicyandiamide and / or cyanamide in ammonia, which is guided in a jet against the layer, whereby there is a decrease in the degree of purity of the obtained raw melamine as well as an increase of the melam content of which would result. To achieve a higher purity of the raw melamine it is therefore necessary to constantly scrape the melamine off the heating surface,

z. B. by means of a stirrer.

Therefore, in the practical implementation of the invention on an industrial scale, an autoclave, which with inevitable heating means to heat the inner surfaces of the autoclave and further with a stirrer with blades, which are preferably as close as possible to the inner surfaces are provided, is provided, advantageously used.

The most preferred embodiment is that the preheated solution of dicyandiamide and / or cyanamide in ammonia is continuously directed in the jet against the inner surfaces in an amount which does not lower the temperature of the inner surfaces below 300 ° C, while the Zone of the inner surfaces of the autoclave on which the solution impinges and the neighboring zones in the autoclave at a temperature above 300 ° C, but below the melting point of melamine, and the pressure of the gas, which consists mainly of ammonia gas, in the autoclave above 50 kg / cm ² , in particular above 55 kg / cm ² .

The invention is illustrated in more detail below with the aid of a few examples; it can be seen that the following examples are given for the purpose of illustration only and that the The invention is in no way limited thereby.

For example

811 of a solution of dicyandiamide in ammonia (30 kg of dicyandiamide, 35 kg of liquid ammonia), which was heated to 130 ° C in advance, were continuously with an addition rate of 10 l / min, against the heated inner surface of an autoclave Made of corrosion-resistant steel with a capacity of 2001, which is equipped with a heating jacket and a scraper was fed; the

In particular, the solution was fed to those points on the inner surface where the melamine produced was located does not accumulate.

While maintaining the ammonia pressure in the autoclave by controlling the Ammoniakgasauslaßventils stable at 70 kg / cm ² and the heating jacket at 335-345 ⁰ C, the reaction temperature is 330 ± 2 ° C was. Furthermore, by continuously working the scraper located in the autoclave, which has a clearance of about 2 to 3 mm from the inner surface, the adherence and build-up of a layer of melamine on the inner surface of the autoclave was prevented.

After all of the preheated dicyandiamide solution had been added, the excess pressure was released and taken out the melamine formed. 29.5 kg of raw melamine were obtained which had a purity of 98.3%; it contained the following impurities:

Melam 0.2%

Mieren 1.3%

Ammeline 0.13%

Comparative experiment

^: Under similar conditions as in Example 1, a preheated solution of 30 kg of dicyandiamide and 35 kg of liquid ammonia was continuously introduced into a similar autoclave, with the exception that in this experiment the clearance between the scraper and the inner surface of the autoclave was on average 10 until 15 has now been expanded. 29.4 kg of a product were obtained which had a purity of 95.7% and contained the following impurities:

Melam ...: 2.0%

Cherries 1.6%

Ammeline 0> 4%

Example2

An autoclave provided with a scraper as used in Example 1 was charged with 811 of a solution of 30 kg of dicyandiamide and 35 kg of liquid ammonia which had been preheated to 120 ° C., the rate of addition being 3 l / min , fraud. The reaction was carried out under a pressure of 60 kg / cm ^{a of} the ammonia gas. The reaction temperature was maintained at 325 to 330 ° C by raising the temperature of the heating jacket to 34O ⁰ C was adjusted to 330th After the feeding of all the solution was completed in this way, the ammonia gas was released to reduce the pressure, and the product was taken out. The crude melamine obtained in this way had a purity of 98.0% and contained as impurities:

Melam 0.4%

Mieren 1.4%

Ammeline 0.15%

Example3

a solution of 122 kg of dicyandiamide and kg liquid ammonia, which was preheated to 130 ⁰ C were fed in a vessel equipped with a scraper autoclave at a flow rate of 3 l / min, and under a pressure of 70 kg / cm ^2. The reaction temperature was 320 to 33O ⁰ C and those of the heating jacket 335-345 ° C.

After all the solution of the dicyandiamide was introduced into the autoclave, the autoclave became relaxed and 120 kg of the produced, raw melamine removed.

The purity of the product was 97.5% and the impurities were present in the following proportions:

Melam 0.6%

Mieren 1.4%

Ammeline 0.4%

Claims (1)

Claim: A process for the preparation of melamine by heating a preheated solution of dicyandiamide and / or cyanamide in ammonia in a under a pressure of about 50kg / cm ^a, particularly 50 to 150 kg / cm ^a property, mainly composed of ammonia gas gas atmosphere, characterized in that that the solution is heated uniformly and rapidly to the reaction temperature in the range from 300 to 345 ^° C. by passing the solution against a ^{solid heating surface heated to over 300 °} C. and at least that which is formed on it while the solution is being supplied to the solid heating surface Melamine scraped off as much as possible. Considered publications:
German Patent No. 943 354;
British Patent Nos. 732,989, 523,448;
Swiss Patent No. 202 245. 609 749/412 12.66 © Bundesdruckerei Berlin