DE2536689C3 - Process for the production of melamine from cyanamide and / or dicyandiamide - Google Patents

Description

R ¹

or

S = O

15th

R ₂ O

wherein Ri and R2 are alkyl radicals having 1 to 10 carbon atoms mean, which can be the same or different or together also form a ring, is carried out in the simultaneous presence of alkali metal or alkaline earth metal hydroxide.

2. The method according to claim 1, characterized in that the solvent is dimethyl sulfoxide, Dimethyl sulfone, sulfolane or mixtures thereof can be used.

3. The method according to claim 1, characterized in that a by deducting a small Amount of cyanamide and / or dicyandiamide stabilized solvent is used.

4. Process according to claims 1 to 3, characterized in that cyanamide in the form of its aqueous solution is used.

5. Process according to claims 1 to 4, characterized in that the reaction is gentle Vacuum is carried out.

6. The method according to any one of the preceding claims, characterized in that 0.02 to 0.2 Moles of alkali metal or 0.05 to 0.5 moles of alkaline earth metal hydroxide are used per mole of cyanamide or dicyandiamide will.

7. The method according to any one of the preceding claims, characterized in that a Solution or suspension of the starting product in a solution preheated to the reaction temperature of the hydroxide is entered.

8. The method according to any one of the preceding claims, characterized in that the method is carried out continuously without the separation of non-precipitated melamine.

The invention relates to a process for the production of melamine from cyanamide and / or dicyandiamide about atmospheric pressure.

Process for the production of melamine based on lime, coal and electrical energy with the Intermediate stages calcium carbide and calcium cyanamide gain due to the changed raw material situation again in importance.

The process for the production of melamine from calcium cyanamide, which has hitherto been carried out on an industrial scale, is multi-stage: By hydrolysing the calcium cyanamide, cyanamide is initially formed, which is converted into dicyandiamide and isolated. The dicyandiamide is atü after drying in high-pressure autoclave at 200 and 300 ⁰ C in about 95% yield to melamine reacted A subsequent recrystallization of the obtained Rohmelamins of water is necessary. Since the solubility of melamine at 90 ⁰ C, only about 4% are required large amounts of water and high amounts of energy for the melamine purification.

To avoid the high pressures of the conventional process, attempts have been made to carry out the reaction of the dicyandiamide to form melamine in suitable solvents. In particular, methanol and isobutanol have been proposed for this purpose (Ulimann, Vol. 12, pp. 281-282 [1960]). Because of the required presence of ammonia and a minimum temperature of 160 ⁰ C is still relatively high pressures were always necessary, the melamine yield but only max was. 80%.

In US-PS 22 06 005 the pressureless production of melamine from dicyandiamide in benzyl alcohol described as a solvent. Even in this process only yields of melamine are below 80% preserved, as large amounts of ammeline and ammelide are formed as by-products.

According to the method of DT-PS 9 33 866 cyanamide and / or dicyandiamide, crystalline or in dissolved in an organic solvent, in relatively large amounts of an inert liquid such as tetrahydronaphthalene, diphenyl ether or higher saturated aliphatic hydrocarbons are preferred introduced at 190 to 250 ° C under normal pressure. The resulting melamine is a maximum of 95% because it is contaminated by deamination products formed with the elimination of ammonia, which is a Requires recrystallization.

The method proposed in DT-PS 9 55 952 for the production of melamine by atomization of a aqueous solution of cyanamide and / or dicyandiamide in a heat transfer medium and buffer hot gas stream at temperatures between 275 and 300 ° C required for evaporation of the whole Water so high amounts of energy that the process is already uneconomical from this point of view; In addition, the resulting product contains larger proportions of impurities, which in turn is a Requires recrystallization.

All processes described in the literature which improve the production of melamine from calcium cyanamide Aimed at, were able to differ from the traditional one, especially for economic reasons High-pressure processes did not prevail and therefore did not find their way into technology.

The invention is therefore based on the object of developing a process for the production of melamine, which at about atmospheric pressure in a suitable solvent, starting from crystalline or cyanamide and / or dicyandiamide dissolved in water in high yield directly high-purity melamine supplies. The expression "about atmospheric pressure" is in relation to the very high pressures of the high pressure process to see, and there is therefore one underneath it

to understand quite a wide range, namely 1OO Torr up to 5 ata.

According to the invention this object is achieved in that the implementation of dicyandiamide and / or Cyanamide in at least one solvent of the general formula

R ¹

R.

S = O

R ₂ O

20th

where Ri and R _{2 are} alkyl groups with 1 to 10 carbon atoms, which can be the same or different and together can also form a ring, in the simultaneous presence of alkali metal or alkaline earth metal hydroxide. The preferred organic solvent is dimethyl sulfoxide, the alkali metal hydroxide, potassium hydroxide or sodium hydroxide. The yields achievable in the conversion of cyanamide and / or dicyandiamide in the abovementioned organic solvent / alkali or alkaline earth metal hydroxide system are up to 97% of a melamine with a purity of more than 99.5%. This melamine obtained in this way can be used for further processing after the solvent has been separated off without further cleaning operations, e.g. B. for the condensation reaction with formaldehyde. The total yield of melamine in the process carried out continuously reaches values of up to more than 98%, based on the starting materials used, cyanamide or dicyandiamide.

The radicals Ri or R ₂ in the compounds of the general formula which can be used as organic solvents

S = O

R,

R ₂

preferably contain t to 4 carbon atoms, e.g. B. methyl, ethyl, propyl or butyl groups, where Ri and R2 be the same or different or together can also form a ring. Typical representatives of this as Compounds suitable for solvents are compounds such as dimethyl sulfoxide, diethyl sulfoxide, dimethyl sulfone, Diethyl sulfone or sulfolane. Particularly suitable as reaction medium are those compounds which are liquid at room temperature and have a boiling point of about 150 to 250 ° C. To the Mixtures of the solvents mentioned can also be used to set the corresponding melting and boiling points can be used, or the reaction is carried out at reduced or elevated pressure as mentioned above Framework carried out.

So that the reaction leads to particularly pure melamine with high yield, the presence of alkali or Alkaline earth hydroxide requirement. All alkali hydroxides such as lithium, sodium, Potassium hydroxide, cesium hydroxide or all sufficiently basic alkaline earth metal hydroxides such as calcium, strontium or Barium hydroxide as pure compounds or as a mixture of alkali or alkaline earth metal hydroxides with one another be used. Without these compounds, which act as catalysts, none or only in Traces of melamine. In order to achieve an optimal yield and purity of melamine, certain Amounts of the alkali or alkaline earth metal hydroxides mentioned are adhered to, with the implementation of Cyanamide to melamine less catalytically active base is required than in the reaction of Dicyandiamide to melamine. So z. B. when using potassium hydroxide when using Dicyandiamide 0.1-0.2 moles of potassium hydroxide per mole Dicyandiamide is used, while when using cyanamide as the starting product 0.02-0.05 mol of potassium hydroxide can be used per mole of cyanamide. Slightly higher amounts of catalytically active base are expedient, namely 0.05-0.5 mol, if instead of alkali metal hydroxide alkaline earth metal hydroxide, preferably calcium or barium hydroxide can be used.

The reaction of dicyandiamide or cyanamide to melamine begins in said system starting at a temperature of about 100 ° C At temperatures of about 180 ^c C the reaction is complete in 10-30 minutes. Under such reaction conditions, the pressure in the apparatus can also be increased to values higher than atmospheric pressure, e.g. B. increase to 3 to 5 ata. On the other hand, working under a slight vacuum (e.g. up to 100 torr) gives melamine in high yield. Instead of using dicyandiamide, the reaction can be carried out using cyanamide or mixtures of dicyandiamide and cyanamide. The conversion of the cyanamide to melamine has proven to be particularly advantageous. The use of alkali is lower than when using dicyandiamide as the starting substance. It is also possible to work with aqueous cyanamide solution and in this way, without isolating a solid intermediate product from the aqueous cyanamide solution obtained during the hydrolysis of calcium cyanamide, after concentration to 50 to 80% cyanamide content, the crystalline melamine can be obtained directly. The strongly exothermic reaction of the trimerization of the cyanamide to form melamine is sufficient to evaporate the water in the cyanamide solution and to maintain the desired reaction temperature. Surprisingly, under the reaction conditions mentioned, neither ammeline nor ammelide are formed in spite of the presence of water. The melamine primarily obtained from the solution is so pure that it does not need to be recrystallized for further processing.

When using crystalline cyanamide or dicyan-

(, 0 diamid the following working method is recommended: cyanamide or dicyandiamide are dissolved or mashed in the relevant solvent in the cold and in a A mixture of alkali metal or alkaline earth metal hydroxide in the same solvent heated to the reaction temperature

(15 and a trace of cyanamide or dicyandiamide. After the addition has ended, the mixture is left at the chosen reaction temperature for a further 10-30 minutes react again. The small amount of cyanamide or

Dicyandiamide in the template causes thermal stabilization of the solvent.

When working with aqueous cyanamide solution, it is preferably hot in about 180 ⁰ C mixture of alkali metal or alkaline earth metal hydroxide and solvent, which also contains a small amount of cyanamide or dicyandiamide to stabilize, to the extent allowed to run in, that the reaction temperature condition in the Bleibl Any water escaping in vapor form is drawn off.

The melamine that forms under the reaction conditions separates into crystalline ones on cooling Shape. It is separated, e.g. B. by filtering off or centrifugation, and then either slurried in water and filtered again or in the Centrifuge washed with water. This product obtained in this way is completely colorless and has a melamine content of over 993%; it is free from both Hydrolysis products such as ammeline and ammelide than colored deamination products such as Mieren, Melam or Melon and represents a product that can be sold immediately, as it does not require any further Purification operations used for condensation reactions and thus for melamine resin production can be used.

With continuous operation, the small amount of melamine remaining in the mother liquor (approx.

3 to 5%) not isolated, but the next batch added Since there is no concentration of impurities, several consecutive Approaches obtained yields of over 97% of high-purity melamine; also reduced the need for alkali metal or alkaline earth metal base required as a catalyst.

The process according to the invention for the production of melamine from calcium cyanamide or aqueous cyanamide solution represents a significant technical advance compared to the previously practiced and described method because

1. the melamine is produced in a one-step reaction from cyanamide,

2. the isolation of solid substances as intermediate products is unnecessary,

3. Much lower temperatures are required and generally working below about atmospheric pressure is sufficient,

4. Capital, wage and operating costs as a result of the one-step process from cyanamide solution be reduced considerably,

5. high purity! Melamine as a reaction product without Recrystallization is obtained.

The following examples are intended to explain the process:

example 1

In a quickly ^{heated to 180 0} C mixture of 80 g of dimethyl sulfoxide, 6 g of powdered potassium hydroxide and

4 g of dicyandiamide is a solution of 60 g of dicyandiamide in 120 g of dimethyl sulfoxide within 22 Dripped in minutes while maintaining the reaction temperature at about 180 ° C. After the end of the reaction will allowed to react for a further 5 minutes at 180 ° C and then cooled. During the cooling process, most of the melamine separates out in crystalline form. It is filtered off with suction, stirred up with water and filtered again. 50.8 g of pure colorless melamine are obtained, corresponding to a yield of 79.4%. The filtrate still contains 8.25 g of melamine or 12.8%. the The total yield of melamine is therefore 92.2%. in the 1.37 g or 2.1% of unreacted dicyandiamide were found in the filtrate.

. Example 2

A solution of 63 g of solid cyanamide in 40 g of dimethyl sulfoxide is heated to 180.degree Mixture of 100 g dimethyl sulfoxide, 4 g potassium hydroxide and 1 g cyanamide within 8 minutes

ίο dropped in. After a further 12 minutes reaction time at 180 ° C is cooled. The precipitated melamine is filtered off, slurried with water and sucked off. After drying, 56 g of melamine crystals, which corresponds to a yield of 87.7%, receive. The filtrate and wash water contain another 2.5 g or 3.9% melamine. the The total yield is therefore 91.4%. In the filtrate 3.9% diamide is also detectable.

Example 3

A solution of 63 g of solid cyanamide in 40 g of SuJfolan is added dropwise to a mixture, heated to 180 ° C., of 100 g of sulfolane and 4 g of potassium hydroxide over the course of 12 minutes. After a reaction time of 30 minutes at approx. 180 ^° C., the mixture is cooled. The melamine which has crystallized out is filtered off, slurried with water and suction filtered. After drying, 50 g of melamine, corresponding to a 79.4% yield, are obtained. 2.4 g or 3.8% of melamine are still dissolved in the filtrate and wash water. The total yield is therefore 83.2% melamine. The filtrate still contains 9.3% unreacted dicyandiamide.

Example 4

A solution of 30 g of dicyandiamide and 30 g of solid cyanamide in 120 g of dimethyl sulfoxide is introduced into a mixture, heated to 180 ° C., of 4.0 g of potassium hydroxide, 4.0 g of dicyandiamide in 80 g of dimethyl sulfoxide. After a further 5 minutes' reaction time at 180 ^° C., the mixture is cooled. The precipitated melamine is filtered off, slurried with water and filtered again. The first yield is 53.8 g of melamine or 84.1%. The second yield found in the filtrate and wash water was 5.7 g or 8.9% of melamine, so that the overall yield was 93.0%. Unreacted dicyandiamide was found to be 1.43 g or 2.24%.

Example 5

113 g of dimethyl sulfone is melted and heated to 100 ⁰ C. 63 g of dicyandiamide and 5.6 g of powdered potassium hydroxide are introduced into the melt. The temperature of the mixture rises rapidly to 210 ^° C. as a result of the exothermic reaction. After cooling to 6O ⁰ C, the reaction mass is taken up with water, filtered off with suction and freed by washing with water of dimethylsulfone. After drying, 44 g of melamine are obtained, which corresponds to 70% of the theoretically possible amount. 8.31 g or 13% of melamine are still present in the filtrate, so that the overall yield is 83%. Dicyandiamide can no longer be detected in the filtrate.

Example 6

In a heated to 18O ⁰ C mixture of 140 g of dimethyl sulfoxide, 2.0 g of potassium hydroxide and 1 g of solid cyanamide is 84 g of a 50% strength aqueous cyanamide solution at such a rate einge-

drops that the reaction ^{temperature of 180 0} C is maintained. During the dropping time, which lasts 25 minutes, water and some dimethyl sulfoxide distill off. The reaction is allowed to continue for 10 minutes at 180 ° C. and the mixture is cooled. After dilution with a little water, the melamine crystals are filtered off, washed and dried. 36.4 g of melamine are obtained, corresponding to a yield of 84.6%. The filtrate still contains 3.82 g or 8.8% melamine, so that the total yield is 93.4%. 0.77 g or 1.8% of dicyandiamide were still found in the filtrate.

Example 7

In a heated at 180 ⁰ C mixture of 280 Dimethyisuifoxid g, 3 g and Kaiiumhydroxid ig solid cyanamide 105 g of a 80% strength aqueous cyanamide solution at such a rate dropped, that the reaction temperature is maintained from 180 ° C. During the dropping time of about 30 minutes, the water is distilled off with a little dimethyl sulfoxide. The reaction is allowed to continue for 10 minutes at 180.degree. ^C. and then cooled. After cooling to 60 ° C is diluted with water and further cooled to 20 ⁰ C. After filtering off and drying, 78.1 g of melamine are obtained, which corresponds to a 92% yield. The filtrate still contains 3.15 g of melamine or 3.7%, so that the overall yield is 95.7%. In addition, 0.8 g of dicyandiamide or 0.95% are still dissolved in the filtrate.

Example 8

A solution of 63 g of solid cyanamide in 40 g of dimethyl sulfoxide is added dropwise to a mixture of 100 g of dimethyl sulfoxide and 5 g of calcium hydroxide heated to 180 ^{° C. over the course of 12 minutes.} After a further reaction time of 10 minutes at approx. 180 ^° C., the mixture is cooled. The melamine which has precipitated is filtered off, slurried with water and filtered again. After drying, 24.2 g or 38.4% melamine are obtained. The filtrate still contains 6.64 g or 104% melamine. The total yield is therefore 48.9%. The filtrate still contains 43.6% (27.5 g) dicyandiamide.

Example 9

In a heated at 180 ⁰ C mixture of 280 dimethylsulfoxide g, 4 g of sodium hydroxide and 1 g of solid cyanamide is 105 g of a 80% strength aqueous cyanamide solution to the extent dripped in that the reaction temperature is maintained from 180 ⁰ C during the 30 minute dropwise addition, the water distilled from the cyanamide solution with a little dimethyl sulfoxide. After cooling to 6O ⁰ C is diluted with water and cooled to 20 ° C. After filtration and drying 733 g of melamine are obtained. In the filtrate, a further 8 g of melamine are dissolved so that the overall yield of the melamine is 96.2%. The filtrate still contains 04 g of dicyandiamide.

Example 10

A solution of 83 g of solid cyanamide in 57 g of dimethyl sulfoxide is heated at 160 g in a ⁰ C mixture of 130 of dimethylsulfoxide, 5.2 g Kaliumhy hydroxide and 1 g of cyanamide are added dropwise within 45 minutes. After a further 15 minutes' reaction time at 160 ^° C., the mixture is cooled. The precipitated melamine is filtered off, slurried with water and filtered again. After drying, 59 g of melamine, corresponding to 70.2%, are obtained. There are still 4 g of melamine or 4.8% dissolved in the filtrate and wash water. The overall yield is accordingly 75%.

Example 11

In a heated to 135 ° C mixture of 187 g of dimethyl sulfoxide, 10.6 g of potassium hydroxide and 1 g of dicyandiamide 83 g of diamide are entered within 40 minutes. After a further 5 minutes reaction time the mixture is cooled at 135 ° C. The melamine which has precipitated is filtered off with suction, stirred up with water and again filtered. After drying, 48.3 g of melamine corresponding to 57.5% are obtained. In the filtrate and wash water there are still 6.45 g or 7.7% melamine in solution. The total yield is therefore 65.2%.

Example 12

A solution consisting of 3.5 g of potassium hydroxide, 1 g of crystalline cyanamide and 300 g of diethyl sulfoxide is kept at the boil at about 150 ° C. by applying a slight vacuum of about 200 torr. 84 g of cyanamide in the form of its 80% strength aqueous solution are added to this mixture in such a way that the reaction ^{temperature of 150 °} C. is maintained and the water is immediately distilled off, some dimethyl sulfoxide also passing over. After completion of the addition of the cyanamide solution, the reaction mixture is held for another 60 minutes at 150 ⁰ C long, then cooled to 6O ⁰ C, diluted with 200 ml of cold water, and centrifuged at 20 ° C. 79.7 g of highly pure melamine are obtained. The filtrate contains a further 3.7 g of melamine, so that the overall yield is 98.2%; the filtrate also contains 0.3 g of dissolved dicyandiamide

Example 13

In the following series of tests, the reaction was carried out with an aqueous 80% strength cyanamide solution. the solvent is dimethylsulfoxide and Kaiiumhydroxid as a catalyst carried out, the reaction temperature was 180 ⁰ C. In the first experiment the cyanamide in the heated to 180 ° C mixture of Kaiiumhydroxid and dimethyl sulfoxide was allowed to enter. Quantities and reaction conditions are recorded in the table below. The filtrate from the previous experiment was always used in the following reactions Melamine is centrifuged off and the filtrate is used again. This melamine obtained primarily was filtered into a slurry of water and dried. It is referred to as melamine I. The melamine in the filter is given as melamine II.

9 g dimethyl
sulfoxide 25 36 689 g Fillrat 10 left drip time,
Min. Post-reaction
at 180 ⁰ C,
Min. 1960
830.4
913.4
976.9 g cyan
amide g potassium
hydroxide
(total) 1129.6
1046.6
983.1 56
32
28
24 ο ο ο ο a) (1st implementation)
b) (2nd implementation)
c) (3rd implementation)
d) (4th implementation) 595
595
595
595 21
21
21
21

The following yields of melamine, based in each case on the cyanamide used, were obtained:

Melamine I Melamine II Dicyandiamide
ο / ο a) 87.6 7.0 0.9 93 5.2 0.5 C) 95.3 3.7 0.8 d) 97.2 2.1 0.8

In the case of continuous production of melamine by the process according to the invention with reuse No accumulation of by-products was found in the filtrate.

Claims (1)

Patent claims: 1. A process for the preparation of melamine from crystalline or dissolved in water cyanamide and / or dicyandiamide at about atmospheric pressure in an organic solvent at 100 to 250 ⁰ C, characterized in that the reaction in solvents or mixtures of solvents of the general formula