JPH08333351A - Purification of melamine - Google Patents

Abstract

PURPOSE: To provide a method for purifying melamine capable of reducing an acidic impurity contained in melamine by extraction with a small amount of water without making the crystal particle diameter of melamine, especially melamine by a dry method so large and obtaining an excellent result in a quality test by resinification rate at a low cost. CONSTITUTION: Melamine in an amount of 1.5-10 times a saturated dissolved amount of melamine at 80-150 deg.C, especially obtained by a dry method is mixed with water and adjusted to pH7 to 12 to give a melamine slurry. The slurry is heated to 80-150 deg.C while stirring, cooled to <=80 deg.C, crystallized melamine is filtered and dried. The heating and the cooling may be repeated as required.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an industrially advantageous method for recovering and purifying melamine. More specifically, it relates to a method for obtaining high-quality melamine by extracting and removing acidic impurities in melamine with water or an alkaline aqueous solution at 80 to 150 ° C.

[0002]

BACKGROUND OF THE INVENTION In the industrial synthesis of melamine, urea and / or its pyrolysis products are heated to a temperature of 350 to 400 ° C. in the presence of a catalyst and added ammonia.
As a method for recovering melamine from the produced crude melamine, a recrystallization method obtained by cooling with water or an aqueous medium and dissolving in water to crystallize melamine, and a synthesis gas containing melamine vapor at 170 to 250 ° C. There is a dry method in which melamine is cooled and selectively crystallized and separated from a gas stream.

[0003]

The crystallization and recovery of crude melamine by the recrystallization method requires a large amount of water and an excessive cooling device, and further requires a crushing step because the crystal size of the obtained crystals is large. There is a drawback that Further, the amount of melamine-condensed impurities (ureide melamine, melam, melem, etc.) contained in the obtained melamine is considerably higher than that in the dry method.

On the other hand, although water is not required for the recovery of melamine by the dry method, a large amount of acidic impurities such as ammelide and cyanuric acid are contained, which adversely affects the resinification rate which is one of the quality evaluation standards. I'm giving. The resinification rate is the resinification (methylolization) by reacting 40 g of melamine with 88 ml of formalin at a temperature of 85 ° C.
It is a value obtained by measuring the time required until the above is recognized, and it is generally said that the resinification rate is slower and the quality is better as the amount of acidic impurities is smaller.

In the dry method, a synthesis gas containing melamine produced by the thermal decomposition of urea or a thermal decomposition product of urea is mixed with a cold inert gas and cooled to a temperature at which unreacted urea does not condense, and melamine is separated by crystallization. To do. The off-gas composed of carbon dioxide, ammonia, unreacted urea and the like after separating melamine is recycled for the urea production process and the above cold inert gas. The melamine obtained by the crystallization separation contains a trace amount of impurities, which can be reduced by changing the conditions of the reaction and other steps.
For example, deammonification condensates of melamine such as melam, melem, cyanomelamine, and ureidomelamine can be reduced by increasing the amount of ammonia present during melamine synthesis. Further, the production of acidic impurities such as cyanuric acid and ammelide, which are caused by the hydrolysis of melamine, can be suppressed by this method.

However, since the amount of the acidic impurities in the melamine obtained by the dry method is much larger than that in the recrystallization method, the resinification rate is still high. Specifically, the recrystallized product has a cyanuric acid content of 15 ppm or less and ammelide of 10 ppm or less, whereas the dry product has a content of 20 ppm.
Thus, the resinification rate is about 90 minutes for the recrystallized product, and about 60 to 70 minutes for the dry product.

[0007]

Means for Solving the Problems The inventors of the present invention have intensively studied to solve the above problems, and as a result, the impurities in the melamine obtained by the dry method, especially the acid type impurities, are converted into a small amount of water or an alkaline aqueous solution. The present invention was completed by solving these problems without increasing the melamine crystal grain size by extracting and removing it in a slurry state. That is, the method for purifying melamine of the present invention comprises mixing melamine in an amount of 1.5 to 10 times the saturated dissolution amount at the heat treatment temperature with water to obtain a pH of 7
8 while stirring the melamine slurry adjusted to ~ 12
After being heated to 0 to 150 ° C., it is cooled to 80 ° C. or lower, and the crystallized melamine is filtered and dried.

Hereinafter, the present invention will be described in detail. The solubility of melamine in water under normal pressure is very low, eg 30 ° C.
0.5wt%, 100wtC 5wt%, 160degC 30
wt%. Therefore, in the recrystallization method, melamine is completely dissolved in a large amount of water, recrystallized and filtered to obtain crystals. In the present invention, since melamine is suspended and heated in a slurry in a small amount of water to extract impurities into an aqueous phase, and then cooled and filtered to obtain melamine, a large amount of water and an excessive cooling device are not necessary.

The concentration of melamine in this slurry method is
Solubility at a heat treatment temperature of 80 to 150 ° C. of 1.5 to
It is selected at a range of 10 times so that the slurry can be easily stirred. This slurry is 80 to 150 ° C., preferably 90 to 1
After heating and stirring at 30 ° C., 80 ° C. or lower, preferably 30
Cool to ~ 50 ° C and filter the crystallized melamine to dryness. Impurity extraction by the present slurry method can be more suitably performed with an alkaline aqueous solution (pH 9 to 12) to which sodium carbonate is added, and under alkaline conditions, the reduction amount of acidic impurities increases. Condensation-type impurities such as ureidomelamine, melam, and melem are not removed even by alkali dissolution, but there is no problem because the content in dry process melamine is originally small.

The heating and cooling operations in this slurry method were repeated a plurality of times (3 to 13 times) and compared, but no significant difference was observed in the quality of impurities and the like due to the number of operations. Further, the particle size of the recrystallized product after complete dissolution is about 10 times as large as that of the raw material melamine, but it is about twice as large in the present slurry method, and the product crushing step required in the recrystallization method is unnecessary.

[0011]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Example 1 8 g of 200 g of melamine having a particle size of 21 μm produced by the dry method was used.
A 10% aqueous sodium carbonate solution is added to a slurry liquid suspended in 00 g of water to adjust the pH to 10. The slurry is heated to 95 ° C. in a water bath with stirring and then cooled to 30 ° C. This heating and cooling operation is 3
After repeated, filtered and washed with 700 g of water, 10
Dry at 0 ° C. The quality of the melamine obtained is shown in Table 1.

Examples 2 to 4 The same procedure as in Example 1 was repeated except that heating and cooling were repeated a plurality of times. The quality of the melamine obtained is shown in Table 1.

Comparative Example 1 50 g of melamine is mixed with 1200 g of water, and a 10% sodium carbonate aqueous solution is added to adjust the pH to 10. While stirring, this was heated to 95 ° C in a water bath to completely dissolve melamine, then cooled to 30 ° C, filtered, and
After washing with 00 g of water, it is dried at 100 ° C. Table 1 shows the quality of the obtained melamine crystals.

Comparative Example 2 The same procedure as in Comparative Example 1 was carried out without adding sodium carbonate. Table 1 shows the quality of the obtained melamine crystals.

Comparative Example 3 600 g of melamine was mixed with 1 kg of water, steam was blown into the solution to raise the temperature of the solution to 95 ° C., and then 10 with stirring.
% Sodium carbonate aqueous solution is added to adjust the pH to 10. After completely dissolving the melamine in this way, it is cooled to 30 ° C., filtered, washed with water, and dried at 100 ° C. Table 1 shows the quality of the obtained melamine crystals.

[0017]

[Table 1]

[0018]

EFFECTS OF THE INVENTION According to the method of the present invention, acidic impurities are extracted and reduced with a small amount of water without increasing the crystal grain size of melamine, particularly melamine by the dry method, and the quality can be easily reduced at low cost. It can be improved and is industrially extremely advantageous.

Claims (2)

[Claims] 1. A saturated dissolution amount of 1.5 at a heat treatment temperature.
Mixing melamine in an amount of 10 times with water, and stirring the slurry of melamine adjusted to pH 7 to 12 to 80 to 150.
A method for purifying melamine, which comprises heating to 80 ° C., cooling to 80 ° C. or lower, filtering and drying the crystallized melamine. 2. The method for purifying melamine according to claim 1, wherein melamine produced by a dry method is used.