Method for the production of urea granules

 

(57) Abstract:

Usage: the invention relates to a method of producing urea granules from a melt or solution of urea, which are used as fertilizers alone and in combination with mineral additives. The essence of the method is that in the melt or solution of urea before granulation enter granulation additive selected from a range that contains formaldehyde, metrolotion, condensation products of urea and formaldehyde, hexamethylenetetramine in this period of time, to the mixture of the additive and melt or solution of urea had contact during at least 20 but not more than 20 minutes Granulation are spraying in the form of very fine droplets in a fluidized bed of particles of urea. 5 C.p. f-crystals, 1 tab., 1 Il.

The invention relates to a method of manufacturing granules of urea.

There are various methods of production of urea granules. In the past the predominant method was granulating drying droplets of sprayed molten salts, which essentially anhydrous molten urea (having a water content of not more than 0.1 to 0.3 wt.%) sprayed from the top of the column granulation curing was caputserpentis with the formation of the so-called granules (pellets). These granules (pellets) had a relatively small maximum diameter and were mechanically weak.

The urea granules having larger dimensions and better mechanical properties currently receive granulation essentially anhydrous molten urea or an aqueous urea solution in the fluidized bed (e.g., U.S. patent N 4219589). In the method presented in this patent, an aqueous solution of urea having a concentration of urea 70 to 99.9 wt.%, preferably 85 to 96 wt.%, sprayed in the form of very fine droplets having an average diameter of 20 to 120 μm, in a fluidized bed of particles of urea at a temperature at which water evaporates from the solution, sprayed on the particles, and urea cures on the particles to form granules having a desired size, which may be 2.5 mm. Because in this way formed a fairly large amount of flying dust, especially if the urea solution used as starting material contains more than 5% by weight, in particular more than 10 wt.% water to the urea solution is preferably added retarder crystallization or granulation additive to urea, in particular water-soluble additive or prod the Finance flying dust. In the result of the presence of granulation additive granules remain plastic during formation, by rolling and/or collisions during the formation can be obtained mechanically strong, smooth and round granules.

The resulting granules have a high resistance to crushing, high impact strength and a low tendency to form flying dust by abrasion, and, moreover, they are good even during prolonged storage, although urea shows a strong natural tendency to caking.

Although the method presented in the description of the cited U.S. patents, is a big step forward in relation to the properties of the granules of urea, it was found that further improvement, particularly in relation to the amount of flying dust from the final product and dust formation during granulation.

The purpose of this invention is the provision of a method of production of urea granules by granulation techniques, preferably such as described in U.S. patent N 4219539, when the amount of dust formed during granulation, and the number of fly dust from the final product decreases.

is spalania melt or solution of urea, which also contains as a granulation additive formaldehyde, metrologiya or hexamethylenetetramine, in the form of very fine droplets, in a fluidized bed of particles of urea, with granulation additive is added before granulation during such period, to the mixture mentioned granulation additives and melt or solution of urea was preconditional for at least 20 but not more than 20 minutes

It was found that by adding a granulation additive in such period to the granulation mixture of the additive and urea had the time specified contact will receive an improved method and an improved product.

More specifically, the method is more economical by reducing the number of dust during granulation. It also reduces the amount of liquid used for washing gas from the granulator and, consequently, the amount of steam for evaporation of water from the above-mentioned urea solution.

In addition, significantly reduces the amount of dust from abrasion of the final product, which is an important advantage for further treatment of the material.

The optimal value of the time of contact depends on various fakebake and chemical composition of the additive.

In particular, it was noted that at a shorter time of contact is not received reduced dust formation and reduced the amount of flying dust.

The increase in time of contact of more than 20 min resulted in unacceptably high quantities biureta and triuret.

Preferred time of contact is between 25 and 20 min, because in this range receive optimal results.

Preferred granulated additives are formaldehyde and water-soluble adducts and/or condensation products of formaldehyde and urea. Production of water-soluble adducts of formaldehyde and urea is known (for example, U.S. patent N 3067177), and the production of water-soluble condensation products of formaldehyde and urea are presented in U.S. patent N 3112343.

You can also use adducts of formaldehyde and urea, obtained first in an alkaline medium and then condensed in an acid environment with formation from thin to syrupy liquids, for example, liquid glues used in the manufacture of recycled cardboard.

The method of the present invention can be used in the traditional production of urea in normal g is s synthetic urea solution, received from the factory for the production of urea, the concentration of which is optionally subsequently brought to the desired water content, mixed with granulation additive, preferably in a quantity of 0.3 - 0.8 wt. % calculated in terms of formaldehyde, relative to the weight of the final granules.

Mixing can be done at any step of the method, provided that the necessary time of contact. If you want, you can include a separate buffer capacity after devices for mixing and before the fluidized layer. Alternative granulation additive can be added before evaporation of water from the urea solution, provided that the time of contact remains within the specified range.

In addition, it was found that in the process of the present invention is formed of the composition of the granules, which perfectly restored, whereby also prevents the formation flying dust, the obtained granules of urea have very high resistance to crushing and a very high density. Another feature is that the urea granules obtained in accordance with the invention, are good even when normalidad, calculated with respect to the final product. If desired, can be used a higher ratio, but it does not make any special advantages. The additive can be added in any suitable form, but preferably in the form of concentrated aqueous solutions, resulting in the concentration is in the range of 30 to 85 wt.%.

Granules after their formation is preferably cooled to a temperature that is compatible with the ambient temperature, for example up to 40 - 45oC for tropical conditions or before 30oC for temperate climatic conditions, for example, by a flow of air, the moisture content of which is preferably reduced to such an extent that during the cooling process, the granules do not absorb moisture from the air.

The urea granules obtained in accordance with the present invention, can be advantageously used in combination with superphosphate and triple superphosphate. Additionally, there may be used, and other components, for example potassium fertilizer (usually KCl).

Granulation of urea can be accomplished by spraying the aqueous solution onto a fluidized bed germ crystallization of urea, while the above solution elemosina 85 - 96 wt. %.

The urea solution is sprayed gas such as air. Preferably the solution is sprayed into the fluidized layer of nuclei of crystallization, as the spray layer includes risk, consisting in the fact that the sprayed droplets are fond of air fluidized, exiting. The spraying air pressure is preferably 118 - 392 kPa (1,2 - 4,0 bar). This pressure has a very significant impact on the size of the sprayed droplets. The higher the pressure, the smaller the sprayed droplets. The average diameter of the droplets is preferably 25 to 250 μm.

The size of nuclei of crystallization of urea fed to a fluidized bed in which the granulation, is usually 0.2 to 3 mm, but can be more when you get the urea granules are larger in size.

The temperature of the fluidized bed germ crystallization of urea is typically in the range 70 - 115oC, preferably 80 to 110oC. Within these temperature limits may be lower, since the concentration of urea in solution, a spray on embryos crystallization above. The temperature of the fluidized bed can be adjusted by appropriate choice of temperature pseudoo the nuclei of crystallization of urea in the form of very fine droplets. Under the influence of the temperature prevailing in the fluidized bed, the water evaporates from the solution and urea crystallizes on the surface of nuclei of crystallization of urea. Due to the small size of the droplets they usually cover only a portion of the surface of the individual nuclei of crystallization of urea. Thus, preventing the formation of patterns of granules, like an onion, in which the nuclei of crystallization consistently covered essentially superimposed layers. As a result, the granules in accordance with the present invention do not have the stresses inherent in the structure, similar to the onion. I believe that the excellent mechanical properties of granules of urea in accordance with this invention are obtained through the absence of these stresses. Another advantage of the smallest droplet size of the spray of the urea solution is that it can be completely evaporate the water within a short period of time.

The product obtained by the method in accordance with the present invention, contains only a small amount of free NH3, CO2, moisture and biureta and has such mechanical properties that are suitable for pneumatic transpot method in accordance with the present invention is during the granulation almost completely prevented the formation of biureta. Thus, by spraying the urea solution having, for example, the content of urea 75 - 85% and content biureta less than 0.1%, it is possible to obtain granules of urea content biureta less than 0.1%.

The urea granules obtained by the method of this invention are particularly suitable for coating, for example, gray, for the formation of slowly released granules, due to the excellent sphericity and a closed surface which minimizes the required amount of coating material.

The method in accordance with the present invention may be implemented in any type of granulator, fluidized bed. One example of a suitable apparatus is shown schematically in the drawing, which shows a granulator 1, is divided into many chapters 2, 3 and 4 for granulation and subsequent separation 5 and 6 for subsequent cooling and drying of the granules of urea. Granulator 1 contains the grating 7, which facilitates fluid layer and passes the air fluidization may preheated in one or more heaters (not shown) and supplied through the pipe 8. Space is I, in this case, the air fluidized served in each of these departments. The granulator 1, in addition, provided at the bottom of the pneumatic systems of the atomizer 9, 10 and 11, which extend above the grating 7. You can use two or more systems atomizer in each Department.

Through these nozzles of the urea solution, to which is added granulation additive supplied through the pipe 13, is sprayed with an air flow supplied through the pipe 14, in the granulation process 2, 3 and 4. The urea solution is served in the buffer tank 13cthrough the line 13a. At the same time granulation additive is introduced into the buffer tank through the line 13in. The sizes of buffer capacity is such that it is guaranteed sufficient time of contact. Fluidized bed consists of nuclei of crystallization of urea.

For further conditioning, cooling and possible drying of the granules in the branches 5 and 6 granulator 1 is supplied by a pipe 16 to supply air.

To remove air and possibly increased dust particles pellet mill 1 has a discharge pipes 17 and 18, which are attached to the scrubber 19, in which the air is washed resbalon the lo to achieve high efficiency washing, water can be sprayed into the air through the nozzles 23. The air removed from the dust that can be released through the discharge pipe 24, and formed a dilute solution of urea release through line 25.

The granulator 1, in addition, contains a bottom slope 26 granular urea, located above the vibrating chute 27, where the pellets are transported to a screening device (rumble) 28 where it is separated into fractions, namely on the lower poagregatnoy fraction having the desired dimensions, and the upper aggregato faction. The fraction having the desired dimensions, not necessarily pass through refrigerator 29 to the storage location where you can perform further separation into fractions carried out for different purposes. Optionally, the refrigerator can be placed upstream from the screening device (screen).

Top aggregato fraction of granules, separated in the screening device (thunder) 28, is transported to the crusher 30, in which this fraction is crushed to the required size, so that it could serve as a fresh embryos crystallization. Bottom poagregatnoy fraction separated in the screening device (thunder) 28, is passed through conduit 32 to the conduit 31 VM is guilt, deleted from the system in the store, replace with fresh embryos crystallization of urea from the crusher.

The granule size of the product depends on a number of factors, for example, the number of nuclei of crystallization of urea in the fluidized bed, the size of nuclei of crystallization, the amount of the urea solution sprayed per unit of time, and time spent germ layer. Thus, for example, product granules of a larger size you can get, if you reduce the number of nuclei of crystallization in a fluidized bed and to increase the residence time. In order to maintain a predetermined granulometric composition of the product, it is necessary that the content of the layer was kept constant as much as possible, both in terms of particle size distribution and the ratio of the number of nuclei of crystallization. This can be achieved by ensuring that the amount by weight of embryos crystallization of urea with the correct particle size, add in a fluidized bed, always in accordance with the amount by weight of granules of the product removed from the layer.

If for one reason or another happens discard is both: if the product becomes large, larger aggregato fraction separated in the screening device (thunder) 28, and the load of the crusher 30 will increase, and through line 31 into the fluidized bed in the granulator 1 serves a greater number of nuclei of crystallization, whereby decreasing the average diameter of the granules. The operation of the crusher 30 regulate properly: if the crushed product is too fine-grained, in the fluidized bed is fed too much dust, from whence it is carried away by the gas fluidization or causes agglomeration, if the crushed product is too coarse, in a fluidized bed serves very few nuclei of crystallization.

In the installation, which is shown in the drawing, grained 1700 t/d of urea. To urea added granulation additive, based on the condensation of urea and formaldehyde, in an amount corresponding to 0.50 wt.% CH2O, calculated with respect to the final product. The time of contact of granulation additives and melt the urea modified. The table presents the results of various experiments.

1. Method for the production of urea granules from a melt or solution of urea by spraying the melt or solution of urea and granulation additives, the, hexamethylenetetramine, in the form of very fine droplets in a fluidized bed of particles of urea, wherein the granular additive is introduced into the melt or solution of urea before granulation in such a period of time to the granulation mixture of the additive and melt or solution of urea had contact during at least 20 but not more than 20 minutes

2. The method according to p. 1, characterized in that the contact time is 25 to

15 minutes

3. The method according to p. 1 or 2, characterized in that the quantity of granulation additive is 0.3 to 0.8 wt. calculated in terms of formaldehyde, relative to the weight of the urea granules.

4. The method according to PP. 1 to 3, characterized in that the time required to provide contact to the presence of a buffer tank between the cell for mixing and fluidized bed.

5. The method according to PP. 1 to 4, characterized in that the time required to provide contact by adding a granulation additive to urea solution before evaporation of water to obtain a melt or solution of urea supplied to the spray.

6. The method according to PP. 1 to 5, characterized in that the size of very small droplets of 25 to 250 microns.

 

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