FI71721C - Process for the preparation of urea granules and their use in fertilizer mixture. - Google Patents

Description

1 71721

Method for preparing urea granules and their use in a fertilizer mixture

The invention relates to a process for preparing urea granules. Various methods are known for the preparation of urea granules. Thus, for example, in the process described in Dutch patent application 7 806 213, corresponding to F1 patent publication 65 713, an aqueous urea solution having a urea content of 70 to 99.9% by weight, preferably 85 to 96% by weight, is sprayed in very small droplets with an average a diameter of 20 to 120 microns, in a fluidized bed of urea particles at a temperature at which water evaporates from the solution sprayed onto the surface of the particles and the urea solidifies on the surface of the particles to form granules of the desired size, which may be 25 mm or larger. Since relatively large amounts of airborne dust are formed in this process, especially if the urea solution used contains more than 5% by weight of water, in particular more than 10% by weight of water, a urea crystallization retarder, in particular formaldehyde and 2 71721 urea water-soluble addition or condensation products, is preferably added to the urea solution, whereby the formation of air dust is prevented almost completely. Due to the crystallization retarder, the granules remain plastic when they are formed, so that mechanically strong, smooth and round granules can be formed during formation due to rotation and / or shocks.

The obtained granules have a high crushing resistance, a high impact resistance and only a slight tendency to form airborne dust due to abrasion. In addition, they do not clump-nu during storage, although urea has a strong inherent tendency to clump.

Urea granules prepared by any known method cannot be used to prepare heterogeneous dual and triple fertilizer mixtures such as N-P or N-P-K mixtures by mixing them with cheap superphosphate or triple superphosphate because such urea granules are incompatible with these phosphates. Such mixtures of urea particles and superphosphate or triple superphosphate granules hydrate after some time, forming a difficult-to-handle and unusable sludge. G. Hoffmeister and H.G. According to Megar's presentation at the meeting "The Fertilizer Industry Round Table" on 6.11. 1975 In Washington D.Cr, this incompatibility is caused by a reaction according to the following equation

Ca (H 2 PO 4) 2 * H 2 O + 4 CO (NH 2) 2 ->

Ca (H 2 PO 4) 2-4 CO (NH 2) 2 + H 2 O 1 mole of monocalcium phosphate monohydrate, which is the main constituent of superphosphate and triple superphosphate, reacts with 4 moles of urea to form a urea mono-calcium phosphate adduct, releasing 1 mole of water. Because the adduct has a high degree of solubility, it readily dissolves in the water released, forming a large amount of solution that wets the mixture granules, thereby continuing the reaction at an even higher rate.

71721 To date, no commercially acceptable means has been invented to make urea compatible with superphosphate or triple superphosphate. Thus, in fact, only the more expensive phosphate fertilizers monoammonium phosphate and diammonium phosphate can be largely mixed with urea.

It is an object of the present invention to provide a process for the preparation of urea granules having unique properties, including compatibility with superphosphate and triple superphosphate granules.

According to the invention, there is provided a process for preparing urea granules by spraying an aqueous urea solution to which a urea crystallization retardant has been added, in the form of very small droplets, into a fluidized bed of urea particles at a temperature where water evaporates from solution and urea solidifies into granules of desired size. The process according to the invention is characterized in that MgO is used as the crystallization retardant as such or as selectively or completely calcined dolomite in an amount corresponding to 0.1-2% by weight of MgO, calculated on the urea in solution.

Surprisingly, it has been found that the granules according to the invention are compatible with superphosphate and triple superphosphate granules, which makes them suitable for mass mixing with these phosphate fertilizers. Mixtures of urea granules obtained according to the invention and superphosphate or triple superphosphate granules, which were subjected to the "TVA" flask test at 27 ° C, were still dry after 7 weeks, while the corresponding mixtures containing urea granules not prepared according to the invention were completely hydrogenated. After 3 days.

71721

In addition, it has been found that when magnesium oxide is present during the granulation of the urea melt or solution, the formation of granules proceeds smoothly and the formation of air dust is inhibited, in addition, the obtained urea granules have a very good crushing resistance and a very high apparent specific gravity. Another very surprising feature is that the urea granules prepared according to the invention do not clump even after long storage.

Magnesium oxide can be added as such (MgO) or as fully calcined dolomite (MgO + CaO) or as selectively calcined dolomite (MgO + CaCO 2). The advantageous effect is already noticeable when the amount of additive corresponds to 0.1% by weight of MgO, based on the weight of the urea of the melt or solution. Preferably, the additive is used in an amount corresponding to 0.5-2% by weight of MgO, based on the weight of the urea of the solution or melt. The additive may be added as a powder to the urea melt or solution prior to spherical granulation or granulation.

Preferably, after formation, the granules are cooled to a temperature of 30 ° C or lower, for example by means of an air stream, the moisture content of which is preferably reduced to such an extent that the granules do not absorb moisture from the cool air during the cooling step.

The invention also relates to a heterogeneous fertilizer mixture of urea granules with one or more other granular fertilizers.

In addition to urea and superphosphate or triple superphosphate, the mixture most often contains a potassium fertilizer such as KCl. To prevent separation of the mixture, the grain sizes of the components to be mixed must be matched to each other.

71721

The following examples illustrate the effectiveness of the method according to the invention. The purpose of the "TVA" bottle test mentioned in the examples is to find out the compatibility of urea granules with superphosphate and triple superphosphate granules. The mixture of urea granules and superphosphate or triple superphosphate granules to be examined in this experiment was kept in a closed 120 cm flask at 27 ° C, and the condition was monitored periodically. The monitored condition was evaluated as follows:

Mixture state D = dry, free-flowing W-1 = moisture spots but usable W-2 = moist and slightly sticky but probably usable W-3 = thoroughly moist and sticky, unusable W-4 = very moist, unusable H = firmly clumped together , unusable

In the "sack test" mentioned in the examples, the tendency of the granules studied to agglomerate was determined. In this experiment, urea granules were packed in 35 kg bags stored under 1000 kg weight at 27 ° C. After 1 month, the average number of lumps per bag was determined and the average hardness of the lumps was measured. Hardness is understood here as the force exerted by the dynamometer in kilograms, which breaks up a 7 x 7 x 5 cm lump.

The crystallization retarder F 80 mentioned in the examples is a clear viscous liquid commercially available under the name "Formurea 80" which is stable between -20 ° C and + 40 ° C and which, after analysis, is found to contain about 20 parts by weight per 100 parts by weight. parts of water, about 6,71721 23 parts by weight of urea and about 57 parts by weight of formaldehyde, wherein about 55% of the formaldehyde is bound as trimethylurea and the remainder is present unbound.

Example I

Experiments were performed in which an aqueous solution of urea containing and not containing a known crystallization retarder (F 80) and containing magnesium oxide as a crystallization retarder was sprayed onto the fluidized bed of urea granules. The granulation conditions and the physical properties of the obtained granules are shown in Table A.

Table A

Crystallization retarder Not at all 1% F80 0.6% MgO 1% MgO

Granulation conditions urea solution - concentration,% by weight 94.6 94.5 94.5 94.5 - temperature, ° C 130 130 130 130 - flow rate, kg / h 280 280 280 280 spray air 3 - flow rate, Nm / h 130 130 130 130 - temperature, ° C 140 140 140 140 fluidizing air 3 - flow rate, Nm / h 850 850 850 850 - temperature, ° C 45 64 54 58 layer temperature, ° C 108 105 104 104

Product characteristics - apparent density, g / cm3 1.23 1.26 1.30 1.31 - crushing resistance dia 2.5 mm, kg 2.1 2.8 4.0 4.3 - air dust, g / kg 5, 4 <0.1 0.1 <0.1 - sack test - lumps,% 100 10 0 0 - hardness, kg 22 0.1 0 0 7 71721

Table A (continued)

Crystallization retarder Not at all 1% F80 0.6% MgO 1% MgO

Product Features - TWA Bottle Test with Super Phosphate (SSP) 50/50

After 1 day W-2 W-2 D D

After 3 days W-4 W-4 D D

After 7 days W-4 W-4 D D

After 14 days W-4 W-4 W-1 D

After 21 days W-4 W-4 W-1 D

After 50 days W-4 W-4 W-1 D

with triple superphosphate (TSP) 50/50

After 1 day W-2 W-2 D D

After 3 days W-4 W-4 D D

After 7 days W-4 W-4 D D

After 14 days W-4 W-4 W-1 D

After 21 days W-4 W-4 W-1 D

After 50 days W-4 W-4 W-1 D

Example II

Next, a series of experiments were performed in the same manner as described in Example I, but using selectively calcined dolomite and fully calcined dolomite as crystallization retarders instead of magnesium oxide. The granulation conditions and the physical properties of the obtained granules are shown in Table B.

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Claims (3)

71 721 A process for preparing urea granules by spraying an aqueous urea solution to which a urea crystallization retardant has been added, in the form of very small droplets, into a fluidized bed of urea particles at a temperature at which water evaporates from the solution and urea solidifies to form granules of the desired size, characterized in that as the crystallization retardant, MgO is used as such or as selectively or completely calcined dolomite in an amount corresponding to 0.1-2% by weight of MgO, calculated on the urea in solution. Process according to Claim 1, characterized in that the amount of additive corresponds to 0.5 to 2% by weight of MgO, based on the urea in solution. Use of urea granules prepared by the process according to claim 1 or 2 in a heterogeneous fertilizer mixture with one or more other granular fertilizers, characterized in that the mixture contains urea granules prepared by the process according to Claim 1 or 2 and 1- or triple superphosphate granules.