Chemistry: fertilizers – Processes and products – Organic material-containing
Patent
1995-05-25
1997-08-05
Bos, Steven
Chemistry: fertilizers
Processes and products
Organic material-containing
71 6405, 71 6406, C05C 900
Patent
active
056537816
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
For the production of urea granules various methods are known. In the past prilling was the predominant method, wherein a substantially anhydrous urea melt ( having a water content of no more than 0.1 to 0.3% by weight) is sprayed from the top of a prilling column in a rising stream of air of ambient temperature in which the droplets solidify to form so-called prills. These prills have a relatively small maximum diameter and are mechanically rather weak.
Urea granules having larger dimensions and better mechanical properties are presently being produced by the granulation of a substantially anhydrous urea melt or of an aqueous urea solution in a fluidized bed, for example as described in U.S. Pat. No. 4,219,589. In the process described therein, an aqueous urea solution having a urea concentration of 70-99.9% by weight, preferably 85-96% by weight, is sprayed in the form of very fine droplets having an average diameter of 20-120 .mu.m into a fluidized bed of urea particles at a temperature at which the water evaporates from the solution sprayed onto the particles, and urea solidifies on the particles to form granules having a desired size which my be 2.5 mm and more. As in this process rather large amounts of fly dust are formed, especially if the urea solution used as the starting material contains more than 5% by weight of water, in particular more than 10% by weight of water, preferably a crystallization retarder or granulation additive for the urea, in particular a water-soluble addition or condensation product of formaldehyde and urea, is added to the urea solution, whereby the formation of fly dust is considerably lower, or even suppressed. The result of the presence of the granulation additive is that the granules remain plastic as they are being formed, so that owing to rolling and/or impacts during the formation mechanically strong, smooth and round granules may be formed.
The resulting granules have a high crushing strength, a high impact resistance, and little tendency of forming fly dust through abrasion, and moreover do not cake together, not even upon prolonged storage, although urea exhibits a strong natural tendency of caking together.
Although the process of the cited U.S. patent specification resulted in large improvement in the properties of the urea granules, further improvement was still found to be necessary, especially with respect to the amount of fly dust of the final product and the dust formation during granulation.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a process for the production of urea granules by a granulation technique, preferably as described in U.S. Pat. No. 4,219,589, wherein the amount of dust formed during granulation is reduced, as well as the amount of fly dust of the end product.
The invention is directed to a process for the production of urea granules from a melt or solution of urea by spraying the urea melt or solution, which also contains as granulation additive formaldehyde, methylolureum, formurea, or hexmethylenetetramine, in the form of very fine droplets into a fluidized bed of urea particles, in which process the granulation additive is added to the melt or solution prior to the granulation at such a stage 50 that the mixture of the said granulation additive and the urea melt or solution has a contact time of at least 20 sec., but not more than 20 min.
Surprisingly it has been found that by adding the granulation additive at such a stage that the mixture of granulation additive and urea has a specified residence time, an improved process and an improved product are obtained.
More specifically the process is much more economic due to the reduced amount of dust in the granulation. This reduces the amount of liquid to be used for washing the gas from the granulator, and consequently the amount of steam for evaporating the water from said urea solution.
Also, the dust from abrasion of the end product is reduced considerably, which is an important advantage for further material handling.
The o
REFERENCES:
patent: 3067177 (1962-12-01), Greco et al.
patent: 3112343 (1963-11-01), Allgeuer et al.
patent: 4217127 (1980-08-01), Kono et al.
patent: 4219589 (1980-08-01), Niks et al.
patent: 4619843 (1986-10-01), Mutsers
patent: 4701353 (1987-10-01), Mutsers et al.
Antonus Roger Alfons Camille
Kayaert Andre Firmin
Bos Steven
Hydro Agri Sluiskil B.V.
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