The present invention relates to a method for granulating fertilizer products and more particularly, the present invention relates to a method for impregnating urea into a fertilizer material and granulating a pellet.

The augmentation of fertilizer compounds with a treatment of urea, typically by spraying or aerosol has been practiced in the art. The most undesirable aspect of the treatment stems from the fact that urea is water soluble and since it is currently applied topically, much of the compound is simply wasted by washing away from rain, irrigation etc. This is a costly procedure with limited efficacy. A product and procedure whereby the urea may be mixed or crystallized within the fertilizer matrix is required to overcome the wastage problem. The present invention solves this problem.

US 3,785,796 discloses a process for the production of urea-ammonium sulfate granules from urea and ammonium sulfate by several granulation methods. The process set forth in this reference only provides a prilled matrix of urea within which is interspersed ammonium sulfate.

US 4,554,004 discloses a process for the granulation of fertilizer materials. The process results in the preparation of granulated materials where urea phosphate is used as a granulation agent for assisting in the agglomeration of solid particles into relatively uniformly dimensioned granules. Urea phosphate acts as a coating for the divided particles and granulation is accomplished at a temperature within the thermal plastic range of urea phosphate.

EP 289074 discloses a method for producing fertilizer granules containing urea and ammonium sulfate by granulation in a fluidized bed of particles. The ammonium sulfate particles are firstly introduced to the fluidized bed where urea is applied by spraying an aqueous urea system over the particles.

The present invention has applicability in the fertilizer industry.

The object of the present invention is to provide an improved fertilizer method of incorporating urea from a source thereof into the fertilizer.

According to the present invention, the method of granulating a fertilizer feedstock, comprises the steps of:

• providing a compound containing urea;
• providing a fertilizer feedstock consisting of ammonium sulfate in a size distribution of 3 mesh to about 10 mesh;
• introducing said compound containing urea and said fertilizer feedstock into a drum granulator;
• maintaining a temperature in said drum granulator sufficient to keep said urea containing compound in a liquid state;
• mixing said urea containing compound in a liquid state with said fertilizer feedstock in said drum granulator so that liquid urea is introduced into the fertilizer feedstock matrix, obtaining a mixture, and
• cooling said mixture to form a granulated fertilizer feedstock containing ammonium sulfate granules having urea crystallized therein.

In order to facilitate easier particle rolling within the drum, the drum may be lined with rubber, Teflon™, neoprene, inter alia.

In view of the fact that the granulated pellets contain a relatively high degree of empty interstitial space by virtue of the crystalline structure, this space provides a particularly useful volume within which to recrystallize urea.

In terms of the type of urea compounds that may be used in the practicing of the present Invention, examples include urea formaldehyde, urea-ammonium orthophosphate, urea-ammonium polyphosphate, ureaform etc. Other suitable examples which are compatible with the chemistry involved will be appreciated by those skilled in the art.

For enhancing wettability of the feedstock, a surfactant may be added. The surfactant lowers the surface tension of the fertilizer product sufficiently so that the urea can flow into the fertilizer. This is of significant value when the mixture of urea and fertilizer is cooled to above the melting point of the urea since the result is recrystallization of the urea within the matrix of the fertilizer.

Intra matrix recrystallization of the urea has been found to substantially increase desirable properties of the so formed granules. Typically, the break strength of the urea bearing pellets or granules exceeds 7 kilograms. This feature is beneficial in that hard particles do not create large quantities of dust; do not break, crumble, etc. during shipping; provide a modified solubility and facilitate a use for urea which would not have been possible based on the current methodology in the art.

Having thus generally described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments.

• Figure 1 is a schematic illustration of the apparatus for practicing the method; and
• Figure 2 is a photograph of an ammonium sulfate granule with urea recrystallized therein.

Similar numerals employed in the text denote similar elements.

Referring now to Figure 1, numeral 10 represents the fertilizer feedstock which may be potassium chloride, potassium sulfate, ammonium sulfate, inter alia. The feedstock 10 is taken from a source thereof, for example, a pan granulation circuit (not shown).

The feedstock 10 is in a size distribution of between about 3 mesh to about 10 mesh. The material is introduced into a drum granulator 12. Suitable drum granulators will be appreciated by those skilled in the art. In addition to the feedstock 10 being introduced into drum 12, there is also introduced a source of urea bearing compound 14. The urea may be introduced into granulator 12 in a liquid state or granulator 12 may be heated sufficiently so that the urea in the urea bearing compound is liquified. As a further possible alternative, the initial feedstock may be heated sufficiently to induce melting of the urea in the compound.

As an optional step, depending upon the nature of the chemistry involved, a suitable surfactant 16 may be introduced int granulator 12 in order to facilitate transportation of the liquid urea into the fertilizer feedstock matrix. The mixture, broadly denoted by numeral 18 is then introduced into a cooling apparatus 20. The apparatus may comprise a further drum granulator, fluidized bed or any other suitable cooling apparatus which facilitates movement of the product as well. In order to induce crystallization of the urea within the feedstock product, cooling fluid is introduced to granulator 20 at inlet 22 in a countercurrent manner. The cooling fluid may comprise any fluid which is inert relative to the chemistry within the cooling apparatus 20. As an example, air may be introduced to the apparatus 20 for cooling the urea.

Crystallization occurs at any point above the melting of urea (typically 110°C) and the cooling air within the apparatus 20 reduces the temperature to approximately 65°C to produce a product exiting the apparatus at 24.

Rotation and inclination of the drum granulator 12 and/or 20 will be appreciated by those skilled in the art. These parameters will depend on the nature of the feedstock among other factors.

The product 24 is photographically illustrated in Figure 2 where the granules are shown in a substantially spherical state as well as in cross section. It is clear from an examination of the cross sections that the urea is crystallized within the matrix of the fertilizer. As is known, the fertilizer granules typically have anywhere from 15% to 25% interstitial empty volume. This provides an ideal opportunity to introduce liquid urea for eventual recrystallization in the feedstock matrix.

It has been found that significant advantages flow from incorporation of the urea, which advantages include, for example, increased particle break strength. The break strength of ammonium sulfate pellets is typically about 2 kilograms to about 3 kilograms, whereas once the urea has been crystallized within the matrix, the break strength of the treated pellet generally exceeds 7 kilograms. In the case of potassium chloride and potassium sulfate, the pretreatment break strength of the granules is from about 2 kilograms to about 3 kilograms and the urea bearing granules demonstrate a break strength from about 7 kilograms to about 8 kilograms.

As illustrated in the graph, urea quickly dissolves. With the augmentation of potassium chloride, the rate of dissolution is significantly reduced, thus providing a longer lasting slow release fertilizer product which does not breakup/disintegrate, but rather is progressively dissolved with a substantially uniform reduction in diameter over time.

As indicated above, there are significant additional advantages that flow from the incorporation of the urea. One of the most attractive advantages with the present invention is the fact that the NPKS ratio can be effectively customized to yield any desired nitrogen, phosphorous, potassium, and sulfur ratio. In the example of Figure 2, the NPKS ratio is 30-0-0-20.

Although embodiments of the invention have been specifically set forth herein, it will be understood that the invention is subject to significant vanation without departing from the spirit, nature or scope of the claimed and described invention.