Method for the manufacture of slow release fertilizers

A method for the manufacture of physically prepared slow-release fertilizer by coating fertilizer particles with at least one layer of a substantially water insoluble metal salt formed in-situ from an organic acid having between 6 and 30 carbon atoms and a metal oxide or carbonate. Optionally, the coating layer contains micronutrients and inert materials. In a preferred embodiment, an additional layer of paraffin is added on the coating of metal salt of organic acid. The method is applicable for most of the known fertilizers. The method enables to vary the rate of fertilizer release and the release period time according to specific requirements.

The present invention relates to an improved method for the manufacture of 
slow-release fertilizers. More particularly, the invention relates to an 
improved method for the manufacture of physically prepared slow-release 
fertilizers possessing the property of releasing the fertilizers after 
long periods of time. 
BACKGROUND OF THE INVENTION 
It is a well-known fact that no fertilizer, of whatever composition, is 
ever utilized by the crop with a complete efficiency. This occurs 
particularly with nitrogen-based fertilizers, although it is encountered 
also with all water-soluble fertilizers. The main reason for this 
deficiency is the rapid dissolution of the fertilizer in the soil, where 
only a part thereof is actually utilized, the balance being lost in the 
draining of rain or irrigation water. Two main solutions were suggested to 
overcome this disadvantage: 
(a) use of chemically prepared slow-release fertilizers, such as ureaform, 
isobutylidene diurea, oxamide etc., and 
(b) use of physically prepared slow-release fertilizers, by coating the 
fertilizer granules with sulfur, wax or synthetic polymers, which 
decreases the removal of the fertilizer from the soil by rain or 
irrigation water. 
The disadvantage of the chemically prepared slow-release fertilizers is 
mainly connected with their costs, since quite expensive raw materials 
and/or manufacturing costs are involved. Thus for instance, in case of 
nitrogen-based fertilizer, it was stated that using nitrogen compounds of 
reduced solubility is at least twice as costly as using standard soluble 
nitrogen fertilizers. 
Whereas the present invention deals with physically prepared slow-release 
fertilizers, more discussion and relevant prior art references will be 
presented on this approach. 
PRIOR ART REFERENCES 
The literature is quite abundant with many references, patents and reviews, 
describing the method of producing physically prepared slow-release 
fertilizers. The general idea is to provide an insoluble coating on 
granules of water-soluble fertilizers. A variety of materials has been 
found suitable for coating purposes, the most important of these being 
wax, sulfur and organic polymers of different types. Among the polymers 
the following are mentioned: copolymers of dicyclopentadiene and glyceryl 
ester of an unsaturated organic acid (U.S. Pat. No. 3,223,518) 
epoxy-polyester resin (U.S. Pat. No. 3,259,482) urethane coating (U.S. 
Pat. No. 3,264,089) polystyrene coating (U.S. Pat. No. 3 158 462). The 
main disadvantage of the organic polymers is their relatively high costs 
which render them not to be economically feasible to use for a cheap 
product such as fertilizer. The mechanism by which the polymer acts is as 
follows: the polymer coats the particles of fertilizer with a skin or 
glove which skin persists on the granules until substantially all the 
water soluble material has been leached out. 
The use of sulfur coating is encountered mainly with urea, where it has 
been shown to have significant advantages over conventional fertilizers on 
certain crops. A typical discussion describing in a detailed manner this 
method is given in two papers by Lynch T.B.(1) and by Meisen and Mathur 
(2) presented at the British Sulphur Corporation (Second International 
Conference on Fertilizers, London, 4-6 December, 1978). The main 
disadvantage of sulfur is the fact that it was found not to be adequately 
resistant to moisture penetration. Accordingly, addition of an oily-wax 
sealant over the sulfur coating is required. Also, sulfur can not be 
utilized in fertilizers containing nitrate where explosive compositions 
might result. 
The use of wax is the most popular coating for various water-soluble 
fertilizers. An improved method is described in the U.S. Pat. No. 
3,242,237. The method involves the formation of a fluid dispersion of the 
water-soluble fertilizer in molten wax and then dropping such dispersion 
in the form of droplets into water. As each droplet of the fluid 
dispersion contacts the water, it immediately solidifies and becomes a 
solid particle comprising a dispersion of solid fertilizer in solid wax. 
One disadvantage of using wax-coated fertilizer is that the wax content of 
the coated fertilizer must often be quite high in order to provide a 
satisfactory reduction in the rate at which the fertilizer is released to 
the plant. In areas of relatively heavy rainfall, wax contents in the 
order of 50% may be necessary. Such quantities of wax make the use of 
fertilizer quite expensive. 
The use of plant-derived wax such as palm, carnauba or sugar cane as the 
coating material, instead of paraffin, is described in the U.S. Pat. No. 
3,096,171. It is claimed that in certain respects these compounds are 
superior to paraffin wax for coating purposes. 
Finally, tung oil (called also wood oil) is suggested as a coating to 
provide an effective slow-release fertilizer (U.S. Pat. No. 3,321,298). 
The advantage claimed for this oil is the fact that it does not 
necessitate the use of flammable solvents. As known, tung oil is a drying 
oil obtained from the seed of the tung tree and consists of glycerides 
that polymerize to a hard gel on heating. For optimum results, it is 
suggested to incorporate small amounts of driers prior to its application. 
Examples of such driers are lead, manganese and cobalt salts, as generally 
used in the art of paint and varnish manufacture. 
The above brief review clearly indicates the great interest attributed in 
the last forty years to the object of providing slow-release fertilizers. 
This interest becomes more accentuated in these days, when the costs of 
the fertilizers and their application to the soil increase significantly. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for the 
manufacture of physically prepared slow-release fertilizers. It is another 
object of the present invention to provide a simple method for the 
manufacture of slow-release fertilizers, the release of the fertilizers 
occuring after prolonged period of time. It is yet another object of the 
present invention to provide a simple method for the manufacture of 
slow-release fertilizers, wherein the release of the fertilizer can be 
"tailor made" according to the specific purpose. Thus, the invention 
consists of a method for the manufacture of physically prepared 
slow-release fertilizers, wherein the fertilizer particles are coated by 
at least one layer of a substantially water-insoluble metal salt of an 
organic acid containing between 6 and 30 carbon atoms, the metal salt 
coating being formed in-situ, from the organic acid and a metal oxide or 
carbonate. It was unexpectedly found that when said coating was obtained 
in-situ, the fertilizer will persist for prolonged periods of time which 
are longer than those encountered by the slow-release fertilizers obtained 
by the known methods. Moreover, as will appear from the comparative 
example 8 presented in the experimental part, when the coating of the 
fertilizer particles was done by a metal salt previously prepared, using 
the same organic acid and metal cation, at the same coating thickness, the 
fertilizer will persist for shorter periods of time than that according to 
the present invention. The term of metal oxide does include also a metal 
hydroxide which does react in-situ with the organic acid to produce the 
corresponding metal salt. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to a preferred embodiment, the coating consists of two or three 
layers of an insoluble metal salt of an organic acid (containing between 6 
and 30 carbon atoms) formed in-situ. It is further preferably that an 
additional coating of wax or paraffin over the metal salt will also be 
added, its purpose being as a sealant. Of course, the amount of the 
sealant compound should be only a small part of the coating composition, 
generally being in a range of between 3-15% by weight thereof. The main 
object of the wax or paraffin coating is only to obtain a thin film which 
should cover the pores of the coating to avoid entrance of water to the 
fertilizer and providing a smooth surface to the resulted slow-release 
granules. 
The method according to the present invention can be easily incorporated in 
any existing fertilizer plant, the equipment required being mainly a 
granulator device of any type such as pan granulator, revolving drum, 
fluidized bed etc. The fertilizer particles, preferably in a granular 
form, will be admixed with a metal oxide (carbonate or hydroxide) and 
subsequently by spraying a stream of an organic acid (having between 6 and 
30 carbon atoms), generally under a slight heating, coated granules by the 
respective metal salt will result. It is also possible that together with 
the metal oxide, other inert materials such as talc, limestone, dolomite 
or clays, or micronutrients, could be incorporated. The thickness of the 
coating may be varied according to the specific requirement, by varying 
the respective amounts of the organic acid and metal oxide present in the 
system. 
In addition to the improved slow-release property of the products obtained 
according to the present invention, it was found that they possess 
excellent storage and handling characteristics and might even be handled 
in bulk under humid climate. 
The rate of nutrient release and release period time can be varied by the 
selection of the coating constituents, coating thickness and of course the 
particular fertilizer. Accordingly, a great flexibility in formulations 
can be achieved. The release of nutrients from the coated particles is 
initiated by movement of water vapor through the coating which will 
dissolve soluble core and subsequently the nutrients in solution will 
diffuse outward through the coating into the soil. 
The inventors are not yet in a position to present a theoretical 
explanation why the coating of metal salt of the organic acid formed 
in-situ, will retain the fertilizer for longer period of time than the 
case when the coating is obtained by the direct application of the 
corresponding metal salt prepared separately. It seems that in case of 
in-situ formation of the metal salt coating, some polymerising effect 
occurs through the unsaturated bonds of the organic acid. The organic acid 
can be selected from the aliphatic or aromatic acids possessing between 6 
and 30 carbon atoms and preferably between 12 and 20 carbon atoms. Of 
course, it is not necessary to use a pure acid, and any mixture of 
technical organic acids (containing the above range of carbon atoms) can 
be useful. In this case, also the costs of the acid will be much lower 
than those of the pure acids. It is also possible to utilize crude mixed 
vegetable and animal acid oils containing the above organic acids such as 
tall oil, tung oil etc. As known, tall oil is a resinous admixture of 
resin fatty acids, high molecular-weight alcohols and other materials, 
derived from wood-pulping waste liquors; it is found in abundant 
quantities in several places its costs being quite low. 
As already mentioned, the coating of the fertilizer is carried out in a 
granulator. Generally the coating is carried out under heating at a 
temperature in the range of between 35 and 200 degrees C. The heating 
assists the formation of a more homogeneous coating. The exact temperature 
will be selected according to the system involved as well as the 
particular fertilizer to be coated. The most preferred temperature range 
will be between 50 and 100 degrees C. 
The metal oxide to be used may be in its impure form or even a natural 
material such as limestone, dolomite or iron oxide which will form with 
the organic acid a corresponding calcium salt or mixed calcium-magnesium 
salt. 
There are many organic acids, possessing between 6 and 30 carbon atoms, to 
be utilized in the method according to the present invention, most of them 
being available in bulk. Typical examples of such acids are: oleic acid, 
lauric acid, palmitic acid, stearic acid, caprilic acid, miristic acid, 
EMERY 140 (Trade Mark, produced by Procter & Gamble Ltd.) which is a 
mixture of stearic and palmitic acids), DIACID 1550 (Trade Mark, produced 
by Westvaco, being a mixture of dimeric acids containing 15 carbon atoms 
etc. The last two organic acids are commercially available in bulk being 
also less expensive than the pure organic acids. As will be realized, the 
melting points of the organic acids will vary, generally increasing with 
an increase in the number of carbon atoms. A person skilled in the art, 
after reading the present specification, will be in a position to select 
the proper acid according to the specific use, cost and availability at 
site. 
The slow-release fertilizers obtained according to the present invention 
possess many advantageous characteristics. First, it will reduce the 
damage to plants, by a rigorous control of the fertilizer present in the 
metal salt coating, due to the avoidance of high local concentrations of 
the soluble fertilizer materials. By a proper selection of the coating 
component, the number of layers as well as the thickness of each layer, it 
will be possible to regulate the exact release of the fertilizer. 
Moreover, it will be possible to release the exact amount according to the 
growth of the plant. Thus for instance a slow release of nitrogen will be 
desirable when the plant is young and small, and a fast-release when it is 
growing rapidly. Also, the very large number of substantially insoluble 
metal salts of the above organic acids, enable the versatility of the 
method so that a "tailor-made" slow-release fertilizer could be produced. 
Another advantage of the method according to the present invention, is the 
possibility to obtain the granules of the coated fertilizer at the desired 
size. It is possible to start with small particles of a desired 
fertilizer, the granulation and coating being carried out subsequently at 
the same pan granulator. It is possible to form first small granules and 
thereafter to form larger pellets, according to the specific use. As 
commonly used in the art of granulation, known binders will be required to 
be incorporated. Preferably, the binder materials should be water 
repellent. Typical examples of such materials are heavy hydrocarbon 
residues, asphalts, waxes etc. These materials may be heated to liquefy, 
or they may be dissolved in a low boiling solvent, so that during the 
heating of the pan granulator, the solvent evaporates out leaving the 
binder on the granules. 
As mentioned above, it is also possible to incorporate during the 
granulation various inert materials such as limestone, phosphate rock, 
gypsum, clays, talc or desired micronutrients, etc, the amount thereof 
being variable, generally being in the range of between 5 and 50% by 
weight of the composition. The inert material will also influence to some 
extent, the rate of the fertilizer release from the granules. 
The particle size of the coated fertilizer granules may vary appreciable, 
tut are preferably below about 5 cm diameter. Most preferably the particle 
size of the pellets will be in the range of between 1 to 5 mm diameter. It 
should be pointed out that the pellets may be also in the form of any 
other geometric configuration (pills, cylinder etc) and not only beads or 
granules, although the latter is preferred. 
The fertilizer to be coated according to the present invention, may be 
selected from the well-known water-soluble fertilizers such as: urea, 
potassium nitrate, potassium sulfate, ammonium nitrate, monopotassium 
phosphate, ammonium phosphates, mixtures of N:P:K. fertilizers, etc. 
Whereas the coating constituents are quite inert material, the existent 
problem encountered with sulfur, where possible explosive compositions 
might be obtained, is completely eliminated. 
The invention will be hereafter illustrated by a number of Examples, being 
clearly understood that no limitation should be understood, since many 
variations could be conceived, without being outside the scope of the 
present invention. Example 8 presented below, is not part of the invention 
and is given only for comparative purposes in order to compare the 
dissolution time when the coating of the fertilizer was produced by a 
metal salt of organic acid, prepared prior to the coating (i.e. not 
in-situ as claimed according to the present invention).

The concentrations given in the Examples are by weight, unless otherwise 
stated. 
EXAMPLE 1 
An amount of 150 g of potassium nitrate (prills) was introduced in a 
laboratory rotating pan and heated at about 75 degrees C. To the prills of 
potassium nitrate an amount of 8 grams of stearic acid was added together 
with an amount of 2 grams of calcium hydroxide. After the entire amount of 
oil was consumed, by its reaction with the calcium hydroxide, free flowing 
granules were obtained consisting of coated potassium nitrate by calcium 
stearate, formed in-situ from the two reagents. 
To the coated granules in the pan granulator, an amount of 6 g of paraffin 
was added together with a small quantity (about 2 g) of calcium hydroxide 
(as inert constituent) and the pan granulator continued to operate until 
all the paraffin was consumed. After cooling, the granules were taken out 
from the granulator, the composition of the coating being as follows: 
31 g of calcium hydroxide, 
8 g of stearic acid, and 
6 g of paraffin. 
The granules of coated fertilizer were tested for their dissolution in 
water and it was found that their substantially complete dissolution 
occured after 432 hours. 
EXAMPLE 2 
The experiment as described in Example 1 was repeated using a commercial 
fatty acid (mixture of stearic and palmitic acids) sold under the Trade 
Mark EMERY 140 by Procter & Gamble Ltd. The composition of the coating was 
as follows: 
8 g of EMERY 140. 
6 g of paraffin, and 
47 g of calcium hydroxide. 
The substantially complete dissolution in water of the coated granules 
produced was 312 hours. 
EXAMPLE 3 
The experiment as described in Example 1 was repeated using as the organic 
acid a dimeric acid containing 15 carbon atoms (Diacid 1550 Trade Mark, 
produced by Westvaco). The composition of the coating was as follows: 
8 g of the Diacid 1550, 
6 g of paraffin, and 
32.2 g of calcium hydroxide. 
The substantially complete dissolution in water of the coated granules was 
about 600 hours. 
EXAMPLE 4 
The experiment as described in Example 1 was repeated, but in this case the 
calcium hydroxide used as the inert material in the step of the coating 
with paraffin, was replaced by 15.1 g of talc. Also, the stearic acid was 
replaced by 10 g of a mixture of fatty acids (EDENOR UKD 3510, Trade Mark 
produced by Henkel, West-Germany). 
The composition of the coating was as follows: 
10 g of EDENOR UKD 3510, 
22.9 g of calcium hydroxide, 
6 g of paraffin, and 
15.1 g of talc. 
The granules of coated fertilizer were tested for their dissolution in 
water and found that their substantially complete dissolution occurred 
after 984 hours. EXAMPLE 5 
The same equipment as in the previous Examples was utilized, but in this 
case the coating consisted of 3 layers. In the first layer, the coating 
contained 3.4 g of stearic acid, about 8.8 g of calcium hydroxide, 2 g of 
paraffin and 6.8 g of talc. The granules obtained were treated again in a 
similar manner, with two separate additional portions, of the above four 
reagents, one after the other resulting the granules coated by three 
layers. The overall composition of the three layers was as follows: 
10 g of stearic acid, 
26.29 g of calcium hydroxide, 
6 g of paraffin, and 
20.4 g of talc. 
The resulted coated granules dissolve in water substantially complete after 
about 1200 hours. 
EXAMPLE 6 
The experiment as described in Example 1 was repeated, using 50 g of urea 
(instead of potassium nitrate), the pan granulator being heated at about 
50 degrees C. The coating consisted of calcium salt of tall oil formed 
in-situ and had the following composition 6 g of tall oil, 9.9 g of 
calcium hydroxide, and 3 g of paraffin. The substantially complete 
dissolution in water of the resulted granules of coated urea was about 
1200 hours. 
EXAMPLE 7 
A water-soluble fertilizer composition containing N:P:K (15:15:15) was 
prepared and introduced in the pan granulator heated at about 60 degrees 
C., as described in Example 1. The coating composition was as follows: 
10 g of tall oil, 
15.7 g of calcium hydroxide, 
6 g of paraffin, and 
15.7 g of talc. 
The substantially complete dissolution of the coated granules occured after 
340 hours. 
EXAMPLE 8 
A comparative experiment as described in Example 1 was performed, with the 
same pan laboratory granulator as in the previous Examples, the fertilizer 
being also 150 g of potassium nitrate (prills), but the reagent used for 
the coating was calcium stearate i.e. the already formed salt. Into the 
granulator containing the resulted coating an amount of 6 g paraffin was 
added together with 2 g of calcium hydroxide (as inert constituent). The 
analysis of the coating was as follows: 
31 g of calcium hydroxide, 
8 g of stearic acid, and 
9 g of paraffin. 
The granules obtained possess indeed a smooth surface, due to the paraffin 
coating, but their strength was quite poor. Their dissolution in water was 
only 48 hours (compared with 432 hours in Example 1).