METHOD FOR PRODUCING FERTILIZERS CONTAINING
DISPERSED MICRONIZED SULPHUR
CROSS REFERENCE TO RELATED APPLICATION
This application claims priority to United States Patent Application Serial
No. 61/377,509 filed on August 27, 201 0 the disclosure of which is incorporated
herein by reference for all purposes.
FIELD OF THE INVENTION
The present invention relates to a method of producing fertilizers
containing dispersed elemental sulphur fines, i.e., micronized sulphur, that are
integrated within the fertilizer.
DESCRIPTION OF PRIOR ART
The majority of the commercial fertilizer manufacturing processes involve
reactions between acidic and basic starting constituents to generate a
neutralized or partially neutralized final product that is either granulated, prilled,
rotoformed or spray dried. Additionally, considerable effort has been expended
on attempts to develop processes for making sulphur containing fertilizers due to
the benefits associated with the influence of sulphur on crop yields, i.e., the more
efficient uptake of other plant nutrients such as nitrogen and phosphorus and its
ability to lower the pH of alkaline soils.
One of the most common examples of a fertilizer is ammonium sulphate
with the ammonium cation contributing nitrogen to the plants and the sulphate
anion contributing the sulphur to the plants. Being highly soluble in water,
ammonium sulphate can provide plants with a rapid dose of both constituents.
Unfortunately, heavy rain can accelerate the leaching of the sulphate from the
soil and dilute the potential maximum benefit plants can attain from the presence
of sulphur in the sulphate form. This rapid but non-sustaining dose of sulphate to
the plants has driven the efforts behind developing elemental sulphur containing
fertilizers. The advantage of having elemental sulphur as part of the fertilizer
composition is that the elemental sulphur will oxidize over time, providing a
sustained, time release sulphate dose to the plants. Elemental sulphur is also
less prone to being washed away so will be resident near the plants for a longer
period of time.
One of the problems in working with elemental sulphur is the economic
and safe generation of fine sulphur particles. Current methods used include
physical grinding (ball milling) of solid sulphur or spraying fine molten sulphur
particles into water. These processes pose disadvantages which include but are
not limited to safety, consistency of product and capital and operating expenses.
Adding these sulphur particles to fertilizers poses additional difficulties
associated with integrity of the sulphur coating, the amount of elemental sulphur
that can be incorporated into the fertilizer and difficulties in achieving uniform
distribution of elemental sulphur throughout the fertilizer, e.g., the pellet.
The particle size of the elemental sulphur that is incorporated within the
fertilizer granules will also affect its ability to be oxidized in a timely manner so
that sulphate is available to the plants during the growing season. The finer
(smaller mean particle diameter) these elemental sulphur particles are, the easier
they are oxidized to sulphate. On a relative basis, sulphur particles described in
existing commercial fertilizers are expected, at best, to start contributing sulphate
sulphur to the plants six to eight weeks after application.
U.S. Patent 3,333,939, incorporated herein by reference for all purposes,
teaches a method that requires the handling of solid sulphur particles as well as
an alternate method utilizing molten sulphur. The former has the hazards
associated with handling fine dusts whereas the latter inhibits even distribution of
the fertilizer components to the soil after application.
Patent Application 2006/01441 08, incorporated herein by reference for all
purposes, teaches two methods whereby liquid (molten) sulphur is added either
to an ammonium phosphate reaction vessel or to a granulator to disperse the
elemental sulphur within the resulting granules. The phosphoric acid in the
reactor is combined with either concentrated aqueous ammonia or anhydrous
gaseous ammonia. The second method described relates to a slurry of
elemental sulphur particles in water being added to an agitated pre-neutralizer
reactor containing phosphoric acid and ammonia. By adding the sulphur slurry to
the pre-neutralizer, higher sulphur loadings were achieved without experiencing
plugging because of perceived better mixing.
As illustrated by the prior art discussed above, it is advantageous to have
an improved method for the manufacturing of fertilizers that incorporate fine
(micronized) sulphur particles.
It is well known that elemental sulphur is soluble in anhydrous ammonia
(Ruff and Hecht, "Uber das Sulfammonium and seine Beziehungen zum
Schwefelstickstoff", Z. anorg. Chem. Bd 70, 19 11; U.S. Patent No. 4,824,656;
U.S. Patent Application Publication 2006/00443002; Proceedings of the 2nd
International Symposium on Phosphogypsum held in Miami, Fl, Dec. 10 - 12,
1986 p 143; and WO 2004/1 0971 4).
SUMMARY OF THE INVENTION
In one aspect the present invention provides a method of making a
fertilizer composition containing micronized sulphur particles.
In one aspect of the present invention, there is provided a method by
which the mean particle size of the sulphur fines in a fertilizer composition can be
controlled and varied by varying the dissolved sulphur concentration in certain
solutions.
In a further aspect of the invention, discrete elemental sulphur particles
and floes can be incorporated into fertilizer compositions, to give a traditional
fertilizer product with elemental sulphur integrated therein.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As taught by Ruff and Hecht, elemental sulphur from all sources can be
dissolved into either liquid anhydrous ammonia or liquid hydrous ammonia to
create ammonia-sulphur solutions, hereinafter referred to as AMS solutions.
AMS solutions can have varying concentrations of elemental sulphur dissolved in
them which is controlled by the temperature, pressure and water content of the
AMS solutions.
As used here, "anhydrous ammonia" refers to ammonia having less than
about 0.3 wt % by water while "hydrous ammonia" refers to ammonia containing
from about 0.3 up to about 70 wt. % water, preferably from about 0.3 up to about
10 wt % water. Although preferred solvents for this invention are anhydrous and
hydrous ammonia as described above, other solvents may include liquid sulphur
dioxide, liquid or super critical carbon dioxide, carbon disulphide,
dimethyldisulfide, etc., including blends of various solvents as described above
including blends with water.
It has been found that by varying the concentration of dissolved sulphur in
the AMS solutions as well as the type of media into which the AMS solution is
introduced to create micronized elemental sulphur particles, the mean sulphur
particles size can be varied in a controlled manner.
By way of example, AMS solutions can be "blown down" into deionized
water; water containing varying concentrations of cations, anions and amphoteric
species; water containing varying concentrations of cations, anions and
amphoteric species with some near or at their respective saturation points;
suitable gaseous phases such as nitrogen as well as into evacuated containers,
to create discrete sulphur particles with mean submicron particle sizes as well as
floes (loose agglomerations of discrete micronized sulphur particles) of up to 150
microns. In addition to the "blow down", manipulation of temperature and
pressure of the AMS solutions into or in the presence of the above fluids can also
be used to create the desired micronized sulphur particles.
It will be understood that in forming the AMS solutions, the AMS will be
under sufficient pressure to maintain the AMS solution in the liquid state until the
reaction with the acidic compound. In this regard, and as is well known,
anhydrous ammonia, in liquefied form, must be under pressure to maintain it in
that state. The AMS solutions of the present invention can be formed either by
dissolving solid, elemental sulphur, regardless of its form, in the AMS solution, or
by introducing liquid sulphur into the AMS solution. With respect to the use of
solid elemental sulphur, such sulphur sources include sulphur blocks, prills,
slated sulphur, sulphur pad bottoms containing impurities such as dirt, etc. WO
2008/041 132, incorporated herein by reference for all purposes, discloses
various methods for forming micronized sulphur using both solid and liquid forms
of sulphur.
It has been found that the solubility of the elemental sulphur and
anhydrous ammonia decreases as hydrous ammonia is created with increasing
water content. In general, the amount of sulphur present in the sulphur solvent
will be the maximum that can be achieved while still maintaining a true solution.
Thus, the sulphur in the AMS solution can be present up to the point of saturation
provided the conditions are such that the saturated solution maintains the sulphur
as a true solution.
With respect to the micronized sulphur produced according to the present
invention, it will generally have an average particle size of less than about 9,999
mi to sub-colloidal range, preferably less than 1,000 mi to sub-colloidal range,
more preferably less than 100 mi to sub-colloidal range, and still more preferably
less than 25 mi to sub-colloidal range. Especially preferred is micronized
sulphur of less than 10 mi to sub-colloidal range.
As part of the present invention and depending on the particle size of the
micronized sulphur created, dispersants or emulsifiers may or may not be utilized
for periods of time to keep the sulphur in suspension. By way of example,
suspensions of submicron sulphur particles stay relatively homogeneous for
several hours with separation occurring after being quiescent for 8 hours.
It has been found that as the concentration of the elemental sulphur
dissolved in the AMS solutions increases, the form of the micronized sulphur
particles changes from discrete particles to loose assemblages of these discrete
particles, hereinafter referred to as floes. Floes are also created after the
discrete micronized sulphur particles have settled out of solution and resuspension
is attempted. It is believed that Van der Waal interactive forces
induce these particles to agglomerate into the floes seen.
It is well known that the particle size of elemental sulphur directly impacts
the rate at which the sulphur particle to be oxidized into sulphate. An unexpected
finding was that the floes formed have virtually the same oxidation rates as the
discrete elemental sulphur particles that make up the floes. Given that the mean
particle size ranges of the micronized sulphur can be varied in a controlled
manner, discrete elemental sulphur particles, as well as floes formed therefrom,
can be selectively created to provide a blended product with a tailored oxidation
rate of elemental sulphur to sulphate, i.e., a time release product.
In forming the fertilizer compositions of the present invention, the AMS
solution is reacted with an acidic compound that has at least one plant growth
constituent to thereby simultaneously form a fertilizer composition comprising a
fertilizer compound and micronized sulphur. The fertilizer composition recovered
after drying can be formed into various shapes, e.g., pellets, prills, or any other
shape, form, or size by well known methods and equipment.
One of the features of the present invention is the ability to provide a timerelease
fertilizer composition. In this regard, an AMS solution having one sulphur
concentration which can produce micronized sulphur of one average particle size
and a second AMS solution of a second sulphur concentration which can
produce micronized sulphur of a second, different, average particle size, can be
used to react with the acidic components to form the fertilizer composition. Since
the amount of sulphur in the AMS solution dictates, to a certain extent, the
particle size of the micronized sulphur produced, there will then be produced in
the fertilizer composition, micronized sulphur of the different average particle
sizes. As noted above, the smaller particle size will oxidize more rapidly into the
sulphate form in the soil, while the larger average particle size micronized
sulphur will oxidize more slowly. Thus it will be a sustained formation of sulphate
in the soil over a longer period of time. Furthermore, by forming floes in
combination with micronized particles, the sustained, time release effect can also
be achieved.
The acidic component reacted with the AMS to form the fertilizer
compound can be any acidic component which when reacted with the ammonia
present in the AMS will form a fertilizer compound, wherein the fertilizer
compound is defined as a compound which when admixed with soil or applied to
plants will enhance the growth of the plants. Non-limiting examples of typical
acidic compounds which have at least one plant growth constituent and which
can react with the ammonia in the AMS to produce a fertilizer compound include
sulphuric acid, phosphoric acid, nitric acid, carbonic acid, and their various
derivatives as well as mixtures thereof. Thus, as is well known, fertilizer
compounds such as ammonium sulphate, ammonium nitrate, ammonium
phosphate, ammonium carbonate, etc. can be formed.
One of the clear advantages of the present invention is the fact that by
incorporating the micronized sulphur into the fertilizer composition according to
the present invention, the micronized sulphur is essentially uniformly dispersed
within the matrix of the final fertilizer form (i.e. pellet, prill, etc.), a clear advantage
in terms of ensuring that all treated soil or plants receive substantially the same
amount of micronized sulphur as well as the fertilizer compound.
According to the present invention, once the fertilizer composition has
been formed, it is possible to add additional AMS solution during the drying step
to provide a larger quantity of micronized sulphur in the fertilizer composition.
It will also be recognized that other components typically used in fertilizer
compositions and containing plant nutrients can be added to the fertilizer
composition once it is formed and preferably prior to it being formed in the
desired shapes such as pellets, prills, etc.
Example 1:
Several 0.2 wt% to 10 wt% AMS solutions were injected into water,
aqueous ammonium sulphate slurries and aqueous ammonium sulphate slurries
having sufficient sulphuric acid to stoichiometrically react with all of the ammonia
added via the AMS solutions. In all cases micronized elemental sulphur was
created. As the sulphur concentration in the AMS solutions increased so did the
tendency to create floes.
Floes created during these tests were isolated and examined
photomicrographically. It was seen that discrete micronized elemental sulphur
particles had agglomerated into the floes, but were uniformly distributed in the
fertilizer composition containing ammonium sulphate.
Example 2:
Addition of the AMS solutions to the aqueous ammonium sulphate slurry
resulted in the formation of both discrete micronized elemental sulphur as well as
the floes. The presence of floes was confirmed by dissolving the resulting
ammonium sulphate-sulphur mixture in an excess of water. Distinct floes were
noted.
Example 3:
Addition of the AMS solutions to the aqueous ammonium sulphate slurries
containing excess sulphuric acid showed a marked increase in viscosity,
indicating more solids production and the successful reaction between the AMS
solutions and the sulphuric acid. Micronized elemental sulphur was uniformly
distributed within the slurry.
WHAT IS CLAIMED IS:
1. A method of producing fertilizer compositions containing micronized
sulphur comprising:
dissolving elemental sulphur in an anhydrous or hydrous ammonia
solution to form an ammonia/sulphur solutions (AMS solution);
reacting said AMS solution with an acidic compound having at least one
plant growth constituent to simultaneously form a fertilizer composition
comprising a fertilizer compound and micronized sulphur;
drying said fertilizer composition.
2. The method of Claim 1, wherein said AMS solution is under
sufficient pressure to maintain said AMS solution as a liquid until reacted.
3. The method of Claim 1, wherein said plant growth enhancing
constituent is selected from a group consisting of sulfate, nitrate, phosphate,
carbonate, and mixtures thereof.
4. The method of Claim 1, comprising adding additional AMS solution
to said fertilizer composition during the drying step.
5. The method of Claim 1, wherein said micronized sulphur has an
average particle size of less than 1,000 mi .
6. The method of Claim 5, wherein said average particle size is less
than 10 mi .
7. The method of Claim 1, wherein other components containing plant
nutrients can be added to said fertilizer composition.
8. The method of Claim 1, wherein said AMS solution is formed by
dissolving solid elemental sulphur.
9. The method of Claim 1, wherein said AMS solution is formed by
dissolving molten elemental sulphur.
10. The method of Claim 1, wherein there is a first AMS solution having
a first sulphur concentration to produce micronized sulphur having a first average
particle size and a second AMS solution having a second sulphur concentration
to produce a micronized sulphur having a second, larger average particle size.
11. The method of Claim 1, wherein said micronized sulphur is in the
form of floes.