Note: Descriptions are shown in the official language in which they were submitted.
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1 33746~J
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PARTICULATE FERTILIZER DUST CONTROL
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to a method for controlling
fugitive dust emic~sion from granular (particulate) fertilizers. The
present invention is specifically directed to the use of an aqueous
solution of a mixture of lignosulfonate and urea as a fugitive dust
suppressant for granular fertilizers.
2. Description of Related Art
Fugitive dust emiccion from granular fertilizers has, in recent
years, been an increasing concern because of the growing reliance on
bulk h~n~lling of fertilizers, in preference to bags, and because of the
heightened awareness of the potential health hazards ~t airborn dust.
Dustiness, in large part, is due to inefficient removal of fines during
fertilizer manufacture, poor granule strength, abrasion of fine surface
crystals, and poorly adherent anticaking additives. As a consequence,
a substantial amount of dust is created during h~n~lling and transpor-
tation of granular fertilizers.
Techniques are known for reducing the dustiness of prilled urea
fertilizer. For example, it is l~nown to add a small amount of formal-
dehyde to urea during urea manufacture to increase hardness,
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although safety and health concerns have limited this
approach. Urea also has been treated with a small
amount of lignosulfonate to increase its hardness, see
U.S. Patent 4,587,358. Numerous attempts to reduce
dustiness of inorganic fertilizers, however, through
improvements in the fertilizer manufacturing process,
have been largely unsuccessful.
The benefits of lignosulfonate as a
granulation aid have long been known in the fertilizer
art. U.S. Patent 3,725,029 discloses using a
concentrated lignosulfonate solution as a binder for
granulating ammonium sulfate. Recent U.S. Patent
4,846,871 claims that the caking tendency and dustiness
of inorganic fertilizers, including phosphates, nitrates
and the like can be reduced by adding lignosulfonate
during the granulation procedure to distribute it
homogeneously throughout the fertilizer granules.
Russian Patent 1,096,265 describes using an aqueous
lignosulfonate-urea solution as a binder to consolidate
a potassium chloride powder (dust) fraction and fine
grain potassium chloride, assisted by mechanical
pressing.
One of the most commonly used procedures for
fertilizer dust control, especially for inorganic
fertilizers, has been simply to spray the fertilizer
with a small amount, generally about 0.5% by weight, of
a petroleum oil to bind the dust to the granules. This
treatment is only short-lived, because of evaporative
loss of the oil, and cannot be used at all with high
nitrate fertilizers (e.g., ammonium nitrate) because it
creates an explosion hazard. Other liquids also have
been used for fertilizer dust control including
lignosulfonate solutions, amines, surfactants, waxes,
wax emulsions and water alone, but have not proved
completely satisfactory. One prevalent problem is that
the use of aqueous treatment solutions tends to
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exacerbate the caking problem often encountered with
inorganic fertilizers. Thus, there remains a need in
the art for a treatment which effectively reduces the
level of fugitive dust emission from inorganic
fertilizers.
DESCRIPTION OF THE INVENTION
In accordance with the present invention, a
method for reducing fugitive dust emission from
granular, inorganic fertilizers is provided which
comprises applying a dust reducing amount of an aqueous
solution containing a mixture of urea and lignosulfonate
to said granular inorganic fertilizers, said aqueous
solution containing between about 45% to 80% by weight
solids, said mixture containing at least about 0.3 part
by weight up to about 40 parts by weight lignosulfonate
per part by weight urea.
Because aqueous solutions of urea and
lignosulfonate have a low volatility, they retain their
effectiveness long after application. Moreover, since
urea is a common fertilizer component and because
lignosulfonates are approved as an animal feed additive,
this treatment does not present any environmental
problems. Contrary to other aqueous treatments, using
aqueous solutions of urea and lignosulfonate to control
fertilizer dust emission also has not exacerbated
fertilizer caking.
As used throughout the specification and
claims, the term "lignosulfonate" is intended to
encompass both "sulfonated lignin" obtained by
sulfonating alkali lignin, hydrolysis lignin and
solvolysis lignin using for example sulfite or bisulfite
compounds, and "sulfite lignin" obtained directly from
the sulfite pulping of wood as a principle constituent
of spent sulfite liquor. Alkali lignin is obtained from
the alkaline pulping liquor (black liquor) of the Rraft,
soda and other
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well-known alkaline pulping processes. Hydrolysis lignin is obtained
from the hydrolysis of lignoce~ lose materials, and solvolysis lignin is
obtained by solvent extraction techniques. Both hardwood and
softwood lignins are suitable lignocellulose sources.
Lignosulfonates are available æ salts with such cations as mag-
nesium, calcium, sotlillm~ pot~ci-lm, ammonium, zinc, iron, copper
and the like in either aqueous solution or in dried powder forms. The
pre~sent invention does not require any particular source or form of
lignosulfonate. For example spent sulfite liquor may be used directly
or non-lignosulfonate constituents may be removed from any
lignosulfonate source to a desired degree by various methods, such æ
by oxidation, dialysis or fermentation.
The inorganic fertilizers treated in accordance with the
present invention include phosphates such as bone meal, ammonium
phosph~te, including mono~mmonium phosphate (MAP) and
diammonium ph~sph~te (DAP), and single and triple superphosphates,
nitrates, such æ ammonium nitrate and pot~csillm nitrate, sulfates,
such æ ammonium sulfate, and pota~iium magnesium sulfate, chlo-
rides such æ pot~ccil~m chloride, limestone, dolomite and mixtures
thereof. The fertilizers may contain other constituents, such as urea.
Methods of manufacturing these inorganic fertilizers are well-known
to those skilled in the fertilizer art, as are the methods for processing
these fertilizers into particles or granules using well known prill
and/or granulation techniqu~s. Granular micronutrients also can be
treated in accordance with the present invention.
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For example, when conventionally producing granular ammo-
nium phosph~te, phosphoric acid is neutralized with ammonia in a
series of ammoniation reaction tanks where the heat of reaction
evaporates a part of the water introduced with the acid. The extent
of ammoniation is controlled to yield a slurry composition ranging
f rom monoammonium phosphate to about two-thirds diammonium
phosphate, depending on the grade of product desired. This slurry
flows to a pugmill or blunger in which it is granulated by mixing with
recycled product fines. The granules are then dried and screened to
provide a product fraction and a finer recycle fraction. Recycle
rations in the order of 8:1 are required for proper control of granula-
tion. In one variation of the process, a rotary drum granulator is sub-
stituted for the blunger. Products made in this m~nn~r typically con-
tain 11% N and 21% P, predominantly mono~mmonium phosph~te; and
16% N and 21% P, about one-third mono~mmonium and two-thirds
diammonium phosph~te. Other grades such as one containing 16% N
and 8.7% P and another with 13% N and 17% P may be made by adding
sulfuric acid to the ammoniators. in which case the product also con-
tains ammonium sulfate. In still dnother variation, unfiltered extract
from a phosphoric acid plant is used to produce lower grades that
contain calcium sulfate.
Di~mmonil-m phosphate containing 18% N and 20.1% P is also
made by a process in which phosphoric acid is neutralized with ammo-
nia in a rotary drum granulator. Heated air and dry recycled fines are
introduced in the granulator to reduce the moisture content to the
proper level for granulation. The gases leaving the granulator are
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scrubbed with the incoming acid. The product is dried and then
screened.
An alternate process for a material containing 18% N and
20.1% P also uses a rotary granulation drum. Phosphoric acid is neu-
tralized in a prereactor with anhydrous ammonia to an NH3:H3PO4
mole ratio of about 1.3, a point near the maximum solubility of the
process. The slurry is fed to the granulator along with recycled fines.
Additional ammonia is added in the granulation step to give a mole
ratio of about 2, a point of minimum solubility. The exces~s ammonia
required to drive the reaction to diammonium ph~ph~te is recovered
by scrubbing the exhaust gases with the incoming acid before it is fed
to the prereactor. The product from the granulator is dried and
screened. In addition to the material containing 18% N and 20.1% P,
a large variety of grades rich in diammonium phosph~te can be con-
ventionally produced by neutralizing with ammonia-ammonium
nitrate solutions rather than anhydrous ammonia, substituting sulfuric
acid for part of the phosphoric acid, or adding various solid fertilizer
ingredients to the ammonia~or.
Ammonium nitrate may be conventionally produced in granular
form substantially as described by the above processes by initially
starting with the neutralization of nitric acid with ammonia in the
ammoniation reaction tanks. Similarly, pot~csi-lm nitrate may be
conventionally produced in granular form from pot~cillm chloride and
sodium nitrate while AmmOnilJm phosph~te may be conventionally
produced in granular ~orm from ammonia and ph~sphoric acid.
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Granular inorganic fertilizers produced by these and other pro-
cedures, well known, to those skilled in the art, such as crystallization
techniques or fertilizers which are recovered naturally in a granular
form can be treated to reduce fugitive dust emiccion in accordance
with the method of the present invention.
The method of the present invention generally is used to reduce
fugitive dust emicsion from granulated (particulate) inorganic fertiliz-
ers, i.e., fertilizers having the bulk (more than 50 weight percent) of
their particles greater than about 35 mesh and usually greater than
about 28 mesh (Tyler Sieve Size), which contain significant amounts
of fine particulates, i.e., particles with a size below about 100 mesh
(i.e., below about 150 microns) and particularly with a size below
about 150 mesh (i.e., below about 100 microns) (Tyler Sieve Size).
Fertilizers which are especially prone to fugitive dust emission during
h~nrlling are those containing above about 0.05%, and especially above
about 2.0%, by weight of such fines.
According to the method of the present invention, an effective
dust reducing amount ~f an aqueous solution of a mixture of urea and
lignosulfonate is appll~d, e.g., simply by spraying, to a granular, inor-
ganic fertilizer containing fine particles in order to reduce the level
of fugitive dust emiccion from the fertilizer.
The aqueous urea and lignosulfonate solution can be prepared
simply by dissolving urea in a lignosulfonate solution. Adding urea to
an aqueous lignosulfonate solution produces a viscosity reduction in
the solution. Consequently, solutions of higher solids contents can be
prepared and effectively applied using a mixture of urea and ligno-
1 33746~
sulfonate, then can be done using lignosulfonate alone. Suitable solu-
tions will contain about 45 to 80% solids. If the solids content of the
solution is too low, excess water in the solution may create a caking
problem for the treated fertilizer; while at high solids contents the
solution may be too viscous to apply effectively. Preferably a solution
with a solids content between about 50 to 65% will be used.
The solution should contain at least about 0.3 part and up to
about 40 parts by weight, and more preferably between about 0.5 part
and 10 parts by weight, lignosulfonate per part by weight urea.
Because lignosulfonate is less expensive than urea, it is preferred to
use solutions containing the least amount of urea needed to enhance
fertilizer dust suppression.
The aqueous solution can also include other constituents such
as fertilizer supplements, including sulfur, pesticides, selective herbi-
cides and micronutrients. Micronutrient elements are considered to
include iron, manganese, molybdenum, boron, copper, and zinc. These
elements normally are supplied in their oxide or salt forms. Suitable
salts include the sulfate, nitrates, chlorides, molybdates and borates.
The aqueous solution can be applied to the granular inorganic
fertilizer using any suitable technique, such as by spraying, sprinkling,
or by brushing. Generally, the aqueous solution is applied by spraying
in an amount to provide about 0.05 to 5.0 parts urea and
lignosulfonate solids per 100 parts fertilizer solids, preferably between
about 0.1 to 3.0 parts of the dry mixture per 100 parts of fertilizer.
Normally this level of treatment can be obtained by applying about
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0.2 to 5.0 weight percent of an aqueous solution onto the granular
fertilizer.
The aqueous solution usually is applied to the fertilizer, eg. by
spraying, at some point after granulation or prilling, for example by
spraying onto a rolling bed of the granular fertilizer, such as in a
rotary coating drum or mixer. Alternate methods might involve
spraying in screw conveyors, on belt conveyors or at any other mate-
rial transfer point where reasonable coverage of the fertilizer can be
obtained. In a bulk blen~ing operation, the solution can be applied
through a spray nozzle or sprayer in the mixer. Finally, it is possible
to spray the solution into the fertilizer in the granulator itself at the
end of the granulation process. Other ways of obtaining good cover-
age of the particulate fertilizer with the aqueous dust suppressant
solution will be apparent to those skilled in the art and can be used in
connection with the present invention.
The following ex~mple is provided for illustration and is not
intended as a limitation on the scope of the present invention
Example
Sep~rate 450 gram s~mples of a granular triple superphosphate
fertilizer (commercial designation 0-46-0), containing 5.3% by weight
fines (particle size less than 150 mesh), were sprayed with various
aqueous solutions of urea and lignosulfonate using a Wagner power
sprayer. The granular fertilizer was rotated in a pan granulator dur-
ing spraying. Three different aqueous solutions were used. Solution A
contained 40% by weight Ca lignosulfonate solids and 20% by weight
urea. Solution B contained 40% by weight Ca lignosulfonate solids and
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10% by weight urea. Solution C contained 35% by weight Ca ligno-
sulfonate solids and 30% by weight urea. The granular fertilizer was
sprayed with between 0.25 to 1.5 parts by weight of urea and
lignosulfonate solids per 100 parts fertilizer. The level of dusting was
determined by measuring the mass of particles passing through a 60
mesh screen after treatment. The results are presented in the table
below.
Level of Dustin~ ~L Application Level of
0.25% 1.0% 1.5%
Solution A 4.7 2.~ 0.~
Solution B 4.9 2.4 0.9
Solution C 3.5 2.6 0.5
While certain specific embodiments of the invention have been
described with particularity herein, it will be recognized that various
modifications thereof will occur to those skilled in the art, and it is to
be understood that such modifications and variations are to be
included within the purview of this application and the spirit and
scope-of the appended claims.
