CA1337460C - Particulate fertilizer dust control - Google Patents

Abstract

The present invention pertains to a method for reducing dust emissions from granular inorganic fertilizers, such as nitrates, phosphates, sulfates, chlorides, limestone, dolomite and mixtures thereof, wherein an aqueous solution containing a mixture of urea and lignosulfonate solids, generally in an amount of 45% to 80% by weight, are applied to the granular fertilizer in a dust reducing amount, generally between about 0.05 part to about 5.0 parts of said urea and lignosulfonate solution per 100 parts by weight of the granular fertilizer.

Description

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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, 1 337~6~
_ - 2 -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 _ - 3 -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 . .
" ;~, ~ 4 ~ 1 3 3 7 4 6 0 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 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.

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 ~ 337460 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 -lo- I 337460 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.

Claims (8)

1. A method for reducing dust emission from granular inorganic fertilizers which comprises applying to the granular fertilizer a dust reducing amount of an aqueous solution containing a mixture of urea and lignosulfonate solids, 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.

2. The method of Claim 1 wherein said dust reducing amount comprises from about 0.05 part by weight to about 5.0 parts by weight urea and lignosulfonate solids per 100 parts by weight of said fertilizer.

3. The method of Claim 1 wherein said inorganic fertilizer is selected from the group consisting of nitrates, phosphates, sulfates, chlorides, limestone, dolomite and mixtures thereof.

4. The method of Claim 1 wherein said lignosulfonate is selected from the group consisting of sulfite lignin and a sulfonated lignin.

5. The method of Claim 1 wherein said granular inorganic fertilizer contains at least 0.05% by weight of particles less than 100 mesh.

6. The method of Claim 3 wherein said granular inorganic fertilizer contains at least 2.0% by weight of particles less than 100 mesh.

7. The method of Claim 1 wherein said inorganic fertilizer is selected from the group consisting of ammonium nitrate, ammonium phosphate, ammonium sulfate, potassium nitrate, potassium sulfate, potassium magnesium sulfate, potassium chloride, bone meal, triple superphosphate, single superphosphate, limestone, dolomite and mixtures thereof.

8. The method of Claim 6 wherein said inorganic fertilizer is selected from the group consisting of ammonium nitrate, ammonium phosphate, ammonium sulfate, potassium nitrate, potassium sulfate, potassium magnesium sulfate, potassium chloride, bone meal, triple superphosphate, single superphosphate, limestone, dolomite and mixtures thereof.