Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Agitating to form larger particles
Reexamination Certificate
1999-08-10
2001-08-21
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Agitating to form larger particles
C264S118000
Reexamination Certificate
active
06277311
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to urea having a low biuret content and, more particularly, to a method of converting crystalline urea into a physical form that has desirable flow properties, resistance to caking and dust formation without any attendant increase in the concentration of biuret.
2. Description of the Prior Art
Urea finds use in a variety of applications. For example, urea is widely used as an agricultural fertilizer. When used as a fertilizer, urea can simply be broadcast on the ground or dissolved in a solution that is sprayed on the foliage of growing plants. In the former case, biuret content is relatively unimportant. However, when used as a foliage spray, the biuret content becomes critical. It is well known that biuret, formed by the condensation of two molecules of urea with the loss of one molecule of ammonia, is noxious to plant life since it exhibits a very active phytotoxic action. Accordingly, to produce a foliar grade urea, it is generally necessary that the urea have a maximum biuret content of 0.25% by weight, more preferably 0.15% by weight or less.
In addition to its use for agricultural purposes, urea has numerous other applications wherein it is required that the biuret content be low. For example, low biuret ureas are necessary in the production of certain synthetic resins and plastics, in pharmaceutical products, in solutions for textile treating and finishing, etc.
High purity crystals of urea can be produced by a process of reacting ammonia and carbon dioxide at high pressures and temperatures to form ammonium carbamate, which, under the reaction conditions, is converted into urea and water. The resulting mother liquor containing urea, ammonium carbamate, ammonia, and water is then treated by various processes to obtain solid urea crystals.
Although urea crystals of high purity and low biuret content can be produced, they are not satisfactory for normal handling, storage, or shipment due to their tendency to agglomerate. Accordingly, most of the industrial and agricultural grades of urea are in the form of prills made in prilling towers or the like well known to those skilled in the art. An inherent problem with the prilling of urea is that the urea is usually heated to or near its melting point with a consequent increase in the biuret content.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method for converting crystalline urea into a physical form that is commercially acceptable in terms of flow properties and has a low biuret content.
Another object of the present invention is to provide a method for converting crystalline urea into a form that is low in biuret content and resistant to caking and dust formation.
Still another object of the present invention is to provide a method for producing urea having a low biuret content that is free-flowing and has ready solubility in water.
The above and other objects of the present invention will become apparent from the description given herein and the appended claims.
In accordance with the present invention, there is provided a method for producing urea having a low biuret content comprising providing solid crystals of urea and forming said solid crystals into discrete, cohesive masses by the use of mechanical compacting techniques. One advantage of the present invention is that melting of the urea crystals is avoided, thereby virtually eliminating any increase in biuret content over and above that which is initially present in the solid crystals.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is based on the surprising finding that urea crystals can be formed by mechanical compacting techniques into larger discrete masses that exhibit good flow properties and resistance to caking and dust formation while still maintaining acceptable solubility in water and other aqueous media without increasing the biuret content of the urea.
The production of urea crystals having low biuret content, e.g., less than 0.15% by weight, is well known by those skilled in the art. Typically, in the process the crystalline urea is recovered from a mother liquor that, in addition to the urea, contains biuret and small amounts of ammonia and water. To separate the urea crystals from the mother liquor, it is common, as well known to those skilled in the art, to evaporate the mother liquor using subambient pressure and then separate the solid urea crystals by techniques such as centrifugation, filtration, decantation, or other techniques commonly used to separate solids from solutions. To remove impurities such as biuret from the solid urea crystals, it is also common to wash the crystals with small amounts of water.
Once the solid urea crystals have been washed, they can be dried, as, for example, using heated air, at a temperature of from 55-95° C. Solid urea crystals produced in this fashion typically have a biuret content of less than 0.2%, commonly less than 0.15% by weight. Generally, urea crystals have a biuret content of from 0.07-0.12% by weight. Accordingly, the biuret content of the urea crystals produced according to the process described above are eminently suited, at least as to biuret content, for agricultural, pharmaceutical, industrial, and virtually any other usage to which urea is put. However, as noted above, urea crystals cannot be easily handled, stored, or shipped due to their tendency to pack or agglomerate, and, accordingly, it becomes economically and commercially necessary to form the solid urea crystals into prills, which, as noted above, raises the biuret content to a level that for many applications makes the urea undesirable for use at best and totally unusable at worst.
To solve the above problems, the present invention contemplates compacting of the solid urea crystals by virtually any mechanical technique in which the solid crystals of urea are formed into larger masses without the use of heat, other than as may be generated in the mechanical compacting technique and, in any event, at a temperature substantially below the melting point of urea. The size enlargement—i.e., the formation of the solid urea crystals into larger, discrete masses—can be accomplished by techniques such as nodulizing, pelletizing, tableting, briquetting, roll pressing, etc. Of these various mechanical compacting techniques, pelletizing is preferred.
In the case of nodulizing, the solid urea crystals can be gathered into larger discrete masses of more or less spherical form by working them together, as, for example, in a ball mill or similar apparatus. However, this form of compacting is less preferred due to the tendency for heat buildup to occur during the working of the solid urea crystals. Moreover, there is a tendency of the end product to have low crush strength and be less flowable.
The more preferred methods of mechanical compacting involve the use of pressure, with or without shear and mixing, such as occurs in extrusion/pelletizing.
In the case of tableting, tableting presses of the rotary type can be employed.
Another type of mechanical compacting that can be employed is roll pressing, which utilizes the principle of press-agglomeration. In this process, the solid urea crystals are introduced into the nip of two counter-rotating rollers by means of a suitable feed—i.e., a screw. As the solid urea crystals are compacted, the pressure in the compacted masses increases. The product is typically a flat sheet of 5-20 mm thickness. The sheet produced by the roller press method can then be broken up into smaller pieces and classified into the desired particle size range.
Briquetting, another technique for forming the discrete masses of solid urea crystals, is a variation of the roll pressing technique wherein the solid urea crystals are fed into the space between and above counter-rotating rolls that have peripheral cavities. As the rolls separate, the briquets are discharged. Typically, the briquets are soft in their centers and hard on the surfaces and edges, where there are greater pressure and p
Anderson Mark C.
Chandler Douglas E.
Eimer Kenneth E.
Nelson Harold G.
Valpiani Sante P.
Coblenz Michael
Costal States Management Corporation
Gardere Wynne Sewell
Sickler Jennifer S.
Tentoni Leo B.
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