CA1241663A - Production of animal feed grade biuret - Google Patents

Abstract

ABSTRACT
Preparation of a composition particularly suitable for use as feedstock in the production of animal feed grade biuret by solid state pyrolyzation thereof in a recirculating oven, which composition comprises from about 37% to about 25% urea, from about 45% to about 60% biuret, and from about 3% to about 20% cyanuric acid, by weight, such preparation involving sparging air or other non-reactive gas through an urea charge at a temperature of from about 145°C. to 165°C. and at a rate of between about two to about ten cu. ft. of gas/hr/lb or urea for a period of at least four hours, then cooling and comminuting the product.

Description

r it 3 PRODUCTION OF ANIMAL FEED GRADE BIURET

Backqround Of the Invention Field of the Invention The present invention relates to the production of a partially pyrolyzed urea composition particularly suitable for use as the feedstock for the production of animal feed grade biuret, and containing controlled amounts of biuret, cyanuric acid, triuret, and outer homologsO Production of such composition involves the controlled pyrolysis of urea at elevated temperatures above the melting point of urea and at a pressure above atmospheric while subjected to sparged air or other non-reactive gas at the rate of about two to about ten cubic feet of gas per hour pound of urea, and cooling and comminuting the resulting product.

Description of the Prior Art Harmon U.S. Patent No. 2,145,392 dated January 31, 1939 discloses a basic process for pyrolysis of urea wherein urea is heated in the temperature range of 130-205C. while under partial vacuum in the absence of any catalyst to produce principally biuret, cyanuric acid, and other related compounds. Biuret is a useful chemical compound having considerable utility in prepared feeds for ruminant animals.
Crude technical mixtures of urea, biuret, triuret, and cyanuric acid have long been used as cattle feed supplements. The United States Department of Agriculture, Food and Drug Administration has approved pyrolyzed urea compositions having not more than 15% urea, not less than

-2-55~ biuret, and not more than 30~ of the sum of cyanuric acid, triuret, or other related compounds, and not more than 0.5~ of oil, by weight, as a cattle feed additive.
Garbo UOS. Patent No. 2,525,049 dated October 3, 1950 disclose much the same process for producing biuret from urea as is disclosed by Harmon 2rl45~392~ with the added feature of accelerating the reaction by the use of one or more catalysts and in some instances use also of a hydrocarbon fluidizing medium such as naphthalene or kerosene.
Olin U.S. Patent NoO 2,370,065 dated February 20, 1945 presents another teaching of use of an entraining agent such as toluene or naphtha to aid in the removal of ammonia when pyrolyzing urea to produce biuret.
Kluge U.S. Patent No. 3~150,177 dated September 22, 1964 presents a biuret production process similar to that disclosed by Harmon U.S. Patent No. 2rl45,392 coupled with the use of disodium phosphate or boric acid as a catalyst.
Kamlet U.S. Patent No. 2,768,895 dated October 30, 1956 presents a rather omnibus disclosure of use of biuret rich urea condensation products as animal feed. Kamlet V.S.
Patent No. 3,453,098 dated July 1, 1969 presents a variation of the process wherein biuret rich urea autocondensation products are dissolved in boiling water and then cooled to recrystallize the biuret products, which are then said to contain not less than 95% biuret and be characterized by complete removal of ammonium cyanate, the product thus being rendered suitable for use a depot fertilizer without the phytotoxic consequences said to be characteristic of such a product when substantial ar~monium cyanate is present.

Colby et al, U.S. Patent No. 2,861,886 dated November 25, 1958 presents another early disclosure of use of biuret rich compositions used a constituent ingredients of animal feed supplements.
Great Britain Patent No. 1,155,907 published 25 June, 1969 discloses the pyrolysis of urea to product a biuret rich reaction product, including the separation of a urea containing mixture from the reaction product, and recycling of the qeparated urea containing mixture to the reactor as a continuous processO
Japan Patent Publication 47-41888 (19723 discloses the production of purified biuret. The English language abstract published with Japan Patent Publication 47-41888 states that this publication discloses a process for preparing purified biuret by treating urea decomposition products with hot water, filtering the obtained mixture, treating the filtrate with aqueous ammonia and cooling it to below 40C., separating the precipitated biuret, recovering ammonia from the supernatant liquor by evaporating water, and recycling residual urea after decomposing it by heating.
Substantially pure biuret (separated from cyanuric acid and triuret) is obtained. Such abstract continues to give the following example. 100 grams urea is decomposed at 150C.
while introducing 22.0 l./min. air for 2 hours to give 38.8 grams biuret, 4.0 grams cyanuric acid, 3.8 grams triuret and 44.0 grams unreacted urea. The products are partly dissolved in 150 ml. water at 70C. The mixture is filtered and the filtrate is treated with 5 grams NH3 and cooled to 40C. The precipitate (24.1 grams; yield 98.8%3 is biuret containing 1.2% urea and only traces of cyanuric acid and triuret~

Formaini et al in U.S. 3,057,918 proposes a continuous process for the production of biuret wherein new urea plus recycled previously pyrolyzed urea is sparged with air while molten at elevated temperature to produce a feedstock for a pressure digestion step whereby a relatively pure biuret is separated from the mother liquor of the ammonia digestion step by a controlled vacuum crystallization and the unwanted byproduct is evaporated to give the sold recycle material added along with urea to made the feedstock for the initial pyrolysis step. The urea content of the typical feedstock entering the ammonia digestion step has the following analysis:
Biuret 28-42%
Urea 65-40%
Cyanuric Acid 3-14%
Triuret 5-7%
This is to be contrasted with the typical analysis for the feedstock of the present invention which is:
Biuret 45-60%
Vrea 37-25%
Cyanuric Acid 3-20~
Triuret 3-10%
The pyrolysis of urea to product biuret~ triuret, cyanuric acid, ammelide, melamine and other homologs, is well known.
When urea is pyrolyzed ammonia is always an accompanying byproduct It was early established by Harmon in U.S.
2,145,392 that the pyrolysis of urea to produce biuret was aided by the application of controlled vacuum whereby tne byproduct ammonia was effectivPly removed from the reactor thus lowering the partial pressure of ammonia and thereby driving the pyrolysis reaction toward higher biuret content.
The use of volatile entraining agents such as toluene to sweep out byproduct ammonia ln biuret manufacture was early suggested by Olin in U.S. 2,370,065. The use of gases such as nitrogen or air to weep out byproduct ammonia formed during urea pyrolysis ls suggested in U.S. 2,918,467 by ~ibbitts et al in the production of a pyrolyzed urea feedstock used in the production of melam$ne. For~aini U.S.

3,093,941 uses air as the stripping gas to remove byproduct ammonia formed during u ea pyrolys~ in the production of a pyrolyzed urea feedstock used in the production of cyanuric acid. Formaini ~Sr 3,057,918 uses air a the stripping gas to remove byproduct ammonia formed during continuous urea pyrolysis in the production of a pyrolyzed urea feedstoclc used in an extraction proce~ to product substantially pure biuret~

Summary of the Invention In tbe present invention air or like non-reactlve 9a8 i8 used as the stripping gas to remove byproduct ammonia formed during urea pyrolysis to produce a feedstock having a controlled fusion point which ic attained by controlling the analysis of the feedstock in regard to the urea, biuret, triuret, and cyanurlc acid content A suitable feedstock product for use in the process claimed in our co-pending Canadian Patent Application 484,244 is one that has the following characteri~tic~s Chemical Anal~si3 urea 37-25~
~Biuret 45-60%

%Cyanuric Acid 3-20%
%Triuret 3-10%
Melting Point 110-130C.

Feed grade biuret is a mixture of nitrogen containing chemicals produced by the pyrolysis of urea. The Food and Drug Administration of the U.S. Department of Agriculture has promulgated a specification for feed grade biuret (see Code of Federal Regulations, Title 21, Section 573,220), as follows:
Minimum biuret content 55 Maximum urea content 15 Maximum cyanuric acid, triuret~ tetrauret, and others 30%
Maximum oil content 0.5~

In the two--stage process described in our

4~
aforesaid application ~5~ 50 partially pyrolyzed urea containing not over 25% cyanuric acid and more than ~0% urea by weight is converted to animal feed grade biuret by subjecting the partially pyrolyzed urea feedstock in particulate form to a mild heat treatment in the substantially solid state with forced air flow through the particulate reaction mass. A product results having a high concentration of biuret and a product with a minimum of 55%
biuret, a maximum of 15% urea, and a maximum of 30% cyanuric acid and similar urea pyrolysis byproducts is easily produced. Such product is hydrocarbon~free and eminently suitable as a feed additive for cattle and needs no additional separation step to remove excess urea and D 'I 3 cyanuric acid. In such process a feedstock consisting of partially pyrolyæed urea in dry particulate form and containing not over about 2S~ cyanuric acid is treated in an oven with forced air recirculation at a temperature at or slightly below the softening point of particles, e.g. at a temperature between about 100C. and about 140C., and preferably between 115C. and 125C. for a period of time of generally between about 15 hours to about 200 hours and preferably from about 24 hours to 180 hours. During this stage of pyrolyzation it is essential that the feedstock be in an essentially solid state with at most only incipient surface fusion of particles, as distinguished from the molten, i.e. liquid state so that substantially sublimation can occur as well as evolution of ammonia from the product.
The ground feedstock is suitably placed in trays or the like to a bed depth o between 1/2 inch and 3 feet or more, and placed in a forced air circulating oven or the like with hot air circulated through the particulate mass in each tray, preferably upwardly through each tray. The temperature is preferably thermostatically controlled to within ~3C. within the oven.
The comminuted feedstock is placed in or on a container porous to air, such as a tray or other box-type container with a screen or like foraminous bottom and open top, which arrangements provide what may be generically termed a fixed bed. Alternatively, the feedstock bed may be arranged on a wire screen or like foraminous conveyor, or in a fluidizing chamber, which arrangements provide what may be generically termed a movable bed.
The depth of the bed of the particulate material can be any desired depth consistent with the need to --En maintain substantially and continuing forced air flow in contact with the material surfaces, and considering also that under a given operating condition a given total amount of contact of moving air with the surfaces of the particles is necessary to achieve the result of substantial urea sublimation and urea conversion to biuret, which considerations involve seYeral interrelated factors such as average mesh size of the particles, the temperature of the air, the depth of the particle bed and the volume of air flow past the particles. Thus, for example, in a situation where a fixed bed, 2 feet in depth, is composed of partlcles having an average mesh siæe of 8 mesh, a pressure drop of 0.16 psig per foot of bed has been found satisfactory for the operating condition where the air and particles are heated to a temperature of 127C. and for 36 hours.
Correspondingly, however, when the average particle size is 4 mesh, an optimized pressure drop through a bed 2 feet thick to accomplish a similar end product at the same temperature has been fcund to be 0.13 psig per foot of bed, and the heating should continue for a period of 50 hours.
Comminution of the solidified and broken up pieces of the partially pyrolyzed reaction product resulting from the first stage of reaction of urea and the addition thereto of feed grade biuret powder to increase the melting point and expedite cooling of the reaction product, can be carried out in any appropriate mechanical disintegrator such as a jaw crusher or rotary crusher, or hammermill or the like.
Regarding the powdered material added to expedite crystallization and cooling of the partially pyrolyzed reacticn product to make the feedstock for the solid state _9_ b~3 heating stave of the process, other powdered or comminuted materials can be used as the additive if they do not substantially lower the meltins point of the reaction mass during he solid state pyrolyzation and provided they are advantageous or at least not deleterious Jo the end use of the final reaction product, such as for animal feed or the like. In the case of the end use being animal feed, for example, advantageous additives may be calcium carbonate or calcium phosphate or other known animal feed additive.
however, the powdered or comminuted additive introduced to the partially pyrolyzed reaction product in making up the feedstock is preferably feed grade biuret such as readily available byproduct fines from earlier sizing processing, and offers the advantage of increasing the 15 melting point of the reaction mass during the solid state pyrolyzation since the proportion of biuret and its homologs is thereby increased in the mass) which in turn permits it being heated during the solid state pyrolyzation to a somewhat higher temperature without melting, thus 20 accelerating the heat conversion.
We have found that the two-stage pyrolyzation technique described results in a product which when comminuted needs no further processing before use as animal grade biuret. In general, the initial stage of the reaction 25 process is carried out at a temperature above the melting point of urea, forming a partially pyrolyzed reaction product comprising urea, cyanuric acid and biuret. The second stage of the process involves the continued pyrolyzation of the reaction product in particulate, essentially solid state, with forced hot air flow ,," ,~ q~5 ,~-~

interstitially through the particles, the net effect of which is to reduce the urea content and enhance the biuret content of the reaction mass, while only slightly increasing the cyanuric acid, triuret and other byproduct content.
During the solid state heat ~reatment~ some urea is sublimed as may also be a slight amount of biuret and possihly others. The effect of this sublimation in reducing the urea content of the product is substantial and is an important part of the process when one is attempting to produce FDA
acceptable material having not more than 15% urea.
In practicing the present invention, the pyrolysis of urea to give the desired feedstock is accomplished by heating urea above its melting point (132.6C.) but not over about 165C. and at an absolute pressure of between 16.5 psia to about 24.5 psia while simultaneously blowing air through the molten urea mass at the rate of from about 2 to about 10 cubic feet of air/hour/pound of molten urea.
The progress of the pyrolysis may be followed by monitoring the disappearance of urea in the melt as well as by the liberation of byproduct ammonia.
In carrying out this invention the raw material urea may be charged to the reactor in either liquid form or in the solid form as urea crystals or urea prills of commerce. When the solid form is charged to a reactor the reactor functions as a melter during the meltup stage and the temperature remains close to the melting point until all of the solid urea has melted. It is usually desirable to have some air sparging through the charge during the meltup stage in order to prevent any urea from fouling or plugging up the air sparger. The air sparger is suitably simply a 3f~

tube which carrles the air from a pressuxized air supply to its point of release at or near the bottom of the reactor, well beneath the surface of the urea mass When urea crystals or prill8 are charged to a oombined melter/reactor

5 due account should be taken of the fact that the bulk density of the solid urea it about 50~/cu.ft.~ whereas the den lty of the molten urea i8 about 80~/cu.tto In other word, a reactor charged full of urea prills will be only approximately half full when 2nelted down. A practical method of sully utilizing the working capacity of a melter/reactor i8 ~cO repeatedly add urea zither a liquid or prills to the reactor a the original charge of solid urea melts down. The temperature of the air sparged urea pyroly~is may vary between abou 145Co and about 165C.
with the preferred temperature ranqe between about 155C. to 165C. teat may be supplied to melt the urea and to initiate the biuret producing reaction by heat transfer through the wall of the meltee/reactor. Conventional heat transfer sy8te~8 such a team, Dowtherm heat exchange fluid, electrical resistance heating, or the llke, may be used. Electrical resistance heating has the advantage of easy thermostatic control and precise control of the end point of the reaction a a function of kilowatt input. A
significant amount of energy may be transferred as contained heat in the sparged air as a result of the heat of compression of the air cominy from a pressure blowers for instance a lS0 horsepower blower produclng alr under a pressure of 10 psig will raise the temperature of the sparging air from 21C. to 60C. when delivering air at the rate of 1680 C.F.M. in the aeration of charge of 18,000 lbs. of prilled urea in a * Trade Mark 2500 gallon reactor. Since the compression heated sparging air comes in contact with virtually all of the individual prills of urea, the heat transfer in the early state of the heat-up period is markedly improved over compressed air from an air storage tank where the heat of compression has been lost to the surrounding atmosphere. Significant heat may be recovered by taking the hot off-gas existing from one batch reactsr operating at 155C~ and using this hot gas to heat up another batch reactor charged with urea prills and starting a meltup and reaction cycle.
The sparging gas delivPry nozzle or nozzles can be directed downwardly or radially of the reaction vessel and can also be oriented to accomplish air-lift stirring, if desired, in a manner known per se. Separate mechanical stirring of a reacting urea batch undergoing air sparging may also be used but is not necessary. The heat transfer through the walls of a reactor is enhanced by the stirring of the molten urea by the sparging air. Where the hot off-gas from one reactor is used to heat up another, cold reactor, any sublimed urea or urea dust in the hot off-gas from one reactor is trapped in the bed of urea prills of the second reactor which acts thereby as a filter medium.
An important function of sparging air through molten urea undergoing reaction to produce biuret according to the following equationo 2 NH2CONH2 NH2CON~CONH2 + NH3 is that the removal of byproduct NH3 is facilitated. This is in contrast with the practice of Harmon in U.S. 2,145,392 which employs a vacuum to remove the byproduct NH3. Ammonia plus water vapor is by nature a corrosive system and it is ,3 necessary to protect the metal parts of any associated blower system. If one attempts to catch the off-gas ammonia in an aqueous trap operating under vacuum with the trap between the reactor and the vacuum source the amount of vacuum attainable is limited by the vapor pressure of the water and ultimately only a dilute ammonia water solution is produced. This severely limits the potentially attainable vacuum. When air sparging is used to hast2n the removal of byproduct ammonia in the pyrolysis of urea the blower is never in contact with any ammonia released during the formation of biuret and the air passing through the reactor and containing the byproduct ammonia may be trapped in a aqueous trap without any corrosion of the blower parts.
An important part of this invention is the recovery of byproduct ammonia as agueous ammonia, also known in the trade as aqua ammonia for use as agriculture fertilizer. This is accomplished by sparging air at a rate of from about 2 to about 10 cubic feet (measured at standard temperature and pressure) per hour per lb. of urea undergoing pyrolysis, and passing the hot off-gas containing sublimed urea and sublimed biuret through a water solution of ammonla in equilibrium with the ammonia in the off-yas and containing urea, biuret~ and other pyrolysis reaction products in solution whereby any sublimed urea and biuret or any urea dust is trapped in the solution but the gaseous ammonia is not. In practice this urea and biuret particulate and sublimate scrubbing should proceed with the aqueous solution at a temperature corresponding to the wet bulb temperatures of the hot off-gas. The cooled and scrubbed gas existing from the scrubber is then admitted to f to the bottom of a counter-current absorber column in which water entering at the top of the absorber column is the makeup for the liquid phase. A relatively concentrated aqueous ammonia phase exists from the bottom of the absorber 5 column. The counter-current absorber column has sufficient equivalent absorber plates to remove the ammonia in the gas stream to thy desired degree.

Description of the Preferred Embodiments EXAMPLE I
600 pounds of urea prills were charged into a 100 gallon 316 stainless steel reactor equipped with electrical resistance heaters rated at 36 kilowatts. The 100 gallon reactor was 24" inside diameter in size and had conical heads top and bottom, each having a depth of 3.5". The height of the vertical side between the top and bottom heads was 48n. The reactor was equipped with a top-mounted anchor type stirrer operating at 160 RPM. The upper head had one 8" flanged opening, one 4" flanged opening and three 2"
flanged openings. The bottom head had one 2" flanged opening in the center which was used to introduce air into the urea mass from below and also to drain the molten product from the reactor. 600 pounds of urea amounts to 53.1 gallons when melted and filled the reactor to a depth of about 2.44' above the bottom drain.
The charge of 600 pounds of urea was melted and heated to a temperature of 152C. and held at that temperature for a period of 4.5 hours, during which time the evolved ammonia gas was removed by air introduced at 15.82 psia and bubbled through the mass at the rate of 175 cubic feet per minute. The off-gas was run through a water trap.
At the end of the 4.5 hour pyrolyzation period, 520 pound of molten pyroly8i8 product was recovered it a temperature of 149C., and upon analysis had the following composition;
33.7% urea, 14.8% cyanuric acid, 46.4~ b~uret, and 5.1% other homologs, by weight, with a ~of~ening point ox 128C. The molten pyrolysi~ product way mixed with 60 pounds of feed grade biuret powder analyzing 12~ urea, 18.8% cyanuric acid, 63.3% biuret, and 5.9~ other homolo~s, by welght, and then mixed with 123 pounds of powdered material analyzing ~4O5%
urea, 12~9~ cyanuric acid, 47.9% b~uret, and 5.5~ other homologs~ by weight, in order to provide crystallization centers to hasten crystallization, and was then allowed Jo cool to about 72C. This composite product was then co~minu~ed Jo a prcduct having a mesh size between 1 and 4 mesh and used as a feed~tock in the oven pyrolyzation pro-cess of our Canadian patent application 484,244, noting particularly the 801id state treatment thereof a jet forth in the last portion of example 5 of said application 484,244. Specifically, as stated in Example 5, 600 lbs. of the comminuted feedstock material was placed in a 4'x4' steel box with a screen bottom and the box containing this bed of product, with a bed thlckness of 11~ was placed in a recirculating oven and heated to a temperature of 130C. by forced air recirculation upwardly through the bed at a rate of 6000 cubic feet per ml~ute. after 24 hour of such recirculation of the heated sir, the particle were slightly fused together end were manually broken apart by stirring with a shovel. The heating W~8 then contlnued by further forced air recirculation at the same temperature for a total a to period of 40 hours, during which time some 28 lbs. of ammonia and some ~2 lbs. of sublimate evolved. The final product, still in discrete particle form with only slight, readily broken surface fusion of the particles, analyzed 5 14.3% urea, 17.6% cyanuric acid, 62.7% biuret, and 5.4%
other homologs, by weight. The aeration rate during this second phase of the process was 17.5 cubic feet/hour/lb. of feedstock. As Jill be noted, the resultant product has a composition well within the feed grade biuret specification established by the Food and Drug Administration of the U.S.
Department of Agriculture and may be used for this purpose without further treatment, except for further comminution of the product, if desired.

E~MPLE II
In the following example the reaction was carried out in a 750 gallon reactor made of 316 stainless steel having an internal d:iameter of 58~ and dished heads top and bottom or radius 54n~ The height between the dished heads was 65n. The upper dished head had an 18" manhole, one 10"
centrally mounted flanged opening, two 8" flanged openings, and four 3" flanged openings. The bottom dish had one centrally mounted 2" flanged opening and two other 2"
flanged openings. An air sparger line made of 5" O.D. pipe projected through one of the I" flanged openings downward as far as possible and then made a right angle turn to support horizontally a closed end piece of 5" O.D. pipe having six 5/8" dia. holes and fourteen 1/2" dia. holes in its 56~
length. All of the air at 645 C.F.M. was exhausted through these twenty holes and provided intimate contact of the air Pi .3 with the molten urea. the urea was charged through the 18"
manhole in the upper head and compressed air at 25.7 psia was introduced through the sparger. The reactor was heated by electric resistance strip heaters attached to the outside shell. Heaters of 135 KW rating were placed on the vertical side wall and heaters of 15 KW rating placed on the bottom dish for a total of 150 RW energy input. All of the heaters were thermostatically controlled responsive to a temperature probe immersed in the reaction mass.
An initial charge of 4700 lbs. o urea prills was heated to 148.9C. in 2 hours 41 minutes while subjected to an airflow of 645 C.F.M~ at which time the air flow was stopped and 1184 lbs. additional urea prills were added to give a total charge of 58&4 lbs. urea. The temperature dropped to 128.9C. The airflow was then resumed and continued to the end of the reaction period. 1 hour and 4 minutes after the air flow was resumed, the temperature had risen to 150.5C. at which time a total of 534 KW hours of electrical energy had been consumed. The temperature was maintained at 150.5C. for an additional 2 hours, after which the temperature varied between 150.5C. and 156.1C.
over a period of an additional 2 hours, at which time the product was removed from the reactor and cooled to a temperature of 37C. and analyzed using the well known HPLC
method. The resulting analysis was: urea 31.3%, cyanuric acid 14.0%, biuret 46.2%, triuret 8.5%, by weight. The heating up time to reaction temperature was 3 hours 44 minutes. The reaction time was 4 hours making the total cycle time 7 hours 44 minutes. This product was eminently suitable for conversion to feed grade biuret according to the procedure set forth in our co-pending ~.~. patent 4~4l!_44 application . The product was a tan cream color, was substantially odorless, and was easily comminuted into through 1 mesh particle size for the oven treatment to produce feed grade biuret. The air pressure on the reactor was 24.7 psia at the start during the meltup period and then declined to 21.7 psia during the reaction period. With 5884 lbs. of urea prills charged and 645 C.F.M. of air sparged, the aeration rate was 6.58 cu. it of air/hour/lb. of urea, producing a product analyzing 31.3% urea.

EXAMPLE I I I
A charge of 4800 pounds of urea prills was placed in the 750 gallon reactor described in Example II and heated from 22C. to 149~4C. in 3 hours during which time 175 C.F.M. of air was passed thru the molten urea. The temperature of the molten urea was then maintained at about 150C. for an additional 8.0 hours. The product was then removed from the reactor and cooled. The product analyzed 44.3% urea and 37.85% biuret, by weight. This shows the effect of aeration at the rate of 2.19 cubic feet of air/hour/lb. or urea on the disappearance of urea and production of biuret when the reaction is carried out at about 150C. During the run the air pressure in the reactor was 21.7 psia at the start and 20.7 at the conclusion. The off-gas ammonia was absorbed in water In carrying out the invention we have discovered that the melting point of the crude urea pyrolysis product to be used as the feedstock for biuret producing oven pyrolyzation should preferably be between about 110C and about 130C and preferably as high as possible to prevent fusion during the oven treatment, slnce Eusion interferes with heat transfer from the hot oven air and the sublimation of urea and byproduct ammonia gas removal. The melting point of the feedstock is affected mostly by the proportionate amounts of urea, biuret, and cyanuric acid.
Generally, increased urea content lowers the melting point whereas increased cyanuric acid content raises the melting point and is a function of the phase diagram characteristic of all the urea pyrolysis products. For example, the M.P.
of the lowest melting eutecti~ mixture of urea and biuret is 111.1C. and has the composition 30~ urea and 70% biuret, by weight. For the feedstock of utility in our invention, it is considered that the urea analysis should be not over about 37% and may have any lower value down to about 25~, by weight. The cyanuric acid analysis of a feedstock of utility in our invention may vary from about 3% to 20%, by weightO The triuret analysis may vary from about 3% to 10%, by weight.
EXAMPLES IV - IX
The following reaction runs Nos. 4 through 9 were made in the equipment of Example It and following the same experimental procedure as in Example II. The results of the runs are tabulated in the followins TABLE ONE together with the published results of the example given in Japan 47-41888.
An important part of our invention is the discovery that the rate of aeration in cubic feet of air/hour/lb. of urea, over a relatively narrow range (2-10 cut ft./hour/lb. urea) is the major factor in controlling the physical and chemical properties of the feedstock product which render it suitable for solid state pyrolysis Jo produce feed grade biuret.
As will be readily understood by those skilled in the art, variations and modifications are possible in practice of the present invention. Thus, simply by way or further example, while air has been the primary sparging gas referred to in the foregoing Examples and is believed preferable for practice of the invention because of economic considerations, it is considered technically possible to utilize other gases or mixtures of gases for sparging purposes in the process, so long as the gas or mixture of gases is nonreactive in relation to the pyrolysis reactions.
Another example of such a nonreactive gas is nitrogen and also the class of gases known as inert gases.

Jo h to I o o us O
aa U
P. o o o o ox us O
on ,~ o Jo o us V l _1 Z _t a ED Us 00 dP ~3 O
m I O N
dP _l ,~

O or r ~~D
US
. . . r 0 I? l IQ In l ED OD N
I: Pi ;~ I
'I I' O O O O O O
a I; O I
O O Us O 1 o o f 0 a o u- ED ' r1 ~c u . o O o O o O
I Us U 7 O O O O N N
K DZ Pl ,1 go o o u-::~ ~~ Us I` O

.
. o C) i I; Z us

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The process of preparing animal feed grade biuret, comprising; preparing a feedstock by heating a urea charge at a temperature of between about 145°C. and about 165°C. while simultaneously blowing a nonreactive gas at superatmospheric pressure through the molten urea charge at an aeration rate of from about two to about ten cubic feet of gas/hour/lb. of urea for at least about four hours and until the urea content of the product is reduced to between about 37% and about 25% urea, the cyanuric acid content is between about 3% and about 20% and the melting point of the product is between about 110°C. and about 130°C., by weight;
cooling and comminuting the product to form such feedstock, containing the comminuted feedstock as a bed of several inches thickness in a recirculating oven, and passing heated recirculated gas upwardly through the bed of feedstock at a temperature slightly below the melting point of the feedstock until the urea content thereof is less than about 15% by weight.

2. The process of claim 1, wherein the temperature at which the urea is heated during gas sparging thereof is from about 155°C. to about 165°C.

3. The process of claim 1, wherein the time of heating and gas sparging is from about 4 hours to about 16 hours.

4. The process of claim 1, wherein the sparging gas is air.

5. The process of claim 1, comprising interrupting the gas sparging and cooling the product when the urea content of the product is reduced to about 30% by weight.

6. The process of preparing animal feed grade biuret, comprising; preparing a feedstock by heating a urea charge at a temperature of between about 145°C. and about 165°C. while simultaneously blowing air at superatmospheric pressure through the molten urea charge at an aeration rate of from about two to about 10 cubic feet of gas/hour/lb. of urea for at least about four hours and until the urea content of the product is reduced to between about 37% and about 25% urea, the cyanuric acid content is between about 3% and about 20%, and the melting point of the product is between about 115°C. and about 125°C., by weight; cooling and comminuting the product to form such feedstock;
containing the comminuted feedstock as a bed of several inches thickness in a recirculating oven; and passing recirculated air through the bed of feedstock at a temperature slightly below the melting point of the feedstock until the urea content thereof is less than about 15% by weight.

7. The process of claim 6, wherein the time of heating and gas sparging is from about 4 hours to about 16 hours.

8. The process of claim 7, comprising interrupting the gas sparging and cooling the product when the urea content of the product is reduced to about 30% by weight.