Патент USA US2373869код для вставки
April 17, 1945~ A. c. WINTRINGHAM 2,373,859 MELAMINE PROCESS F11-ed April 1o, 1942 WASH WÄTFF /wï AVM/N5 BY 24,1. f JM , 'Patented Aint-1,7, v -,uiufrlszo ' ~sm'rrfzgs -Pli'rairr"Íorticav i ,y ' v¿Mascia mouse , ` cwinmnrnam Glen man, N. vJ., ’u signor to American'oyanamid CompannNew: York, N. Y., a >corporation of Maine j _ ' application april 1o, 1942, serial _Nm 43am ‘ 1 s claims. iwi. 26o-249.5) bonate radical itself, or guanidine carbonate This invention relates tothe manufacture of formed by the reaction ofammonium carbonate melamine by4 processes which involve the heating with the cyanamide or dicyandiamide in the auto of cyanamide or dlcyandlamide v‘in a solvent com clave,-v or to a combination of these or other fac prising liquid ammonia at high'y temperatures and tors. At any eventI found rthat the presence pressures, and more particularly to processes of -oi.' substantial amounts of carbon dioxide or car this type ‘in’ which the‘solvent is recovered and bonate radical in the liquid ammonia-containing reused. ' ` ' l It is known that melamine can be obtained by heating a mixture oi’y about l part of cyanamide or dicyandiamide with from l to 2 parts of liq-~ uid ammonia in an autoclave, with or without solvent causes` an _increase in the amount of me , tallic impurities in the melamine-~ product and in the recirculating solvent liquor. It is a principal object of the present inven tion to improve the process of melamine manu facture outlined above- by obviating this source _ the addition of a lower aliphatic alcohol orl other'-` non-aqueous diluent to reduce the autoclave pres-l` sure. In the commercial operation of thisproc . of metallic impurities.v It is a further object of l ' containing the ammonia from the melamine >and ysolvent; liquor after its separation from the mela ess it is now the practice to separate the solvent 15 to pass this solvent into a stripping column where in'the ammonia,Í or the ammonia-methanol mix- 1 ture,A can beseparated-from the wash water or other solvents thatmay be used in the process. 20 The, recovered ammonia or yammonia-methanol the invention to treat the ammonia-containing mine but before completion of the stripping step with va fixed in'order to remove the bonate radical therefrom. Only a small amount of carbonate radical is present in the liquor obtained as a filtrate from theL melamine slurry formed in the autoclave, which for convenience will be referred to .here inafter as mother liquor. Nevertheless, I found autoclave charge. i When a'melamine plant equipped with an auto 25 thatby adding a strong alkali such as sodium, potassium, or calcium hydroxide to thev other clave composed of a high-chromium alloy steel liquor after its separation4 fromv the melamine was operated continuously for some time using butbefore the `completion -of the stripping step the process outlined-'above with recovery’and re use of the methanol-ammonia solvent, it lwas 30 the content of carbonate radical could. be reduced to one-tenth of its former value, and by using found that the melamine was contaminated in the a large excess of strong valkali this ligure could autoclave by iron and other metallic impurities. ' mixture is then used t0 -dissolve further amounts of cyanamide or dicyandiamide to prepare a newr By discharging the contents of the autoclave di rectly into a large body of water containing an be reduced even -further. My invention in its broader aspects, therefore, consists in the addi tion of a strong alkali to the mother liquor from organic hydroxy compound capable of‘forming a _slurry prepared by heating a charge complex water-soluble iron salt, as described in 35 aof melamine cyanamide or dicyandiamide in a solvent com Patent No. 2,324,450, it was possible to retain the greater part yof the metallic impurities vin the aqueous methanol-ammonia -f solution separated from the melamine product, and during a typical prising liquid ammonia, followed by stripping and recovering ammoniaor a mixture of ammonia andv _non-aqueous diluent> substantially Y free -from week’s operation the iron content of this " solu 40 carbony dioxide or carbonate radical for reuse in> tion` averaged about 60 parts per million. Since theprocess. extensive tests had shown that high-chromium alloy steels 'are ordinarily resistant kto attack by the reagents involved in the melamine-forming reaction, this corrosion was entirely unexpected. After a careful investigation, I finally d‘scov ered that the ammonia-containing solvent ex hibited a tendency to pick up carbon dioxide dur y - 1 . - , The invention will be> illustrated in greater de tail with reference to the accompanying drawing, the’ single ligure of which is a flow sheet :illus trating VÍdiagrammatically a preferred method of recovering .a carbonate-free methanol-ammonia solution from ythe mother liquor obtained by the ñltration of'a melamine slurry. In the pro’cessy ing its reuse in the process, and that it was the " outlined on this ilowsheet dicyandiamide is dis carbonate radicalthat caused the corrosion re 50 solved in a water-free mixture of methanol and ammonia from storage tank l in a mixing tank sulting in an excessively high contento! iron in’` 2, and the` resulting solution is charged into an the autoclave product. I have not as yet deñnlte autoclave 3. In this autoclave the charge is heat ed at temperatures_above> 100° C., and prefer it may be due to a corrosive action of the car 55 ably above 120‘ C. for several hours, the opti 1y determined the reason why the carbonate radi cal causes an increase of 'iron' in the melamine: 2 2,373,869 vmum temperature range being i60-200° C. The dicyandiamide is converted into melamine by this heat treatment, and since the melamine is not weight of/dicyandiamide wereL dissolved in the mixing ta‘nk 2 in 270 parts of an alcohol-am monia soution analyzing 54% ammonia and 46% methanol. 'I'his charge was then pumped into soluble in liquid ammonia or in a methanol-am monia-mixture. it is obtained in the form of a the autoclave l, which was a large autoclave composed of a chrome-iron alloy containing about SluIlrfìorder to remove small quantities of alkaline 18% chromium, about 8% of nickel, about 0.2% carbon, the balance consisting substantially of iron and stabilizer for the alloy. The autoclave . clave are discharged directly into a large volume 10 charge was heated at 180-200’ C. for several hours impurities of the type of guanidine, biguanide and guanylurea, the entire contents of the auto of water in the closed quenching tank 4. A small quantity of glycerine, mannitol or other water „ at a pressure of 1600-1900 lbs./sq. in. to form a soluble hydroxy- compound is dissolved in this water to prevent the precipitation of iron. .The parts ‘by weight of water containing 1 part of melamine slurry which was then lblown into 600 \ mannitol in the quenching tank 4 during a period ammonia and methanol from -the autoclave are 15 ‘of about 15 minutes. ._The solution in the quench- y of course dissolved in the water during the y -ing tank was agitated and cooled by recirculat quenching operation, and the resulting aqueous ' ing through a cooler to reduce the temperature slurry is òooled to 40-70" c. and altered in s nner » - to about 40-70° C. in order to avoid excessive losses of. melaminev while retaining any uncon verted dicyandiamide in solution and was then filtered in the filter press 5. The mother liquor tank 4 and the wash water is stored in a storage i was passed through the stripping coluinn'! and tank l, from which it -is introduced into the lthe alcohol and ammonia vapors were condensed quenching tank 4 to receive the next batch of in condenser I2 and returned to the storage tank melamine from the autoclave.` The nlter cake from , 25 I and used to dissolve more dicyandiamide to the iilter press 5 is dried in a drier >`l, preferably form another autoclave charge. . at temperatures of about 100° C., and is obtained When the foregoing process was operated with as a pure melamine product. Y a fresh ammonia-methanol mixture containing no In accordance with the present invention the carbon dioxide the amount of, iron picked up by 30 mother liquor obtained-as a ?ltrate from the filter corrosion in the autoclave» was very small. How press 5 is mixed with sodium hydroxide, potassium ever, upon continuous daily operation of the plant hydroxide, milk of lime 'or other strong alkali and for a period of time without the addition of strong this alkali addition may advantageously take place alkali to the mother liquor from the i‘llter press in a closed mixing tankl. Any alkali'capable of 5 it was noted that the iron content oi' the mother press 5 to separate the solvent from the melamine. The illter cake is washed with a quantity of cold water about equal to that used in the quenching combining with carbondioxide inpreference to liquor became progressively higher, amounting to ammonia or capable of liberating ammonia from about 62 parts per million over anaverage week’s ì ammonium carbonate may be employed and the operation. At the same time, the amount of iron in the melamine product was increased, despite term “strong alkali”is intended to define an alkali such as the oxide or _hydroxide of an alkali the use of mannitol to hold back as much iron as forming metal which is capable oi' accomplishing 40 possible in the quenching tank 4; during an aver this result. 'I'he sodium hydroxide or other strong age week’s operation without the addition of alkali should of course be'added in quantities `at alkali to` the mother liquor the content of iron least suillcient to combinewith all the carbonio in the melamine product averaged 28 parts per acid radical that may be present .in the mother million. During this period the recovered meth liquor and preferably a> large `excess of strong snol-ammonia mixture in the storage tank l alkali is added because of the low concentration was found to contain 0.043% oi' combined carbon of ammonium carbonate in. the mother liquor. dioxide. Amounts on the order of 1 part by weight of NaOH for each 25-50 parts of methanol-ammonia mix ture have given good results in practice. ' \ , f ' , , ‘ After the present invention was made, the 50 After the strong alkali has been added the mother liquor is introduced near the center vof a stripping column 9, which is preferablyfheated by steam, in which vapors of methanol and am monia. are separated from water containing the sodium carbonate formed in the mixing tank 8. The water and sodium carbonate are discharged to waste from the base of the' column through the outlet l0 while the carbonate-free ammonia practice was adopted of adding 5 parts by weight of 100% sodium hydroxide for each batch of mother liquor from the quenchingtank '4 amount ing‘to approximately 860 parts of nltered mother liquor. After the strong alkali had been added`to several successive `batches of mother liquor in this manner it 'was found that the carbonate ' content of thel recovered methanol-ammonia blend had-been reducedto an average of about 0.023%, and aftercontinued additions of sodium hydroxidek to successive lbatches this iigure was and methanol' vapors ar'e y discharged througl'lV 60 reduced .toas lowV as 0.0127 . After the meth vapor line Il toa condenser I2, wherein they are snol-ammonia mixture had been rendered sub condensed to a methanol-ammonia solvent liquid stantially carbonate-free in this manner the which is returned to the storage tankv i. By> this average- iron content of the mother liquor, over a long periodof operation, was only 27 parts per method a `.substantially carbonate-free alcohol 65 million. , `At the same time the average iron con ammonia mixture -is obtained from the tent of the melamine product was reduced to 13 liquor from the filter press Sand excessive cor parts per million. 'I'he addition of- a strong alkali rosion in the autoclave 3 and' contamination of to the mother liquor from the ñlter press 5 there the melamine by excessive amounts of iron' are avoided. ` ` ' f ` fore resulted in a total reduction of the average 70 iron pick up in the autoclave 3 from 90 parts per As a further illustration of the practical advan tages obtainedby the present invention, the fol lowing -s’peciñc example is given. `A melamine plant in continuous daily production was oper ated on the following schedule: 150 parts by 75 million to 40 parts per` million, with a corre sponding improvement in the purity of the mela mine product. . What I claim is: 1. In a melamine manufacturing process which 2,373,869 ' 3 charge consisting of a solution of a member of « ammonia in amounts in excess of that necessary to combine with all the carbonio acid radical pres the group consisting of cyanamide and dicyandi ent, then distilling said solvent and recovering a l amide in a liquid solvent comprising anhydrous ammonia to form melamine, separating the mela mine from said solvent, and thereafter dissolving drous ammonia and non-aqueous diluent, and comprises the steps of heating in an autoclave a 7 further amounts of a member of the group con sisting of cyanamide and dicyandiamide in said substantially carbonate-free mixture of anhy dissolving further amounts of a member of the ‘ group consisting of cyanamide and dicyandiamide in said mixture to form another charge, whereby excessive metallic impurities in the melamine sr lvent to form another autoclave charge, the method of avoiding excessive corrosion of the au 10 product due to the presence of carbonate radical during the heating step are avoided. toclave d'ue to the presence of carbonate radical in 3. A process for the *manufacture of melamine said solvent which 4consists in adding to the sol which comprises the steps of heating a charge vent, after separating the melamine therefrom but consisting of a solution of a member of the group before the new charge is formed, an alkali which combines with carbon dioxide in preference to 15 consisting of cyanamide and'dicyandiamide in a f liquid mixture of anhydrous ammonia and a lower ammonia, said alkali being added in amounts in aliphatic alcohol to form a melamine slurry, dis excess of that necessary to combine with all the charging the slurry into water, separating the re carbonate radical present in the solvent, and then sulting aqueous solution of ammonia and alcohol distilling the solvent to separate it from the added alkali and the carbonate radical combined there 20 from the melamine, adding an alkali which com bines with carbon dioxide in preference to ammo with. t nia to the solution in amounts in excess of that 2. A process for the'manufacture of melamine necessary to combine with all the carbonio acid which comprises the steps of heating a charge consisting of a solution of a member of the group consisting of cyanamide and dicyandiamide in a solvent comprising a liquid mixture of anhydrous ammonia and a non-aqueous diluent to form a melamine slurry, separating the solvent from the melamine, adding to the solvent an alkali ‘which combines with carbon dioxide in preference to radical present and stripping and recovering .a substantially carbonate-free alcohol-ammonia mixture from the solution, whereby excessive l metallic >impurities inthe melamine product due to the presence of the carbonate radical during the heating step are avoided. ANDREWS C. W'IN‘I'RINGHAM.