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Патент USA US2373869

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April 17, 1945~
F11-ed April 1o, 1942
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24,1. f
JM ,
'Patented Aint-1,7,
-,uiufrlszo ' ~sm'rrfzgs -Pli'rairr"Íorticav i ,y
' v¿Mascia
, `
cwinmnrnam Glen man, N. vJ., ’u
signor to American'oyanamid CompannNew:
York, N. Y., a >corporation of Maine
' 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
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
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
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:
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.
a fresh ammonia-methanol mixture containing no
In accordance with the present invention the
carbon dioxide the amount of, iron picked up by
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.
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. '
After the present invention was made, the
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
` '
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
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
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.
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