Process for the production of azodicarbonamide

A process for the preparation of azodicarbonamide by oxidation of hydrazodicarbonamide in an aqueous suspension containing hydrogen peroxide wherein the reaction is carried out in the presence of iodine at a temperature between 50.degree. and 95.degree. C. while the reaction mixture has a pH ranging from 1.0 to 5.0.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The invention relates to a process for the preparation of azodicarbonamide 
by oxidation of hydrazodicarbonamide with hydrogen peroxide. 
DISCUSSION OF THE PRIOR ART 
Azodicarbonamide is a known foaming agent for the production of mouldings 
which have a porous or sponge-like structure (German Patent Specification 
871,835). It can be obtained both by electrolytic oxidation and by 
chemical oxidation of hydrazodicarbonamide (DT-OS (German Published 
Specification No.) 2,016,764 and DT-OS (German Published Specification 
No.) 2,341,928). 
According to JA-AS (Japanese Published Specification) 6,166/67, oxidation 
with hydrogen peroxide in the presence of hydrobromic acid or its salts is 
particularly advantageous since, in accordance with the equation 
##STR1## 
only water is formed as a by-product and it is said that the amount and 
degree of acidity of the resulting waste water are also less than in the 
case of the processes of the state of the art (compare DT-OS (German 
Published Specification No.) 2,341,928, page 3, 2nd paragraph). 
However, when the oxidation with hydrogen peroxide is carried out in the 
presence of a bromine compound, it is also necessary for an acid which has 
a strength at least equal to that of acetic acid to be present (DT-OS 
(German Published Specification No.) 2,341,928, page 6, final paragraph) 
and this is used in a concentration of 5 to 45% by weight, and preferably 
of 10 to 40% by weight in the case of sulphuric acid. Therefore, the use 
of hydrogen peroxide as the oxidising agent also leads to an acid mother 
liquor being obtained and the elimination of this poses problems in 
respect of the load on the effluent and of environmental protection, apart 
from the fact that the acid, which is used merely as an auxiliary for the 
oxidation, is lost. 
Thus, the problem of finding a process for the oxidation of 
hydrazodicarbonamide to azodicarbonamide which does not pollute the 
environment and in which, as far as possible, no auxiliaries, or only very 
small amounts of auxiliaries, are used has not yet been solved hitherto, 
even by the use of hydrogen peroxide as the oxidising agent. 
SUMMARY OF THE INVENTION 
It has now been found that hydrazodicarbonamide can be oxidised in aqueous 
suspension with hydrogen peroxide to give azodicarbonamide in a 
particularly advantageous manner when the reaction is carried out in the 
presence of iodine at temperatures of between 50.degree. and 95.degree. C 
and in the pH range between 1.0 and 5.0. 
Hydrazodicarbonamide and its preparation are known (German Patent 
Specification No. 945,235, DT-OS (German Published Specification No.) 
2,057,979 and DT-OS (German Published Specification No.) 2,210,317). 
Iodine is used in an amount of 0.01 to 0.3% by weight, and preferably 0.04 
to 0.08% by weight, relative to the hydrazodicarbonamide employed. 
The amount of iodine which is used in the process according to the 
invention can also depend on the amount of hydrazodicarbonamide which is 
employed. If only a small amount of hydrazodicarbonamide is oxidised and 
if the reaction volume is small, the possibility that iodine will vaporise 
from the reaction mixture and that its concentration will fall very 
rapidly is generally greater than when, with a large amount of 
hydrazodicarbonamide, a correspondingly large amount of iodine is also 
employed in the same concentration ratio. Therefore, it is generally 
appropriate, when the amounts reacted are small, to keep to the upper 
region of the concentration range indicated for iodine, whilst in the case 
of larger batches it can also be advantageous to employ lower 
concentrations of iodine. 
In place of iodine one can also use compounds which contain iodine and 
which supply iodine under the reaction conditions. Compounds which can be 
used are, in particular, hydriodic acid and its water-soluble salts; 
appropriately, readily accessible salts are chosen, for example alkali 
metal iodides and alkaline earth metal iodides, in particular lithium 
iodide, sodium iodide and potassium iodide. If compounds containing iodine 
are used in place of iodine, the amount employed is, of course, that which 
is equivalent to the amount of iodine to be used. 
In general, the process according to the invention is appropriately carried 
out in the temperature range between 60.degree. and 90.degree. C. It is 
possible for the temperature to fall below or to exceed the stated 
temperature but in general this is not appropriate since the rate of 
reaction decreases as the temperature falls and at a higher temperature 
substances start to be lost due to thermal decomposition and, furthermore, 
the acid consumption increases. 
The reaction is preferably carried out in the temperature range between 
70.degree., and especially 75.degree., and 85.degree. C. 
In general, the process according to the invention is carried out at a pH 
value of the aqueous suspension of between 1.5 and 5.0. The reaction is 
preferably carried out in the pH range of 2.0 to 4.0. 
However, the pH values can also fall below or exceed the abovementioned 
values. However, this is generally not appropriate since, at lower pH 
values, the rate of reaction becomes lower and the acid consumption, in 
order to maintain the pH values becomes greater and because, at higher pH 
values, the ease with which hydrogen peroxide undergoes thermal 
decomposition becomes greater and the catalytic action of the iodine is 
reduced. 
However, one can compensate for the choice of a less favourable pH value by 
prolonging the reaction time and/or by a higher catalyst concentration. 
In general, it is not important which acid is used to adjust the pH. In 
principle, all acids of adequate strength can be used. However, the acid 
chosen will, of course, be an acid which is readily accessible, which does 
not react with hydrogen peroxide and which has an anion which poses as few 
effluent problems as possible. In particular, hydrochloric acid, 
phosphoric acid and, preferably, sulphuric acid can be used. 
The concentration of the acid used is not an important factor. 
Appropriately, the acid is used in the customary concentration. However, 
it can be advantageous to choose as high as possible a concentration in 
order to avoid an unnecessary increase in the volume of the reaction 
solution. 
The amount of acid used depends on the given conditions in a particular 
case since, as has been stated, the acid is required only to adjust the 
pH. In general, when sulphuric acid is used, about 0.7 to 3.0% by weight, 
relative to the hydrazodicarbonamide employed, are necessary. 
Hydrogen peroxide can be employed both in the pure form and as an aqueous 
solution. Appropriately, the commercially available 30 to 70% strength by 
weight concentrates are used since the concentration of hydrogen peroxide 
in the reaction mixture is not of fundamental importance. 
In general, it is appropriate to use H.sub.2 O.sub.2 in excess of the 
stoichiometrically required amount of one mol per mol of 
hydrazodicarbonamide. Advantageously, the excess is 5 to 25 mol % of 
H.sub.2 O.sub.2. 
Although the azodicarbonamide formed is not attacked by H.sub.2 O.sub.2, 
reactions of impurities in the hydrazodicarbonamide employed and products 
of the thermal decomposition of small amounts of the azodicarbonamide 
formed, as well as the decomposition of H.sub.2 O.sub.2, can, however, 
give rise to a consumption of H.sub.2 O.sub.2 which exceeds the 
stoichiometrically required amount, so that the use of an excess of 
H.sub.2 O.sub.2 is advantageous. The amount of this excess which is 
appropriately to be employed will depend on the given conditions in the 
particular case and it will be possible to determine this easily by a few 
experiments. As already mentioned, too high an excess is not harmful but 
is unnecessary. 
In general, the process according to the invention is carried out as 
follows. 
Hydrazodicarbonamide is suspended in water and iodine or the chosen 
compound containing iodine which supplies iodine under the reaction 
conditions, is added, the suspension is heated to the chosen reaction 
temperature and, after the pH value has been adjusted, hydrogen peroxide 
is added and, during the addition, the said pH value is maintained by 
adding further acid. 
The amount of water used to prepare the aqueous suspension is not 
important. Of course, sufficient water is used to obtain a stirrable 
suspension, so that homogeneous distribution of the hydrogen peroxide, 
which is added, in the reaction mixture can be achieved by stirring. In 
general, a weight ratio of hydrazodicarbonamide:water of about 1:1 has 
proved advantageous. However, one can also use a smaller or greater amount 
of water but an increase in the amount of water needlessly requires 
greater reaction volumes and amounts of heat for the same conversion. 
Iodine, or the chosen compound containing iodine, can be added at room 
temperature prior to heating or only at the reaction temperature. 
The pH value can be adjusted immediately after the iodine or the compound 
containing iodine has been added. In general, a small amount of acid is 
required for this, as has already been mentioned above. 
In the case where hydriodic acid or another acid iodine compound has been 
chosen as the compound containing iodine and the pH value of the aqueous 
suspension is below 1.5, or the intended pH value, one can either adjust 
the pH to the desired higher value by adding a corresponding amount of an 
alkaline solution, preferably an alkali metal hydroxide solution, 
especially sodium hydroxide solution or potassium hydroxide solution, 
until the desired value is reached or wait for the pH value or rise to the 
desired value during the oxidation and then maintain the pH value at the 
desired value by adding acid. In fact, in general, the pH value of the 
reaction mixture rises during the reaction due to the formation of small 
amounts of impurities which have a basic reaction. 
Since the reaction is exothermic it is generally not necessary to supply 
further heat after heating to the reaction temperature and the reaction 
temperature can be maintained by appropriate control of the addition of 
hydrogen peroxide. However, it can also be advantageous to remove the 
resulting heat of reaction by cooling and to add hydrogen peroxide 
correspondingly more rapidly. 
Iodine can vaporise at the reaction temperature and is appropriately 
collected in an absorption solution which reduces it to iodide. Absorption 
solutions of this type are known; an aqueous solution of sodium 
pyrosulphite (Na.sub.2 S.sub.2 O.sub.5) is advantageously used. 
When the reaction has ended, the azodicarbonamide formed can be separated 
off from the mother liquor in the customary manner, for example by 
filtration. 
The iodine contained in the mother liquor can be driven out, if necessary 
after adding further hydrogen peroxide in order to oxidise iodide, by 
means of steam or a stream of gas, for example air or nitrogen, and 
recovered. One can also heat the mother liquor to the boil, in which case 
the iodine passes over with steam. 
One can, of course, transfer all of the iodine contained in the mother 
liquor into the abovementioned absorption solution and re-use the iodine 
solution, thus obtained, for oxidation of an equal amount of 
hydrazodicarbonamide, so that the same amount of iodine theoretically can 
be used to oxidise unlimited amounts of hydrazodicarbonamide. 
Excess H.sub.2 O.sub.2 contained in the mother liquor already decomposes 
when the mother liquor is heated in order to drive out the iodine or can 
be decomposed, after neutralising the mother liquor, by boiling. 
After the iodine has been driven off and the H.sub.2 O.sub.2 has been 
decomposed, the mother liquor contains, in addition to small amounts of 
organic impurities which were contained in the hydrazodicarbonamide or 
have been formed by thermal decomposition of azodicarbonamide, only the 
anions of the acid used in order to adjust the pH, preferably sulphate. 
The amount is generally about one % by weight, relative to the 
hydrazodicarbonamide employed. 
The iodine employed can be recovered particularly advantageously as follows 
and, at the same time, the mother liquor can also be re-used. 
The absorption solution, in which the iodine which vaporises during the 
reaction has been collected and which contains, in a sufficient amount, 
the reducing agent required to reduce iodine to iodide, is added to the 
mother liquor and, thus, all of the iodine contained in the mother liquor 
is reduced to iodide, if necessary water is distilled off until the 
original volume of the mother liquor or the volume of the amount of water 
used to prepare the aqueous suspension of hydrazodicarbonamide has been 
reached and this solution is used in place of water to prepare the aqueous 
suspension of hydrazodicarbonamide. In this way, fresh water can also be 
saved. It is true that the salt content of the mother liquor increases 
every time it has been re-used. However, the advantage of the saving of 
fresh water can outweigh the disadvantage of the higher salt content in 
the effluent. 
In particular, the recovery, which has been described above, of the iodine 
employed and of the mother liquor can be advantageous for continuous 
operation of the process according to the invention. In order to prevent 
the salt content of the aqueous solution rising too high one can, for 
example, continuously withdraw from the system that amount of the total 
volume, which is finally obtained, of mother liquor, washing water and 
absorption solution which is in excess of the original volume of the 
amount of water used. 
As already mentioned, the particular advantage of the process according to 
the invention is that the auxiliaries consumed are only small amounts of 
an acid, preferably sulphuric acid, in order to maintain the pH range 
during the reaction, and of a reducing agent for iodine, preferably sodium 
pyrosulphite, and that, accordingly, only small amounts of salts, 
preferably sulphates, are contained in the mother liquor. The extent to 
which the process pollutes the environment is thus particularly small. 
A further advantage of the process according to the invention, that is to 
say the quantitative recovery of the iodine, has already been mentioned. 
Furthermore, in order to carry out the process according to the invention 
it is not necessary to isolate the hydrazodicarbonamide after it has been 
prepared. If, for example, hydrazodicarbonamide is prepared from urea and 
hydrazine or hydrazine hydrate at elevated temperature and under elevated 
pressure by reacting urea with hydrazine or hydrazine hydrate in an 
aqueous medium at 105.degree. to 140.degree. C and under a pressure of 1.2 
to 3.8 bars, the reaction product can be employed direct, without 
isolation and purification of the resulting hydrazodicarbonamide, as an 
aqueous suspension of the hydrazodicarbonamide in accordance with the 
process of the invention. It is true that, in this case, because the 
suspension may contain excess urea or hydrazine hydrate, ammonium salts 
and other oxidisable impurities, a higher consumption of H.sub.2 O.sub.2 
is to be expected and, thus, a larger excess is necessary than when 
hydrazodicarbonamide is employed in a fresh water suspension, but this 
disadvantage can be more than compensated by the operations which are 
dispensed with. In contrast to the oxidation with chlorine, which is 
carried out industrially, the formation of explosive nitrogen-halogen 
compounds is not to be expected with the small amount of iodine which is 
used.

EXAMPLE 1 
900 g (7.63 mols) of hydrazodicarbonamide are suspended in 1,000 ml of 
water and 1.25 g of 57% strength by weight hydriodic acid are added. 
Whilst stirring vigorously, the suspension is warmed to about 80.degree. C 
and the pH value is adjusted to 3.0 by the dropwise addition of 10% 
strength by weight sodium hydroxide solution. 546 g of 50% strength by 
weight hydrogen peroxide (7.98 mols of H.sub.2 O.sub.2) are then added in 
the course of about 4 hours. By regulating the addition of hydrogen 
peroxide, the reaction temperature is kept between about 78.degree. and 
85.degree. C without supplying heat and the temperature is prevented from 
rising above 85.degree. C, if necessary by means of external cooling. At 
the same time, the pH value of the suspension is controlled and kept at 
about 3.0 by adding 12% strength by weight sulphuric acid. The pH should 
not fall outside the range of 2.5 to 3.5. 
During the oxidation, small amounts of iodine sublime and these are passed 
through a tube, which has been warmed to about 80.degree. C, into an 
absorption vessel which contains a solution of 0.4 g of Na.sub.2 S.sub.2 
O.sub.5 in 60 ml of H.sub.2 O. 
After all of the hydrogen peroxide has been added dropwise, the mixture is 
stirred for about a further 2 hours at the reaction temperature, during 
which time the pH value is maintained, as described above. A total of 
about 49 g of 12% strength by weight H.sub.2 SO.sub.4 are consumed. 
The reaction product is then filtered off, washed with 200 ml of water and 
dried. In this way 875 g (98.8% of theory) of azodicarbonamide which has a 
melting point of 230.degree. C (decomposition) are obtained. 
5 g of 50% strength by weight hydrogen peroxide are added to the filtrate 
and the washing water and the mixture is heated to the boil, the 
distillate being collected in the abovementioned absorption solution. In 
the course of about 15 minutes, the iodine from the mother liquor, 
together with about 60 ml of water, has distilled over quantitatively and 
been reduced. The absorption solution can be added to a new batch in place 
of iodine. 
The mother liquor is then neutralised at about 90.degree. C and kept at 
this temperature for about 45 minutes in order to decompose excess H.sub.2 
O.sub.2. The mother liquor contains about 9 g of salts, corresponding to 
one % by weight, relative to the hydrazodicarbonamide employed. 
The total consumption of H.sub.2 SO.sub.4 is about 6.1 g, which corresponds 
to about 0.7% by weight or 0.8 mol %, relative to the hydrazodicarbonamide 
employed. 
EXAMPLES 2-6 
The amount of hydrazodicarbonamide indicated in Table I which follows was 
suspended in the indicated volume of water and 1.25 g of 57% strength by 
weight hydriodic acid were added. Whilst stirring vigorously, the 
suspension is warmed to approximately the temperature indicated in Table I 
and the pH value is adjusted to 3.0 by the dropwise addition of 10% 
strength by weight sodium hydroxide solution. 105 mol % of the 
stoichiometrically required amount of 50% strength by weight hydrogen 
peroxide are then added in the course of the indicated reaction time. 
Without supplying heat, the reaction temperature is kept, on average, at 
about the indicated temperature by regulating the addition of hydrogen 
peroxide and the temperature is prevented from exceeding the indicated 
temperature by more than 5.degree. C, if necessary by means of external 
cooling. At the same time, the pH value of the suspension is controlled, 
and kept at about 3.0, as described in Example 1. 
Subsequently, as described in Example 1, the reaction product is filtered 
off, washed and dried and its azodicarbonamide content is determined by 
analysis. The yield of azodicarbonamide which is determined in this way is 
given in % of theory in Table I which follows. 
Table I 
__________________________________________________________________________ 
Hydrazodi- Reaction 
Reaction 
Mol % of HI, 
Yield of 
Example 
carbonamide 
Water 
temperature 
time relative to hydrazo- 
azodicarbon- 
No. g ml .degree. C 
hours 
dicarbonamide 
amide, % of theory 
__________________________________________________________________________ 
2*a 225 500 50 10 1.6* 84 
b 225 1,000 
50 12 1.6* 94 
3 225 500 60 8 0.32 93 
4 450 500 70 8 0.16 95 
5 900 1,000 
90 6 0.08 94 
6 450 500 95 5 0.16 89 
__________________________________________________________________________ 
*Note: 
In contrast to the general description given above, 6.25 g of 57% strengt 
by weight hydriodic acid were used in Example 2, in place of 1.25 g. 
EXAMPLES 7 and 8 
450 g of hydrazodicarbonamide were suspended in 500 ml of water in each 
case and 1.25 g of 57% strength by weight hydriodic acid were added. 
Whilst stirring vigorously, the suspension is warmed to approximately the 
temperature indicated in Table II and the pH value is adjusted to 4.0 by 
the dropwise addition of 10% strength by weight sodium hydroxide solution. 
The amount of 32% strength by weight hydrogen peroxide which corresponds 
to 115 mol % of the stoichiometrically required amount of H.sub.2 O.sub.2 
is then added in the course of the indicated reaction time. 
The reaction temperature is kept at, on average, approximately the 
indicated temperature by supplying heat. At the same time, the pH value of 
the suspension is controlled, as described in Example 1, and kept at about 
4.0. 
Subsequently, as described in Example 1, the reaction product is filtered 
off, washed and dried and its azodicarbonamide content is determined by 
analysis. The yield, determined in this way, of azodicarbonamide is given 
in % of theory in Table II which follows. 
Table II 
______________________________________ 
Mol % of HI, 
Reaction Reaction relative to 
Yield of azodi- 
Ex. temperature 
time hydrazodi- 
carbonamide, 
No. .degree. C (hours) carbonamide 
% of theory 
______________________________________ 
7 60 12 0.16 99 
8 70 12 0.16 98 
______________________________________ 
EXAMPLES 9 to 12 
The amount of hydrazodicarbonamide indicated in Table III which follows was 
suspended in the indicated amount of water and 1.25 g of 57% strength by 
weight hydriodic acid were added. Whilst stirring continuously, the 
suspension was warmed to about 80.degree. C and the pH value was adjusted, 
by the dropwise addition of 10% strength by weight sodium hydroxide 
solution, to the value indicated in Table III. Subsequently, 105 mol % of 
the stoichiometrically required amount of 50% strength by weight H.sub.2 
O.sub.2 were added in the course of the indicated reaction time and, 
without supplying heat, the reaction temperature was kept at about 
80.degree. C by regulating the addition of hydrogen peroxide, as described 
in Example 1; in Example 12 only, 120 mol % of the stoichiometrically 
required amount of H.sub.2 O.sub.2 were added. At the same time, the pH 
value of the suspension was controlled and, in the manner described in 
Example 1, kept at the value indicated in Table III. 
Table III 
__________________________________________________________________________ 
Hydrazodi- Mol % of HI, 
Yield of 
Example 
carbonamide 
Water 
Reaction time 
relative to hydrazo- 
azodicarbon- 
No. g ml hours pH 
dicarbonamide 
amide, % of theory 
__________________________________________________________________________ 
9 450 500 6 1.5 
0.16 97 
10 450 500 5 2.0 
0.16 96.5 
11 450 500 5 4.0 
0.16 95.5 
12 450 500 5 4.5 
0.16 95.5 
__________________________________________________________________________ 
EXAMPLE 13 
225 g (1.91 mols) of hydrazodicarbonamide are suspended in 500 ml of water 
and 1.25 g of 57% strength by weight hydriodic acid (0.32 mol % of HI, 
relative to hydrazodicarbonamide) are added. Whilst stirring vigorously, 
the suspension is warmed to about 80.degree. C and the pH value is 
adjusted to 3.0 by the dropwise addition of 10% strength by weight sodium 
hydroxide solution. Subsequently, 137 g of 50% strength by weight hydrogen 
peroxide are added in the course of about 3 hours, the reaction 
temperature being kept at about 80.degree. C and the pH value being kept 
at 3.0 in the same manner as has been described in Example 1. The reaction 
mixture is then worked up in the same way as has been described in Example 
1. 215 g (96.5% of theory) of azodicarbonamide are obtained. 
EXAMPLE 14 
900 g of hydrazodicarbonamide are suspended in 1,000 ml of water and 0.45 g 
of sodium iodide (0.04 mol %, relative to hydrazodicarbonamide) are added. 
Whilst stirring vigorously, the suspension is warmed to about 90.degree. C 
and the pH value is adjusted to 3.0 by the dropwise addition of 10% 
strength by weight sodium hydroxide solution. In the same manner as 
described in Example 1, 575 g of 50% strength by weight hydrogen peroxide 
(8.35 mols of H.sub.2 O.sub.2) are added in the course of 6 hours. The 
reaction temperature is kept at 90.degree. C and the pH value is kept at 
3.0 in the same way as has been described in Example 1. The reaction 
mixture is then stirred for a further 1 hour and cooled to about 
80.degree. C and 5 g of 50% strength by weight H.sub.2 O.sub.2 are added. 
Subsequently, 100 l of air are blown in one hour from a submerged frit 
through the warm suspension which is at 80.degree. C. The stream of air 
containing iodine is passed through an ice-cooled washing bottle filled 
with a solution of 0.4 g of sodium pyrosulphite in 60 ml of water and in 
this bottle the iodine is absorbed and reduced to sodium iodide. 
The reaction mixture is then cooled to about 50.degree. C and the reaction 
product is filtered off. 841 g (95% of theory) of azodicarbonamide are 
obtained. 
The mother liquor does not contain either iodine or iodide ions. The iodine 
employed has been removed quantitatively from the mother liquor. 
EXAMPLE 15 
450 g (3.81 mols) of hydrazodicarbonamide are suspended in 500 ml of water 
and 1.25 g of 57% strength by weight hydriodic acid (0.16 mol % of HI, 
relative to hydrazodicarbonamide) are added. Whilst stirring vigorously, 
the suspension is warmed to about 80.degree. C and the pH value is 
adjusted to 3.0 by the dropwise addition of 10% strength by weight sodium 
hydroxide solution. 280 g of 50% strength by weight hydrogen peroxide 
(4.11 mols of H.sub.2 O.sub.2) are then added in the course of about 4 
hours. The reaction temperature is kept at about 80.degree. C and the pH 
value is kept at about 3.0, as described in Example 1. 
After all of the hydrogen peroxide has been added dropwise, the mixture is 
stirred for a further 1 hour at the reaction temperature, the pH value is 
then adjusted to 4.0 with 10% strength by weight sodium hydroxide solution 
and, without further pH control, the mixture is stirred for a further hour 
at about 80.degree. C. The reaction product is then filtered off, washed, 
rinsed with 100 ml of water and dried. 432 g (97% of theory) of 
azodicarbonamide are obtained. 
The mother liquor from which the reaction product has been filtered off, 
the washing water and the Na.sub.2 S.sub.2 O.sub.5 solution, in which the 
iodine which sublimes has been collected, as described in Example 1, are 
combined and concentrated to the original volume of 500 ml by 
distillation. The resulting distillate is salt-free and iodine-free. 
EXAMPLE 16 
450 g of hydrazodicarbonamide are suspended in the concentrated mother 
liquor obtained according to Example 15 and oxidised, whilst stirring 
vigorously, at 80.degree. C and at a pH value of 3.0, in the course of 4 
hours with 280 g of 50% strength by weight hydrogen peroxide, as described 
in Example 15. 
437 g (98.5% of theory) of azodicarbonamide are obtained. 
Example 17 
(a) 1,634 g (27.5 mols) of urea, 649 g (13 mols) of 100% strength hydrazine 
hydrate and 980 ml of water are heated, in a 10 l steel autoclave, to 
about 120.degree. C, whilst stirring well. The pressure is allowed to rise 
up to 2.0 bars and is then adjusted to 1.2 bars by means of a reducing 
valve. The aqueous ammonia which distils off is condensed in a steel 
condenser and collected in a measuring vessel. The amount of water in the 
autoclave is kept approximately constant by pumping in fresh water. The 
reaction has ended after 7 hours. A total of about 4.5 l of water are 
distilled off. The hydrazodicarbonamide which has precipitated is filtered 
off and dried; 1,475 g (96.5% of theory, relative to the hydrazine hydrate 
employed) of hydrazodicarbonamide are obtained. The mother liquor has a 
volume of 1.4 l and is diluted to 1.6 l. 
(b) 1.25 g of 57% strength by weight hydriodic acid are added to 900 g of 
the hydrazodicarbonamide obtained according to (a) and 1,000 ml of the 
mother liquor from (a) and the mixture is warmed to about 80.degree. C, 
whilst stirring vigorously. The pH value is adjusted to 3.0 as described 
in Example 1 and, as described in Example 1, is kept at this value during 
the reaction. Whilst maintaining a reaction temperature of about 
80.degree. C, as described in Example 1, 600 g of 50% strength by weight 
hydrogen peroxide (8.77 mols of H.sub.2 O.sub.2) are added in the course 
of about 4 hours. Initially a vigorous evolution of nitrogen and carbon 
dioxide takes place and has to be controlled by intensive stirring and 
appropriate regulation of the addition of H.sub.2 O.sub.2, in order to 
prevent the suspension foaming over. 
Subsequently, the mixture is stirred for a further 2 hours at about 
80.degree. C whilst maintaining a pH value of 3.0 and the reaction product 
is then filtered off, washed with 200 ml of water and dried. In this way, 
873 g (98.5% of theory) of azodicarbonamide are obtained. 
As is shown by the evolution of nitrogen and carbon dioxide at the start of 
the oxidation with H.sub.2 O.sub.2, the mother liquor from the preparation 
of hydrazodicarbonamide contains impurities which can be oxidised more 
easily than hydrazodicarbonamide. A larger excess of H.sub.2 O.sub.2 than 
in Example 1 has therefore also been used. In the same way, a larger 
amount of H.sub.2 SO.sub.4 is required in order to maintain the pH value 
of 3.0; 26 g of concentrated sulphuric acid were consumed, which 
corresponds to about 3% by weight of H.sub.2 SO.sub.4, relative to the 
amount of hydrazodicarbonamide employed.