Process for the continuous production of sodium azide

The present invention deals with the continuous production of sodium azide from sodium amide and nitrous oxide. This reaction takes place on a support material for sodium amide consisting of a mixture of sodium azide and sodium hydroxide. This mixture is passed through a reactor maintained at temperatures of between 200.degree. and 270.degree. C.; a portion of the reaction product and the support material are transferred out of the system at an end of the reactor, and a primary quantity of the reaction product and support material are returned to the reactor inlet where the material is combined with fresh, heated sodium amide in an amount corresponding to the quantity of sodium azide transferred-out with the reaction product and support material. With the aid of this continuous process, higher space-time yields of sodium azide can be obtained than in case of the conventional methods without increasing the danger of no longer controllable explosions.

BACKGROUND OF THE INVENTION 
This invention relates to a continuous process for the production of sodium 
azide wherein sodium amide on a support material of sodium azide and 
sodium hydroxide is reacted with nitrous oxide or dinitrogen monooxide 
(N.sub.2 O). 
The production of sodium azide has heretofore been conducted by reacting 
sodium amide with nitrous oxide at temperatures of above 190.degree. C. At 
this temperature, sodium amide is present in the molten phase. For this 
reason, it is applied to a support material of sodium azide and sodium 
hydroxide. The reaction is performed discontinuously in mixing reactors 
permitting because of the size of the reactors maximally the conversion of 
130 kg of sodium amide within 8 hours. 
During the reaction between sodium amide and nitrous oxide, water is formed 
in addition to the target product, sodium azide; this water reacts with 
sodium amide leading to the formation of ammonia and sodium hydroxide. 
Since ammonia and nitrous oxide form an explosive mixture, the 
aforedescribed mode of operation for producing sodium azide has given rise 
to explosions in case the two gases are in an explosive mixing range. 
Consequently, enlargement of the reactor volume to increase the space-time 
yield in this conventional process would considerable heighten the danger 
of explosions, which are no longer controllable. 
Therefore, the problem presents itself of guiding the reaction between 
sodium amide and nitrous oxide in such a way that sodium azide is obtained 
in a considerably larger space-time yield than in the disclosed methods 
without incurring the aforedescribed disadvantages. 
SUMMARY OF THE INVENTION 
In meeting this objective, in accordance with the invention a continuous 
process for the production of sodium azide has now been found wherein 
nitrous oxide (N.sub.2 O) is reacted with sodium amide arranged on a 
support material of sodium azide and sodium hydroxide in a reactor at 
temperatures of between 200.degree. and 270.degree. C., characterized by 
separating at least about 5% by weight of the solid material discharge 
from the reactor to provide a first product stream containing a mixture of 
sodium azide and sodium hydroxide (which include newly formed products and 
support material) and a second product stream with the remainder of the 
discharged material; the first product stream being removed from the 
system and the second product stream being recycled to the reactor and 
NH.sub.3 and excess N.sub.2 O being discharged from the reactor in a zone 
of an inlet section of the reactor. 
It is possible with the aid of this process to increase the space-time 
yield in the reaction of sodium amide with nitrous oxide by a multiple as 
compared with the above-disclosed known process without increasing the 
total gas volume of ammonia and nitrous oxide to such an extent that 
uncontrolled gas explosions occur. 
In the reactor which preferably is a mixing/conveying reactor, molten 
sodium amide provided on the support material - - - denoted hereinbelow 
also as "solid reaction material" - - - is transported in the direction 
toward a product stream divider, whereby it is reacted with nitrous oxide 
(N.sub.2 O) to form sodium azide (NaN.sub.3) and sodium hydroxide (NaOH). 
Appropriate mixing-conveying units are known to a person skilled in the 
art; examples that can be cited are screw conveyors or kneader mixers 
preferably located in a tubular reactor. 
The mixing/conveying reactor is maintained at an internal temperature of 
between 200.degree. and 270.degree. C.; the reaction is preferably 
performed at a temperature of between 230.degree. and 270.degree. C. 
The solid reaction material and the reaction products including newly 
formed sodium azide and sodium hydroxide pass from the mixing reactor into 
a product stream divider directly adjoining the outlet section of the 
reactor; the product stream divider can also optionally be integrated into 
the reactor on an end facing away from the inlet section. The reaction 
material, after leaving the reactor, need no longer be heated to the 
reaction temperature; however, a special cooling step is likewise 
unnecessary since the main portion of the solid reaction material is from 
there again conducted to the inlet section of the reactor where this 
portion must again be heated to the reaction temperature. 
In the product stream divider, from 5 to 10% by weight of the reaction 
mixture obtained at that point is transferred out of the system. From the 
removed material, the sodium azide contained therein is obtained by 
conventional methods. For example, after cooling, the mixture is dissolved 
in water and subsequently such a quantity of water is distilled off from 
the resulting aqueous solution that the sodium azide is precipitated in 
the aqueous medium. The sodium azide is then filtered off and can be 
purified, if desired, by recrystallization. 
The main quantity of solid reaction material and the remainder of the newly 
formed sodium azide and sodium hydroxide leaving the product stream 
divider are reintroduced into the inlet section of the mixing reactor by 
way of suitable conveying means. It is advantageous to heat the conveying 
means so that the material being conveyed will not cool down too much and 
a minimum of additional energy needs to be expended during heating of the 
material in the reactor. 
Before the recycled material is made to contact the nitrous oxide in the 
mixing reactor, fresh, preheated sodium amide is added to this material 
stream in a quantity corresponding to the quantity of sodium azide 
transferred out in the product stream divider. Suitably, this addition 
takes place shortly prior to entrance of the recycled solid material into 
the reactor. However, it is also possible to effect the addition in the 
inlet section of the reactor. 
The sodium amide added to the returned reaction material is preferably 
heated to a temperature at which it is present in the liquid phase. In 
principle, it is also possible to conduct the heating step to a 
temperature corresponding to the temperature exhibited by the recycled 
material. The nitrous oxide is passed countercurrently to the stream of 
solid material through the reactor. Preferably, the nitrous oxide is 
introduced into the system at the product stream divider; however, it can 
also be introduced at the outlet section of the reactor. A stoichiometric 
excess is utilized; the excess gas in withdrawn, together with the ammonia 
formed during the reaction in the reactor at the inlet portion of the 
mixing reactor. 
It is advantageous to separate out of the withdrawn gaseous mixture, the 
nitrous oxide from the ammonia in a manner known per se and to recycle 
this nitrous oxide into the process. 
The amount of nitrous oxide to be employed is to be at least so large that 
one mole of nitrous oxide is used per two moles of sodium amide. An excess 
past this stoichiometric ratio is advantageous. The excess can range up to 
50% above this ratio. 
A mixture of sodium azide and sodium hydroxide, also called crude azide, 
serves as the support material for the sodium amide. Preferably, these two 
compounds in this mixture are in a molar ratio of 1:1. Exceeding this 
ratio in the upward or downward direction by up to 10% is readily 
possible. 
At the beginning of the reaction, the crude azide is charged into the 
reactor and heated to the reaction temperature. Since both compounds or 
components of the crude azide are constantly newly formed during the 
process in a molar ratio of 1:1, neither of the two compounds needs to be 
supplemented during the process.

DETAILED DESCRIPTION OF THE INVENTION 
The tubular reactor 1 of a testing plant, initially rendered inert with 
nitrogen gas, is filled with 320 kg of crude azide (NaN.sub.3 /NaOH in a 
molar ratio of 1:1) as support material, and interior of the reactor as 
well as the recycle screws 3 of a screw conveyor for recycling solid 
material from the product divider to the reactor are preheated to a 
temperature of 240.degree. C. Subsequently, 0.3 kg of sodium amide at a 
temperature of 300.degree. C. is added, in metered amounts, via conduit 6 
and N.sub.2 O is introduced via conduit 4 in a quantity stream of 200 
l/min. After onset of the reaction--recognizable by a temperature 
elevation in the reaction zone and by analyzing the waste gas in conduit 7 
for ammonia--molten sodium amide (0.78 l/min corresponding to about 60 
kg/h) is continuously added to the recycled reaction mixture of sodium 
azide and sodium hydroxide via conduit 6, the solid reaction products and 
support material are recycled via the recycle screws 3, and the N.sub.2 O 
stream is increased to a value of 435 l/min corresponding to an excess of 
34%. The reactor is then continuously operated in this way by withdrawing 
from the product stream divider 2, integrated into the reactor 1, 
continuously via the gate valve 5 under complete reaction of the sodium 
amide, 1.6 kg/min of a mixture of NaOH and NaN.sub.3 (equimolar mixture) 
and removing at the reactor inlet section via conduit 7 the excess N.sub.2 
O as well as 0.2175 kg/min of NH.sub.3 ; the reaction material and newly 
formed solid products that have not been withdrawn are recycled into the 
reactor 1 by way of the recycle screws 3. In separator 8, the ammonia is 
separated from the nitrous oxide, and the nitrous oxide is introduced into 
conduit 4. 
The reaction is terminated by interrupting the amide feed, and the reactor 
can be taken out of operation after a post reaction period of 30 minutes 
during which time the feed of gaseous N.sub.2 O is not interrupted. 
If one compares the continuous addition of sodium amide at about 60 kg/h, 
corresponding to approximately 480 kg within 8 h, with the throughput 
according to the discontinuous method according to the prior art, 
amounting to 130 kg in 8 h, the pilot plant itself will increase output by 
a factor of about 3.6.