246 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVEScapacity already containing the spent acid from the previous nitration. The mixture
is stirred at high speed. During this operation the extraction of dinitrobenzene
from the spent acid is taking place. After one hour of stirring the nitrator contents
are allowed to remain at rest for 1 hr during which separation of the liquid into
two layers takes place. The lower, acid layer is drained off to a spent acid tank
of 25 m3
capacity; then 6500 kg of nitrating mixture of the composition:
HNO 3 30%
H 2 SO 4 67%
H 2 O 3%
is fed slowly into the nitrator over a period of 4 to 5 hr. The nitrator contents are
then cooled and stirred at high speed. A temperature below 40°C is maintained
during the addition of acid. By the end of the operation a rise in temperature up
to 80°C may be allowed. After all the acid has been added the contents of the
nitrator are stirred again at 80-90°C until the nitration of nitrobenzene to dinitro-
benzene has been completed. This can be achieved within 1 to 2 hr. Nitrobenzene
is tested for by smelling.
After the nitration has been completed, 1000 l. of water is added gradually
during 1 hr, the temperature being maintained at 80-90°C. The nitrator contents
are allowed to remain at the above temperature for 1 hr. Then the lower layer,
i.e. the spent acid, is drained off to a tank from which it can be transferred by
compressed air to the nitrator for the next nitration operation. The product left
in the nitrator is passed to a purification plant, where it is washed with alkali and
purified by sulphitation. Finally the product is dried.
Purification. Washing dinitrobenzene with a solution of sodium hydroxide is car-
ried out in the following way. The crude, molten dinitrobenzene in the nitrator is run
in a tightly closed, brick lined iron vessel, heated internally by a stainless steel steam
coil. From this vessel it is transferred in steam heated, stainless steel pipes to a
washing tank of 16 m^3 capacity, filled with 3-4 m^3 of water and 400 1. of a 33%
solution of sodium hydroxide, heated to 50°C. The dinitrobenzene is run in
the washing tank over a period of half an hour and this is accompanied by con-
tinuous vigorous stirring. The temperature should not rise above 80-85°C. All the
time the solution in the washing tank should be alkaline (phenolphthalein test).
If all the sodium hydroxide is used up, an additional quantity of it must be added.
After all the crude dinitrobenzene has been introduced to the washing tank its
contents are stirred for a further period of 30 min to one hour.
Sulphitation of dinitrobenzene (Griesheim method) is carried out in the same
washing tank. Cold water is run in until the temperature falls to 66°C. Then a so-
lution of sodium sulphite, prepared by mixing 1000-1100 l. of a 40% solution of
sodium hydrogen sulphite with 400-500 kg of 33% sodium hydroxide, is added.
During sulphitation a temperature of 66-73°C is maintained. After 2 hr of
mixing at this temperature the contents of the vessel are cooled to 50°C. The puri-fied dinitrobenzene crystallizes in rather large crystals. The melting point of the
product should be 88-89°C. The mother liquor is pumped off through a pipe closed