350 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES- Another method of TNT separation from spent acid consists in bringing
the TNT to crystallization point by stirring and adding water to the nitrator con-
tents until cool. The reaction mixture is then transferred to filters to separate the
crystals from the acid.
From 100 kg of DNT 102 kg of TNT are obtained (theoretical yield 124 kg),
which makes about 82% of the theoretical yield.
When calculated on toluene, 190 kg of TNT are obtained from 100 kg (instead
of 246 kg calculated theoretically) which makes the actual yield nearly 77% of
theory.
Acid usage control. The French method is based on the use of the spent acid
from trinitration for dinitration after it has been fortified, and on further utilization
of the spent acid from dinitration in the mononitration stage. The composition
of the mixtures is calculated as described above.
The spent acid obtained from mononitration by the method described should
contain traces of nitric acid (0.2-0.4% of HNO 3 ). Its absence may be an indication
that the amount of nitric acid used for the nitration was insufficient and consequently
that part of the toluene has remained unnitrated. In such cases the portion of acid
used for the next batches should be richer in HNO 3 , since in the old French method
the unreacted toluene is not distilled off from the mononitrotoluenes.
The H 2 SO 4 , content in the spent acid should not be less than 70%. A lower
content is not advisable, for a less concentrated sulphuric acid may attack the iron
equipment.
FIG. 78. Wooden tank for washing TNT (Pascal [51).Washing the TNT and preparing it for final purification. Crude, separated
TNT, as described above, is transferred to a separator - a heated iron vessel, in
which a temperature of 80°C or somewhat higher is maintained for about 3 hr in
order to keep the product therein in a molten state. The remaining spent acid flows
down to be then drawn off and added to the main portion of the spent acid. The
molten TNT is transferred to a tank in which a preliminary washing (deacidifi-
cation) is effected. For this purpose wooden tanks, shaped like truncated cones.
are used. They are of about 20 m^3 capacity and are equipped with stirrers (Fig. 78).
15 m^3 of water heated to 85°C are run into the tank where this temperature