TNT MANUFACTURE^379FIG. 92. Continuous crystallization of TNT (Bofors-Norell method [26]).is dissolved. The solution flows to the crystallizer (1) through an overflow weir (23).
Mechanical impurities are held back by the weir. The crystallizer decks are provided
with apertures (24). Vat (1) is heated, vat (2) is cooled. Here crystallization begins,
and is ended in vat (5).SULPHITATION AND DRYING OF TNTThere are various methods of purification of the TNT with sodium sulphite.
French methodIn the French method the ground TNT is mixed with a 7.5% solution of
Na 2 SO 3 at room temperature for 2 hr. The operation is carried out in wooden
vats, 1.3 m high and 1.3 m in diameter (Fig. 78), using 750 kg of TNT and 750 1.
of the solution at a time. Then the mixture in the vat is drained off to a vacuum
filter, where the TNT is filtered off, washed with water four times, and sent to a drier.
For drying, an iron vessel of construction similar to that of the nitrator, is
charged with TNT (2250 kg). The TNT is melted by passing steam through the
jacket of the drying vessel, after which dry air is passed from the bottom through
the molten product.
The vessel is covered by a hood with a ventilation pipe through which the
moisture escapes (Fig. 93). Initially a temperature of 90°C is maintained, and toward
the end of the operation it is raised to 105°C to let the rest of moisture escape.
@he-dry, molten TNT is -subsequently flaked. The equipment for this, operation
consists of a rotating steel drum (I), cooled by water from inside. The lower part
of the drum is dipped in molten TNT, the temperature of which is about 80°C
(Fig. 94). The cooled surface of the drum causes the TNT to solidify on it, and
a bronze knife (2) scrapes off the TNT coating in fine flakes, which fall into hopper (3).
Flaked TNT is the most convenient form for transport, storage and further use,and is therefore the most widely used form of this substance. Alternatively pure
TNT is sometimes used in the form of granules, obtained by granulating in cold